This is occurs even at very low temperatures, hence it is called cold creep. The rate of creep is initially fast but slows down later-on. Transient or cold creep extends from A to B. It does not considered the plastic deformation in the vicinity of the crack. It is justified for completely brittle materials only.
Stress fracture definition crack#
It cannot explain well the fracture mechanism if the crack in the material is spheroidal. Some of the limitations of Griffith theory: Irwin and Orowan suggested that Griffith criterion can be applied to brittle material undergone plastic deformation before fracture. Also plastic deformation requires time so the amount of plastic deformation that can occur at the crack tip will depend on rate of advancement of the crack. If the material in which crack is propagating can deform plastically, the crack tip form changes because of plastic strain.
All metals are not ideally brittle and normally fails with certain amount of plastic deformation. Griffith has furnished a criterion for the propagation of preexisting crack in brittle material (glass) according to which brittle fracture occurred when the energy release rate during crack growth exceeded the rate than the rate at which energy was required. Fracture resistance of a material increases by making blunt cracks, fillets or notches of some radius. Then the glass breaks very easily in two pieces along the scratch. The paper is then teared apart very easily along the scratch.Ī window glass panel is scratched by a mechanical nail to introduce stress concentration. Folding and pressing results in creating a scratch that induces higher stress concentration. It is because the most effective cracks are eliminated in the first fracture.ĭuring tearing of a piece of paper, we fold it and press. One of its broken pieces will require more applied stress than the previously applied stress to fracture. Hence surfaces of machine components should be finely finished.Ī material fractures at a certain value of applied stress. Surface cracks in nature are twice as effective as the internal cracks. Brittle cast iron fails with a plane surface having granular appearance. Wrought iron shows fibrous fracture ends. It disappear finally in high carbon steel. With increasing percentage of carbon it becomes shallower. Percentage of carbon in steel has an impact on the profile of cup and cone. The cup and cone fracture is common in plain carbon steels. Cup and cone fracture, crack spread in direction 45° to the tensile axis These fractured ends are of following types and are shown in below figure:
0 kommentar(er)
