Question

(a) Define fatigue and fracture and specify the conditions under which it occurs.

(b) Describe the different fracture modes and the mechanism of crack propagation in different fracture modes

Answer 1

Part (a)

Definitions and Conditions of Occurrence

Fatigue

Fatigue is the progressive and localized structural damage that occurs in a material subjected to cyclic or fluctuating stresses where the maximum stress value is below the ultimate tensile strength (and often below the yield strength). It results from the initiation and growth of cracks, leading to sudden failure without significant gross plastic deformation.

Typical Characteristics:

Failure occurs after a large number of repeated loading/unloading cycles.
Often initiates at stress concentrators like notches, keyways, welds, or surface defects.
The fracture surface typically shows macroscopic "beach marks" or microscopic striations, which indicate progressive crack growth.

Conditions Under Which Fatigue Occurs

Fatigue failure requires a combination of the following conditions:

Cyclic or Reversing Stress: Alternating loads (tensile–compressive, bending, or torsional) that fluctuate over time.
Stress Level Below Yield Strength: Repeated loading, even at stresses significantly lower than the yield strength, can initiate failure.
Presence of Stress Concentration: Surface irregularities, sharp corners, holes, or internal inclusions that magnify the local stress.
Environmental Effects: Factors like corrosion, high temperature, or humidity can accelerate crack initiation and growth (corrosion fatigue).
Material Type: High-strength steels and alloys are often more susceptible to fatigue than very ductile metals.
Example: Crankshafts, turbine blades, and connecting rods frequently experience fatigue due to fluctuating operational loads.

Fracture

Fracture is the separation or breaking of a material into two or more parts under the action of stress. It can be categorized based on the degree of associated plastic deformation:

Ductile fracture: Occurs after significant plastic deformation.
Brittle fracture: Occurs without significant plastic deformation.

Conditions Under Which Fracture Occurs

General fracture occurs when:

The applied stress exceeds the material's ultimate strength.
Pre-existing defects or cracks are present, acting as significant stress concentrators that reach a critical size.
The material is subjected to conditions that promote brittle behavior, such as low temperature or a high strain rate.
The material has been compromised by factors like improper heat treatment or adverse residual stresses.
Example: A ship's hull plating cracking or a turbine rotor fragmenting due to overload or flaw propagation.

Part (b)

Fracture Modes and Crack Propagation Mechanisms

Fracture is generally classified into distinct modes based on the material's ductility and the loading conditions.

1. Ductile Fracture

Characteristic	Description
Occurs	After significant plastic deformation (necking).
Mechanism	1. Crack initiation at voids/inclusions. 2. Growth of micro-voids. 3. Coalescence into a main crack.
Surface	Rough, fibrous texture with a characteristic "cup-and-cone" shape (in tension). Evidence of intense plastic flow.
Propagation	Slow and stable, requiring continuous energy input; the crack blunts easily.
Common in	Mild steels, aluminum alloys, copper.
Example	A bolt failure following long-term overload.

2. Brittle Fracture

Characteristic	Description
Occurs	Suddenly with little or no plastic deformation.
Mechanism	1. Crack initiation at a flaw/grain boundary. 2. Rapid crack propagation along crystallographic planes (cleavage).
Surface	Shiny, granular, and flat appearance. Chevron marks often point toward the crack origin.
Propagation	Fast and catastrophic, typically running perpendicular to the applied tensile stress.
Favored by	Low temperature, high loading rate, triaxial stress state (e.g., at a notch).
Example	Sudden, catastrophic fracture of a high-carbon steel component in a cold climate.

3. Fatigue Fracture

Characteristic	Description
Occurs	Under cyclic loading (even at low stresses).
Mechanism	1. Crack Initiation at surface defects/stress concentrations. 2. Stable Crack Propagation through cyclic plastic deformation (striations). 3. Final Fracture when the remaining cross-section yields under the load.
Surface	Distinct regions: Crack initiation site, "beach marks" (macroscopic progressive growth), and final rapid fracture zone.
Propagation	Stable growth governed by the stress intensity factor range and the number of cycles (Paris' Law).
Example	Failure of propeller shafts or gear teeth due to stress repetition.

4. Creep Fracture

Occurs under a constant static load at high temperature over extended periods. Crack growth is typically intergranular (along grain boundaries) due to atomic diffusion and void coalescence.

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