VIBRATORY STRESS RELIEVING -lt's ADVANTAGES ASAN ALTERNATIVE TO THERMAL TREATMENT

j .S Hornsey B.Sc (Mech Eng), SAIW, SAIMech.Eng, CGLI (Dist Weld ) CGLI (Dist Met allurgy)

VSR(Africa)cc December 2004

ABSTRACT In an introductory review, the techniques and equipment for vibratory stress re/íeving are described and app/ications exemplified with case histories. lt has been proven that the process gives stabílisation results comparable with and in many cases exceedíng those obtained wíth therma/ treatment, whílst be;ng quicker, cheaper, more versatife as the equipment is complete/y portable and the technique offers many advantages when machined parts or heavy fabrications are in volved.

INTRODUCTION Over the last 60 years, vibratory stress relieving has evolved from a little known art into an indispensable basic process, which is now a well tried and established alternative to thermal treatment for the treatment of castings, fabrications, components reqUinng intricate machining operations and non-ferrous metals. lt is important to emphasize that vibratory stress relieving is not claimed to be a substitute for all thermal t reatments although there is sorne common ground just as there are areas where each process is and will remain predominant. Thermal and Vibratory treatment share a capabi lity in three areas, namely overall stress reduction, dimensional control and dimensional stabi lisation. Although total stress relief is almost impossible to obtain by using any commercial process, vibratory stress relieving can stabilise and stress relieve the component at any stage of the manufacturing or machining process without changing the materials metallurgical condition, without scaling or discoloration and without distortion at low cost and with minimal time restraints to the manufacturer. Conversely only thermal treatment will change a material's metallurgical properties and thermal treatment is also more effective than vibratory stress relieving when used to prevent incidences of brittle fracture, although more often correct material selection is a prerequisite for the prevention of this type of fai lure. Additionally, materials that derive their mechanical properties from transformation hardening or cold working cannot be so thoroughly stabilised. Thus the complementary nature of the two processes can be appreciated. However instability in these types of materials can be successfully treated using vibratory stress relieving. The vibratory process involves inducing metal structures into one or more resonant and sub resonant conditions usinq oortable hioh force exciters. Treatment periods are short and frequencies generally in the range l0-230Hz.

With modern exciters, correctly sited, and the component virtually undamped by means of rubber isolation mounts, equal and often better results than are to be expected from commercia l thermal treatments are possible and are more often obtained. One of the main factors which has and in sorne cases sti ll hinders the acceptance of the process is the reluctance by engineers to accept that a low cost vibratory treatment, using only 220v and often lasting less than thirty minutes could possibly replace an extended thermal treatment lnvolvlng high energy consumption.

A recent survey carried out by the US Department of Energy has shown energy savings experienced by using vibratory stress relieving in sorne cases exceed 500:1. This article discusses the practica! benefits of vibratory stress relieving as compared with thermal treatment and attempts to dispel sorne of the myths associated with vibratory stress relieving it also highlights sorne of the discrepancies in the various vibratory systems available. A section dealing with vibratory stress relieving equipment is included although it is assumed that the reader will be broadly conversant with installations for thermal treatment. Sorne of the variations in specifications for thermal treatment are also included which will hopefully expose the ignorance in these specifications.

PAST RESEARCH lt is possible to f ind technical papers, ostensibly written about vibratory stress relieving dating as far back as 1934, but in fact, there have been few genuine research programs into the process. The bulk of the papers concern either general oscillatory testing of metals or work which the author thought to be related to vibratory stress relievinq without prooerlv evaluatinc¡ and appreciating the process. Many tests were limited to simple test bars. which due to restricted

