Evaluation of pretension losses in heated screw joint

Abstract: The heated flow within the silencers of heavy transport vehicles can easily reach temperatures of hundreds of degrees Celsius. The thermal loads can cause problems for certain components on a silencer. In this case it has been seen in previous testing that screw joints attaching suspension brackets onto the silencer tend to lose pretension when heat is applied. The goal with this project is to investigate possible causes of the pretension losses and gain a deeper understanding of the joint’sbehavior. The project starts with a pre study with the purpose of obtaining knowledge of what is usually taken into consideration when designing joints for heated applications thatcould cause pretension losses. This was followed by an experimental part. Two main tests are made, firstly a friction test to ensure that the friction properties ofthe studs used in previous clamping force measurements were up to specification. Comparisons to a second batch of studs is also conducted. The second test is an oven test were test specimens of the joint is heat cycled in different temperatures for different assembly torques. This, in order to gain a deeper understanding of the joints´plastic behavior. The test pieces are cycled a total of five cycles and clamping force measurements are made after the first and fifth cycle. Measurements of clamping force are conducted through both ultrasonic and micrometer measurements of screw elongation. The results of the oven test are compared to the results of a finite element simulation. The finite element model is a model of the same test piece as in the tests, it captures the clamping force losses due to plastic deformation of components at heat up. However, the finite element model has its´ limitations, especially in the way the threads are modelled. This is to be taken into consideration and possibly utilized when interpreting the results. The takeaways from the pre study are that creep and stress relaxation, as well asembedding is highly unlikely primary causes for the pretension losses. More instant effects like plasticity due to the applied load is deemed more likely. The friction tests concluded that both of the batches´ friction properties are up to specification, but onethe batches thread friction is on the lower side.The results from oven test shows a temperature dependency in the losses and clea rsigns pointing towards a plastic collapse. All of the clamping force seems to be lost at initial heat up, no further signs of losses after 5 cycles. In comparison with the finite element model the plastic tendencies shows yet again, but most of the losses is not captured in the simulation. By taking into account the known weaknesses of the model, the two threaded components, weld nut and flange nut, are considered to be possible subjects of plastic deformation causing the pretension losses. The two measurement techniques show big deviations between them. It is therefore hard to interpret the absolute results of the pretension losses. In order to that a furtherstudy of how to value the two techniques should be conducted.