Chapter 8 Flashcards
What is the Charpy test?
- Test used to measure impact energy/notch toughness
- Blow is administered through a weighted pendulum
- Sample is held horizontally with notch facing away
What is the Izod test?
- Test used to measure impact energy/notch toughness
- Blow is administered through a weighted pendulum
- Sample is held vertically with notch facing towards
What is brittle fracture?
Fracture that occurs by rapid crack propagation and without appreciable macroscopic deformation
What is case hardening?
Hardening of the outer surface of a steel component by a carburizing or nitriding process, improves wear and fatigue resistance
What is corrosion fatigue?
A type of failure that results from the simultaneous action of a cyclic stress and chemical attack
What is creep?
The time-dependent permanent (plastic) deformation that occurs under stress; for most materials it is important only at elevated temperatures.
What is ductile fracture?
Failure that is preceded by necking or other forms of plastic deformation. Occurs in malleable materials
What is the ductile-to-brittle transition?
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.
What is fatigue?
Failure, at relatively low stress levels, of structures that are subjected to fluctuating and cyclic stresses. The most common cause of engineering failures.
What is fatigue life?
The total number of stress cycles that cause a fatigue failure at some specified stress amplitude.
What is fatigue limit?
For fatigue, the maximum stress amplitude level below which a material can endure an essentially infinite number of stress cycles and not fail.
What is fatigue strength?
The maximum stress level that a material can sustain without failing, for some specified number of cycles.
What are fracture mechanics?
A technique of fracture analysis used to determine the stress level at which preexisting cracks of known size will propagate, leading to fracture.
What is fracture toughness?
The measure of a material’s resistance to fracture when a crack is present.
What is impact energy?
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.
What is an intergranular fracture?
Fracture of polycrystalline materials by crack propagation along grain boundaries.
What is plane strain?
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.
What is plane strain fracture toughness?
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.
What is a stress raiser?
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.
What is thermal fatigue?
A type of fatigue failure in which the cyclic stresses are introduced by fluctuating thermal stresses.
What is a transgranular fracture?
Fracture of polycrystalline materials by crack propagation through the grains.
What are the mechanisms by which materials fail?
- Direct overload (fracture)
- Loading history-dependent processes (fatigue)
- TIme-dependent processes (creep)
What is simple fracture?
The separation of a body into two or more pieces in response to a static stress and propagation of cracks accompanies fracture
What are the two types of fractures?
- Ductile
- Brittle
What are the characteristics of ductile fracture?
- Slow crack propagation and gradual failure
- Significant plastic deformation
- Fails with warning
What are the characteristics of brittle fracture?
- Rapid crack propagation and catastrophic failure
- Little or no plastic deformation
- Fails without warning
What are the stages of ductile fracture?
1) Necking
2) Void nucleation
3) Void growth and coalescence
4) Crack propagation
5) Fracture
What do brittle fractures look like?
- Chevron markings on surface pointing towards crack initiation site
- Intergranular and transgranular crack propagation
Why is stress concentration higher for sharp cracks?
They propagate at lower stresses than cracks with blunt tips. When the material is ductile the tip will become blunted, reducing stress.
What is critical stress?
The amount of stress that will cause a crack to propagate in a brittle material, largest, most highly stressed cracks grow first.
What can cause ductile materials to have brittle failures?
- High strain/loading rate
- Deformation at low temperatures
- Presence of a notch
What effect does temperature have on material fracture?
Colder temperatures increase the chance of brittle fracture and higher temperatures increase the chance of ductile fracture.
How can fatigue life be improved?
- Reducing the magnitude of mean stress
- Surface treatments (nitriding, carburizing, peening, etc)
- Design changes (ex: rounding corners)
What are the three stages of creep?
- Primary creep (decreasing)
- Secondary creep (constant)
- Tertiary creep (increasing)
