David C. Erlich

Cut Resistance of High-strength Yarns

The cut resistance of three high-strength yarns under tension-shear loading conditions was measured by pressing a knife blade transversely at a constant rate against a yarn gripped at its ends. The load-deflection relation and the energy required to cut through the yarn were determined for Kevlar (aramid), Spectra (polyethylene), and Zylon (polybenzobisoxazole, or PBO). The cut energy and strain to initiate cutting were highest for Zylon, least for Kevlar, and depended on the slicing angle, the sharpness of the blade, and the pre-tension in the yarn. The dependencies are explained by changes in failure mode of the fibers within the yarn. The test provides input needed for computational simulations of ballistic response of fabrics to sharp fragments and should be useful in designing slash-resistant gloves and clothing.

Slow Penetration of Ballistic Fabrics

A quasistatic penetration test is designed and implemented to provide insight into the evolution and phenomenology of fabric deformation and failure during penetration. The results are needed to guide development of a physics-based computational model of fabric response to projectile impact. The test involves slowly pushing a rigidly held fragment simulator into and through a single ply fabric specimen. The stroke and load on the penetrator are recorded during the test, and a videocamera and microphone enable the fabric failure phenomena to be time-correlated with the load-stroke history. The three distinct modes of fabric failure observed in these tests—local yarn rupture, remote yarn failure, and yarn pullout—are the same modes observed in impact tests, although the extent to which each mode occurs is different in static and dynamic tests. The conditions under which the different failure modes occur and the effects on the load-stroke curve and the energy absorbed are determined.

Fracture-induced and thermal decomposition of NTO using laser ionization mass spectrometry

Abstract A surface analysis by laser ionization (SALI) apparatus has been used to obtain, for the first time, real-time photoionization mass spectra of the shear-induced molecular-fragment emission from an explosive. NTO (5-nitro-1,2,4-triazol-3-one) was chosen for these experiments because of its potential utility as a reasonably energetic, but very insensitive, explosive. Using vacuum ultraviolet single-photon ionization, the shear-induced NTO spectra were obtained with a spring-driven shearing device installed in a SALI chamber directly beneath the mass spectrometer sampling region. For comparison, we also obtained spectra under either slow-heating or rapid pulsed-laser heating conditions. The shear-induced spectra are dominated by a peak at m/z 99, which is not seen in the thermal- or laser desorption spectra. This peak is assigned to the closed-shell traiza-diketone produced by a nitro-nitrite rearrangement, followed by NO loss and then by rapid bimolecular H-atom removal. The stability of the cyclic diketone intermediate thus generated could help to explain the shock insensitivity of NTO. Laser-desorption spectra were also obtained both on fresh NTO samples and on samples that have been recovered from marginally sub-critical drop-weight impact tests. Comparison of spectra obtained with and without laser desorption, and as a function of temperature, demonstrate that the sequences of fragment ions observed under laser desorption conditions are the result of thermal decomposition, not of ion-fragmentation. The sequence of thermally generated fragments is dominated by M-16, M-30, M-45, M-46, and M-59. This series suggests several decomposition pathways, dominated by the same nitro-nitrate rearrangement and NO loss as the shear-induced decomposition. However, under the lower-density, but higher temperature, thermal or laser-desorption conditions, subsequent bimolecular H-atom removal to produce the closed-shell diketone is evidently slower than unimolecular ring-opening adjacent to the carbonyl group. We show how this sequence satisfactorily explains (1) the “initial” formation of CO2 that has been previously reported, (2) the results of nitrogen double-labeling experiments, and (3) the fact that neither NO2 nor HONO have been seen as substantial initial products of NTO decomposition.

Dynamic Tensile Failure of Glycerol

A tensile strain was induced in shock‐loaded glycerol by the intersection of two rarefaction waves, one of which was caused by the partial reflection of a shock from an interface with octane, a material of lower shock impedance. The maximum tensile stress and the subsequent tensile stress relaxation due to void nucleation and growth were measured indirectly by a stress gauge placed in the octance. The dynamic tensile strength of the glycerol was measured to be ≈ 0.25 kbar for an initial tensile stress rate on the order of 107 kbar/sec. Calculations indicated that spherical voids in glycerol of between 0.01 and 10 μ in radius follow a viscous growth law under tensile strain, and computations using this growth law showed the influence of fractional void volume and strain rate upon the macroscopic stress history during cavitation. Several possible void nucleation mechanisms are discussed.

