Mechanical properties of metallic materials refer to the various properties exhibited under the action of external forces, which determine the suitability and durability of the materials in different environments. The following are the main indicators of mechanical properties of metallic materials:
1. Strength and yield strength
Strength is the ability of a metallic material to resist the action of an external force (load) and avoid excessive plastic deformation or fracture. It is the most basic and one of the most important indicators of the mechanical properties of metal materials. Strength can be divided into various types, including tensile strength, compressive strength, flexural strength and shear strength, among which tensile strength is the most commonly used.
- Tensile strength (σb or Rm): refers to the maximum value of stress that a material can withstand during tension, i.e., the maximum stress that can be achieved before pulling off. It reflects the ability of the material to resist fracture, symbolised as Rm or σb, and unit is MPa (megapascals). Tensile strength is an important parameter for evaluating the toughness and plastic deformation capacity of a material.
- Yield strength (σs or ReL, Rp0.2): refers to the material in the tensile process, when the stress reaches a certain critical value, even if the stress is no longer increased, the material will continue to undergo significant plastic deformation of the stress value. For materials with obvious yielding phenomenon, yield strength is the yield point of stress; for materials without obvious yielding phenomenon, it is usually stipulated to produce 0.2% residual deformation of the stress value as its yield limit, known as the condition of yield strength. Yield strength is the beginning of plastic deformation of the material is an important symbol, but also the structural design of the commonly used strength indicators.
2. plasticity and elongation
Plasticity refers to the ability of a metallic material to produce significant plastic deformation without fracture when subjected to external forces. A material with good plasticity can absorb a large amount of energy through plastic deformation, thus improving its ability to resist impact and fatigue.
- Elongation (δ): The percentage of the total elongation of a material after tensile rupture in relation to the original marked length. It is an important indicator of the plasticity of a material.Engineering usually δ ≥ 5% of the material is called plastic materials, such as mild steel, aluminium, copper, etc.; and δ ≤ 5% of the material is called brittle materials, such as cast iron, glass, ceramics, etc..
- Section shrinkage (ψ): refers to the percentage of the maximum reduced area of the section to the original fracture area of the material after tensile fracture. It is also an important index to measure the plasticity of the material. Together with the elongation, it can comprehensively evaluate the plastic deformation ability of the material.
3. Hardness
Hardness is the ability of a material to resist other harder objects pressed into its surface. It is an indicator of the degree of hardness and softness of the material, and is also an important parameter reflecting the wear resistance and cutting performance of the material.
- Brinell hardness (HBS, HBW) and Rockwell hardness (HRA, HRB, HRC): are two commonly used hardness testing methods.Brinell hardness is applicable to softer materials, while Rockwell hardness is applicable to harder materials. The hardness test can be used not only to evaluate the mechanical properties of the material, but also to check the quality of the surface layer of the material, such as decarburisation and carburisation.
4. Impact toughness
Impact toughness is the ability of a material to resist impact loads. It is an important indicator of the fracture resistance of materials under dynamic loading.
- Impact toughness value (Ak): usually in joules / square centimetre (J / cm²) as a unit, indicating the ability of the material to absorb energy under the action of impact load.A material with good impact toughness has a high resistance to impact fracture and is suitable for applications where impact loads need to be applied.
5. Modulus of elasticity
The modulus of elasticity is the ratio of stress to strain during the elastic deformation phase of a material. It is an important measure of the stiffness of a material.
- Modulus of elasticity (E): usually expressed in Pascals (Pa) or Gigapascals (GPa). For common metallic materials, such as steel, the modulus of elasticity is typically between 200-210 GPa. Materials with a high modulus of elasticity also have high stiffness and high resistance to elastic deformation.
6. Fracture toughness
Fracture toughness is the ability of a material to resist crack extension when it contains cracks. It is an important indicator of a material's resistance to brittle fracture.
- Fracture toughness (KIC): indicates the stress intensity factor of the material under plane strain conditions, when the crack begins to expand. Materials with high fracture toughness also have strong resistance to brittle fracture and are suitable for applications that require high stress or low temperature environments.
7. Fatigue strength
Fatigue strength is the ability of a material to resist fatigue damage under alternating loads. It is an important indicator of the long-term service life of a material.
- Fatigue limit (σ-1): indicates the maximum stress value of a material under infinite times of alternating load without fatigue damage. Materials with high fatigue strength also have a long long-term service life and are suitable for occasions where they need to withstand alternating loads.
Mechanical property indicators of metal materials include strength, plasticity, hardness, impact toughness, modulus of elasticity, fracture toughness and fatigue strength.These indicators together determine the applicability and durability of metal materials in different environments. In practical applications, it is necessary to select the appropriate metal materials according to the specific requirements of use, in order to achieve the best product quality and use effect.