budgets were treated in fatigue test machines at fixed frequencies and amplitudes. Recent testing (Oct 2003) in conjunction with The Anglo American Corporation of South Africa, again due to budget restraints used a simple welded specimen two plates 150rnm x 150mm x 25mm thick, butt welded together. This is certainly not a good example of actual components owing to perfect welding conditions with the welding carried out by personnel frorn The South African lnstitute of Welding and consequently very low inductions of stress. The results obtained showed strain redistribution of up to 90% and a reduction in stress of 42% In some tests fixed foundry knock-out shakers and de-burring barreis have been used and the work audaciously clairned to relate to vibratory stress relieving! There are obvious reasons why most of these low budget methods did not succeed in producing the desired resu lt. Recent tests carried out amongst others by the University of Strathclyde and the Department of Trade and lndustry has redressed sorne of the inadequacies of the older test methods, a list of papers and tests will be included in the bibliography at the end of thís paper. EQUIPMENT Although any equipment can satisfy the "easy to treat 1 little need" category, only the best equipment with optirnum force 1 frequency characteristics and rnaxirnum "g" tolerance successfully treats the rnost challenging end of the range. As exarnples and research show, it ís a range that spans the entire rnaterials and engineering spectra. Gone are the days when heat treatment contractors took an adversial attitude to vibratory stress relieving. Some have purchased their own VSR equipment; and rnany others use an on-site service. As well as enabling thern to treat parts hitherto too large for their furnace, vibratory stress relieving opens up completely new areas of business. However, for coded components such as pipework and pressure vessels etc. thermal stress relief must be used as only th is gives the required metallurgical benefits. There have been many requests to include vibratory stress relieving into the various codes but the reluctance to do so is still dominant in the industry Stability is the main requirernent for which vibratory stress rel ieving is applied. When VSR is used stability more than matches that of thermal stress relieving. Stabi lity can be improved by re­applying VSR to components in near finished condition thus saving components that might otherwise have been scrapped. Vibratory stress relieving does not reduce rigidity or affect material properties or fatigue life.

There are various VSR systerns, sorne effective some less so. The only common denorninator being that the component to be treated is placed upon rubber isolators and subjected to a cyclic force. Recent research has identified the successful and not so successful processes. The three rnain VSR approaches are resonant (R­VSR), modal sub-resonant (SB-VSR) and sub harmonic (SH-VSR). The British "VCM series" is the only equipment range that is specifically designed for R-VSR. lt has superior frequency/force ranges and a remarkable tolerance to high "g" torces. The formula 62 and Fourierrnatic systems cla im to be successful for resonant VSR but research mentioned below cast's doubts on their effectiveness - possibly because of poor frequency range, "g" tolerance etc. Practice seems to support th is.

RESONANT VSR This has evolved over a 40-year period. For the VCM series mid 1997 saw major research-led changes in both approach and equipment specification. In well defined areas of application. R-VSR is now 100% successful in its rnain objective stress relief-component stabilisation. The treatrnent of components from less than 1kg to in excess of 100 ton is cornmonplace. Procedures stipulate a progression up the peaks to resonance, consisting of a pause at the foot to allow any crit ically high stresses to diminish, prior to treating at the mid height region and then a short defined number of cycles at the actual peak. As long as the mean stress is allowed to float the resulting cyclic imposed, straíns progressively add to the residual strains in the

00<

e ~ "' " z

0111

~ ¡:¡

OOJ ..l < ~ ~ z

"' l 00!

e "' ~ 000 > <

~

ASCAST HEATTlfAIED Vll - WFTM.AX VSR - I'C\100

material to cause stress reduction and distribution as with TSR. For the most uniform stress relief and stabil ity, as many as the natural frequencies as possible are reached. The greater the equipment's range and the more cornplex the loading pattern then the better the treatment.

2

Research and over 40 years of application have shown that there is no damage due to high resonance. This is because critically high-imposed stresses are impossible to achieve as damping increases dramatically with high cyclic strain. R-VSR is normally applied before machining, ideally though it should be applied after rough machining as it then also reduces machining stresses. Application before fina l grinding achieves even closer tolerances. Treatment at this or the finished stage eliminates micro movement occurring between leaving the customer or in service. The most accurate and stable components are R-VSR treated. In general, even using old style R-VSR, where su itable components have been excited at one or more resonant frequencies, the resu lts 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 showed an 80% reduction with mild steel welded specimens and a 60% reduction in stainless steel welded pieces. Zveginceva found over a 40% decrease and Zubchenko 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 to cause substantial reduction and redistribution of stresses. At the limit 1% stress relief makes the difference between instabil ity and stabil ity. With the advent of the 5-220Hz range of VSR machines, jesensky Bonthuys Ohol and Sagalevich 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 . Manufactures of vibrating plant use R-VSR for stress relief and fi tness for purpose testing and thereby extend warranties on screen, deck support frames, moulds etc.

