Fatigue and Fracture

University Of KhartoumUniversity Of KhartoumBuilding And Road Research InstituteBuilding And Road Research Institute

M.SC in building TechnologyM.SC in building Technology

Presented by: Mohamed AbdulBagi Presented by: Mohamed AbdulBagi Mahmoud TahaMahmoud Taha

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Contents:Contents: Section One (Fracture in steel structures):Section One (Fracture in steel structures):1.1. Introduction. Introduction. 2.2. Fracture definition ( ductile and brittle fracture and Fracture definition ( ductile and brittle fracture and

their behavior). their behavior). 3.3. Transition temperature between ductile and brittle Transition temperature between ductile and brittle

fracture.fracture.4.4. Fracture Mechanics (linear elastic and elastic plastic Fracture Mechanics (linear elastic and elastic plastic

fracture mechanics).fracture mechanics).5.5. Testing methods for procuring fracture properties Testing methods for procuring fracture properties

( ( Charpy testCharpy test, , fracture mechanics testingfracture mechanics testing and and other other relative testsrelative tests).).

6.6. Key factors for safe fracture design.Key factors for safe fracture design. Section Two ( Fatigue in steel structures):Section Two ( Fatigue in steel structures):1. Introduction.2.2. Assessment of fatigue life of structural components (S-

N curves and strain life 3.3. Improvement techniques (Grinding, weld toe remelting

and Hammer peening)

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Introduction:Introduction:

Sometimes, structures which were properly designed to avoid both excessive elastic deflection and plastic yielding fail in a catastrophic way by fast fracturefast fracture .

Common to these failures - of things like welded ships, welded bridges and gas pipelines and pressure vessels under large internal pressures - is the presence of cracks, often the result of imperfect welding.

Fast fractureFast fracture is caused by the growth - at the speed of sound in the material - of existing cracks that suddenly became unstable.

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Fracture definition ( ductile and brittle Fracture definition ( ductile and brittle fracture). fracture).

FractureFracture is the (local) separation of a body into two, is the (local) separation of a body into two, or more pieces under the action of stress.or more pieces under the action of stress.

Fracture is classified into two categories which are:Fracture is classified into two categories which are:

1.1. Brittle Fracture:Brittle Fracture: Fast , unstable fracture that occur with very little energy absorption ( no plastic deformation occurrence).

2.2. Ductile Fracture:Ductile Fracture: Small process that absorbs a considerable amount of energy usually through plastic deformation.

Brittle fracture in a structure may be unexpected and catastrophic failure and on the other hand Ductile fracture can somehow provides an ample warning for alternatives measures to be taken.

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ductile and brittle fracture behavior:ductile and brittle fracture behavior:

Ductile Fracture behavior features :-Ductile Fracture behavior features :-• Is normally preceded by extensive plastic deformation.Is normally preceded by extensive plastic deformation.

• Slow, and generally results from the formation and Slow, and generally results from the formation and coalescence of voids.coalescence of voids.

• The fracture path of a ductile crack is often The fracture path of a ductile crack is often irregularirregular, and , and the presence of a large number of small voids gives the the presence of a large number of small voids gives the fracture surface a fracture surface a dull fibrousdull fibrous appearance. appearance.

• Most engineering applications aim for selecting metals Most engineering applications aim for selecting metals that exhibit plastic deformation and work hardening that exhibit plastic deformation and work hardening because they are extremely valuable as a safeguard because they are extremely valuable as a safeguard against design oversight, accidental overloads or failure by against design oversight, accidental overloads or failure by cracking due to fatigue , corrosion or creep. cracking due to fatigue , corrosion or creep.

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Brittle Fracture behavior features :-Brittle Fracture behavior features :-• Rapid propagation of cracks which take the form

transgranular (Cleavage) or intergranular, without any plastic deformation at a stress level below the yield stress of the material

• Shows very limited, plastic deformation ahead of the crack tip.

