【Daily Discussion】Utilities - Civil Engineering - 20210421

Mechanical properties are the most important performance characteristics of steel, including tensile strength, plasticity, toughness, and hardness. (1) Tensile strength. The indicators that represent the tensile properties of steel include yield strength, tensile strength, yield-to-tensile ratio, elongation, and reduction of area. Yield refers to the phenomenon in which, during the tensile testing of a steel specimen, the load stops increasing while the specimen continues to deform. The minimum stress at which yield occurs is known as the yield point or yield strength; in structural design, the yield strength is generally used as the basis for design. Tensile strength refers to the ratio of the maximum load that a specimen can withstand before breaking, during stretching, to the original cross-sectional area of the specimen. The ratio of the yield point (yield strength) of steel to its tensile strength is called the yield strength ratio. The higher the yield ratio, the greater the reliability of structural components. Generally, the yield ratio for carbon steel is 0.6–0.65, for low-alloy structural steel it is 0.65–0.75, and for alloy structural steel it is 0.84–0.86. Elongation refers to the percentage increase in the length of the gauge section of a metal material after it is pulled and breaks, compared to its original gauge length ； The reduction of area refers to the percentage by which the maximum decrease in the cross-sectional area at the necking region of a metal specimen after it is pulled apart, compared to its original cross-sectional area. The greater the elongation and reduction of area, the better the plasticity of the steel. (2) Cold bending performance. Cold bending strength refers to the ability of steel to resist bending deformation at room temperature, and it indicates the plasticity of the steel under harsh conditions. After the steel is bent at a specified bending angle a and a bend center diameter d, it is evaluated by checking for cracks, delamination, or fractures on the outer and side surfaces of the bent area. Cold bending can reveal defects such as internal stress and impurities in steel, and it can also be used to inspect the welding quality of steel, identifying defects like cracks and impurities on the bent surface of the welded parts. (3) Impact toughness. Impact toughness refers to the ability of steel to resist impact loads without breaking. In engineering, the single pendulum impact bending test is commonly used to determine a material’s ability to resist impact loads, that is, to measure the impact energy Ak required for the specimen to break under such loads, with the unit being joules (J). The impact toughness of steel is an indicator for assessing the quality of steel; it is particularly important to evaluate the impact toughness of components that are frequently subjected to load impacts, such as heavy crane girders. The greater the impact toughness, the better the impact resistance of the steel. (4) Hardness. Hardness refers to a metal’s ability to resist penetration by hard objects into its surface. It is not a mere physical quantity; rather, it is a comprehensive performance indicator that reflects properties such as elasticity, strength, and plasticity. Methods for expressing hardness include Brinell hardness, Rockwell hardness, Vickers hardness, and Shore hardness. The most commonly used method is Brinell hardness, which involves using a sphere of a certain diameter (either a steel ball or a cemented carbide ball) to apply a specified testing force to the surface of the specimen. After a defined holding time, the testing force is removed, and the hardness value is calculated by measuring the diameter of the indentation left on the surface. (5) Fatigue failure. Under alternating stress, steel fails suddenly at stresses far below its tensile strength under static load, or even at stresses below its yield strength under static load; this type of failure is known as fatigue failure. The stress parameter for fatigue failure of steel is expressed as fatigue strength (or fatigue limit), which refers to the maximum stress value under which a specimen can withstand alternating stresses without experiencing fatigue failure. Generally, fatigue strength is defined as the maximum stress that a steel material can withstand without failing under alternating loads for 1×107 cycles. When designing structures that are subjected to alternating loads and require fatigue analysis, it is necessary to understand the fatigue strength of the steel used.

A. Cold drawing rate B. Cold bending performance C. Elastic retraction rate D. Yield strength E. Tensile strength