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
In some alloys, crack propagation is along grain boundaries; this fracture is termed ____.
Intergranular.
Ref: Pg 215
The measured fracture strength for most materials are significantly _____ than those predicted by theoretical calculations based on atoms bonding energies.
Lower
Ref: Pg 216
Explain why the measured fracture strength for most materials is significantly lower than those predicted by theoretical calculations.
This discrepancy is explained by the presence of microscopic flaws or cracks that always exist under normal conditions at the surface and within the interior of a body of material.
Ref: Pg 216
A crack tip is sometimes called a “stress raiser.” Explain why.
At crack tips, applied stress is significantly amplified.
Ref: Pg 216
True or False
The effect of a stress raiser is more significant in brittle than in ductile materials.
True
Ref: Pg 217
Describe in simple terms the ductile-to-brittle transition.
For some materials, whether or not they would exhibit a ductile or brittle fracture depends on the temperature of the material.
Ref: Lecture Slides
What is the Charpy Test used for?
Used to study the energy absorption of materials at differing temperatures.
Ref: Lecture slides
Ductile materials have a ____ energy absorption.
High
Ref: Lecture Notes
Brittle materials have a ____ energy absorption.
Low
Ref: Lecture Notes
What are the three steps to yield fatigue failure?
- Crack formation due to stress concentrations and the presence of free surfaces.
- Crack Growth
- Fracture
Ref: Lecture Slides
What is fatigue?
A form of failure that occurs in structures subjected to dynamic and fluctuating stresses.
Ref: Pg 229
True or False Under fatigue (cyclic loading) it is possible for failure to occur at a stress level considerably lower than the tensile or yield strength for a static load.
True
Ref: Pg 229
What type of failure is most common?
Fatigue failure
Ref: Pg 229
True or False
For some ferrous, and titanium alloys, the S-N curve becomes horizontal at higher N values. This is a limiting stress level called the fatigue or endurance limit.
True
Ref: Pg 231
The fatigue limit represents ______.
The largest value of fluctuating stress that will not cause failure for essentially an infinite number of cycles.
Ref: Pg 232
True or False
Most non-ferrous alloys do not have a fatigue limit.
True
Ref: Pg 232
Define “Fatigue Strength.”
For non-ferrous alloys that do not have a fatigue limit, it is the point at which failure occurs for some specified number of cycles.
Ref: Pg 232
Define “Fatigue Life.”
The number of cycles to cause failure at a specified stress level.
Ref: Pg 232
Give some examples of places where crack nucleation sites would be.
Surface scratches
sharp fillets
keyways, threads, and dents
Ref: Pg 236
When do beachmarks take place?
When the material experiences interruptions in the crack propagation.
Ref: Pg 236
Each beachmark band represents ____.
A period of time over which crack growth occurred.
Ref: Pg 236
What is a striation?
The tiny advancing distance of a crack front during a single load cycle.
Ref: Pg 236
True or False
There may be thousands of striations within a single benchmark.
True
Ref: Pg 236
An area having a dull fibrous texture is indicative of ____.
Rapid failure
Ref; Pg 237
True or False
Benchmarks and striations do not appear on the region over which the rapid failure occurs.
True
Ref: Pg 237https://assets.grammarly.com/emoji/v1/1f610.svg
What is “creep” with respect to material deformation?
The tendency of a solid material to move slowly or deform permanently under the influence of persistent mechanical stresses. It can occur as a result of long-term exposure to high levels of stress that are still below the yield strength of the material. More severe in materials that are subjected to heat for long periods.
Ref: Wikipedia
What is “primary creep?”
The rate of creep decreases with time because the material goes through strain hardening.
Ref: Lecture Slides
Describe “secondary creep.”
The rate of creep has a constant slope. It does not speed up or slow down. This is referred to as the work stage o the material.
Ref: Lecture Slides
Describe “tertiary creep.”
The slope of the creep rate increases with time.
Ref: Lecture Slides
Which stage of creep has the longest duration?
Secondary
Ref: Pg 241
What is the most important parameter from a creep test?
The rate of creep with respect to time during secondary creep. This is often called the minimum or steady-state creep rate.
Ref: Pg 241
