MME 223 Module 5

Fatigue life

tells us how long a component survives at a particular stress

Fatigue strength

The maximum stress level that a material can sustain without failing, for some specified number of cycles.

Fatigue

Failure of a material at stresses below its yield strength due to repetitive, cyclic loading

Fracture toughness

measures the ability of a material containing a flaw to withstand an applied load

Creep

The time-dependent permanent deformation that occurs under stress

Endurance limit

The stress below which the material will not fail regardless of the number of load cycles

1. Material selection
2. Component Design
3. Processing Method Design

Importance of fracture mechanics in material selection

Grain boundary sliding

At high temperature the grain boundary becomes weaker than the grain interior and two grains can slide past one another

Endurance ratio

The ratio of endurance limit to tensile strength

1. Specimen thickness
2. Temperature
3. Grain size
4. Flaws size
5. Strength and ductility

Fracture toughness factors

Creep rate

slope of the steady-state portion of stress-strain curve of a creep test

Diffusional creep

In response to the applied stress, vacancies preferentially move from grain boundaries transverse to the stress axis to grain boundaries parallel to the stress axis

Rupture time

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

Fracture Mechanics

Discipline concerned with behavior of materials containing cracks or small flaws