Chapter 8

Question 1

What is the Charpy test?

Answer 1

• Test used to measure impact energy/notch toughness
• Blow is administered through a weighted pendulum
• Sample is held horizontally with notch facing away

Question 2

What is the Izod test?

Answer 2

• Test used to measure impact energy/notch toughness
• Blow is administered through a weighted pendulum
• Sample is held vertically with notch facing towards

Question 3

What is brittle fracture?

Answer 3

Fracture that occurs by rapid crack propagation and without appreciable macroscopic deformation

Question 4

What is case hardening?

Answer 4

Hardening of the outer surface of a steel component by a carburizing or nitriding process, improves wear and fatigue resistance

Question 5

What is corrosion fatigue?

Answer 5

A type of failure that results from the simultaneous action of a cyclic stress and chemical attack

Question 6

What is creep?

Answer 6

The time-dependent permanent (plastic) deformation that occurs under stress; for most materials it is important only at elevated temperatures.

Question 7

What is ductile fracture?

Answer 7

Failure that is preceded by necking or other forms of plastic deformation. Occurs in malleable materials

Question 8

What is the ductile-to-brittle transition?

Answer 8

The transition from ductile to brittle behavior with a decrease in temperature exhibited by some low-strength steel (BCC) alloys; the temperature range over which the transition occurs is determined by Charpy and Izod impact tests.

Question 9

What is fatigue?

Answer 9

Failure, at relatively low stress levels, of structures that are subjected to fluctuating and cyclic stresses. The most common cause of engineering failures.

Question 10

What is fatigue life?

Answer 10

The total number of stress cycles that cause a fatigue failure at some specified stress amplitude.

Question 11

What is fatigue limit?

Answer 11

For fatigue, the maximum stress amplitude level below which a material can endure an essentially infinite number of stress cycles and not fail.

Question 12

What is fatigue strength?

Answer 12

The maximum stress level that a material can sustain without failing, for some specified number of cycles.

Question 13

What are fracture mechanics?

Answer 13

A technique of fracture analysis used to determine the stress level at which preexisting cracks of known size will propagate, leading to fracture.

Question 14

What is fracture toughness?

Answer 14

The measure of a material’s resistance to fracture when a crack is present.

Question 15

What is impact energy?

Answer 15

A measure of the energy absorbed during the fracture of a specimen of standard dimensions and geometry when subjected to very rapid (impact) loading; is important in assessing the ductile-to-brittle transition behavior of a material.

Question 16

What is an intergranular fracture?

Answer 16

Fracture of polycrystalline materials by crack propagation along grain boundaries.

Question 17

What is plane strain?

Answer 17

Where all the shape changes of a material happen on a single plane (displacement is parallel to a given plane). Occurs when material thickness is significantly greater than crack dimension.

Question 18

What is plane strain fracture toughness?

Answer 18

For the condition of plane strain, the measure of a material’s resistance to fracture when a crack is present. Is relatively high for ductile materials and low for brittle ones.

Question 19

What is a stress raiser?

Answer 19

A small flaw (internal or surface) or a structural discontinuity at which an applied tensile stress will be amplified and from which cracks may propagate.

Question 20

What is thermal fatigue?

Answer 20

A type of fatigue failure in which the cyclic stresses are introduced by fluctuating thermal stresses.

Question 21

What is a transgranular fracture?

Answer 21

Fracture of polycrystalline materials by crack propagation through the grains.

Question 22

What are the mechanisms by which materials fail?

Answer 22

• Direct overload (fracture)
• Loading history-dependent processes (fatigue)
• TIme-dependent processes (creep)

Question 23

What is simple fracture?

Answer 23

The separation of a body into two or more pieces in response to a static stress and propagation of cracks accompanies fracture

Question 24

What are the two types of fractures?

Answer 24

• Ductile
• Brittle

Question 25

What are the characteristics of ductile fracture?

Answer 25

• Slow crack propagation and gradual failure
• Significant plastic deformation
• Fails with warning

Question 26

What are the characteristics of brittle fracture?

Answer 26

• Rapid crack propagation and catastrophic failure
• Little or no plastic deformation
• Fails without warning

Question 27

What are the stages of ductile fracture?

Answer 27

1) Necking
2) Void nucleation
3) Void growth and coalescence
4) Crack propagation
5) Fracture

Question 28

What do brittle fractures look like?

Answer 28

• Chevron markings on surface pointing towards crack initiation site
• Intergranular and transgranular crack propagation

Question 29

Why is stress concentration higher for sharp cracks?

Answer 29

They propagate at lower stresses than cracks with blunt tips. When the material is ductile the tip will become blunted, reducing stress.

Question 30

What is critical stress?

Answer 30

The amount of stress that will cause a crack to propagate in a brittle material, largest, most highly stressed cracks grow first.

Question 31

What can cause ductile materials to have brittle failures?

Answer 31

• High strain/loading rate
• Deformation at low temperatures
• Presence of a notch

Question 32

What effect does temperature have on material fracture?

Answer 32

Colder temperatures increase the chance of brittle fracture and higher temperatures increase the chance of ductile fracture.

Question 33

How can fatigue life be improved?

Answer 33

• Reducing the magnitude of mean stress
• Surface treatments (nitriding, carburizing, peening, etc)
• Design changes (ex: rounding corners)

Question 34

What are the three stages of creep?

Answer 34

• Primary creep (decreasing)
• Secondary creep (constant)
• Tertiary creep (increasing)