Chapter II Mechanism of Vibration stress relief

Vibration stress relief, also known as vibratory stress relief(VSR), is a technology, with which vibration treatment is done to workpieces (castings, forgings, welding structure, etc. included) for a certain time at its natural frequency and it eliminates and homogenizes the residual stress, making it more stable for the size and precision of the pieces. The technology boasts such features as low energy consumption, short period and significant effect. It has been rapidly developed and widely applied both in China and other countries.

A large number of practices in China and other countries have proven that Vibration stress relief is helpful to eliminate and homogenize the residual stress as well as stabilize the dimensional precision. In order to decrease and homogenize the internal stress within the piece itself and keep the precision of the size, the measures adopted in production fall in the following two categories. The first one is to relieve internal stress as much as 30% to 80% in general, such as heat aging, during which the workpiece is heated to a certain temperature, kept for some time at the temperature and then gradually cooled down to the room temperature. The second one is to homogenize part of the internal stress, thus increasing the relaxation rigidity of the piece such as natural aging and load treatment. And the mechanism of Vibration stress relief is the combination of the above two measures, by which it means that not only does it relieve and homogenize the internal stress (decreasing it to 30% to 60%), but efficiently increase the relaxation rigidity, thus reinforcing resistance to dynamic load deformation. The mechanism of vibration can be comprehensively analyzed from the following perspectives:

From the microcosmic perspective, Vibration stress relief can be considered as an additional dynamic stress imposed on the workpiece by cyclic loading. It is well known that the materials utilized in projects are not ideal elastomers, with all kinds of internal micro-defects. Therefore, there are varying degrees of stress concentration near the microscopic defects within steel, cast iron or other metals.When the piece is subjected to vibration, the alternative stress imposed on it and the residual stress within overlap. When the overlapping part of these two stresses reaches a certain value, the part where the stress concentration at the site is the most intense will came into plastic deformation as a result of exceeding the material’s yield limit. The deformation will bring down the residual peak stress of the place and thus enhance the metal body. Afterwards, vibration plays the same role in some places where stress concentration is relatively intense, until the overlap of the additional stress and the residual stress can not bring any further plastic deformation. At that time, vibration can no more play the role in relieving and homogenizing the residual stress and reinforcing the metal. The above explanation has been proven to be true by a great quantity of tests.

From the macro perspective, Vibration stress relief do the plastic deformation to the workpieces, decreasing and homogenizing the residual stress and increasing the ability to resist deformation, which is no doubt the fundamental factor for stabilizing the dimensional precision of the piece. From analyzing the residual stress relaxation and the deformation, we can tell that the existence of residual stress and its instability lead to stress relaxation and redistribution, hence making the plastic deformation of the piece. Therefore, heat aging are usually employed to relieve and reduce residual stress, especially the dangerous peak stress. Vibration stress relief can reduce residual stress as well. After vibration processing, the residual stress in the workpiece will decrease by 30% to 60% in general and the peak stress will decrease at the same time, making the stress distribution homogenized. ..............

Learn more.........