The
Bauschinger effect refers to a property of
materials where the material's stress/strain characteristics change as a result of the microscopic
stress distribution of the material. For example, an increase in tensile
yield strength occurs at the expense of compressive
yield strength. The
Bauschinger effect is named after the German engineer
Johann Bauschinger. While more tensile cold working increases the tensile yield strength, the local initial compressive yield strength after tensile cold working is actually reduced. The greater the tensile cold working, the lower the compressive
yield strength. The Bauschinger effect is normally associated with conditions where the yield strength of a metal decreases when the direction of strain is changed. It is a general phenomenon found in most polycrystalline metals. The basic mechanism for the Bauschinger effect is related to the
dislocation structure in the
cold worked metal. As deformation occurs, the
dislocations will accumulate at barriers and produce dislocation
pile-ups and tangles. Based on the cold work structure, two types of mechanisms are generally used to explain the Bauschinger effect. First, local back stresses may be present in the material, which assist the movement of dislocations in the reverse direction. Thus, the dislocations can move easily in the reverse direction and the yield strength of the metal is lower. The pile-up of dislocations at grain boundaries and
Orowan loops around strong precipitates are two main sources of these back stresses. Second, when the strain direction is reversed, dislocations of the opposite sign can be produced from the same source that produced the slip-causing dislocations in the initial direction. Dislocations with opposite signs can attract and annihilate each other. Since strain hardening is related to an increased dislocation density, reducing the number of dislocations reduces strength. The net result is that the yield strength for strain in the opposite direction is less than it would be if the strain had continued in the initial direction.
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