Metal material characteristics
Many mechanical parts and engineering components work under alternating loads. Under the action of alternating loads, although the stress level is lower than the yield limit of the material, after a long period of repeated stress cycles, sudden brittle fracture will occur. This phenomenon is called fatigue of metal materials. The characteristics of fatigue fracture of metal materials are:
(1) The load stress is alternating;
(2) The action time of the load is longer;
(3) The fracture occurs instantaneously;
(4) Whether it is a plastic material or a brittle material, it is brittle in the fatigue fracture zone. Therefore, fatigue fracture is the most common and dangerous form of fracture in engineering.
The fatigue phenomenon of metal materials can be divided into the following types according to different conditions:
(1) High cycle fatigue: refers to fatigue with stress cycle cycles above 100,000 under low stress (working stress is lower than the yield limit of the material, or even lower than the elastic limit). It is the most common type of fatigue failure. High cycle fatigue is generally referred to as fatigue.
(2) Low cycle fatigue: refers to fatigue under high stress (working stress is close to the yield limit of the material) or high strain conditions, the number of cycles of stress is below 10,000 to 100,000. Since alternating plastic strain plays a major role in this fatigue failure, it is also called plastic fatigue or strain fatigue.
(3) Thermal fatigue: refers to the fatigue damage caused by the repeated action of thermal stress caused by temperature changes.
(4) Corrosion fatigue: refers to the fatigue damage of machine parts under the combined action of alternating loads and corrosive media (such as acid, alkali, seawater, active gas, etc.).
(5) Contact fatigue: This refers to the contact surface of the machine parts, under the repeated action of contact stress, pitting or surface crushing and peeling occur, resulting in failure and destruction of the machine parts.
'Plasticity' refers to the ability of metal materials to produce permanent deformation (plastic deformation) without being destroyed under the action of external force. When a metal material is stretched, its length and cross-sectional area will change. Therefore, the plasticity of a metal can be measured by two indicators: the elongation of the length (elongation) and the shrinkage of the section (reduction of area).
The greater the elongation and reduction of area of a metal material, the better the plasticity of the material, that is, the material can withstand greater plastic deformation without breaking. Generally, metal materials with elongation greater than 5% are called plastic materials (such as low carbon steel, etc.), and metal materials with elongation less than 5% are called brittle materials (such as gray cast iron, etc.). A material with good plasticity can produce plastic deformation in a large macroscopic range, and at the same time as plastic deformation, the metal material is strengthened by plastic deformation, thereby improving the strength of the material and ensuring the safe use of parts. In addition, materials with good plasticity can be smoothly processed by certain forming processes, such as stamping, cold bending, cold drawing, and straightening. Therefore, when selecting metal materials as mechanical parts, certain plasticity indexes must be met.