Re‐Calibration of the UK Large Scale Gap Test

A new donor explosive, ROWANEX 3601 has been selected for use in the large scale gap test (LSGT). It was necessary to calibrate the peak pressure recorded on the central axis of the PMMA gap using calibrated piezo‐resistive pressure transducers as a function of the gap thickness. The stress history within the PMMA gap was measured and the peak pressure plotted against gap thickness as a calibration. Further effects were noted relating to the curvature of front exiting the donor charge and the validity of the measurement of the transmitted shock at small gap thicknesses.     

Energetic Material Detonation Characterization: A Laboratory‐Scale Approach

A novel energetic‐material detonation and air‐blast characterization technique is proposed through the use of a laboratory‐scale‐based modified “aquarium test.” A streak camera is used to record the radial shock wave expansion rate at the energetic material air interface of spherical laboratory‐scale (i.e., gram‐range) charges detonated in air. A linear regression fit is applied to the measured streak record data. Using this in conjunction with the conservation laws, material Hugoniots, and two empirically established relationships, a procedure is developed to determine fundamental detonation properties (pressure, velocity, particle velocity, and density) and air shock wave properties (pressure, velocity, particle velocity, and density) at the energetic material air interface. The experimentally determined properties are in good agreement with published values. The theory’s applicability is extended using historical experimental test data due to the limited number of experiments able to be performed. Predicted detonation wave and air shock wave properties are in good agreement for a multitude of energetics across various atmospheric conditions.     

On the Use of Composite Charges to Determine Insensitive Explosive Material Properties at the Laboratory Scale

Laboratory‐scale air‐blast experiments an gram‐range composite explosive charges are presented. The composite charges consist of a spherical booster charge surrounded by a concentric, spherical “candidate material” shell charge. By way of composite charge explosive characterization, the candidate explosive material is able to be characterized through the “removal” of the known booster effects. Using peak shock wave pressures, a method is developed to remove the booster effects from the composite charge’s signature to yield the sole effects of the candidate explosive material, permitting its characterization. Air‐blast explosive tests are conducted using digital high‐speed shadowgraph visualization to measure the resulting shock wave radial position as a function of time. Booster and composite charge data are converted to Mach number versus shock wave radius profiles and subsequently to peak shock wave pressure versus shock wave radius profiles for characterization of the shell material. Explosives tested include: PETN, RDX, HMX, and Alliant Bullseye® SP.     

Predictions of two nonlinear viscoelastic constitutive relations for polymers under multiaxial loadings

Two viscoelastic constitutive relations in differential form are further developed here to include material nonlinearity and distinction between loading and unloading regimes, which is a characteristic of polymers. The effects of hydrostatic pressure and anisotropy in tension and compression on the deformation response of polymers are accounted for through the definition of a pressure‐dependent equivalent stress. In the uniaxial stress state, these constitutive relations reduce to the two well‐known mechanical analogue representations: “Kelvin–Voigt‐type” and “Maxwell‐type” rheological models. The predictive capabilities of these constitutive relations are then assessed against a wide range of experimental results, which include both uniaxial and biaxial stress states subjected to quasi‐static and cyclic‐loading conditions. The predictions of both models are found to be in good agreement with the test data. POLYM. ENG. SCI., 47:593–607, 2007. © 2007 Society of Plastics Engineers.     

Interfacial instabilities in coextrusion flows of low‐density polyethylenes: Experimental studies

A fundamental investigation into the interfacial instability phenomenon was performed. Coextrusion experiments were carried out using well‐characterized low‐density (LDPE) resins in an effort to gain a better understanding of interfacial instability phenomena. The resins used were chosen carefully and included materials of high and low viscosity as well as broad and narrow molecular weight distributions (MWD). The experiments involved the coextrusion of either the same material in both layers or various combinations of the four materials and the focus of the work was to elucidate the effects of flow rates, molecular weight (MW) and MWD on interfacial instability. The effect of the geometry at the point where the materials merged was also investigated. It was concluded that there are essentially two types of interfacial instabilities and that the MW had the strongest effect on the occurrence of the “zig‐zag” instability due to high interfacial stress while the breadth of the MWD had a strong effect on the appearance of the “wave” instability. Broad MWD materials had a greater tendency to exhibit interfacial instability, which is more due to layer ratio than processing conditions or die geometries. The results suggest that the origin of the “wave” type of interfacial instability is due to an extreme extensional deformation of the minor layer at the merge point and that the viscoelastic properties of adjacent layers determine the instability development.     

