プラスチックの動き

When a load is removed before the yield point, stress and strain decrease linearly, and the material returns to its original state. This elastic behavior demonstrates that no permanent deformation occurs. The material's elastic limit has not been exceeded, allowing complete recovery of its original dimensions.

Elastic deformation is reversible—the material returns to its original shape when the load is removed. Plastic deformation is permanent; the strain does not return to zero after unloading. This permanent set occurs when stress exceeds the yield point, causing irreversible changes in the material's structure.

Strain-hardening occurs when a material is loaded beyond its initial yield point, increasing its proportional and elastic limits. Upon reloading, the specimen follows the previous unloading curve until near the yield point, then aligns with the original stress-strain curve. This indicates a higher yield point than initially observed, though ductility is reduced.

The rupture point represents the material's ultimate tensile strength and remains constant despite strain-hardening. While strain-hardening increases the yield point and elastic limits, it reduces ductility—the material's ability to deform before breaking. This trade-off means the material fails at the same maximum stress level.

The Bauschinger effect occurs when a second load is applied in the opposite direction to the first load. This reversal causes yield strength to decrease, and the stress-strain diagram becomes curved without a distinct yield point. The material's response to directional stress changes demonstrates its complex mechanical behavior under alternating loads.

When substantial initial loading causes strain-hardening, the total stress change between the highest tensile and maximum compressive stress equals twice the yield stress. While compressive stress remains less than the yield stress, this relationship illustrates the material's symmetric response to significant stress variations in opposite directions.

Plastic deformation is influenced by the maximum stress value applied and the time elapsed before load removal. Materials subjected to higher stresses experience greater permanent set. The duration the material remains under stress also affects plastic behavior, with longer loading periods potentially increasing permanent deformation.