Fracture and Failure

Question 1

What are the 2 steps in a fracture?

Answer 1

Crack Formation and crack propagation

Question 2

Ductile and brittle describe either end of a spectrum, but what is the spectrum a measure of?

Answer 2

The ability for a material to undergo plastic deformation before a fracture.

Question 3

Facts of ductile fracture

Answer 3

Most metals; extensive plastic deformation; the crack is stable (it stills until further force is applied.)

Question 4

Facts of brittle fracture

Answer 4

Ceramics, ice, cold metals; little plastic deformation; the crack is unstable (will propagate further at same force.)

Question 5

In most applications which is preferred?

Answer 5

Ductile

Question 6

What shape would expect from either end of a very ductile fracture? Which materials would this include?

Answer 6

Tapered to a point; soft metals such as gold, glass at high temperature (See lecture 7 slide number 5 for a picture)

Question 7

What shape would you expect from a brittle fracture? Which material is this failure typical of?

Answer 7

Flat ends with no tapering; cold metals and ceramics (See lecture 7 slide number 5 for a picture)

Question 8

What shape would you expect from a moderately ductile fracture? Which material is this failure typical of?

Answer 8

Tapered near the fracture but a perpendicular rough tear; typical for ductile metals (See lecture 7 slide number 5 for a picture)

Question 9

What are the steps in a ductile fracture failure?

Answer 9

Necking - Void Nucleation - Void Coalescence - Crack Propogation - Separation

Question 10

What are the steps in a brittle fracture failure?

Answer 10

Crack propagates quickly; nearly perpendicular to direction of stress; with no appreciable plastic deformation.

Question 11

What are the two types of brittle fracture?

Answer 11

Transgranular and Intergranular, respectively, fracture passes through grains and frack propagates along boundaries.

Question 12

What causes stress-concentration?

Answer 12

A reduction in cross-sectional area due to strain causes increasing stress.

Question 13

What is a stress raiser?

Answer 13

A stress-concentration. Commonly a crack or pore.

Question 14

The max stress at a stress raiser is:

Answer 14

2 x sigma_0 x (a/p_t)^1/2 a is the half-length of the crack, and p_t is the radius at the crack tip. (Think this is only for cylinders???)

Question 15

Alloys have higher or lower ductile-to-brittle transition temperatures than their pure metal alternatives?

Answer 15

Higher

Question 16

What is the result of fatigue?

Answer 16

Failure can occur at loads considerably lower than the yield strength of the material under a static load.

Question 17

What causes fatigue?

Answer 17

Fluctuating/cyclic stresses

Question 18

What are the stages of a fatigue failure?

Answer 18

Crack initiation, incremental crack propagation, final catastrophic failure

Question 19

What are the three types of cyclic stress?

Answer 19

Reversed stress cycle: periodic and symmetrical about zero stress. Repeated stress cycle: periodic and asymmetrical about zero stress. Random stress cycle: random fluctuation.

Question 20

What is the fatigue limit?

Answer 20

The maximum stress below which the material will not fail, irrelevant of the number of stress cycles it experiences.

Question 21

Fatigue strength:

Answer 21

Increase load at which fracture occurs after a specific number of cycles.

Question 22

Fatigue life:

Answer 22

Number of cycles to fail at a specified stress level.

Question 23

Crack propagation:

Answer 23

Stage 1: Initial slow propagation along crystal planes involving just a few grains. Stage 2: Faster propagation perpendicular to the applied stress. Crack eventually reaches critical dimension and propagates very rapidly.

Question 24

Explain N_f = N_i + N_p

Answer 24

N_f = N_i + N_p The total number of cycles until failure are counted as the number of cycles as the crack initiates and the number of cycles the crack propagates.

Question 25

What factors affect fatigue life? (Think of the K-values from design)

Answer 25

Magnitude of stress; quality of surface finish; thermal fatigue (creep); corrosion.

Question 26

Explain creep:

Answer 26

Creep is time-dependant and permanent. It is the deformation of a material when subjected to a constant load at a high temperature > 0.4 of its absolute melting point.

Question 27

What are the stages of creep?

Answer 27

1. Instantaneous deformation, mainly elastic; 2. Primary/ transient creep: strain slows as time increases (a form of work-hardening); 3. Secondary/steady-state creep: Rate of strain v time is steady; 4. Tertiary: Rapidly accelerating strain rate until failure (formation of cracks).

Question 28

Which creep stage is the most important parameter for long-life applications:

Answer 28

Secondary/steady-state

Question 29

How is creep minimised?

Answer 29

High melting point and high elastic modulus

Question 30

Examples of creep resistant materials:

Answer 30

Stainless steel; and cobalt and nickel based alloy