IIW History 1948-1958

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\\'Orking group and, later on, of Sub-Commission A, five years were necessary before a first edition of the complete report could be approved by the parent Commission. In the meantime, however, many reports bad been circulated for study by the members of the Sub-Commission and discussed within national committees. The final text, entitled Recommended Welded Connexions for Pressme Vessels, contains fifty-nine drawings of recommended welded con– nexions comprising screwed and screwed and studded connexions for branches, flanges, jacket ed vessels, tube fo tube plate connexions and tube plate to shell connexions. For each t ype of assembly, the manufacturer has a choice between so many and such varied connexions that there is no possibility of bis not being able to find one corresponding to his own practice. The document shows the· dimensigns and types of the welds with, where necessary, one or more alternatives. In view of its terms of reference, the Sub-Commission did not set out to specify connexions which could be considered as mandatory or t o restrict in any way the possibilities of improvement, but it rather gave examples of sound and normally accepted practice. At the first meeting mentioned above, delegates noted that the values. adopt ed in the diffe rent countries with regard to allowable stresses were then so different that it would be well to concentrate attention on this question. The working group thought that a detailed study of allowable stresses was necessary, their value being dependant not only on the parent metal but also on the weld metal. At the same time, it was necessary to consider the influence· of pre- and post-heat treatment, of the dressing of welds, of non-destructive t esting etc., which, according to the national codes, had to be taken into account in order to fix the value of the weld efficiency factor when calculating the nominal allow– able stress. The first step ·was to prepare a table of all the values of the allowable stresses. and weld efficiency factors appearing in the national codes. These values were· carefully examined and compared, account being taken of the differences between. the weld efficiency factors in the calculation formulae used and of the differences in general constructional practice. Only those values were considered which had been shown to be reliable in practice by at least t en years of application. This investigation allowed members of the working group t o compare the values accepted in the different countries and to derive from them a curve synthesising: the various national practices, from which it was possible to find the maximum. stresses which could be considered as acceptable and safe . At the same time, a fundamental theoretical investigation was undertaken. All available data were assembled regarding the measurement of the stresses on. the various parts of pressure vessels, such as domed ends, nozzles, supports, etc. The comparison between the measured stress raisers on the one hand and the· conYentional formulae and allowable stresses given in the various national codes on the other resulted in a fairly precise calculation of the allovvable stress peak,. this peak also being based on good practice. It 'vas found that the all°'vable stress peak was above the ultimate tensile strength of the metal so that it was not possible to apply a fact or of safety based on the yield point or upon the U.T.S. It " ·as realised that such high stress raisers would result in partial yield of the material when the first load was applied and sometimes in reverse y ield at un– loading. The stress would then vary behveen a negative and a positive value in the course of further stress cycles. It was established that it was not the stress itself "·hich was important, but the stress range which had a higher value. The study 'vas continued on the basis of this new principle. It was noted that one of the criteria of calcul ation of allowable stresses was high strain fatigue, this being a logical consequence of the use of the stress range instead of the maximum