Representation is a psychological term, which refers to the description or representation of objective entities in human cognitive environment. Material characterization, that is, through the relevant performance indicators, structural morphology, composition and other information, a relatively complete and accurate description or reproduction of a material. Generally speaking, there is a brick in front of you. According to the characterization of this brick, anyone who has never seen this brick can distinguish it from many different materials. For example, the properties of materials include physical properties (density, porosity, hydrophilicity, water absorption, water resistance, thermal conductivity, heat resistance, heat storage, sound absorption, sound insulation, light transmission, conductivity, ferromagnetism, etc.). ), mechanical properties (strength, stiffness, hardness, plasticity, toughness, wear resistance, etc. ), durability (weather resistance, corrosion resistance, etc. ) and the structure of surface properties materials include macroscopic structure (above mm scale, such as dense, porous, layered structure, fiber structure, stacked structure, etc. ), microstructure (mm~ micron scale, such as metallographic structure, wood fiber, microcracks, etc. ) and microstructure (below micron scale, such as crystal structure, colloid, nano-materials and so on. ); The morphology of the material includes appearance, fracture morphology and micro-morphology. The composition of the material includes chemical composition, mineral composition, material composition (proportion) and so on. Because there is more than one kind of material and a certain material has more than one purpose, it should be characterized in many ways.