Chapter 8 Flashcards
Define “simple fracture.”
The separation of a body into two or more pieces in response to an imposed stress that is static and at temperatures that are low relative to the melting temperature of the material.
Ref: Pg 211
For metals, what two fracture modes are possible?
Brittle and ductile.
Ref: Pg 211
What is the general difference between brittle and ductile fracture?
Ductile metals typically exhibit substantial plastic deformation with high energy absorption before fracture. However, there is normally little or no plastic deformation with low energy absorption accompanying a brittle fracture.
Ref: Pg 211
What are the two steps to any fracture process?
Crack formation and propagation.
Ref: Pg 211
With respect to crack propagation, describe ductile fracture.
Extensive plastic deformation in the vicinity of an advancing crack. The process proceeds slowly as the crack length is extended.
Ref: Pg 211
When is a crack said to be “stable?”
When it resists any further extension unless there is an increase in the applied stress.
Ref: Pg 211
With respect to crack propagation, describe brittle fracture.
Cracks may spread extremely rapidly, with very little accompanying plastic deformation.
When is a crack considered to be “unstable?”
Crack propagation, once started, continues spontaneously without an increase in the magnitude of the applied stress.
Ref: Pg 211
What are the two reasons why ductile fracture is usually preferred to brittle fracture?
- Brittle fracture can occur suddenly and catastrophically without any warning.
- More strain energy is required to induce ductile fracture.
Ref: Pg 211
What is a microvoid?
A small cavity that forms in the interior of a structure under tensile plastic deformation.
Ref: Pg 212
Microvoids enlarge, come together, and coalesce to form an _____, which has its long axis _____ to the stress direction.
Eleipical crack
perpecdicular
A “cup and cone” fracture is related to which type of fracture?
Ductile
Ref: Pg 212
When surveying a “cup and cone” fracture under an electron microscope which characteristic of plastic deformation can be observed?
Numerous small spherical dimples. Each dimple is on half of a microvoid that formed and then separated during the fracture process.
Ref: Pg 213
In brittle fracture, the direction of crack motion is very nearly ____ to the direction of the applied tensile stress.
Perpendicular.
Ref: Pg 213
True or False
Fracture surfaces of materials that fail in a brittle manner have distinctive patterns that differ from ductile failure.
True
Ref: Pg 213
What are some examples of surface features indicating a brittle failure?
A series of V-shaped chevron marking may form near the center f the fracture cross-section that points back toward the crack initial site.
Lines or ridges that radiate from the origin of the crack in a fanlike pattern.
Ref: Pg 213
True or False
For very hard and fine-grained metals there is no discernible fracture pattern.
True
Ref: Pg 214
Brittle fracture in amorphous materials, such as ceramic glasses, yields a _______.
Relatively shiny smooth surface.
Ref: Pg 214
Define “cleavage” with respect to brittle crystalline materials.
For most brittle crystalline materials, crack propagation corresponds to the successive and repeating breaking of atomic bonds along specific crystallographic planes.
Ref: Pg 214
Cleavage is said to be a transgranular fracture. Explain what this means.
The fracture cracks pass through the grains.
Ref: Pg 214