Material Failure

Question 1

What are the two fracture modes?

Answer 1

Ductile and Brittle

Question 2

What is the main difference between ductile and brittle fracture?

Answer 2

Ductile materials exhibit substantial plastic deformation with high energy absorption before fracture there is little to no plastic deformation in brittle fracture.

Question 3

What are the two steps of a fracture?

Answer 3

Crack formation and propagation.

Question 4

What are the five stages of ductile fracture?

Answer 4

1) Initial necking
2) Small cavity formation
3) Coalescence of cavities to form crack
4) Crack propogation
5) Final shear fracture at 45’ angle relative to tensile direction

Question 5

Describe a cup and cone fracture

Answer 5

The characteristic shape left after ductile fracture. The central interior region of the surface has an irregular and fibrous appearance, the edges are shear.

Question 6

Describe the characteristics of brittle fracture

Answer 6

Brittle fracture takes place without any appreciable deformation, and by rapid crack propagation. The direction of crack motion is very nearly perpendicular to the direction of the applied tensile stress and yields a relatively flat fracture surface

Question 7

What is fatigue?

Answer 7

Fatigue is a form of failure that occurs in structures subjected to dynamic and fluctuating stresses. Under these circumstances it is possible for failure to occur at a stress level considerably lower than the tensile or yield strength for a static load.

Question 8

What is the fatigue level of a material?

Answer 8

A value of below which a material will not succumb to fatigue failure. It is around 35%-60% in steels.

Question 9

Explain the role of dislocations in the strengthening of metals.

Answer 9

In general terms, the goal is to impede the movement of dislocations. Dislocations facilitate slip. Tie these up, and more stress must be applied resulting in strengthening of the material.

Question 10

A material supplier has advised you that the Fatigue Strength of a particular metal is 250MPa. What is wrong with this statement?

Answer 10

Fatigue strength is associated with a particular number of cycles to failure. This number is missing from the material suppliers information.

Question 11

If you were asked to examine the failure surface of a shaft, what features on the surface would suggest to you that fatigue failure had occurred?

Answer 11

Would look under the microscope for lines (striations – sharp cracks) which has grown incrementally until final (rapid) failure.