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Notes: The anomalous, low-stress, shock initiation of detonation observed in earlier studies of pentaerythritol tetranitrate single crystals was examined in more detail experimentally. Time-resolved particle-velocity histories were obtained for [110], [001] and [100] orientations of single-crystal pentaerythritol tetranitrate explosive for shock input stresses of 4–7 GPa using laser interferometry instrumentation. At about 4.2 GPa an elastic-plastic, two-wave structure was noted in [110] and [001] orientations, and a single shock wave for [100] orientation. The two-wave structure provides an explanation for the anomalous shock initiation sensitivity and intermediate velocity transition previously observed in [110] orientation at this stress level. It also explains details of fluorescent emission histories from [110] and [001] crystals previously measured. The orientation-dependent results are consistent with the model of steric hindrance to shear at the molecular level. Fits to the elastic Hugoniot data in [110] and [001] orientations are given as well as a revised fit for the bulk Hugoniot. © 1997 American Institute of Physics.

Notes: The elastic precursor shock strengths of pentaerythritol tetranitrate explosive crystals were measured for [100], [101], [110], and [001] orientations using velocity interferometer system for any reflector instrumentation for samples 3–6 mm thick. Input shock strength was 1.14 GPa. Measured precursor amplitudes were 0.38, 0.58, 0.98, and 1.22 GPa, respectively, for the four orientations. Critical shear stress for the slip system with the maximum resolved shear stress for each shock orientation was computed. Details of the elastic and plastic wave profiles are discussed. Molecular mechanics modeling of the shear induced by the uniaxial strain of a plane shock wave in this molecular crystal was also performed using the amber code. This may be the first application of molecular mechanics computation to a shear problem. The modeling correctly predicts the dependence of the precursor amplitude on crystal orientation for the cases considered. The results confirm the importance of steric hindrance to shear in controlling the orientation-dependent strength in molecular crystals and sensitivity to shock initiation of detonation in molecular explosive crystals. Details of the molecular deformations and contributions to the energy barrier to inelastic shear for different orientations are given. The computational results also explain why the {110} 〈11¯1〉 slip system is observed in quasistatic deformation in spite of having the longest Burgers vector. The dynamics of sterically hindered, shock-induced shear is considered.

Notes: Two cases of rupture of uterus due to placenta in the second trimester are presented. The significance of pain in this period is discussed.

Notes: Objective To assess the incidence of urinary incontinence in pregnancy and after spontaneous vaginal delivery and its relation with changes in the static and dynamic function of the pelvic floor.Design The second part of a prospective longitudinal study.Setting University Hospital Groningen and Martini Hospital Groningen, the Netherlands.Population A cohort of 62 women before and after spontaneous vaginal delivery at term and 27 nulliparous non-pregnant controls.Methods Urinary incontinence was measured by a questionnaire and by a 24 hour pad test. The position and mobility of the urethrovesical junction were measured by perineal ultrasound and related to simultaneously measured abdominal pressure changes. Serial investigations were done at 38 weeks of gestation and at 6 weeks and 6 months after delivery.Main outcome measures Urinary incontinence and its relation with the position of the urethrovesical junction at rest and with the mobility of the urethrovesical junction during Valsalva and during coughing, indicated by the displacement/pressure coefficient and with obstetric variables.Results After delivery, reported urinary incontinence was reduced from 26% at 38 weeks of gestation to 16% and 15% at 6 weeks and 6 months after delivery, respectively. Even lower rates were measured by the 24 hour pad test, which revealed a decrease from 14% at 38 weeks to 10% and 5% at 6 weeks and 6 months postpartum, respectively. Six weeks and six months after delivery, the angle of the urethrovesical junction at rest was significantly increased compared with the non-pregnant control women. Compared with the antenatal measurements, the displacement/pressure coefficients during coughing and during the Valsalva manoeuvre were significantly increased six weeks after delivery. Six months after delivery, only the coefficient for coughing was still significantly greater than the antenatal value and the value in the non-pregnant control group. No relations were found between urethrovesical junction measurements and obstetric variables and subjective or objective urinary incontinence parameters.Conclusion Although pregnancy and spontaneous vaginal delivery significantly increased the degree of bladder neck descent during coughing, urinary incontinence, quite common during pregnancy, occurs less frequently postpartum.

Notes: Trichloride vapor-phase epitaxy has been employed in a single chamber reactor to achieve the continuous (noninterrupted) growth of n−InP/n−In0.53Ga0.47As/InP superlattice structures for avalanche photodetector applications. Very low background doping has been achieved in both the InP (n≈1×1014 cm−3) and the superlattice layers (n=1×1015 cm−3). We report the details of the epitaxial crystal growth as well as analysis of the structures by secondary ion mass spectrometry and transmission electron microscopy. Avalanche photodiodes have been fabricated by Zn diffusion in the InP layer and mesa etching. The devices exhibit an intrinsic response time (full width at half maximum) of 66 ps at λ=1.5 μm which is the shortest so far achieved in superlattice photodetectors. Dark currents of 50 nA at unity gain, multiplication factors of 6, and breakdown voltages exceeding −120 V have been measured.

Notes: The shock response of single crystals of the explosive pentaerythritol tetranitrate was studied from 3.4 to 19 GPa. Four orientations, 〈110〉, 〈001〉, 〈101〉, and 〈100〉 were studied. The shock Hugoniot and shock initiation sensitivities were measured in wedge tests. Intermediate velocity transitions were noted. A sensitivity anomaly near 4 GPa was discovered for 〈110〉 orientations. Chemiluminescent phenomena associated with this anomaly were studied using framing photography and uv/visible spectroscopy. Both plane-wave explosive and gas-gun impact experiments were done. Analysis of the steric hindrance occurring during edge dislocation motion in the shear flow associated with shock-induced uniaxial strain was done. The results corroborate the hypothesis that the orientation dependence of shock initiation sensitivity is due to the anisotropy of plastic flow, that is, the relative ease of slip in different orientations.

Notes: The plane shock response of PBX 9502 was measured from 0.5 to 25 GPa. The explosive is 95-wt. % triamino-trinitrobenzene powder coated with Kel-F 800 plastic; porosity was 2.6%. Shock initiation sensitivity, the Hugoniot, and dynamic yield behavior were studied. Experiments done were explosively driven wedge tests and particle velocity history measurements using electromagnetic gauges at a light-gas gun. A new analysis of the wedge test was implemented. Based on wedge test results, an approximation for the particle velocity at the shock front is constructed, which is used in functional composition with the Hugoniot to calculate the shock path of each experiment. Optimization on the parameters of the approximation and the Hugoniot simultaneously ensures agreement of the Hugoniot with the overall kinematics of the shock growth as well as the particle velocity/shock velocity data. The individual shock paths are used to examine the single-curve buildup assumption; the assumption is not valid for PBX 9502. A ramping elastic precursor of 0.073 GPa strength was observed. The yield and weak shock behavior are interpreted in terms of porosity, crystalline anisotropy, and residual strain in the pressed, plastic-bonded explosive.