Week 4 Flashcards
What happens at a flaw: stress is concentrated at _____ _____
Crack tips
What are these arrows points to
Region of high local stress
Uniform applied stress= number of ______ per unit _____
Lines
Width
What variable equation answers the question of “how much is the stress multiplied at a crack tip”
(Variable)
Kt
Stress concentration:
Sharp crack
Formula for sharp crack
(Small Row sub t)
What’s the difference in critical flaw size in ceramics vs. metals
Ceramics: microns to tens of microns
Metals: millimetres to cm
Why do metals perform better in tension than ceramics?
Metals have a smaller critical flaw size (mm to cm)
(While ceramics are microns to tens of microns)
What are the 3 loading modes for cracks
(and their corresponding numbers)
Opening (I)
Shearing (II)
Tearing (III)
Cracks: what loading mode is this?
Opening
Cracks: what loading mode is this
Shearing
Cracks: what loading mode is this?
Tearing
What variable answers the question of
“plane strain fracture toughness”
Kc
Critical stress intensity factor: what does picture mean
K in mode 1 fracture
(Where K= critical stress intensity factor)
When does a crack fail instantaneously
Plane strain fracture toughness: refers to ________ samples
Thick
What does DBBT stand for
Ductile to brittle transition temperature
BCC: metals get ______ at low temps
Brittle
What graph relates to temp dependence
(Axis titles)
Y: impact energy
X: temperature
Temperature dependence graph: describes the 3 lines and what they represent
BBC: low strength _____
Steels
FCP and HCP: low strength ______
Metals
More carbon = stronger _____
Steel
Describe the composition dependence graph for carbon steels
(Described general trend)
(And what do the decimals mean)
More carbon = stronger steel
Decimal= carbon content in weight percent (wt%)
Define fracture toughness
Resistance to crack propagation
Define impact toughness
Abilities to deform without breaking under rapid loading
What type of toughness does this picture show
Impact toughness
What the of toughness does this picture show
Fracture toughness
What are the 3 types of toughnesses
Tensile
Fracture
Impact
Define cyclic loading
Loading at low stresses (below yield strength)
Can cyclic loading still cause failure
Yes
What type of testing does this show
Fatigue testing
Where are these on a stress vs. time graph
Do cracks grow in compression
Compression
What are the axis for an S-N curve
Y: stress amplitude
X: cycles to failure, N
Describe the 3 mean stress lines
(And greatest to least)
Do BCC metals have a fatigue limit
YES
Where is the fatigue limit for BCC’s on an S-N curve
Extend horizontal line to y axis
Do FCC’s have a fatigue limit
(And why)
NO
(No horizontal line to extrapolate)
What does the S-N graph for FCC’s look like
What fatigue fracture is this
Beach marks
What fatigue fracture is this
Striations
Fatigue failure are ALWAYS from what type of stress?
Cyclic stresses
What type of failure is this
Fatigue
Define crack initiation
A crack forms, or already exists in the material
Fatigue crack growth: define propagation
The crack advances a small amount each loading cycle
Define fast fracture
Part can no longer handle the applied load
Name the three parts of fatigue crack growth here
Crack initiation (top)
Crack propagation (middle)
Reputed to failure (squiggly lines)
List the 3 stages of fatigue crack growth in order
1) crack initiation
2) propagation
3) fast fracture
Fatigue fracture surfaces : define origin
Origin: at a stress concentration
Fatigue fracture: define beach marks (how wide are they usually)
A discrete period of crack growth
(A few mm wide)
Striations: 1 Striation = _______ loading _____
One loading cycle
What equipment do you need to be able to see striations
An electron microscopy
(Or else way to small)
Striations are NOT tree ______, they shouldn’t not be _______
Rings
Counted
Say if they have a fatigue limit or not
BCC metals
FCC metals
BCC metals: has a fatigue limit
FCC metals: no fatigue limit
What question does the “Paris Law” answer
How quickly will a crack grow
What question does the “critical stress intensity factor” answer?
How SENSITIVE is a material to sudden, uncontrollable crack growth
List the four ways to avoid fatigue
Lower the mean stress
Surface treatments
Reduce exposure to corrosive environments
Reduce thermal gradients, especially for insulating materials
Avoiding fatigue: explain lowering the mean stress
Avoiding fatigue: explain surface treatments
Place the surface into compression (shot owen or carburize)
The stress at a crack tip APPROACHES ________
Infinite
Name the 3 things in this picture
Describe the local stress vs. r graph for this picture
Creep happens above……
(And simple formula for this)
Creep happens below ………
The yield strength of the material
Creep: failure is often at _____ ______
Low strains
List the 3 requirements for creep
Time, temp, stress
When does lead experience creep
Room temperature
Is creep testing the same as tension testing?
NO
Describe tension testing
Describe creep testing
List the 5 stages of creep
Elastic deformation
Primary creep
Secondary creep
Tertiary creep
Final failure
Stages of creep: describe elastic deformation
Bonds stretch
Stages of creep: describe primary creep
Slope decreases, work hardening happens
Stages of creep: describe secondary creep
(What is constant)
(And what is it also called)
Work hardening cancelled out by recovery
(Constant strain rate)
(Also called steady state creep)
Stages of creep: describe tertiary creep
Increases fast, sample fails
What variable represent strain RATE
Stages of creep: describe final failure
Voids are a vacuum, there is nothing in them
Creep: what type of final failure is this
Intergranular
Creep: what type of final failure is this
Transgranular
Define critical flaw
Max size of a flaw that can exist under loading before immediate failure occurs
Define work hardening
Increase in hardness of a metal
