Ferrous metals, also known as iron and steel materials, include industrial pure iron with a total impurity content of less than 0.2% and a carbon content of not more than 0.0218%, steel with a carbon content of 0.0218% and 2.11%, and cast iron with a carbon content of more than 2.11%. In a broad sense, ferrous metals also include chromium, manganese and their alloys.
(2) Nonferrous metals refer to all metals and their alloys except iron, chromium and manganese, which are usually divided into light metals, heavy metals, precious metals, semi-metals, rare metals and rare earth metals, etc. the strength and hardness of non-ferrous alloys are generally higher than those of pure metals. and the resistance is high, the temperature coefficient of resistance is low.
(3) Special metal materials include structural metal materials and functional metal materials with different uses. Among them, there are amorphous metal materials obtained by rapid condensation process, as well as quasicrystal, microcrystalline and nanocrystalline metal materials, as well as special functional alloys such as stealth, hydrogen resistance, superconductivity, shape memory, wear resistance, vibration damping and metal matrix composites.
Many mechanical parts and engineering components work under alternating loads. Under the action of alternating load, although the stress level is lower than the yield limit of the material, the sudden brittle fracture will occur after repeated stress cycling for a long time, which is called the fatigue of metal materials.
The fatigue fracture characteristics of metal materials are as follows:
1 the load stress is alternating.
2 the action time of the load is longer.
(3) the fracture occurs instantly.
(4) both plastic and brittle materials are brittle in the fatigue fracture zone.
Therefore, fatigue fracture is the most common and dangerous fracture form in engineering.
The fatigue phenomena of metal materials can be divided into the following categories according to different conditions:
1 High cycle fatigue: refers to fatigue with a stress cycle of more than 100000 under the condition of low stress (the working stress is lower than the yield limit of the material, or even lower than the elastic limit). It is the most common kind of fatigue failure. High cycle fatigue is generally referred to as fatigue.
2 low cycle fatigue: it refers to the fatigue under the condition of high stress (the working stress is close to the yield limit of the material) or high strain, the number of stress cycles is less than 10000mm 100000. Because the alternating plastic strain plays a major role in this kind of fatigue failure, it is also called plastic fatigue or strain fatigue.
Thermal fatigue: refers to the fatigue damage caused by the repeated action of thermal stress caused by temperature changes.
Corrosion fatigue: refers to the fatigue damage of machine parts under the joint action of alternating load and corrosion medium (such as acid, alkali, sea water, active gas, etc.).
5 contact fatigue: this refers to the contact surface of machine parts, under the repeated action of contact stress, there are pitting spalling or surface crushing and spalling, resulting in failure and damage of machine parts.