When metals are subjected to external forces (loads) at a certain temperature, the ability to resist deformation and fracture is called the mechanical properties of metal materials (also known as mechanical properties). There are many forms of loads on metal materials, which can be static or dynamic, including tensile stress, compressive stress, bending stress, shear stress, torsional stress, friction, vibration, impact, etc., so the main indicators to measure the mechanical properties of metal materials are as follows:
This is the maximum resistance to deformation and failure of materials under external forces, which can be divided into tensile strength limit (σ b), bending strength limit (σ bb), compressive strength limit (σ bc) and so on. Because the metal material has a certain law from deformation to failure under the action of external force, it is usually determined by tensile test, that is, the metal material is made into a sample of a certain specification and stretched on a tensile testing machine until the sample is broken. the main strength indexes measured are as follows:
(1) strength limit: the maximum stress that a material can resist fracture under external force, which generally refers to the tensile strength limit under tensile force, which is expressed by σ b. For example, the strength limit corresponding to the highest point b in the tensile test curve is commonly used as MPa, and the conversion relations are as follows: 1 MPA, 1N, M2 = 9.8-1kgf/mm2 or 1kgf/mm2=9.8MPa.
(2) yield strength limit: when the external force borne by the metal sample exceeds the elastic limit of the material, although the stress no longer increases, the specimen still undergoes obvious plastic deformation, which is called yield, that is, when the material bears external force to a certain extent, its deformation is no longer proportional to the external force and produces obvious plastic deformation.
The stress at the time of yield is called the yield strength limit, which is expressed by σ s, and the S point corresponding to the tensile test curve is called the yield point. For materials with high plasticity, there will be an obvious yield point on the tensile curve, but there is no obvious yield point for low plastic materials, so it is difficult to calculate the yield limit according to the external force of the yield point. Therefore, in the tensile test method, the stress of 0.2% plastic deformation of the gauge length on the specimen is usually specified as the conditional yield limit, expressed by σ 0.2.
The yield limit index can be used as the design basis for requiring parts not to produce obvious plastic deformation in work. However, for some important parts, it is also considered that the yield ratio (σ s / σ b) is small to improve its safety and reliability, but the utilization rate of the material is also low at this time.
(3) Elastic limit: the ability that the material will deform under the action of external force, but the ability to restore its original state after removing external force is called elasticity. The maximum stress that the metal material can maintain elastic deformation is the elastic limit, corresponding to the e point in the tensile test curve, expressed as σ e, the unit is MPa: in σ e=Pe/Fo formula, Pe is the maximum external force when keeping elasticity (or the load of the material with maximum elastic deformation).
(4) modulus of elasticity: this is the ratio of stress σ to strain δ (unit deformation corresponding to stress) within the elastic limit, expressed by E, where α is the angle between the Omure line on the tensile test curve and the horizontal axis Omurx in the formula of unit MPa: e = σ / δ = TG α. Elastic modulus is an index that reflects the rigidity of metal materials (the ability of metal materials to resist elastic deformation under stress is called rigidity).
The maximum capacity of a metal material to produce permanent deformation without failure under external force is called plasticity, which is usually expressed by the elongation δ (%) (elongation δ = [(L1-L0) / L0] x 100%) and section shrinkage ψ (%) of the specimen in the tensile test. This is the ratio of the difference between the standard distance length L1 and the original standard distance length L0 (growth) to L0 after the fracture of the sample is broken in the tensile test.
In the actual test, the elongation measured by the tensile specimens of the same material but with different specifications (diameter, section shape-such as square, circle, rectangle and gauge length) will be different, so special injection is generally needed. for example, for the most commonly used circular cross-section specimen, the elongation measured when the initial gauge length is 5 times the diameter of the sample is expressed as δ 5. The elongation measured when the initial gauge length is 10 times the diameter of the sample is expressed as δ 10.
Section shrinkage ψ = [(F0-F1) / F0] x100%, which is the ratio of the difference between the original cross-sectional area F0 and the minimum cross-sectional area F1 at the fine neck of the fracture surface (the reduction of cross-section) to F0 during the tensile test. In practice, the most commonly used circular cross-section specimens can usually be calculated by diameter measurement: ψ = [1-(D1/D0) 2] x100%, where: D0-the original diameter of the specimen; D1-the minimum diameter at the fine neck of the specimen after tension. The higher the values of δ and ψ, the better the plasticity of the material.