2- Mechanical Behavior, Testing, Manufacturing

Front 1

forming

Back 1

applying external forces to a workpiece
ex: drawing wire for making nails

Front 2

tension test

Back 2

method for determining mechanical properties of materials

Front 3

mechanical properties

Back 3

SDTEmSh
-strength
-ductility
-toughness
-elastic modulus
-strain-hardening capability

Front 4

test specimen

Back 4

for tension test

Front 5

original gage length

Back 5

Lo (little "l" sub "o")
usually 50 mm

Front 6

cross-sectional area

Back 6

Ao (A sub o)
usually with diameter = 12.5 mm

Front 7

linear elastic behavior

Back 7

if load is removed, specimen returns to original length and shape
ex: rubber band

Front 8

engineering stress

Back 8

o~ = P / Ao
P = load

Front 9

nominal stress

Back 9

= engineering stress

Front 10

engineering strain

Back 10

e = (l - lo) / lo
l = instantaneous length

Front 11

nonlinear elastic deformation

Back 11

stress and strain are no longer proportional but still returns to original shape

Front 12

proportional limit

Back 12

the point where linear elastic behavior ends and nonlinear elastic behavior starts

Front 13

yield stress

Back 13

Y, point where plastic deformation starts

Front 14

UTS

Back 14

ultimate tensile stress,
maximum engineering stress

Front 15

neck

Back 15

cross-sectional area is no longer uniform along gage length (smaller in necked region)

Front 16

When does necking occur?

Back 16

at loads beyond a specimens UTS

Front 17

breaking or fracture stress

Back 17

engineering stress at fracture

Front 18

modulus of elasticity (definition)

Back 18

E, young's modulus
ratio of stress to strain in the elastic region

Front 19

modulus of elasticity (formula)

Back 19

E = o~ / e

Front 20

hooke's law

Back 20

modulus of elasticity

Front 21

stiffness

Back 21

modulus of elasticity

Front 22

poisson's ratio

Back 22

v = lateral strain / longitudinal strain

Front 23

ductility

Back 23

the extend of plastic deformation a material undergoes before fracture
ex: chalk = 0, gum = high

Front 24

what are the 2 ways to measure ductility?

Back 24

total elongation, reduction of area

Front 25

total elongation

Back 25

elongation = [(lf - lo) / (lo)](100)
lf = length at fracture

Front 26

reduction of area

Back 26

[(Ao - Af) / (Ao)](100)

Front 27

true stress

Back 27

o~ = P / A
A = actual cross-sectional area

Front 28

true strain

Back 28

soft "E",
E = ln(l / lo)

Front 29

true stress-strain curve equation

Back 29

o~ = K E^n
K = strength coefficient
n = strain-hardening exponent

Front 30

specific energy

Back 30

energy per unit volume

Front 31

toughness

Back 31

amount of energy per unit volume the material dissipates prior to fracture

Front 32

strain at onset of necking in tension test

Back 32

slope of load-elongation curve = 0 at UTS,
true strain = n,
n = strain hardening exponent

Front 33

what effect does increasing temp have on stress-strain curves?

Back 33

a. ductility and toughness increase
b. yield stress and modulus of elasticity decrease,
n (strain hardening exponent) decreases with increasing temp

Front 34

deformation rate

Back 34

speed at which a tension test is being carried out (m/s, ft/min)

Front 35

strain rate

Back 35

function of the specimen's length

Front 36

strain-rate hardening

Back 36

increasing the strain rate increases the strain of the material

Front 37

strain-rate sensitivity exponent

Back 37

m, value obtained from log-log plots, provided the vertical and horizontal scales are the same

Front 38

strength coefficient

Back 38

C in o~= C(soft E with dot)^m

Front 39

true strain rate

Back 39

soft "E" with dot on top in o~= C(soft E with dot)^m

Front 40

superplastic forming (of sheet metal)

Back 40

exploits ductility enhancement caused by the high strain-rate sensitivity of some materials

Front 41

superplasticity

Back 41

-capability of some materials to undergo large uniform elongation prior to necking and fracture in tension
-from a few 100 % to 2000%
-bubble gum, glass, thermoplastics

Front 42

what effect does increasing hydrostatic pressure have on the mechanical properties of materials?

