Experimental Measurement of Pressure Loading from Near-Field Blast Events: Techniques, Findings and Future Challenges

Abstract

The accurate characterisation of pressure loads imposed on structural members following the detonation of a high explosive is critical to our ability to design protective systems. This poses serious challenges for experimentalists, due to the high magnitude and short duration of loading. If the distance from the detonation to the target is relatively large, the loading is imparted through the interaction of a shock wave travelling away from the detonation through the surrounding medium, say air. Here, pressure magnitudes are typically in the range 10 3-10 6 Pa and are measurable using conventional, commercially available piezo-electric or piezo-resistive pressure transducers. A considerable effort has been expended on experimentally characterizing these "far-field" loads and consequently, we have a strong understanding of the mechanisms and magnitudes of loading. However, we are also interested in the loading when the target is very close to the detonation, for a range of protection applications, from aviation security to the design of personnel protection. Here, very different physical processes dominate. At these so-called "near-field" distances from a detonation (<~1 m/kgTNT 1/3) the high temperature gaseous detonation products are still violently expanding, and loading on a target is generated by the impingement of both the shocked surrounding material and these products themselves. Blast pressures are often higher that the yield strength of structural materials and temperatures can reach several thousand Kelvin. Furthermore, loading can vary by an order of magnitude over very short distances and timescales. This paper will describe experimental work conducted at University of Sheffield on developing approaches to accurately measure and predict near-field blast loading and gain a better understanding of the underlying mechanisms of loading. The challenges inherent to this field of work will be discussed and an attempt made to identify some of the emerging themes for future research.

Behaviour of a blast-driven ball bearing embedded in rear detonated cylindrical explosive

“…Assuming a TNT equivalency value of 1.2 for PE4, the Hopkinson scaled distance (Z) values for the detonations above 10g are below 0.44 m/kg 1/3 . It is known that the (initially very hot) fireball continues to expand until it reaches Z of 1 m/kg 1/3 [31], so it is plausible the fireball is at sufficiently high temperature to mark the surface of the aluminium alloy witness plate for 0.31 m/kg 1/3 < Z < 0.44 m/kg 1/3 .…”

Frequency Bandwidth of Pressure Sensors Dedicated to Blast Experiments

“…In our work, we will use the simplified Kingery and Bulmash model reported in [ 22 ], where each of the measured descriptors , and of blast waves are conveniently given for both side-on and reflected pressure measurement, and for hemispherical surface bursts and spherical airbursts. K&B data are a reference standard [ 7 ] and are used, for example, in the simulation software ConWep [ 23 ], in the LOAD_BLAST module of LS-DYNA [ 24 ] and in many research works (see, e.g., [ 3 , 4 , 6 , 20 ]).…”

“…During an explosion, the measurement of the overpressure at the front of the generated blast wave by using pressure sensors is a standard and well-controlled technique (see, e.g., [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 ]). Assessing the magnitude of the overpressure peak is crucial to characterize explosives, because this magnitude can be used to derive a plethora of physical quantities at the blast wave front from the Rankine–Hugoniot relationships [ 9 ].…”

The effect of explosive charge backing in close-proximity air-blast loading

“…The Hopkinson-Cranz scaled distance, Z = R/W 1/3 (where R is the SOD from the charge to the target and W is the TNT equivalent charge mass) is a dimensional scaled distance. Explosions are considered 'near-field' for Z < 1 m/kg 1/3 , and in this region the blast pressure and impulse distribution is extremely nonuniform and fireball effects are significant (8) (9) (10). Spherical blast waves propagate in the axial and radial directions after the detonation of a cylindrical charge (9) (8).…”