Computational Mechanics and Design Group

Department of Civil & Structural Engineering

Research Seminar - Dr Samuel Rigby and Dr Andy Tyas

Date/time

07/10/2014 - 11:00 to 12:00

Speakers

Dr S E Rigby & Dr A Tyas

Venue

Joseph Husband Room, Department of Civil and Structural Engineering, Mappin Street. Sheffield

About the event

Both presentations are also to be presented at 6th International Conference on Protection of Structures Against Hazards.

Presentation 1: Validation of semi-empirical blast pressure predictions for far-field explosions - is there inherent variability in blast wave parameters?

S. E. Rigby

A considerable amount of scientific effort has been expended over many decades on developing means of predicting the loading generated when a blast wave impinges on a structure. Semi-empirical ‘look-up’ predictive methods, such as those incorporated in the UFC-3-340-02 manual, the ConWep code or the *LOAD_BLAST module of LS-DYNA, offer a simple means for predicting the blast loading generated in geometrically simple scenarios. However, reported test data frequently show considerable spread and lack of repeatability, which is often attributed to some inherent variability in the blast waves developed from detonations, although no definitive physical interpretation has been forwarded as to the source of such inherent variation. As such, the semi-empirical predictions are often viewed as only ‘ball-park’ or ‘order of magnitude’ estimations.
This paper presents experimental measurements of reflected pressure-time histories from a series of well-controlled small scale blast tests. Data fitting techniques are used to obtain experimental reflected pressure and impulse values which are compared to corresponding semi-empirical predictions. We find that it is possible to produce reliable and highly consistent, repeatable results that match predictions remarkably well and therefore show that existing semi-empirical blast predictions can be used with confidence as a first-order approach for quantifying the blast load a structure will be subjected to. Our results presented here suggest that for small scale far-field loading in simple geometrical scenarios, test-to-test variability can be reduced by ensuring that test parameters are tightly controlled.

Presentation 2: Testing apparatus for the spatial and temporal pressure measurements from near-field free air explosions

A Tyas

Accurate quantification of the loading on a structure resulting from the impingement of a blast wave following a high explosive detonation is crucial if analysts are to be able to determine the viability of protective structures. This is of particular importance in the case of near-field explosive detonations, where the magnitude of the loading is extremely high, and highly spatially non-uniform over the face of the target. This loading can result in localised failure of structural targets due to brisance or rear-face spalling (predominantly load magnitude related phenomena) or shear failure due to spatially non-uniform impulse take-up of the target (predominantly impulse related phenomenon). However, no clear and simple guidance exists on how to define the magnitude and spatial variation of very near-field blast loading. Whilst it is possible to use numerical modelling approaches to simulate the detonation, air-shock propagation and shock-structure interaction, little definitive, well controlled experimental data exists to validate such models.
This paper presents an experimental methodology that has been developed in part to enable such experimental data to be gathered. The experimental rig comprises an array of Hopkinson Pressure Bars, fitted through holes in a target, with the loaded faces of the bars flush with the target face. Thus, the bars are exposed to the normally or obliquely reflected shocks from the impingement of the blast wave with the target. Pressure-time recordings are presented along with associated Arbitary Langrangian Eulerian modelling using the LS-DYNA explicit numerical code. A new finite element based method is introduced which allows for correction of the effects of dispersion of the propagating waves in the pressure bars, enabling accurate characterisation of the peak pressures and impulses from these loadings.