Blast Wave Clearing Effects on Finite-Sized Targets Subjected to Explosive Loads

A high explosive detonation is characterised by the rapid release of energy as a mass of explosive material is converted into a high pressure, high temperature gas. As this gas expands it displaces the surrounding air, causing a high pressure shock wave to travel through the air away from the explosive at supersonic speed. This shock wave can potentially cause significant damage as it impacts a structure -- it is the challenge of the engineer to ensure that our infrastructure is robust enough to be able to withstand such extreme loading.
The first aspect of blast engineering is to be able to predict and quantify the spatial and temporal variation of the load acting on the target to a sufficient level of accuracy. Whilst experimental trials and higher order numerical schemes offer useful insights, the time and expense associated with such methods renders them unusable for the early stages of design. Accordingly, semi-empirical blast predictions are more often favoured.
These semi-empirical predictions assume that the target forms a reflecting surface that is effectively infinite in dimensions perpendicular to the direction of travel of the blast wave. For finite-sized targets, however, the presence of target edges is known to significantly alter the pressure acting on the loaded face in a process known as blast wave clearing -- diffraction of the blast wave around the target which causes a relief wave to sweep in from the edges of the reflecting surface.
Current methods for predicting blast wave clearing fail to capture the physical process, and as such are inadequate at providing valid blast pressure predictions. Approximating the relief wave as an acoustic pulse allows for accurate predictions which are based on physically valid principles. Accurate prediction of cleared blast pressure loads has enabled the effect of blast wave clearing to be identified and quantified.
Secondly, the target response to this load must be predicted. The single-degree-of-freedom (SDOF) method approximates the distributed properties of the real life system into single point equivalent values. This procedure is well established for the target properties, and by transforming the spatial variation of cleared blast pressure in a similar manner (by conserving energy between real life and equivalent systems), clearing blast pressure loads can be modelled in SDOF analyses. The marked effect of clearing on structural response has been clearly demonstrated in this thesis.
The combined improvements to both load prediction and response modelling has allowed a full parametric study to be conducted on finite-sized targets subjected to blast loads. Neglecting clearing can be largely over-conservative for small targets, and for targets with plastic resistance significantly less than the applied transient blast force. It has been observed, however, that neglecting clearing can sometimes be non-conservative through a combination of target rebound, plasticity and early negative pressures caused by the clearing waves.
Findings in the PhD thesis should be used to highlight the complex nature of blast-target interaction, particularly when blast wave clearing is concerned, and should dispel the myth that a design will be safe if clearing is neglected. Results presented in the study can also be used by practising engineers to determine the likely effect that blast wave clearing will have on any configuration of explosive mass, stand-off, target size and dynamic properties, and the numerical models developed within have the potential for widespread use in existing commercial software.