Surface pressures from laser‐supported detonations

The surface pressures directly behind a Laser‐Supported Detonation (LSD) were measured with a system of carbon piezoresistive gauges. A quartz piezoelectric gauge was also used to measure surface pressures far away from the laser spot. The small size of the carbon gauges, relative to the laser beam diameter, permitted simultaneous resolution of the surface pressure history at several locations within the laser spot. A comparison of the experimental data with recent theories indicates that the theories predict peak pressures close to those measured but overestimate the impulse intensity at the target surface.

Effect of grain size on high strain rate deformation of copper

Experiments were performed to observe the deformation characteristics of oxygen-free high-conductivity (OFHC) copper at high strain rates (up to 40,000 s−1) and to relate differenc in grain size with differences in deformation behavior. The rod impact and torsional Hopkinson bar test methods were used in these experiments. Results show that grain size reductions substantially reduce surface irregularities that develop during deformation. The effect of grain size on the yield stress and on the strain-hardening behavior of copper is small and is similar to the effect of grain size in copper at quasistatic strain rates. The observation that grain size has a substantial effect on surface irregularities may have important implications for applications in which stable deformation of thin sections is of concern.

Thermal fatigue behavior of J-lead solder joints

Thermal cycling environments were applied to test specimens with J-lead eutectic Sn-Pb solder joints to assess the effect of lead and package material selection. Metallographic examination and finite element analysis were used to evaluate the effect of thermal cycling environments on solder joints. Matching the coefficient of thermal expansion (CTE) of the lead and solder was found to be crucial to reducing thermal cycling damage for the J-lead solder joints examined. Matching the CTE of the package and the board was substantially less important. The number of thermal cycles 1.9 nucleate cracks in the J-lead solder joints correlated reasonably well with the number of cycles to produce a 50% drop in the load range in isothermal strain controlled mechanical fatigue tests on Sn/Pb solder. >

Inhomogeneities of shock‐wave deformation in Fe‐32 wt. % Ni‐0.035 wt. % C alloy

Low‐pressure plane impact experiments performed on Fe‐32 wt. % Ni‐0.035 wt. % C alloy revealed, after recovery, markings which are attributed to shock‐induced inhomogeneities. Shear of the material does not occur homogeneously, but in preferential planar regions. These regions are made visible by a martensitic transformation [fcc (austenite)→bcc (martensite)] produced by the tensile pulses generated by the reflection of the compressive shock wave at a free surface. The bands with higher plastic deformation served as preferential nucleation sites for martensitic transformation. The formation of these bands is attributed to inhomogeneous yielding due to work softening of the material during tensile loading.

An experimental technique for studying shock propagation in large‐scale samples of snow and other highly‐porous materials

Electromagnetic particle‐velocity gages were emplaced along a diagonal within large (roughly 40 by 60 by 10 cm) specimens of artificially‐made snow (ρ0≊0.35 g/cm3) and a snow‐matching grout (ρ0≊0.25 g/cm3), to record transmitted pulse histories at various depths in the specimen. The snow was produced by a leased cryogenic snow‐making system at a newly‐established permanent cold‐temperatures facility at SRI’s remote explosives test site. Nearly uniaxial strain loading was achieved by a line detonation wave running through dilute explosives tile (DET) squares in contact with a slanted top face of the specimen block. Peak particle velocities ranged from ≊0.6 mm/μs at the He interface to ≊0.25 mm/μs at depths of ≊6 cm. Corresponding peak stresses were calculated to be from ≊1.0 to ≊0.25 kbar.

THREE-DIMENSIONAL ANALYSES OF PLATE IMPACT EXPERIMENTS WITH CIRCULAR AND STAR GEOMETRIES

Flier impact experiments are used in the study of dynamic material response and failure. In general, experiments study the material response to the shock processes within the one-dimensional response region of the target before lateral waves from the plate edges arrive. Prior work has indicated that star-shaped flier and target plates reduce the magnitude of the edge effects or extend the one-dimensional response time within the central region of the target plate. To further quantify this effect, we performed three-dimensional finite element simulations of plate impact experiments with circular and two different star geometries for the flier and target plates. Calculations showed that star-shaped plates indeed reduce the magnitude of the edge effects compared to circular plates. However, the star geometries can reduce the one-dimensional response time at the center of the target plate and are more difficult to fabricate than circular geometries.