Lecture 12- Types of Fatigue and SN Curves

Question 1

What is fatigue?

Answer 1

the process of slow crack growth due to cyclic loading

Question 2

What are the 2 conditions in which fatigue can occur?

Answer 2

Fatigue of cracked structures
Fatigue of uncracked structures

Question 3

What are the 2 conditions for fatigue in uncracked structures?

Answer 3

High cycle fatigue
low cycle fatigue

Question 4

describe high cycle fatigue, how many cycles till failure

Answer 4

when stresses applied are below yield strength

more than 100,000 cycles till failure

Question 5

describe low cycle fatigue, how many cycles till failure

Answer 5

when stresses applied are above yield strength but below tensile strength

less than 100,000 cycles till failure

Question 6

How can fatigue be measured?

Answer 6

carry out tests on smooth samples at a given stress amplitude and measure the number of cycles till failure

Question 7

wheres the fatigue limit on an sn curve

Answer 7

where the curve flattens, some materials have a fatigue limit

Question 8

What is the relationship between ultimate tensile stress and fatigue limits?

Answer 8

The higher the ultimate tensile stress, the higher the fatigue limits.

Question 9

What are 5 ways to improve fatigue?

Answer 9

increase UTS
-reduce applied tensile stresses
-improve surface finish
-Avoid stress concentrations
-Refine microstructure, eg material with small grain size