Fatigue Questions

Question 1

In cyclic loading, how is the mean stress calculated?

Answer 1

Question 2

In cyclic loading, how is the stress amplitude calculated ?

Answer 2

Question 3

In cyclic loading, how is the stress range calculated?

Answer 3

Question 4

How is the stress ratio calculated ? What is fully reversed loading? And what is zero-based loading ?

Answer 4

Question 5

What is high cycle fatigue ?

Answer 5

Question 6

What is low cycle fatigue ?

Answer 6

Question 7

What is Basquins law?

Answer 7

Question 8

What is Coffin Mansons law?

Answer 8

Question 9

What is Goodmans law?

Answer 9

Question 10

What is the cyclic stress intensity factor?

Answer 10

Question 11

What is Paris’s law?

Answer 11

Question 12

How do you derive the number of cycles until failure for a pre-cracked component from Paris’s law?

Answer 12

Question 13

What is Miner’s law?

Answer 13

Question 14

What is fatigue failure?

Answer 14

When a fracture grows slowly due to cyclic loads

Question 15

What is fatigue?

Answer 15

The tendency of a material to experience progressive and localised failure by means of brittle fracture under cyclic stress

Question 16

Is fatigue failure below the typical yield stress of a material?

Answer 16

Yes

Question 17

What are the 3 stages of fatigue?

Answer 17

Microcracks form, then prorpagate then fracture occurs

Question 18

What is a stress life theory?

Answer 18

A graph which estimates the number of cycles to initiate and grow a crack

Question 19

If the endurance level is reach in a stress-life diagram, what does this mean?

Answer 19

Fracture will not occur

Question 20

What is high-cycle fatigue?

Answer 20

When the material undergoes many cycles before fatigue, mainly elastic deformation

Question 21

What is low-cycle fatigue?

Answer 21

When the material will not survive many cycles, mainly plastic deformation

Question 22

What is Ni?

Answer 22

The number of cycles corresponding to the i’th block of constrant stress

Question 23

What is Nfi?

Answer 23

The number of cycles till failure at a certain stress

Question 24

What does Hf equal?

Answer 24

Sum of Ni / Nfi

Question 25

When do you use coffin-manson’s law?

Answer 25

For high cycles

Question 25

When do you use Basquins’s law?

Answer 25

For low-cycle fatigue

Question 25

What does the Goodman graph plot and show?

Answer 25

On the y axis is stress amplitude, with the line starting at the endurance stress, on the x axis is the mean stress with the line extending to the ultimate tensile stress. If material is below the line, it is safe

Question 25

When do you use Goodman’s rule?

Answer 25

For when the mean stress is not 0

Question 25

What do large structures always have?

Answer 25

Cracks

Question 26

Based on an initial crack, what can we estimate?

Answer 26

The safe life of a material

Question 26

What do we plot to estimate the safe life of a material?

Answer 26

Log growth rate (da/dN) on the y-axis, against the log of the intensity factor.

Question 26

What is Da/DN?

Answer 26

The crack growth raqte

Question 27

What does da/dN equal?

Answer 27

Question 28

What does Nf equal in terms of da?

Answer 28

Question 29

In safe life design, what is the assumption?

Answer 29

The initial part is perfect and its life will be defined by the time tp initiate and propagate cracks.

Question 30

In safe life design, what do we use to estimate the time till failure?

Answer 30

S-N curves

Question 31

What happens when a structure reaches their safe life?

Answer 31

They are replaced

Question 32

Why is the process of safe life design an economic problem?

Answer 32

Good parts are often discarded

Question 33

In damage tolerant design, what is the assumption?

Answer 33

Cracks are present

Question 34

What do you do in damage tolerant design?

Answer 34

Carry out inspection on cracks, use crack growth rate to estimate number of cycles till failure