6. Failure of materials

Question 1

ductile tensile fracture surface

Answer 1

is rough

Question 2

define toughness

Answer 2

the quantity of mechanical energy needed to break a given cross-section of a material

Question 3

what is stable crack growth?

Answer 3

in ductile fracture where the crack propagates by being continually supplied by energy

Question 4

stages of tensile fracture of a ductile material

Answer 4

1. in the necking region, microvoids form around oxide inclusions and at grain boundaries
2. these microvoids join up (coalesce) to form a large central void which propagates towards the surface
3. Final fracture occurs along ~45 degree shear planes to give a typical ‘cup-and-cone’ fracture

Question 5

Why is brittle fracture associated with pre-existing cracks or flaws(pores)?

Answer 5

local stresses at stress-concentrating cracks or flaws can exceed the engineering fracture stress of the material, causing it to fracture well below the yield point

Question 6

What does the impact test measure?

Answer 6

toughness

Question 7

Why is a notch involved in the impact test’s sample?

Answer 7

to concentrate the stress, making the component more susceptible to brittle failure even if the material is ductile TO SIMULATE AN EXTREME TEST

Question 8

Effect of carbon content, manganese content and impurities respectively on the transition temperature of steel

Answer 8

higher carbon content and more impurities increase transition temperature. Increasing manganese content decreases transition temperature.

Question 9

effect of grain size on transition temperature

Answer 9

decreasing grain size decreases transition temperature

Question 10

How does faster loading affect transition temperature?

Answer 10

Faster loading increases transition temperature (as dislocations need time to move to produce slip)

Question 11

length of edge crack and internal crack in terms of a

Answer 11

edge crack length = a; internal crack length = 2a

Question 12

What is K1?

Answer 12

Stress Intensity Factor. Measure of the degree to which an external stress is amplified at a crack tip

Question 13

What is K1C?

Answer 13

The critical value of K1 where the crack will propagate rapidly (at the speed of sound in the material) and result in fast, uncontrollable fracture.

Question 14

What is K1C dependent on?

Answer 14

The material

Question 15

What else is K1C known as?

Answer 15

Fracture toughness

Question 16

Why are fracture mechanics important to an engineer? 3 reasons

Answer 16

1. Engineer can design with imperfect materials (accounting for K1C) so real world engineering
2. Material’s strength (UTC) and fracture toughness must be considered when designing load-bearing components
3. Fracture may occur before the material’s yield strength is reached

Question 17

Situations where you can get fatigue failure

Answer 17

when the component is exposed to a corrosive environment

when there is a oscillating or cyclic pattern of stress (fatigue loading)

Question 18

four requirements for fatigue failure to occur

Answer 18

cyclic stress
tensile component (to open crack)
maximum cyclic stress is less than yield stress
a crack must either pre-exist or form and it must grow to critical size (for brittle fracture)

Question 19

What is an fatigue/endurance limit and what metals does this apply to?

Answer 19

below this endurance limit fatigue will not occur, applies to ferrous metals (iron/steel etc)

Question 20

define fatigue strength

Answer 20

the max stress that will allow a minimum number of cycles N to be achieved before failure

Question 21

What are ‘beach’ marks?

Answer 21

periods of major crack advancement, can be seen by eye usually

Question 22

what are fine fatigue striations?

Answer 22

microscopic advance of the crack - shows how much the crack grows with each cycle

Question 23

What type of stress cycle produces a shiny or polished fracture surface

Answer 23

reverse stress component, the compression components remove the beach marks and fatigue striations, producing a polished part.

Question 24

3 ways of improving fatigue properties

Answer 24

improving surface finish (polishing, grinding)
putting surface in residual compression (reducing effective tensile strength)
good design and manufacturing

Question 25

what is shot peening

Answer 25

firing tiny pieces of metal at the surface, deforms it slightly and puts it into compressive.

Question 26

When does a pure metal begin to creep, and when does a ceramic begin to creep?

Answer 26

pure metal: service temp > 0.3-0.4 of melting temp

ceramic: service temp > 0.4-0.5 of melting temp

TEMPERATURES IN KELVIN

Question 27

four parts of a creep curve

Answer 27

1. Instantaneous elastic strain
2. Primary non-linear creep (small bend)
3. Secondary/ steady state creep (most important)
4. Tertiary creep (small bend) ends in creep failure

Question 28

What is the failure time of a creep curve called?

Answer 28

time to rupture

Question 29

What is dislocation climb?

Answer 29

At sufficiently high temperature (creep temp), atoms move via diffusion into vacancies which can then cause the dislocation to move up from a slip plane with an obstacle, effectively bypassing it

Question 30

What is stress-induced migration of atoms and vacancies and how does this cause creep?

Answer 30

At high temperatures vacancies increase and atoms have enough energy to diffuse - atoms diffuse in direction towards the tensile stress ELONGATING THE SAMPLE. vacancies diffuse perpendicular to the tensile stress, REDUCING THE CROSS-SECTIONAL AREA

Question 31

Where is diffusion the most rapid?

Answer 31

at grain boundaries, as these are regions of disorder and they have greater energy due to strained inter-atomic bonds

Question 32

Describe grain boundary sliding

Answer 32

Rapid diffusion at grain boundaries leave trail of vacancies (as grain boundaries are not smooth like slip planes), These vacancies grow and combine to form voids or microscopic cracks leading to creep fracture.

Question 33

How to make creep resistant material?

Answer 33

High melting point alloys (creep occurs at > 0.3-0.4 of melting point)

maximum obstacles to dislocation movement and dislocation climb

prefer alloys with covalent bonding (stronger bonds, harder for dislocation movement)

LARGE grain size so atoms have to diffuse further before reaching a grain boundary (where diffusion is faster) also less sliding

USE single crystal (above reasons)

precipitates at grain boundaries (makes sliding difficult)