TECHNICAL ARTICLEMaterial Australia January / February 1998

Vibratory Stress Relief –An Authoritative Overview

Roger Claxton

Roger Claxton is a director of the Vibratory Stress Relieving Co. Worcester, U.K. Involvement with research at universities and in industry introduced him to VSR nearly 30 years ago and he has been involved with VSR research projects ever since. This has led to the development and manufacture of the VCM905 range of AC VSR equipment. In recent years he has been adviser and assessor to several European Community and British DT1 research projects and is presently collaborating with two privately sponsored projects and two university projects.

ABSTRACT

Vibratory stress relieving (VSR) of components, welded or otherwise, is common place in most industrialised countries as a speedy and low cost alternative to thermal stress relieving (TSR). Additionally VSR has a unique role where thermal processing is not possible. The author describes the research that has taken place, the procedures and equipment involved in mainstream VSR and provides examples of its use.

INTRODUCTION

There are three distinct VSR processes, only one consistently works across the applicable component spectrum – resonant VSR (R-VSR). Its use has evolved over a forty-year period due to a succession of empirical and research led refinements. Use of this mainstream vibratory stress relieving process on welded fabrications is commonplace – but often misapplied. In well-defined areas of applications, R-VSR is 100% successful in its main objective – component stabilisation. With its 100:1 time saving and over 1000:1 fuel saving as against thermal stress relieving, it has a great appeal. However, overzealous sales claims, particularly by US equipment manufacturers, have led to mistrust when VSR processes fail to do what extremists claim they do – i.e. completely remove stresses, increase fatigue life by 500% and refine grain structure (in the case of vibrating during welding). No independent, well-exercised research programme has shown resonant VSR to be other than 100% reliable if applied as directed and certainly 100% stability is the aim. With the enhanced force and frequency ranges now introduced even treatment of traditionally difficult areas has been facilitated. The treatment of components of up to one hundred tonnes is commonplace.Not only is R-VSR convenient, obviating the need for expensive manufacturing delays and transport costs, it is also clean and does not reduce the rigidity of a component or distort it significantly, as might occur using heat treatment. Noise problems are largely overcome as the refined resonant treatment is a modal sub-resonant/resonant treatment a progression to resonance consisting of treatment at the foot of the peak to reduce critically high peak stresses, at the mid flank of peak to reduce stresses further and finally at the peak itself to reduce and redistribute the lower and previously modified stress field. Sub-harmonic and sub resonant treatment has been shown experimentally to reduce peak stresses by only a few percent whilst resonant VSR has been shown to reduce stresses comparable with TSR. After the subsequent induction of machining stress components can be re-treated even in the finish size condition. As a result R-VSR can ensure supreme accuracy and stability.

There are clearly defined areas in which VSR cannot be used: e.g. it does not bring about metallurgical changes that would enable it to be used on pressure vessels, pipework or in any other case where such changes are required.

THE VSR PROCESSES

The only common denominator between the various VSR processes is that the component to be treated is isolated from its surroundings and subjected to a cyclic force, the effect of which is monitored. The three main VSR approaches are resonant (R-VSR) modal sub-resonant (SR-VSR) and sub-harmonic (SH-VSR).

