The Bauschinger effect is named after a well-known German engineer named Johann Bauschinger. The Bauschinger effect refers to the property of a material in which the material’s stress, as well as the strain characteristics, change as a result of the microscopic stress distribution of that material. For instance, we can say that the increase in tensile yield strength occurs at the cost of compressive yield strength. While more tensile cold working (deformation of plastic) tends to increase the tensile yield strength, the compressive yield strength initially after tensile cold working is actually reduced. The greater the tensile cold working (deformation), the lower will be the compressive yield strength.
The Bauschinger effect is normally said to be associated with the conditions where the yield strength of a metal decreases when the direction of strain acting on it is changed. The Bauschinger effect is a general phenomenon that is found in most of the polycrystalline metals. The basic mechanism occurring for the Bauschinger effect is related to the dislocation structure in the cold-worked metal. A cold-worked metal means a deformed metal. As soon as the deformation occurs, all the dislocations will accumulate at various barriers and produce dislocation pile-ups and thus will produce tangles. Based on the cold work structure, generally, we use two types of mechanisms to explain the Bauschinger effect.
Severe kind of cold working in a single direction results in accumulation of dislocation at the barriers to dislocation movement when the stress are applied in the reverse direction, the dislocations tend to be acted upon by the back stress that was already present at the dislocation barriers previously and also because the back stresses at the dislocation barriers in the back are not seen to be as strong as compared to that of the previous one. Hence the dislocations glide over easily, thus resulting in the lower yield stress for the plastic or metal deformation for the reversed direction of the loading.
THE FORMING PROCESS
Metal forming or deforming operations often result in situations that tend to expose the metal workpiece to stresses of a reversed sign. The Bauschinger effect contributes to the work softening of the workpiece. The main processes in which it helps are the straightening of drawn bars or the rolled sheets, where rollers are used to subject the workpiece to alternate bending stress. It thereby reduces the yield strength and enables a greater cold draw-ability to the workpiece.
When plastic deformation in one direction is just followed by the plastic deformation in the opposite direction (or to a nearly opposite direction) such as during the process of deformation, the internal stress distribution inside the material changes which results in the change in the properties of the respected material. This directional strain hardening results in an increase in the yield strength in the direction in which the force is applied and a drop in yield strength in the opposite direction. This drop-in yield strength is what we call it as the Bauschinger effect.
If we talk about it in a graphical way, then we can say that the tensile stress and the strain both are treated as positive and the compressive stress-strain are treated to be negative. Thus, in case if an annealed specimen is loaded with some kind of tension which is beyond its elastic limit and then it is unloaded, then we can clearly notice the resultant condition by studying its graph. In a graph, the elastic limit of the material (in tension) is represented by the change Set.
A same kind of reasoning would apply if the annealed specimen which is initially loaded in compression past the elastic limit, is unloaded and then again loaded in some tension. Here the resulting elastic limit in tension would be much smaller than the elastic limit of the annealed material which is in compression. Thus we can say that either a tensile or compressive load is applied beyond the elastic limit, the corresponding tensile limit or the compressive limit is this reduced and the more the amount of load that we make to exceed the elastic limit, the greater will be the reduction, but up to a limit.
THE STRESS-STRAIN CURVE
The Bauschinger effect as seen originally was stated in terms of the elastic limit, the term yield strength is often used interchangeably (the reason for this is that the elastic limit and the yield are found to be located in close approximation to each other as seen on the stress-strain curve). It is applied to very small strains only. Bauschinger effect can be said that it is a phenomenon of pile-ups of those dislocations that happen during the process so it can also be called the dislocation pile-ups. The predicted stress-strain response is said to be sensitive to the grain aspect ratio, and the grain boundaries give rise to back stress that inhibits the nucleation at the dislocation sites.
The back stress as a result of the dislocation pile-ups provides the driving force for the dislocation activity during the early stages of the unloading of the stress. First, dislocation pile-ups stretch out in a collective manner with the dislocations gliding back on their planes. This subsequently gives new dislocations with an opposite sign as compared to those generated during loading. On reloading it there is a significant peak in the stress-strain curve due to the dislocations that are present at that point.
At high temperatures, the microstructure of various materials can become unstable. The aging of materials may result in precipitation in the grain structure and at the grain boundaries in the solid solution alloys, the solute dislocation interaction gives rise to the dynamic strain aging. Precipitate strengthened alloys may often end up soften due to the effect of partial dissolution.