The performance of metal materials plays a crucial role in determining their application range and the practicality of their usage. These properties are generally categorized into four main types: mechanical, chemical, physical, and technological properties.
Mechanical Properties
When an object is subjected to external forces, the internal resistance per unit area is referred to as stress. This can be further divided into working stress, which occurs due to external loads, and internal stress, which exists within the material without any external force applied. Internal stresses may include things like thermal stress, residual stress from manufacturing processes, or structural stress.
The mechanical properties of metals refer to their ability to withstand various types of loads under specific temperature conditions. This includes resisting deformation and failure when subjected to forces such as tension, compression, bending, shear, torsion, impact, vibration, and more. To evaluate these properties, several key indicators are commonly used.
Strength
Strength represents the maximum resistance a material has against deformation or failure under external forces. It can be measured in different forms, such as tensile strength, compressive strength, and bending strength. The most common method for measuring strength is through a tensile test, where a standardized sample is stretched until it breaks. Key measurements obtained from this test include:
(1) Tensile Strength: This is the maximum stress a material can endure before breaking. It is denoted by σb and corresponds to the peak point on the stress-strain curve. The unit is typically megapascals (MPa), with 1 MPa equal to 1 N/m² or approximately 0.1 Kgf/mm². The formula for tensile strength is σb = Pb / Fo, where Pb is the maximum load at fracture and Fo is the original cross-sectional area of the specimen.
(2) Yield Strength: When a material is subjected to stress beyond its elastic limit, it begins to deform plastically even if the stress remains constant. This point is known as the yield point, and the corresponding stress is called the yield strength (σs). For ductile materials, there is a clear yield point on the stress-strain curve, while for brittle materials, the yield point may not be obvious. In such cases, the conditional yield strength (σ0.2) is defined as the stress at 0.2% plastic deformation. This value is often used in design to ensure that parts do not undergo significant plastic deformation during operation.
(3) Elastic Limit: This is the maximum stress a material can withstand without permanent deformation. After the load is removed, the material returns to its original shape. The elastic limit is represented by σe and is calculated as σe = Pe / Fo, where Pe is the maximum load applied while the material remains in the elastic state.
(4) Elastic Modulus: Also known as Young’s modulus, it measures the stiffness of a material. It is the ratio of stress to strain within the elastic range and is expressed as E = σ / δ. A higher elastic modulus indicates greater rigidity, meaning the material resists elastic deformation more effectively.
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