Fatigue test refers to the determination of σ-1 of metal material by metal material experiment, draws the S-N curve of the material, and then observes the phenomenon of fatigue failure and fracture, and then learns the method of measuring the fatigue limit of metal material under symmetrical cycle. The testing equipment generally has a fatigue testing machine and a vernier caliper.
Under the action of large enough alternating stress, micro-cracks may be induced due to large stress concentration in the shape of the metal member or the surface scratch or internal defect. Dispersed microscopic cracks will form macroscopic cracks after assembly and communication. The formed macro crack gradually expands slowly, and the cross section of the member is gradually weakened. When a certain limit is reached, the member suddenly breaks. The aforementioned failure phenomenon of metal due to alternating stress is called metal fatigue. A material with good plasticity under static load, when subjected to alternating stress, often breaks suddenly when the stress is lower than the yield limit without significant plastic deformation. Fatigue fractures are clearly divided into two regions: a smoother crack extension zone and a coarser fracture zone. After the crack is formed, the alternating stress causes the two sides of the crack to be closed when they are opened, and is repeatedly pressed and rubbed, and the smooth region is thus formed. The discontinuity and size of the load, leaving a number of crack fronts in the smooth zone. As for the rough fracture zone, it is the last sudden fracture. Statistics show that about 70% of the failure of mechanical parts is caused by fatigue, and most of the accidents caused are catastrophic. Therefore, it is practical to study the fatigue resistance of metal materials through experiments.
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