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What are the general problems in obtaining the yield strength of materials?
There are the following problems in calculating the yield strength of materials in general: 1. Confuse the yield point of metal material with that of plastic. Because the properties of metal materials and plastics are very different, their definitions of yield are also different. For example, metal materials have the concepts of yield, upper yield and lower yield. And plastic is only defined by the concept of yield. In addition, the yield strength of metal materials must be less than the ultimate strength, while the yield strength of plastics may be less than or equal to the ultimate strength (both are the same point on the curve). Because of the unfamiliarity with the standard, some mistakes often appear in the output of test results, such as taking the plastic yield concept (upper yield) as the yield concept of metal materials (generally lower yield), or comparing the maximum strength of unyielding metal materials to the yield value of metal materials according to the definition of plastic yield strength, resulting in a joke that the yield value of metal materials is consistent with the maximum value. To sum up, the yield value plays a very important role in the mechanical properties test of materials, but there are also many problems in the calculation. Therefore, whether it is the national standard formulation department, the research and development manufacturer of testing machine or the user department of testing machine, they should try their best to solve the existing problems from their own perspective, so as to calculate the yield point accurately, quickly and conveniently and create good conditions for the safe use of materials. 2. Taking the trend of discontinuous yield as the definition of yield in the national standard with yield point, it is pointed out that when deformation continues and the force remains constant or fluctuates, it is called yield. However, in some materials, this phenomenon will occur. Although the deformation continues and the force value continues to increase, the increase range of the force value is from large to small and then to large. From the curve, it is a bit like the trend of yield, which does not conform to the definition of constant force value at yield. In the third influence, it is mentioned that there is no quantitative index for the condition of "constant force value", so there is often a debate about whether this phenomenon is yield and how to get the yield value. 3. Confuse non-proportional stress and yield. Although non-proportional stress and yield are both indicators of the transition state between elastic and plastic stages of materials, they are essentially different. Yield is the inherent property of materials, and non-proportional stress is the result of artificially specified conditions. Non-proportional stress does not need to be calculated when the material has a yield point, but only when the material has no obvious yield point. Some testers don't understand this deeply, and think that yield point, upper yield, lower yield and non-proportional stress exist in every test and need to be calculated.
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