Strength includes material strength and structural strength. Material strength refers to various mechanical properties of materials under different influencing factors. The influencing factors include chemical composition, processing technology, heat treatment system, stress state, load nature, loading rate, temperature and medium. Strength grade: the technical index of concrete structure strength. Refers to the maximum stress that the standard specimen can bear per unit area under pressure until it is destroyed. It is used as a technical index to evaluate the quality of concrete. According to the properties of materials, material strength can be divided into brittle material strength, plastic material strength and cracked material strength. ① Strength of brittle materials: brittle materials such as cast iron suddenly break after loading, and there is almost no plastic deformation. Brittle materials take their strength limit as the standard for calculating strength. There are two kinds of strength limits: the maximum nominal stress before tensile specimen fracture is called the tensile strength limit of material, and the maximum nominal stress of compressive specimen is called the compressive strength limit. ② Strength of plastic materials: Qin Gang and other plastic materials have large plastic deformation before fracture and cannot disappear after unloading, which is also called residual deformation. Plastic materials take their yield limit as the standard for calculating strength. The yield limit of a material is the stress when the tensile specimen yields (the phenomenon of strain increasing under the condition of constant stress). For plastic materials that have not yielded, the stress corresponding to 0.2% plastic deformation is taken as the nominal yield limit, which is expressed as 0.2. ③ The strength of cracked materials: often lower than the strength limit of materials, and the fracture toughness of materials should be considered when calculating the strength (see Fracture Mechanics ... Strength includes material strength and structural strength. Material strength refers to various mechanical properties of materials under different influencing factors. The influencing factors include chemical composition, processing technology, heat treatment system, stress state, load nature, loading rate, temperature and medium. Strength grade: the technical index of concrete structure strength. Refers to the maximum stress that the standard specimen can bear per unit area under pressure until it is destroyed. It is used as a technical index to evaluate the quality of concrete. According to the properties of materials, material strength can be divided into brittle material strength, plastic material strength and cracked material strength. ① Strength of brittle materials: brittle materials such as cast iron suddenly break after loading, and there is almost no plastic deformation. Brittle materials take their strength limit as the standard for calculating strength. There are two kinds of strength limits: the maximum nominal stress before tensile specimen fracture is called the tensile strength limit of material, and the maximum nominal stress of compressive specimen is called the compressive strength limit. ② Strength of plastic materials: Qin Gang and other plastic materials have large plastic deformation before fracture and cannot disappear after unloading, which is also called residual deformation. Plastic materials take their yield limit as the standard for calculating strength. The yield limit of a material is the stress when the tensile specimen yields (the phenomenon of strain increasing under the condition of constant stress). For plastic materials without yield phenomenon, the stress corresponding to 0.2% plastic deformation is taken as the nominal yield limit, which is expressed as 0.2. ③ The strength of cracked materials: often lower than the strength limit of materials, and the fracture toughness of materials should be considered when calculating the strength (see fracture mechanics analysis). For the same material, if different heat treatment systems are adopted, the higher the strength, the lower the fracture toughness. Different materials have different grading methods.