The document discusses three main types of material failure: fatigue failure, failure under fluctuating stress, and creep failure. Fatigue failure occurs when a material breaks under cyclic stresses that are lower than the material's static load capacity. Creep failure happens at high temperatures over long periods of time due to constant stresses. The key factors that influence fatigue life and creep behavior include stress levels, temperature, surface quality, microstructure, and environment. Crack initiation and propagation play an important role in fatigue failure. Different creep mechanisms dominate depending on the material, stress levels, and temperature.

1. Fatigue rapture
Failure under Fluctuating Stress
Creep rapture

2. Muhammad Umair Bukhari
Engr.umair.bukhari@gmail.com
www.bzuiam.webs.com
03136050151

3. The failure of metal under alternating stresses is
known as Fatigue.
Under fluctuating / cyclic stresses, failure can occur
at lower loads than under a static load.
90% of all failures of metallic structures (bridges,
aircraft, machine components, etc.)
Fatigue failure is brittle-like –
even in normally ductile materials. Thus sudden
and catastrophic!
Fatigue
Failure under fluctuating stress

4. Fatigue: Cyclic Stresses
Characterized by maximum, minimum and mean
Range of stress, stress amplitude, and stress ratio
Mean stress m = (max + min) / 2
Range of stress r = (max - min)
Stress amplitude a = r/2 = (max - min) / 2
Stress ratio R = min / max
Convention: tensile stresses  positive
compressive stresses  negative

5. Fatigue: S—N curves (I)
Rotating-bending test  S-N curves
S (stress) vs. N (number of cycles to failure)
Low cycle fatigue: small # of cycles
high loads, plastic and elastic deformation
High cycle fatigue: large # of cycles
low loads, elastic deformation (N > 105)

6. Fatigue: S—N curves (II)
Fatigue limit (some Fe and Ti alloys)
S—N curve becomes horizontal at large N
Stress amplitude below which the material never fails,
no matter how large the number of cycles is

7. Fatigue: S—N curves (III)
Most alloys: S decreases with N.
Fatigue strength: Stress at which fracture occurs after
specified number of cycles (e.g. 107)
Fatigue life: Number of cycles to fail at specified stress
level

8. Fatigue: Crack initiation+ propagation (I)
Three stages:
1. crack initiation in the areas of stress concentration (near
stress raisers)
2. incremental crack propagation
3. rapid crack propagation after crack reaches critical size
The total number of cycles to failure is the sum of cycles at the first
and the second stages:
Nf = Ni + Np
Nf : Number of cycles to failure
Ni : Number of cycles for crack initiation
Np : Number of cycles for crack propagation
High cycle fatigue (low loads): Ni is relatively high. With increasing
stress level, Ni decreases and Np dominates

9. Fatigue: Crack initiation and propagation (II)
 Crack initiation: Quality of surface and sites of stress concentration
(microcracks, scratches, indents, interior corners, dislocation slip steps,
etc.).
 Crack propagation
 I: Slow propagation along crystal
planes with high resolved shear
stress. Involves a few grains.
Flat fracture surface
 II: Fast propagation perpendicular
to applied stress.
Crack grows by repetitive blunting
and sharpening process at crack tip.
Rough fracture surface.
 Crack eventually reaches critical dimension and propagates very
rapidly

10. Factors that affect fatigue life
 Magnitude of stress
 Quality of the surface
Solutions:
 Polish surface
 Introduce compressive stresses (compensate for applied tensile
stresses) into surface layer.
Shot Peening -- fire small shot into surface
High-tech - ion implantation, laser peening.
 Case Hardening: Steel - create C- or N- rich outer layer by atomic
diffusion from surface
Harder outer layer introduces compressive
stresses
 Optimize geometry
Avoid internal corners, notches etc.

11. Factors affecting fatigue life
Environmental effects
 Thermal Fatigue. Thermal cycling causes expansion and
contraction, hence thermal stress.
Solutions:
 change design!
 use materials with low thermal expansion coefficients
 Corrosion fatigue. Chemical reactions induce pits which act as
stress raisers. Corrosion also enhances crack propagation.
Solutions:
 decrease corrosiveness of medium
 add protective surface coating
 add residual compressive stresses

12. The Macroscopic Character of Fatigue Failure
 Because of the manner in which the fracture develops, the
surfaces of a fatigue fracture are divided into two areas
with distinctly different appearances.
 In most cases, the surface will have a polished or burnished
appearance in the region where the crack grew slowly.
 In the last stage, the surfaces developed are rough and
irregular.

13. Fractograph of fatigue failure in SAE 1050 pin, induction hardened to a depth
of 5 mm ( 3/16 in.) and surface hardness of 55 HRC. Core hardness: 21 HRC.
Fatigue initiated inside the grease hole at the metallurgical notch created by
the very sharp case-core hardness gradient.

14. Schematic representation of fatigue fracture surface marks produced on
smooth and notched components with circular cross sections under various
loading conditions.

15. Creep
Creep testing
Furnace
Time-dependent deformation due to
constant load at high temperature
(> 0.4 Tm)
Examples: turbine blades, steam generators.
Creep test:

16. Stages of creep

17. 1. Instantaneous deformation, mainly elastic.
2. Primary/transient creep. Slope of strain vs. time
decreases with time: work-hardening
3. Secondary/steady-state creep. Rate of straining
constant: work-hardening and recovery.
4. Tertiary. Rapidly accelerating strain rate up to failure:
formation of internal cracks, voids, grain boundary
separation, necking, etc.
Stages of creep

18. Parameters of creep behavior
Secondary/steady-state creep:
Longest duration
Long-life applications
(creep rate)
Time to rupture ( rupture lifetime, tr):
Important for short-life creep
t/s

tr
/t

19. Creep: stress and temperature effects
With increasing stress or temperature:
The instantaneous strain increases
The steady-state creep rate increases
The time to rupture decreases

20. Creep: stress and temperature effects
Stress/temperature dependence of the steady-state creep rate can be
illustrated by

21. Mechanisms of Creep
Different mechanisms act in different materials and under different
loading and temperature conditions:
 Dislocation Glide
 Dislocation Creep
 Diffusion Creep
 Grain boundary sliding
Different mechanisms  different n, Qc.
Grain boundary diffusion Dislocation glide and climb

22. Dislocation glide- Involves dislocations moving along
slip planes and overcoming barriers by thermal
activation. This mechanism occurs at high stress
levels.
Dislocation creep- Involves the movement of
dislocations which overcome barriers by thermally
assisted mechanisms involving the diffusion of
vacancies or interstitials.
Mechanisms of Creep

23. Diffusion creep- Involves the flow of vacancies and
interstitials through a crystal under the influence
of applied stress. This mechanism occurs at high
temperatures and low stress levels.
Grain boundary sliding- Involves the sliding of
grains past each other.
Mechanisms of Creep