Resonant VSR. Every engineering component has a series of inherent natural frequencies. A resonant peak occurs when the induced frequency of the vibrator coincides with the structure’s natural frequency and this can be seen, felt and displayed on meters and recorders. At resonance the cyclic input force (e.g. for heavy components – 1.8 tonnes at 110 Hz.) will be magnified by between fifty and one hundred times. Treatment at and about these resonant peaks is the basis of resonant VSR. Peaks are approached slowly, with a pause at the foot (sub-resonant zone) to allow any critically high stresses to diminish, prior to treating at the mid-height region and then the actual peak for the number of cycles specified in the handbook charts for the component type and material. The resulting cyclically imposed strains add to the residual strains in the material within the component. To achieve the highest percentage of stress relief, the cyclic properties of the material need to be stimulated by a constant high cyclic amplitude and accurate frequency stability. Also the mean stress must be allowed to float. For the most uniform stress relief and for supreme stability, as many of the natural frequencies as possible are sought. The greater the equipment’s frequency range the better is the treatment, as more loading patterns (modes) are established and different areas are acted on. Simulating service loading patterns is obviously beneficial. More loading patterns means that fewer cycles per mode are needed, (often as low as 1000 cycles). The higher the frequency, the more complex is the loading pattern and the more uniform the treatment [1]. In exacting ‘thin plate’ type applications, the ratio of the mass of the vibrator to the mass of the component is such that repositioning the vibrator can lead to extra mode shapes being achieved.Depending on accuracy, stability required and convenience, R-VSR is normally applied before machining – as is any other VSR system. Ideally, for maximum benefit, VSR should be applied after rough machining as it then also reduces the machining stresses. If TSR has been used and fails to confer the required accuracy or long term stability then R-VSR (or modal SR-VSR) can be applied just prior to finish machining to achieve total stability. Research has consistently shown resonant treatment to be most effective when the foregoing considerations have been taken into account.For treatment, the component is supported on rubber isolators and the vibrator is attached at the edge of the component, with its axis parallel to, but away from, the nearest anticipated node line. Rigid clamps are used to make the vibrator and component as one. A piezo-electric sensor mounted on the component will identify the resonant conditions as the frequency range is scanned.Any straightening of a distorted component or fabrication should be carried out before VSR is applied. In case of straightening shafts and bars, application before and after straightening speeds up the process of establishing the required stable shape. Components are not damaged by resonance because the small movement at the foot of the resonant peak starts to reduce and redistribute critically high stresses before the high cyclic strains of full resonant amplitudes are to be experienced. The highest stresses are reduced by the lowest imposed amplitudes and conversely the lowest stresses are reduced by the highest imposed amplitudes. Unlimited motion is impossible because of the absorption of energy (damping) by the material itself, the rubber isolators and by air displacement. Jesensky [2] and others [3] have started that VSR does not cause fatigue. In nearly 30 years of applications, the author knows of no such failures of good welds.

Modal Sub-Resonant VSR. This process is best carried out with the same equipment used for R-VSR processing. If when attempting R-VSR, only the base and the flank of a resonant peak is achievable (due to the peak being just out of the range of the equipment), this treatment would be classed as modal sub-resonant VSR – because the mode shape would be evident but the peak not achievable. Four to ten times the number of cycles required for R-VSR is a good guide, applied in inverse proportion to the magnitude of the cyclic response. Again obviously the better the frequency range of the equipment the more successful is the process. If mode shapes can be modified by re-sitting the vibrator (e.g. plate type components) this can dramatically influence the quality of the result. Initial set up conditions and general principles are the same as for R-VSR except that it is even more important to position the vibrator as far from a node line as possible. The mechanism by which benefit is derived is similar to that of R-VSR but the cyclic properties of the material are not usually invoked.

Sub-Harmonic VSR. If neither of the above conditions are met i.e. there is no modal response at all (due to resonant responses being way beyond the range of the equipment), conventional wisdom indicates that no stress relief is possible. The process is said to depend on energy absorption being at a maximum at the foot of a sub-harmonic peak. As vibrator force increases with the square of the speed one might expect the highest sub-harmonic peak to be the most effective for treatment. Strangely, one of the lowest peaks is infact chosen. The mechanism by which SH-VSR is said to work has no connection with either R-VSR or modal SR-VSR. It is claimed to vibrate the atoms and move them relative to one another in the strained crystal lattice of the material. The energy used is so low that the vibration usually cannot be either felt or heard. Both SR-VSR and SH-VSR processes are used for vibrating during welding. The latter being less effective.

