2: Mechanical Properties
No description yet. This curated deck is ready for daily study.
Card preview (24)
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