Unit 6 - Ductile and Brittle Fracture

Question 1

Three sections of a stress strain graph

Answer 1

1-2 elastic deformation
2-3 plastic slip - dislocations move
3 start of failure - dislocations locked

Question 2

4 factors of a brittle material

Answer 2

1. very little plastic flow
2. the two sides of the fracture surface fit together very well after failure
3. the fracture surface appears faceted - you can make out individual grains and atomic planes
4. in many materials, fracture occurs along certain crystallographic planes.

Question 3

4 factors of ductile material

Answer 3

1. extensive plastic flow occurs prior to fracture
2. there is usually evidence of considerable necking in the specimen
3. fractures don’t fit together
4. the fracture surface has a dimpled appearance - you can see little holes often with second phase particles inside them

Question 4

5 steps of ductile fracture

Answer 4

1. necking
2. cavity formation
3. cavity coalescence to form a crack
   4, crack propagation
4. fracture

Question 5

4 stages of fracture in BFF and FCC

Answer 5

1. Yield; dislocations move
   2 failure starts; specimen starts to neck and microvoids form in the centre
2. microvoids coalesce into an elliptical shaped crack with the long axis perpendicular to the stress
3. final fracture by crack propagation and shear failure in the plane of maximum shear

Question 6

How do cracks affect the integrity of a structure

Answer 6

reduce stiffness
strain energy decreases
energy is being used to create new surfaces

Question 7

what is a crack?

Answer 7

Localised area of bond breakage in a material