What is residual stress
Residual stresses are those stresses that are locked into the component as a result of the manufacturing processes. Residual stresses therefore form the datum on which the applied loading stresses need to be superimposed. As is often the case, a highly detrimental situation may result if these stresses are additive.
Residual stresses generally arise when conditions in the outer layer of the material differ from those encountered internally. They are principally caused by mechanical, chemical, heat treatment, welding or casting processes and can result in residual stress values reaching yield. Residual stresses in welds, with magnitudes in the region of 200 MPa, are common. Origen has recently measured a 550 MPa residual stress in the weld of a 14mm butt welded high yield strength plate that was restrained during welding.
The relief of unfavourable residual stress is essential if
- the full working strength of the component is to be utilised,
- high machining tolerances are to be achieved; and
- stress corrosion cracking and fatigue are to be prevented, or at least significantly delayed
How Does Vibration Stress Relief (VSR) Work?
At present there are no conclusive theories fully explaining the underlying principles of VSR. Origen adheres to a synthesis of empirical procedures developed and refined during the forty-five years that VSR has been successfully employed in Europe and the USA.
In order to ensure the efficacy of our service, Origen has embarked on an extensive research programme to investigate the underlying principles of VSR.
The most current, internationally accepted theories relate the stress relief achieved by VSR to either
- localised overstressing of the material at or near resonance
- diffusionless phase changes within the material, or
- vibration induced dislocation movement.
What Is Resonance?
When a dynamic force, with a frequency similar or equal to the natural frequency of the system is applied to the system the system will resonate. When the system is in resonance the deflection caused by the applied force is large and incommensurate with the magnitude of the force and will, if not carefully controlled, cause the failure of the system. The systems response to the applied force is dependent on the stiffness of the system and the degree of damping present.
The natural frequencies of any system are dependent on its dimensions, stiffness and mass – as a result the natural frequencies may vary from component to component. Depending on the systems complexity it may have ten or more resonant frequencies.
How Is Resonance Recorded?
During the VSR process the resonant frequencies are indicated using an accelerometer that measures the dynamic forces associated with the vibration induced deflections of the component. The signal from the accelerometer is displayed on the meters of the VSR equipment and can, on request, be recorded on a chart recorder.
How Long Has VSR Been Operational World-Wide And In South Africa?
VSR has a long proven track record in Europe, USA, Britain, India and South Africa. Research into the field of VSR dates back to 1939. VCM 80/90 VSR systems, that preceded the VCM 905 that is used by Origen, have been in production for thirty years and were first introduced into South Africa in the late 80’s. VSR is now extensively used in Gauteng, Kwazulu Natal and Namibia.
Does VSR Affect Fatigue Life?
The number of cycles that the component undergoes during a normal VSR treatment does not normally significantly reduce its potential fatigue life. The gains attained by reducing the residual stress outweigh any minor reduction in the number of cycles to failure.
How Do You Know That VSR Works?
As is the case with TSR there is no simple visible sign that indicates that the residual stress has been relieved. There may be a slight change in the resonant frequency of the component indicating partial residual stress relief. Distortion of the component as a result of stress relief may in certain instances indicate stress relief. The extent of stress relief obtained with VSR is not always as great as that obtained with TSR and the associated distortion is usually reduced. As there are no reliable indicators of stress relief, one either has to rely on published accounts and prior experience of improved performance or measure the residual stress directly.
Origen has the ability to measure residual stress using an air abrasive centre hole drilling technique. To ascertain quantifiably how successful any stress relief process has been, measurements of the residual stress before and after the application of the stress relief can be made, at the client’s request.
Beside Time And Cost Are There Any Other Savings Offered By VSR?
Besides the direct time and cost savings offered by VSR over those of TSR, the indirect costs related to having stress relief carried out on site and reduced surface preparation are of great significance.