• Normally only occurs when a critical combination of the following exist:

1. A severe stress concentration due to a notch or severe structural discontinuity.

2. A significant tensile force occurs across the plane of the notch (or equivalent).

3. Low fracture toughness of the steel at the service temperature.

4. Dynamic loading.

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Ductile fracture

By voids growth and Coalescence

Ductile fracture

By necking and shearing

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Brittle fracture pattern

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Transition temperature between ductile and Transition temperature between ductile and brittle fracture:-brittle fracture:-• An important feature of ferritic steels.An important feature of ferritic steels.

• Allows the designers to be able to select a material which Allows the designers to be able to select a material which will be ductile at the required operating temperatures for a will be ductile at the required operating temperatures for a given structure.given structure.

• The traditional procedure for assessing the ductile to brittle The traditional procedure for assessing the ductile to brittle transition in steels is by impact testing small notched beams. transition in steels is by impact testing small notched beams.

• The energy absorbed during the fracture process is a The energy absorbed during the fracture process is a measure of the toughness of the material and varies from a measure of the toughness of the material and varies from a low value at low temperatures to a high value as the low value at low temperatures to a high value as the temperature is raised.temperature is raised.

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Lower shelf

Upper shelf

• Lower shelf: low temperature, brittle behavior.

• Upper shelf: High temperature, Ductile behavior.

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Fracture Mechanics (linear elastic):-Fracture Mechanics (linear elastic):-

• The science of dealing with relatively large cracks in essentially elastic bodies is linear elastic fracture mechanics.

• A crack in a solid can be opened in three different modes

Tensile Shear Mixed

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Fracture Mechanics (linear elastic) Fracture Mechanics (linear elastic) continue:-continue:-

• The commonest is when normal stresses open the crack. This is termed mode I.

• The essential feature of linear elastic fracture mechanics is that all cracks adopt the same parabolic profile when loaded in mode I and that the tensile stresses ahead of the crack tip decay as a function of 1/√(distance) from the crack tip.

• The absolute values of the opening displacements and the crack tip stresses depend on the load applied and the length of the crack.

• The relation that put all parameters in one form is :

aYK I aYK I

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Fracture Mechanics (linear elastic) Fracture Mechanics (linear elastic) continue:-continue:-• Where :

: The scaling factor for the stress field and crack displacements.

Y : a geometric correction term to account for the proximity of the boundaries of the structure and the form of loading applied.

Subscript I : Denotes the mode I, or opening mode of loading.

a : Crack length.

IK

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Fracture Mechanics (linear elastic) Fracture Mechanics (linear elastic) continue:-continue:-• It is important to note that the convention is that an

embedded crack that has two tips has a length of 2a, and an edge crack which only has one tip has a length of a.

• The limiting stress intensity required to cause crack extension in plane strain at the tip of a crack when the stress is transverse to the crack is known as fracture toughness and is represented by

• fracture toughness value is influenced by the dimensions of the specimen. In particular, the size and thickness of the specimen and the length of the remaining uncracked section, called the ligament, control the level of constraint at the crack tip.

• Thick specimens, which generate high constraint, give lower values of fracture toughness and vise versa.

ICK

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Fracture Mechanics (linear elastic) Fracture Mechanics (linear elastic) continue:-continue:-• The least dimensions to give the maximum constraint,

and hence the minimum value of toughness, are when:

• The practical applications of linear elastic fracture mechanics are in assessing the likelihood that a particular combination of loading and crack size will cause a sudden fracture. Given that a structure is at risk of failing if :

• Linear elastic fracture mechanics is of much greater use on the lower shelf of the ductile-brittle behavior of steels than on the upper shelf.

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Fracture Mechanics (elastic- plastic):-Fracture Mechanics (elastic- plastic):-• Is related to fracture resistance of materials outside the

limits validity of plane strain linear elastic fracture mechanics.

• Approaches to fracture mechanics when significant plasticity has occurred have been to consider either the crack tip opening displacement or the J-integral.

• Significant yielding at a crack tip leads to the physical separation of the surfaces of a crack, and the magnitude of this separation is termed the crack tip opening displacement (CTOD).

