Stress Strengthening
Stress strengthening is the phenomenon of an increase of an objects strength due to its exposure to an applied or inherent stress. This weakened strengthening can occur in a variety of materials, both solids and liquids, and is the manifestation of an adequate alteration in the materials microstructure in reaction to the forces applied to it. In most cases, the resulting strength increase is of smaller magnitude than the actual stress magnitude (the applied deformations), and is a result of a combination of different strengthening mechanisms.
The design of engineered components and structures takes into account the effects of stress yielding and the consequent stress strengthening, which has implications in the material selection process for a specific application, since the appropriate strength levels can be obtained through the maintenance of a given level of loading.
Stress Strengthening Mechanisms
The major mechanisms that account for stress strengthening observed in materials include:
• Plastic-Elastic Deformations: when a material is initially subjected to an applied stress level exceeding the unyielding strength, its initially elastic response becomes plastically deformed. This plastic deformation reduces the materials dimensions closest to the deformation region by reducing its thickness, which results in strengthening.
• Dislocation Densification and Slip Band Development: dislocations are structural defects that act as sources for the plastic deformation in materials, since they are able to move in order to eliminate applied force differences. Therefore, the densification of dislocations in regions of high stress magnitude leverages the bulk plasticity of the material, promoting stress hardening and resulting in weaker material in regions with less stress.
• Recrystallization: this refers to the process of grain refinement and rearrangement of newly formed grains at the material surface in direct response to the applied stress level. This refinement of the materials grain size increases its strength due to the fact that boundaries between grains (which act as sites for dislocation movement) form smaller and more numerous, which further leverages the material plasticity and promotes increased strain hardening.
• Solid State Phase Transformations: these refer to the transformation of various structures of a solid material under extreme stress conditions. These transformations can include transformations from one crystal structure to another, as well as the formation of amorphous structures from existing crystalline forms. This results in an increase of stiffness and strength of the material due to the alteration of its fundamental material properties.
• Diffusion and Precipitation Strengthening: this refers to the process by which compressive stress triggers the diffusion, segregation, precipitation and coagulation of existing material clusters and impurities in the materials structure. These suspended particles further increase the materials stiffness and strength since most of the materials stress is displaced as the particles aggregate and form networks throughout the material.
Conclusion
Stress strengthening is an overlooked mechanism of considerable significance in the design of components and materials. A proper understanding of the underlying strengthening mechanisms can be of tremendous importance in improving the performance of the material, due to the optimization of stress withstanding and hardening capabilities of the components and structures exposed to external and internal stress magnitudes.
