Steel has two forms of damage: plastic damage and brittle damage.
Plastic damage is under the action of external forces, the cross-sectional stress of the member reaches the yield point of the material, resulting in excessive deformation, exceeding the strain capacity of the material or member, the final stress reaches the tensile strength of steel and damage. The member produces large plastic deformation before damage, and the fracture after fracture is fibrous, dark in color, and sometimes the trace of slippage can be seen. Before plastic damage, due to the occurrence of large plastic deformation, and deformation of a longer duration, easy to find in time and take remedial measures, without causing serious consequences. In addition, the plastic deformation of the member under static load after its internal force redistribution, so that the stress in the member tends to be uniform, improving the ability of the structure to withstand static loads.
Brittle damage is under the action of external forces, the member before the damage deformation is very small, or even no plastic deformation, cross-sectional stress is even less than the yield strength of the material, the member from the stress concentration of sudden fracture and damage. There is no warning before the damage, the damage is sudden, the fracture is flat and shiny grain shape. Defects arising from metallurgy and machining processes, especially notches and cracks, are often the origin of fracture. Cold and low temperatures, fatigue problems under repeated loading, dynamic loading, and welding residual stresses often cause brittle damage. As brittle damage is often low-stress damage, no obvious signs of yield deformation, it is difficult to detect and take remedial measures in time, and the fracture of individual members often causes continuous collapse of the entire structure, with serious consequences. In the design, construction and use of steel structures, special attention should be paid to preventing brittle damage.