Residual stress test
Introduction
Residual stresses are stresses that remain in a solid material after the cause that produced said stresses has ceased. These residual stresses can be desirable or undesirable. For example, "laser writing") generates residual compressive stresses that are often beneficial and on metals, such as the turbine blades of a jet engine or generator. Compressive stresses are also generated on the surface of glasses to improve their strength (thin, scratch- and crack-resistant glass screens used in smartphones). However, unwanted residual stresses in some structures can cause them to fail prematurely.
Residual stresses can be produced by different mechanisms including plastic deformations, temperature gradients (during thermal treatments) or structural changes (phase transformations). The heat from welding can cause localized expansion, which is absorbed during welding by the molten metal or the location of the parts being welded. When the finished weld cools, some areas cool and contract more than others, leaving residual stresses. Another example occurs during semiconductor and microsystem manufacturing [1] when thin film materials with different thermal and crystalline properties are deposited sequentially under different process conditions. The variation of stresses across a stack of film materials can be very complex and can vary between compressive and tensile stresses from layer to layer.
Applications
While uncontrolled residual stresses may be undesirable, some designs rely on them. In particular, brittle materials may be hardened by compressive residual stresses, as in the case of toughened glass and prestressed concrete. The predominant mechanism for failure in brittle materials is brittle fracture, which begins with initial crack formation. When an external tensile force is applied to the material, a stress concentration occurs inside the crack, increasing the tensile stresses locally at that point above the average of the material. This causes the initial crack to lengthen rapidly, propagating while the material surrounding the fissure or crack is overstressed by the stress concentration, leading to fracture of the material.
In a material that has residual compressive stresses, brittle fracture can be prevented because the initial crack forms under compressive stress (negative stress). To cause a brittle fracture by crack propagation of the initial crack, the stress associated with additional external forces must exceed the compressive residual stress before the crack tips experience sufficient tensile stress to propagate.