Lecture 17 Flashcards

1
Q

What is fatigue the process of?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What is fatigue made up of?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What is deltasigma?

A

= sigma(max) - sigma(min)

Change in stress

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What is sigma(m)?

A

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

Mean stress

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What is sigma(a)?

A

Stress amplitude

deltasigma/2

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

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

A

Slip in grains

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

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

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What is the scale of short crack growth?

A

The scale of the mircostructure

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What is stable crack growth?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

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

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

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

A
Elastic = stretching bonds
Plastic = breaking & reforming bonds
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

How can slip planes contribute towards crack initiation?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What are some other initiation triggers?

A
  • Cracking / debonding of second phase particles
  • Scratches / machining marks on surface
  • corrosion pits or intergranular attack
  • Porosity from casting
  • Brittle surface layers
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

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

A

Stage I: Shear crack

Stage II: Tensile crack

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What is the process of growth for a tensile crack?

A
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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

What is safe life?

A

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

17
Q

What is fail safe?

A

Provides redundant load paths

Design to fail into a safe condition then survive until repair

18
Q

What is deflect tolerance?

A

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

19
Q

What is S-N?

A

The Total Life method

Relates nominal or local elastic stress to total life

20
Q

What is epsilon-N?

A
Crack Initiation (or Strain-Life) method
Relates local strain to crack initiation life
21
Q

What is LEFM?

A

Crack Propagation method

Relates stress intensity to crack propagation rate

22
Q

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

A

They all rely on similitude

23
Q

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

A

Total life = crack initiation + crack growth

24
Q

State the Paris equation

A

da/dN = C(deltaKI)^m

25
What is da/dN?
Crack growth rate
26
What can KI be replaced by?
Y*deltasigma*sqrt(pi*a)
27
What is needed to calculate lifetimes?
``` Initial crack size Final crack size Stress range K calibration Material growth law ```
28
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