When a metal is plastically deformed in tension, unloaded and then compressed in the reverse direction, the yield stress in compression is lower than the flow stress it would have had if tension had continued. Which is the correct explanation of this phenomenon (the Bauschinger effect)?

Answer: A. Dislocations piled up against obstacles during tension leave an internal stress in the opposite direction (back stress), which assists deformation in the reverse direction

During tension, dislocations pile up against grain boundaries, precipitates and clusters of other dislocations, and Orowan loops remain around hard particles. These create an internal stress (back stress) that opposes the tensile deformation and assists deformation in the reverse direction. So when the loading is reversed, the back stress acts to push the dislocations back, and compression begins at a lower stress. Gathering or collapse of vacancies cannot explain it. Grain refinement, through the Hall–Petch relationship, raises both the tensile and the compressive yield stress, so it cannot be why only the compressive one falls. The Bauschinger effect must be taken into account in analyzing members under cyclic loading, in reverse bending of sheet and in assessing the strength of formed steel pipe.