Unit 4 - Cracks and Fracture Mechanics

Question 1

What is the Griffith Criterion?

Answer 1

A crack will grow if the energy supplied by the material exceeds that required to form new surfaces.

If energy supplied&raquo_space; surface energy - Catastrophic failure

Question 2

What are and what do Kt and K describe?

Answer 2

Kt (Stress concentration) describes 3D features

K (Stress Intensity Factor) relates to atomically sharp cracks.

Question 3

Describe the 3 stressing modes

Answer 3

1 - Opening or tensile mode - Crack surface moves perpendicular to the plane of the crack

2 - Sliding or in plane shear mode - Crack surfaces move over each other in the plane of the crack and perpendicular to the leading edge of the crack

3 - Tearing or anti-plane shear mode, crack surfaces again move in the plane of the crack but this time parallel to the leading edge.

Mode 1 dominates where alpha >60
Mode 3 - Pure shear