Materials ch3

Engineering vs True Stress and Strain

Engineering vs True Stress and Strain

Engineering ssumes Ao is constant while True does not

Tensile Test

The material sample is secured between a pair of clamps. The upper clamp is attached to a fixed bar and a load cell. The lower grip is attached to a movable bar that slowly pulls the material downward. The load cell records force, and an extensiometer records the elongation of the sample

Engineering vs True

Engineering stress and strain assume constant area

Elastic Stretching

The region on a stress-strain curve in which no permanent changes to the material occur.

Plastic Deformation

The region on a stress-strain curve in which the material has experienced a change from which it will not completely recover.

Yield Strength

The stress at the point of transition between elastic stretching and plastic deformation.

Tensile Strength

The stress at the highest applied force on a stress-strain curve.

Breaking Strength

The stress at which the material breaks completely during tensile testing.

Ductile

Materials that can plastically deform without breaking.

Brittle

Materials that fail completely at the onset of plastic deformation. These materials have linear stress-versus-strain graphs

Necking

The sudden decrease in cross-sectional area of a region of a sample under a tensile load

Bend Test

Used to test brittle materials. As the sample begins to deflect under an applied force, the bottom experiences a tensile stress while the top experiences compressive stress

Compressive Test

Uses the same apparatus as tensile testing, but instead of pulling the sample apart, the sample is subjected to a crushing load.

Hardness Test

A method used to measure the resistance of the surface of a material to penetration a tungsten-carbide sphere of 10 mm diameter under a static force.

Creep Test

Creep refers to plastic deformation of a material over time (usually at elevated temperatures). When a continuous stress is applied to a material at elevated temperature, it may stretch and ultimately fail below the yield strength.

Impact Test

Toughness defines a material’s resistance to a blow. In an impact test, a hammer is secured to a pendulum at some initial height and released. The orientation of the sample varies depending on specific testing techniques.

Flexural strength

The amount of flexural stress a material can withstand before breaking. Measured through the bend test

Hardness

The resistance of the surface of a material to penetration by a hard object under a static force

Primary Creep

The first stage of creep, during which the dislocations in a material slip and move around obstacles

Secondary Creep

The stage in which the rate that dislocations propagate equals the rate at which the dislocations are blocked, resulting in a fairly linear region on the strain-time plot.

Tertiary Creep

The final stage of creep, during which the rate of deformation accelerates rapidly and continues until rupture

Larson Miller Parameter

A value used to characterize creep based on time, temperature, and material-specific constants.

Toughness

A property defining a material’s resistance to a blow that is measured by an impact test.