Trends in pressure vessel applications involving higher pressures, lower service temperatures, thicker walls, new materials, and cyclic loading require the development of new bases in the supporting scientific and technological areas. This report presents a “broad look” analysis of the opportunities to apply new scientific approaches to fracture-safe design in pressure vessels and of the new problems that have arisen in connection with the utilization of higher-strength steels. These opportunities follow from the development of the fracture analysis diagram which depicts the relationships of flaw size and stress level for fracture in the transition range of steels which have well-defined transition temperature features. The reference criteria for the use of the fracture analysis diagram is the NDT temperature of the steel, as determined directly by the drop-weight test or indirectly by correlation with the Charpy V test. Potential difficulties in the correlation use of the Charpy V test are deduced to require engineering interpretation of Charpy V test data rather than to involve basic barriers to the use of the test. The rapid extension of pressure vessel fabrication to Q&T steels is expected to provide new problems of fracture-safe design. These derive from the susceptibilities of steels within this family to tear fractures of low energy absorption. This fracture mode does not involve a transition temperature and is therefore relatively independent of temperature. It is emphasized that such susceptibilities are not inherent to the family of Q&T steels of low and intermediate strength levels, but are related to specific metallurgical conditions of the plate and particularly the HAZ (heat-affected-zone) regions of Q&T steel weldments.
Public safety agencies in many large metroplitan cities need a mobile system which is capable of safely transporting “terrorist-type” bombs from a discovery point to a disposal area. In view of the requirement of such a system by the Government of the District of Columbia, Metropolitan Police Department (DC-MPD), the Naval Research Laboratory has provided a solution to this problem by designing and fabricating a prototype explosives containment system. The system capability was successfully demonstrated by proof tests using dynamite. The critical elements of the system are an ultrahigh-strength, highly fracture-resistant steel pressure vessel held in a specially fabricated support base made of similar steel. Materials for the system were selected and evaluated on the basis of advanced metals characterization procedures to ensure fracture-safe performance in this unique application. A duplicate system has been donated to the DC-MPD. This system is considered to be the most reliable, highest-strength and lightest-weight mobile explosives containment system available to any metropolitan public safety organization.
The fracture behavior of thick-walled nuclear vessels is considered for the case of a radiation-induced toughness gradient through the wall which characteristically results from neutron attenuation by the wall material itself. Fracture-safe design analyses based on linear elastic formulations or extrapolations of these formulations to the elastic-plastic regime are not sufficiently developed to characterize the integrated behavior of a wall whose toughness can range from brittle at the inner surface to highly ductile at the outer surface. Solutions to the problem in the foreseeable future will be obtained only by experimental means. The present approach uses the Fracture Analysis Diagram (FAD) together with a new interpretative method for fracture extension resistance based on modified dynamic tear specimens as the tools for gradient assessments. With these techniques the significance of the toughness gradient through the wall is assessed in terms of thick section mechanical constraint, and fracture characteristics of the complete wall are predicted. Characterization of a hypothetical 8-in. vessel wall is based on measured through-thickness fluence levels; this behavior is correlated with fracture toughness degradation for steels of varying sensitivity to irradiation using the FAD. This analysis indicates that major portions of the vessel wall remain above the FTE temperature, which dictates yield stress loading requirements for fracture, when the wall temperature is maintained at the limiting value, NDT + 60 deg F, for the inside surface as suggested by current AEC criteria. Fracture extension resistance measurements based on data from a 3-in. thick plate having a metallurgically induced toughness gradient suggest that nuclear vessels with analogous gradients will not fracture in an unstable fashion and will not generate missiles capable of breaching the containment system. Additional research is necessary to fully develop the approach for application to the individual reactor vessel.
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