Mechanics Fracture under stress (class) Flashcards by Niamh Fitzpatrick

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

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

Describe the characteristic appearance of ductile fracture

necking
flat granulated central portion
small shear lip
–> which gives cup and cone surface

How well did you know this?

Not at all

Perfectly

Describe the characteristic appearance of brittle fracture

flat surface, perpendicular to the applied load
granular appearance
chevron pattern

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

What is the term for stress when a material fractures?

rupture strength

How well did you know this?

Not at all

Perfectly

give an example of

a) ductile fracture
b) brittle fracture

a) material, plastic shopping bag handles

b) snapping a frozen chip, glass, ceramics, concrete

How well did you know this?

Not at all

Perfectly

What is a ductile fracture ?

a fracture that occurs after considerable plastic deformation characterised by necking

How well did you know this?

Not at all

Perfectly

What is a brittle fracture ?

A fracture that occurs abruptly without any plastic deformation ie no necking

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

What is a stress riser?

a change in shape (eg notch or hole) that results in increased stress concentration

How well did you know this?

Not at all

Perfectly

where are stress concentrations highest?

at the tips of cracks or notches

How well did you know this?

Not at all

Perfectly

What is an impact load?

a sudden intense blow

How well did you know this?

Not at all

Perfectly

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 well did you know this?

Not at all

Perfectly

How do stress concentrations weaken structures?

any sudden change in shape increases stress concentration making fracture more likely

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

What is a fatigue fracture?

fracture caused by repeated loading (load is smaller)

How well did you know this?

Not at all

Perfectly

how can fracture propagation be stopped?

by including smooth holes in the structure

How well did you know this?

Not at all

Perfectly

Describe the characteristic appearance of a fatigue fracture

2 regions

one smooth with repeated conchoidal markings
granular/fibrous region

How well did you know this?

Not at all

Perfectly

What do conchoidal markings indicate?

the various positions at which the crack has stopped

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

Why do bones get fatigue fractures?

frequency of repetition of load is too fast for remodelling eg continuous period of strenuous physical activity

How well did you know this?

Not at all

Perfectly

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