Mechanical Behavior, Testing, & Manufacturing of Materials

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29 Νοε 2013 (πριν από 3 χρόνια και 6 μήνες)

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Strength

Hardness

Toughness

Stiffness

Strength/Density


Specimen can be round, flat or
tubular


ASTM specifications


Stress
-
Strain Curve


Elastic region


Plastic region


Necking






Proportional limit


the stress that the
specimen undergoes nonlinear elastic
deformation


Permanent(plastic) deformation


occurs
when the yield stress of the material is
reached


Y(yield stress)
is often determined using
the offset method (.2% elongation) figure
2.2



The maximum engineering stress is called the
tensile strength or ultimate tensile strength
and is the maximum stress found from the


σ
-
ε

diagram




The true strain at the onset of necking is
numerically equal to the strain
-
hardening
exponent, n, of the material. Thus, the
higher the value of n, the higher the strain
that a piece of material can experience
before it begins to neck.


Note: from table 2.3 these have high n values
and can be stretched uniformly to a greater
extent than can the other metals listed.


Annealed copper


Brass


Stainless steel


Increasing the temperature…


Ductility increases


Toughness increases


Yield stress decreases


Modulus of elasticity decreases


n decreases (strain
-
hardening
exponent)



Deformation rate
is defined as the speed at
which a tension test is being carried out
(
ft
/min, m/sec…)


Strain rate
is a function of the specimen’s
length. A short specimen elongates
proportionally more during the same period
than does a long specimen.


Superplasticity

refers to the capacity of
some materials to undergo large uniform
elongation prior to necking and fracture in
tension (examples: bubble gum, glass,
thermoplastics at room temperature)


Solid cylindrical specimen between two well
lubricated flat dies (platens)


Because of friction between the specimen
and the platens, the specimen’s cylindrical
surfaces bulge (barreling)


Slender specimens buckle


For ductile materials, the true stress
-
true
strain curves coincide


Brittle materials are generally stronger and
more ductile in compression


Disk test is also used to test
compressive
stress




Used for brittle materials


Three point or four point


Rectangular cross section
specimens


Modulus of rupture is the stress
at fracture


Defined as the resistance to permanent
indentation


Hardness tests use different indenter
materials and shapes


Brinell


Rockwell


Vickers


Knoop





UTS=3.5(HB) SI units (UTS in MPa)


UTS=500(HB) English units (UTS in
psi)


HB is Brinell hardness


Since hardness is the resistance to
permanent indentation it can be
likened to performing a compression
test on a small volume on the
surface of a material


Rapid fluctuating cyclic or periodic loads


Parts fail at a stress level below that at
which failure would occur under static
loading


Failure is found to be associated with cracks
that grow with every stress cycle and
propagate through the material


FATIGUE FAILURE
-
responsible for the majority
of failures in mechanical components


Rotating machine elements under constant
bending stresses as with shafts


Testing specimens under various states of
stress, usually in a combination of tension
and bending



Stress amplitudes S


Number of cycles N


S
-
N Curves


Endurance limit (fatigue limit): the maximum
stress the material can be subjected without
fatigue failure, regardless of N


CREEP is the permanent elongation
of a component under a static load
maintained for a period of time.


Metals, thermoplastics, rubbers


Occurs at any temperature


Recall: creep at elevated
temperatures is attributed to grain
-
boundary sliding


The test generally consists of subjecting a
specimen to a constant tensile load at
elevated temperature and measuring the
changes in length at various time increments


Primary stage/Secondary stage/Tertiary stage


STRESS RELAXATION
-
the stresses resulting
from loading of a structural component
decrease in magnitude over a period of time,
even though the dimensions of the
component remain constant (example: piano
wire)


A typical impact test consists of placing a
notched specimen in an impact tester and
breaking the specimen with a swinging
pendulum


CHARPY


IZOD


Impact Toughness
-
the energy dissipated by
breaking the specimen


Materials with high impact resistance
generally have high strength, ductility,
toughness


Fracture
-

through either internal or external
cracking


Ductile
-
plastic deformation which proceeds to
failure


Brittle

little or no gross plastic deformation


Buckling


a long slender column under
compressive loads


Many metals undergo a sharp change
in ductility and toughness across a
narrow temperature range


Occurs mainly in bcc and hcp metals


Occurs with little or no gross plastic
deformation


In tension fracture takes place along the
crystallographic plane (cleavage plane) on
which the normal tensile stress is a maximum


In general low temperature & high
deformation rate promote brittle fracture


DEFECTS explain why brittle materials are
weak in tension compared to compression


CATASTROPHIC FAILURE
-
under tensile stresses
cracks propagate rapidly


Residual stresses are those that remain in a
workpiece after it has been plastically
deformed and then has had all external
forces removed


Eliminated by stress
-
relief annealing, further
plastic deformation, or relaxation over time