Lecture 17 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
What is safe life?
Evaluate expected life
Use a margin of safety
Design to survive expected service life
Retire
What is fail safe?
Provides redundant load paths
Design to fail into a safe condition then survive until repair
What is deflect tolerance?
Assume flaws do exist
Design to live with some crack growth below critical size
Requires regular inspections
What is S-N?
The Total Life method
Relates nominal or local elastic stress to total life
What is epsilon-N?
Crack Initiation (or Strain-Life) method Relates local strain to crack initiation life
What is LEFM?
Crack Propagation method
Relates stress intensity to crack propagation rate
What do S-N, epsilon-N and LEFM have in common?
They all rely on similitude
What does N(f) = N(i) + N(p) mean?
Total life = crack initiation + crack growth
State the Paris equation
da/dN = C(deltaKI)^m
What is da/dN?
Crack growth rate
What can KI be replaced by?
Ydeltasigmasqrt(pi*a)
What is needed to calculate lifetimes?
Initial crack size Final crack size Stress range K calibration Material growth law
What are some limitations for designing against fatigue?
Life calculations are less precise than strength calculations
Fatigue properties cannot be inferred from static mechanical properties
Lab tests often exhibit scatter and are difficult to translate to full-sized components
