The performance of the metal material determines the scope of application and the rationality of the application. The performance of metal materials is mainly divided into four aspects, namely: mechanical properties, chemical properties, physical properties, process properties.
Mechanical behavior
The concept of stress, the force on the internal cross-sectional area of ​​an object is called stress. The stress caused by the external force is called the working stress, and the stress that is balanced inside the object without the external force is called the internal stress (for example, the tissue stress, the thermal stress, the residual stress remaining after the machining process ends, etc.).
Second mechanical properties, the ability of metal to withstand external forces (loads) under certain temperature conditions, the ability to resist deformation and fracture is called the mechanical properties of metal materials (also known as mechanical properties). Metal materials can be loaded in a variety of forms, from static loads to dynamic loads, including tensile stress, compressive stress, bending stress, shear stress, torsional stress, and friction, vibration, and Impact and so on, so the indicators for measuring the mechanical properties of metal materials are mainly the following:
Intensity
This is the maximum ability to characterize the material against deformation and damage under external forces. It can be divided into tensile strength limit (σb), bending strength limit (σbb), compressive strength limit (σbc) and so on. Since the metal material has a certain regularity from deformation to failure under the action of external force, it is usually measured by a tensile test, that is, the metal material is made into a sample of a certain specification, and stretched on a tensile test machine until the test Sample fracture, the measured strength indicators are:
(1) Strength limit: The maximum stress that the material can resist fracture under the action of external force, generally refers to the tensile strength limit under tensile force, expressed as σb, such as the strength limit corresponding to the highest point b in the tensile test curve, the common unit is mega Pa (MPa), conversion relationship: 1MPa = 1N / m2 = (9.8) - 1Kgf / mm2 or 1Kgf / mm2 = 9.8MPa σb = Pb / Fo where: Pb? C to the maximum stress when the material breaks (or is The maximum load that the specimen can withstand); the original cross-sectional area of ​​the Fo?C tensile specimen.
(2) Yield strength limit: When the external force of the metal material sample exceeds the elastic limit of the material, although the stress does not increase any more, the sample still undergoes obvious plastic deformation. This phenomenon is called yielding, that is, when the material is subjected to external force to a certain extent. The deformation is no longer proportional to the external force and produces significant plastic deformation. The stress at the time of yielding is called the yield strength limit, expressed as σs, and the point S corresponding to the tensile test curve is called the yield point. For a material with high plasticity, a significant yield point appears on the tensile curve, while for a low plastic material, there is no obvious yield point, making it difficult to determine the yield limit from the external force of the yield point. Therefore, in the tensile test method, the stress at the time of 0.2% plastic deformation of the gauge length on the sample is usually specified as the conditional yield limit, which is represented by σ0.2. The yield limit indicator can be used as a basis for designing parts that do not produce significant plastic deformation during operation. However, for some important parts, it is also considered that the yield ratio (ie σs/σb) is small to improve its safety and reliability, but the utilization rate of the material is also low.
(3) Elastic limit: The material will deform under the action of external force, but the ability to recover the original force after removing the external force is called elasticity. The maximum stress that the metal material can maintain elastic deformation is the elastic limit, which corresponds to the point e in the tensile test curve, expressed as σe, and the unit is MPa (MPa): σe=Pe/Fo where Pe is the elastic state. The maximum external force (or the load when the material is elastically deformed).
(4) Elastic modulus: This is the ratio of the stress σ and the strain δ (the unit deformation amount corresponding to the stress) in the elastic limit range of the material, expressed in 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 the metal material (the ability of the metal material to resist elastic deformation when subjected to force is called rigidity).
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