Failure of Engineering Materials

Question 1

Describe the two fracture modes. Which is preferable?

Answer 1

Ductile fracture, where there is a lot of plastic deformation before fracture, and brittle fracture, where there is little plastic deformation before fracture. Brittle failure is bad, because it is sudden and unexpected.

Question 2

Describe the fracture process

Answer 2

Crack creation and propagation until failure.
In ductile failure, the material necks, develops cavities, develops cracks, cracks propagate, fails at an angle. (cup and cone shape)
Brittle fracture (cleavage fracture) occurs perpendicular to the load, and flat. It can be intergranular or intra granular.

Question 3

What type of failure does cyclic loading result in.

Answer 3

Fatigue failure, which is a brittle failure.

Question 4

What are the types of cyclic loading?

Answer 4

Reverse stress cycles, Repeated stress cycles, Random stress cycles

Question 5

Define fatigue strength and fatigue life

Answer 5

Fatigue Strength: stress level at which failure will occur for a certain number of cycles
Fatigue life: number of cycles till failure
Fatigue life is greater for lower fatigue stresses

Question 6

Define fatigue limit

Answer 6

For some metals the fatigue limit is the largest stress for which a material can be loaded infinitely without failure. ex steel

Question 7

Describe the process of fatigue failure

Answer 7

Cracks are created at points of high stress concentration, stress concentration increases with every loading, internal stress reaches failure stress and failure occurs.

Question 8

Define beachmarks and striations

Answer 8

Beachmarks are macroscopic lines that indicate periods of repetitive loading.
Striations are microscopic lines within beachmarks that represent individual loading cycles. They are created by the movement of the cracks.