How should secondary stress be understood?
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I saw an introduction to secondary stress: the allowable limit for secondary stress is based on periodic and fatigue fracture patterns; it does not depend on the stress level over a certain period of time, but rather on the range of alternating stresses and the number of cycles of variation. How should we understand this sentence? ①Why doesn’t it depend on the stress level? If a thermal pipeline is fixed at both ends with no compensation in the middle, the secondary stresses will definitely be very high, and these stresses cannot be compensated for through deformation. If the two fixing points are strong enough not to break, then the pipe will undergo plastic deformation and eventually break, right? Well, it’s related to the stress levels during that period. ②Why are there so many iterations? I took a look; the cycles required for fatigue failure are quite high. Even in the case of low-cycle fatigue, it’s 10 to the power of 4, which means 10,000 cycles. But does a cycle refer to the condition when it is cold during installation, becomes hot while in operation, and then returns to a cold state – counting that as one cycle (excluding pipes that experience vibration such as pumps and compressors for now)? If it is running stably, the temperature and the phase state of the medium should not change significantly. So, there’s probably a major overhaul every three years, and low-cycle fatigue also requires 10,000 cycles over three years. This sentence appears in many sources, so it should be correct; it’s likely my own understanding that is flawed. So where exactly is the mistake in my understanding? Thanks to everyone for your advice.
Is there no one? Is there anyone who wants to discuss this?
OP, we need to note that what is being explained here is the limit related to secondary stress. The concept of secondary stress limit was introduced to address another form of material failure (mainly fatigue failure), and it is related to the number of times stress is applied repeatedly as well as the mechanical properties of the material itself. Generally speaking, as long as there is no repeated loading, secondary stresses will not cause the pipeline to fail even if they lead to plastic deformation of the material. However, when the displacement load is extremely high, local yielding or slight deformation may be insufficient to meet the requirements of displacement constraints or the continuity of deformation itself, and in such cases the pipeline can fail after just one loading cycle (as in the first example you gave). The system conditions we have to deal with vary greatly – some have few cycles while others have many. To find a universal solution, it is necessary to adopt a strategy that accounts for every possible scenario; therefore, only some representative cycle values can be chosen to determine the secondary stress limit. The influence of the number of cycles on the secondary stress limit is expressed through the stress range reduction factor “f”, with different ranges of cycle counts corresponding to different “f” values. ）