Abstract
An equation to predict blast effects from cased charges was first proposed by U. Fano in 1944 and revised by E.M. Fisher in 1953 [1]. Fisher’s revision provides much better matches to available blast impulse data, but still requires empirical parameter adjustments. A new derivation [2], based on the work of R.W. Gurney [3] and G.I. Taylor [4], has resulted in an equation which nearly matches experimental data. This new analytical model is also capable of being extended, through the incorporation of additional physics, such as the effects of early case fracture, finite casing thickness, casing metal strain energy dissipation, explosive gas escape through casing fractures and the comparative dynamics of blast wave and metal fragment impacts. This paper will focus on the choice of relevant case fracture strain criterion, as it will be shown that this allows the explicit inclusion of the dynamic properties of the explosive and casing metal. It will include a review and critique of the most significant earlier work on this topic, contained in a paper by Hoggatt and Recht [5]. Using this extended analytical model, good matches can readily be made to available free-field blast impulse data, without any empirical adjustments being needed. Further work will be required to apply this model to aluminised and other highly oxygen-deficient explosives.
Highlights
An equation to predict blast effects from cased charges was first proposed by U
Many real bomb casings are made from metals with significant yield strength and these will fracture at expansion radii where internal driving pressure of the explosive gases is significant and the simple energy balance between gases and casing fragments predicted by Gurney [3] has not been reached
In a further paper [6] it will be shown that the work energy EC remaining with the explosive gases at the radius of casing fracture can be expressed by the following modified version of equation (1): EC EC
Summary
The equation derived in [2] for the blast impulse I from a cased charge as a fraction of the impulse I0 from the same charge without a casing is: I= I0. This equation applies where the casing metal is very ductile and expands to a radius at which the internal driving pressure of the explosive gases is negligible. It applies only to explosive compositions that are neither aluminised nor otherwise highly oxygen deficient, since these generate additional blast energy through exothermic reactions with the surrounding air (i.e. after-burn). Many real bomb casings are made from metals with significant yield strength and these will fracture at expansion radii where internal driving pressure of the explosive gases is significant and the simple energy balance between gases and casing fragments predicted by Gurney [3] has not been reached
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