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EASA Part 66 - Materials and Hardware.doc Issue 1 - 21 March,
2013 Page 1
JAR 66 CATEGORY B1 MODULE 6
MATERIALS AND HARDWARE
engineering uk
CONTENTS
1 INTRODUCTION
.........................................................................................
1-1 2 PROPERTIES OF METALS
........................................................................
2-1
2.1 BRITTLENESS
..........................................................................................
2-1
2.2 CONDUCTIVITY
........................................................................................
2-1
2.3 DUCTILITY
................................................................................................
2-1
2.4 ELASTICITY
..............................................................................................
2-1
2.5 HARDNESS
...............................................................................................
2-1
2.6
MALLEABILITY..........................................................................................
2-1
2.7 PLASTICITY
..............................................................................................
2-1
2.8 TENACITY
.................................................................................................
2-1
2.9 TOUGHNESS
............................................................................................
2-2
2.10 STRENGTH
...............................................................................................
2-2 2.10.1 Tensile Strength
...........................................................................
2-2 2.10.2 Yield Strength
..............................................................................
2-2 2.10.3 Shear Strength
.............................................................................
2-2 2.10.4 Bearing Strength
..........................................................................
2-2
3 TESTING OF MATERIALS
.........................................................................
3-1
3.1 TENSILE TESTING
...................................................................................
3-1 3.1.1 Tensile Strength
...........................................................................
3-1
3.2 LOAD/EXTENSION DIAGRAMS
............................................................... 3-4
3.2.1 Ductility
........................................................................................
3-7 3.2.2 Proof Stress
.................................................................................
3-7
3.3 STIFFNESS
...............................................................................................
3-9
3.4 TENSILE TESTING OF PLASTICS
........................................................... 3-9
3.5 COMPRESSION TEST
............................................................................
3-10
3.6 HARDNESS TESTING
............................................................................
3-10 3.6.1 Brinell Test
.................................................................................
3-10 3.6.2 Vickers Test
...............................................................................
3-11 3.6.3 Rockwell Test
.............................................................................
3-11 3.6.4 Hardness Testing on Aircraft
...................................................... 3-12
3.7 IMPACT TESTING
...................................................................................
3-13
3.8 OTHER FORMS OF MATERIAL TESTING
............................................. 3-14 3.8.1 Creep
.........................................................................................
3-14 3.8.2 Creep in Metals
..........................................................................
3-14 3.8.3 Effect of Stress and Temperature on Creep
............................... 3-15 3.8.4 The Effect of Grain Size
on Creep .............................................. 3-16 3.8.5
Creep in Plastics
........................................................................
3-16 3.8.6 Fatigue
.......................................................................................
3-16 3.8.7 Fatigue Testing
..........................................................................
3-17
3.9 S-N CURVES
..........................................................................................
3-18
3.10 CAUSES OF FATIGUE FAILURE
............................................................
3-20
3.11 VIBRATION
.............................................................................................
3-20
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EASA Part 66 - Materials and Hardware.doc Issue 1 - 21 March,
2013 Page 2
JAR 66 CATEGORY B1 MODULE 6
MATERIALS AND HARDWARE
engineering uk
3.12 FATIGUE METALLURGY
........................................................................
3-21
3.13 FATIGUE PROMOTERS
.........................................................................
3-22 3.13.1 Design
........................................................................................
3-22 3.13.2 Manufacture
...............................................................................
3-23 3.13.3 Environment
...............................................................................
3-23
3.14 FATIGUE PREVENTERS
........................................................................
3-23 3.14.1 Cold Expansion (Broaching)
....................................................... 3-24
3.15 DO'S AND DONT'S PREVENTING FATIGUE FAILURES
.................... 3-25
3.16 STRUCTURAL HEALTH MONITORING (SHM)
....................................... 3-25 3.16.1 Fatigue Meters
...........................................................................
3-25 3.16.2 Strain Gauges
............................................................................
3-25 3.16.3 Fatigue Fuses
............................................................................
3-25 3.16.4 Intelligent Skins Development
.................................................... 3-25
4 AIRCRAFT MATERIALS - FERROUS
........................................................ 4-1
4.1 IRON
.........................................................................................................
4-1 4.1.1 Cast Iron
......................................................................................
4-1 4.1.2 Nodular Cast Iron
.........................................................................
4-1
4.2 STEEL
.......................................................................................................
4-1 4.2.1 Classification of Steels
.................................................................
4-2 4.2.2 Metallurgical Structure of Steel
..................................................... 4-3 4.2.3
Structure and Properties Slow-Cooled Steels
............................ 4-3 4.2.4 Effects of Cooling Rates on
Steels ............................................... 4-4
4.3 HEAT-TREATMENT OF CARBON STEELS
.............................................. 4-4 4.3.1 Associated
Problems - Hardening Process .................................. 4-5
4.3.2 Tempering
....................................................................................
4-6 4.3.3 Annealing
.....................................................................................
4-6 4.3.4 Normalising
..................................................................................
4-6
4.4 SURFACE HARDENING OF STEELS
....................................................... 4-7 4.4.1
Carburising...................................................................................
4-7 4.4.2 Nitriding
........................................................................................
4-8 4.4.3 Flame/Induction Hardening
.......................................................... 4-8
4.4.4 Other Surface Hardening Techniques
.......................................... 4-8
4.5 ALLOYING ELEMENTS IN STEEL
............................................................
4-9
4.6 CARBON
...................................................................................................
4-9 4.6.1 Low-Carbon Steel
........................................................................
4-9 4.6.2 Medium-Carbon Steel
..................................................................
4-9 4.6.3 High-Carbon Steel
........................................................................
4-9
4.7 SULPHUR
.................................................................................................
4-9
4.8 SILICON
....................................................................................................
4-9
4.9 PHOSPHORUS
.......................................................................................
4-10
4.10 NICKEL
...................................................................................................
4-10 4.10.1 Nickel Alloys
...............................................................................
4-10
4.11 CHROMIUM (CHROME)
.........................................................................
4-11 4.11.1 Nickel-Chrome Steel and its Alloys
............................................ 4-11
4.12 COBALT
..................................................................................................
4-11
4.13 VANADIUM
..............................................................................................
4-12
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EASA Part 66 - Materials and Hardware.doc Issue 1 - 21 March,
2013 Page 3
JAR 66 CATEGORY B1 MODULE 6
MATERIALS AND HARDWARE
engineering uk
4.14 MANGANESE
..........................................................................................
4-12
4.15 MOLYBDENUM
.......................................................................................
4-12
4.16 CHROME AND MOLYBDENUM
..............................................................
4-12
4.17 TUNGSTEN
.............................................................................................
4-13
4.18 MARAGING
STEELS...............................................................................
4-13
5 AIRCRAFT MATERIALS - NON-FERROUS
............................................... 5-1
5.1 PURE METALS
.........................................................................................
5-1 5.1.1 Pure Aluminium
............................................................................
5-1 5.1.2 Pure Copper
.................................................................................
5-2 5.1.3 Pure Magnesium
..........................................................................
5-2 5.1.4 Pure Titanium
...............................................................................
5-2
5.2 ALUMINIUM ALLOYS
................................................................................
5-3
5.3 IDENTIFICATION OF ELEMENTS IN ALUMINIUM ALLOYS
.................... 5-3
5.4 CLAD MATERIALS
....................................................................................
5-5
5.5 HEAT-TREATMENT OF ALUMINIUM ALLOYS
......................................... 5-5 5.5.1 Solution
Treatment
.......................................................................
5-6 5.5.2 Age-Hardening
.............................................................................
5-7 5.5.3 Annealing
.....................................................................................
5-7 5.5.4 Precipitation Treatment
................................................................
