Lecture 18 Flashcards
How is similitude used in the S-N method?
So long as a specimen and real structure have the same material are subject to the same nominal stress, their predicted lives are the same
What is the region below the line on an S-N curve?
The safe region
What cycle-lives are S-N curves used for?
High cycle life - more than 100,000 cycles
What happens at the fatigue limit?
Below this stress, no fatigue damage appears to occur
What is the tendency of steels and aluminium alloys in terms of fatigue limits?
Steels tend to have a fatigue limit
Aluminium alloys tend not to
What is fatigue limit associated with?
The difficulty a crack has in getting past the first grain boundary (or a dominant microstructural boundary)
Do cracks develop below the fatigue limit?
Yes, but cannot grow
How is fatigue limit and UTS related?
There is a general positive trend
What can S-N curves be created for?
Both materials and structures (eg bolted joints)
What is K(t) (equation)?
The ratio of the local notch root stress to the applied nominal stress
K(t) = max stress/nominal stress
What are K and K(t)? Are they the same thing?
NO
K(t) is the stress CONCENTRATION
K is the stress INTENSITY FACTOR
What is the effect of mean stress?
Higher mean stress means the material must work harder, so the cycles till failure decreases with increasing amplitude
What is a Goodman plot used for?
Estimating fatigue limit at different mean stresses
Where does the max stress amplitude come from on a Goodman plot?
The fatigue limit from experimental data at zero mean stress
How is a line for all combinations to give a certain life plotted?
Experimental data for that number of cycles at a certain mean stress and stress amplitude is plotted
A line is then drawn from tensile strength through this/these point/s