EQUIPMENT

Successful VSR machines drive high performance rotating mass vibrators. The equipment is portable and can be divided into AC and DC motor types. Only those with AC motor exciters are capable of withstanding the enormous “g” forces required to put the process first – over 80g. DC motors tend to fail at round 10g and therefore the treatment procedure is tailored to prevent this happening. For low frequency applications either can be used but with DC types frequencies are limited to 130 Hz. Many components need treatment at frequencies well in excess of 130Hz, because the stiffer a part is the higher stresses it can hold without yielding. British AC equipment can produce frequencies of up to 250Hz if required (5-220Hz. As standard). Because of this, AC resonant VSR machines tend to be the most effective and reliable.Non-resonant VSR is advocated for equipment with poor frequency range. This low frequency, quiet, approach is claimed to have applications on fairly solid castings and for vibration during welding where it is claimed to reduce distortion, lower stresses, refine weld material, improve dilution, reduce cracking and increase deposit rates. An AC system with its greater “g” tolerance and frequency range can do everything that a DC system can do and more. Reliability is now second to none with automatic fault protection and detection built in.A wide speed range and selection of exciters makes a system ideal for frequency response testing as well as stress relieving. Some DC vibrator equipments only have one powerful fixed frequency/force ranged vibrator, which can damage certain categories of component. Some have two vibrators with adjustable weights – one vibrator for the majority of work and an extremely heavy one for the largest components. The most versatile equipment is an AC vibrator system that not only has three different vibrators available for middle ground applications, over an extended frequency range, but it also has a low mass 5–220Hz vibrator for light aluminum components – e.g. 5x2x1m tubular antenna. Additionally a twinning facility is available whereby two vibrators can be run in tandem off one console to treat exceptionally heavy or long components – e.g. generator bedplates 30x4x1m, weighing up to 40 tonnes.

RESEARCH

Research into VSR has produced four theories about what happens inside materials during process: A cyclic version of a simple stress overload A beneficial effect of vibrations on the distorted crystal lattice of the material. A compound all embracing theory. Nothing at all.

Until the mid 1980’s researchers almost invariably used whatever equipment was available rather than purpose built resonant-VSR machines, some experiments accurately reproduced the process, many others did not [4]. The equipment was often not capable of speeds high enough to reach the components resonant frequencies, and even when it was, these frequencies were sometimes not used for treatment, usually due to DC vibrators failing because of high ‘g’ forces. Incredibly some researchers designed test programmes for R-VSR involving components with resonant frequencies too high to be treated and then proclaimed the process to be ineffective [5]. Because of this the outcome often indicated that R-VSR was unreliable and its results were inconsistent – nothing could be further from the truth. It was the research that was unreliable and inconsistent. [6] & [7].In general, even using old style R-VSR, where suitable components have been excited at one or more resonant frequencies, the results have been stress reductions of 30% or more depending mainly on the equipment used. Meanwhile an AC vibrator system with a ‘g’ tolerance of over 80g can obviously be expected to be the most efficient means of stress relief Strachen [8] showed an 80% reduction with mild steel welded specimens and a 60% reduction in stainless steel welded pieces. Zveginceva [9] found over a 40% decrease and Zubchenko [10] showed a 73% reduction with large mild steel welded bedplates. Treatment at a succession of modes, each having a different strain pattern was shown by Polnov [1] to cause substantial reduction and redistribution of stresses. In the limit 1% stress relief makes the difference between instability and stability.With the advent of the 5-220Hz range of VSR machines, Jesensky [2] Bonthuys [11], Ohol [12] and Sagalevich [13] have shown reductions of 40-80% using resonant frequencies. The higher percentage figure will not be achieved if the researchers did not invoke the cyclic properties of the material. Much is to be learned from the excellent research by Walker, Waddell & Johnstone [14].

Where only modal sub-resonant treatment is possible Waddell [14] has proved that, given sufficient cycles, considerable stress relief occurs with no reduction in fatigue life. Practice supports this. The time for treatment varies equipment to equipment.Strain measurements have indicated that modal SR-VSR is most effective against high tensile stresses, whereas R-VSR works well either on both high tensile and high compressive stresses [2], [8], [10-12] & [15]. For stability after machining and in service, both tensile and compressive stress peaks must be lowered if they are approaching yield value. After all, stability is the main requirement for which R-VSR or modal SR-VSR is applied. When resonance is used, stability more than matches that of thermal stress relief as it can be re-applied near finished machine size [2], [8], [12], [16-18]. VSR does not reduce rigidity or affect material properties – e.g. fatigue life [2].

