There are number of different methods and procedures in vibration analysis, where the natural frequencies of the specimen or the system are one of the key parameters. It is known that these frequencies can change under load, for example in response to pre-stressing, but the effect of residual stresses is less known. By developing a suitable method, natural frequencies can be used to predetermine residual stress, therefore this method can be used for example predicting whether it will cause deformation during machining of a part, whether it requires increased attention or how to set the parameters well for vibratory stress relief. The results can be significant cost and time savings, as well as the improvements of the quality. Natural frequency is the frequency of free vibration of an undamped linear vibration system, or in other words at which a system left alone will vibrate after excited by an external force [1]. Metal castings or welded structures may have several natural frequencies which appear as frequency bands or ranges on the measurement images. Based on these, to determine the natural frequency of a component or system, we need to excite a frequency as close as possible to the natural frequency for the resonance to occur. When the resonance is reached, the amplitude of the system is at its maximum, and the natural frequencies of the workpiece can be measured. Traditionally, sensors, usually accelerometers are used to measure the natural frequency. The continuous development of information technology has made it possible to replace these sensors with an acoustic diagnostic system. During this research, we have developed an acoustic diagnostic system and procedure, which can generate the acoustic measurement images. We have evaluated the measurement images in many ways, and many different types of components and materials (mostly iron alloys) were analyzed. In addition, the changes of natural frequencies show a similar pattern in the case of parts before treating with vibratory stress relief as for load tests.

In this study, the natural frequency changes, induced by vibratory stress relief, were analysed. Vibratory stress relief (VSR) is a possible alternative to thermal stress relief (TSR), which can be applied to welded structures and castings to reduce residual stresses. In this paper, acoustic-based natural frequency measurements will be presented to investigate the effect of VSR treatment. Natural frequency measurements were taken on large disc-type grey cast iron components, before and after the VSR treatment and we have analysed the results for five similar castings from the same batch. The natural frequencies change when the stress state of the workpiece changes. Changes in the frequencies have been examined, and we have identified a pattern for a specific treatment parameter setting. Measurement images – spectrograms - were generated to visualize the frequency response, where stress-induced variations appeared as shifts in resonance bands. Across all five tested components, a consistent decrease in dominant natural frequencies was observed. While the results mostly align with theoretical expectations, they also highlight the complex, time-dependent behaviour of residual stress redistribution. Further long-term monitoring is recommended to clarify post-treatment evolution and material-specific responses. The results can open new directions for the automotive and heavy industry to effectively reduce residual stresses and monitor the performance of the workpieces.