MODAL SUB·RESONANT VSR lf when attempting R-VSR, only the base of the peak is achievable (dueto the peak being just out of range). Treatment wou ld be classed as modal sub-resonant VSR i.e. the mode shape would be evident, but the peak not achievable. Optimum results are obtained if up to 10 times the number of cycles required for R-VSR are applied ·in inverse proportion to the magnitude of the cycl ic response. Where only modal sub-resonant treatment is possible Waddell has proved that, given sufficient

cycles, considerable stress relief occurs with no reduction in fat igue life. Practice supports this. The time for treatment varíes from 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. 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 wh ich R­VSR 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 f inished machine size. lt is best carried out with the equipment used for R-VSR processing because of its superior frequency range. VCM 90/905 machines which have twice the range of any other equipment.

SUB-HARMONIC VSR (MetaLax) lf neither of the above conditions are met (due to resonant responses being way beyond the range of the equipment), conventional wisdom indicates that no stress relief is possible. Th is seems to be the domain of sub-harmonic VSR. Treatment is said to take place at the foot of a minute sub­harmonic of a true resonant peak. Sales literature states that the process depends on energy absorption being ata maximum near the foot of a sub-harmonic peak. Because exciter force increases with the square of the speed one might logically expect the highest sub-harmonic peak to be the most effective for treatment, however the manufacturers, actually advocate treatment at a low one. This possibly indicates that their equipment has poor 'g' tolerance. The mechanism by which SH-VSR is said to work has no connection with either R-VSR or modal SR­VSR. The diagram used to promete the process and its mechanism appears unconvincing if drawn to scale. SH-VSR claims to vibrate the atoms and move them relative to one another in the stra ined crystal lattice of the material. This seems farcical, as the energy used is so low that the vibration usually cannot be either felt or heard.

RESEARCH Researchers ha ve investigated aspects of VSR for over 40 years. Sorne were legitimately exploring its boundaries but others have toyed with test­pieces and procedures not remotely connected with VSR resulting in sorne misconceptions. All the research reported below was conducted with actual VSR equipment, assisted by the equipment

3

manufactures or their direct agents. Whereas historically research projects in the mid/late nineties have consistently disproved the effectiveness of American de resonant, non­resonant and sub harmonic equipment. A 2-year Dutch/German EU project tested two de types of equipment - SRE Co, Formula 62 and VSR Eng Martín LT120/MX800 re: stress reduction and fatigue of components. Little or no benefit was found. British EU and later DTI projects tested two other de types; both automatic Bonal Meta-lax sub harmonic system and VSR Eng. KD16 Fourier sean re stress reduction and stability. The projects !asted nearly six years and little or no benefit was found. Particularly difficult components were tested as an adjunct to the DTI project.

They were treated using the Meta-lax and VCM 90 equipment. The results showed that Meta-lax brouqht about little chanqe whereas the VCM 90 was on par with thermal stress relief (see bar diagram).

Sumarising recent research, it clearly shows that:

• VSR can be as effective as TSR glven the best R·VSR equipment.

• A cycl ic version of a simple stress overload is one mechanism that is at work given sufficient amplitude.

• Given sufficient energy, a beneficia! effect on the distorted crystal lattice of the material occurs.

• No reduction in fatigue life occurs using any form of modern VSR equipment.

INDUSTRIAL EXAMPLES The widespread use of, and the general satisfaction with vibratory stress relieving has been shown by the extent to which it has been accepted by virtually all sectors of industry; so extensive in fact that it is impossible to tru ly represent the entire spectrum here. No specific example of mild steel fabrications or cast iron 1 cast steel castings is given here as it is well accepted that where no metallurgical changes are requ ired, vibratory stress relieving is as good as thermal stress relieving for stabilising and stress relieving beams, bases, columns, gearboxes, bedplates etc. But it is quicker, cleaner and cheaper as witnessed by thousands of regular users over the last 45 years in virtually every applicable englneering field. The acceptance and usage of VSR in South Africa alone has increased by an average of 69% annually since 1992.

Fan lmpellors and Rotating Equipment: Vibratory Stress Relieving is used to stabilise fans and impellors ranging in size from 800mm diameter x lOOmm to 2m di a meter x 900mm in fabricated mild steel and stainless steel. Sometimes these are repaired components, and sometimes replacements. After fabricating, but prior to dynamic balancing, the components are subjected to VSR. Since introducing this treatment, no fans or impellors have gone out of balance in service, even under hot conditions -hitherto a troublesome area. lnstallations are now much quieter and last longer between overhauls. Novenco Aerex, the UK's largest fan and impellor manufacturer, have had their own VSR unit for many years and endorse the benefits stated above. Their Canadian plant also uses VSR. In both cases the system paid for itself in 4 - 5 months. Rubber coated, steel fan blades have been treated to overcome instabili .