The research clearly shows that: A cyclic version of a simple stress overload is one mechanism that is at work given sufficient

amplitude. A beneficial effect of vibrations on the distorted crystal lattice of the material is supported by

Waddell. A compound all embracing theory encompassing the previous two and more.

EXAMPLES – VSR AND WELDED FABRICATIONS

The widespread use of, and general satisfaction with R-VSR has been shown by the extent to which it has been accepted by virtually all sectors of industry (Claxton [19-21]); so extensive in fact that it is impossible to truly represent the entire spectrum here. Presented below are just a few examples of how resonant VSR has been applied in the industrial sector.

EXAMPLE 1 – FERROUS COMPONENTS

No specific example of mild steel fabrications or cast iron / cast steel castings is given here as it is well accepted that where no metallurgical changes are required, R-VSR is as good as thermal stress relieving for stabilising beams, bases, columns, gearboxes, bedplates etc. But it is quicker , cleaner and cheaper as witnessed by thousands of regular users over the last 25 years in virtually every applicable engineering field.

EXAMPLE 2 – PUMP BODIES

One particular area where R-VSR can be especially valuable is the treatment of materials whose properties would be impaired by a full thermal stress relieving treatment. An example of this illustrated by precision-machined vane pump bodies manufactured from precipitation hardened aluminum alloy castings (LM25TF). Over a number of years the pump bodies, measuring approximately 375 x 300 x 150mm, had been subjected to highly skilled machining and extensive inspection, but were frequently rejected following delivery to the customer because of unacceptable distortion up to 0.15mm across the machined surface.Careful investigation revealed that the cast pump bodies were stress relieving themselves over a period of time after the extensive machining operations, and this highlighted the need to relieve the internal stresses induced during a quenching operation. Unfortunately, the delicate metallurgical balance of quenching and ageing required to achieve the desired mechanical properties does not allow any further heat treatment to be carried out and the only alternative left was to attempt vibratory stress relief. R-VSR has been found to be indispensable for stabilising weld repaired pump housings. Aluminium, stainless steel, cast steel and chrome molybdenum steel pump housings passing corrosive or abrasive media wear badly in service and are built up by MMA and MIG welding then machined back to correct dimensions. Prior to the introduction of VSR, service leaks occurred and service life of a repaired pump was one third of that of a new one. Housings treated after rough machining, at frequencies between 80 and 170Hz., were found to change shape up to 0.8mm TIR during VSR. On finish machining and in service, components have been stable with no leaks. New and reconditioned, stellite deposited plastic moulding machine screws and barrels are similarly treated both to stabilise and to make straightening easier. This is a world wide application.

EXAMPLE 3 – COPPER AND COPPER ALLOY COMPONENTS

The stability of copper and copper alloy components features prominently in the production problems of electrical resistance spot welding machines. For more than 15 years the main fabricated steel structural elements of the machines have been vibratory stress relieved internationally in all the main centres of manufacture. However only in the past eight years with the development of new higher frequency R-VSR equipment has the full potential of the technique with respect to the stabilisastion of copper and copper alloys been realised. A large variety of plate, bar, fabricated. And cast components ranging in weight from less than a kilo to 20 kg or more have been vibratory stress relieved, usually after some pre-machining and straightening stresses. A large number of composite mild steel and bronze brazed components are now treated that were a problem before companies bought R-VSR units.

EXAMPLE 4 – FURTHER COMPOSITE COMPONENTS

The problem of stabilising composite materials often comes the way of VSR for obvious reasons. From 50mm diameter plastic coated steel valve seats to 1m diameter bore cast iron/white metal lined marine bearings, from 2m wide tunneling machine skirts having a mild steel body fabrication and a welded on hard manganese cast blade edge to 10m long Corten/MS lifting beams and other auxiliary devices for container handling – R-VSR has been used in all these areas for ten to twenty five years to overcome instability and high stresses.