Micromechanics of ITZ–Aggregate Interaction in Concrete Part I: Stress Concentration

At the macroscopic scale, concrete appears as a composite made of a cement paste matrix with embedded aggregates. The latter are covered by interfacial transition zones (ITZs) of reduced stiffness and strength. Cracking in the ITZs is probably the key to the nonlinear stress–strain behavior in the prepeak regime. For a deeper understanding of this effect triggered by tensile microstress peaks, we here employ and extend the framework of continuum micromechanics, as to develop analytical solutions relating the macroscopic stresses acting on a piece of concrete, to microtractions at the aggregates' surfaces and to three‐dimensional stress states within the ITZs. In the latter context, a new aggregate‐to‐ITZ stress concentration tensor is derived based on the separation‐of‐scale principle, which implies that ITZs may be modeled as two‐dimensional interfaces at the concrete scale, but as three‐dimensional bulk phases at the scale of a few micrometers. Microtensile peaks occur both under uniaxial macroscopic tension and compression. To describe the respective microtraction and microstress fields, it is suitable to define aggregate's “poles” and “equator” by an “axis” through the aggregate center, directed in the uniaxial macroscopic loading direction. Accordingly, tensile microtraction peaks, induced by macro‐tension and macro‐compression, respectively, occur at the “poles” and at the “equator”, respectively. The largest tensile ITZ‐microstresses occur at an offset of about π/8 from the “poles” and the “equator”, respectively. These fields of microtractions and ITZ microstresses are prerequisites for upscaling ITZ‐related strength to the macroscopic concrete level, as presented in the companion paper (Part II).     

Mixing behaviour of solids in multiple spouted beds

Stimulus response experiments are conducted in four different rectangular columns having two and three spout cells. A pink‐coloured polymer material is used as bed material with ambient air as the spouting fluid. A pulse input of dark blue colour polymer material is used as the stimulus, when the column is operating under steady flow conditions, and the response measured. A mathematical model “plug flow‐mixed flow in series” is used to fit the experimental data and the model parameters are evaluated.     

Effect of Scale and Confinement on Gap Tests for Liquid Explosives

The factors influencing initiation of detonation in gap tests for liquid explosives are investigated experimentally. A calibrated donor charge (nitromethane) and PMMA attenuator disk arrangement are used to transmit shocks of known strength (2–10 GPa) into a test explosive of nitromethane sensitized with 5% diethylenetriamine. The test explosive is contained in capsules of different wall materials (PVC, Teflon, aluminum), and the dimensions of the charges vary from 25 mm to 100 mm in diameter. For the small‐scale charges, the presence of the confining wall of the test capsule is seen to have a pronounced effect on the detonation initiation. Certain wall materials (PVC, Teflon) exhibit a multi‐valued critical gap thickness, meaning that a weaker shock may result in initiation while a stronger shock does not. The effect of the wall materials could not be correlated with their acoustic or shock impedance, and the only way to eliminate these effects was to make the diameter of the test charge larger than the donor charge. When the size of the donor charge was increased, the critical pressure required for initiation decreased. These results could be correlated to “ideal” shock initiation experiments that use flyer plates as shock sources assuming that lateral rarefactions quench detonation initiation if they reach the central axis of the charge before the onset of detonation is complete.     

Mechanical properties of unidirectional continuous fiber self‐reinforced polyethylene graded laminates

The aim of this study is to prepare of self‐reinforced polyethylene graded composite laminates (SrPEGCL) by adopting both concepts of “graded” and “self‐reinforced” and analyze their mechanical properties under tensile loading. Three different kinds of fiber volume fractions were employed to prepare continuous fiber unidirectional symmetry SrPEGCL with two graded directions. Tensile experiments were carried out to investigate tensile properties of SrPE composites in longitudinal, transverse, and 45‐bias direction. The microscopic failure mechanism of SrPEGCL were studied and observed by Scanning Electron Microscope (SEM). Laminate stress analysis with ply‐by‐ply discount method was adopted to investigate the damage mechanism using failure criteria and parallel spring model. Observations and conclusions about the effect of graded structure and graded direction on mechanical properties of SrPEGCL under tensile loading were discussed. Compared to common self‐reinforced polyethylene composites, SrPEGCL with the same or even less overall fiber volume fraction exhibited 10–20% higher tensile strength under longitudinal, transverse and 45‐bias loading direction, while graded direction had an effect on the mechanical strength of SrPEGCL as well. POLYM. COMPOS., 36:128–137, 2015. © 2014 Society of Plastics Engineers     

