【Mechanical Equipment Technology Exchange Edition】Mechanical Equipment 【Daily Question】20190616

The main mechanical properties of commonly used metal materials are as follows: 1. Strength. Strength refers to the ability of a metal material to resist deformation and fracture under static loads. The strength index is generally expressed as the load, or force, per unit area, denoted by the symbol σ, with the unit being MPa. Common strength indices used in engineering include yield strength and tensile strength. Yield strength refers to the stress at which a metal material begins to yield under external forces, or the minimum stress value at which plastic deformation starts to occur; it is denoted by σs. Tensile strength refers to the maximum stress value that a metal material can withstand under tensile force before it breaks, and is denoted by σb. For most mechanical parts, plastic deformation is not allowed during operation; therefore, the yield strength serves as the basis for designing the strength of such parts ； For parts that fail due to fracture, tensile strength is used as the basis for their strength design. 2. Plasticity: Plasticity refers to the ability of a metal material to undergo plastic deformation under external forces without breaking. Common plasticity indicators used in engineering are elongation and reduction of area. Elongation refers to the percentage ratio of the elongation of a specimen after it is pulled to break to its original length, and is denoted by the symbol δ. The reduction of area refers to the ratio of the reduced area of the cross-section after the specimen is pulled apart to its original cross-sectional area, denoted by y. The greater the elongation and reduction in area, the better the plasticity ； Conversely, the lower the plasticity. Good plasticity is a necessary condition for metal materials to be processed under pressure, and it is also essential to ensure the safe operation of mechanical parts and prevent sudden brittle fracture. 3. Hardness: Hardness refers to the ability of a material’s surface to resist being pressed by an object that is harder than it. Hardness is an important mechanical property indicator of materials. Generally, the higher the hardness of a material, the better its wear resistance. The higher the strength of a material, the greater its resistance to plastic deformation, and the higher its hardness value as well. 4. Impact toughness: The ability of a metal material to resist impact loads is referred to as impact toughness, denoted by ak, with the unit being J/cm2. Impact toughness is commonly determined by a single pendulum impact bending test: the material to be tested is shaped into a standard impact specimen, which is broken by a pendulum in one strike; the impact energy AK required to break the specimen is measured, and then the impact toughness is expressed as the impact energy ak per unit cross-sectional area F at the notch of the specimen. The higher the AK value, the better the toughness of the material. Materials with a low AK value are called brittle materials, while those with a high AK value are called ductile materials. Many components, such as gears and connecting rods, are subjected to high impact loads during operation; therefore, they need to be made from materials with a high ak value. The AK value of cast iron is very low; the AK value of gray cast iron is close to zero, so it cannot be used to manufacture parts that are subject to impact loads.

(1) Yield strength (2) Tensile strength (3) Elongation after fracture (4) Reduction of area (5) Hardness (6) Impact toughness

(1) Strength: refers to the ability of a metal material to resist plastic deformation and failure under load； (2) Plasticity: refers to the ability of a metal material to deform under load without breaking, and to retain its deformed shape even after the load is removed ； (3) Hardness: refers to the ability of a metal material to resist the penetration of external objects into its surface ； (4) Toughness: refers to the ability of a metal material to resist impact forces without being damaged ； (5) Fatigue: The phenomenon of material fracture under variable loads. (6) Castability: Refers to a metal’s ability to produce satisfactory castings using casting methods ； (7) Formability: refers to the property of a metal to change its shape without cracking after being forged ； (8) Weldability: refers to the property of a metal material that determines whether it can be welded into high-quality joints using welding methods ； (9) Machinability: refers to the property of a metal regarding its ease of being cut by cutting tools.

(1) Yield strength (2) Tensile strength?? (3) Elongation after fracture (4) Reduction of area?? (5) Hardness (6) Impact toughness??

