IIW History 1948-1958

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presence of a defect, would not lead to brittle fractu re if the material n·as used at a temperature well above t he fract ure appearance transition temperature. So far, however, t hese conclusions can only be stated t entatively, and much more work is required to establish them with complet e reliability. The crux of the problem seems t o lie in the definition of transition temperature. It can be taken as generally agreed that, for any given structure, there is some t eni.perature above which brittle fracture would be impossible even in the presence of a defect and residu al stresses; but the fundamental difficulty is t o define this t emperature, since it is not easily relat ed t o a transition t emperature such as th f' Cha rpy V-Not ch fracture appearance transition, or the fracture appearance transition in the not ched t ensile t est , or, fo r that matter, any criterion of a multitude of ot her t ypes of not ch t est s which a re in use in different countries by different investigators. Basically, t herefore, t he problem is the establishment of a relation beb·veen t he behaviour t o be expect ed in service in t he p resence of severe defects and high residual stresses, and the properties of the mat eri al as det ermined by ome test or ot her which may have to be quite different from any of the tests in current use today. A subsidiary p roblem to which the Commission will have to continue giving attention is the effect of different stress relieving treatments on the inhibition of brittle fracture. The t wo methods v1rhich were felt t o be complet ely reliable - namely stress reli eving in the furnace at 650° C, and mechanical overst rain beyond t he yield point at a temperature well above the transition t emperature of the mat eri al, cannot be applied in many cases, and large vessels fo r nuclear reactors are a case in point \vhere it is becoming increasingly difficult t o produce a uniform tempe rature of 600° C throughout the vessel. Consideration will have to be given therefore t o t he possibilities of stress relieving at lower temper– atures t h an 600° C, and the possibilities of carrying out stress reli ef by local heating. \"\ h ilst an appreciable amount of info rmation is now available on the brittle fracture beh aviour of different t ypes of mild st eel up t o r in (2 .54 cm .) in thick– ness, t his work must be extended to st eels of great er thickness and t o st eels of great er t ensile strength. It is hoped that the international co-operation which h as now been so firmly established within t he Commission will encourage increasing participation of other count ries in the work on these two ext ensions of t he problem. For a number of years the Commission had to adopt the attitude that it could not be said definit ely t h at residual stresses could have an important del– eterious effect on t he fatigue strength of welded construction. On the other hand, it could not be said, on t he basis of the available evidence, t hat residual stresses exercised no influence on fatigue strength. Recent ly, experimental results emanating from Russia and from Czechoslovakia \Vere published which indicat e quite definit ely that residual stresses in welded structures can have an important influence on fatigue strength in the presence of severe notch effects . The results obtained are so startling that it h as become an urgent problem fo r the Commission t o pursue this problem with renewed energy . Similar work carri ed out in the United Kingdom and stimulat ed by these results appears t o confirm wh at has been found by the Russian and Czechoslovakian invest igations. It must t herefore be the aim of the Commission t o devote renewed and energetic efforts t o the further elucidation of these fact s.