The maximum ability of metallic materials to produce permanent deformation without damage under the action of external force is called plasticity, usually based on the elongation δ (%) of the gauge length of the sample and the shrinkage of the sample section ψ (%) during the tensile test. =[(L1-L0)/L0]x100%, this is the difference between the gauge length L1 and the original gauge length L0 of the sample (increase) after the sample is broken during the tensile test. The ratio to L0. In the actual test, the measured elongation of the tensile specimens of the same material but different specifications (diameter, cross-sectional shape-such as square, round, rectangular and gauge length) will be different, so it is generally necessary to add special notes, such as For the most commonly used round cross-section specimens, the elongation measured when the initial gauge length is 5 times the sample diameter is expressed as δ5, and the elongation measured when the initial gauge length is 10 times the specimen diameter is expressed as δ10 . The reduction of area ψ=[(F0-F1)/F0]x100%, which is the difference between the original cross-sectional area F0 and the minimum cross-sectional area F1 at the neck of the fracture after the sample is broken during the tensile test (the reduction in cross-section) and F0 Ratio. In practice, the most commonly used round cross-section samples can usually be calculated by diameter measurement: ψ=[1-(D1/D0)2]x100%, where: D0- original diameter of the sample; D1- fracture after the sample is broken The smallest diameter at the neck. The larger the value of δ and ψ, the better the plasticity of the material.
The ability of metal materials to resist damage under impact load is called toughness. The impact test is usually used, that is, when a metal sample of a certain size and shape is subjected to an impact load on a specified type of impact testing machine and broken, the impact energy consumed per unit cross-sectional area on the fracture characterizes the toughness of the material: αk=Ak/ The unit of F is J/cm2 or Kg·m/cm2, and 1Kg·m/cm2=9.8J/cm2αk is called the impact toughness of metal materials, Ak is the impact energy, and F is the original cross-sectional area of the fracture. 5. Fatigue strength limit metal material under long-term repeated stress or alternating stress (the stress is generally less than the yield limit strength σs), the phenomenon of fracture without significant deformation is called fatigue failure or fatigue fracture, which is due to A variety of reasons cause local stresses (stress concentration) greater than σs or even greater than σb on the surface of the part, causing plastic deformation or microcracks in this part. As the number of repeated alternating stresses increases, the cracks gradually expand and deepen (crack tip Stress concentration at the local area) causes the actual cross-sectional area of the local stress to be reduced, until the local stress is greater than σb and fracture occurs. In practical applications, the sample is generally subjected to repeated or alternating stress (tensile stress, compressive stress, bending or torsion stress, etc.) within a specified number of cycles (usually 106 to 107 times for steel, and for non-ferrous metals Take 108 times) The maximum stress that can be withstood without fracture is taken as the fatigue strength limit, expressed by σ-1, in MPa. In addition to the above five most commonly used mechanical performance indicators, for some particularly strict materials, such as metal materials used in aerospace, nuclear industry, and power plants, the following mechanical performance indicators are also required: Creep limit: Under temperature and constant tensile load, the phenomenon that materials slowly produce plastic deformation over time is called creep. High temperature tensile creep test is usually used, that is, the creep elongation (total elongation or residual elongation) of the sample within a specified time under constant temperature and constant tensile load, or relatively constant creep elongation rate The maximum stress when the creep speed does not exceed a certain specified value is used as the creep limit, expressed in MPa, where τ is the test duration, t is the temperature, δ is the elongation, and σ is the stress; or Indicated by, V is the creep speed. High-temperature tensile endurance strength limit: The maximum stress that the sample can reach the specified duration without breaking under the action of constant temperature and constant tensile load, expressed in MPa, where τ is the duration, t is the temperature, and σ Is stress. Metal notch sensitivity coefficient: Kτ is the ratio of stress between a notched sample and an unnotched smooth sample when the duration is the same (high temperature tensile endurance test): where τ is the test duration, which is notched test Such stress is the stress of a smooth specimen. Or it can be expressed as: under the same stress σ, the ratio of the duration of the notched specimen to the duration of the smooth specimen. Heat resistance: the resistance of a material to mechanical loads at high temperatures.