1. Strength refers to the ability of a metal material to resist failure (excessive plastic deformation or fracture) under static loads. Since loads can act in forms such as tension, compression, bending, and shear, strengths are also classified into tensile strength, compressive strength, bending strength, shear strength, etc. There is often a certain relationship among different strengths, and in practice, tensile strength is generally used as the most fundamental strength indicator. 2. Plasticity refers to the ability of metal materials to undergo plastic deformation (permanent deformation) under load without breaking. 3. Hardness: Hardness is an indicator used to measure the softness or hardness of metal materials. The most commonly used method for measuring hardness in current production is the indentation hardness test. In this method, a indenter of a specific geometric shape is pressed into the surface of the metal material under a certain load, and its hardness value is determined based on the degree of indentation. Commonly used methods include Brinell hardness (HB), Rockwell hardness (HRA, HRB, HRC), and Vickers hardness (HV). 4. Fatigue: The strength, plasticity, and hardness discussed earlier are all mechanical property indicators of metals under static loads. In fact, many machine parts operate under cyclic loads, and under such conditions the parts experience fatigue. 5. Impact toughness: A load that acts on a component at a very high speed is called an impact load, and the ability of metal to resist destruction under such impact loads is referred to as

Mechanical properties, also known as mechanical characteristics, include four main indicators: 1. Strength indicators, which are divided into tensile strength, yield strength, etc； 2. Hardness indicators include Brinell hardness, Rockwell hardness, and Vickers hardness, among others ； 3. Plasticity indicators, including reduction of area and elongation ； 4. Toughness index, which is impact toughness.

1. Strength: Strength refers to the ability of a metal material to resist deformation and fracture under static loads. The strength index is generally expressed as the load, or force, per unit area, denoted by the symbol σ, with the unit being MPa. Common strength indices used in engineering include yield strength and tensile strength. Yield strength refers to the stress at which a metal material begins to yield under external forces, or the minimum stress value at which plastic deformation starts to occur; it is denoted by σs. Tensile strength refers to the maximum stress value that a metal material can withstand under tensile force before it breaks, and is denoted by σb. For most mechanical parts, plastic deformation is not allowed during operation; therefore, the yield strength serves as the basis for designing the strength of such parts ； For parts that fail due to fracture, tensile strength is used as the basis for their strength design. 2. Plasticity: Plasticity refers to the ability of a metal material to undergo plastic deformation under external forces without breaking. Common plasticity indicators used in engineering are elongation and reduction of area. Elongation refers to the percentage ratio of the elongation of a specimen after it is pulled to break to its original length, and is denoted by the symbol δ. The reduction of area refers to the ratio of the reduced area of the cross-section after the specimen is pulled apart to its original cross-sectional area, denoted by y. The greater the elongation and reduction in area, the better the plasticity ； Conversely, the lower the plasticity. Good plasticity is a necessary condition for metal materials to be processed under pressure, and it is also essential to ensure the safe operation of mechanical parts and prevent sudden brittle fracture. 3. Hardness: Hardness refers to the ability of a material’s surface to resist being pressed by an object that is harder than it. Hardness is an important mechanical property indicator of materials. Generally, the higher the hardness of a material, the better its wear resistance. The higher the strength of a material, the greater its resistance to plastic deformation, and the higher its hardness value as well. 4. Impact toughness: The ability of a metal material to resist impact loads is referred to as impact toughness, denoted by ak, with the unit being J/cm2. Impact toughness is commonly determined by a single pendulum impact bending test: the material to be tested is shaped into a standard impact specimen, which is broken by a pendulum in one strike; the impact energy AK required to break the specimen is measured, and then the impact toughness is expressed as the impact energy ak per unit cross-sectional area F at the notch of the specimen. The higher the AK value, the better the toughness of the material. Materials with a low AK value are called brittle materials, while those with a high AK value are called ductile materials. Many components, such as gears and connecting rods, are subjected to high impact loads during operation; therefore, they need to be made from materials with a high ak value. The AK value of cast iron is very low; the AK value of gray cast iron is close to zero, so it cannot be used to manufacture parts that are subject to impact loads.

Strengths are classified into: tensile strength, compressive strength, bending strength, shear strength, etc.

Elastic limit, yield limit, strength limit. Elastic limit: It indicates the maximum extent to which a material can undergo purely elastic deformation. Yield limit: Used to indicate a material’s ability to resist minor plastic deformation. Strength limit: The maximum capacity of a material to resist external forces that cause damage.