BMFB 4283 NDT & FAILURE ANALYSIS

BMFB 4283NDT & FAILURE ANALYSIS

Lectures for Week 10

Prof. Qumrul Ahsan, PhD Department of Engineering MaterialsFaculty of Manufacturing Engineering

10.0 Fatigue Failure

10.1 Introduction10.2 Factors Causing Fatigue Failure10.3 S-N Curve10.4 Fatigue Crack Growth10.5 Factors Influence Fatigue Properties

Issues to address

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

Fatigue failure is characterized by three stages Crack Initiation Crack Propagation Final Fracture

Ken Youssefi MAE dept., SJSU 4

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.

Appearance of Fatigue Fracture Surface

1.0-in. diameter steel pins from agricultural equipment.Material; AISI/SAE 4140 low allow carbon steel

Fracture surface of a failed bolt. The fracture surface exhibited beach marks, which is characteristic of a fatigue failure.

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 SurfaceCrack initiation site

Fracture zoneBeach Mark

Both marks indicate the position of the crack tip. It appears as concentric ridges (circular or semicircular

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

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928 Porsche timing pulley

Crack started at the fillet

Fatigue at Stress Raiser• Notches decrease fatigue life

through stress concentration

• Remove stress concentrators.

bad

bad

better

better

corrosion either increases the rate at which material is removed from the crack tip and/or it produces material on the crack surfaces that forces the crack open (e.g. oxidation).

Thermal cycling causes expansion and contraction, hence thermal stress, if component is restrained.

•eliminate restraint by design•use materials with low thermal expansion coefficients

MAE dept., SJSU

Jack hammer component, shows no yielding before fracture.

Crack initiation site

Fracture zonePropagation zone, striation

VW crank shaft – fatigue failure due to cyclic bending and torsional stresses

Fracture areaCrack initiation site

Propagation zone,

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.

bicycle crank spider arm

MAE dept., SJSU 42

Gear tooth failure

Crank shaft

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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.