Carl M. Cady

A VISCOELASTIC MODEL FOR PBX BINDERS

Abstract. Stress-strain measurements done at different rates and temperatures along with measurementsof the rate- and temperature-dependent dynamic storage modulus have allowed us to construct a generalizedMaxwell model for the linear viscoelastic response of plasticized estane. A theoretical analysis ispresented to include effects of impurites.INTRODUCTIONComplete knowledge of the thermo-mechanicalbehavior of the constituents of PBX-9501 is requiredfor any micromechanics method to be a useful toolfor modeling its behavior. The primary constituentsof PBX 9501 are the explosive cyclotetramethylene-tetranitramine (HMX) crystals and the inertplasticized estane binder matrix. Estane 5703 is apolyester polyurethane elastomer manufactured bythe B.F. Goodrich Company with a density of 1.19gm/cm 3 . The polymeric binder shows dramaticsensitivity to changes in strain rates andtemperatures. For example, a change in thetemperature from -50 C to 50 C will have anassociated change in the shear modulus of five ordersof magnitude. Obviously, a successful theory forPBX 9501 must account for this behavior. Becauseof recent experimental effort, much high-qualitystress-strain data has become available for theplasticised binder. A primary goal was to use thisdata to formulate a generalized Maxwell model(GMM) thermo-mechanical constitutive law for thebinder. While a GMM constitutive law hasimmediate applications for PBX 9501, ourtheoretical analysis used to obtain the constitutivelaw has interest to the general community involvedwith plastic bonded high explosives.The aforementioned stress-strain data was measuredby the LANLs Material Structure/Property Group(MST-8) and was obtained by several differentexperimental methods. An Intron 5567 testingmachine was used for measuring uniaxial stress-strain data for rates in the range of 1

DISLOCATIONS IN MO5SIB2 T2 PHASE

Abstract Dislocation structures in a nearly single phase annealed Mo5SiB2 T2 alloy have been investigated by transmission electron microscopy (TEM). The dislocations have been subjected to Burgers vector and trace analyses to determine the slip directions and planes with the aid of image simulations generated using single crystal elastic constants derived from first principles calculations. The experimental results are compared to predicted slip directions and planes from anisotropic elasticity calculations. Thermal expansion coefficients have been measured by dilatometry and are compared to both calculated and previous experimental values measured using diffraction techniques. Lastly, preliminary compression testing has been performed on the single phase material at 1200°C.

Influence of temperature on the high-strain-rate mechanical behavior of PBX 9501

High-strain-rate (2000 s−1) compression measurements utilizing a specially-designed Split-Hopkinson-Pressure Bar have been obtained as a function of temperature from −55 to +50 °C for the plastic-bonded explosive PBX 9501. The PBX 9501 high-strain-rate data was found to exhibit similarities to other energetic, propellant, and polymer-composite materials as a function of strain rate and temperature. The high-rate response of the energetic was found to exhibit increased ultimate compressive fracture strength and elastic loading modulus with decreasing temperature. PBX 9501 exhibited nearly invariant fracture strains of ∼1.5 percent as a function of temperature at high-strain rate. The maximum compressive strength of PBX 9501 was measured to increase from ∼55 MPa at 50 °C to 150 MPa at −55 °C. Scanning electron microscopic observations of the fracture mode of PBX 9501 deformed at high-strain revealed predominantly transgranular cleavage fracture of the HMX crystals.

Influence of Temperature and Strain Rate on the Compressive Behavior of PMMA and Polycarbonate Polymers

Compression stress‐strain measurements have been made on commercial polymethylmethacrylate (PMMA) and polycarbonate (PC) polymers as a function of temperature (−197C to 220C) and strain rate. A split‐Hopkinson‐pressure bar (SHPB) was used to achieve strain rates of about 2500 s−1 and a servo‐hydraulic tester was used for lower strain rate testing (0.001 to 5 s−1). The mechanical response of these transparent polymers is quite different. The strength of PC is weakly dependent on strain rate, only moderately dependent on temperature, and remains ductile to −197C. In contrast, the strength of PMMA is linearly dependent on temperature and strongly dependent on strain rate. Significantly, PMMA develops cracking and fails in compression with little ductility (7–8% total strain) at either low strain rates and very low temperatures (−197C) or at high strain rates and temperatures very near ambient.

Influence of temperature and strain rate on the mechanical behavior of PBX 9502 and Kel-F 800™

Compression measurements were conducted on plastic-bonded explosive PBX 9502 and its binder, Kel-F 800™, as a function of temperature from −55 °C to +55 °C using an improved split Hopkinson pressure bar at high strain rates (≈1400 s−1) and at low strain rates (≈0.001 to 0.1 s−1) at ambient temperatures. PBX 9502 exhibits lower dynamic compressive strength, but is much less sensitive to strain rate and temperature, than PBX 9501. In contrast, the mechanical response of the Kel-F 800™ binder is stronger than pure (or plasticized) Estane™, but is again less strain rate and temperature dependent. The effects of longitudinal and transverse loading orientations (due to preferred orientation of TATB) and virgin versus recycled TATB on the properties of PBX 9502 are presented.

