2: Mechanical Properties Flashcards
define mechanical properties
how does a material react to external forces
what are the types of reactions to external forces
material will ALWAYS deform.
- elastic
- plastic
- fracture
describe elastic deformation
material deforms under force but regains its original form when the force is removed
atomic bonds stretch under the force but no bond break
describe plastic deformation
deformation that is permanent
there must be some breaking of the bonds, but generally they reform after force removed
describe fracture
material breaks in pieces
exceed max force the bonds can withstand so they break
name types of forces on material
tension (stretch)
compression (contract)
shear
torsional
a shear force is _____ to a plane
parallel
a normal/linear force is _____ to a plane
perpendicular
how are mechanical properties measured
uniaxial tensile testing
pulls the material and records deformation (elongation) and force used
is length or area more important in tensile test sample
cross-sectional area.
length doesnt matter bc if you have a chain of atoms , the amount of force that pulls on each bond will be the same.
the area is more important bc it is how many atoms the force is distributed between
def engineering stress
sigma = F/Ao
sigma = eng stress
A0 = initial area
proportional to the amount of force each atom sees
tensile stress is pos or neg ? compressive?
tensile = pos
compressive = neg
describe engineering strain
how we describe deformation
e = deltaL/lo
percentage elongation of a material
describe shear stress and shear strain
tau = F/Ao
y = tan(theta) = a/b
note - shear happens at constant volume
describe hooke’s law. when is it used ?
engineering stress = (engineering strain)(E)
E = modulus of elasticity, young’s modulus
used during the elastic region (elastic deformation)
describe elastic modulus. what does it tell us ?
slope of elastic region deformation
characteristic of material
higher E = need more force for same type of deformation
describe shear modulus
tau = Gy
def yield strength
highest stress a material can sustain before undergoing plastic deformation
how is yield strength determined (hint - graph)
either a minimum point after the elastic region
OR
the intersection of the stress-strain curve and a line parallel to the elastic curve offset by a certain number (generally 0.2%)
def tensile strength. found how on graph?
highest stress a material can sustain before breaking.
highest point on the stress-strain curve
describe strain hardening
(elastic recovery after deformation)
if you add stress until you reach the plastic region, then stop and remove stress, some of the deformation will be plastic (permanent) and some will be elastic.
now that the bonds were broken at the weak points the first time, the material is stronger.
now more stress is needed to get back to the same point of strain.
so, the new yield strength will be higher.
describe poissons ratio
applying a force stretches the bonds in one direction, to compensate, bonds in the other direction get smaller
v = -ex/ez = -ey/ez
e,lateral = (-v)(e,tensile)
describe relationship between elastic and shear modulus for isotropic materials
E = 2G(1+v)
what are typical values for poisons ratio
between -1 and 0.5, but generally between 0-0.5 for most materials and 0.25-0.35 for metals
what does a poisson ratio of 0.5 mean
no volume change with strain
ex. rubber band
what does a poisson ratio of 0 mean
no lateral strain with axial strain
ex. cork - when compressed in one direction it won’t expand in the other
what does a poisson ration below zero mean
positive lateral strain with positive axial strain
ex. foam - expands in both directions at the same time force is applied
ductility
measure of the degree of plastic deformation that has been sustained at fracture
fracture strain <5% = brittle
more brittle vs more ductile
more brittle can withstand more force before breaking, but ductile can withstand more energy
how to calculate ductility
%EL= lf-lo/lo x 100
%RA = Ao-Af/Ao x 100
resilience
capacity of a material to absorb energy when deformed elastically
modulus of resistance
strain energy per unit volume required to stress a material to the point of yielding
see notes for eon
toughness
capacity of a material to absorb energy up to the point of fracture. also the Intel over the curve but the function is non-linear.
engineering stress/strain vs true stress/strain (dont name formulas)
eng stress and strain are normalized to initial sample dimensions.
as the sample deforms, true stress and strain change
formula true stress, strain
stress = F/A
(A is not og area)
strain = ln(li/lo)
review real vs eng stress, strain
describe hardness test
pushing a hard object with a round or pointy shape in a material creating an indentation mark.
the initial stress is always high and plastic deformation occurs.
as point goes in, the stress decreases and equilibrium is reached.
the shallower the mark, the harder the material.
NON DESTRUCTIVE, simpler than tensile.
how to obtain shear modulus
using a torsion tester, a hollow cylindrical sample (a tube) can be used to obtain the shear modulus, G, from the applied torque, T, and the angle of twist
G= (T)(length)/2(pi)(r^3)(angle)(thickness)
describe the safety factor, why is it needed ?
there are uncertainties in measured mechanical properties and variabilities in materials produced, so it is necessary to apply a safety factor, N, when designing a piece.
thus we use the resulting working stress, (sigma,w) instead of the yield strength to design a piece:
(sigma,w) = (yield stress)/N
safety factor depends on what ? what numbers is it ?
depends on application
N : 1.2 - 5
describe stress-strain curve for ceramics, why
linear only
ceramics are brittle, they have elastic deformation until the point of fracture
describe stress-strain curve for polymers, why
variety:
- linear, almost straight up
- linear, then horizontal
- mostly horizontal with some curves
polymers can be brittle, plastic, elastic, depending on their structure
review how to choose material
