Fatigue Flashcards
(19 cards)
What is material fatigue
A form of failure that occurs under repeated or cyclic stress, it can occur at a stress level significantly lower than Yield or tensile strength
Why is aluminium at very high risk of fatigue failure
It has no endurance limit, it will eventually fail by fatigue regardless of the stress
What is fatigue stress defined as
The stress level at which failure will occur for some specific number of cycles
What are the 3 main steps in fatigue failure process
Crack initiation, Crack propagation, final rapid fracture
What is the total fatigue life a sum of
Number of cycles for crack initiation + Number of cycles for crack prorogation
What is high cycle fatigue
HCF occurs when the stress is less than the yield stress, the number of cycles to failure is greater than approx 10^4
The fatigue life is approximately the same as the crack initiation life
What is Low cycle Fatigue
LCF occurs when the stress is greater than the yield stress and the number of cycles to failure is less that approx 10^4 cycles
What is stage 1 crack growth
Cracks grow parallel to persistent slip bands (PSB) subjected to the highest shear stress
What is stage 2 crack growth
The crack turns and becomes a mode 1 crack perpendicular to the direction of maximum tensile stress and propagates rapidly. striations are formed q
Describe the fracture surface for stage 1 crack growth and stage 2 crack growth
For stage 1 it is smooth stage 2 has visible striations
What are the 2 types of surface features present of fracture surfaces
Benchmarks and striations
What are striations
A surface feature that represents the materials advance distance during single load cycle.
They are of microscopic scale and width increases with stress range
What are bench marks
A macroscopic feature that forms on components that experience applied stress interruptions, each bench mark may contain many thousands of bench marks
What factors effect fatigue strength
Stress concentration
surface roughness
surface treatment- carbonising and nitriding
Environment
What is Basquins law
For HCF when the SN curve is plotted on log log scales we get an approximate straight line. This is valid at a constant stress amplitude about a zero mean stress
How is fatigue effected by mean stress
To achieve the same fatigue life for a higher mean stress the amplitude of cyclic stress has to be reduced.
What is miners rule of cumulative damage
Fatigue failure will occur when the sum of the fractions of lifetimes is equal to one
Sum of Ni/Nfi = 1 then fatigue failure will occur
What is coffin Manson Law
For LCF a straight line is achieved when plastic strain range is plotted against Nf on log scales
What is crack growth rate a measure of
The crack growth per cycle