5-8
5.6 IDENTIFICATION OF HEAT-TREATED ALUMINIUM ALLOYS
................. 5-9
5.7 MARKING OF ALUMINIUM ALLOY SHEETS
.......................................... 5-10
5.8 CAST ALUMINIUM ALLOYS
...................................................................
5-11
5.9 MAGNESIUM ALLOYS
............................................................................
5-11
5.10 COPPER ALLOYS
...................................................................................
5-12
5.11 TITANIUM ALLOYS
.................................................................................
5-13
5.12 WORKING WITH TITANIUM AND TITANIUM ALLOYS
........................... 5-13 5.12.1 Drilling Titanium
.........................................................................
5-14
6 METHODS USED IN SHAPING METALS
.................................................. 6-1
6.1 CASTING
...................................................................................................
6-1 6.1.1 Sand-Casting
...............................................................................
6-1 6.1.2 Advantages/Disadvantages of Sand-Casting
............................... 6-3 6.1.3 Typical Casting
Defects................................................................
6-3 6.1.4 Shell-Moulding
.............................................................................
6-3 6.1.5 Centrifugal-Casting
......................................................................
6-3 6.1.6 Die-Casting
..................................................................................
6-4 6.1.7 Investment-Casting (Lost Wax)
.................................................... 6-4
6.2 FORGING
..................................................................................................
6-5 6.2.1 Drop-Stamping
.............................................................................
6-6 6.2.2 Hot-Pressing
................................................................................
6-6 6.2.3 Upsetting
......................................................................................
6-6
6.3 ROLLING
...................................................................................................
6-7
6.4 DRAWING
.................................................................................................
6-7
6.5 DEEP DRAWING/PRESSING
...................................................................
6-7
6.6 PRESSING
................................................................................................
6-7
6.7 STRETCH-FORMING
................................................................................
6-7
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EASA Part 66 - Materials and Hardware.doc Issue 1 - 21 March,
2013 Page 4
JAR 66 CATEGORY B1 MODULE 6
MATERIALS AND HARDWARE
engineering uk
6.8 RUBBER-PAD FORMING
.........................................................................
6-7
6.9 EXTRUDING
.............................................................................................
6-8 6.9.1 Impact-Extrusion
..........................................................................
6-8
6.10 SINTERING
...............................................................................................
6-8
6.11 SPINNING
.................................................................................................
6-9
6.12 CHEMICAL MILLING
.................................................................................
6-9
6.13 ELECTRO-CHEMICAL MACHINING
......................................................... 6-9
6.14 ELECTRO-DISCHARGE MACHINING E.D.M.
........................................ 6-10
6.15 CONVENTIONAL MACHINING
...............................................................
6-11
6.16 SUPERPLASTIC FORMING
....................................................................
6-12
7 AIRCRAFT MATERIALS - COMPOSITE AND NON-METALLIC
................ 7-1
7.1 PLASTICS
.................................................................................................
7-1 7.1.1 Thermoplastic Materials
............................................................... 7-2
7.1.2 Thermosetting Materials
............................................................... 7-3
7.1.3 Resins
..........................................................................................
7-4 7.1.4 Elastomers
...................................................................................
7-6
7.2 PRIMARY ADVANTAGES OF PLASTICS
................................................. 7-7
7.3 PRIMARY DISADVANTAGES OF PLASTICS
........................................... 7-7
7.4 PLASTIC MANUFACTURING PROCESSES
............................................. 7-8
7.5 COMPOSITE MATERIALS
........................................................................
7-9 7.5.1 Glass Fibre Reinforced Plastic (GFRP)
........................................ 7-9 7.5.2 Carbon Fibre
Reinforced Plastic (CFRP) .................................... 7-10
7.5.3 Aramid Fibre Reinforced Plastic (AFRP)
.................................... 7-11 7.5.4 General Information
...................................................................
7-11 7.5.5 Laminated, Sandwich and Monolithic Structures
........................ 7-12
7.6 NON-METALLIC COMPONENTS
............................................................ 7-13
7.6.1 Seals
..........................................................................................
7-13
8 DETECTING DEFECTS IN COMPOSITE
MATERIALS.............................. 8-1
8.1 CAUSES OF DAMAGE
..............................................................................
8-1
8.2 TYPES OF
DAMAGE.................................................................................
8-1
8.3 INSPECTION METHODS
..........................................................................
8-3 8.3.1 Visual Inspection
..........................................................................
8-3 8.3.2 Ring or Percussion Test
............................................................... 8-3
8.3.3 Ultrasonic Inspection
....................................................................
8-3 8.3.4 Radiography
.................................................................................
8-3
8.4 ASSESSMENT OF DAMAGE
....................................................................
8-4
9 BASIC COMPOSITE REPAIRS
..................................................................
9-1
9.1 REPAIR OF A SIMPLE COMPOSITE PANEL
........................................... 9-2
9.2 REPAIR OF A SANDWICH PANEL
........................................................... 9-3
9.3 GLASS FIBRE REINFORCED COMPOSITE REPAIRS
............................ 9-5
9.4 TYPES OF GLASS REINFORCEMENT
.................................................... 9-5 9.4.1
Uni-Directional Cloth
....................................................................
9-5 9.4.2 Bi-directional Cloth
.......................................................................
9-6 9.4.3 Chopped Strand Mat
....................................................................
9-6 9.4.4 Resin
............................................................................................
9-6
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EASA Part 66 - Materials and Hardware.doc Issue 1 - 21 March,
2013 Page 5
JAR 66 CATEGORY B1 MODULE 6
MATERIALS AND HARDWARE
engineering uk
9.5 POT
LIFE...................................................................................................
9-7
9.6 CURING
....................................................................................................
9-7
9.7 GEL COAT
................................................................................................
9-8
9.8 STORAGE OF GFRP MATERIALS
........................................................... 9-8
9.8.1 Storing Resin
...............................................................................
9-8 9.8.2 Storing Hardener
..........................................................................
9-8 9.8.3 Storing Fabrics
.............................................................................
9-8
9.9 PREPARATION FOR REPAIR
..................................................................
9-9 9.9.1 Surface Preparation
...................................................................
9-11
9.10 TECHNIQUES OF LAMINATING GLASS FIBRE
..................................... 9-11
9.11 PRE-WETTING GLASS FIBRE
...............................................................
9-12
10 ADHESIVES AND SEALANTS
.................................................................
10-1
10.1 THE MECHANICS OF BONDING
............................................................ 10-1
10.1.1 Stresses on a Bonded Joint
....................................................... 10-1 10.1.2
Advantages of Adhesives
........................................................... 10-3
10.1.3 Disadvantages of Adhesives
...................................................... 10-3 10.1.4
Strength of Adhesives
................................................................
10-4
10.2 GROUPS AND FORMS OF ADHESIVES
................................................ 10-4 10.2.1
Flexible Adhesives
.....................................................................
10-4 10.2.2 Structural Adhesives
..................................................................
10-4 10.2.3 Adhesive Forms
.........................................................................
10-4
10.3 ADHESIVES IN USE
...............................................................................
10-5 10.3.1 Surface Preparation
...................................................................
10-5 10.3.2 Final Assembly
...........................................................................
10-5 10.3.3 Typical (Abbreviated) Process
.................................................... 10-6
10.4 SEALING COMPOUNDS
.........................................................................
10-6 10.4.1 One-Part Sealants
......................................................................
10-7 10.4.2 Two-Part Sealants
......................................................................
10-7 10.4.3 Sealant Curing
...........................................................................
10-7
11 CORROSION
............................................................................................
11-1
11.1 CHEMICAL (OXIDATION)
CORROSION................................................. 11-1
11.1.1 Effect of Oxide Thickness
........................................................... 11-2
11.1.2 Effect of Temperature
................................................................
11-3 11.1.3 Effect of Alloying
........................................................................
11-4
11.2 ELECTROCHEMICAL (GALVANIC) CORROSION
................................. 11-5 11.2.1 The Galvanic Cell
.......................................................................
