SHOCK WAVE MEASUREMENTS 24s 



clearly wrong if peak pressure or energy are more significant factors. 

 It must also be remembered that ratios of this kind may depend sig- 

 nificantly on the pressure, and statements true for low pressures at con- 

 siderable distances from the charge may not be true at smaller dis- 

 tances. 



More precise values of peak pressure and other ratios than are given 

 by similarity curves can be obtained by direct comparisons in which 

 charges of different explosive materials are fired under as nearly the same 

 experimental conditions as possible. A vast amount of experimental 

 data has been acquired in this way, not only for comparison of explosive 

 materials, but also to determine the effects of differences in methods of 

 preparing the materials, effects of charge shape and orientation, and so 

 on.^ All that can be given here is a brief discussion of the ways in which 

 results of such comparisons can be presented, and their significance. 



B. Shock wave ratios for equal volumes of explosive. In many cases, 

 explosive comparison tests are performed by using equal volumes of ex- 

 plosives in cases of the same dimensions. The ratios of shock wave 

 parameters at equal distances will evidently be the same as for equal 

 weights only if the two explosives have the same density. On an equal 

 volume basis, as compared to equal weight, the material of higher den- 

 sity is evidently favored because of the greater mass of material, and 

 this difference further illustrates the necessity for caution in being sure 

 what one really means or is concerned with when comparing explosives. 



The equal volume basis is somewhat more convenient when com- 

 paring shock wave parameters with theoretical results, which are natur- 

 ally obtained directly in terms of charge dimensions, and this repre- 

 sentation will be considered first. Strictly, peak pressure and other 

 ratios should be stated for a specific value of ao/R, where ao is the charge 

 radius (or other characteristic dimension) and R the distance. It is al- 

 most always found to be the case that in the pressure range 1,000 to 

 15,000 lb. /in. 2 these ratios do not change significantly with pressure 

 level, and for such pressures these results can usually be accepted with- 

 out more specific limitation. The standard deviation of mean ratios 

 obtained in tests at Woods Hole depend on the explosive and parameter, 

 but are of the order 2 per cent for peak pressure, 4 per cent for impulse 

 (integrated in most cases to approximately five times the time constant), 

 and 6 per cent for the energy function. 



As in the case of comparisons for equal weights of explosives, the 

 ratio of one explosive to another depends on the shock wave parameter 

 considered, the values being progressively larger for impulse and energy 

 as compared to peak pressure, owing to increase in both peak pressure 



® The most complete summary of such data is given in a report by J. S. Coles 

 (23) , in which results from tests at Woods Hole and from representative British and 

 German work are given. 



