1. The concept of stress, the force on the internal cross-sectional area of an object is called stress. The stress caused by the external force is called the working stress, and the stress that is balanced inside the object without the external force is called the internal stress (for example, the tissue stress, the thermal stress, the residual stress remaining after the machining process ends, etc.).

2. Mechanical properties, the ability of metal to withstand external forces (loads) under certain temperature conditions, the ability to resist deformation and fracture is called the mechanical properties of metal materials (also known as mechanical properties). Metal materials can be loaded in a variety of forms, from static loads to dynamic loads, including tensile stress, compressive stress, bending stress, shear stress, torsional stress, and friction, vibration, and stress alone or simultaneously. Shock and so on.

The mechanical properties of metal materials are the main basis for the design and selection of parts. Different applied load properties (such as tensile, compression, torsion, impact, cyclic loading, etc.), the mechanical properties required for metallic materials will also be different. Common mechanical properties include: strength, plasticity, hardness, impact toughness, multiple impact resistance and fatigue limit.