11-5 11.2.2 Factors Affecting the Rate of Corrosion in a Galvanic
Cell. ........ 11-6
11.3 TYPES OF CORROSION
........................................................................
11-8 11.3.1 Surface Corrosion
......................................................................
11-8 11.3.2 Dissimilar Metal Corrosion
......................................................... 11-8
11.3.3 Intergranular Corrosion
.............................................................. 11-9
11.3.4 Exfoliation Corrosion
.................................................................11-10
11.3.5 Stress Corrosion
.......................................................................11-10
11.3.6 Fretting Corrosion
.....................................................................11-11
11.3.7 Crevice Corrosion
.....................................................................11-11
11.3.8 Filiform Corrosion
......................................................................11-11
11.3.9 Pitting Corrosion
.......................................................................11-12
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JAR 66 CATEGORY B1 MODULE 6
MATERIALS AND HARDWARE
engineering uk
11.3.10 Corrosion
Fatigue......................................................................11-13
11.3.11 Microbiological
Contamination...................................................11-13
11.3.12 Hydrogen Embrittlement of Steels
.............................................11-13
11.4 FACTORS AFFECTING CORROSION
...................................................11-14 11.4.1
Climatic
.....................................................................................11-14
11.4.2 Size and Type of Metal
.............................................................11-14
11.4.3 Corrosive
Agents.......................................................................11-14
11.5 COMMON METALS AND CORROSION PRODUCTS
............................11-15 11.5.1 Iron and Steel
............................................................................11-15
11.5.2 Aluminium Alloys
.......................................................................11-15
11.5.3 Magnesium Alloys
.....................................................................11-16
11.5.4 Titanium
....................................................................................11-16
11.5.5 Copper Alloys
............................................................................11-16
11.5.6 Cadmium and Zinc
....................................................................11-16
11.5.7 Nickel and Chromium
................................................................11-17
11.6 CORROSION REMOVAL
.......................................................................11-17
11.6.1 Cleaning and Paint Removal.
....................................................11-17 11.6.2
Corrosion of Ferrous Metals
......................................................11-18 11.6.3
High-Stressed Steel Components
.............................................11-18 11.6.4 Aluminium
and Aluminium Alloys
..............................................11-18 11.6.5 Alclad
........................................................................................11-19
11.6.6 Magnesium Alloys
.....................................................................11-19
11.6.7 Acid Spillage
.............................................................................11-20
11.6.8 Alkali Spillage
............................................................................11-20
11.6.9 Mercury Spillage
.......................................................................11-21
11.7 PERMANENT ANTI-CORROSION TREATMENTS
................................11-22 11.7.1 Electro-Plating
...........................................................................11-22
11.7.2 Sprayed Metal Coatings
............................................................11-22
11.7.3 Cladding
....................................................................................11-22
11.7.4 Surface Conversion Coatings
....................................................11-23
11.8 LOCATIONS OF CORROSION IN AIRCRAFT
.......................................11-23 11.8.1 Exhaust Areas
...........................................................................11-23
11.8.2 Engine Intakes and Cooling Air Vents
.......................................11-23 11.8.3 Landing Gear
............................................................................11-24
11.8.4 Bilge and Water Entrapment Areas
...........................................11-24 11.8.5 Recesses in
Flaps and Hinges
..................................................11-24 11.8.6
Magnesium Alloy Skins
.............................................................11-24
11.8.7 Aluminium Alloy Skins
...............................................................11-24
11.8.8 Spot-Welded Skins and Sandwich Constructions
......................11-25 11.8.9 Electrical Equipment
.................................................................11-25
11.8.10 Miscellaneous Items
..................................................................11-25
12 AIRCRAFT FASTENERS
..........................................................................
12-1
12.1 TEMPORARY JOINTS
............................................................................
12-1
12.2 PERMANENT JOINTS
.............................................................................
12-1
12.3 FLEXIBLE JOINTS
..................................................................................
12-1
12.4 SCREW THREADS
.................................................................................
12-2 12.4.1 The Inclined Plane and the Helix
................................................ 12-2
12.5 SCREW THREAD TERMINOLOGY
......................................................... 12-4
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EASA Part 66 - Materials and Hardware.doc Issue 1 - 21 March,
2013 Page 7
JAR 66 CATEGORY B1 MODULE 6
MATERIALS AND HARDWARE
engineering uk
12.5.1 Screw Thread Forms
..................................................................
12-6 12.5.2 Other Thread Forms
...................................................................
12-8 12.5.3 Classes of Fit
.............................................................................
12-8 12.5.4 Measuring Screw Threads
......................................................... 12-9
12.6 BOLTS
....................................................................................................12-10
12.6.1 British Bolts
...............................................................................12-10
12.6.2 Identification of BS Unified Bolts
...............................................12-10 12.6.3
American Bolts
..........................................................................12-13
12.6.4 Identification of AN Standard Bolts
............................................12-14 12.6.5
Special-to-Type Bolts
................................................................12-16
12.6.6 Metric Bolts
...............................................................................12-17
12.7 NUTS
......................................................................................................12-18
12.7.1 Stiffnuts and Anchor Nuts
..........................................................12-19
12.8 SCREWS
................................................................................................12-22
12.8.1 Machine Screws
........................................................................12-22
12.8.2 Structural Screws
......................................................................12-24
12.8.3 Self-Tapping Screws
.................................................................12-24
12.9 STUDS
...................................................................................................12-25
12.9.1 Standard Studs
.........................................................................12-26
Waisted Studs
........................................................................................12-26
12.9.3 Stepped Studs
..........................................................................12-27
12.9.4 Shouldered
Studs......................................................................12-27
12.10 THREAD INSERTS
................................................................................12-27
12.10.1 Wire Thread Inserts
...................................................................12-27
12.10.2 Thin Wall Inserts
.......................................................................12-28
12.11 DOWELS AND PINS
..............................................................................12-29
12.11.1 Dowels
......................................................................................12-29
12.11.2 Roll Pins
....................................................................................12-29
12.11.3 Clevis Pins
................................................................................12-30
12.11.4 Taper
Pins.................................................................................12-30
12.12 LOCKING DEVICES
...............................................................................12-31
12.12.1 Spring Washers
........................................................................12-31
12.12.2 Shake-Proof Washers
...............................................................12-32
12.12.3 Tab Washers
.............................................................................12-33
12.12.4 Lock Plates
...............................................................................12-34
12.12.5 Split (Cotter) Pins
......................................................................12-34
12.13 LOCKING WIRE
.....................................................................................12-35
12.13.1 Use of Locking Wire with Turnbuckles
.......................................12-37 12.13.2 Use of Locking
Wire with Locking Tabs. ....................................12-37
12.13.3 Thin Copper Wire
......................................................................12-38
12.14 QUICK-RELEASE FASTENERS
............................................................12-38
12.14.1 Dzus Fasteners
.........................................................................12-38
12.14.2 Oddie Fasteners
.......................................................................12-39
12.14.3 Camloc Fasteners
.....................................................................12-40
12.14.4 Airloc Fasteners
........................................................................12-41
12.14.5 Pip-Pins
....................................................................................12-41
12.14.6 Circlips and Locking Rings
........................................................12-42
12.14.7 Keys and Keyways
....................................................................12-43
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EASA Part 66 - Materials and Hardware.doc Issue 1 - 21 March,
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JAR 66 CATEGORY B1 MODULE 6
MATERIALS AND HARDWARE
engineering uk
12.14.8 Peening
.....................................................................................12-44
12.15 GLUE/ADHESIVE BONDED JOINTS
.....................................................12-45 12.15.1
Locking by Adhesives
...............................................................12-45
12.15.2 Loctite
.......................................................................................12-46
12.15.3 Synthetic Resin Adhesives
........................................................12-46
12.15.4 Testing of Adhesive Joining Techniques
...................................12-46
12.16 METAL-TO-METAL BONDED JOINTS
...................................................12-46 12.16.1
Welding
.....................................................................................12-46
12.16.2 Soft Soldering
...........................................................................12-47
12.16.3 Hard Soldering
..........................................................................12-47
13 AIRCRAFT RIVETS
..................................................................................
