Discussion on primary stress and secondary stress

For specific details, please refer to the \"Code for Design of Industrial Metal Piping\" GB50316. Let me explain it roughly: Primary stress: The primary stress (SL) must not exceed the allowable stress (Sh) of the pipe material at the design temperature; that is, SL ≤ Sh. Secondary stress: SE ≤ Sa = f(1.25 SC + 0.25 Sh), where f represents the allowable stress taking into account the total number of cycles over the expected service life; SC is the allowable stress of the pipe material at 20°C, in MPa ； Sh—allow yield stress of the pipe material at the design temperature, MPa

Primary stress + secondary stress = total stress. In fact, in the world, stresses are not originally divided into primary and secondary stresses; it is merely for the convenience of studying (handling) problems that people classify stresses as such. It’s just quadratic. (Haha, it can actually be said that the concept of stress didn’t exist in the world originally; it was created by people for the convenience of studying (handling) problems.) It is recommended that the original poster read the book \"Stress Analysis of Pressure Pipelines\" by Tang Yongjin, specifically Section 2 of Chapter 5 (The origin of the criteria for checking secondary stresses in pipes). It is believed that after reading it, the original poster will gain a deeper understanding of primary and secondary stresses. Regarding the second question raised by the original poster, I suddenly remembered that for my undergraduate thesis, I used ANSYS to analyze a piping system. I took it out and checked it now, and realized that I was wrong from the analysis to the conclusion; I misinterpreted primary and secondary stresses once, twice, haha. From your second question, it seems you haven’t clearly distinguished between the concepts of primary stress, secondary stress, and allowable stress. Take Ansys as an example; after the pipe model is created. First, apply pressure, gravity, and other external loads to the model to determine the primary stresses; then, SL ≤ Sh can be used to verify these primary stresses ； Then, by applying displacement loads such as thermal expansion, thermal contraction, and end-point displacement to the model, the total stress will be obtained. By subtracting the primary stress obtained earlier from this total stress, the secondary stress can be determined. The secondary stress can then be checked using the formula SE ≤ Sa = ƒ(1.25 SC + 0.25 Sh). These are just my personal opinions; I welcome any corrections.

Thanks for the answer from the 3rd floor; I’ve learned it now