Digital Speckle Radiography Research Articles

The significance of grain morphology and moisture content on the response of silica sand to ballistic penetration

The dynamic response of sand is of interest for a wide range of applications, from civil engineering to asteroid impact, in addition to defense and industrial processes. Granular dynamics are controlled by a complex network of intergrain force chains; yet, our understanding of how grain morphology, moisture, rate, and loading geometry affect the response to rapid compaction remains limited. Here, we show how just 1% moisture can significantly reduce penetration resistance in silica sand, while smoother-grained material—with a similar bulk density, grain size, and mineralogy—exhibits markedly improved stopping power. Cylindrical targets are impacted by spherical steel projectiles, with Digital Speckle Radiography employed to determine both the penetration depth and the sand bed displacement at a series of incremental time steps after impact. The results provide substantial insight into how slight adjustments to grain-grain contact points can affect the bulk dynamic response of brittle granular materials.

The effect of rod nose shape on the internal flow fields during the ballistic penetration of sand

This paper discusses the technique of Digital Speckle Radiography (DSR) and its application to the measurement of the internal flow fields in penetration of sand by long-rod projectiles at velocities up to 200 m/s. Three different rod nose shapes were studied: flat-ended, ogive-2, and hemispherical. Impacts performed on gelatine and concrete gave significantly different displacement fields to sand. Sand, therefore, cannot either be modelled as a fluid or as a conventional solid. Simulations performed using a code written by two of the authors (Bobaru and Promratana) showed that the velocity distribution has a very different appearance to the force distribution. This suggests that processes such as reorganisation, sliding and void filling take place, allowing the grains to move in directions other than the applied force. The resulting velocity distribution bears a strong resemblance to the experimentally measured displacement fields.

The use of digital speckle radiography to study the ballistic deformation of a polymer bonded sugar (an explosive simulant)

This paper reports an initial study into the benefits of determining two-dimensional flow fields for low velocity impact on a small-scale model of explosive reactive armour (ERA) using digital speckle radiography (DSR). The model system consisted of a polymer-bonded sugar (PBS) (otherwise known as a sugar mock) confined between two mild steel plates. The DSR technique relies upon creating a layer within the specimen that is seeded with lead particles. So although radiography itself is mechanically non-invasive, the lead layer needed may change the mechanical properties of the material. DSR revealed where regions of intense shear occurred in normal impact. These regions are likely to be where a polymer-bonded explosive (PBX) would initiate.

Response of PBXs and inert substitutes in launch and impact scenarios

The physical behaviour of a polymer bonded explosive (PBX) fill is important for the development of safe and reliable shells and penetrators. The strain rate environment during launch or impact is generally in the intermediate regime, below shock but above quasi-static. Confidence in the predictions of the constitutive response of PBXs is a necessary pre-requisite for accurate predictions of the likelihood of reactions. The level of deformation and the rate of that deformation can cause ignitions. A number of experiments have been completed with different levels of confinement of both PBXs and PB inerts to obtain data on their behaviour at these strain rates. Instrumentation has included flash X-ray, Digital Speckle Radiography (DSR), VISAR, high speed video and pressure gauges. Experimental data is presented together with hydrocode modelling comparisons. Necessary features of the material models are discussed together with lessons learnt in its application across the strain rate regimes present during launch and impact.

Measurement of granular flow in a silo using Digital Speckle Radiography

In this paper, the flow of a powder through a silo is investigated using Digital Speckle Radiography (DSR). This technique allows displacement measurements to be made on the sub-mm scale to an accuracy of 0.06 μm and a spatial resolution of 26 μm. The method performs an image cross-correlation on a random seeding of X-ray opaque material as it flows out of the silo with the powder. The flow is captured digitally using a continuous X-ray source and an image intensifier and Charge Coupled Device (CCD) camera to give real time measurements. The powder used is Al 2O 3 with an average particle size of 50 μm and the seeding material is tungsten powder, also with an average particle size of 50 μm. Clear flow behaviour is observed for two different sizes of outlet and the flow rate and strains occurring during the flow are also investigated.

Digital speckle radiography—a new ballistic measurement technique

Digital speckle radiography is a measurement technique which is capable of visualizing internal flow fields within an opaque material undergoing a ballistic impact. This recent development has been successfully applied to the study of polyester, cement, sand and glass targets. The technique is based on digital speckle photography but, instead of producing the required speckle pattern by traditional means, the specimen is seeded with a layer of X-ray opaque filings. By utilizing two X-ray heads and varying the delays and position of the seeded layer, a full three-dimensional displacement map can be built up. The theories behind the technique are introduced, and results presented from two-dimensional displacement measurements on sand subjected to rod impact and three-dimensional measurements of cement being impacted by a shaped charge jet.

Some aspects of flow in granular materials in hoppers

The main part of the paper is devoted to the description of the localized deformation zones in plane (slow) flow of granular materials from hoppers. Specifically, two types of zones are discussed i...