CHE3166: HANDOUT 4 Mechanical Properties and Testing (Hardness, Impact, Creep and Fatigue) LEARNING OBJECTIVES Material’s response to: • Excessive Loading: Tensile Test • Localized Loading: Hardness Test • Sudden Intense Loading: Impact Test • Loading at High Temperatures: Creep Test • Cyclic Loading: Fatigue Test
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CHE3166: HANDOUT 4
Mechanical Properties and Testing
(Hardness, Impact, Creep and Fatigue)
LEARNING OBJECTIVES
Material’s response to:
• Excessive Loading: Tensile Test
• Localized Loading: Hardness Test
• Sudden Intense Loading: Impact Test
• Loading at High Temperatures: Creep Test
• Cyclic Loading: Fatigue Test
Localized Loading: Hardness / Hardness Test
Hardness: Resistance to
• localized plastic deformation
• penetration of the surface
Hardness Test Data are used for:
• rough but quick comparison of strength
of materials
• characterization of wear resistance
• quick examination of heat treatment
• quick testing for materials quality control
Hardness Tests are quick and simple
Hardness / Hardness Test
Hardness Test
Brinell Hardness Test: 10mm sphere of steel or tungsten carbide
Vickers Micro-hardness Test: Diamond pyramid
P is the applied load
Correlation between
Hardness and Tensile StrengthTS (MPa) = 3.45 x HB TS (psi) = 500 x HB
Response to Sudden Loading: Impact
Increase in Loading
Rate results in:- increase in sy and TS
- decreases %EL
Why?
A rapid loading gives less time for dislocations to
move past obstacles: thus hardens the material
So, The response to sudden loading is also a
measure of: • Lack of Ductility and
• Toughness
Very low toughness:unreinforced polymers
Engineering tensile strain, e
Engineering
tensile
stress, s
Low toughness: ceramics
High toughness: metals
Impact Resistance: Toughness
• Energy to break a unit volume of material:
Energy is absorbed for movement of dislocation
• The lesser the energy absorbed the poorer the toughness
Impact Tests
Impact test conditions are chosen to represent
severe conditions for fracture:
1. Deformation at a relatively low temperature: less
opportunity for movement of dislocations
2. Very rapid deformation: less opportunity for
movement of dislocations
3. Presence of a notch: existing crack initiation site
• Charpy (common)
• Izod (for non-metallic)
Two Standard Testing Methods:
Heavy pendulum:
• starting at height
h, swings through
its arc
• strikes and
breaks the
specimen
• reaches a final
elevation, h'
• h' is a measure
of the ability of
the material to
withstand the
impact (energy
absorbed)
Ductile-Brittle Transition (DBT) Temperature
Impact Energy vs DBT Temperature
DBT Temp for Steel: Role of Carbon
Creep
Deformation of materials at elevated temperatures
• plastic deformation,
• time-dependent,
• for metallic materials creep temperature > 0.4 Tm
(Tm = melting point on absolute temp. scale, K),
• life limiting factor for several critical high
temperature components
Examples of in-service creep:
• steam generators in thermal power plants
• turbine rotors in jet engines
Assessment of Creep
Measurement of deformation or strain as a function of time at:
• Constant temperature and
• Constant load or stress
Constant Load Creep Behaviour
Instantaneous Deformation
On loading, instantaneous elastic deformation
Constant Load Creep: Primary RegionPrimary or Transient Creep Region
Continuously decreasing:
• creep strain with time
• creep/strain rate with time, the slope decreases with time
Indicates creep resistance: Strain Hardening
Constant Load Creep: Secondary Region
Secondary or Steady-state Creep region:
Creep or strain rate is constant with time
• the plot takes almost a linear shape
• the region of longest duration
• for practical applications, the regions should be as long
as possible
The constancy of creep
rate is attributed to the
balance between
• Strain hardening
• Recovery (a softening
process)
Constant Load Creep: Secondary Region
Recovery:
stored strain energy is removed by virtue of dislocation motion and enhanced atomic diffusion at elevated temperatures, resulting in annihilation (vanishing off) of dislocations and improved ductility
Strain Hardening:
ductile material becoming harder when plastically
deformed as a result of extensive generation of
dislocations and straining in the crystalline structure.
Also, called work hardening (cold working)
Constant Load Creep: Tertiary Region
Tertiary Creep region / Creep Rupture:
• Rapid acceleration of creep rate, leading to failure/rupture
• Rupture results from microstructural changes:
• grain boundary separation
• internal cracks, cavity cracks, leading to decrease in the
effective load-bearing area
• For practical applications,
the onset of tertiary creep
should be delayed as long
as possible
Creep Parameters and
Design of High Temperature Components
For long-life components (e.g., steam generators):
• Steady-state creep rate (e/t) is the design parameter
• The creep tests need not last to failure
For short-life components (e.g., rocket motor nozzle):
• Time to rupture (creep-rupture life) is the design parameter
• Creep rupture tests required
Design data for creep is commonly represented as
log stress
vs
• log Steady-state Creep Rate (e/t) or
• log Rupture Time
Creep Data Extrapolation
For long-life components, generating actual creep
data under actual conditions (temperature/stress)
would require impractically long-term tests
Solution:
• Short-term accelerated tests at higher
temperatures and comparable stresses
• At different temperatures, determine Larsen-
Miller parameter
• T (C + log tr): Larsen Miller Parameter
(T: temp. in K, C: constant (~20), tr: rupture
time in hours)
• Extrapolation of the data
Creep Data Extrapolation
Larson-Miller
Parameter
vs
log stress
Cyclic LoadingFluctuations in:
Temperature (inside and outside)
Loading
Pressure
Hip implant:
Cyclic loading
during
walking
Examples of Cyclic Loading
Fatigue
• Damage or failure caused due to dynamic and
fluctuating stress
(e.g., bridges, aircraft and machine components)
• Failure may occur at a stress level considerably
lower than the tensile or yield strength
• Failures normally occur after a lengthy period of
repeated stress or strain cycling
• 90% of metallic components fail due to fatigue
• Failures are sudden
S-N Curve: Fatigue Parameters
Fatigue Life: Number of cycles to cause fatigue failure at a
given stress level
Fatigue Strength: Maximum stress at which no fatigue
failure occurs for a given number of cycles (say, 107)
S-N Curve: Fatigue Parameters
Fatigue Limit: Stress below which no fatigue failure occurs