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Strength Hardness Toughness Stiffness Strength/Density

# 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

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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