Mechanics Materials: Properties and Testing

This set of vocabulary flashcards covers the fundamental concepts of mechanics and materials, including elastic and plastic deformation, stress-strain testing, and hardness measurements based on the lecture notes.

Elastic behavior

A reversible type of deformation where bonds stretch under a small load and return to their original state upon unloading; it can be linear or non-linear.

Plastic behavior

A permanent type of deformation that occurs when bonds stretch and atomic planes shear; the material does not return to its original shape after the load is removed.

Modulus of elasticity ($E$)

Also known as Young’s modulus, it is the ratio of stress to strain in the linear elastic range, measured in units such as $GN\,m^{-2}$.

Yield strength ($\sigma_y$)

The stress level at which noticeable plastic deformation occurs, typically defined when permanent strain ($\epsilon_p$) is $0.002$.

Tensile strength (UTS)

The maximum stress point (ultimate elastic stress) on a stress-strain curve, also referred to as the ultimate strength.

Rupture strength

The specific stress value at which a material fails or breaks.

Neck

A localized reduction in cross-sectional area that occurs during a tensile test and acts as a stress concentrator.

Ductility ($\%EL$)

A measure of the degree of plastic deformation that has been sustained at fracture, calculated as $\%EL = \frac{L_f - L_0}{L_0} \times 100$.

Factor of safety ($N$)

A design factor used to account for uncertainties, calculated as $N = \frac{\sigma_y}{\sigma_{working}}$; it is often between $1.2$ and $4$.

Toughness

A property approximated by the area under the stress-strain curve; metals generally have larger toughness compared to ceramics or polymers.

Hardness

A measure of a material's resistance to localized plastic deformation, such as a small dent or scratch, determined by applying a known force to an indenter.

Brinell Hardness ($HB$)

A hardness test using a $10\text{-mm}$ sphere of steel or tungsten carbide as an indenter with the formula $HB = \frac{2P}{\pi D[D - \sqrt{D^2 - d^2}]}$.

Vickers microhardness ($HV$)

A hardness test using a diamond pyramid indenter with a $136^\circ$ angle, calculated using the formula $HV = 1.854P/d^2$.

Knoop microhardness ($HK$)

A hardness test using a diamond pyramid indenter where the ratio of the indentation lengths is $l/b = 7.11$, calculated as $HK = 14.2P/l^2$.

Rockwell Test

A hardness measurement method using either a diamond cone ($120^\circ$) or steel spheres of various diameters ($1/16$, $1/8$, $1/4$, $1/2\text{ in.}$).

Extensometer

A device used in a tensile test machine to measure the change in the length of the specimen's gauge length.

Stress and Strain

Size-independent measures used instead of load and deformation to characterize material properties.