Principles of fracture mechanics

Question 1

What characterizes a ductile fracture?

Answer 1

• Large amount of plastic deformation before
  crack development
• Further amount of plastic work for crack
  growth.
• The final fracture is anticipated by large
  macroscopic straining

Question 2

What characterizes a brittle failure?

Answer 2

• Crack growth is much more rapid, accompanied by little amount of plastic deformation
• The process of crack development is unstable: once the crack is nucleated, it will
  spontaneously propagate even without any
  increase in applied load

Question 3

When do we have to consider the stress consentration?

Answer 3

Whenever discontinuities in stress distribution of a body are present, either on a macro- or on a micro-scale.

Question 4

In what type of material is the stress concentration effect more significant? Why?

Answer 4

Brittle materials. In ductile metals, once the yield stress is reached in the region of the notch, plastic flow is set at the crack tip and a more uniform distribution of stress is developed.

Question 5

What is the notch effect in a perfectly plastic metal?

Answer 5

Plastic flow around a defect would lead to uniform deformation

Question 6

What happens when a perfectly elastic metal is strained?

Answer 6

It stores elastic strain energy, especially close to crack tip due to stress intensity factor.

Question 7

What does the Griffith theory say?

Answer 7

In a perfectly elastic material, a crack can propagate only when the elastic energy
released (associated to crack growth) is at least as high as the surface energy required for the generation of the new crack faces

Question 8

In engineering metals, why is the fracture energy at fracture by orders of magnitude higher than that calculated by Griffith’s theory?

Answer 8

Due to significant contribution of plastic strain at crack tips.

Question 9

What is the stress intensity factor K ?

Answer 9

K allows specifying the magnitude of stress distribution close to a defect in an elastic material when it is stressed with a known load.

Question 10

What is K_IC?

Answer 10

Critical value of the stress intensity factor K that gives the conditions for unstable crack growth i.e. catastrophic (brittle) fracture for the tested material.