2: Mechanical Properties

Question 1

define mechanical properties

Answer 1

how does a material react to external forces

Question 2

what are the types of reactions to external forces

Answer 2

material will ALWAYS deform.

• elastic
• plastic
• fracture

Question 3

describe elastic deformation

Answer 3

material deforms under force but regains its original form when the force is removed

atomic bonds stretch under the force but no bond break

Question 4

describe plastic deformation

Answer 4

deformation that is permanent

there must be some breaking of the bonds, but generally they reform after force removed

Question 5

describe fracture

Answer 5

material breaks in pieces

exceed max force the bonds can withstand so they break

Question 6

name types of forces on material

Answer 6

tension (stretch)
compression (contract)
shear
torsional

Question 7

a shear force is _____ to a plane

Answer 7

parallel

Question 8

a normal/linear force is _____ to a plane

Answer 8

perpendicular

Question 9

how are mechanical properties measured

Answer 9

uniaxial tensile testing

pulls the material and records deformation (elongation) and force used

Question 10

is length or area more important in tensile test sample

Answer 10

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

Question 11

def engineering stress

Answer 11

sigma = F/Ao

sigma = eng stress
A0 = initial area

proportional to the amount of force each atom sees

Question 12

tensile stress is pos or neg ? compressive?

Answer 12

tensile = pos
compressive = neg

Question 13

describe engineering strain

Answer 13

how we describe deformation

e = deltaL/lo
percentage elongation of a material

Question 14

describe shear stress and shear strain

Answer 14

tau = F/Ao
y = tan(theta) = a/b

note - shear happens at constant volume

Question 15

describe hooke’s law. when is it used ?

Answer 15

engineering stress = (engineering strain)(E)

E = modulus of elasticity, young’s modulus

used during the elastic region (elastic deformation)

Question 16

describe elastic modulus. what does it tell us ?

Answer 16

slope of elastic region deformation
characteristic of material

higher E = need more force for same type of deformation

Question 16

describe shear modulus

Answer 16

tau = Gy

Question 17

def yield strength

Answer 17

highest stress a material can sustain before undergoing plastic deformation

Question 18

how is yield strength determined (hint - graph)

Answer 18

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%)

Question 19

def tensile strength. found how on graph?

Answer 19

highest stress a material can sustain before breaking.

highest point on the stress-strain curve

Question 20

describe strain hardening

Answer 20

(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.

Question 21

describe poissons ratio

Answer 21

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)

Question 22

describe relationship between elastic and shear modulus for isotropic materials

Answer 22

E = 2G(1+v)

Question 23

what are typical values for poisons ratio

Answer 23

between -1 and 0.5, but generally between 0-0.5 for most materials and 0.25-0.35 for metals

Question 24

what does a poisson ratio of 0.5 mean

Answer 24

no volume change with strain
ex. rubber band

Question 25

what does a poisson ratio of 0 mean

Answer 25

no lateral strain with axial strain
ex. cork - when compressed in one direction it won’t expand in the other

Question 26

what does a poisson ration below zero mean

Answer 26

positive lateral strain with positive axial strain
ex. foam - expands in both directions at the same time force is applied

Question 27

ductility

Answer 27

measure of the degree of plastic deformation that has been sustained at fracture

fracture strain <5% = brittle

Question 28

more brittle vs more ductile

Answer 28

more brittle can withstand more force before breaking, but ductile can withstand more energy

Question 29

how to calculate ductility

Answer 29

%EL= lf-lo/lo x 100

%RA = Ao-Af/Ao x 100

Question 30

resilience

Answer 30

capacity of a material to absorb energy when deformed elastically

Question 31

modulus of resistance

Answer 31

strain energy per unit volume required to stress a material to the point of yielding

see notes for eon

Question 32

toughness

Answer 32

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.

Question 33

engineering stress/strain vs true stress/strain (dont name formulas)

Answer 33

eng stress and strain are normalized to initial sample dimensions.
as the sample deforms, true stress and strain change

Question 34

formula true stress, strain

Answer 34

stress = F/A
(A is not og area)

strain = ln(li/lo)

Question 35

review real vs eng stress, strain

Question 36

describe hardness test

Answer 36

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.

Question 37

how to obtain shear modulus

Answer 37

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)

Question 38

describe the safety factor, why is it needed ?

Answer 38

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

Question 39

safety factor depends on what ? what numbers is it ?

Answer 39

depends on application

N : 1.2 - 5

Question 40

describe stress-strain curve for ceramics, why

Answer 40

linear only

ceramics are brittle, they have elastic deformation until the point of fracture

Question 41

describe stress-strain curve for polymers, why

Answer 41

variety:
- linear, almost straight up
- linear, then horizontal
- mostly horizontal with some curves

polymers can be brittle, plastic, elastic, depending on their structure

Question 42

review how to choose material