Vibration stress relieving, Vibration stabilization and Relaxation of stress

Residual stresses, by definition, are those stresses in an elastic body that is free from external force or restraint and temperature gradients.

An incompatibility of regions in the metal created by non-homogenous plastic deformation is the principal cause of these internal stress systems, whether they are in an individual part or in an assembly of parts.

This mismatch or misfit between adjacent regions of the same part distorts the neighbouring regions. This condition can be either extremely damaging or very beneficial to the part, depending on the magnitude and direction. Compressive stresses created by shot peening can be good while tensile stresses created during welding can be bad.

Residual stresses are hard to visualize, difficult to measure and extremely difficult to calculate or predict, yet they are just as important in the function of a part as are externally applied forces that are more easily measured and calculated.

Residual stresses are fundamentally introduced into the material in one or more of the following ways: thermal, metallurgical, mechanical and chemical. Since these are the processes that make up our metalworking trades, it is only right to assume that, at some point in time, a stress relief treatment may be required.

"FORMULA 62" is a resonance based method of vibratory stress relief developed by Stress Relief Engineering Co Workpieces are subjected to low frequency, high amplitude vibrations for a short period of time based on the weight of the workpiece. This allows the residual stresses to be reduced to a much lower level where static equilibrium is restored. The resonance method is used by researchers around the world in stress relief studies using vibration and is currently considered an industry standard.

Low frequency vibrations are used as a carrier to deliver high amplitude energy to a metal fabrication, or machined part. The heavy vibrations produce a load that is superimposed on the existing stress patterns that result in a reduction of peak residual stresses. This produces a more dimensionally stable product and reduces the random distortion that often occurs in unstable workpieces.

Resonant vibrations have been found to be the most effective means for reducing residual stresses by vibration. The resonant frequency vibration method bas a much more pronounced stress redistribution compared to the subresonant frequency methods. High amplitude resonant vibrations are very efficient in significantly reducing peak residual stresses in weldments.

There are some limitations as with any metalworking process. The process is not recommended for extrusions or severely cold worked items. On very large, very long or open space frame type structures, the vibration may need to be applied at several locations which does require more time. Very small items in large quantifies are more easily treated thermally in batches.

In welding situations where vibration is used during the welding process, this method is most compatible with SMAW, GMA and GTAW welding processes. Other welding practices may present formidable logistical problems.

The process can be used on a wide range of ferrous and non-ferrous metals. Typical materials are: carbon steels, stainless steel, aluminium, cast iron, manganese, incolnel, etc. These are a few of the metals that can be treated in a variety of conditions such as: wrought or cast, forged, formed, welded, ground, polished or machined.

As with any treatment process, one must follow a given recipe to achieve success. In a thermal process, treatments are governed by a practice that dictates so many degrees per hour per inch of thickness. In the vibratory or non-thermal mode, the treatment is predicated on a time vs. weight basis where the aggregate weight of the part, including any associated tooling or fixturing attached to the part, is used to determine the length of time for treatment after resonance bas been determined.

Parts may be stress relieved virtually at any point in the manufacturing process where the part is accessible. The most typical applications allow for stress relief at key junctures in the manufacturing process, i.e. after rough machining, boring, grinding, etc. For welded fabrications, stress relieving can be performed during welding which is very helpful in preventing residual stress build-up that can cause weld cracking or distortion of some sections.

Because the fusion process produces large temperature gradients in a short period of time, residual stresses are more dynamically active which can require stress relief during welding, immediately after welding or in an ongoing program of routine stress relief on a daily basis.

As the time to completion increases for a fabrication, so does the risk of distortion related problems. Since large magnitude tensile residual stresses can reduce the fatigue life of welded assemblies, ample thought should be given to stress relieving all welded assemblies.

The actual length of time for treatment is based on weight per each application.

Treatment time can range from a short fifteen-minute period to an hour or more depending on the size of the item and whether or not it needs to be treated at more than one location.

The most direct way, and the most rewarding, is to resolve a basic dimensional instability problem. This is quite apparent when standard measurement techniques confirm that the part is now holding dimensions that previously moved in random fashion. However, in most stress relief applications, there is no major dimensional problem to overcome and the part looks the same before and after treatment. What now? Is there any simple way to tell how well we've done? Unfortunately, no! There have been some very technically weak and scientifically unsupported methods promoted over the years that allegedly indicate when stress relief occurs. Some of these methods claim that motor current changes are a means to indicate when the part is relieved or that vibration frequencies change as the stiffness of the part changes due to less résistance from stress or the part undergoes a vibrational frequency change from an unnatural mode to a natural mode. While we all would like to see some evidence of change, it just isn't that simple.

Real quantitative proof of stress relief efficiency can be obtained for virtually any metal using the right analytical tools. Whether you are using heat, vibration or a cryogenic technique for stress relief, you should only rely on known and approved analytical methods for determining when stresses have been reduced. This is very important because, not only do we need to know the magnitude of the stress concentration, but we also need to know whether it is tensile or compressive. There is no simple tool or technique available that can supply all of these data requirements for a three dimensional object.

Measurement techniques used by researchers are well known and we strongly support them. We highly recommend x-ray diffraction per ASTM E915-83, high-speed hole drilling with strain gauges per ASTM E837-85 or for ferromagnetic materials, the Barkhausen Noise Analysis method, for making any kind of determination that stresses have been reduced.

Not at all! There are a few rules that must be followed and they apply to all applications. The first requirement is to isolate the workpiece, as much as is practically possible, so it is free to vibrate. In cases where this is not possible, the vibration unit must be located as close as possible to the work area. The unit must be directly coupled to the workpiece in order to transfer the vibrational energy into the workpiece. This can be accomplished by using clamps, fasteners or adaptive fixtures.

Once these conditions are met, the part must be vibrated for a minimum period of time based on weight. Parts can be vibrated for longer periods without suffering any fatigue damage or loss of tensile strength.

The equipment is designed for ease of operation and is used both in shop environments and field applications by your personnel.

The "Formula 62" equipment can be used on a wide variety of shapes and sizes from small castings, shafts, gears, or OEM products to very large welded and machined fabrications that are too large for thermal treatment. Using a special vibration table designed specifically to hold smaller parts, the vibration system can be set up right in the production area which saves time and makes it possible to treat numerous parts at one time. The table bas a capacity of 2000 lbs. or 910 Kg., which makes it ideal for moulds, die sets or other heavy workpieces.

In cases where the thermal method is used solely to promote dimensional stability of the part for alignment purposes, surface integrity or service conditions, the vibration method can be used as a much more cost effective replacement. This method also allows the machinist or fabricator to treat parts at various points throughout the manufacturing process that otherwise would be too costly or impossible to do, due to surface considerations.

The thermal stress relief process, while adequate for many metals, does have some serious limitations that make vibratory stress relief a far better choice. For example: AWS DI. 1 does not recommend thermal treatment of A514, A517, A709 or Grade 100 material. Other carbon or low alloy steels may undergo undesirable changes in microstructure, causing a deterioration of mechanical properties, cracking, or both. Additional consideration must be given to possible distortion, oxidation, loss of corrosion résistance or intergranular cracking that may result from the heat treatment. While the method is good, it's not perfect. AWS D14,1 talks about it for dimensional stability.

Many code-required heat treatments go beyond stress relief and can have a pronounced effect on the overall performance of the part. In cases where stress relief and changes in mechanical properties or metallurgical composition are needed, there can be no substitution. Existing codes neither endorse nor disapprove of using vibratory stress relief methods. Questions concerning the possibility of using vibration in code-related cases should be directed to the factory.