1) The main failure form: the failure of mechanical parts to complete the specified function within the specified time and under the specified conditions is called failure. The main failure modes of mechanical parts are as follows:

① The whole is broken. Under the action of the load, the fracture of the part caused by the stress on the dangerous section greater than the ultimate stress of the material is called the overall fracture, such as the fracture of the bolt, the fracture of the gear teeth, the fracture of the shaft, etc.

② Excessive elastic deformation or plastic deformation. Mechanical parts deform elastically when they are loaded. When the elastic deformation exceeds the allowable range, the part or machine cannot work properly.

③ The surface of the parts is damaged. Surface damage is a failure that occurs on the working surface of mechanical parts, and is divided into wear, pitting and corrosion. Wear is the phenomenon of loss or transfer of material on the surface of a part due to friction during the relative movement of two contact surfaces. Pitting corrosion is a localized fatigue failure phenomenon that occurs on the surface of a part under the action of variable contact stress. Corrosion is an electrochemical or chemical attack that occurs on metal surfaces.

④ Failure caused by disrupting normal working conditions. Some parts can only work properly under certain working conditions, and if these necessary conditions are violated, different types of failures will occur. 2) Design criteria: In order to avoid the failure of mechanical parts, the mechanical parts should be designed with sufficient working capacity. For various failure modes of parts, corresponding calculation criteria are proposed respectively. Commonly used calculation criteria are: strength criterion, stiffness criterion, service life criterion and vibration stability criterion.

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