1. Behavior of Materials in Service (2)
Lecture 10

2. Failure under Fluctuating Load
Fatigue Failure
Failure under Fluctuating Load
 Failure occurs at prolonged application of dynamic and fluctuation stress, the value of which is much lower than tensile or yield stress of material (for a static load)
bridges, aircrafts, machine components
 Single largest cause of material failure (90% of all material failure)
 It is catastrophic and insidious, occurring very suddenly and without warning
Brittle-like failure, even in ductile materials
 Failure process occurs by the initiation and propagation of surface-initiated crack, and the fractured surface is usually perpendicular to the direction of the applied stress.

3. Failure of the Tacoma Narrows Bridge, Washington, USA

4. Stages of Fatigue Failure
Fatigue failures typically occur in three stages.
1. Crack initiation: A tiny crack initiates or
nucleates often at a time well after loading begins. Normally, nucleation sites are located
at or near the surface, where the stress is at a maximum, and include surface defects such
as scratches or pits, sharp corners due to poor design or manufacture, inclusions, grain
boundaries, or dislocation concentrations.
Quality of surface is important.
Fig:Fatigue fracture surface in a steel shaft, showing the initiation region, the propagation of the fatigue crack (with beach markings), and catastrophic rupture when the crack length exceeds a critical value at the applied stress.
Beach marks always suggest
a fatigue failure, but-unfortunately-the absence of beach marks does not rule out fatigue failure!!

5. 2. Crack propagation: Crack advances incrementally with each stress cycle
Stage I – initially slow, involving few grains
Stage II – faster propagation perpendicular to the applied stress by repetitive blunting and sharpening of process of crack tip
3. Final failure
occurs very rapidly once the advancing crack has reached a critical value when the remaining cross-section of the material is too small to support the applied load.

6. Fatigue life
The fatigue life of a component or a material is defined as the total number of stress cycles to cause failure. This life can be separated into three stages, consisting of crack initiation, crack propagation and rapid fracture.
The fatigue life in terms of number of cycles can therefore be as:
Nf = Ni + Np
Nf : No. of cycles to failure
Ni : No. of cycles for crack initiation
Np: No. of cycles for crack propagation

7. dull, fibrous brittle failure smooth circular “beachmark”
Fractograph of Fractured Surface
practical example of fatigue failure
direction of rotation
final rupture
crack origin
dull, fibrous brittle failure
smooth circular “beachmark”

8. periodic and asymmetrical about zero axis random stress fluctuation
periodic and symmetrical about zero axis
periodic and asymmetrical about zero axis
random stress fluctuation
Laboratory fatigue test
rotating bend test
LOAD
Result is commonly plotted as:
S (stress) vs. N (# of cycles to failure) graph
Low cycle fatigue
high loads, plastic and elastic deformation
High cycle fatigue
low loads, elastic deformation (N > 105)

9. The S-N Curve stress below which fatigue failure would not occur
Example: Steel
Fatigue limit, or endurance limit, Sfat
stress below which fatigue failure would not occur
for steels, Sfat  35-60% of TS
Most nonferrous materials do not show any fatigue limit (i.e., Sfat = 0 !!)
Example: Aluminium
Fatigue strength
stress to cause fracture after specific # of cycles
Fatigue life
number of cycles to cause failure at a specific stress

10. Fatigue Life and Fatigue Strength Explanation:

11. The S-N Curve: An Example

12. Factors Improving Fatigue Life
 Reducing working stress (mean stress level)
(magnitude, amplitude)
 Improving design of surface (Design factors)
(removing defects e.g., sharp edge, notch, groove, etc.; applying surface treatments)
Surface Effects
Imposing compressive surface stress (by shot peening, case hardening, etc.)
(to suppress crack growing)
 Removing environmental effects
(thermal fluctuations, corrosive environment)

15. Failure under Constant Load At High Temperature
Creep Failure
Failure under Constant Load At High Temperature
Creep is a time-dependent and permanent deformation of materials when subjected to prolonged constant load/constant stress at a high temperature (T > 0.4 Tm).
It can also happened at room temperature for soft metals such as Lead.
It is a slow process, where deformation changes with time. Hence, Time dependent.
Objects commonly failed under creep:
Creep is important in applications such as: turbine blades (jet engines), engines), gas turbines, turbines, steam generators, power plants (boilers and steam lines) which must operate at high stresses and high temperatures without any changes in dimensions.
Creep is an undesirable phenomenon and a limiting factor in the lifetime of a part.

16. Salient Features of Creep failure
Creep fracture is defined as the fracture which takes place due to creeping of materials under steady loading.
It occurs in metals like iron, copper & nickel at high temperatures. The tendency of creep fracture increases with the increase in temperature and higher rate of straining.
The creep fracture takes place due to shearing of grain boundary at moderate stresses and temperatures and movement of dislocation from one slip to another at higher stresses and temperatures.

17. Salient Features of Creep failure
The movement of whole grains relation of each other causes cracks along the grain boundaries, which act as point of high stress concentration.
When one crack becomes larger it spreads slowly across the member until fracture takes place.
This type of fracture usually occurs when small stresses are applied for a longer period.
The creep fracture is affected by grain size, strain hardening, heat treatment and alloying.
Creep behavior of a metal is determined by measuring the strain (ε) deformation as function of time under constant stress.

18. Generalized Creep Failure
Constant load
Generalized Creep Failure
Obtaining creep (e-t) curve in laboratory experiment
1 Instantaneous deformation
mainly elastic.
2 Primary creep
decreasing creep strain with time due to work-hardening
3 Secondary (steady-state) creep
rate of straining is constant: balance of strain hardening and recovery
(longest stage in duration)
4 Tertiary creep
rapidly accelerating strain rate up to failure due to micro-structural changes
(formation of internal cracks, voids, cavities, grain boundary separation, necking, etc.)
Steady-state creep rate, De/Dt
Time of rupture, tr

19. Effect of Temperature and Applied Stress
Creep Characteristics
Effect of Temperature and Applied Stress
Dependency of steady-state creep rate on s and T:
es = K1 sn .
es = K2 sn exp
- Qc RT
K1, K2 and n = materials constant
Qc = activation energy for creep
Constant T: .
Constant stress: es =A .
- Qc RT
At temperature below 0.4 Tm and after the initial deformation, the strain is virtually independent of time. With increasing stress or temperature:
 The instantaneous strain increases
 The steady-state creep rate increases
 The time to rupture decreases

20. Factors reducing creep rate/failure (better creep resistance)
The rate of deformation is a function of the material properties, exposure time, exposure temperature and the applied structural load.
Unlike brittle fracture, creep deformation does not occur suddenly upon the application of stress. Instead, strain accumulates as a result of long-term stress. Therefore, creep is a "time-dependent" deformation
Factors reducing creep rate/failure (better creep resistance)
 High-melting point of material
 Increased Young’s modulus
 Coarse-grained structure
(reduces grain boundary sliding)
(Opposite effect to strength !!)
Smaller grains permit more grain boundary sliding resulting in higher creep rates.