Back 42

it substantially increases the strain at fracture, for both ductile and brittle metals

Front 43

what effect does radiation have on mechanical properties of metals?

Back 43

-increases yield stress, tensile strength, hardness
-decreases ductility, toughness

Front 44

what effect does radiation have on plastics?

Back 44

detrimental effects

Front 45

compression test

Back 45

-specimen is subjected to compressive load
-cylindrical specimen is compressed between 2 platens
-gives information useful for estimating forces and power requirements in these processes

Front 46

what are platens?

Back 46

well-lubricated flat dies

Front 47

barreling

Back 47

-bulging of cylindrical surface caused by friction between specimen and platens in compression test

Front 48

Are brittle materials stronger (and more ductile) in tension or compression?

Back 48

compression

Front 49

bauschinger effect

Back 49

-phenomenon where when a metal is subjected to tension into the plastic range, and then the load is released and a compressive load is applied, the yield stress in compression is found to be lower than that in tension
-applies to all metals and alloys

Front 50

strain softening (work softening)

Back 50

- another name for bauschinger effect
-named because of the lowered yield stress in the direction opposite that of the original load application

Front 51

disk test

Back 51

-for brittle materials such as ceramics and glasses
-disk is subjected to compression between 2 platens
-tensile stresses develop perpendicular to the vertical centerline along the disk causing the disk to split in half vertically

Front 52

tensile stress of disk (formula)

Back 52

o~ = 2P / (pi)(d)(t), p = load at fracture, d = diameter, t = thickness

Front 53

torsion test

Back 53

-test used to determine properties of materials in shear
-thin tubular specimen

Front 54

shear strain of torsion specimen (examples and formula)

Back 54

ex: punching holes in sheet metal, swaging, metal cutting
y (gamma) = [r (phi)] / l, l = length of tube, o/ = phi, angle of twist in radians

Front 55

shear stress of torsion specimen (formula)

Back 55

t (tau, one-legged pi) = T / (2(pi)r^2t, T = torque, r = avg radius, t = thickness of tube at narrow section

Front 56

angle of twist (in radians)

Back 56

phi, o/

Front 57

shear modulus, G

Back 57

ratio of shear stress to shear strain in the elastic range

Front 58

modulus of rigidity

Back 58

shear modulus, G

Front 59

forgeability

Back 59

refers to the number of twists a metal is able to sustain prior to failure

Front 60

bend test

Back 60

-for brittle materials like ceramics and carbides
-apply vertical load at 1 (three-point) or 2 (four-point bending) point
-longitudinal stresses are tensile at lower surfaces and compressive at upper surfaces

Front 61

flexure test

Back 61

= bend test

Front 62

modulus of rupture

Back 62

stress at fracture in bend test

Front 63

transverse rupture strength

Back 63

= modulus of rupture

Front 64

hardness

Back 64

-resistance to scratching
-steel > aluminum > lead

Front 65

what is the difference between hardness tests?

Back 65

they use different indenter materials and shapes

Front 66

what are the common hardness tests?

Back 66

-brinell test, rockwell test, vickers test, knoop test, scleroscope and leeb test, mohs hardness, shore test and durometer, hot hardness

Front 67

brinell test

Back 67

-indenter: steel ball with 10 mm diameter
-load: 500kg, 1500kg, 3000kg
-measures diameter of indentation
-smaller impression = greater hardness

Front 68

rockwell test

Back 68

-indenter: diamond cone, steel ball
-load: 60-150 kg
-measures depth of penetration

Front 69

superficial hardness

Back 69

rockwell test at lighter loads

Front 70

vickers test

Back 70

-indenter: pyramid shaped diamond
-load: 1-120 kg

Front 71

knoop test

Back 71

-indenter: elongated pyramid shaped indenter
-load: 25g-5kg

Front 72

microhardness test

Back 72

-knoop test because of the very light loads applied
-suitable for very small/thin specimens and brittle materials (carbides, ceramics, glass)