13-1
13.1 SOLID RIVETS
........................................................................................
13-1
13.2 RIVET IDENTIFICATION
.........................................................................
13-2 13.2.1 Solid Rivets (British)
...................................................................
13-2 13.2.2 Rivet Identification (British)
......................................................... 13-3
13.2.3 Rivet Material Identification (British)
........................................... 13-3 13.2.4 Solid
Rivets (American)
.............................................................. 13-5
13.2.5 Rivet Identification
(American).................................................... 13-6
13.2.6 Rivet Material Identification (American)
...................................... 13-6
13.3 HEAT-TREATMENT/REFRIGERATION OF SOLID RIVETS
................... 13-7 13.3.1 Heat-Treatment.
.........................................................................
13-8 13.3.2 Refrigeration.
.............................................................................
13-8 13.3.3 Use of Different Types of Rivet Head
......................................... 13-8
13.4 BLIND AND HOLLOW RIVETS
...............................................................
13-9 13.4.1 Friction Lock Rivets
...................................................................13-10
13.4.2 Mechanical Lock
Rivets.............................................................13-11
13.4.3 Hollow/Pull-Through Rivets
.......................................................13-12 13.4.4
Grip
Range................................................................................13-12
13.4.5 Tucker Pop Rivets
...................................................................13-13
13.4.6 Avdel Rivets
..............................................................................13-14
13.4.7 Chobert Rivets
..........................................................................13-15
13.4.8 Cherry Rivets
............................................................................13-16
13.5 MISCELLANEOUS FASTENERS
...........................................................13-16
13.5.1 Hi-Lok Fasteners
.......................................................................13-16
13.5.2 Hi-Tigue Fasteners
....................................................................13-17
13.5.3 Hi-Shear Fasteners
...................................................................13-18
13.6 SPECIAL PURPOSE FASTENERS
........................................................13-19
13.6.1 Jo-Bolts
.....................................................................................13-19
13.6.2 Tubular Rivets.
..........................................................................13-20
13.6.3 Rivnuts
......................................................................................13-21
14 SPRINGS
..................................................................................................
14-1
14.1 FORCES EXERTED ON, AND APPLIED BY, SPRINGS
......................... 14-1
14.2 TYPES OF SPRINGS
..............................................................................
14-1 14.2.1 Flat Springs
................................................................................
14-1 14.2.2 Leaf Springs
...............................................................................
14-2 14.2.3 Spiral Springs
.............................................................................
14-2 14.2.4 Helical Compression and Tension Springs
................................. 14-2 14.2.5 Helical Torsion
Springs
..............................................................
14-2
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14.2.6 Belleville (Coned Disc) Springs
.................................................. 14-2 14.2.7
Torsion-Bar Springs
...................................................................
14-2
14.3 MATERIALS FROM WHICH SPRINGS ARE MANUFACTURED ............
14-2 14.3.1 Steels used for Cold-Wound
Springs.......................................... 14-2 14.3.2
Steels used for Hot-Wound Springs
........................................... 14-3 14.3.3 Steels used
for Cold-Rolled, Flat Springs ...................................
14-3 14.3.4 Non-Ferrous Metals used for Springs
......................................... 14-3 14.3.5 Composite
Materials used for Springs ........................................
14-4
14.4 CHARACTERISTICS OF TYPICAL AEROSPACE SPRINGS
.................. 14-5
14.5 APPLICATIONS OF SPRINGS IN AIRCRAFT ENGINEERING
............... 14-6
15 PIPES AND UNIONS
................................................................................
15-1
15.1 RIGID PIPES
...........................................................................................
15-1
15.2 SEMI-RIGID FLUID LINES (TUBES)
....................................................... 15-2 15.2.1
Flared End-Fittings
.....................................................................
15-2 15.2.2 Flare-Less Couplings
.................................................................
15-3
15.3 FLEXIBLE PIPES (HOSES)
.....................................................................
15-4 15.3.1 Low-Pressure Hoses
..................................................................
15-5 15.3.2 Medium-Pressure Hoses
............................................................ 15-5
15.3.3 High-Pressure Hoses
.................................................................
15-6
15.4 UNIONS AND CONNECTORS
................................................................
15-7 15.4.1 Aircraft General Standards (AGS)
.............................................. 15-8 15.4.2 Air
Force and Navy (AN)
............................................................ 15-8
15.4.3 Military Standard (MS)
...............................................................
15-8
15.5 QUICK-RELEASE COUPLINGS
..............................................................
15-8
16 BEARINGS
................................................................................................
16-1
16.1 BALL BEARINGS
....................................................................................
16-2 16.1.1 Radial Bearings
..........................................................................
16-2 16.1.2 Angular-Contact Bearings
.......................................................... 16-2
16.1.3 Thrust Bearings
..........................................................................
16-2 16.1.4 Instrument Precision
Bearings....................................................
16-2
16.2 ROLLER BEARINGS
...............................................................................
16-3 16.2.1 Cylindrical Roller Bearings
......................................................... 16-3
16.2.2 Spherical Roller Bearings
........................................................... 16-3
16.2.3 Tapered Roller Bearings
............................................................
16-3
16.3 BEARING INTERNAL CLEARANCE
....................................................... 16-4 16.3.1
Group 2 (One Dot) Bearings
..................................................... 16-4 16.3.2
Normal Group (Two Dot) Bearings
........................................... 16-4 16.3.3 Group 3
(Three Dot) Bearings
.................................................. 16-4 16.3.4
Group 4 (Four Dot) Bearings
.................................................... 16-4
16.4 BEARING MAINTENANCE
......................................................................
16-5 16.4.1 Lubrication
.................................................................................
16-5 16.4.2 Inspection
..................................................................................
16-5
17 TRANSMISSIONS
....................................................................................
17-1
17.1 BELTS AND PULLEYS
............................................................................
17-1
17.2 GEARS
....................................................................................................
17-3 17.2.1 Gear Trains and Gear Ratios
..................................................... 17-3 17.2.2
Spur
Gears.................................................................................
17-4
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17.2.3 Helical Gears
.............................................................................
17-4 17.2.4 Bevel Gears
...............................................................................
17-4 17.2.5 Worm and Wheel Gears
............................................................. 17-4
17.2.6 Planetary (Epicyclic) Reduction Gear Train
................................ 17-5 17.2.7 Spur and Pinion
Reduction Gear Train ....................................... 17-6
17.2.8 Accessory Unit Drives
................................................................
17-6 17.2.9 Meshing Patterns
.......................................................................
17-7
17.3 CHAINS AND SPROCKETS
....................................................................
17-8 17.3.1 Typical Arrangements - Chain Assemblies
................................. 17-9
17.4 MAINTENANCE INSPECTIONS
.............................................................17-10
18 CONTROL
CABLES..................................................................................
18-1
18.1 TYPES OF CABLES
................................................................................
18-1
18.2 CABLE SYSTEM COMPONENTS
........................................................... 18-2
18.2.1 End-Fittings
................................................................................
18-2 18.2.2 Turnbuckles
...............................................................................
18-3 18.2.3 Cable Tensioning Devices
.......................................................... 18-4
18.2.4 Cable Fairleads
..........................................................................
18-5 18.2.5 Pulleys
.......................................................................................
18-6
18.3 FLEXIBLE CONTROL SYSTEMS
............................................................ 18-7
18.3.1 Bowden Cables
..........................................................................
18-7 18.3.2 Teleflex Control Systems
...........................................................
