Lecture 17

Question 1

What is fatigue the process of?

Answer 1

Repeated vibrations in loading causing failure even when the nominal stresses are below the material yield strength

Question 2

What is fatigue made up of?

Answer 2

Crack initiation and subsequent crack growth as a result of cyclic, plastic deformation

Question 3

What is deltasigma?

Answer 3

= sigma(max) - sigma(min)

Change in stress

Question 4

What is sigma(m)?

Answer 4

(sigma(max)+sigma(min))/2

Mean stress

Question 5

What is sigma(a)?

Answer 5

Stress amplitude

deltasigma/2

Question 6

What can cause crack initiation for a fatigue crack from a smooth surface?

Answer 6

Slip in grains

Question 7

What can cause crack initiation in a notched surface for a fatigue crack?

Answer 7

High concentrations of strain at the root of the notch, with possible fracture of inclusions

Question 8

What is the scale of short crack growth?

Answer 8

The scale of the mircostructure

Question 9

What is stable crack growth?

Answer 9

‘Long’ cracks in the body of a metal growing regularly as the stress cycles

Question 10

What is ‘end of life’ for a fatigue crack?

Answer 10

Rapid bursts of crack growth under massive plastic strain or brittle failure

Question 11

What is the difference between elastic and plastic deformation in reference to bonds?

Answer 11

Elastic = stretching bonds
Plastic = breaking & reforming bonds

Question 12

How can slip planes contribute towards crack initiation?

Answer 12

They essentially form a notch when they slip due to the application of a tensile stress

Question 13

What are some other initiation triggers?

Answer 13

• Cracking / debonding of second phase particles
• Scratches / machining marks on surface
• corrosion pits or intergranular attack
• Porosity from casting
• Brittle surface layers

Question 14

What are the common stages of a crack growing under a tensile stress?

Answer 14

Stage I: Shear crack

Stage II: Tensile crack

Question 15

What is the process of growth for a tensile crack?

Answer 15

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

Question 16

What is safe life?

Answer 16

Evaluate expected life
Use a margin of safety
Design to survive expected service life
Retire

Question 17

What is fail safe?

Answer 17

Provides redundant load paths

Design to fail into a safe condition then survive until repair

Question 18

What is deflect tolerance?

Answer 18

Assume flaws do exist
Design to live with some crack growth below critical size
Requires regular inspections

Question 19

What is S-N?

Answer 19

The Total Life method

Relates nominal or local elastic stress to total life

Question 20

What is epsilon-N?

Answer 20

Crack Initiation (or Strain-Life) method
Relates local strain to crack initiation life

Question 21

What is LEFM?

Answer 21

Crack Propagation method

Relates stress intensity to crack propagation rate

Question 22

What do S-N, epsilon-N and LEFM have in common?

Answer 22

They all rely on similitude

Question 23

What does N(f) = N(i) + N(p) mean?

Answer 23

Total life = crack initiation + crack growth

Question 24

State the Paris equation

Answer 24

da/dN = C(deltaKI)^m

Question 25

What is da/dN?

Answer 25

Crack growth rate

Question 26

What can KI be replaced by?

Answer 26

Ydeltasigmasqrt(pi*a)

Question 27

What is needed to calculate lifetimes?

Answer 27

Initial crack size
Final crack size 
Stress range 
K calibration 
Material growth law

Question 28

What are some limitations for designing against fatigue?

Answer 28

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