A Review of UFC-3-340-02 Blast Wave Clearing Predictions

When a blast wave impinges on a target of finite lateral or vertical dimension, diffraction of the blast wave around the target edge causes a relief wave to propagate inwards along the face of the target. This relief wave reduces the pressure acting at any point it passes over as the higher reflected pressures migrate towards the lower incident pressure regions in a process known as blast wave clearing. UFC-3-340-02 predictions provide a method for adjusting the reflected pressure to account for the global reduction in impulse by assuming that pressure relief acts uniformly across the target face. This has been shown exper-imentally to be inaccurate for targets whose dimensions are similar to the length of the blast wave, and hence the validity of the UFC-3-340-02 clearing method is questionable. This paper presents results from a series of numerical analyses on the mechanism of blast wave clearing on different sized targets. It is found that the UFC-3-340-02 method is only applicable for target sizes ap-proaching 1/250 of the scaled distance, and that the apparent features witnessed in the large-scale high explosive and nuclear trials from which the method was developed are simplifications that cannot be translated to typical explosion events. The importance of correctly predicting clearing relief is highlighted with results from experimental and numerical work on the deflection of finite-sized plates subjected to cleared blast loads.