Mechanics Fracture under stress (class) Flashcards
How can a material specimen be tested under a tensile load?
tensile load is applied to specimen using tensile testing equipment
the load is gradually increased until the specimen fractures
the length of the specimen is measured throughout so that strain can be calculated
the stress is calculated using the applied load and the cross-sectional area
this is plotted on a stress-strain curve
Explain the mechanism of ductile fracture
tensile load causes small holes to form at the centre of the bar, then these grow as the local stress increases, eventually the contact area is too small to support the applied load and fracture occurs
+ shearing deformation contributes
Describe the characteristic appearance of ductile fracture
necking
flat granulated central portion
small shear lip
–> which gives cup and cone surface
Describe the characteristic appearance of brittle fracture
flat surface, perpendicular to the applied load
granular appearance
chevron pattern
Define the terms
a) notch sensitivity
b) fracture propagation
a) concentrated stress at the tip of a crack or notch
b) when a fracture develops at a crack or notch and then spreads
What are the 3 factors that are important in determining whether a material will fail?
what else
- the magnitude of the applied load
- the rate of speed at which the load is applied
- the number of times that the load is applied
wear + corrosion
What is the term for stress when a material fractures?
rupture strength
give an example of
a) ductile fracture
b) brittle fracture
a) material, plastic shopping bag handles
b) snapping a frozen chip, glass, ceramics, concrete
What is a ductile fracture ?
a fracture that occurs after considerable plastic deformation characterised by necking
What is a brittle fracture ?
A fracture that occurs abruptly without any plastic deformation ie no necking
When does a normally ductile material respond like a brittle material?
if it has been exposed to fatigue loading
if it has a notch/crack
decreasing temperature and increasing strain
What is a stress riser?
a change in shape (eg notch or hole) that results in increased stress concentration
where are stress concentrations highest?
at the tips of cracks or notches
What is an impact load?
a sudden intense blow
Describe the charpay impact test
a heavy pendulum is released from a known height and as the pendulum reaches the bottom of its trajectory it strikes and breaks the test specimen until it reaches the peak of its swing
How do stress concentrations weaken structures?
any sudden change in shape increases stress concentration making fracture more likely
How does temperature affect the ability of a material to absorb energy? why?
increase temperature = increased energy absorbed
because material changes from brittle to ductile
What is a fatigue fracture?
fracture caused by repeated loading (load is smaller)
how can fracture propagation be stopped?
by including smooth holes in the structure
Describe the characteristic appearance of a fatigue fracture
2 regions
- one smooth with repeated conchoidal markings
- granular/fibrous region
What do conchoidal markings indicate?
the various positions at which the crack has stopped
What causes
a) a granular appearance
b) a fibrous appearance
in a fatigue fracture?
a) rapid brittle fracture that occurs once the remaining cross-section can no longer support the load
b) material undergoes ductile fracture
Why is the frequency of repetition of load application important in bone?
bone has ability to remodel to acquired load so if they are spaced out the vine will repair any damage
Why do bones get fatigue fractures?
frequency of repetition of load is too fast for remodelling eg continuous period of strenuous physical activity
How does muscle fatigue affect fatigue fractures?
muscle wont contract to neutralise the stresses
general loss of shock-absorbing capacity
movements cause abnormal loading
Explain wet corrosion
anode (oxidation) and cathode (reduction) are joined by an aqueous electrolyte
the metal gradually dissolves away into the solution weakening the structure
When is corrosion particularly serious?
when two metals have been joined together
How do imperfections on metal surfaces affect corrosion?
imperfections prone to attack and develop into crevices as metal ions migrate away
difference in oxygen concentration accelerates the process
crevices are prone to stress concentration and fracture
What is the endurance limit?
below the limit, the material will never fail regardless of how many fatigue cycles it goes through
What is the fatigue life?
the no of cycles of fatigue loading that a material can go through before failing
How does corrosion affect the fatigue behaviour of metals?
corrosion reduces fatigue resistance of metals which results in lower fatigue life and no endurance limit
How do metals used in implants resist corrosion?
they have an inert layer (usually an oxide) called a passivation layer that forms a film covering the surface of the metal
Identify a corrosion weak spot in an implant
damage to the inert layer by chemical attack or mechanical wear
allows a crevice to form between the layer and the metal which allows chemical attack to corrode the metal
What is the difference between a ferrous and non-ferrous metal?
ferrous metals are composed of iron while non-ferrous metals contain no iron
What is a metal alloy?
elements added to a basic metal to improve the mechanical and corrosive properties
What is steel an alloy of? how is it classed?
iron and carbon so it is a ferrous alloy
classed depending on carbon content
What is stainless steel?
steel alloys that contain between 12-18% chromium which gives it resistance to corrosion
What non-ferrous alloy is commonly used in orthopaedics? why?
titanium alloys
- less dense than steel
- higher strength-weight than aluminium
- excellent corrosion resistance
What are the disadvantages of using titanium based alloys?
high material cost
high fabrication cost
low young’s modulus
What are polymers? give 2 examples
lightweight, corrosion-resistant, electrical insulations with low tensile strengths and melting points
eg plastics and elastomers
Why are polymers popular?
easy to manufacture
low density
Describe the stress-strain behaviour of polymers
non-linear
time dependent
elastic and plastic
What are the 2 main types of plastic?
thermoplastics: can be formed at high temp, cooled and then reformed
thermoset: harden at high temperatures so cannot be reformed
What is the main quality of elastomers? give an example of one
can deform by enormous amounts without permanently changing shape
eg rubber band
Describe the structure of ceramics and give an example of one
hard, brittle material’s with a high melting point, low electrical and thermal conductivity
eg diamond
What would the young’s modulus of a ceramic be?
very high
How can production of ceramics affect the toughness?
reduce grain size reduces pore size as pores are defects which lower the tensile strength and toughness
What are the 3 types of composite materials?
particulate
fibre
laminar
Describe particulate composite materials and give an example
particles of high brittle material dispersed within a softer more ductile material
eg concrete
Describe fibre composite materials and give an example
fibres of a strong, stiff, brittle material within a softer, more ductile material
eg. wood, fibre-glass
Describe laminar composite materials and give an example
a very thin coating may cover a material to improve corrosion resistance
thicker layers may be laminated to improve strength
eg plywood
Compare the properties of bone cement and bone
overall bone is 3-5x stronger, stiffer as higher YM, more brittle material that can sustain twice as many fatigue cycles
In ductile materials is the rupture or ultimate strength bigger?
ultimate strength
due to necking meaning stress is actually less at fracture
What is fatigue loading?
repeated loading and unloading
How do we calculate the energy absorbed by the specimen in the Charpay impact test?
PE = W (H0-HF)
Is an anode or cathode negative?
cathode
Is the young’s modulus higher for titanium or steel?
steel
Do metals obey Hooke’s law?
yes
What is the main shortcoming of ceramics?
they are not tough
Are biological materials stronger under a quick or gradually applied load?
quick makes it stronger and stiffer but more energy stored in system
Why do we carry out the Charpay test?
to determine the resistance of structure to impact loading
