Materials Selection- Constant Life Diagrams

Question 1

When doing fatigue tests, why don’t you keep doing cycles beyond about 10^9?

Answer 1

Because the testing machine itself may fail by fatigue

Question 2

How are some endurance limits defined for materials that don’t show a well-defined endurance limit?

Answer 2

Taken to be the stress amplitude that causes failure in 10^6, 10^7 or 10^8 cycles

Question 3

Conditions for fully reversed cyclic loading

Answer 3

Mean stress σm is 0. Stress ration R is -1.

Question 4

Conditions for zero-tension loading

Answer 4

Minimum stress is 0. Mean stress σm is half of maximum stress. Stress ration R is 0.

Question 5

Applied stress vs time graphs for fully reversed and zero-tension loading

Answer 5

Fully reversed is like sine wave starting from origin. Zero-tension is like sine wave shifted up so that the troughs are at 0 applied stress.

Question 6

What does a positive (tensile) mean stress cause?

Answer 6

A reduction in fatigue life and fatigue endurance limit. Greater positive mean stress enhances this effect.

Question 7

Describe the stress amplitude vs mean stress graph

Answer 7

Has data points representing fatigue endurance limit for each mean stress. General negative correlation between mean stress and stress amplitude.

Question 8

Where are the Gerber, Goodman and Soderberg lines on the stress amplitude vs mean stress graph?

Answer 8

All start at point on y axis corresponding to fatigue endurance limit for fully reversed loading. Soderberg is straight diagonal line down to the yield stress on the x axis. All data points are above the line. Goodman is straight diagonal line down to UTS on x axis. Most data points are above this line. Gerber is a curve down to the UTS and most data points are below it and is upper limit. Above Gerber is unsafe region and below Soderberg is safe region.

Question 9

The Soderberg model

Answer 9

Any combination of mean stress (Sm) and stress amplitude (Sa) that lies on or below the Soderberg line will have infinite life.
kfSa/Se + Sm/Sy =1
Se is FEL for fully reversed loading
kf is safety factor
Sy is yield strength

Question 10

The Goodman model

Answer 10

Any combination of mean stress (Sm) and stress amplitude (Sa) that lies on or below the Goodman line will have infinite life.
kfSa/Se + Sm/SUTS =1
Se is FEL for fully reversed loading
kf is safety factor
SUTS is ultimate tensile strength (UTS subscript)

Question 11

The Gerber model

Answer 11

Any combination of mean stress (Sm) and stress amplitude (Sa) that lies on or below the Gerber line will have infinite life.
kfSa/Se + (Sm/SUTS)^2 =1
Se is FEL for fully reversed loading
kf is safety factor
SUTS is ultimate tensile strength (UTS subscript)

Question 12

When should the three models be used?

Answer 12

Only for tensile mean stress values