Fatigue behavior of neat and long glass fiber (LGF) reinforced blends of nylon 66 and isotactic PP

This paper focuses on the study of the fatigue behavior of neat and long glass fiber (LGF) reinforced nylon 66/PP-blends. The fatigue was characterized using Parislaw plots in the stable crack growth acceleration range. The fatigue crack propagation (FCP) is presented as a function of the crack growth per cycle (da/dN), the amplitude of the stress intensity factor ΔK, and of the strain energy release rate ΔG. It was also of interest to compare the order of performance found in fatigue to that in the static fracture test. The fracture surfaces were characterized with SEM to determine the failure mechanisms. Further, thermographic camera recordings were used to study the size of a “heated” area (ΔT = 2°C) that developed around the crack tip during the cyclic loading of LGF-PP with different amounts of maleic anhydride grafted PP (PP-g-MAH). For the neat materials, a different order of performance was detected under static and cyclic loading. This was explained by the different failure mechanisms observed after static and cyclic fracture that were related to different stress states of the specimens during the fracture process. On the other hand, the LGF-blends showed a similar order of performance during the static and the fatigue test. This was explained by the observation that similar fiber related failure mechanisms occurred in the composite, both after failure caused by the static and cyclic loading, respectively. For the LGF-PPs with varying PP-g-MAH content, the order of performance in fatigue did not correspond to the size of the “heated area” around the crack tip. This was caused by a change in the composite failure mechanisms, which contributed differently to the size of the “heated area” and to the fatigue performance.     

Shaped Charge Jet Initiation on Explosive Charges equipped with barriers made up of various materials

This paper describes initiation tests on cast TNT/RDX (35/65) explosive charges applying shaped charge jets with test set-ups on which the HE charge was arranged either in contact to a 50-mm thick barrier or after a 100-mm thick barrier in a 15-mm air gap. A variety of materials was attached to the barrier's rear side which, on one band, resulted in a varying shock wave attenuation and also in different bulging effects that are responsible for the differences in the initiation mechanisms observed on the two test arrangements. Materials with a lower density also provide, due to a less precompression of the IIM charge used on the arrangement “test charge in contact”, shorter buildup distances than materials with a higher density. An exception to this is a high ductile material such as e.g. steel. The build-up distances, however, remain constant when arranging the explosive charge with an air gap. This backs up the hypothesis that most of all, bulging of the target is responsible for the sensitivity reduction observed on the test HE charges in contact with the barrier.     

Molecular origin of decreased diffusivity with loading for benzene/HY

On the basis of our recently proposed “ideal” and “insertion” adsorption mechanisms of aromatics in HY zeolites, changes in energetic and dynamic properties with loading were investigated through Monte Carlo and molecular dynamic (MD) calculations. Loading‐dependent isosteric heat could be divided into three linear ranges. The two inflection points were attributed to adsorbate interactions and inherent adsorption mechanism changes. With regard to the loading dependence of diffusivity, diffusivity decreased faster at high loadings than at low and medium loadings, separated by an inflection point. This result confirmed a two‐stage diffusion mechanism based on the distribution of adsorbate from MD simulations which was able to qualitatively predict the further restriction of the mobility. This study provided insights into the modeling of mobility at high loadings on the basis of site‐hopping mechanism. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2778–2785, 2016     

Modifications of the weissenberg rheogoniometer for measurement of transient rheological properties of molten polyethylene under shear. Comparison with tensile data

Improvements to the Weissenberg rheogoniometer are necessary in order to measure the transient rheological properties of polymer melts correctly. The improvements reported concern the mechanical design, a new heating system, a new normal force measuring system, and additional equipment for the relaxation test. Reliable short-time results require sufficiently stiff torque and normal force springs, and a small radius and relatively large angles of the cone-and-plate gap. The behavior of the LDPE melt under test is “linear viscoelastic,” if shear rate or total shear are small: The relaxation modulus, the stress growth at the onset of constant shear rate, the stress relaxation after cessation of steady shear flow, and, in addition, dynamic shear data (from an oscillation viscometer) all show consistent results when correlated by means of formulae from the theory of linear viscoelasticity. Shearing in the nonlinear range with constant shear rate leads to pronounced maxima of the shear stress p₁₂ and of the first normal stress difference p₁₁ ? p₂₂ which occur at constant total shear, almost independent of shear rate. Comparison of shear and tensile data (from extensional rheometer) confirms the Trouton relation in the linear-viscoelastic case. In the nonlinear case, there is a “work softening” in shear and a “work hardening” in extension.     