Courtesy Rotary Ma South A frica

Rolls, bars and shafts: Bowing of shafts whether during machining, weld depositing of worn items or in service had proved to be a virtually insurmountable problem. Particularly difficult materials such as duplex stainless steel, nitronic50, E4340PQ etc. are stabilised using this method, saving companies a fortune in material, time and labour costs. The following photograph shows one of twenty­four unstable EN19 steel, forged drive shafts, being VSR treated and monitored using surface strain gauges. The results showed that VSR reduced surface stress to safe limits, stabi lising the component while not reducing fatigue life or altering material properties. Also, VSR and stra in measurements clearly identified the shafts which had been correctly TSR'd and those which had no t.

4

20 off EN19 Shafts treated prior to final machining

Vibratory stress re/íeving being carried out on a cast iron precision machine bed.

Sorne twenty three years ago, Dean Smith & Grace became disillusioned with thermal stress relieving when cast iron saddles, consistently in tolerance on final inspection in the UK, were 20% out on arrival in the USA, necessitating rework. VSR solved the problem and today, DSG rely on it solely to stabilise saddles, beds, etc. Before that t ime QA records showed that, using thermal stress relief 98% of beds were reworked in-house after final machining due to movement during handling. Subsequently, only one of 533 beds was reworked - 0.2%. Dean Smith & Grace's subcontract machine shop also finishes mild steel fabricated beds up to 10m long and 1m x 900mm section, basically in 12mm plate, but with sideway sections up to 100 x 300mm and weight up to 12 tons. A 10m bed has a welding t ime of approximately so hours. The fabrication is solely VSR treated. Operation procedure is to fabricate, apply VSR, inspect, rough machine removing up to 35mm to produce sideway profile, ship to Dean Smith & Grace, apply VSR and fin ish machine to five microns in 6m by grinding. No machinability problems are encountered at any stage, despite extensive machining of flame cut edges up to 100mm th ick.

Gearbox Casinqs

ted pump and gearbox bases.

In this example the manufacturer had a reclamation problem involving the rewelding and fin ish machining of lightweight gearboxes already in a part machined cond ition. The components had already undergone one heat treatment prior to the reclamation operation, during which distortion had taken place sufficient to indicate that a further thermal treatment would render them suitable only for scrap. After consu ltation VSR was attempted on a reclaimed sample of the weakest component, the gearbox hood, following a rigorous dimensional check. The hood was satisfactorily crack detected and finish machined and all other items successfully treated, thereby avoiding complete remanufacture. VSR is used extensively for th is type of fabrication which requires close machining tolerances.

Picture courtesy Sasol Synthetic Fuels South A frica

Treating the parts that thermal stress relieving cannot treat There are thousands of components in need of stabilising that cannot be thermally stress relieved but can be treated using a VSRS. Here follow sorne typica l examples:

(a) Precision conveyor rolls for nuclear waste disposal, having an outer 304L stainless­steel shell, of 819mm diameter x 884mm face, welded to 789mm-diameter mild-

5

steel end plates and bosses with integral En8 120mm-diameter shaft. AC-VSRS was specified by Sandvik, approved by British Nuclear Fuels and NIS based on Sandvik's twelve years of complete satisfaction with the AV-VSRS.

(b) Mild-steel rolled hollow-section (RHS) fabricated 'A' Frames with reinforcements for a vehicle front chassis, with integrally­welded cast steel "Rose' universal joints, are manufactured in a jig to tight tolerance. Prior to VSRP being applied, 106 sets were produced and all distorted in service, causing wear. For over 1000 sets, VSRS treating at three resonances between 35 and 180Hz has rendered all completely stable.

(e) Three designs of steel armour-grade investment casting, one with a welded-on t ie-bar in the fully heat-treated and final metallurgical condition, were found to be grossly unstable during machining. The largest had a 300 x 400mm picture-frame tace, associated bore and pad faces 400mm apart, in the first batch of four, movement continued for two months after machining. TIR allowable in all planes is better than 3 microns. A special VSRP was applied, prior to machining, by mounting the component at its center of gravity on a small pad on a 400 x 400mm j ig table with a vibrator mounted on the underside. Treatment: 11 modes of vibration between S and 220Hz, which has rendered all subsequent batches completely stable.

