Fatigue Overview Flashcards
What is fatigue the process of?
Repeated vibrations in loading causing failure even when the nominal stresses are below the material yield strength
What is fatigue made up of?
Crack initiation and subsequent crack growth as a result of cyclic, plastic deformation
What is deltasigma?
= sigma(max) - sigma(min)
Change in stress
What is sigma(m)?
(sigma(max)+sigma(min))/2
Mean stress
What is sigma(a)?
Stress amplitude
deltasigma/2
What can cause crack initiation for a fatigue crack from a smooth surface?
Slip in grains
What can cause crack initiation in a notched surface for a fatigue crack?
High concentrations of strain at the root of the notch, with possible fracture of inclusions
What is the scale of short crack growth?
The scale of the mircostructure
What is stable crack growth?
‘Long’ cracks in the body of a metal growing regularly as the stress cycles
What is ‘end of life’ for a fatigue crack?
Rapid bursts of crack growth under massive plastic strain or brittle failure
What is the difference between elastic and plastic deformation in reference to bonds?
Elastic = stretching bonds
Plastic = breaking & reforming bonds
How can slip planes contribute towards crack initiation?
They essentially form a notch when they slip due to the application of a tensile stress
What are some other initiation triggers?
-Cracking / debonding of second phase particles
-Scratches / machining marks on surface
-corrosion pits or intergranular attack
-Porosity from casting
-Brittle surface layers
What are the common stages of a crack growing under a tensile stress?
Stage I: Shear crack
Stage II: Tensile crack
What is the process of growth for a tensile crack?
As stress increases, the crack opens
Shear stresses at the tip grow
The crack is opened and blunted
As stresses fall, the crack closes
It has grown by a very small amount