EXAMPLE 5 – MINE SWEEPING MULTIPLANES

Multiplanes for Royal Navy minesweeping are subject to severe vibration and stress during operation, and tend to premature failure. It is likely, in view of the operating conditions of these items, that the cracking is of ament of thermal treatment by R-VSR has already resulted in a doubling of service life, and shot peening the weld toes to induce compressive stresses into the surface is being used after R-VSR in an attempt to obtain further increases. This is a particularly interesting example of the effects of R-VSR since the stress reducing characteristics of the process appear to be more significant than any metallurgical effects of thermal treatment. Since the latter would not be expected to be expected to be very great in stainless steels of this type anyway, the result is perhaps not entirely surprising.

REFERENCES

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2. Jesensky, M ‘Vibratory lowering of residual stresses in weldments’ Proceedings of IIW conference, Sofia, July 1987.

3. Gnirss, G. ‘Vibration and vibratory stress relief. Historical development, theory and practical application.’ Welding in the World/Le Soudage dans le Monde, 1988, 26 (11-12) 284-291.

4. Buhler, H. Et al. ‘Investigations into the reduction of welding stresses.’ Schweissen and Schneiden, May 1964.5. Gifford, DJ. ‘Vibratory Stress Relief.’ Metals Australia, April 1984.6. Wahlstrom,LE. ‘Dimensionsstabiliering genom vibrationer.’ Report No. 83 Jan. 1976. Institutionen for

Svetsteknologi Kunliga Tekniska Hogsoko – Jan. Swedish Welding Institute.7. Leide, NG. ‘The significance of residual welding stresses – some experimental results and practical experience in a

shipyard.’ Proceedings of paper 15, WI conference ‘Residual stresses in welded constructions and their effects.’ London, Nov. 1977.

8. Strachen, RW. Report on the Efficiency of vibrational stress relief. General Dynamics report no. U413-68-059. 1976

9. Zveginceva, KV. Svarochnoe Proizvodstvo. 1968 (11).10. Zubchenko, OL. Et al. ‘Vibrating loads used for relieving stresses in welded frames.’ Automatic Welding, 1974,

27(9), 59-62.11. Bonthuys, BF, Vibratory stress relief study. ISCOR South Africa 1989 Proprietary report.12. Ohol, RD. et al. ‘Measurement of vibration-induced stress in the heavy fabrication industry.’ Proceedings of

International Symposium on mechanical relaxation of residual stresses. Cincinnati, Ohio, April 1987.13. Sagalevich, VM et al ‘ Eliminating strains in welded beam structures by means of vibration.’ Svarochnoe

Proizvodstvo, 1979, (9), 9-11.14. Walker, Waddell & Johnstone. ‘Vibratory stress relief – An investigation of the underlying process.’ Proceedings

Institute Mechanical engineers vol209, pp51-58 1995.15. Dawson, R. ‘Residual stress relief by vibration.’ Ph.D. Thesis, Liverpool University 1975.16. Sedek, P. Vibtrational stabilisation of welded structures – experiments and conclusions.’ Proceedings of IIW

conference, Sofia, July 1987.17. Ananthagopal, KP et al. ‘Effect of Vibratory stress relieving on dimensional stability of fabricated structures.’

Proceedings of the National Welding Seminar, Indian Institute of Welding October 1986.18. De. Rudder, A. Et al. Studie van het utrillen van lasspanningen.’ Hoger Technisch Institut, Oostende, 1970-71.19. Claxton, RA. ‘Vibratory stress relieving of metal fabrications.’ Welding & Metal Fabrications, 1991.20. Claxton, RA. ‘Vibrations reduce stress levels.’ European Surface Treatment, Winter 1992/321. Saunders, GG & Claxton, RA. ‘VSR. A current state of the art appraisal.’ Proceedings of Paper 29. Internationsl

WI Conference ‘ residual stresses in welded constructions and their effects.’ London Nov. 1977.