• The J-integral is a mathematical expression that may be used

to characterize the local stress and strain fields around a crack front

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Fracture Mechanics (elastic- plastic) Fracture Mechanics (elastic- plastic) continue:-continue:-

• The relationships between KI , d and JI under linear elastic conditions are:

• Where GI is the strain energy release rate, which was the original energy based approach to studying fracture.

• In all cases, either linear elastic or general yielding, there is a parameter that describes the loading state of a cracked body. This might be KI, d or JI as appropriate to the conditions.

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Testing methods for procuring fracture Testing methods for procuring fracture propertiesproperties Charpy V- Notch impact Test :Charpy V- Notch impact Test :

• The test consists of measuring the energy absorbed in breaking by one blow from a pendulum a test piece notched in the middle and supported at each end.

• The energy of the swinging mass is 294 J for the Charpy test.

• Test piece is a standard test piece that has a 10 mm square cross-section.

• The test can be carried out at a range of temperatures to determine the transition between ductile and brittle behavior for the material, and the value obtained from the test is denoted by the symbol CV and is measured in joules.

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Testing methods for procuring fracture Testing methods for procuring fracture propertiesproperties Fracture mechanics testing :Fracture mechanics testing :

• The principle behind the tests is that a single edge-notched bend or compact tension specimen is cyclically loaded within prescribed limits until a sharp fatigue crack is formed. The specimen is then subjected to a displacement controlled monotonic loading until either brittle fracture occurs or a prescribed maximum force is reached.

• The particular type of test piece used depends upon the form, the strength and the toughness of the material under test.

• The fracture toughness, KI value is calculated from this load by equations which have been established on the basis of elastic stress analysis of the data.

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Testing methods for procuring fracture Testing methods for procuring fracture properties:-properties:-

Fracture mechanics testing (continue) :Fracture mechanics testing (continue) :

• The obtained data can be used to evaluate the critical CTOD or critical J for the material.

other test for procuring fracture other test for procuring fracture properties:properties:

• Other test can be carried out for such purpose is the Wide plate test for testing welded plate joint.

• Developed by Wells at the welding Institute.

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Testing methods for procuring fracture Testing methods for procuring fracture properties:-properties:-• The test involve using a large full- thickness plate , The test involve using a large full- thickness plate ,

typically 1m square, is butt welded using the process typically 1m square, is butt welded using the process and treatments to be used in the production weld.and treatments to be used in the production weld.

• The test has an advantage of representing failure of The test has an advantage of representing failure of an actual welded joint without the need for machining an actual welded joint without the need for machining prior to testing.prior to testing.

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Key factors for safe fracture design:-Key factors for safe fracture design:-

• There are several key factors which need to be considered when determining the risk of brittle fracture in a structure. These are:

1. minimum operating temperature.2. loading – in particular, rate of loading.3. metallurgical features such as parent plate, weld

metal or HAZ (Heat affected zone).4. thickness of material to be used.

• Each of these factors influence the likelihood of brittle fracture occurring.

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Key factors for safe fracture design:-Key factors for safe fracture design:-

• once a weld is deposited on a steel plate, the toughness of the plate surrounding the weld, particularly in the heat affected zone (HAZ), will be reduced.

• If larger defects may be present in the weld area than the parent plate, appropriate procedures should be adopted to ensure that a welded structure will perform as designed. These could involve non-destructive testing including visual examination .

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Introduction:Introduction:• A structural member subjected to cyclic loadings may

eventually fail through initiation and propagation of cracks. This phenomenon is called fatigue and can occur at stress levels considerably below the yield stress.

• Fatigue life can be defined as the number of cycles and hence the time taken to reach a pre-defined failure criterion.

• In the analysis of a structure for fatigue there are three main areas of difficulty in prediction:

1. The operational environment of a structure and the relationship between the environment and the actual forces on it.

2. The internal stresses at a critical point in the structure induced by external forces acting on the structure.

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3. The time to failure due to the accumulated stress history at the critical point.