(d) A VCM80 AC-VSRS was specified by Short Bros. For this work at their subcontractors and they have used their own VCM80 for stabilising mild-steel and aluminum composite fabrications for many years. A VCM90 system was ordered in january 1991.

(e) A failure rate of approximately 40% has been reduced to zero on parts of mining and quarrying equipment since Trellex (Trelleborg Group) and Skega introduced the VSRP to complex mild-steel fabricated components, often only 14mm x 2mm in section x 4m long.

(f) In the same industry, sorne vibrating screens now carry a three-year guarantee, thanks to the AC-VSRP.

Vibrating Screen VSR after assembly

(g) Typically, screens are mild-steel fabrications from 1m x 3m to 3m x 10m and 100 to 200mm deep - usually a complex lattice of RHS, angle and tubular members. lf thermally stress relieved, they usually distort badly and need mechanical or thermal straightening, often defeating the object of the original thermal treatment. When no stress relief or thermal treatment was used, butt welds that lacked preparation and had their bead ground off, leaving a weak joint, failed in service. Now, with the introduction of VSRP, welders know that a poor joint will break in the weld shop so they ensure good joints. This 'fitness for purpose' testing is regarded by Goodwin Barsby, Parker, Kue Ken, Pegsons, Babcock Power, etc. as a good reason to use the VSRP, as in-service life has, on average, tripled.

(h) Beams, Sm long x 140 x 300mm section, fabricated from RTQ60 material, bowed 2mm during rough machining. Thermal stress relieving was not permissible on metallurgical grounds. The VSRP has completely solved the problem for British Steel.

(i) Deloro Stellite use an AC-VSRS at both their UK and Canadian plants to stabilise

6

carpet knife blades. These are typical ly mild­steel bar, 5m x 150 x lOmm, grooved out and deposited with stell ite along one long edge. Th is edge is ground to expose the stellite and form a cutting edge. The mild-steel section behind is slotted to give adjustments for the holding screws. Up to ten year ago, it was common for 50% of the wear tolerance to be lost due to movement taking place during t ransport (typically UK to ltaly). Since introducing the VSRP, no movement has occurred and t ighter tolerances are maintained.

Hundreds of examples are on file: large copper plates fully machined, screwed and dowelled; flow-brazed aluminum instrument frames; powder-coated enameled 25mm x 25mm mild­steel angle instrument frames for Marconi; mixed-metal fabrications for Helio tank turrets and a wide variety of materials such as Jnconel, Zeron, duplex stainless steel, Ferraliu, titanium, P20 (l. 7% Cr steel), aluminum in TF condition, composite metal 1 plastic, metal 1 rubber fabrications, etc. for these and many other applications, the VSRP is invaluable. However, it must be remembered that the VSRP cannot be used on pressure vessels, pipework or any parts where metallurgical change is necessary. To its benefit VSRP can be used on all non-ferrous materials and on the hardened materials that are commonplace on most mining and quarrying components. No reduction or softening of material properties occur with the correct treatment

Treatment of La

119,000kgs Tippler cage VSR after welding prior to machlnlng

Treatment of the above tippler cage and its associated components was carried out on behalf of Saldanha Steel at DCD Dorbyl. The tippler cage weighed 119,000kgs and treatment was in the range of 2 hours at resonant frequencies, the complete assembly including the end rings was in

excess of 400,000kgs all of which was VSR treated. A modern VSR system has the capacity to treat a singular component of up to 200ton. Opencast dragline buckets with weights of up to 70,000kgs are treated on a regular basis. Many are treated after a major repair sorne are treated after fabrication at the OEM suppliers. VSR has been proven to reduce cracking and some manufactures in the USA are claiming an increase in service life of 400% a figure suspected to be grossly exaggerated although published in a leading welding publ ication. Feedback received from mines in South Africa would suggest a figure of around 45% to be more realistic as VSR cou ld in no way influence the (wear and tear) characteristics of a bucket with the exception of no softening of the materials occurring during the stress relief process

CONCLUSIONS Based on hitherto attained research resu lts and experience in practica! use of VSR weldments the introduction of VSR into practice can be recommended. The Vibratory stress relieving can be employed for stabilisation of the size of suitable weldments prior to their machining and servicing as a replacement of stress relief annealing. The VSR process is used for lowering of residual stresses and stabilisation of the size of different weldments such as trames of forming machines, machine trames, grey cast iron castings, etc. which were up to now subjected to stress relief annealing. VSR does not negatively affect the static dynamic strength of welded joints and weldments, fracture and notch toughness and homogeneity of welded joints.