18-9
19 ELECTRICAL CABLES & CONNECTORS
............................................... 19-1
19.1 CABLE SPECIFICATION
.........................................................................
19-1
19.2 CABLE IDENTIFICATION
........................................................................
19-1
19.3 DATA BUS CABLE
..................................................................................
19-5
19.4 CONDUCTOR MATERIAL & INSULATION
............................................. 19-6
19.5 WIRE SIZE
..............................................................................................
19-7
19.6 WIRE RESISTANCE
...............................................................................
19-8
19.7 CURRENT CARRYING CAPABILITY
...................................................... 19-8
19.8 VOLTAGE DROP
...................................................................................19-10
19.9 WIRE IDENTIFICATION
.........................................................................19-11
19.10 WIRE INSTALLATION AND ROUTING
..................................................19-12
19.11 OPEN WIRING
.......................................................................................19-12
19.12 WIRE & CABLE CLAMPING
...................................................................19-13
19.13 CONDUIT
...............................................................................................19-14
19.14 CONNECTORS
......................................................................................19-16
19.15 CRIMPING
..............................................................................................19-19
19.16 CRIMPING TOOLS
.................................................................................19-20
19.17 WIRE SPLICING
.....................................................................................19-21
19.18 BEND RADIUS
.......................................................................................19-22
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1 INTRODUCTION
The variety of materials and hardware used in aircraft
engineering is vast, and this module will only deal with a broad
group of materials, their main characteristics, identification and
uses. These materials can be classed into the three main categories
of Ferrous Metals, Non-Ferrous Metals and Non-Metallic materials.
Additionally, combinations (Composites) of many of these materials
will be found, in use, in the aerospace industry. The usefulness of
any materials may be enhanced as a result of the addition of other
materials that alter the basic characteristics to suit the specific
requirements of the aircraft designer. A metals usefulness is
governed principally by the physical properties it possesses. Those
properties depend upon the composition of the metal, which can be
changed considerably by alloying it with other metals and by
heat-treatment. The strength and hardness of steel, for example,
can be intensified by increasing its carbon content, adding
alloying metals such as Nickel and Tungsten, or by heating the
steel until red-hot and then cooling it rapidly. Apart from the
basic requirement of more and more strength from metals, other,
less obvious characteristics can also be added or improved upon,
when such features as permanent magnetism, corrosion resistance and
high-strength whilst operating at elevated temperatures, are
desired. Composites make up a large part of the construction of
modern aircraft. In the early days, composites and plastics were
limited to non-structural, internal cosmetic panels, small fairings
and other minor parts. Today there are many large aircraft, which
have major structural and load-carrying parts manufactured from
composites. Composite materials, in addition to maintaining or
increasing component strength, contribute to the important factor
of weight saving. There are also many modern light aircraft that
are almost totally manufactured from composites and contain little
metal at all.
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INTENTIONALLY BLANK
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2 PROPERTIES OF METALS
The various properties of metals can be assessed, by accurate
laboratory tests on sample pieces. The terminology, associated with
these properties, is outlined in the following paragraphs.
2.1 BRITTLENESS
The tendency of the metal to shatter, without significant
deformation. It will shatter under a sudden, low stress but will
resist a slowly-applied, higher load.
2.2 CONDUCTIVITY
The ability of a metal to conduct heat, (thermal conductivity)
and electricity. Silver and copper are excellent thermal and
electrical conductors.
2.3 DUCTILITY
The property of being able to be permanently extended by a
tensile force. It is measured during a tensile, or stretching,
test, when the amount of stretch (elongation), for a given applied
load, provides an indication of a metals ductility.
2.4 ELASTICITY
The ability of a metal to return to its original shape and size
after the removal of any distorting force. The Elastic Limit is the
greatest force that can be applied without permanent
distortion.
2.5 HARDNESS
The ability of a metal to resist wear and penetration. It is
measured by pressing a hardened steel ball or diamond point into
the metals surface. The diameter or depth of the resulting
indentation provides an indication of the metals hardness.
2.6 MALLEABILITY
The ease with which the metal can be forged, rolled and extruded
without fracture. Stresses, induced into the metal, by the forming
processes, have to be subsequently relieved by heat-treatment. Hot
metal is more malleable than cool metal.
2.7 PLASTICITY
The ability to retain a deformation after the load producing it
has been removed. Plasticity is, in fact, the opposite of
elasticity.
2.8 TENACITY
The property of a metal to resist deformation when subjected to
a tensile load. It is proportional to the maximum stress required
to cause the metal to fracture.
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2.9 TOUGHNESS
The ability of a metal to resist suddenly applied loads. A
metals toughness is tested by impact with a swinging pendulum of
known mass.
2.10 STRENGTH
There are several different measurements of the strength of a
metal, as may be seen from the following sub-paragraphs
2.10.1 TENSILE STRENGTH
The ability to resist tension forces applied to the metal
2.10.2 YIELD STRENGTH
The ability to resist deformation. After the metal yields, it is
said to have passed its yield point.
2.10.3 SHEAR STRENGTH
The ability to resist side-cutting loads - such as those,
imposed on the shank of a rivet, when the materials it is joining
attempt to move apart in a direction normal to the longitudinal
axis of the rivet.
2.10.4 BEARING STRENGTH
The ability of a metal to withstand a crushing force.
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3 TESTING OF MATERIALS
The mechanical properties of a material must be known before
that material can be incorporated into any design. Mechanical
property data is compiled from extensive material testing. Various
tests are used to determine the actual values of material
properties under different loading applications and test
conditions.
3.1 TENSILE TESTING
Tensile testing is the most widely-used mechanical test. It
involves applying a steadily increasing load to a test specimen,
causing it to stretch until it eventually fractures. Accurate
measurements are taken of the load and extension, and the results
are used to determine the strength of the material. To ensure
uniformity of test results, the test specimens used must conform to
standard dimensions and finish as laid down by the appropriate
Standards Authority (BSI, DIN, ISO etc). The cross-section of the
specimen may be round or rectangular, but the relationship between
the cross-sectional area and a specified "gauge length", of each
specimen, is constant. The gauge length, is that portion of the
parallel part of the specimen, which is to be used for measuring
the subsequent extension during and/or after the test.
3.1.1 TENSILE STRENGTH
Tensile strength in a material is obtained by measuring the
maximum load, which the test piece is able to sustain, and dividing
that figure by the original cross-sectional area (c.s.a.) of the
specimen. The value derived from this simple calculation is called
STRESS.
Note: The units of Stress may be quoted in the old British
Imperial (and American) units of lbf/in2, tonf/in2 (also psi and
tsi), or the European and SI units such as kN/m2, MN/m2 and kPa or
MPa.
)2(mm c.s.a. Original
(N) Load Stress =
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Example 1 A steel rod, with a diameter of 5 mm, is loaded in
tension with a force of 400 N. Calculate the tensile stress.
Exercise 1 Calculate the tensile stress in a steel rod, with a
cross-section of 10 mm x 4 mm, when it is subjected to a load of
100 N. Exercise 2 Calculate the cross-sectional area of a tie rod
which, when subjected to a load of 2,100N, has a stress of 60
N/mm2. Note: When calculating stress in large structural members,
it may be more convenient to measure load in Mega-Newtons (MN, or
N6) and the area in square metres (m2). When using such units, the
numerical value is identical to that if the calculation had been
made using Newtons and mm2. i.e. A Stress of 1 N/mm2 = l MN/m2
Example 2
A structural member, with a cross-sectional area of 05m2, is
subjected to a load of 10 MN. Calculate the stress in the member
in; (a) MN/m2 and (b) N/mm2
(a)
(b)
Area
Load Stress = 2
22/3720
52
400400mmN
r=
=
Area
Load Stress = 2m/MN20
50
10====
222 N/mm 20 Stress So MN/m 1 N/mm ==1
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As the load in the tensile test is increased from zero to a
maximum value, the material extends in length. The amount of
extension, produced by a given load, allows the amount of induced
STRAIN to be calculated. Strain is calculated by measuring the
extension and dividing by the original length of the material.