Assessment of fatigue life of structural Assessment of fatigue life of structural components:-components:-

• There are three approaches to the assessment of fatigue life of structural components:-

1. The traditional method, called the S–N approach, was

first used in the mid 19th century. This relies on empirically derived relationships between applied elastic stress ranges and fatigue life.

2. A development of the S–N approach is the strain–life method in which the plastic strains are considered important. Empirical relationships are derived between strain range and fatigue life.

3. The third method, based on fracture mechanics, considers the growth rate of an existing defect. The concept of defect tolerance follows directly from fracture mechanics assessments.

Fatigue loading :-Fatigue loading :-

• Fatigue as a design consideration is affected by the magnitude of the stress range, the number of load cycles, and the severity of the stress concentration associated with a particular structural detail.

• Stress range is defined as the magnitude of the change in stress (ignoring sign) due to the application or removal of the live load.

• Unfactored live loads and determination of stresses by elastic analysis is used for fatigue design in both LRFD and ASD specifications.

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• The stress range is calculated as the algebraic difference between minimum and maximum stress, or the numerical sum of maximum shearing stresses of opposite direction at the point of probable crack initiation.

• For convenience, loadings are usually simplified into a load spectrum, which defines a series of bands of constant load levels and the number of times that each band is experienced.

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Nature of fatigue:Nature of fatigue:

• Fatigue failure is a process of crack propagation due to the highly localized cyclic plasticity that occurs at the tip of a crack or metallurgical flaw.

• Propagation of a defect within the microstructure of the material; slow incremental propagation of a long crack; and final unstable fracture.

• a crack starts to grow from the first loading cycles and

continues right through to failure.

• In welded structures or cast components the initiation phase is bypassed as substantial existing defects are already likely to be present.

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Assessment of fatigue life of structural Assessment of fatigue life of structural components:-components:- S-N curves:S-N curves:

• The total cyclic stress range (S) is plotted against the number of cycles to failure (N).

• A series of specimens is subject to cycles of constant load amplitude to failure.

• A sufficient number of specimens are tested for statistical analysis to be carried out to determine both mean fatigue strength and its standard deviation.

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Assessment of fatigue life of structural Assessment of fatigue life of structural components:-components:- S-N curves (continue):S-N curves (continue):

• Fatigue limit or (Endurance limit) is known as the stress at which the strength had reduced after exposing to a certain number of cycles N and then remains constant.

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Assessment of fatigue life of structural Assessment of fatigue life of structural components:-components:-

S-N curves:S-N curves:

• Welded steel joints are usually regarded as containing small defects due to the welding process itself. It has been found after much experimental work that the relationship between fatigue life and applied stress range follows the form:

• where Nf is the number of cycles to failure, DS is the applied stress range, and a and b are constants which depend on the geometry of the joint. The value of a is in the range of 3 to 4 for welded joints in ferritic steels.

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Fatigue design curves for welded joints according to BS 5400-10: 1980 and BS 7608:1993

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Assessment of fatigue life of structural Assessment of fatigue life of structural components:-components:-

• Strain-life:-Strain-life:-

• The notion that fatigue is associated with plastic strains led to the strain–life approach to fatigue.

• A common empirical curve fit to the fatigue lifetime data takes the form:

• where E is Young’s modulus and , b and c are considered to be material properties

Assessment of fatigue life of structural Assessment of fatigue life of structural components:-components:- Strain-life (continue):-Strain-life (continue):-

• The technique is:-1. Determine the strains in the structure, often by finite

element analysis;2. Identify the maximum local strain range, the ‘hot

spot’ strain;3. Read, from the strain–life curve, the lifetime to first

appearance of a crack at that strain at that position.

• The strain–life, or local strain, method has wider use in fatigue assessments in the engineering industry than the S–N method and is available in commercial computer software.

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Improvement techniques:-Improvement techniques:-

• The fatigue performance of a joint can be enhanced by the use of weld improvement techniques.

