ANALOGY BETWEEN LOW TEMPERATURE AND FATIGUE EFFECTS By

Wide-spread use of welding multiplied the number of brittle or fatigue fractures in steel structures, giying new initiatives for research on preyention measures. Several methods have been developed for the elimination of brittle fracture, among them that by PELLINI [1], [2] is the closest to traditional engineering approach. PELLINI'S diagram known in the literature as FAD (Fracture Analysis Diagram) indicates mean strength of low-grade steel specimens exhibiting the adverse conditions of real welded structures as a function of temperature T and fault size G. (Fig. 1), permitting the engineer to apply structural engineer-ing formulae at low temperatures hence for brittle or less tough material con-ditions. U c values of PELLINI's diagram are between an upper and a lower bound-ary curve in the T-uc co-ordinate system. Obviously, the upper boundary curve involves a minimum fault size value G.min still likely of impairing the specimen's mean strength. Where this diagram intersects the yield curve UF of the faultless material, elements including the fault undergo brittle fracture in any case. This point of temperature is NDT (Nil Ductility Transition Temper-ature). The lower houndary curve termed CAT (Crack Arrest Temperature Curve) points to a given mean stress value required to produce failure even for hig fault sizes. Thus, CAT may he considered as a minimum Uc helonging to a fault G. max or over. As a matter of fact, for "wide and thin members, the two houndary curves meet the tensile strength diagram UB of the faultless material at a sufficiently high temperature FTP (Fracture Transition Plastic Temperature). Its physical purport is obvious: with increasing temperature, the material grows in toughness, hence the effect of practically encountered faults becomes increasingly negligihle. FAD has been composed by PELLINI and al. on the basis of test results hut the correctness of this conception is also supported by fracture mechanics. 238 PLATTHY 0rnox l/DT flt... FT