BMFB 4283
NDT & FAILURE ANALYSIS
Lectures for Week 10
Prof. Qumrul Ahsan, PhD
Department of Engineering Materials
Faculty of Manufacturing Engineering
Issues to address
10.0 Fatigue Failure
10.1 Introduction
10.2 Factors Causing Fatigue Failure
10.3 S-N Curve
10.4 Fatigue Crack Growth
10.5 Factors Influence Fatigue Properties
Fatigue Failure
Failures occurring under conditions of dynamic
loading are called fatigue failures.
metal subjected to a repetitive or fluctuating
stress will fail at a stress much lower than
that required to cause failure on a single
application of load.
Fatigue failure is characterized by three stages

Crack Initiation

Crack Propagation

Final Fracture
Hawaii, Aloha Flight 243, a Boeing 737, an upper part of the plane's cabin
area rips off in mid-flight. Metal fatigue was the cause of the failure.
Ken Youssefi
MAE dept., SJSU
4
Appearance of Fatigue Fracture Surface
Fracture surface of a failed bolt. The
fracture surface exhibited beach marks,
which is characteristic of a fatigue failure.
1.0-in. diameter steel pins from
agricultural equipment.
Material; AISI/SAE 4140 low
allow carbon steel
The final fracture area is clearly
identified by its colour - the
surface of the crack will be dull
due to the effects of air and
moisture being in contact and
setting up mild corrosion, while the
newly failed surface will show
signs of new, clean brittle failure
Fatigue Crack Nucleation
• Flaws, cracks, voids can all act as
crack nucleation sites, especially at the
surface.
• Therefore, smooth surfaces increase
the time to nucleation; notches, stress
risers decrease fatigue life.
• Dislocation activity (slip) can also
nucleate fatigue cracks.
Fatigue: Crack initiation and propagation
• Three stages of fatigue failure:
– crack initiation in the areas of stress concentration (microcracks,
scratches, indents, interior corners, dislocation slip steps, etc.).
Quality of surface is important.
– incremental crack propagation
– final 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
Crack propagation
• Stage II a: initial slow propagation
along crystal planes with high
resolved shear stress. Involves just
a few grains, and has flat fracture
surface
• Stage II b: faster propagation
perpendicular to the 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 and causes fatigue failure.
• Visually a fatigue fracture region is
smoother than the overload
fracture region
Indication of Fatigue Failure Surface
Crack initiation site
Both marks indicate the
position of the crack tip. It
appears as concentric ridges
(circular or semicircular
Beach Mark
Fracture zone
Beach Mark
•Also called calmshell marks
•Can be observed using unaided eye
•Components experienced interruption
during the crack propagation
•Each beachmark band represents a
time over which crack growth occured
Striations
• Microscopic in size
•Each striation represents crack
growth rate
• stress range  striations width
• Multiple strations within a beach
mark
Fatigue at Stress Raiser
• Notches decrease fatigue life
through stress concentration
• Remove stress concentrators.
bad
better
bad
better
928 Porsche timing pulley
Crack started at the fillet
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Thermal cycling causes expansion and contraction,
hence thermal stress, if component is restrained.
•eliminate restraint by
design
•use materials with low
thermal expansion
coefficients
Jack hammer component,
shows no yielding before
fracture.
Crack initiation site
Fracture zone
Propagation zone, striation
MAE dept., SJSU
VW crank shaft – fatigue failure due to cyclic bending and torsional stresses
Propagation
zone,
Crack initiation site
Fracture area
bicycle crank spider arm
This long term fatigue crack in a high quality component took a
considerable time to nucleate from a machining mark between the spider
arms on this highly stressed surface. However once initiated propagation
was rapid and accelerating as shown in the increased spacing of the 'beach
marks' on the surface caused by the advancing fatigue crack.
Crank shaft
Gear tooth failure
MAE dept., SJSU
42
Fatigue: summary
• Initiated at surface
• Fatigue failure controlled either by initiation or by
propagation
• Fatigue Fracture Features: Beachmark and
Striation
• Intrinsic and Extrinsic features control fatigue
failure and fatigue life.
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