Evaluation of blast protection using novel-shaped water-filled containers: experiments and simulations

A combined experimental and numerical investigation was conducted into evaluating the influence of the geometry of a water-filled container on maximising the reduction in deformation it provides to a high-strength steel plate subject to localised blast loading. Experiments were conducted with a range of novel container shapes including a cone, inverted cone, diamond and mushroom. In addition to these container shapes, an array of water bottles known as a kinetic energy defeat device (KEDD) and a high performing quadrangular container design were also evaluated. The performance of each container was evaluated in terms of both the reduction in deformation of a steel target plate and the efficiency of the mitigation in terms of the reduction per unit mass of water. The numerical simulations were found to provide adequate predictions for the novel container shapes. They were then used to isolate the differences in target loading for each container type. Further numerical simulations were then performed to identify improvements in the design of the best performing containers. The best performing novel geometries were the mushroom and inverted cone shaped containers, which are more effective at radially spreading the water. However, the mushroom shaped container was the only container found to outperform the most efficient quadrangular container on a mass efficiency basis. The results of this investigation can be used to assist in the design of water-filled containers that are used as part of a near-field blast protection system on an armoured vehicle or other protected structure

Blast mitigation with fluid containers: effect of mitigant type

The effect of filled external containers on the deformation induced in a steel plate under near-field blast loading has been investigated through a combined numerical and experimental study. Six different fill materials (mitigants) were considered for inducing near-field blast mitigation. The mitigants evaluated were bulk water, aerated water, sand, expanded polystyrene (EPS), a combination of EPS and water, and shear thickening fluid. The performance of the mitigants depended on their mass, with sand providing the best mitigation and EPS the worst for a given volume. Bulk water provided the greatest reduction of the peak deformation per unit of added mass. The mitigant material also had a significant effect on the deformation-time history of the steel plate. The sand and ½ EPS + ½ water containers were found to significantly delay the arrival of the pressure wave at the target surface due to their compressibility and low sound speed. Numerical analysis reveals that different mechanisms induce blast mitigation, and these are identified for each of the different mitigant materials

High strain rate and high temperature response of two armour steels: Experimental testing and constitutive modelling

Under ballistic impact or blast loading, the high strain rate and high temperature behaviour of armour steels is key to their response to a given threat. This experimental and numerical investigation examines the tensile response of a class 4a improved rolled homogenous armour steel (IRHA) and a high hardness armour steel (HHA). Cylindrical tensile specimens were tested at a range of strain rates from 0.001 s-1 to 2700 s-1. Quasi-static, elevated temperature tests were performed from room temperature up to 300° C. While the HHA is strain rate insensitive, the IRHA displays a significant increase in strength across the range of loading rates reducing the ultimate strength difference between the materials from 19% at 0.001s-1 to 4.6% at 2700s-1. An inverse numerical modelling approach for constitutive model calibration is presented, which accurately captured the dynamic material behaviour. The modified Johnson-Cook strength and Cockcroft-Latham (C-L) fracture models were capable of predicting the ballistic limit of each material to within 5% of the experimental result and to within 10% for deformation under blast loading. The blast rupture threshold of both materials was significantly over-estimated by the C-L model suggesting stress state or strain rate effects may be reducing the ductility of armour steel under localised blast loading

High strain rate and high temperature response of two armour steels: Experimental testing and constitutive modelling

Under ballistic impact or blast loading, the high strain rate and high temperature behaviour of armour steels is key to their response to a given threat. This experimental and numerical investigation examines the tensile response of a class 4a improved rolled homogenous armour steel (IRHA) and a high hardness armour steel (HHA). Cylindrical tensile specimens were tested at a range of strain rates from 0.001 s-1 to 2700 s-1. Quasi-static, elevated temperature tests were performed from room temperature up to 300° C. While the HHA is strain rate insensitive, the IRHA displays a significant increase in strength across the range of loading rates reducing the ultimate strength difference between the materials from 19% at 0.001s-1 to 4.6% at 2700s-1. An inverse numerical modelling approach for constitutive model calibration is presented, which accurately captured the dynamic material behaviour. The modified Johnson-Cook strength and Cockcroft-Latham (C-L) fracture models were capable of predicting the ballistic limit of each material to within 5% of the experimental result and to within 10% for deformation under blast loading. The blast rupture threshold of both materials was significantly over-estimated by the C-L model suggesting stress state or strain rate effects may be reducing the ductility of armour steel under localised blast loading

Characterisation of the behaviour of welded aluminium structures under dynamic loading

The presence and quality of welds in metallic structures has the ability to influence their likelihood of failure under dynamic loading. This investigation focused on characterising the behaviour of a welded aluminium structure. Samples were taken from the parent metal, heat affected zone (HAZ) and the weld bead and high strain rate characterisation testing was performed to determine the Johnson-Cook (JC) strength and failure model parameters for each material. However, significant scatter was found in the data for the weld bead due to porosity within the samples. Additional tensile tests were performed using a rotating fly wheel machine with four larger samples, which were machined from the welded aluminium structure and contained HAZs on either side of the weld bead, located in the centre of the specimen. Three of the four samples had the weld bead ground flush to the level of the base plate. Digital image correlation was used to determine the surface strain within each region of the sample and identified significant strain localisation at the interface between the weld metal and the HAZ, as well as within the weld bead. Comparisons between the ground welded specimens and those with the weld reinforcement showed a different failure mode between the two specimens. For the ground specimens, the strain localisation in the weld bead initiated failure prior to the strain localisation occurring at the interface between the weld bead and HAZ. Sectioning of the welds indicated that the strain localisation in the weld bead may have been caused by significant levels of porosity within the weld bead. Preliminary numerical simulations of the ground specimens indicated that the force-time history could be well captured. However, as the strain localisation due to porosity is not captured using a JC model, in addition to the scatter in the characterisation data for the weld bead, failure was not accurately predicted numerically

Characterisation of the behaviour of welded aluminium structures under dynamic loading

The presence and quality of welds in metallic structures has the ability to influence their likelihood of failure under dynamic loading. This investigation focused on characterising the behaviour of a welded aluminium structure. Samples were taken from the parent metal, heat affected zone (HAZ) and the weld bead and high strain rate characterisation testing was performed to determine the Johnson-Cook (JC) strength and failure model parameters for each material. However, significant scatter was found in the data for the weld bead due to porosity within the samples. Additional tensile tests were performed using a rotating fly wheel machine with four larger samples, which were machined from the welded aluminium structure and contained HAZs on either side of the weld bead, located in the centre of the specimen. Three of the four samples had the weld bead ground flush to the level of the base plate. Digital image correlation was used to determine the surface strain within each region of the sample and identified significant strain localisation at the interface between the weld metal and the HAZ, as well as within the weld bead. Comparisons between the ground welded specimens and those with the weld reinforcement showed a different failure mode between the two specimens. For the ground specimens, the strain localisation in the weld bead initiated failure prior to the strain localisation occurring at the interface between the weld bead and HAZ. Sectioning of the welds indicated that the strain localisation in the weld bead may have been caused by significant levels of porosity within the weld bead. Preliminary numerical simulations of the ground specimens indicated that the force-time history could be well captured. However, as the strain localisation due to porosity is not captured using a JC model, in addition to the scatter in the characterisation data for the weld bead, failure was not accurately predicted numerically