Chapter 8 Flashcards
What is a simple fracture?
The separation of a body into two or more pieces in response to a static stress
Propagation of cracks accompanies fracture
Two general types of fracture
What are the two general types of fracture?
Ductile and brittle
What are three properties of ductile fracture?
1) slow crack propagation
2) accompanied by significant plastic deformation
3) fails with warning
What are three properties of brittle fracture?
1) rapid crack propagation
2) little or no plastic deformation
2) fails without warning
Are ductile or brittle cracks more desirable?
ductile fracture
What kinds of fracture have large necking? No necking?
Very ductile: large necking
Moderately ductile: slight necking
Brittle: no necking
What’s the name for the shape a ductile metal makes at fracture?
cup-and-cone fracture
What does a brittle fracture look like?
flat surface
What could a pipe’s ductile fracture look like? Brittle fracture?
ductile: one piece, large deformation
brittle: many pieces, small deformations
What are the five stages of moderately ductile failure?
1) necking
2) void nucleation
3) void growth and coalescence
4) crack propagation
5) failure
What do the surface photographs of a brittle fracture look like?
They display V-shaped, chevron markings
The V features point to the crack initiation site
What are the two types of crack propagation?
Intergranular crack propagation (between grains)
Transgranular crack propagation (through grains)
Why does fracture occur?
As a result of crack propagation
Why are measured fracture strengths of most materials much lower than predicted?
1) microscopic flaws (cracks) always exist in materials
2) magnitude of applied tensile stress amplified at the tips of these cracks
How do you calculate the stress at the crack tip?
How do you calculate the stress concentration at the crack tip?
Is stress concentration higher for sharp cracks or blunt tips?
sharp cracks because they cracks spread with lower stresses
What are the tip shapes for cracks of brittle materials? Ductile materials?
brittle: sharp
ductile: blunt (because of a deformed region)
Which cracks grow first?
The largest, most highly stressed cracks
When does crack propagation & fracture occur?
When the stress at the crack tip (σm) > (σc) critical stress for crack propagation
What is fracture toughness?
Measure of material’s resistance to brittle fracture when a crack is present
qualitatively scales with impact resistance
What is the variable for fracture toughness? How do you calculate it?
K sub c
Y=1
When is Kc (fracture toughness) independent of thickness?
When a specimen thickness is much greater than crack dimension
What is plane strain fracture toughness?
characterizes the ability of a material to resist the propagation of a crack or fracture under conditions of plane strain. Plane strain conditions occur when the stress state is such that the deformation in one direction is constrained, meaning that the material cannot expand in that direction. This often happens in thick structures where the deformation is constrained in one dimension
Thickness is greater than the crack dimension so Kc is independent of thickness
K sub (ic)
Is plane strain fracture toughness high or low for ductile materials? brittle?
ductile: high
brittle: low
What is the formula for crack growth condition?
What do the plots of
1) critical stress for crack propagation (σc) v. crack length
2) inverse
look like?
What will the parameter Y be consistent with?
The same component and alloy
What test do you use to test ductile materials for brittle failure? How do you compute the impact energy?
Impact Test
Compute the impact energy from the difference between initial height h and final height h’
- charpy and izod
What are three test conditions promoting brittle fracture in impact tests?
1) high strain rate
2) deformation at low temperatures
3) presence of a notch
What is the influence of T on impact energy?
there are three kinds of behavior observed for the different crystal structures/strength materials
What kind of metals have a ductile-to-brittle transition temperature? At what temperature should they be used?
BCC metals
Only used at temperatures where ductile
What is fatigue?
Failure under lengthy period of repeated stress or strain cycling
What are the key parameters to fatigue?
S (stress amplitude), σm (stress at crack tip), and cycling frequency
What are the two key points about fatigue?
1) It can cause part failure even if they applied stress < critical stress
2) It’s responsible for 90% of mechanical engineering failures
How is fatigue data plotted?
Stress amplitude (S) vs. log of number N of cycles to failure
What are the two types of fatigue behavior observed?
1) Fatigue limit: Sfat:
2) no fatigue limit if S<Sfat
- some materials there is no fatigue limit
What is the fatigue life Nf?
Toal number of stress cycles to cause fatigue failure at specified stress amplitude
What are the three general techniques to improve fatigue life?
1) Reducing magnitude of mean stress: think of the SN graph
2) surface treatments: shot peening and carburizing
3) design changes: removing stress concentrations
How does decreasing mean stress affect fatigue life? What does the graph look like?
It increases fatigue life
How do compressive surface stresses affect surface hardness?
Compressive surface stresses increase surface hardness by suppressing surface cracks from growing
What are the two surface treatments for imposing compressive surface stresses & improving fatigue life?
1) Shot peening: surface compressive stress due to plastic deformation of outer surface layer
2) Carburizing: carbon atoms diffuse into outer surface layers
What is a design change to improve fatigue life?
replace sharp corners with rounded corners to reduce stress concentration
What is creep?
ΔE/Δt
A measure of deformation (strain) vs. time at constant stress
What temperature does creep occur at in most metals?
elevated temperatures
T> 0.4(Tm)
What are the three phases of creep? What are on the x and y axis?
1) primary creep: slope (creep rate) decreases with time
2) secondary creep: steady-state (constant slope)
3) tertiary creep: slope (creep rate) increases with time, so an acceleration of rate
How does creep behave with increasing temperature and stress?
The steady-state creep rate increases with increasing T and stress
The slope of the curve increases and gets shorter
Rupture lifetime (tr) decreases
What is steady-state creep rate?
The graph is linear because strain hardening is balanced by recovery. The slope has a dependence on T and stress (increases)
What does a steady-state creep graph look like?
How do material scientists predict creep rupture lifetime?
It’s impractical to wait years, so they increase the temperature and Larson-Miller parameter (m). Then solve it using the graph (psi v. m) and relevant equation.
What is fatigue failure? What are two important things to remember?
Stress fluctuations with time
1) occurs at applied stress < TS
2) Important parameters: fatigue limit, fatigue strength/lifetime
what is tr?
Creep Rupture lifetime
What are the two categories of fatigue?
Stress-controlled or strain-controlled
What are the most heavily used structural materials?
Copper alloys
What are three reasons for fracture?
static stress, fatigue, and creep
