Strain Life Curves

Question 1

What is considered crack initiation for the Strain-Life method?

Answer 1

Crack is 1-2mm

Question 2

What does the Strain-Life method measure?

Answer 2

The time taken to initiate a crack

Question 3

What types of conditioning are there?

Answer 3

Cyclic softening
Cyclic hardening

Question 4

What is the name for non-cyclic loading?

Answer 4

Monotonic loading

Question 5

How does a strain curve compare to a stress curve?

Answer 5

They are very similar, but strain curves take plasticity into account

Question 6

What steps are involved in the Strain-Life method?

Answer 6

-Look at the whole component, cycle etc
-Stress concentration
-Hysteresis curve
-Stress-strain curve
-Strain life curve

Question 7

Give some examples of factors which could affect data quality

Answer 7

-Material composition and heat treatment
-Specimen preparation
-Quality of test machine (control systems, calibration, fixtures, data captures etc)

Question 8

What does the Morrow Correction for Mean Stress do?

Answer 8

Moves the elastic cycle up/down depending on mean stress

Question 9

What is the limitation of the Morrow Correction?

Answer 9

It only applies to elastic curves

Question 10

What does the Smith Watson Topper Correction do?

Answer 10

Corrects for mean stress

Question 11

What is the nominal stress and strain?

Answer 11

The applied stress and strain - would be at there if the stress concentration wasn’t

Question 12

What is local elastic/plastic stress and strain?

Answer 12

Stress and strain next to notch root

Question 13

How can the notch root local stresses and strains be derived?

Answer 13

1. Measurement from a strain gauge
2. Elastic-plastic FE modelling
3. An empirical rule

Question 14

Which is the best derivation method for notch root local stresses and strains?

Answer 14

An empirical rule, eg Neuber’s rule

Question 15

What is Neuber’s rule?

Answer 15

K(epsilon).K(sigma) = (K(t))^2

Question 16

What is the useful rearrangement of Neuber’s rule and why is it useful?

Answer 16

(K(t))^2.s.e = sigma.epsilon
The terms on the left are usually known

Question 17

What are e and s in Neuber’s rule?

Answer 17

e = nominal strain
s = nominal stress

Question 18

Where is the plastic strain range on a hysteresis loop?

Answer 18

Along the stress = 0 line inside the loop

Question 19

How do fatigue limit and plastic strain range relate?

Answer 19

The larger the plastic strain range, the lower the fatigue limit

Question 20

What is point 1 in the graphical solution for local sigma and epsilon?

Answer 20

s and e- the material response diverges from the linear stress-strain line

Question 21

What is point 2 in the graphical solution for local sigma and epsilon?

Answer 21

K(t)s and K(t)e
The point of intersection of the Neuber equation structural response and the linear stress-strain line

Question 22

What is point 3 in the graphical solution for local sigma and epsilon?

Answer 22

The solution point - epsilon and sigma
The intersection of the material response and the structural response