Shock Wave Behaviour of Polymeric Materials for Detonation Waveshapers

This paper discusses the experimental determination of explosive shock attenuation parameters of four different polymers viz., Teflon, Phenol formaldehyde, Polyethylene foam and Polypropylene foam. These polymers are candidate materials for waveshapers in shaped charge warheads. Cylindrical specimens of the polymer materials were subjected to explosive shock loading by the detonation of RDX:Wax (95:5). Shock arrival time was measured using piezo-wafers positioned at known spatial intervals in the specimens. Initial shock velocity, stabilised shock velocity and attenuation constant were determined. These parameters are essential for the design of waveshapers. Foams have better shock attenuating properties compared to solids due to their cellular structure. Polypropylene foam has the highest shock attenuating characteristic among the four materials studied.

Acute Effects on Impact Accelerations Running with Objects in the Hand

Amateur runners usually run carrying implements in their hands (keys, a mobile phone, or a bottle of water). However, there is a lack of literature about the effects of different handloads on impact accelerations. Thus, this study aimed to analyse the effects of carrying different objects in the hand on impact accelerations during running. Nineteen male recreational runners (age 24.3 ± 6.8 years, training volume of 25 ± 7.38 km/week) performed twenty minutes of running on a treadmill at 2.78 m/s with four different conditions: no extra weight, with keys, with a mobile phone, and with a bottle of water. Impact acceleration and spatio-temporal parameters were analysed through a wireless triaxial accelerometry system composed of three accelerometers: two placed in each tibia and one placed on the forehead. A higher tibia acceleration rate in the dominant leg was observed when participants ran holding both a mobile phone (p = 0.027; ES = 0.359) and a bottle of water (p = 0.027; ES = 0.359), compared to no extra weight. No changes were observed in peak acceleration, acceleration magnitude, and shock attenuation in any other conditions. Likewise, neither stride frequency nor step length was modified. Our results suggest that recreational runners should not worry about carrying objects in their hands, like a mobile phone or a bottle of water, in short races because their effect seems minimal.

Shock Attenuation and Electromyographic Activity of Advanced and Novice Equestrian Riders’ Trunk

Low back pain (LBP) is a prevailing affliction among recreational and professional horse riders. It lowers performance and distracts from the development of the athlete. A lower incidence of LBP has been observed in the professional riding population despite higher training volumes. This paper aims to describe neuromuscular mechanisms through which advanced and novice riders attenuate the shockwave generated between the horse and rider. Six novice riders (age 24 ± 7 years), nine advanced riders (age 31 ± 5 years) and four horses (age 15 ± 3 years) took part in this study. Surface electromyographic activity of the riders’ trunk was recorded along with inertial data from the horses’ leg, saddle and vertebrae of the riders at walk, trot and canter. Analyses of variances revealed a superior shock attenuating ability of the advanced group (F (1,38) > 5.023, p < 0.03) as well as a higher overall muscle tone (F (1,488) > 9.80, p < 0.02). Cross correlation analyses between shock attenuation and electromyographic activity showed that groups differ mostly in timing rather than the intensity of muscle activation. This finding is of great interest for equestrian athletes and coaches, as it points to a greater importance of training muscular anticipation within the trunk stabilizers rather than overall muscle activation.

Numerical Investigation of Remote Ignition in Shock Tubes

Highly resolved two- and three-dimensional computational fluid dynamics (CFD) simulations are presented for shock-tube experiments containing hydrogen/oxygen (H2/O2) mixtures, to investigate mechanisms leading to remote ignition. The results of the reactive cases are compared against experimental results from Meyer and Oppenheim (Proc Combust Inst 13(1): 1153–1164, 1971. 10.1016/s0082-0784(71)80112-1) and Hanson et al. (Combust Flame 160(9): 1550–1558, 2013. 10.1016/j.combustflame.2013.03.026). The results of the non-reactive case are compared against shock tube experiments, recently carried out in Duisburg and Texas. The computational domain covers the end-wall region of the shock tube and applies high order numerics featuring an all-speed approximate Riemann scheme, combined with a 5th order interpolation scheme. Direct chemistry is employed using detailed reaction mechanisms with 11 species and up to 40 reactions, on a grid with up to 2.2 billion cells. Additional two-dimensional simulations are performed for non-reactive conditions to validate the treatment of boundary-layer effects at the inlet of the computational domain. The computational domain covers a region at the end part of the shock tube. The ignition process is analyzed by fields of localized, expected ignition times. Instantaneous fields of temperature, pressure, entropy, and dissipation rate are presented to explain the flow dynamics, specifically in the case of a bifurcated reflected shock. In all cases regions with locally increased temperatures were observed, reducing the local ignition-delay time in areas away from the end wall significantly, thus compensating for the late compression by the reflected shock and therefore leading for first ignition at a remote location, i.e., away from the end wall where the ignition would occur under ideal conditions. In cases without a bifurcated reflected shock, the temperature increase results from shock attenuation. In cases with a bifurcated reflected shock, the formation of a second normal shock and shear near the slip line is found to be crucial for the remote ignition to take place. Overall, the two- and three-dimensional simulations were found to qualitatively explain the occurrence of remote ignition and to be quantitatively correct, implying that they include the correct physics.

Optimization of the Rubber Formulation for Footwear Applications from the Response Surface Method

Impact force remains the primary cause of foot injury and general discomfort with regard to footwear. The footwear industry traditionally relies on modified elastomers (including natural rubber) whose properties can be physically adjusted by varying the constituents in the rubber formulations. This work aims to investigate the effect of filler/plasticizer fractions on shock attenuation of natural rubber soles. The statistical response surface method (RSM) was used to optimize the loading of natural rubber, fillers (carbon black and china clay) and a plasticizer (paraffinic oil). A novel predictive equation addressing the effects of additives on the physical and mechanical properties of the shoe sole was successfully created using the RSM. Our results demonstrate how the concentrations of these components regulate final properties, such as impact force absorption and hardness, in the commercial manufacture of shoe soles. While a higher loading level of plasticizer promotes reductions in hardness and impact force, as well as energy dissipation, in these modified elastomers, these properties were improved by increasing the filler content.

Diffracted Blast Loads Behind Structures

An incident blast wave interacting with a building will diffract around the side walls and roof, resulting in reduced blast loads on the back wall. There is also a region behind the back wall where the blast loads will be attenuated (i.e., lower than the incident blast loads). This paper focuses on defining the attenuated blast load region as a function of the blast wave strength and building dimensions. Characteristic parameters are utilized to present the analysis results, including wave length, wave length normalized by a characteristic building dimension, and normalized standoff distance from the building back wall. Blast load adjustment factors (i.e., ratio of the diffracted to incident blast load) are used to define the blast load attenuation as a function of these characteristic parameters. The purpose of this work was to generate a database of the shock attenuation behind a structure for engineering modeling applications.