Factors affecting fatigue life

The factors affecting fatigue life of a component:

1. Stress Level

Fatigue life is highly dependent on

2. Surface Effects

Surface finish is important because in fatigue, cracks usually start

at the surface.

Design: Notches, discontinuities, grooves, holes, threads

increase the stress concentration, and the sharper the

discontinuity the more severe the stress concentration. Therefore

to design against fatigue, avoid irregularities

3. Environment

Thermal fatigue: Fluctuating temperatures can cause

thermal stresses due to thermal expansion of the

components.

Corrosion fatigue: If the component is exposed to a

corrosive environment, pits caused by corrosion can act

as initiation sites and corrosion can also increase the

crack growth rate.

Surface treatment: Machining introduces scratches and

grooves, therefore polishing a machined surface will increase

fatigue life. Fatigue life can be improved by introducing a

compressive residual stress on the surface layer (shot peening

and case hardening).

Factors affecting fatigue

Summary Fatigue crack growth rate varies with cyclic stress intensity: Paris law Also in position to determine the fatigue life of a component Mechanism of fatigue crack formation in HCF and LCF Fracture surface of fatigued component show peculiar features: Striations mark

Lecture 16 Materials at high

temperatures

Jayant Jain Assistant Professor,

Department of Applied Mechanics, IIT Delhi, Hauz Khas, 110016

Creep is slow, continuous deformation with time: strain depends on stress, temperature and time

Creep

Creep testing and creep curves

Creep test

Metals, polymers and ceramics all show creep curve of this nature In designing against creep the secondary stage is the most important

Variation of creep rate with stress

Creep rate vs. stress

Where n is creep exponent Power law creep

Variation of creep rate with temperature

Creep rate vs. temperature

R gas constant Q activation energy for creep

Combining stress and temperature dependence

Effect of stress and temperature on creep strain

Stress-Rupture Curve

Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

Design data based on creep is generally presented in a stress-rupture curve allows you to identify either the

design stress or rupture life at a given temperature

Creep mechanism

The following are the main mechanisms of creep deformation

1) Dislocation creep

2) Diffusional creep

3) Grain boundary sliding

The rate of (1) and (2) is limited by diffusion of atoms

If we want to make engineering materials more resistant to creep deformation and creep fracture, we must look at how creep and

creep-fracture take place on an atomic level.

Atomic diffusion: Mechanism

How atomic diffusion takes place in crystalline solids??

Interstitial diffusion C, O, N, B and H diffuse interstitially in most crystals

Vacancy diffusion Zn atom diffuses in brass

Jump from one interstice to another

Bulk diffusion takes place by two mechanism:

Movement requires vacancy to sit next to it

Fast diffusion paths: Grain boundary

and dislocation core

Grain boundary diffusion

Dislocation-core diffusion

Dislocation creep

Plastic deformation takes place by the motion of dislocations

Dislocation has to move through various obstacles: GB, PPT, solute atoms, lattice resistance

Diffusion of atoms can Unlock dislocations from obstacles in their path, and the movement of these unlocked dislocations

under the applied stress is what leads to dislocation creep

Dislocation creep also known as power law creep

Dislocation Creep: Edge dislocation

How does the unlocking occurs??

The dependence of creep rate on applied stress is due to climb force

Climb force, more dislocations become unlocked per second, more dislocations

glide per second, higher the strain rate will be