Observation of carbon black agglomerate dispersion in simple shear flows

Experiments aimed at studying the mechanisms of agglomerate breakup due to the application of a simple shear flow field were performed in a cone and plate transparent device. Spherical compacts of carbon black (diameters 1-2 mm) in a range of different porosites were used in the experiments. Two distinct breakup mechanisms, denoted as “rupture” and “erosion”, were observed. The critical stress for erosion was found to be smaller than that for rupture. Once erosion starts, it continues for very long times. Rupture occurs shortly after reaching a critical stress and concludes abruptly. For this analysis of rupture, the dimen-sionless group $\alpha {\rm = \{ }\eta {\rm .}\mathop \gamma \limits^{\rm .} {\rm /K'}\phi ^{\rm 4} {\rm \} }$, which is the ratio of applied stress to cohesive strength, was found to be a significant parameter for determining the final particle size distribution. The size analysis of fragments produced by shearing pellets for 1 minute showed a lognormal distribution function.     

Diagnostics of the Profile and Rotational Symmetry of a Detonation Wave

It is possible to record a detonation wave profile and to conduct a detailed analysis on its various symmetrical levels and on the rotational symmetry of a detonation wave in only “one” test by using a specific streak mask and applying multi streak or flash gap technique.     

Effect of rigid particle size on the toughness of filled polypropylene

The role of rigid particle size in the deformation and fracture behavior of filled semicrystalline polymer was investigated with systems based on polypropylene (PP) and model rigid fillers [glass beads, Al(OH)₃]. The regularities of the influence of particle content and size on the microdeformation mechanisms and fracture toughness of the composites at low and high loading rates were found. The existence of the optimal particle size for fixed filler content promoting both maximum ultimate elongation of the composite at the tensile and maximum toughness at impact test was shown. The decrease of the toughening effect with both decreasing and increasing particle size regarding the optimal one was explained by dual role of particle size, correspondingly as either “adhesive” or “geometric” factors of fracture. The adhesive factor is due by the increase of debonding stress with the particle size decrease and the voiding difficulty resulting in the restriction of plastic flow. The geometric factor consists in the dramatic decrease of the composite strength at break if the void size exceeds the critical size of defect (for a given matrix) at which the crack initiation occurs. The analysis of the filled polymer toughness dependencies upon the particle size revealed that a capacity of rigid particles for the energy dissipation at the high loading rate depends on two factors: (i) ability of the dispersed particles to detach from matrix and to initiate the matrix local shear yielding at the vicinity of the voids and (ii) the size of the voids forming. Based on the findings it was concluded that the optimal minimal rigid particle size for the polymer toughening should answer the two main requirements: (i) to be smaller than the size of defect dangerous for polymer fracture and (ii) to have low debonding stress (essentially lower compared to the polymer matrix yield stress). © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1917–1926, 2004     

Dual drug release from CO2‐infused nanofibers via hydrophobic and hydrophilic interactions

This study investigated the effects of hydrophobic–hydrophilic interactions on dual drug release from CO₂‐infused nanofibers scaffolds (PCL, PCL–gelatin, and PCL “core” PCL–gelatin “shell”) using BODIPY 493/503 and Rhodamine B fluorescent dyes as drug models. Favorable dye–scaffold interactions increased total dye loading and promoted steady, more linear release. Unfavorable dye–scaffold interactions reduced overall loading and led to a greater burst release of dye. However, when CO₂ was used to infuse dye into an unfavorable scaffold, the changes in loading and release were less pronounced. When two dyes were infused, these behaviors were accentuated due to interactions between the dissolved forms of the dyes. Core–shell composite nanofibers displayed radically different release properties versus pure PCL–gelatin fibers when treated with dyes via CO₂ infusion. Dye release from core–shell scaffolds was highly sensitive to both interactions with scaffolds and the phase of CO₂ used to infuse the compounds of interest. By using different phases of CO₂ to partition dyes into hydrophobic and hydrophilic sections of core–shell nanofibers, such interactions can be manipulated to develop a bimodal drug release system with potential application in drug delivery or tissue engineering. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42571.     