• There is a large amount of data available on the influence of weld improvement techniques on fatigue life but as yet little progress has been made into developing practical design rules.

• Modern steelmaking has led to the production of structural steels with excellent weld ability.

• The low fatigue strength of a welded connection is generally attributed to a short crack initiation period. An extended crack initiation life can be achieved by:-

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Improvement techniques (continue):-Improvement techniques (continue):-

1. Reducing the stress concentration of the weld,2. Removing crack-like defects at the weld toe,3. Reducing tensile welding residual stresses or

introducing compressive stresses.• The methods employed fall broadly into two categories:

Improvement techniques

Weld geometry improvement(grinding, weld dressing and profile

Control)

Residual stress reduction(Peening and thermal stress relief)

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Improvement techniques (continue):-Improvement techniques (continue):-

Grinding:-Grinding:-

• The improvement of the weld toe profile and the removal of slag inclusions can be achieved by grinding either with a rotary burr or with a disc.

• To obtain the maximum benefit from this type of treatment it is important to extend the grinding to a sufficient depth to remove all small undercuts and inclusions.

• The degree of improvement achieved increases with the amount of machining carried out and the care taken by the operator to produce a smooth transition.

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Improvement techniques (continue):-Improvement techniques (continue):- Welding toe remelting:-Welding toe remelting:-

• Remelting of the weld toe using either TIG or plasma Remelting of the weld toe using either TIG or plasma welding equipment generally results in large gains in welding equipment generally results in large gains in fatigue strength, for several reasons. fatigue strength, for several reasons.

• Firstly, the smoother weld toe transition reduces the Firstly, the smoother weld toe transition reduces the stress concentration factor; stress concentration factor;

• Secondly, slag inclusions and undercuts are removed; Secondly, slag inclusions and undercuts are removed;

• Thirdly, according to some Japanese publications, the Thirdly, according to some Japanese publications, the higher hardness in the heat affected zone is claimed to higher hardness in the heat affected zone is claimed to contribute to the higher fatigue strength.contribute to the higher fatigue strength.

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Improvement techniques (continue):-Improvement techniques (continue):-

• Plasma dressing generally tends to give better results Plasma dressing generally tends to give better results than TIG dressing.than TIG dressing.

• Weld toe remelting can result in large increases in Weld toe remelting can result in large increases in fatigue strength due to the effect of providing low fatigue strength due to the effect of providing low contact angle in the transition area between the weld contact angle in the transition area between the weld and the plate and by the removal of slag inclusions and and the plate and by the removal of slag inclusions and undercuts at the toe.undercuts at the toe.

Hammer peening:-Hammer peening:-

• Improved fatigue properties of peened welds are obtained by extensive cold working of the toe region. These improved fatigue properties are due to:

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Improvement techniques (continue):-Improvement techniques (continue):-

1. introduction of high compressive residual stresses,2. a flattening of crack-like defects at the toe,3. an improved toe profile.

• It can be shown that weld improvement techniques greatly improve the fatigue life of weldments. For weldments subject to bending and axial loading, peening appears to offer the greatest improvement in fatigue life, followed by grinding and TIG dressing.

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References:-References:- Steel Designer Manual, 6th Edition.Steel Designer Manual, 6th Edition.

ASHBY, M. F. (1998) - Engineering Materials VOL 1.ASHBY, M. F. (1998) - Engineering Materials VOL 1.

Brocken rough & Merritt - Structural Steel Designer's Brocken rough & Merritt - Structural Steel Designer's Handbook 3rd ed. [McGraw Hill 1999].Handbook 3rd ed. [McGraw Hill 1999].

CARVILL, J. (1993) - Mechanical Engineer's Data CARVILL, J. (1993) - Mechanical Engineer's Data Handbook .Handbook .

CRC - The Civil Engineering Handbook.CRC - The Civil Engineering Handbook.

SMITH, E. H. (1994) Mechanical Engineer's Reference SMITH, E. H. (1994) Mechanical Engineer's Reference Book (12th ed).Book (12th ed).