IIW History 1948-1958

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portant alloy steels, by using a method, developed by \i\TELLINGER in Germa– ny, for the study of stress relief as a function of time and temperature. In this method a specimen is subjected to a torque and then heated; the relaxation of stress is measured as a function of temperature and time. vVhen the results of this investigation had become available it was realised that there were differences in the slopes and curves obtained for different materials, and it \Vas considered necessary to repeat some of the experimental work before t h e results of these investigations are made generally available. C) Recommendat ions for s t ress reli ef. The Commission had been asked by Commission XI, (Pressure Vessels, Boilers and Pipelines) to make recommendations with regard to the conditions and circumstances in which stress relieving of 1)ressure vessels should be com– pulsory. This request was transmitted to the Commission after the meeting in Florence in 1954. A resolution incorporating recommendations vvas tentatively adopted by the Commission the following year, but on circulating this resolution to national delegations it became very soon apparent that there was widespread and irreconcilable disagreement; consequently, these tentativ e recommendations were not in fact put forward. The difficulties in making such recommendations arise frorn many different sources . Practices differ in different countries, each of which naturally believes that its own practice is the only safe and reliable one. Moreover, there is very widespread disagreement with regard to the ultimate purpose of stress relieving in pressure vessels : some experts think that it is carried out primarily to prevent brittle fracture, others to prevent failure from fatigue, still others to produce in the mechanical behav iour of pressure vessels a general improvement, resulting p artly from the relief of residual stresses but partly also from metallurgical improvements; some sections of opinion consider that all pressure v essels whose failure may h aye catastrophic consequences should be stress relieved while other experts, on the basis of their experience, are of the opinion that pressure vessels need not be stress relieved provided the shell thickness does not exceed a certain value ; but further disagreement exists with regard to what this shell thickness should be. As a consequence of this experience, the Chairman instituted an extensive international enquiry, which is still in progress, on opinions held con– cerning stress relieving and the effect of residual stresses. The immediate object of the enquiry is to discover whether there are in fact any points of agreement at all, with the ultimate objective of clarify ing the issues forming the basis of di sagreement by means of critical experiments, if such experiments can be devised. By far the most important problem which the Commission must try to solve is that of the effect of residual stresses on brittle fracture. There can be no doubt at all that residual stresses are an important contributory factor in this phenomenon \Nhich is the result of a combination of three circumstances - residual stresses, the presence of a defect where the fracture can be initiated and a material in a suitable condition. It may be claimed, and it is hoped that the Commission will be in a position to produce irrefutable proof of this fact, that brittle fracture cannot take place in the absence of any one of these con– diti ons - that is t o say, in the absence of a defect from which brittle fracture can start, the risk of it occurring in service as a result of residual stresses would be negligible, even in a material prone to brittle fracture. Similarly, in the absence of residual stresses, that is in stress relieved structures, a defect would be most unlikely to prevoke brittle fr acture, even though the material were prone to it. And, as a third variation , one might consider that residual stresses, even in the 4 . WORK IN HAND AND FUTURE PROGRAMME.