Analysis of roll wave characteristics under low liquid loading two‐phase flow conditions

An experimental study is conducted using a 0.152‐m ID facility to investigate the wave characteristics of two‐phase stratified wavy flow in horizontal pipelines. The experiments are conducted under low liquid loading condition, which is very commonly observed in wet gas pipelines. The experiments are conducted with water as the liquid phase, and repeated with 51 wt % of monoethylene glycol (MEG) in the aqueous phase to analyze the effects of MEG presence on wave characteristics. The experimental range of this study covers superficial gas velocity, v_Sg, values of 9–23 m/s and superficial liquid velocity, v_SL, values of 0.01–0.02 m/s. Similar test matrices are completed for the cases with and without MEG in the aqueous phase. A conductivity probe system is used to measure the wave characteristics at the liquid–gas interface. These characteristics include the wave celerity, frequency, amplitude, length, and liquid film thickness. The experimental oil–air wave characteristics data of Gawas et al. (Int J Multiphase Flow. 2014;63:93–104) is also used for comparison purposes. The trends in the resulting wave characteristics with respect to input parameters are investigated, for oil, water, or MEG–water mixture as the liquid phase. Common predictive methods for interfacial wave celerity, including shallow water theory, Watson (Proceedings of the 4th International Conference in Multi‐Phase Flows, Nice, France. 1989:495–512), Paras et al. (Int J Multiphase Flow. 1994;20(5):939–956), Al‐Sarkhi et al. (AIChE J. 2012;58(4):1018–1029), and Gawas et al. (Int J Multiphase Flow. 2014;63:93–104) are evaluated in comparison with the experimental data. The results of the wave frequency correlation of Al‐Sarkhi et al. (AIChE J. 2012;58(4):1018–1029) are also compared with the experimental wave frequency data. Lastly, a correlation is developed to predict the relative wave amplitude, as a function of superficial gas Weber number and liquid velocity number. Most of the commonly used two‐phase stratified flow models are developed with the assumption of steady‐state conditions, and neglect the transient wave effects. This study provides valuable experimental results on wave characteristics of stratified wavy flow for different types of liquid phase. Moreover, a comprehensive analysis of the parameters affecting the wave characteristics of stratified wavy flow is presented. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3177–3186, 2017     

Air Gap Effects in LX‐17

Three experiments done over twenty years on gaps in LX‐17 are described. For the detonation front moving parallel to the gaps, jets of gas products were seen coming from the gaps at velocities 2 to 3 times greater than the detonation velocity. A case can be made that the jet velocity increased with gap thickness but the data are scattered. For the detonation front moving transverse to the gap, time delays were seen. The delays roughly increase with gap width, going from 0–70 ns at “zero gap” to around 300 ns at 0.5–1 mm gap. Larger gaps of up to 6 mm width almost certainly stopped the detonation, but this was not proved. Real‐time resolution of the parallel jets and determination of the actual re‐detonation or failure in the transverse case needs to be achieved in future experiments.     

Reaction Behavior of Polytetrafluoroethylene/Al Granular Composites Subjected to Planar Shock Wave

The shock‐compression responses of PTFE (polytetrafluoroethylene)/Al granular composites subjected to planar shock waves of various pressures are investigated. A 57‐mm diameter single‐stage gas‐gun and 50‐mm diameter plane‐wave lenses are employed to perform planar shock wave experiments. High frequency manganin piezoresistance stress gauges are used to monitor the stress (regarded as pressure in consideration of the high pressure state) at four Lagrangian positions of the PTFE/Al granular composites specimens. Planar shock wave experiments show characteristics of densification at measured input pressure of 0.5 GPa to 1.27 GPa using single‐stage gas‐gun and shock‐induced reaction (SIR) indicated by growth of shock pressure and specific volume expansion at measured input pressure of 7.29 GPa to 12.25 GPa using plane‐wave lenses. The pressure and relative volume states behind the shock wave front are calculated from the experimental recorded pressure profiles using Lagrangian analysis method, which are used to determine the reaction ratios under different shock pressures by comparing with partial reacted Hugoniot calculations. It was shown that the reaction ratios obtained in this research have good agreement with the thermochemical modeling calculations. The corresponding results indicate that the shock‐induced reactions of PTFE/Al granular composites occur in the shock wave rising period and the reaction ratios are intimately related to the shock wave pressure.