Note: Both measurements must be in the same units, though, since
Strain is a
ratio of two lengths, it has no units.
Example 3 An aluminium test piece is marked with a 20 mm gauge
length. It is subjected to
tensile load until its length becomes 2115 mm. Calculate the
induced strain.
Exercise 3 A tie rod 1.5m long under a tensile load of 500 N
extends by 12 mm. Calculate the strain.
Length Original
Extension Strain =
mm 151 20 - 1521 Extension ==
units) (no 05750 20
151
Length Original
ExtensionStrain ====
========
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3.2 LOAD/EXTENSION DIAGRAMS
If a gradually increasing tensile load is applied to a test
piece while the load and extension are continuously measured, the
results can be used to produce a Load/Extension diagram or graph
(refer to Fig. 1). Obviously a number of different forms of graph
may be obtained, depending on the material type and condition, but
the example shows a Load/Extension diagram which typifies many
metallic materials when stressed in tension.
Load/Extension Diagram
Fig 1 The graph can be considered as comprising two major
regions. Between points 0 and A, the material is in the Elastic
region (or phase), i.e. when the load is removed the material will
return to its original size and shape. In this region, the
extension is directly proportional to the applied load. This
relationship is known as Hooke's Law, which states: Within the
elastic region, elastic strain is directly proportional to the
stress causing it. Point A is the Elastic Limit. Between this point
and point B, the material continues to extend until the maximum
load is reached (at point B). In this region the material is in the
plastic phase. When the load is removed, the material does not
return to its original size and shape, but will retain some
extension. After point B, the cross-sectional area reduces and the
material begins to neck. The material continues to extend under
reduced load until it eventually fractures at point C.
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Aircraft structural designers interest in materials does not
extend greatly beyond the elastic phase of materials. Production
engineers, however, are greatly interested in material properties
beyond this phase, since the forming capabilities of materials are
dependent on their properties in the plastic phase. An examination
of a graph, obtained from the results of a tensile test on mild
steel (refer to Fig. 2), shows that considerable plastic extension
occurs without any increase in load shortly after the elastic limit
is reached. The onset of increasing extension, without a
corresponding increase in load, at point `B', is known as the yield
point and, if this level of stress is reached, the metal is said to
have yielded. This is a characteristic of mild steel and a few
other, relatively ductile, materials.
Load/Extension Diagram for Mild Steel
Fig. 2 If, after passing the yield point, the load is further
increased, it may be seen that mild steel is capable of
withstanding this increase until the Ultimate Tensile Stress (UTS)
is reached. Severe necking then occurs and the material will
fracture at a reduced load. The unexpected ability of mild steel to
accept more load after yielding is due to strain-hardening of the
material. Work-hardening of many materials is often carried out to
increase their strength.
Point B Yield Point
UTS
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As previously stated, various forms of load/extension curves may
be constructed for other materials (refer to Fig 3), and their
slopes will depend on whether the materials are brittle, elastic or
plastic.
Load/Extension Graphs for Brittle, Elastic and Plastic
Materials
Fig. 3
(a) represents a brittle material (glass)
(b) represents a material with some elasticity and limited
plasticity (high-carbon steel
(c) represents a material with some elasticity and good
plasticity (e.g. soft aluminium).
Zero Elongation
Small Elongation
Large Elongation
Plastic Region Point of Fracture
(a) (b) (c)
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3.2.1 DUCTILITY
After fracture of a specimen, following tensile testing, an
indication of material ductility is arrived at, by establishing the
amount of plastic deformation which occurred. The two indicators of
ductility are:
Elongation
Reduction in area (at the neck) Elongation is the more reliable,
because it is easier to measure the extension of the gauge length
than the reduction in area. The standard measure of ductility is to
establish the percentage elongation after fracture. Example 4 In a
tensile test, on a specimen with 150.5 mm gauge length, the length
over the gauge marks at fracture were 176.1 mm. What was the
percentage elongation?
3.2.2 PROOF STRESS
Many materials do not exhibit a yield point, so a substitute
value must be employed. The value chosen is the Proof Stress, which
is defined as:
The tensile stress, which is just sufficient to produce a
non-proportional elongation, equal to a specified percentage of the
original gauge length.
Usually a value of 0.1% or 0.2% is used for Proof Stress, and
the Proof Stress is then referred to as the 0.1% Proof Stress or
the 0.2% Proof Stress respectively. The Proof Stress may be
acquired from the relevant Load/Extension graph (refer to Fig 4) as
follows:
If the 0.2% Proof Stress is required, then 0.2% of the gauge
length is marked on the extension axis. A line, parallel to the
straight-line portion of the graph, is drawn until it intersects
the non-linear portion of the curve. The corresponding load is then
read from the graph. Proof Stress is calculated by dividing this
load by the original cross-sectional area.
100 Length Gauge Original
Extension Final elongation Percentage
17% 17.009% 100 5150
150.5 - 176.1 100
Length Gauge
Extension Final Elongation ==
==
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0.1% Proof Stress will produce permanent set equivalent to one
thousandth of the specimen's original length. 0.2% Proof Stress
will produce permanent set equivalent to one five hundredth of the
original length.
Acquiring the Proof Stress from a Load/Extension Graph
Fig. 4
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3.3 STIFFNESS
Within the elastic range of a material, if the Strain is
compared to the Stress causing that extension, it will provide a
measure of stiffness/rigidity or flexibility.
This value, which is of great importance to designers, is known
as the Modulus of Elasticity, or Youngs Modulus, and is signified
by use of the symbol E. Thus E = Stress divided by Strain and,
since Strain has no units, the unit for `E' is the same as Stress.
i.e. lbf/in2, tonf/in2 (also psi and tsi), or the European and SI
units such as kN/m2, MN/m2 and kPa or MPa. The actual numerical
value is usually large, as it is a measure of the stress required
to theoretically double the length of a specimen (if it did not
break first). A typical value of E for steel would be 30 x 106 psi.
or 210,000 MN/m2
Relative stiffness values for some common materials (using
Rubber as a datum), are:
Wood 2000 x
Aluminium 10,000 x
Steel 30,000 x
Diamond 171,000 x
3.4 TENSILE TESTING OF PLASTICS
This is conducted in the same way as for metals, but the test
piece is usually made from sheet material. Although the basic
load/extension curve for some plastics is somewhat similar to metal
curves, changes in test temperature or the rate of loading can have
a major effect on the actual results. Even though the material
under test may be in the elastic range, the specimen may take some
time to return to its original size after the load is removed.
stiffness of measure a is Strain
Stress .ie
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3.5 COMPRESSION TEST
Machines for compression testing are often the same as those
used for tensile testing, but the test specimen is in the form of a
short cylinder. The Load/Deflection graph in the elastic phase for
ductile materials is similar to that in the tensile test. The value
of `E' is the same in compression as it is in tension. Compression
testing is seldom used as an acceptance test for metallic or
plastic materials (except for cast iron). Compression testing is
generally restricted to building materials and research into the
properties of new materials.
3.6 HARDNESS TESTING
The hardness of materials is found by measuring their resistance
to indentation. Various methods are used, but the most common are
those of the Brinell, Vickers and Rockwell Hardness Tests.
3.6.1 BRINELL TEST
In the Brinell Hardness Test (refer to Fig. 5), a hardened steel
ball is forced into the surface of a prepared specimen, using a
calibrated force, for a specified time. The diameter of the
resulting indentation is then measured accurately, using a
graduated microscope and, thus, the area of the indentation is
calculated. The hardness number is determined by reference to a
Brinell Hardness Number (BHN) chart.
