Chapter 7 - Fracture & Failure Flashcards
What is a fracture?
The separation of a body into pieces due to stress at temperatures below melting point
What are the 2 steps in fracture?
- Crack formation
- Crack propagation
What are the 2 fracture models?
Ductile or Brittle
What are the characteristics of a ductile fracture?
- occurs in most metals
- extensive plastic deformation ahead of crack
- Crack is ‘stable’
What are the characteristics of a brittle fracture?
- Occurs in ceramics, ice & cold materials
- Little plastic deformation
- Crack is unstable
What are the characteristics of a ductile matetrial?
- Again a lot of plastic deformation
- and a lot of energy absorption
What are the characteristics of a brittle material?
- Little plastic deformation
- Low energy absorption
Describe a very ductile materials fracture?
It looks like two spear tips pointed towards each other
Describe a moderately ductile materials fracture?
chubbier with teeth
Describe a brittle materials fracture?
straight cut through
What type of dislocation occurs in a ductile fracture?
Mediated dislocation
Describe the dislocation that occurs in a brittle fracture?
Limited dislocation mobility
Describe the process of a brittle fracture?
- No plastic noticeable plastic deformation
- Crack propagation is very fast
- Crack propagates perpendicular to the direction of applied stress
- Crack propagates by cleavage
What does cleavage mean?
The braking of atomic bonds along specific crystallographic planes
What are the 2 types of brittle fracture?
- Transgranular
- Intergranular
Describe a transgranular fracture?
- Fracture cracks pass through the grains
- Fracture surface is faceted
Describe a intergranular fracture?
- Fracture crack propagation is along grain boundaries which are weakened by impurities
What is a stress riser?
- Can be Macro or Microscopic
- these are flaws in th material caused by a reduction of the cross sectional area
What is the fracture strength of a brittle solid related to?
The cohesive forces between atoms
What is a lower fracture strength explained by?
The effect of stress concentration at microscopic flaws
what is the effect of stress concentration at microscopic flaws?
- applied stress is amplified at the tips of micro-cracks, voids, notches etc that are called stress raisers
- The magnitude of the amplification depends on the orientation of the micro-cracks
When temperature decreases what can happen to a material?
It can transition from ductile to brittle
What process can help raise the temperature that the ductile to brittle transition occurs at?
Alloying
What temperature do brittle fractures occur at?
Around 4°C
What applied stressed will cause fatigue?
- Axial (compression/tension)
- Flexural (bending)
- Torsional (twisting)
What are the fatigue failure steps?
- Crack initiation
- Gradual crack propagation
- Final catastrophic failure
What is fatigue?
When a material will fail at a considerably lower stress than the tensile or yield strength due to cyclic stresses
Name the 3 cyclic stresses?
- Reversed stress cycle
- Repeated stress cycle
- Random stress cycle
Describe the characteristics of the reversed stress cycle?
Periodic and symmetric about the zero stress
Describe the characteristics of the repeated stress cycle?
Periodic and asymmetrical around the zero stress
Describe the characteristics of the random stress cycle?
Random stress fluctuations
What is the stress ratio?
R = sigma(min)/sigma(max)
Tensile stresses are ……..
Compressive stresses are ……
Positive
Negative
How are fatigue properties tested?
In rotating bending tests in fatigue testing apparatus
What does a low cyclic fatigue mean?
High loads, plastic and elastic deformation
What does a high cyclic fatigue mean?
Low loads, elastic deformation
What is the fatigue limit?
A maximum stress amplitude below which the material never fails no matter the number of cycles
The fatigue limit only occurs in some materials, name two?
Iron (Fe) & Titanium (Ti) alloys
What is fatigue strength?
The stress at which fracture occurs after a specified number of cycles
What is fatigue life?
The number of cycles to fail at a specified stress level
What quality determines the crack initiation stage?
The quality of the surface of the material
What happens during the first stage of crack propagation?
- Slow propagation along crystal planes
- High resolved shear stress
- Involves just a few grains
- Flat fracture surface
What happens during the second stage of crack propagation?
- Faster propagation perpendicular to the applied stress
- Crack grows (blunting and sharpening process at tip)
- Rough fracture surface
What are the three stages of fatigue failure?
- Crack initiation in areas of stress concentration
- Incremental crack propagation
- Final rapid crack propagation
What are the factors that affect fatigue life?
- Magnitude of stress (Mean, Amplitude etc)
- Quality of the surface (scratches, sharp transition)
- Thermal fatigue
- Corrosion fatigue
What is corrosion fatigue?
- Chemical reactions induce pits which act as stress raisers
- Corrosion also enhances crack propagation
What is thermal fatigue?
- When thermal cycling causes expansion and contraction, hence thermal stress, if the component is restrained
What is creep?
Is a time dependent and permanent deformation when a material is subjected to high temperatures
E.G. Turbine blades
What are creep rates used for?
Evaluating materials for anything that involves high temperatures
What are the stages of creep?
1) instant deformation
2) Primary/transient creep
3) Secondary/steady-state creep
4) Tertiary
How does an increase in stress or temperature effect the creep?
- instant strain increases
- the steady state creep rate increases
- the time to rupture increases
Creep is generally minimized in materials with two factors?
- High melting point
- High elastic modulus
