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Plasticity is the property where a body remains permanently deformed after external forces are removed, unlike elasticity where objects return to original shape. When stress exceeds the yield point, atoms permanently displace from their lattice sites. This irreversible deformation distinguishes plasticity from elastic behavior, making it a key concept in understanding material behavior under load.

The yield point marks where a material transitions from elastic to plastic deformation. Beyond this critical threshold, permanent deformation occurs as atoms shift to new lattice positions. Understanding the yield point is essential for analyzing stress and strain behavior and predicting when materials will undergo irreversible changes.

The ultimate stress point represents the maximum stress a material can withstand before rupturing. Beyond this point lies the fracture point where the body breaks apart. Different materials have vastly different ultimate stress values; for example, steel's ultimate stress is approximately 20.0 × 10⁸ Pa, while aluminum's is only 2.2 × 10⁸ Pa.

During plastic deformation, atoms permanently displace from their original lattice sites and remain at new positions even after external forces are removed. This atomic-level rearrangement creates permanent changes in the material's shape and size, distinguishing plastic deformation from temporary elastic changes that reverse when stress is removed.

Materials have different breaking stresses due to variations in their atomic structure and bonding characteristics. For instance, steel exhibits a breaking stress of 20.0 × 10⁸ Pa, significantly higher than brass at 4.7 × 10⁸ Pa or aluminum at 2.2 × 10⁸ Pa. These differences determine how much stress each material can endure before fracturing.

Ideal plasticity occurs when a material undergoes irreversible deformation without requiring increased stress or load. Once a material exceeds its elasticity limit and experiences plastic deformation, it remains plastically deformed until stress reaches the fracture point, at which the material breaks into pieces.

The stress-strain curve identifies three critical points: the yield point where plastic deformation begins, the ultimate stress point marking maximum sustainable stress, and the fracture point where rupture occurs. By analyzing these points, engineers can determine safe stress limits and predict when materials will fail under load conditions.