This paper presents a modeling approach for simulating the anisotropic thermal expansion of polycrystalline (1,3,5-triamino-2,4,6-trinitrobenzene) TATB-based explosives which utilizes microstructural information including the porosity, crystal aspect ratio and processing-induced texture. A self-consistent homogenization procedure is used to relate the macroscopic thermoelastic response to the constitutive behavior of single-crystal TATB. The model includes a representation of the grain aspect ratio, porosity and, crystallographic texture attributed to the consolidation process. A quantitative model is proposed for describing the evolution of the preferred orientation of basal planes in TATB during consolidation and an algorithm constructed for developing a discrete representation of the associated orientation distribution function. Analytical and numerical solutions using this model are shown to produce textures consistent with previous measurements and characterization for isostatically and uniaxially ‘die-pressed’ specimens. Predicted thermal strain versus temperature results for textured specimens are shown to be in agreement with corresponding experimental measurements. Results from these simulations are used to identify qualitative trends. Key conclusions from this work include the following. Both porosity and grain aspect ratio have an influence on the thermal expansion of polycrystal TATB, considering realistic material variability. The preferred orientation of the single-crystal TATB [0 0 1] poles within a polycrystal gives rise to pronounced anisotropy of the macroscopic thermal expansion. The extent of this preferred orientation depends on the magnitude of the deformation and, consequently, is expected to vary spatially throughout manufactured components much like the porosity. The modeling approach presented here has utility toward bringing spatially variable microstructural features into macroscale system engineering models.
Ionomeric polymers are a promising class of intelligent material which exhibit electromechanical coupling similar to that of piezoelectric bimorphs. Ionomeric polymers are much more compliant than piezoelectric ceramics or polymers and have been shown to produce actuation strain on the order of 2% at operating voltages between 1V and 3V (Akle et al., 2004, Proceedings IMECE). Their high compliance is advantageous in low force sensing configurations because ionic polymers have a very little impact on the dynamics of the measured system. Here we present a variational approach to the dynamic modeling of structures which incorporate ionic polymer materials. To demonstrate the method a cantilever beam model is developed using this variational approach. The modeling approach requires a priori knowledge of three empirically determined material properties: elastic modulus, dielectric permittivity, and effective strain coefficient. Previous work by Newbury and Leo has demonstrated that these three parameters are strongly frequency dependent in the range between less than 1Hz to frequencies greater than 1kHz. Combining the frequency-dependent material parameters with the variational method produces a second-order matrix representation of the structure. The frequency dependence of the material parameters is incorporated using a complex-property approach similar to the techniques for modeling viscoelastic materials. A transducer is manufactured and the method of material characterization is applied to determine the mtaerial properties. Additional experiments are performed on this transducer and both the material and structural model are validated. Finally, the model is shown to predict sensing response very well in comparison to experimental results, which supports the use of an energy-based variational approach for modeling ionomeric polymer transducers.
SUMMARYThis paper presents a new implementation of a constitutive model commonly used to represent plastic bonded explosives in finite element simulations of thermomechanical response. The constitutive model, viscoSCRAM, combines linear viscoelasticity with isotropic damage evolution. The original implementation was focused on short duration transient events; thus, an explicit update scheme was used. For longer duration simulations that employ significantly larger time step sizes, the explicit update scheme is inadequate. This work presents a new semi‐implicit update scheme suitable for simulations using relatively large time steps. The algorithm solves a nonlinear system of equations to ensure that the stress, damaged state, and internal stresses are in agreement with implicit update equations at the end of each increment. The crack growth is advanced in time using a sub‐incremental explicit scheme; thus, the entire implementation is semi‐implicit. The theory is briefly discussed along with previous explicit integration schemes. The new integration algorithm and its implementation into the finite element code, Abaqus, are detailed. Finally, the new and old algorithms are compared via simulations of uniaxial compression and beam bending. The semi‐implicit scheme has been demonstrated to provide higher accuracy for a given allocated computational time for the quasistatic cases considered here. Published 2014. This article is a US Government work and is in the public domain in the USA.
Time-of-flight neutron diffraction, contour method, and surface hole drilling residual stress measurements were conducted at Los Alamos National Lab (LANL) on a lab sized plate specimen (P4) from phase 1 of the joint U.S. Nuclear Regulatory Commission and Electric Power Research Institute Weld Residual Stress (NRC/EPRI WRS) program. The specimen was fabricated from a 304L stainless steel plate containing a seven pass alloy 82 groove weld, restrained during welding and removed from the restraint for residual stress characterization. This paper presents neutron diffraction and contour method results, and compares these experimental stress measurements to a WRS finite element (FE) model. Finally, details are provided on the procedure used to calculate the residual stress distribution in the restrained or as welded condition in order to allow comparison to other residual stress data collected as part of phase 1 of the WRS program.
Ionomeric polymers are a promising class of intelligent material which exhibit electromechanical coupling similar to that of piezoelectric bimorphs. Ionomeric polymers are much more compliant than piezoelectric ceramics or polymers and have been shown to produce actuation strain on the order of 5% at operating voltages between 1 V and 5 V. This performance indicates the potential for self-actuating devices manufactured from ionomeric polymers, such as deformable mirrors or low pressure pump diaphragms. This paper presents a variational approach to the dynamic modeling of ionic polymer plates in rectangular coordinates. A linear matrix equation, which relates displacement and charge to applied forces and voltage, is developed to determine the response of the structure to applied forces and applied potentials. The modeling method is based on the incorporation of empirically determined material properties, which have been shown to be highly frequency dependent. The matrices are calculated at discrete frequencies and solved frequency-byfrequency to determine the response of the ionomeric plate structures.A model of a thin rectangular plate is developed and validated experimentally. Simulated frequency response functions are compared to experimental results for several locations on the plate. The response of the plate at certain frequencies is computed and compared to experimentally-determined response shapes. The results demonstrate the validity of the modeling approach in predicting the dynamic response of the ionomeric plate structure. These spatial solutions are also compared to experimentally determined response shapes.
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