**What is Fatigue Testing?**

**1. From Latin "Fatigare"** meaning "to tire."

**2. Engineering terminology:** - damage and failure of materials under cyclic loads.

**3. Fatigue testing is defined as** the process of progressive localized permanent structural change occurring in a material subjected to conditions that produce fluctuating stresses and strains at some point or points and that may culminate in cracks or complete fracture after a sufficient number of fluctuations.

**Nomenclature to describe the test parameters involved in cyclic loading**

**Fatigue is the progressive, **localized, permanent structural change that occurs in materials subjected to fluctuating stresses and strains that may result in cracks or fractures after a sufficient number of fluctuations. The cyclic stresses are normally well below the yield strength of the material.

**1. Initial fatigue damage** leading to crack nucleation and crack initiation,

**2. Progressive cyclic growth of a crack (crack propagation)** until the remaining un-cracked cross section of a part becomes too weak to withstand the loads applied,

**3. Final, sudden fracture** of the remaining cross section.

stresses and strains.

environment.

5

**Cyclic loading generally produces failure** however low the stress may be. However, with some materials the S-N curve levels off, suggesting that for these materials a limit of stress (load) can be specified - known as the fatigue limit - below which infinite life can be expected.

**The fatigue life is thought to be associated with the phenomenon of strain ageing.**

**Most non ferrous alloys do not show a fatigue limit**. Instead their S-N curves continue to drop at a slow rate (dotted line).

**For these types of materials, the fatigue strength is quoted.** This is the value of stress to which the material can be subjected to for a given number of cycles (10,000,000 cycles is the value often used).

**Strain-Life Approach**

Low cycle, high stress fatigue with appreciable plastic deformation. Uses the cyclic strain range versus number of

cycles to failure.

Total life = crack initiation + crack propagation (90% of

life can be crack initiation).

Failure = Typically a crack of predefined size.

**The strain-life relationship is as follows:**

sf’ is the fatigue strength coefficient

E is the elastic modulus (Young's Modulus)

b is the fatigue strength exponent (Basquin’s exponent)

ef’ the fatigue ductility coefficient and c is the fatigue ductility exponent (the Coffin-Manson exponent)

High cycle, low stress fatigue.

Material deforms elastically.

Cyclic stress range vs number of cycles to failure (S-N Curve)

Total life = crack initiation + crack propagation

Failure = Total separation of specimen

First fatigue design method. Large amount of data available.

**Damage-Tolerant Approach**

Assumes defects present in material.

Linear Elastic Fracture Mechanics approach ( da/dN vs delta K)

Resistance to Fatigue Crack Growth.

Useful Fatigue life = number of cycles to propagate a crack from an initial size to some critical dimension.

Failure = critical crack size based on fracture toughness of material, limit load for particular structural part, allowable strain, change in compliance of a component etc.