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The local approach criterion of fracture mechanics, initially developed by Beremin for brittle cleavage fracture, is applied here to A508 class 3 low-alloy ferritic steel. This criterion, based on the maximum principal stress and Weibull statistics, has previously been verified in the case of uniaxial tests. In this study, it is extended to multiaxial loading tests, that can lead to more significant levels of plastic strain, and thus permit a study of the effect of plastic strain on cleavage fracture. Uniaxial tests on axisymmetric notched tensile bars (AE2-6) were used to determine Beremin's model parameters m and [[Sigma].sub.u]. The cleavage fracture behavior, described by these parameters, was then verified by multiaxial tension-torsion tests carried out on thin tubular specimens. Numerical simulations of the tension-torsion tests, by the finite element method, were also performed, taking into account the nonlinear geometrical effects and the specimen plastic buckling. The buckling critical loads were calculated and used to ascertain whether fracture was associated with the instability phenomenon. Beremin's model is shown to correctly describe experimental data which are not affected by buckling.

The high-temperature design codes are presently considering the use of stress reduction factors for designing welded structures submitted to creep. These reduction factors are derived from creep tests which are generally made on small specimens and are not necessarily representative of large-size geometries. These codes are very likely overconservative, consequently uneconomical and need to be improved; an investigation to assess and quantify the supposed size effect is required. This paper presents an experimental and numerical study on creep behavior at 600 [degrees] C of full-size welded joints taking into account real full-thickness of weldings. The material investigated is the austenitic stainless steel 316L(N) with manual metal arc welds using the 19 Cr 12 Ni 2 Mo electrode grade. The creep laws used in calculations are those obtained from tests using small specimens, but some coefficients of their theoretical formulation have been modified to obtain a better coherence with full-size specimen data. Between small and large full-size specimens, experimental results show no significant difference in time to rupture, and the same location of fracture, at the center of the weldment, is observed. Finite element simulations performed for full-size welded joints provide rupture times that are consistent with measured values. The calculated percentage of the damaged volume in the weld metal as a function of load levels and of creep-time duration is studied; it shows that the creep-rupture times for high stress loading are determined with higher accuracy than for low stress loading.

In order to validate the long-term behavior of fast breeder reactor steam generator tubes, a creep test monitoring program has been undertaken. A series of over one hundred tests was performed on small specimen tubes which were cut from the as-received tube material of lncoloy 800, similar to those used in generator applications. In contrast to uniaxial conditions in which constant load conditions usually apply, multiaxial experiments were performed assuming constant stress conditions. Thin-walled tube approximation was used to estimate the stresses across the tube wall thickness required for given internal tube pressures. This paper deals with a refined numerical modeling of stresses and strains in the pressurized test specimens, taking into account: internal pressure changes, nonlinear geometrical effects, and creep deformation during all the test procedure cycles. Comparison of the numerical results with the experimental measurements shows a good agreement which validates the multiaxial generalization of the RCC-MR strain creep laws which were used in the calculations. A multiaxial creep rupture criterion is discussed.

Powder metallurgy techniques are often used in the fabrication of finished and semi-finished articles. At the CEA, a cold followed by a hot isostatic pressing method is used to produce solid forms from various types of powder. Occasionally, during the hot isostatic pressing part of the process, buckling of the titanium envelope near to the sealed end leads to fracture of the can and incomplete powder consolidation. The aim of this paper is to investigate by numerical finite element simulation the fracture process. A 2-D shell element model with Fourier series taking into account plastic deformation of the can material has been considered and used to determine the buckling critical pressure and the corresponding buckling mode. The simulation has been used to eliminate failure of the can by modifying the temperature-pressure schedule or by changing the can design. The calculations show that reducing the sharp angles of the initial titanium can near the rupture area can resolve the buckling problem; however, this solution is not totally satisfactory due to the development of a zone of constriction (breaks and irregularities) in the compacted powder just behind the modified can wall. This geometrical defect leads to difficulties in machining the final product. A better solution to the problem is to increase the initial can temperature prior to application of the pressure. This leads to a can of enhanced ductility with a better ability to deform. This latter solution, which can be employed with or without can modifications, eliminates both the can's buckling and the zone of constriction. These numerical results have been validated by recent tests performed in our laboratories.