The Use of Cantilevers as Blast Wave Gauges.
Abstract
A study has been made of the response of elastic plastic and brittle cantilevers when subjected to blast wave loading with a view to using such devices as passive blast wave gauges, and of using the deformation of cantilevers to assess the characteristics of accidental explosions. The study was restricted to cantilevers that were circular in crosssection and made of readily available materials. A cantilever, when loaded by a blast wave, either deforms plastically, in which case the amount of deformation is the critical parameter, or fractures, in which case the failing or not failing of the cantilever provides the required information. Two numerical models were developed to describe the deformation of a dynamically loaded cantilever. Both models assume that the plastic deformation is localized in a region near the fixed end, and that the loading force was a function of the dynamic pressure time history and a variable drag coefficient, dependent on the Reynolds number, Mach number and angle of attack. The first numerical model assumed a rigidplastic response of the cantilevers. The model accurately described the response only of cantilevers made of 50/50 lead/tin alloy. It overestimated the deformation of cantilevers made of other materials exposed in both high explosive and shock tube experiments. The second model assumed an elasticplastic response for the blast loaded cantilever with strain hardening effects included. The algorithm was based on the premise that the elastic curvature of the cantilever was limited by the plastic yield stress of the material and that as the curvature approached this limit the cantilever was rotated by the necessary amount to keep the curvature constant and equal to this maximum. The amount of rotation was determined by fitting a fourth order polynomial with a constrained second derivative based on the maximum allowed curvature. The rotation angle were found from the angle derived from the slope of the fitted function at the origin. A rotation by this angle yields a minimum in curvature in the rotated reference frame. This model improved the predictions for cantilevers constructed of aluminum and steel. The numerical models were evaluated by studying the response of cantilevers exposed to shock waves produced in a shock tube, and to blast waves produced by the detonation of two large highexplosive chemical sources. The response of the cantilevers to the shock tube flows was recorded by highspeed photography which showed good agreement between the observed modes of deformation and those predicted by the model. The models which were finally developed also provided good predictions of the deformation or fracture of a wide range of cantilevers exposed to the freefield blast waves. These models were also used to detect any nonradial flows and to study the boundary layers in the blast wave over different surfaces. Finally, it is demonstrated how the numerical modelling can be used to determine the type of cantilever that might be used as a passive gauge for monitoring the blast wave from an explosive event, and for evaluating the deformation of a cantilever exposed to the blast wave from an accidental explosion so as to characterize that explosion.
 Publication:

Ph.D. Thesis
 Pub Date:
 January 1995
 Bibcode:
 1995PhDT........29V
 Keywords:

 Physics: Fluid and Plasma; Engineering: Civil; Engineering: Mechanical