Q.Why are bridges unsafe after a very long use?

The stress-strain curve of a material is obtained by loading a wire in small steps, from zero up to the point of fracture, and plotting the resulting stress against strain at each step. The curve begins as a straight line through the origin (region $OA$), where Hooke's law holds and stress is proportional to strain; the stress at $A$ is the proportional limit. Beyond $A$, up to a point $B$ (the yield point), the curve bends but the material is still elastic overall — unloading anywhere in $OB$ retraces the same path back to zero strain, so this recovery is fully reversible; the corresponding stress is the elastic limit. Beyond $B$, strain increases rapidly for very little further increase in stress, and unloading no longer retraces the loading path — the material keeps a permanent set (residual strain) once unloaded, so this region ($B$ to the fracture point $D$) is called the plastic-flow or plastic-deformation region. …