Experimental demonstration of shakedown in a vessel submitted to impulsive loading

Vessels subjected to internal impulsive loadings, such as those used for controlled-detonation chambers, can be designed for a single impulsive load application or for multiple impulsive loads. Design of a single-use vessel may take advantage of the capability of the vessel material to absorb energy through elastic-plastic behavior, provided that the public health and safety is protected, even though the owner's investment in the vessel may be compromised because of severe distortion and potential loss of containment functionality. However, when the vessel is designed to contain multiple internal impulse loads, the usual design practice is to require completely elastic response or, at most, very localized elastic-plastic behavior. A recently approved ASME Boiler Pressure Vessel Code, Section VIII, Division 3 action (Code Case 2564-2) provides limits for the accumulated plastic strains in such vessels, including a limit on the accumulated plastic strain averaged across the wall thickness of the vessel, that are sufficiently conservative to permit the design of vessels for both single-impulse and for multiple-impulse applications. Analytical or experimental demonstration to meet the Code Case 2564-2 strain limits is straightforward for the single-impulse vessel design and is relatively straightforward for multiple-impulse vessel designs when the vessel response to any of the individual impulsive loads is nearly elastic. However, when the design-basis impulsive loading for a multiple-impulse vessel design leads to significant plastic straining, the demonstration of design adequacy becomes extremely complex, raising issues of impulsive loading sequences (since elastic-plastic response is load-path dependent, what is the temporal order of the impulse loadings?) and demonstration of shakedown to elastic or near-elastic behavior. In such cases, an analytical demonstration of design adequacy may be impractical, while an experimental demonstration may be both practical and illuminating, especially if the demonstration is carried out at a scale that is both economical and convincing. Here, a one-seventh-scale model of a controlled-detonation vessel is used as the basis for demonstrating the effect of shakedown to essentially elastic behavior, with no further accumulation of plastic straining, along with the satisfaction of ASME Code Section VIII, Division 3, local ductility exhaustion requirements. The experiments on a scale model vessel have proved that the phenomenon of shakedown can be demonstrated experimentally, for internal detonation loadings that initially led to plastic strains up to 0.7.