Quantification of Residual Stresses in the Weld by the Hole-drilling Method

Many failures of supporting structures or machines are not caused only by stresses that are invoked by loading, but they often result due to residual stresses. Time needed for reparation of equipment and production losses are important factors, but much more serious are possible injuries or death of people. Residual stresses occur also in the structures without loadings. They arise mostly due to technological processes, e.g. casting, rolling, welding, pressing or forging. Residual stresses are equally danger as are the stresses due to operational loading. They are undesirable from the following reasons: it is not easy to determine them, they are superimposed to the operational stresses. Compressive residual stresses in surface layer of material are sometimes created deliberately, e.g. by shot peening, hardening, and so on and they have positive contribution to the strength properties of machine part. Residual stresses can be dangerous, because they can be relatively hard determined by non-destructive methods. We are not able to predict their magnitudes, type, nor direction. They are able to superimpose with stresses induced by corrosion, fatigue influences and it is very hard to remove them from the structure. Residual stresses should be known during operation of the equipment in order to determine their influence on functionality of machine part. Although there were some attempts to model residual stresses, these models are not ideal. For the quality check during production of machine part it should be suitable to use the methods for determination of residual stresses at the same time. Although there was reached some progress in measurement of residual stresses to this time was not developed non-destructive, cheap and effective method. Regardless their occurrence and importance in determination of reliability of structures, the residual stresses are often not considered in practical computations. Residual stresses are one of the forgotten areas in designing of machine parts. The most spread method for determination of residual stresses is the hole-drilling method. It is based on change of stresses (or strains) after drilling the hole in specimen with residual stresses 1-5 . It is one of the methods based on the removing material from structure. Deformation change can be assessed by photoelastic coatings, brittle layers, or by strain gages. If we know magnitudes and directions of released strains, dimensions of the hole and material properties, we can determine residual stresses on the base of analytical or numerical analysis. This method is often considered to be semi-destructive, because drilled hole can be very small or it can be repaired by pin, bolt, or by welding. Of course, such reparation can change stress distribution. Sometimes are used blind holes although the assessment of residual stresses is rather complicated than for through holes.