Based on the attained data the implementation of VSR procedures as a replacement of stress relief annealing for the stabilisation of weldments, castings and forging leads to high savings of production costs to our national economy. The saving

7

of thermal energy has to be emphasized first of all because in the VSR procedure it does not exceed 1% of the energy required for the annealing of weldments and in average only 0.4% of production costs.

REFERENCES • Polnov, VG. 'Effect of natural oscillations of

welded structures on residual stress relief by vibrations.' Welding lnt. 1989, 3(6) 520-523.

• jesensky, M 'Vibratory lowering of residual stresses in weldments' Proceedings of IIW conference, Sofia, July 1987.

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

• Buhler, H. Et al. ' lnvestigations into the reduction of welding stresses.' Schweissen and Schneiden, May 1964.

• Gifford, DJ. 'Vibratory Stress Relief.' Metals Australia, April 1984.

• Wahlstrom,LE. 'Dimensionsstabiliering genom vibrationer.' Report No. 83 jan. 1976. lnstitutionen for svetsteknologi Kunliga Tekniska Hogsoko - jan. Swedish Welding lnstitute.

• Leide, NG. 'The significance of residual welding stresses - sorne experimental results and practica! experience in a sh ipyard.' Proceedings of paper 15, Wl conference 'Residual stresses in welded constructions and their effects.' London, Nov. 1977.

• Strachen, RW. Report on the Efficiency . of vibrational stress relief. General Dynam1cs report no. U413-68-059. 1976

• Zveginceva, KV. Svarochnoe Proizvodstvo. 1968 (11).

• Zubchenko, OL. Et al. 'Vibrating loads used for relieving stresses in welded trames.' Automatic Welding, 1974, 27(9), 59-62.

• Bonthuys, BF, Vibratory stress relief study. ISCOR South Africa 1989 Proprietary report.

• Ohol, RD. et al. 'Measurement of vibration­induced stress in the heavy fabrication industry.' Proceedings of lnternational Symposium on mechanical relaxation of residual stresses. Cincinnati, Ohio, April 1987.

• Sagalevich, VM et al ' Eliminating strains in welded beam structures by means of vibration.' Svarochnoe Proizvodstvo, 1979, (9), 9-11.

• Walker, Waddell & Johnstone. 'Vibratory stress relief - An investigation of the underlying process.' Proceedings lnstitute Mechanica l eng ineers vol209, pp51-58 1995.

• Dawson, R. 'Residual stress relief by vibration.' Ph.D. Thesis, Liverpool University 1975.

• Sedek, P. Vibtrational stabi lisation of welded structures - experiments and conclusions.' Proceedings of IIW conference, Sofía, Ju ly 1987.

• Ananthagopal, KP et al. 'Effect of Vibratory stress relieving on dimensional stability of

fabricated structures.' Proceedings of the National We lding Seminar, lndian lnst itute of Welding October 1986.

• De. Rudder, A. Et al. Studie van het utrillen van tasspanningen.' Hoger Technisch lnstitut. Oostende, 1970-71.

• Claxton, RA. 'Vibratory stress relieving of metal fabrications.' Welding & Metal Fabrications, 1991.

• Claxton, RA. 'Vibrations reduce stress levels.' European Surface Treatment, Winter 1992/3.

• Saunders, GG & Claxton, RA. 'VSR. A current state of the art appraisal.' Proceedings of Paper 29. lnternationsl Wl Conference ' residual stresses in welded constructions and their effects.' London Nov. 1977.

• Hornsey J. VSR Recent Developments Johannesburg june 2001

• Uoyd S.M Vibratory Stress Relieving an Alternative to Thermal Dissertation submitted to the University of Hertfordshire in partial fulfi llment of the requirements for the degree BSc (Honours) Johannesburg Feb 1998.

Claxton R.A Vibratory Stress Relief -An Authorltative Overview Material Australia january 1 February 1998.

• Hornsey J.S Vibratory Stress Relieving An Alternative to Thermal Treatment March 2002

8