Edge-on Impact Tests Research Articles

Ultra-high speed X-ray imaging of dynamic fracturing in cementitious materials under impact

In this work the dynamic fracturing of an ultra-high strength cementitious material is probed with in-situ ultra-high speed X-ray phase-contrast diagnostics to investigate the phenomenology of dynamic fracture. Gas gun experiments were conducted on two characteristic samples with two different impact speeds, namely 80 and 190 m/s using the edge-on impact test configuration. The samples were placed within the intense X-ray beam providing an observation field of 12.8 mm in width and 8 mm in height. Thanks to equispaced 16 bunches of short X-ray pulses, the samples were imaged through an indirect detector arrangement using the Shimadzu HPV-X2 camera lens-coupled to a fast scintillator capturing through-thickness measurements with an interframe time of 1.06 µs. The comparison of fragmentation patterns between two samples revealed an important insight into velocity dependant spall formation as well as the effects of crack closure and bridging.

Application of microtomography and image analysis to the quantification of fragmentation in ceramics after impact loading.

Silicon carbide ceramics are widely used in personal body armour and protective solutions. However, during impact, an intense fragmentation develops in the ceramic tile due to high-strain-rate tensile loadings. In this work, microtomography equipment was used to analyse the fragmentation patterns of two silicon carbide grades subjected to edge-on impact (EOI) tests. The EOI experiments were conducted in two configurations. The so-called open configuration relies on the use of an ultra-high-speed camera to visualize the fragmentation process with an interframe time set to 1 µs. The so-called sarcophagus configuration consists in confining the target in a metallic casing to avoid any dispersion of fragments. The target is infiltrated after impact so the final damage pattern is entirely scanned using X-ray tomography and a microfocus source. Thereafter, a three-dimensional (3D) segmentation algorithm was tested and applied in order to separate fragments in 3D allowing a particle size distribution to be obtained. Significant differences between the two specimens of different SiC grades were noted. To explain such experimental results, numerical simulations were conducted considering the Denoual-Forquin-Hild anisotropic damage model. According to the calculations, the difference of crack pattern in EOI tests is related to the population of defects within the two ceramics.This article is part of the themed issue 'Experimental testing and modelling of brittle materials at high strain rates'.

Experimental and numerical analysis of the dynamic fragmentation in a SiC ceramic under impact

Silicon carbide ceramics are generally used in armour configurations for the foot-soldier or military vehicles. However, their behaviour during impact is not fully understood. In this work, the dynamic fragmentation of silicon carbide has been characterised through different impact configurations: Edge-on impact tests and normal impact tests have been conducted in open and sarcophagus configuration. In the first configuration, an ultra-high speed camera is used to visualise the fragmentation process with an interframe time set to 1 μs. The sarcophagus configuration provides a post-mortem view of the damage pattern. Moreover, a comparison with numerical results given by an anisotropic damage model shows a good capacity to predict the damage patterns and the cracking densities of the ceramic after impact but also the damage kinetics observed during impact. In addition, an original impact configuration is proposed for characterising the dynamic behaviour of a pre-fragmented ceramic. The tests have been compared to experiments performed with a plane aluminium alloy target. The results underline the good penetration resistance of the fragmented ceramic.

Numerical and experimental study of the plasticity of HMX during a reverse edge-on impact test

The initiation mechanisms of mechanically stressed explosive compositions are largely dependent on the behaviour of the constituents, and the formation of hot spots is linked to the location of anelastic strains. This study takes a first step towards understanding the behaviour of HMX crystals (cyclotetramethylenetetranitramine, C4H8N8O8, octogen) when these are subjected to quasi-static stress. Our analysis couples the numerical simulation of the composite material at the mesoscopic scale with observations made at this same scale during weakly confined quasi-static tests. For the simulation, we have chosen a biphasic description of the composite by separating the large particles from the matrix otherwise assumed to be homogeneous. The matrix is, in fact, composed of smaller particles, binder and porosities. The behaviour of the HMX particles is determined from data in the literature: this opens up three possibilities with respect to its plastic behaviour. Based on triaxial tests and given the macroscopic response of the composite, the behaviour of the matrix is identified for each possibility with regard to the behaviour of the HMX particles. Real time observations made at a mesostructural scale firstly enable the plastification of the HMX particles to be demonstrated and secondly, by comparison with numerical simulations, enable a model to be attributed to these particles.

