Properties of metal materials
The properties of metal materials determine the scope of application of the material and the rationality of its application. The properties of metal materials are mainly divided into four aspects, namely: mechanical properties, chemical properties, physical properties, and process properties.
1. Mechanical properties
(1) The concept of stress. The force per unit area inside an object is called stress. The stress caused by external force is called working stress, and the stress that is balanced inside the object without external force is called internal stress (such as structural stress, thermal stress, residual stress remaining after the end of the processing process...).
(2) Mechanical properties. When metals are subjected to external forces (loads) under certain temperature conditions, their ability to resist deformation and fracture is called the mechanical properties of metal materials (also known as mechanical properties). There are many forms of load that metal materials bear. It can be static load or dynamic load, including tensile stress, compressive stress, bending stress, shear stress, torsional stress, and friction, vibration, and Impact and so on, so the main indicators to measure the mechanical properties of metal materials are as follows:
This is a characterization of the maximum ability of a material to resist deformation and damage under the action of external forces, which can be divided into tensile strength limit (σb), bending strength limit (σbb), compressive strength limit (σbc), etc. Because metal materials have a certain rule to follow from deformation to failure under the action of external force, tensile test is usually used for measurement, that is, the metal material is made into a sample of a certain specification and stretched on a tensile testing machine until the test If the sample breaks, the main strength indicators measured are:
(1) Strength limit: the maximum stress that a material can resist fracture under the action of external force, generally refers to the tensile strength limit under tensile force, expressed in σb, such as the strength limit corresponding to the highest point b in the tensile test curve, a common unit It is megapascals (MPa) and the conversion relationship is: 1MPa=1N/m2=(9.8)-1kgf/mm2 or 1kgf/mm2=9.8MPa.
(2) Yield strength limit: When the external force borne by a metal material sample exceeds the elastic limit of the material, although the stress no longer increases, the sample still undergoes obvious plastic deformation. This phenomenon is called yielding, that is, the material bears the external force to a certain extent. The deformation is no longer proportional to the external force and produces obvious plastic deformation. The stress when yielding is called the yield strength limit, expressed by σs, and the point S corresponding to the tensile test curve is called the yield point. For materials with high plasticity, an obvious yield point appears on the tensile curve, while for materials with low plasticity, there is no obvious yield point, so it is difficult to find the yield limit based on the external force of the yield point. Therefore, in the tensile test method, the stress at which the gauge length on the sample undergoes 0.2% plastic deformation is usually specified as the conditional yield limit, expressed by σ0.2. The yield limit index can be used as a design basis for requiring parts not to produce obvious plastic deformation during work. However, for some important parts, the yield ratio (ie, σs/σb) is also considered to be small to improve its safety and reliability, but the utilization rate of materials is also low at this time.
(3) Elastic limit: The ability of a material to deform under the action of an external force, but the ability to recover its original shape after the external force is removed is called elasticity. The maximum stress that a metal material can maintain elastic deformation is the elastic limit, which corresponds to the point e in the tensile test curve, expressed in σe, and the unit is megapascals (MPa): σe=Pe/Fo where Pe is when elasticity is maintained The maximum external force (or the load at the maximum elastic deformation of the material).
(4) Elastic modulus: This is the ratio of the stress σ to the strain δ (unit deformation corresponding to the stress) of the material within the elastic limit range, expressed by E, in megapascals (MPa): E=σ/δ =tgα where α is the angle between the oe line and the horizontal axis ox on the tensile test curve. The elastic modulus is an index reflecting the rigidity of metal materials (the ability of metal materials to resist elastic deformation when subjected to force is called rigidity).