Materials Chapter 7 Key Terms

chevron pattern

a common fracture feature produced by separate crack fronts propagating at different levels in the material

Griffith flaw

a crack or flaw in a material that concentrates and magnifies the applied stress

tempering

a glass heat treatment that makes the glass safer by creating a compressive stress layer at the surface

S-N curve (aka Wohler curve)

a graph showing the relationship between the applied stress and the number of cycles to failure in fatigue

Weibull distribution

a mathematical distribution showing the probability of a fracture or survival of a material as a function of the stress

rotating cantilever beam test

a method for fatigue testing

stress-rupture curve

a method of reporting the results of a series of creep test by plotting the applied stress versus the rupture time

Weibull modulus (m)

a parameter related to the Weibull distribution; indicator of the variability of the strength of materials resulting from a distribution of flaw sizes.

Larson-Miller parameter

a parameter used to relate the stress, temperature, and rupture time in creep

stress corrosion

a phenomenon in which materials react with corrosive chemicals in the environment, leading to the formation of cracks and lowering of strength

shot peening

a process in which metal spheres are shot at a component which leads to a residual compressive stress at the surface of a component and this enhances fatigue life

toughness

a qualitative measure of the energy required to cause fracture of a material. a material that resists failure by impact is said to be tough // area under the stress strain curve

creep

a time dependent permanent deformation at high temperatures, occurring at constant load or constant stress

endurance limit

an older concept that defined a stress below which a material will not fail in a fatigue test. factors such as corrosion or occasional overloading can cause materials to fail at stresses below the assumed endurance level

microvoids

development of small holes in a material. form when a high stress causes separation of the metal at grain boundaries or interfaces between the metal and inclusions

conchoidal fracture

fracture surface containing a smooth mirror zone near the origin of the fracture with tear lines comprising the remainder of the surface. this is typical of amorphous materials

intergranular

in between grains or along the grain boundaries

transgranular

meaning across the grains

notch sensitivity

measures the effect of a notch, scratch, or other imperfection on a material's properties such as toughness or fatigue life

creep test

measures the resistance of a material to deformation and failure when subjected to a static load below the yield strength at an elevated temperature

fatigue test

measures the resistance of a material to failure when a stress below the yield strength is repeatedly applied

climb

movement of a dislocation perpendicular to its slip plane by the diffusion of atoms to or from the dislocation line

beach or clamshell marks

patterns often seen on a component subjected to fatigue. normally formed when the load is changed during service or when the loading is intermittent, perhaps permitting time for oxidation inside the crack

striations

patterns seen on a fractured surface of a fatigued sample. these are visible on a much finer scale than beach marks and show the position of the crack tip after each cycle

endurance ratio

the endurance limit divided by the tensile strength of the material. the ratio is about .5 for many ferrous materials

fatigue life

the number of cycles permitted at a particular stress before a materials fails by fatigue

delamination

the process by which different layers in a composite will begin to debond

creep rate

the rate at which a material deforms when a stress is applied at high temperatures

factor of safety

the ratio of the stress level for which a component is designed to the actual stress level experienced // a factor used to design load-bearing components

fracture toughness

the resistance of a material to failure when a stress below the yield strength is repeatedly applied

fatigue strength

the stress required to cause failure by fatigue in a given number of cycles

fracture mechanics

the study of a material's ability to withstand stress in the presence of a flaw

rupture time

the time required for a specimen to fail by creep at a particular temperature and stress