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The mechanical properties of metal materials mainly include strength and plasticity, hardness, impact toughness, and fatigue strength. The strength indicators obtained from testing the mechanical properties of metal materials can help us make better material selections in the field of mechanical processing, thereby meeting the requirements regarding the performance of components more effectively. I. Strength and strength indicators: Strength refers to the ability of a metal material to resist plastic deformation or fracture when subjected to static loads. The strength of metal materials is expressed in terms of stress, which can accurately reflect the stress conditions within the metal material. The internal force per unit area (the force within a material that counteracts external forces) is what we call stress. The strength index of metal materials is usually expressed by yield strength. When a metal material begins to yield, the stress at which plastic deformation occurs without an increase in load is known as the yield strength. The yield strength is further divided into upper yield strength and lower yield strength, denoted respectively as Reh and ReL. Generally, the components we use are not allowed to undergo significant plastic deformation. The higher the yield strength of a material, the greater its ability to resist plastic deformation, and the better its performance will be. Therefore, yield strength is the key factor we rely on when designing components and selecting materials. II. Plasticity and Plasticity Indices Plasticity refers to the ability of a metal material to undergo maximum plastic deformation before fracturing under load. Plasticity indices are usually expressed as elongation after fracture and reduction of area. The elongation at break is denoted by A, and it is the percentage of the difference between the gauge length after the specimen breaks and the original gauge length, relative to the original gauge length. The reduction in cross-sectional area is denoted by Z, and it is the percentage of the difference between the original cross-sectional area of the specimen and the minimum cross-sectional area at the fracture site, relative to the original cross-sectional area. Among them, compared with elongation, reduction of area reflects the plasticity of metal materials more accurately. Because the reduction of area is not affected by the size of the specimen. The greater the reduction in cross-sectional area and the elongation, the better the plasticity of this metal material, making it easier to shape it and to produce components with complex geometries. Materials with good plasticity will first undergo plastic deformation when subjected to significant impact forces, rather than breaking suddenly, thus offering safer performance in use. III. Hardness and hardness testing: Hardness refers to the ability of a metal material to resist local plastic deformation and failure. There are various methods for hardness testing, with the most commonly used being Brinell hardness testing, Rockwell hardness testing, and Vickers hardness testing. (1) Brinell hardness testing method ① Principle of Brinell hardness testing. A cemented carbide ball is used as the indenter, which is pressed into the metal surface at a certain test pressure; after holding it in place for a specified time, the test force is removed, and finally the diameter of the indentation surface is measured. The hardness value is then calculated using a mathematical formula. Experiments show that the value of Brinell hardness is only related to the diameter of the indentation. The larger D is, the larger the indentation diameter, which means the lower the hardness. ②Characteristics and application range of Brinell hardness. The testing force and indentation diameter for Brinell hardness are both relatively large, so the indentation is also large, allowing for a more direct and accurate determination of the hardness value. However, due to the relatively large indentation, the degree of damage to the metal surface is also high; this means that the Brinell hardness test is not suitable for measuring the surface quality of components with high requirements or thin-walled components. (2) Rockwell hardness testing method ① Principle of Rockwell hardness testing. The principle of the Rockwell hardness test is similar to that of the Brinell hardness test; the difference lies in the use of a diamond cone indenter instead of a spherical cemented carbide indenter, with the hardness value being determined based on the depth of the indentation. The common Rockwell hardness scales are A, B, and C, with scale C being the most frequently used. Hardness values measured on different scales cannot be compared with each other; they must be converted to the same scale before comparison is possible. ②Characteristics and application range of Rockwell hardness. Rockwell hardness differs from Brinell hardness in that it produces a smaller indentation, resulting in less damage to the components. Therefore, it can be used to measure thin-walled components as well as those with high requirements regarding surface quality. However, due to the small size of the indentation, there are certain limitations, and the hardness values obtained are not entirely accurate. (3) Vickers hardness testing: The difference between Vickers hardness testing and Brinell hardness testing is that the indenter is a diamond regular tetrahedron, and the hardness value is calculated based on the length of the diagonals of the indentation mark. The testing force applied in the Vickers hardness test is relatively low, resulting in indents that are not very distinct; therefore, it is possible to measure thin-walled components, but measuring the diagonals of the indentations is rather complex. IV. Impact Toughness and Fatigue Strength: In the actual processing and use of metal materials, the loads they are subjected to are not limited to static loads; there are also impact loads and alternating loads, or a combination of various types of loads. It is far from sufficient to study static loads only through tensile tests and hardness tests. Therefore, it is necessary to investigate the mechanical properties of metal materials under various types of loads, which underscores the great importance of studying impact loads and fatigue loads. (1) Impact toughness, as the name implies, refers to the ability of a metal material to resist impact loads without suffering damage. Impact toughness is primarily measured through bending tests. For the measurement of impact force, we mainly rely on the value of impact toughness. When metal materials are subjected to high-energy impact loads, their resistance to impact depends primarily on their impact toughness; whereas under repeated impact loads, their resistance to impact is determined mainly by the strength and plasticity of the material. (2) Fatigue strength: In daily life, we often observe this phenomenon – many mechanical components, even though the forces they are subjected to are below the material’s yield strength, still break after operating for a long time. This phenomenon is known as metal fatigue, and fatigue failure is one of the main causes of component damage. The fatigue strength of metal materials is determined by various factors, such as the operating conditions of the components, the internal structure of the material, its microstructural composition, and the stresses it is subjected to. Therefore, improving the structure of the components and employing certain surface strengthening methods can both enhance the fatigue strength of metal materials