Influence of strain rate on the deformation and fracture response of a 6061-T6 Al–50 vol.% Al2O3 continuous-reinforced composite

Abstract The compressive mechanical properties of an aluminum–matrix composite unidirectionally reinforced with Al2O3 fibers have been measured and characterized as a function of loading orientation. The influence of strain rate and fiber orientation on the deformation and fracture response of a 6061 Al–50 vol.% Al2O3 continuous fiber-reinforced metal–matrix composite (MMC) aged to a T6 condition is reported. The stress–strain response of this composite was found to vary substantially as a function of loading orientation; the quasi-static yield changing from nominally 250 MPa transverse to the fibers to ∼1.7 GPa parallel to the fibers under ideal conditions. Increasing the strain rate to 2000 s−1 was observed to only slightly increase the yield strength of the composite for both orientations. The main failure mechanism has been identified to be kinking, although an upper bound seems to be attained when the fibers reach their compressive strength. The experimental results are consistent with a plastic kinking model for strain hardening composites. The failure response of the composite transverse to the fibers, under both uniaxial stress (quasi-static and dynamic) and uniaxial strain loading, displays a protracted but substantial load drop after yield followed by continued degradation in load carrying capacity. Lack of ideal parallel fiber construction was found to lead to systematic buckling failure of the alumina fibers through the sample under uniaxial loading.

The yield stress anomaly in stoichiometric FeAl at high strain rate

Single crystals and large-grained polycrystals of B2-structured FeAl containing initially low vacancy concentrations exhibit a so-called yield anomaly, with a yield strength peak at typically {approximately}0.40--0.45 of the absolute melting point. If the grain size in a polycrystal is fine, declining Hall-Petch strengthening with increasing temperature can obscure this yield anomaly. Similarly, vacancies retained after elevated temperature annealing can raise the low temperature yield strength to such an extent that the yield anomaly is not observed, an effect that has been most clearly demonstrated by Carleton et al. The purpose of the work reported here was to examine whether staining at a higher strain rate would shift the yield strength peak in stoichiometric FeAl to higher temperatures, as predicted by the vacancy-hardening model.

A comparative study of the strain rate and temperature dependent compression behavior of Ti-46.5Al-3Nb-2Cr-0.2W and Ti-25Al-10Nb-3V-1Mo intermetallic alloys

Extensive research over the past two decades has led to the development of alloys such as Ti-46.5Al-3Nb-2Cr-0.2W ({gamma}-TiAl) and Ti-25Al-10Nb-3V-1Mo (super {alpha}{sup 2}-Ti{sub 3}Al), which offer a good balance of formability and high temperature strength. Although the mechanical behavior of these intermetallic alloys at low strain rates has been extensively studied, fewer studies have probed the systematic mechanical behavior of these alloys over a wide range of strain rates and temperatures. The purpose of this study is to compare and contrast the behavior of these two intermetallic alloys over a wide range of strain rates and temperatures. Variations in the mechanical responses of these two structurally different Ti-Al based intermetallics are documented.

Micromechanics simulations of glass–estane mock polymer bonded explosives

Polymer bonded explosives (PBXs) are particulate composites containing explosive particles and a continuous binder. The elastic modulus of the particles, at room temperature and higher, is often three to four orders of magnitude higher than that of the binder. Additionally, the explosive particles occupy high volume fractions, often greater than 90%. Both experimental and numerical determination of macroscopic properties of these composites is difficult. High modulus contrast mock PBXs provide a means of relatively inexpensive experimentation and validation of numerical approaches to determine properties of these materials. The goal of this investigation is to determine whether the effective elastic properties of monodisperse glass–estane mock PBXs can be predicted from two-dimensional micromechanics simulations using the finite element (FEM) method. In this study, the effect of representative volume element (RVE) size on the prediction of two-dimensional properties is explored. Two-dimensional estimates of elastic properties are compared with predictions from three-dimensional computations and with experimental data on glass–estane composites containing three different volume fractions of spherical glass beads. The effect of particle debonding on the effective elastic properties is also investigated using contact analyses. Results show that two-dimensional unit cells containing 10–20 circular particles are adequate for modelling glass–estane composites containing less than 60% glass particles by volume. No significant difference is observed between properties predicted by the two- and three-dimensional models. FEM simulations of RVEs, containing particles that are perfectly bonded to the binder, produce estimates of Youngs modulus that are higher than the experimental data. Incorporation of debonding between particles and the binder causes the effective Youngs modulus to decrease. However, the results suggest that cracks in the composite may play a significant role in determining the effective properties of mock polymer bonder explosives composed of glass and estane. The FEM simulations indicate that two-dimensional models that incorporate debonds and cracks can be used to obtain accurate estimates of the effective properties of glass–estane composites and possibly of PBXs.

The High Strain Rate Deformation Behavior of High Purity Magnesium and AZ31B Magnesium Alloy

The deformation in compression of pure magnesium and AZ31B magnesium alloy, both with a strong basal pole texture, has been investigated as a function of temperature, strain rate, and specimen orientation. The mechanical response of both metals is highly dependent upon the orientation of loading direction with respect to the basal pole. Specimens compressed along the basal pole direction have a high sensitivity to strain rate and temperature and display a concave down work hardening behavior. Specimens loaded perpendicularly to the basal pole have a yield stress that is relatively insensitive to strain rate and temperature and a work hardening behavior that is parabolic and then linearly upwards. Both specimen orientations display a mechanical response that is sensitive to temperature and strain rate. Post mortem characterization of the pure magnesium was conducted on a subset of specimens to determine the microstructural and textural evolution during deformation and these results are correlated with the observed work hardening behavior and strain rate sensitivities were calculated.