AlON: A brief history of its emergence and evolution

In the early 1970s in Japan, the United States and France it was found that additions of nitrogen into aluminum oxide resulted in new spinel-like phases. At about the same time there was much increased interest in oxynitrides, stimulated by Professor K. Jack in the UK and Y. Oyama in Japan. Following these activities a major research program in this area was initiated at the Army Materials and Mechanics Research Center in Watertown, Massachusetts in 1974. These efforts resulted in the first complete Al 2O 3–AlN phase equilibrium diagram and a process to reactively sinter to nearly full density, translucent aluminum oxynitride spinel ceramic, which was named AlON. Subsequently, the Raytheon Company further developed AlON into a highly transparent material (ALON™) with many applications including transparent armor and EM domes and windows, among others—the technology was recently transferred to the Surmet Corporation. This paper will review the early history, phase equilibrium, crystal chemistry, and properties of this material, along with more recent work in our laboratory on transient liquid phase sintering and new data on lattice parameter measurements. In addition, recent results of collaborative work on AlON's dynamic mechanical properties using plate impact, Kolsky bar and edge-on impact (EoI) experimental techniques, including preliminary modeling at the microstructural scale of AlON in the EoI test, will be presented.

Dynamic Fragmentation of an Ultrahigh-Strength Concrete during Edge-On Impact Tests

To understand and model damage generated during impact by a penetrator of ultrahigh strength concrete targets, edge-on impact tests are performed with Ductal concrete, which is unreinforced or reinforced with short fibers. Two edge-on impact configurations are designed with a dynamic confinement system. The first configuration uses aluminum projectiles and allows us to study the dynamic fragmentation that spreads out within the tile without any confined damage close to the impact point. The fragmentation process is composed of numerous oriented millimetric cracks. In the second configuration, steel projectiles are used with a higher impact velocity. Damaged zones are visualized by using an ultrahigh speed camera and a sarcophagus configuration designed to prevent the fragments from moving. Postmortem studies of impacted tiles enabled us to observe an intense fragmentation of the targets and confined damage close to the impact point when steel projectiles were used. Simulations are performed with an anisotropic damage model coupled with a concrete plasticity model. Orientation and crack density are compared with postmortem observations.

A ballistic material model for starphire ®, a soda-lime transparent-armor glass

Experimental observations pertaining to the damage evolution in Starphire ®, a soda-lime transparent-armor glass, made in the recent work of Strassburger et al. [E. Strassburger, P. Patel, J.W. McCauley, C. Kovalchick, K.T. Ramesh, D.W. Templeton, Proceedings of the 23rd International Symposium on Ballistics, Spain, April, 2007; E. Strassburger, P. Patel, J.W. McCauley, D.W. Templeton, Proceedings of the 23rd International Symposium on Ballistics, Spain, April, 2007] in a series of edge-on-impact (EOI) tests and other open literature experimental findings are used to construct a (high strain-rate, high-pressure, large-strain) ballistic constitutive model for this material. The basic components of the model are constructed in such a way that the model is suitable for direct incorporation into typical transient non-linear dynamics finite element-based software packages like ANSYS/Autodyn [ANSYS/Autodyn version 11.0, User Documentation, Century Dynamics Inc. a subsidiary of ANSYS Inc., 2007] or ABAQUS/Explicit [ABAQUS version 6.7, User Documentation, Dessault Systems, 2007]. To validate the material model, a set of finite element analyses of EOI tests was carried out and the computational results compared with their experimental counterparts. It is found that front-shapes and propagation velocities of the longitudinal and transverse waves are quite well represented by the model. The same was found to be the case for front-shapes and propagation velocities of the “ coherent-damage” zones but mainly at shorter post-impact times. Discrepancies at longer post-impact times are attributed to the effects of damage-promoting target-fixturing-induced stresses and cutting/grinding-induced flaws.

Visualization of Impact Damage in Ceramics Using the Edge‐On Impact Technique

Projectile impact generates severe fragmentation in ceramics which propagates at high velocities and precedes the penetration of the projectile. The high‐speed photographic technique of the Edge‐On Impact (EOI) has been developed at the Ernst‐Mach‐Institute (EMI) in order to visualize dynamic fracture in brittle materials. In a typical EOI test the projectile hits one edge of a specimen and fracture propagation is observed during the first 20 us after impact by means of a Cranz‐Schardin highspeed camera. EOI tests allow a characterization of different ceramics by the macroscopic fracture patterns, single crack velocities, and crack front velocities (damage velocities). The phenomenology of damage propagation in several ceramics and a ceramic‐metal composite is discussed. The EOI technique is useful for the evaluation of damage models for brittle materials because it enables a direct comparison of model predictions to experimental data obtained during the impact process.