Explanation of terms of metal materials (VI): Mechanical properties

1. The concept of stress, the force on the unit cross-sectional area inside the object is called stress. The stress caused by the action of external force is called working stress, and the stress balanced in the interior of the object without the action of external force is called internal stress (such as tissue stress, thermal stress, residual stress left after the processing process, etc.).

2. Mechanical properties: the ability of metal to resist deformation and fracture when it is subjected to external force (load) under certain temperature conditions is called the mechanical properties of metal materials (also known as mechanical properties). There are many forms of loads borne by metal materials, which can be static load or dynamic load, including tensile stress, compressive stress, bending stress, shear stress, torsional stress, friction, vibration, impact and so on. Therefore, the indicators to measure the mechanical properties of metal materials mainly include the following:

2.1 strength

This represents the maximum ability of the material to resist deformation and failure under the action of external force, which can be divided into tensile strength limit（ σ b) Bending strength limit（ σ BB) compressive strength limit（ σ BC) etc. Since there is a certain rule for metal materials to follow from deformation to failure under the action of external force, tensile test is usually used for measurement, that is, the metal materials are made into samples of certain specifications and stretched on the tensile testing machine until the samples are broken. The strength indexes measured mainly include:

 (1) Strength limit: the maximum stress that the material can resist fracture under the action of external force. Generally, it refers to the tensile strength limit under the action of tensile force σ B refers to the strength limit corresponding to the highest point B in the tensile test curve. The common unit is MPa. 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 the metal material sample exceeds the elastic limit of the material, although the stress no longer increases, the sample still has obvious plastic deformation. This phenomenon is called yield, that is, when the material bears the external force to a certain extent, its deformation is no longer proportional to the external force and produces obvious plastic deformation. The stress at which yield occurs is called the yield strength limit σ S indicates that the point s 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, while for materials with low plasticity, there is no obvious yield point, 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 when the gauge length on the specimen produces 0.2% plastic deformation is usually specified as the conditional yield limit σ 0.2 indicates. 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, the required yield ratio (i.e σ s/ σ b) It should 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 still return to its original state after removing the external force is called elasticity. E is the point of maximum elastic deformation in the test curve, i.e. the tensile stress of the metal can be maintained σ E is expressed in megapascals (MPA): σ E = PE / fo, where PE is the maximum external force when maintaining elasticity (or the load when the material has maximum elastic deformation).

 (4) Elastic modulus: This is the stress of the material within the elastic limit σ And strain δ (unit deformation corresponding to stress), expressed in E, unit megapascal (MPA): e= σ/δ= tg α Where α Is the angle between the O-E line on the tensile test curve and the horizontal axis o-x. The elastic modulus is an index reflecting the rigidity of metal materials (the ability of metal materials to resist elastic deformation under stress is called rigidity)

2.2 plasticity

The maximum capacity of metal materials to produce permanent deformation without damage under the action of external force is called plasticity, which is usually based on the elongation of specimen gauge length during tensile test δ (%) and reduction of sample area ψ (%) elongation δ= [(l1-l0) / l0] X100%, which is the ratio of the difference (increase) between the gauge length L1 and the original gauge length l0 of the sample after the fracture of the sample is aligned during the tensile test.

In the actual test, the elongation measured by tensile specimens of the same material but different specifications (diameter, section shape – such as square, circular, rectangular and gauge length) will be different, so special notes are generally required. For example, the elongation measured when the initial gauge length of the most commonly used circular section specimen is 5 times the diameter of the specimen is expressed as δ 5. The elongation measured when the initial gauge length is 10 times the sample diameter is expressed as δ 10。 Reduction of area ψ= [(f0-f1) / F0] X100%, which is the ratio of the difference between the original cross-sectional area F0 of the sample after breaking and the minimum cross-sectional area F1 at the fine neck of the fracture (cross-sectional reduction) and F0 during the tensile test.

In practice, the most commonly used circular section sample can usually be calculated by diameter measurement: ψ= [1 – (D1 / D0) 2] X100%, where d0 is the original diameter of the sample; D1 – the minimum diameter at the fine neck of the fracture after the sample is broken. δ And ψ The higher the value, the better the plasticity of the material.

2.3 toughness

The ability of metal materials to resist failure under impact load is called toughness.

Impact test is usually used, that is, when a metal sample with certain size and shape is broken under impact load on a specified type of impact testing machine, the impact energy consumed per unit cross-sectional area on the fracture surface represents the toughness of the material: α K = AK / F unit: J / cm2 or kg · M / cm2, 1kg · M / cm2 = 9.8j/cm2 α K is called the impact toughness of metal material, AK is the impact energy, and F is the original cross-sectional area of fracture.

Fatigue strength limit metal materials under long-term repeated stress or alternating stress (stress is generally less than yield strength σ s) , the phenomenon of fracture without significant deformation is called fatigue failure or fatigue fracture, which is caused by a variety of reasons σ S is even greater than σ The stress (stress concentration) of B causes plastic deformation or microcrack in this part. With the increase of the number of repeated alternating stress, the crack gradually expands and deepens (stress concentration at the crack tip), resulting in the reduction of the actual cross-sectional area under stress at this part until the local stress is greater than σ B to produce fracture.

The maximum tensile stress or alternating stress that can be applied to the non-ferrous metal in the fatigue cycle (106 ~ 107 times) is the maximum tensile stress or alternating stress that can be applied to the non-ferrous metal σ- 1 means, in MPa.

In addition to the above five most commonly used mechanical property indexes, the following mechanical property indexes are also required for some materials with particularly strict requirements, such as metal materials used in aerospace, nuclear industry and power plants:

Creep limit: under a certain temperature and constant tensile load, the phenomenon that the material slowly produces plastic deformation with time is called creep. High temperature tensile creep test is usually used, that is, under constant temperature and constant tensile load, the creep elongation (total elongation or residual elongation) of the sample within the specified time or the maximum stress when the creep speed does not exceed a specified value at the stage of relatively constant creep elongation speed is taken as the creep limit, expressed in MPa, where τ Is the test duration, t is the temperature, δ Is elongation, σ Is the stress; Or expressed in, V is the creep rate.

High temperature tensile endurance strength limit: the maximum stress of the sample that reaches the specified duration without fracture under the action of constant temperature and constant tensile load, expressed in MPa, where τ Is the duration, t is the temperature, σ Is the stress.

Metal notch sensitivity coefficient: K τ Represents the stress ratio of notched specimen and smooth specimen without notch when the duration is the same (high temperature tensile endurance test): where τ Is the test duration, is the stress of the notched specimen and is the stress of the smooth specimen. Or expressed by: that is, under the same stress σ The ratio of the duration of the notched specimen to the duration of the smooth specimen.

Thermal resistance: the resistance of materials to mechanical loads at high temperatures.