Front 73

scleroscope and leeb test

Back 73

-skleros means hard in greek
-scleroscope is dropped onto specimen from a certain height and the height of the rebound is measured
-higher rebound = harder

Front 74

scleroscope

Back 74

diamond tipped indenter (hammer) enclosed in a glass tube

Front 75

leeb test

Back 75

-electronic version of a scleroscope
-carbide hammer impacts surface, and incident and rebound velocities are electronically measured

Front 76

mohs hardness

Back 76

-scale from 1 (talc) to 10 (diamond)
-material with higher hardness number scratches one with lower number
-2-3 = soft metal, 6 = hardened steel, 9 = aluminum oxide

Front 77

shore test

Back 77

-measures hardness of soft nonmetallic materials like rubber, plastic
-uses durometer
-range 0-100
-measures depth of penetration after 1s
-type A (for soft) uses blunt indenter with load of 1kg
-type B (for harder) uses sharp indenter with load of 5 kg

Front 78

durometer

Back 78

-durus = latin for hard
-instrument in shore test

Front 79

hot hardness

Back 79

-hardness tests performed at elevated temperatures
-specimen and indenter are enclosed in a small electric furnace
-important in applications such as cutting tools in machining and dies in hot-working/casting

Front 80

what does the hardness of a cold-worked metal roughly equal?

Back 80

3 times the yield stress, Y
(for annealed metals = 5 Y)

Front 81

zone of deformation

Back 81

-location of specimen under the indenter
-must be allowed to develop freely for hardness test to be meaningful
-2 diameters of the indenter from the edge of the specimen with thickness 10 times depth of penetration

Front 82

surface preparation

Back 82

not important for brinell, important for other tests because of small size of indentations

Front 83

cyclic stresses

Back 83

-caused by fluctuating mechanical loads
-such as (a) gear teeth or reciprocating sliders (b) rotating machine elements under constant bending stresses (c) thermal stresses

Front 84

fatigue failure

Back 84

-phenomenon where parts fail at stress levels below that at which failure would occur under static loading
-associated with cracks that grow with each stress cycle and propagate

Front 85

fatigue tests

Back 85

-testing specimens under various states of stress by varying stress amplitude (S), number of cycles (N)

Front 86

stress amplitude

Back 86

S, maximum stress the specimen is subjected to

Front 87

S-N curves

Back 87

Stress amplitude-Number of cycles

Front 88

creep

Back 88

-permanent elongation of a component under a static load for a period of time
-usually at elevated temperatures

Front 89

grain-boundary sliding

Back 89

most common mechanism of creep at elevated temperatures

Front 90

creep test

Back 90

-consist of subjecting a specimen to a constant tensile load at elevated temperature and measuring change in length at various time increments

Front 91

3 stages of a creep curve

Back 91

primary, secondary, tertiary

Front 92

creep rupture

Back 92

failure of a specimen my necking and fracture in a creep test

Front 93

stress relaxation

Back 93

-closely related to creep
-stresses from loading of a structural component decrease in magnitude over a period of time
-ex: decrease in tensile stress between the ends of a piano wire

Front 94

impact

Back 94

dynamic loading, ex: high speed metal working like making bolt heads

Front 95

impact test

Back 95

placing a notched specimen in an impact tester and breaking the specimen with a swinging pendulu

Front 96

charpy test

Back 96

impact test, specimen is supported at both ends

Front 97

izod test

Back 97

impact test, specimen is supported at 1 end

Front 98

impact toughness

Back 98

energy dissipated (by the amount of swing of the pendulum) in breaking the specimen

Front 99

notch sensitivity

Back 99

sensitivity to surface defects

Front 100

failure types

Back 100

1. fracture- (a) ductile (b) brittle
2. buckling

Front 101

ductile fracture

Back 101

-characterized by plastic deformation which precedes failure
-occurs where shear stress is maximum
-failure is initiated by the formation of voids

Front 102

cup-and-cone fracture

Back 102

the fracture surface of a tension-test specimen

Front 103

inclusions

Back 103

-nucleation sites for voids
-consists of impurities and of second-phase particles (oxides, carbides, sulfides)
-important influence on ductile fracture