The Brinell Hardness Test Fig. 5
Diameter (Area) of resulting Indentation
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3.6.2 VICKERS TEST
The Vickers Hardness Test is similar to the Brinell test but
uses a square-based diamond pyramid indenter (refer to Fig. 6). The
diagonals, of the indentation, are accurately measured, by a
special microscope, and the Hardness Value (HV) is again determined
by reference to a chart.
The Vickers Hardness Test Fig. 6
3.6.3 ROCKWELL TEST
The Rockwell Hardness Test (refer to Fig. 7) also uses
indentation as its basis, but two types of indenter are used. A
conical diamond indenter is employed for hard materials and a steel
ball is used for soft materials. The hardness number, when using
the steel ball, is referred to as Rockwell B (e.g. RB 80) and the
diamond hardness number is known as Rockwell C (e.g. RC 65). Note:
Whereas Brinell and Vickers hardness values are based upon the area
of indentation, the Rockwell values are based upon the depth of the
indentation.
The Rockwell Hardness Test Fig. 7
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No precise relationship exists between the various hardness
numbers, but approximate relationships have been compiled. Some
comparative values between Brinell Vickers and Rockwell are shown
in Table 1. Table 1 COMPARATIVE HARDNESS VALUES
MATERIAL BHN HV ROCKWELL
Aluminium alloy 100 100 B 57
Mild steel 130 130 B 73
Cutting tools 650 697 C 60
Note: There is a good correlation between hardness and U.T.S. on
some
materials (e.g. steels)
3.6.4 HARDNESS TESTING ON AIRCRAFT
It is not normal to use Brinell, Rockwell or Vickers testing
methods on aircraft in the hangar. There are, however, portable
Hardness Testers, which may be used to test for material hardness
on items such as aircraft wheels, after an over-heat condition,
because the over-heat condition may cause the wheel material to
become soft or partially annealed.
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3.7 IMPACT TESTING
The impact test (refer to Fig. 8) is designed to determine the
toughness of a material and the two most commonly used methods are
those using the Charpy and Izod impact-testing machines. Both tests
use notched-bar test pieces of standard dimensions, which are
struck by a fast-moving, weighted pendulum. The energy, which is
absorbed by the test piece on impact, will give a measure of
toughness. A brittle material will break easily and will absorb
little energy, so the swing of the pendulum (which is recorded
against a calibrated scale) will not be reduced significantly. A
tough material will, however, absorb considerably more energy and
thus greatly reduce the recorded pendulum swing. Most materials
show a drop in toughness with a reduction in temperature, though
some materials (certain steels in particular) show a rapid drop in
toughness as the temperature is progressively reduced. This
temperature range is called the Transition Zone, and components,
which are designed for use at low temperature, should be operated
above the materials Transition Temperature. Nickel is one of the
most effective alloying elements for lowering the Transition
Temperature of steels
Impact Test Fig. 8
.
Test Piece
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3.8 OTHER FORMS OF MATERIAL TESTING
Although some of the more important forms of material testing
have already been discussed, there are several other forms of
material testing to be considered, not least important of which are
those associated with Creep and Fatigue Testing.
3.8.1 CREEP
Creep can be defined as the continuing deformation, with the
passage of time, of materials subjected to prolonged stress. This
deformation is plastic and occurs even though the acting stress may
be well below the yield stress of the material. At temperatures
below 0.4T (where T is the melting point of the material in
Kelvin), the creep rate is very low, but, at higher temperatures,
it becomes more rapid. For this reason, creep is commonly regarded
as being a high-temperature phenomenon, associated with
super-heated steam plant and gas turbine technology. However, some
of the soft, low-melting point materials will creep significantly
at, or a little above, ambient temperatures and some aircraft
materials may creep when subjected to over-heat conditions.
3.8.2 CREEP IN METALS
When a metallic material is suitably stressed, it undergoes
immediate elastic deformation. This is then followed by plastic
strain, which occurs in three stages (refer to Fig. 9):
Primary Creep - begins at a relatively rapid rate, but then
decreases with time as strain-hardening sets in.
Secondary Creep - the rate of strain is fairly uniform and at
its lowest value.
Tertiary Creep - the rate of strain increases rapidly, finally
leading to rupture. This final stage coincides with gross necking
of the component, prior to failure. The rate of creep is at a
maximum in this phase.
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3.8.3 EFFECT OF STRESS AND TEMPERATURE ON CREEP
Both stress and temperature have an effect on creep. At low
temperature or very low stress, primary creep may occur, but this
falls to a negligible value in the secondary stage, due to
strain-hardening of the material. At higher stress and/or
temperature, however, the rate of secondary creep will increase and
lead to tertiary creep and inevitable failure. It is clear, from
the foregoing, that short-time tensile tests do not give reliable
information for the design of structures, which must carry static
loads over long periods of time, at elevated temperatures. Strength
data, determined from long- time creep tests (up to 10,000 hours),
are therefore essential. Although actual design data are based on
the long-time tests, short-time creep tests are sometimes used as
acceptance tests.
Three Stages of Creep Fig. 9
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3.8.4 THE EFFECT OF GRAIN SIZE ON CREEP
Since the creep mechanism is partly due to microscopic flow
along the grain boundaries, creep resistance is improved by
increased grain size, due to the reduced grain boundary region per
unit volume. It is mainly for this reason that some modern,
high-performance turbine blades are being made from directionally
solidified (and, alternatively, improved single-crystal)
castings.
3.8.5 CREEP IN PLASTICS
Plastics are also affected by creep and show similar, though not
identical, behaviour to that described for metals. Since most
plastics possess lower thermal properties than metals, the choice
of plastic for important applications, particularly at elevated
temperature, must take creep considerations into account.
3.8.6 FATIGUE
An in-depth survey, in recent years, revealed that over 80
percent of failures of engineering components were caused by
fatigue. Consequences of modern engineering have led to increases
in operating stresses, temperatures and speeds. This is
particularly so in aerospace and, in many instances, has made the
fatigue characteristics of materials more significant than their
ordinary, static strength properties. Engineers became aware that
alternating stresses, of quite small amplitude, could cause failure
in components, which were capable of safely carrying much greater,
steady loads. This phenomenon of small, alternating loads causing
failure was likened to a progressive weakening of the material,
over a period of time and hence the attribution of the term
fatigue. Very few constructional members are immune from it, and
especially those operating in a dynamic environment. Experience in
the aircraft industry has shown that the stress cycles, to which
aircraft are subjected, may be very complex, with occasional high
peaks, due to gust loading of aircraft wings. For satisfactory
correlation with in-service behaviour, full-size or large-scale
mock-ups must be tested in conditions as close as possible to those
existing in service.
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3.8.7 FATIGUE TESTING
An experiment, conducted in 1861, found that a wrought iron
girder, which could safely sustain a mass of 12 tons, broke when a
mass of only 3 tons was raised and lowered on the girder some 3x106
times. It was also found that there was some mass, below 3 tons,
which could be raised and lowered on to the beam, a colossal number
(infinite) of times, without causing any problem. Some years later,
a German engineer (Wohler), did work in this direction and
eventually developed a useful fatigue-testing machine which bears
his name and continues to be used in industry. The machine uses a
test piece, which is rotated in a chuck and a force is applied at
the free end, at right angles to the axis of rotation (refer to
Fig. 10). The rotation thus produces a reversal of stress for every
revolution of the test piece. Various other types of fatigue
testing are also used e.g. cyclic-torsional, tension-compression
etc. Exhaustive fatigue testing, with various materials, has
resulted in a better understanding of the fatigue phenomenon and
its implications from an engineering viewpoint.
Test Piece made to vibrate or oscillate against load (Stress
Cycles).
