Unit 3 - Fracture under stress Flashcards
What factors determine if a material will fail?
Magnitude of load applied
Speed at which loads is applied
Number of times load is applied
(wear & corrosion can also lead to failure)
What 3 types of loading may lead to failure?
Steady loading
Impact loading
Fatigue loading
What is tensile testing?
Tensile load applied to a material and increased until material fractures
Throughout, the length is measured and therefore the strain can be calculated
A stress-strain curve can be produced
What is the point called when a material fractures?
Rupture strength
Why may ultimate strength (max stress calculated) be greater than rupture strength?
Necking (in ductile materials) - material appears to carry less stress when calculated using original CSA rather than the true reduced CSA
What are the 2 types of fracture that may occur to a material when it is subjected to a steady load?
Ductile (when it occurs with necking)
Brittle (when it occurs without necking)
Describe how a ductile fracture comes about
Occurs after considerable plastic deformation, characterised by necking
Begins with formation of microscopic voids at centre of bar (stress causes separation of metal at grain boundaries/interfaces between metal grains & inclusions)
Voids grow & coalesce. Eventually actual metal to metal contact area within bar is reduced so that it is unable to support applied load –> fracture
Deformation by shearing also contributes
Describe how a brittle fracture occurs?
Suddenly without any plastic deformation/necking
Separate crack fronts fan out from origin of crack
What is the difference in appearance between ductile & brittle fractures?
Ductile - necking, flat granulated central portion, small shear lip (gives cup-and-cone surface)
Brittle - flat fracture perpendicular to load, granular appearance with chevron pattern
Under what conditions may a ductile material undergo a brittle fracture?
If material contains notch/crack
Low temperatures
If exposed to fatigue loading
What may sudden changes in shape of an object result in?
Stress concentrations (where stress is higher than average stress of material)
Notches/holes (that result in stress concentrations) are called stress risers
What are stress trajectories?
Diagrams used to show locations of stress concentrations
Points with tightly packed stress trajectories = areas of high stress
Why are cracks & notches prone to fracture propagation?
Sharp changes in shape have high stress concentrations which means fracture is likely to develop from this point & propagate
How may stress concentrations be dissipated & fracture propagation halted?
Build features such as smooth holes into a design in order to blunt a propagating crack (stress no longer concentrated at sharp point)
All materials will contain microscopic defects (scratches, pores, cracks)
What is notch sensitivity?
The phenomenon of concentrated stress at tip of a notch
Can be calculated by comparing energy absorbed by notched & unnotched designs
How may the resistance of a structure to an impact load be assessed?
Charpy impact test:
- pendulum released from known height & allowed to break specimen at bottom of swing
- height reached by pendulum at and of swing is lower
- impact energy = mg(H1-H2)
Impact energy = energy absorbed by specimen
How does temperature effect a materials ability to resist a load?
As temperature increases:
- ductility increases (material is able to absorb more energy)
- strength reduces
- strain increases
Describe the appearance of a fatigue fracture
Smooth region:
- concentric clam shell markings (indicate various positions at which crack has stopped as it intermittently propagated through)
- allows origin of fracture to be located
Granular/fibrous region:
- granular: produced by rapid brittle fracture when remaining cross-section can no longer support load (if whole surface, indicates few load cycles were needed)
- fibrous: if material undergoes ductile fracture
When are fatigue fractures found in bone?
When load is repeatedly applied in short period of time (faster than bone can remodel)
Why is muscle fatigue important in fatigue fractures?
Tired muscles less able to neutralise tensile loads & act as shock absorbers
How can corrosion lead to fracture?
Imperfections on metal surfaces are prone to attack by corrosion
Imperfection develops into a crevice as metal ions migrate away from imperfection
Crevice gives rise to stress concentrations –> fracture
What is a passivation layer?
Inert film (usually an oxide) on surface of metals
What is the difference between ferrous & non-ferrous metals?
Ferrous - contains iron
All steels are alloys of what?
Iron & carbon
Steel = most common ferrous alloy
What makes stainless steel corrosion resistant?
Chromium - gives protective oxide surface
How are steels classed?
Carbon content
Stainless steel 316L (low carbon) used in orthopaedics
What is the difference in material property of low carbon & high carbon steels?
High - high strength & hardness but brittle
Low - low strength & hardness but ductile
(Surface hardening of low carbon steel improves strength and hardness of surface but retains ductility of core)
What are the advantages of titanium in ortho?
Low density (weight)
High strength-weight ratio
High corrosion resistance
What are the disadvantages of titanium in ortho?
High cost
High fabrication cost
Low Young’s modulus (can be compensated for by using thicker sections)
What other non-ferrous metal (not titanium) may be used in ortho implants?
Cobalt
Name 5 properties of polymers & suggest why they are popular
Lightweight Corrosion resistant Low tensile strength Electrical insulator Low melting point
Easy to use in manufacturing
How are polymers made?
Polymerisation of monomers
Lots of smaller molecules put into a long chain
Describe the stress-strain behaviour of polymers
Non-linear
Time dependent
Elastic & plastic
What are the 2 subtypes of polymers
Plastics
Elastomers
What are the 2 categories of plastics?
Describe them
Thermoplastic - can be reheated & reformed (polyethylene, polypropylene)
Thermosets - cannot be reformed. They harden at high temps (during formation at high temps a by product is released)
What is the most important property of elastomers?
Can deform by enormous amounts without permanently changing shape
Better known as rubbers
Describe ceramics
Where may they be used in hip prostheses?
Crystalline structures
Very hard but brittle
Used for head of hips - wear resistant & low friction (would not be used as stem as they would easily snap)
What are the 3 categories of composite materials?
Describe & give example
Particulate:
- particles of hard brittle material dispersed in softer ductile one
- concrete
Fibre:
- fibres of strong stiff brittle material withing soft ductile one
- fibreglass
Laminar:
- thick layers laminated (layers)/coating of material over other
- plywood