Front 104

2 factors that affect void formation

Back 104

a. strength of the bond at the interface between an inclusion and the matrix
b. hardness of the inclusion
(weaker bond, harder inclusion = greater potential for void formation)

Front 105

transition temperature

Back 105

narrow temperature range where many metals undergo a sharp change in ductility (mostly in bcc)

Front 106

strain aging

Back 106

-phenomenon in which carbon atoms in steels segregate to dislocations, thereby pinning the dislocations and increasing the resistance to their movement
-result is increased strength and reduced ductility

Front 107

accelerated strain aging

Back 107

-strain aging that occurs in just a few hours at high temperature, instead of taking place over several days at room temperature

Front 108

blue brittleness

Back 108

-an example of accelerated strain aging
-occurs in the blue-heat range where steel develops a bluish oxide film

Front 109

brittle fracture

Back 109

-occurs with little or no plastic deformation
-low temperature and high rate of deformation promote brittle fracture

Front 110

cleavage plane

Back 110

-crystallographic plane, where brittle fracture happens, where tensile stress is a maximum

Front 111

defects

Back 111

-scratches, flaws, preexisting external or internal cracks
-important factor in fracture
-presence explains why brittle materials experience such weakness in tension compared with their strength in compression

Front 112

catastrophic failure

Back 112

-under tensile stress, cracks propagate rapidly, causing catastrophic failure

Front 113

transgranular fracture

Back 113

-the fracture paths most commonly observed in polycrystalline metals
-cracks propagate through the grains
-transcrystalline or intragranular

Front 114

intragranular fracture

Back 114

type of transgranular fracture where crack propagates along the grain boundaries

Front 115

how does fatigue fracture typically occur?

Back 115

in a brittle way (cracks develop at existing flaws and propagate over time, eventually leading to total sudden failure)

Front 116

beach marks

Back 116

the fracture surface in fatigue, named for it's appearance

Front 117

striations

Back 117

can be seen on fracture surfaces at 1000X, each beach mark consists of several striations

Front 118

ways to improve fatigue strength

Back 118

a. inducing compressive residual stresses on surfaces--by shot peening or roller burnishing
b. case hardening (surface hardening)
c. providing a fine surface finish to reduce the effects of notches
d. selecting appropriate materials free from significant amounts of inclusions, voids, and impurities

Front 119

way to reduce fatigue strength

Back 119

tensile residual stresses on the surface, decarburization, surface pits (due to corrosion) that act at stress raisers, hydrogen embrittlement, galvanizing, electroplating

Front 120

stress-corrosion cracking (season cracking)

Back 120

-causes an otherwise ductile metal to fail in a brittle way
-depends on material, presence and magnitude of tensile residual stresses, and the environment

Front 121

hydrogen embrittlement

Back 121

phenomenon where the presence of hydrogen reduces ductility and causes severe embrittlement
-severe in high strength steels

Front 122

sources of hydrogen embrittlement

Back 122

-melting of metal, pickling, electrolysis in electroplating, water vapor in the atmosphere, moist electrodes and fluxes used during welding

Front 123

pickling

Back 123

-removing of surface oxides by chemical or electrochemical reaction
-possibile source of hydrogen embrittlement, along with electrolysis in electroplating, water vapor from the atmosphere, melting of metal

Front 124

residual stresses

Back 124

-stresses that remain within a part after it has been formed and all the external forces are removed
-caused by subjection of a workpiece to nonuniform plastic deformation

Front 125

warping

Back 125

-removal of a layer of material from a part, such as by machining or grinding disturbs the equilibrium of residual stresses and creates a new radius of curvature in order to balance the internal forces

Front 126

temperature gradients

Back 126

-temperature gradients within a body can cause residual stresses

Front 127

what are ways to reduce residual stresses?

Back 127

stress-relief annealing, deformation of the part (ex: stretching it), given sufficient time at room temperature (relaxation)

Front 128

heat

Back 128

product of mechanical work in plastic deformation

Front 129

temperature rise

Back 129

(delta)T = u / pc, u = specific energy, p = density, c = specific heat