Test Piece
Load
Simple Fatigue Testing Fig 10
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3.9 S-N CURVES
One of the most useful end-products, from fatigue testing, is an
S-N curve, which shows, graphically, the relationship between the
amount of stress (S), applied to a material, and the number of
stress cycles (N), which can be tolerated before failure of the
material. Using a typical S-N curve, for a steel material (refer to
Fig. 11), it can be seen that, if the stress is reduced, the steel
will endure a greater number of stress cycles. The graph also shows
that a point is eventually reached where the curve becomes
virtually horizontal, thus indicating that the material will endure
an infinite number of cycles at a particular stress level. This
limiting stress is called the Fatigue Limit and, for steels, the
fatigue limit is generally in the region of 40% to 60% of the value
of the static, ultimate tensile strength (U.T.S.)
A S-N Curve for a Steel Material Fig. 11
Fatigue Limit
40 60 % UTS
Number of Cycles (N)
Stress (S)
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Many non-ferrous metals, however, show a different
characteristic from steel (refer to Fig. 12). In this instance
there is no fatigue limit as such and it can be seen that these
materials will fail if subjected to an appropriate number of stress
reversals, even at very small stresses. When materials have no
fatigue limit an endurance limit together with a corresponding
number of cycles is quoted instead. It follows that components made
from such materials must be designed with a specific life in mind
and removed from service at the appropriate time. The service
fatigue lives of complete airframes or airframe members are typical
examples of this philosophy. Non-metallic materials are also liable
to failure by fatigue. As is the case with metals, the number of
stress cycles, required to produce a fatigue failure, will increase
as the maximum stress in the loading cycle decreases. There is,
however, generally no fatigue limit for these materials and some
form of endurance limit must be applied. The importance of fatigue
strength can be illustrated by the fact that, in a high- cycle
fatigue mode, a mere 10% improvement in fatigue strength can result
in a 100-times life improvement.
An S-N Curve for an Aluminium Alloy Fig. 12
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3.10 CAUSES OF FATIGUE FAILURE
As the fatigue characteristics of most materials are now known
(or can be ascertained), it would seem reasonable to suppose that
fatigue failure, due to lack of suitable allowances in design,
should not occur. Nevertheless, fatigue cracking occurs frequently,
and even the most sophisticated engineering product does not
possess immunity from this mode of failure. Such failures are often
due to unforeseen factors in design, environmental or operating
conditions, material, and manufacturing processes. Two essential
requirements for fatigue development in a material are:
An applied stress fluctuation of sufficient magnitude (with or
without an applied steady stress).
A sufficient number of cycles of that fluctuating stress. The
stress fluctuations may be separated by considerable time
intervals, as experienced in aircraft cabin pressurisation, during
each take-off (e.g. daily), or they may have a relatively short
time interval, such as encountered during the aerodynamic
buffeting/vibration of a wing panel. The former example would be
considered to be low-cycle fatigue and the latter to be high-cycle
fatigue. In practice, the level of the fluctuating stress, and the
number of cycles to cause cracking of a given material, are
affected by many other variables, such as stress concentration
points (stress raisers), residual internal stresses, corrosion,
surface finish, material imperfections etc.
3.11 VIBRATION
Vibration has already been quoted as being a cause of high-cycle
fatigue and, because most dynamic structures are subjected to
vibration, this is undoubtedly the most common origin of fatigue.
All objects have their own natural frequency at which they will
freely vibrate (the resonant frequency). Large, heavy, flexible
components vibrate at a low frequency, while small, light, stiff
components vibrate at a high frequency. Resonant frequencies are
undesirable (and in some cases could be disastrous), so it is
important to ensure that, over their normal operating ranges,
critical components are not vibrated at their natural frequencies
and so avoid creating resonance. The resonant frequency, of a
component, is governed by its mass and stiffness and, on certain
critical parts, it is often necessary to do full-scale fatigue
tests to confirm adequate fatigue life before putting the product
into service.
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3.12 FATIGUE METALLURGY
Under the action of fatigue stresses, minute, local, plastic
deformation on an atomic scale, takes place along slip planes
within the material grains. If the fatigue stresses are continued,
then micro cracks are formed within the grains, in the area of the
highest local stress, (usually at or near the surface of the
material). The micro cracks join together and propagate across the
grain boundaries but not along them. A fatigue fracture generally
develops in three stages (refer to Fig. 13):
Nucleation
Propagation (crack growth)
Ultimate (rapid) fracture.
The resultant fractured surface often has a characteristic
appearance of:
An area, on which a series of curved, parallel, relatively
smooth ridges are present and are centred around the starting point
of the crack. These ridges are sometimes called conchoidal lines or
beach marks or arrest lines.
A rougher, typically crystalline section, which is the final
rapid fracture when the cross-section is no longer capable of
carrying its normal, steady load.
Nucleation Propagation (crack growth) Ultimate (rapid)
fracture
The Three Stages of Fracture Fig. 13
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The arrest lines are, normally, formed when the loading is
changed, or the loading is intermittent. However, in addition to
these characteristic and informative marks, there are similar, but
much finer lines (called striations), which literally show the
position of the crack front after each cycle. These striations are
obviously of great importance to metallurgists and failure
investigators when attempting to estimate the crack initiation
and/or propagation life. The striations are often so fine and
indistinct that electron beam microscopes are required to count
them. In normal circumstances, a great deal of energy is required
to `weaken' the material sufficiently to initiate a fatigue crack,
and it is not surprising, therefore, to find that the nucleation
phase takes a relatively long time. However, once the initial crack
is formed, the extremely high stress concentration (present at the
crack front) is sufficient to cause the crack to propagate
relatively quickly, and gaining in speed as the crack front not
only increases in size, but also reduces the component
cross-sectional area. A point is eventually reached (known as the
'critical crack length') at which the remaining cross-section is
sufficiently reduced to cause a gross overloading situation, and a
sudden fracture finally occurs. It is not unusual for the crack
initiation phase to take 90% of the time to failure, with the
propagation phase only taking the remaining 10%. This is one of the
major reasons for operators of equipment being relatively
unsuccessful in detecting fatigue cracks in components before a
failure occurs.
3.13 FATIGUE PROMOTERS
As fatigue cracks initiate at locations of highest stress and
lowest local strength, the nucleation site will be:
dictated largely by geometry and the general stress
distribution
located at or near the surface or
centred on surface defects/imperfections, such as scratches,
pits, inclusions, dislocations and the like
3.13.1 DESIGN
Apart from general stressing, the geometry of a component has a
considerable influence on its susceptibility to fatigue. A good
designer will therefore minimise stress concentrations by:
avoiding rapid changes in section and
using generous blend radii or chamfers to eliminate sharp
corners
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3.13.2 MANUFACTURE
While the designer may specify adequate blend radii, the actual
product may still be prone to fatigue failure if the manufacturing
stage fails to achieve this sometimes-seemingly unimportant drawing
requirement. Several other manufacturing-related causes of
premature fatigue failure exist, the most common of which are:
Inherent material faults: e.g. cold shuts, pipe, porosity, slag
inclusions etc.
Processing faults: e.g. bending, forging, grinding, shrinking,
welding, etc.
Production faults: e.g. incorrect heat-treatment, inadequate
surface protection, poor drilling procedures, undue force used
during assembly, etc
In-service damage: e.g. dents, impact marks, scratches, scores,
tooling marks etc.
3.13.3 ENVIRONMENT
One of the most potent environmental promoters of fatigue occurs
when the component is operating in a corrosive medium. Steel
(normally), has a well-defined fatigue limit on the S-N curve but,
if a fatigue test is conducted in a corrosive environment, not only
does the general fatigue strength drop appreciably, but the curve
also resembles the aluminium alloy curve (e.g. the fatigue failure
stress continues to fall as the number of cycles increases). Other
environmental effects such as fretting and corrosion pitting,
erosion or elevated temperatures will also adversely affect fatigue
strength.
3.14 FATIGUE PREVENTERS
If a component is prone to fatigue failure in service, then
several met