Cracks

Question 1

Where do cracks originate?

Answer 1

Cracks originate at stress concentrations

Question 2

What is the equation for the stress intensity factor, K?

Answer 2

K = Ysigmasqrt(pi*a)
where a is the crack length
sigma is the applied stress
Y is the geometrical factor

Question 3

What are sources of stress concentration?

Answer 3

Surface defects and notches

Question 4

How are stress concentrations prevented?

Answer 4

By polishing the surface of the material to reduce the number of surface defects such as notches and by radiussing all sharp corners which can act as stress concentrations

Question 5

How are flaws found?

Answer 5

Using non-destructive testing such as dyes

Question 6

The two mechanisms of crack propagation in composites

Answer 6

1. Deflexion at the matrix-fibre interface

2. Crack bridging and fibre pullout

Question 7

Describe deflextion at the matrix fibre interface

Answer 7

The strengthening fibre stops the growth of the crack/ Laminating at the surface of the interface because it is weaker.

Question 8

Describe crack bridging and fibre pullout

Answer 8

The crack is held together by fibres. Eventually the fibre breaks and absorbs energy

Question 9

Are deflextion and fibre pullout energy absorbing or energy releasing processes?

Answer 9

Energy absorbing processes

Question 10

Is the propagation of cracks energy absorbing or energy releasing?

Answer 10

Energy releasing

Question 11

What is the only energy absoroption mechanism in brittle materials?

Answer 11

The formation of new surfaces. It consumes energy to absorb new surfaces.

Question 12

Other energy absorbing mechanisms in classes of materials

Answer 12

• Plastic Deformation

- Fibre pullout in composites

Question 13

What is Griffith Crack Theory?

Answer 13

Unstable crack growth occurs when the stress intensity is higher than the elastic strain energy. When the elastic energy released by crack growth is greater than the energy required in absorption to form new surfaces.

Question 14

What class of materials does the Griffith Crack Theory apply to?

Answer 14

It applies to brittle materials. It doesn’t apply to ductile materials because plastic deformation in the ductile materials reduces the stress concentration at the tip of the crack.

Question 15

What is fracture toughness?

Answer 15

The fracture toughness is a measure of the resistance of a material to crack propagation.

Question 16

What is the equation for the critical stress intensity factor?

Answer 16

K ic = sqrt(2gammaE)

where gamma is the surface energy

Question 17

Define the critical stress intensity factor

Answer 17

It is the stress intensity required for unstable crack propagation. This is a material property and characterises the fracture toughness of a material.

Question 18

What does the critical stress intensity factor depend on?

Answer 18

It depends on the released during crack growth. The crack growth releases strain energy in the material but consumes energy in creating new surfaces, in plastic deformation

Question 19

When does unstable crack growth occur?

Answer 19

When the energy released iby the crack is greater than the energy required for the formation of new surfaces.

Question 20

The three tests required to determine the fracture toughness of ceramics

Answer 20

1. The tensile test
2. The bend test
3. The compressive test

Question 21

Why is the fracture toughness measured in the tensile test of ceramics usually low?

Answer 21

Ceramics contain a large number of flaws. In tension, the largest flaw in the ceramic will propagate unstably.

Question 22

Why is the fracture toughness measured by the compressive test for ceramics greater than that of the tensile test?

Answer 22

The flaws will propagate stably. Extension of small cracks distributed through the surfaces.

Question 23

Why are there flaws in ceramics?

Answer 23

Production process but also in service and handling

Flaws are locations of stress concentrations.

Question 24

The ranking of the fracture toughness of materials

Answer 24

Polymer foams < Ceramics+Glasses < Polymers < Metals and alloys < Composites

Question 25

The effect of compressive stresses on crack propagation

Answer 25

By introducing compressive residual stresses then cracks are not able to propagate. Prevents the propagation of cracks.