Mechanical Properties of Materials Flashcards
what is resilience?
the capacity to absorb and recover elastic energy
what are the two main standardised test methods for testing the mechanical properties of materials?
- British/European standards (ISO)
- American society for testing and materials standards (ASTM)
what are the main mechanical properties of a material (the standardised language)?
- strength
- hardness
- ductility
- stiffness
what is the most simple mechanical test?
applying a load to a sample which will make it extend
what is normal stress defined as (in terms of testing materials)?
- using a uniform static load applied over a certain area
- stress = force / area
- units: N/m^2
what is strain defined as (in terms of testing materials)?
- strain = extension / original length
- dimensionless
what is the difference between tensile/longitudinal and lateral strain?
tensile/longitudinal/axial strain = change in the length
lateral strain = change in the width
what does elastic deformation mean?
- the deformation is temporary
- the sample will return to its original shape and size when the force has been removed
- obeying Hooke’s law
what is another name for Young’s modulus?
elastic modulus, E
what is the equation that defines Hooke’s law?
E = stress / strain
what does the gradient of a stress-strain graph tell you?
E, elastic modulus (which relates to the stiffness of the material) - a higher gradient = more stiff material
what is the area under the curve of a stress-strain graph?
the toughness of the material
how does bond strength of the material relate to the stress-strain graph of the material?
the slope (gradient is proportional to stiffness) depends on the bond strength of the material
what is poisson’s ratio?
- symbol: v
- ration of lateral to axial strains
v = - Ey / Ex - dimensionless
what is poisson’s ratio for most metals?
0.33
what is poisson’s ratio for most ceramics?
0.25
what is poisson’s ratio for most polymers?
0.40
what is poisson’s ratio for cork?
0
what is an auxetic material?
- materials that have a negative Poisson’s ratio when stretched
- as we stretch the material, it gets wider
what are auxetic materials used in?
- they are developed in the UK (Exeter)
- applications range from explosion curtains to comfortable mattresses
what materials tend to have the biggest elastic modulus?
metals, ceramics, composites
what material tends to have a low elastic modulus?
polymers
what is E, elastic modulus a measure of?
measure of stiffness and bond strength
what happens when we exceed the limits of elasticity when deforming a material?
plastic deformation will occur (permanent, non-recoverable deformation)
what is plastic deformation?
- when Hooke’s law is not being obeyed
- stress is not proportional to strain
- bonds are no longer being stretched, bonds are being broken and bonds are being reformed with new neighbours
- dislocations play a key role here
- metals will undergo elastic deformation first
how far will metals undergo elastic deformation in terms of strain until they start plastically deforming?
elastic deformation until strain of 0.5%
what does UTS stand for?
ultimate tensile stress/strength
generally what are the 3 main regions of a stress-strain curve?
region 1 - straight line, undergoing elastic deformation, obeying Hooke’s law
region 2 - uniform plastic deformation, strain is uniform across material
region 3 - non-uniform plastic deformation, deformation is limited to “neck” region
FAILURE/FRACTURE
where can the yield point be found?
where the strain deviates from being proportional to the stress (the proportional limit) - where the line is not straight anymore
what is proof stress?
the convention we use so we can identify a point at which noticeable plastic deformation has occurred because some materials do not exhibit a distinct transition from the elastic to plastic region - a straight line is drawn parallel to the stress strain curve with 0.2% strain offset
what is the yield strength σy?
the point at which stress is no longer proportional to strain, after this permanent deformation occurs - it is a measure of resistance to plastic deformation
what is the maximum tensile strength or UTS?
it is the maximum stress that a structure can sustain before the material fails or fractures…
for metals: occurs when noticeable necking occurs
for polymers: occurs when polymer backbone chains are aligned and about to break
what is the definition of ductility?
degree of plastic deformation or plastic strain before failure
what type of structure (unit cell) are usually ductile materials?
gold, silver, copper which are FACE CENTRED CUBIC
what is the definition of toughness?
- toughness is the energy to break a unit volume of material
- the level of resistance to crack propagation
- a measure of the ability of a material to absorb energy up to fracture (from a sudden impact)
what is the area under a stress-strain curve approximately equal to?
the toughness of the material
what type of energy is involved in a brittle fracture?
elastic energy
generally what kind of materials are tough and not tough?
generally:
- ductile materials are tough e.g Al
- brittle materials have low toughness e.g Al2O3 ceramic
what is resilience (Ur), or modulus of resilience?
the ability of a material to store energy
in what region of a stress-strain graph is energy best stored?
in the elastic (straight) region, the area under the elastic region is equal to the modulus of resilience
what is the equation for resilience?
approx = 0.5 σy x εy
= (σy)^2 / 2E (USE THIS)
units: Pa or Nm^-2 or Jm^-3
what do resilient materials have in terms of yield strengths and elastic modulus? what are resilient materials used for?
resilient materials have…
- high yield strengths
- low elastic modulus
(they are used in spring materials and diving springboards)
what is hardness?
hardness is the resistance of a material to localised plastic deformation (dents or scratches)
what was Mohs Scale of Hardness?
- he devised a scale of hardness by arranging 10 minerals so that one mineral could only scratch those below it
- it goes from 1 to 10 in intervals of 1
- starts at 1 with talc
- ends with 10 with diamond
what is hardness related to?
σy and E
how do we measure hardness?
qualitatively - we use a diamond indenter to indent the surface and the depth of the indent is a measure of the hardness
what does a large hardness mean?
- high resistance to plastic deformation or cracking in compression
- better wear properties
what can welding do to a material?
affects the hardness
what is the equation relating to the factor of safety we use when working with materials?
σworking = σy / N
where N is the factor of safety (usually between 1.2 and 4)
to minimise deformation what type of material should you pick?
a material with a large elastic modulus (E)
when does plastic deformation occur?
when σy has been reached
what is the equation relating to ductility?
%EL = (Lf - Lo / Lo) x 100
what type of energy is involved in a ductile fracture?
elastic & plastic energy
