The manufacture of some swords utilises a gradient in martensite formation to produce particularly hard edges (notably the katana). To cause brittle fracture by crack propagation of the initial crack, the external tensile stress must overcome the compressive residual stress before the crack tips experience sufficient tensile stress to propagate. This causes the initial crack to enlarge quickly (propagate) as the surrounding material is overwhelmed by the stress concentration, leading to fracture.Ī material having compressive residual stress helps to prevent brittle fracture because the initial crack is formed under compressive (negative tensile) stress. When an external tensile stress is applied to the material, the crack tips concentrate stress, increasing the local tensile stresses experienced at the crack tips to a greater extent than the average stress on the bulk material. The predominant mechanism for failure in brittle materials is brittle fracture, which begins with initial crack formation. In particular, brittle materials can be toughened by including compressive residual stress, as in the case for toughened glass and pre-stressed concrete. While uncontrolled residual stresses are undesirable, some designs rely on them. The stress variation through a stack of thin film materials can be very complex and can vary between compressive and tensile stresses from layer to layer. Another example occurs during semiconductor fabrication and microsystem fabrication when thin film materials with different thermal and crystalline properties are deposited sequentially under different process conditions. When the finished weldment cools, some areas cool and contract more than others, leaving residual stresses. Heat from welding may cause localized expansion, which is taken up during welding by either the molten metal or the placement of parts being welded. Residual stresses can result from a variety of mechanisms including inelastic ( plastic) deformations, temperature gradients (during thermal cycle) or structural changes ( phase transformation). However, unintended residual stress in a designed structure may cause it to fail prematurely. For example, laser peening imparts deep beneficial compressive residual stresses into metal components such as turbine engine fan blades, and it is used in toughened glass to allow for large, thin, crack- and scratch-resistant glass displays on smartphones. Residual stress may be desirable or undesirable. In materials science and solid mechanics, residual stresses are stresses that remain in a solid material after the original cause of the stresses has been removed.
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