strength hardness toughness stiffness strength/density
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StrengthHardness
ToughnessStiffness
Strength/Density
Specimen can be round, flat or tubular
ASTM specifications Stress-Strain Curve
Elastic regionPlastic regionNecking
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 copperBrassStainless steel
Increasing the temperature…Ductility increasesToughness increasesYield stress decreasesModulus of elasticity decreasesn 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 shapesBrinell RockwellVickersKnoop
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 crackingDuctile-plastic deformation which proceeds
to failureBrittle–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
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