Modeling Of Material Response Research Articles

Dynamic properties of polyurea-milled glass composites Part I: Experimental characterization

Polyurea (PU) is an elastomer, which exhibits unique thermo-mechanical properties. It is synthesized from a di-functional amine, e.g. Versalink P-1000 and a diisocyanate, e.g. Isonate143L. In this study, various volume fractions milled glass (MG) was added to create polyuria-milled glass composites (PU-MG). Milled glass is a micro fiber with cylindrical shape. The distribution of the milled glass in the polyurea matrix was observed under the scanning electron microscope. The dynamic properties of pure polyurea and the PU-MG composites were measured by dynamic mechanical analysis (DMA) in the low frequency range (1–20 Hz) and by ultrasonic wave measurement in the high frequency range (0.5–1.5 MHz). Both experiments show that increasing the milled glass volume fraction drastically increases both the storage and loss moduli of the composites. DMA results show that dynamic Young’s modulus increases with increasing frequency. However, longitudinal and shear moduli from ultrasonic wave measurement appears to be insensitive to frequency within the range of 0.5–1.5 MHz. The experimental dynamic moduli master curves of PU and PU-MG composites were constructed and compared. The relaxation function or creep compliance are generally useful than dynamic moduli for modeling of material response under complex histories. It is of practical use to convert dynamic mechanical data from the frequency domain into the time domain. The discrete relaxation spectra of the composites were calculated by fitting Prony series to the master curves, using least square nonlinear regression. Retardation spectra were then calculated using the interrelation between relaxation modulus and creep compliance in Laplace domain. Finally, the time domain relaxation modulus and creep compliance for each composite were obtained from the two spectra. In order to extend our understanding of the dynamic behavior of the PU-MG composite, micromechanical models have been created and are discussed in the accompanying paper Nantasetphong (2016a).

Conformal Phenolic Impregnated Carbon Ablator Arcjet Testing, Ablation, and Thermal Response

A new conformal type of phenolic impregnated carbon ablator was manufactured by resin impregnation of carbon felt. Based on property measurements of the conformal material and the existing model for a standard, rigid phenolic impregnated carbon ablator, a midfidelity material response model for the conformal material was developed. The rigid and conformal materials were arcjet tested simultaneously on a sphere-cone geometry in several environments with frustum heat flux up to . Good agreement between the predictions and data was obtained for recession, surface temperature, and in-depth temperatures. The conformal material has a slightly greater ablation rate but significantly lower thermal diffusivity than the rigid material.

Journal of Dynamic Behavior of Materials: A New Forum for Knowledge Exchange on Dynamic Material Response, High Strain-Rate Effects, Shock, and Materials Under Extreme Loading

We are excited to introduce the Journal of Dynamic Behavior of Materials (JDBM) as the new journal of the Society for Experimental Mechanics (SEM). We wish to invite you to submit your best work on the study of the dynamic behavior of materials to the journal and help in getting the word out to your colleagues. Information on the journal can be found at http://springer.com/40870. The field of dynamic behavior of materials is undergoing a revolution with increasing interdisciplinary focus and unprecedented new level of diagnosis at the bench scale and numerous user facilities. Classic loading techniques including split Hopkinson pressure (Kolsky) bar, gas/ powder guns, and high-explosives are being pushed to increased levels of precision, linked with new and novel diagnostics enabling previously unattainable levels of in situ measurement combined with advanced high resolution and three-dimensional post-mortem observation. National User Facilities are enhancing their capacities for studying materials in extremes with new facilities being developed ranging from SLAC’s Linac Coherent Light Source, the Dynamic Compression Sector at the Advanced Photon Source at Argonne National Laboratory, the Z-Machine at Sandia National Laboratories, the National Ignition Facility at Lawrence Livermore National Laboratory, and Matter-Radiation Interactions in Extremes at Los Alamos National Laboratory. An exciting collaboration at GSI in Darmstadt is incorporating expertise from pRad and LANSCE at Los Alamos National Laboratory in the USA and the Institute for Theoretical and Experimental Physics in Russia using the PRIOR microscope with great potential for studying dynamic materials processes. The Diamond Light Source located at the Harwell Science and Innovation Campus in the UK has been offering great levels of postmortem investigation of damage from dynamic loading. Exciting capabilities exist at these and other facilities around the world to advance the state-of-the-art for studies of dynamic behavior of materials. Advances in modeling & simulation of dynamic loading and material response have paralleled these experimental advances. Continued evolution in computing hardware is enabling calculations with extraordinary fidelity in representation of highly coupled complex physics. Great advances have been made in density functional theory, molecular dynamics, and dislocation dynamics based material models. Simulations based on these theories have historically had to be performed at very high strain-rates and in very short time and length scales. Advances in these models and the hardware on which these simulations are performed have brought them into line with the regimes of many platforms for dynamic experiments. New theories and models of material behavior are being developed to take advantage of understanding material at the mesoscale. Continuum level constitutive models are being proposed and developed with greater levels of physical underpinning and predictive ability. The state-of-the art in modeling & simulation and experiments along with the interdisciplinary approach to modern science and engineering have lead to significant payoff from hybrid experimental-computational approaches. Advances in simulation are enabling the development and optimization of experiments, with better understanding of the underlying physics and data capture. Conversely, advances in experimentation are pushing better models, codes, and simulations. The JDBM offers a single source to bring this broad, vibrant international community together. This peer E. N. Brown (&) Los Alamos National Laboratory, Los Alamos, NM 87545, USA e-mail: en_brown@lanl.gov

Pyrolysis of phenolic impregnated carbon ablator (PICA).

Molar yields of the pyrolysis products of thermal protection systems (TPSs) are needed in order to improve high fidelity material response models. The volatile chemical species evolved during the pyrolysis of a TPS composite, phenolic impregnated carbon ablator (PICA), have been probed in situ by mass spectrometry in the temperature range 100 to 935 °C. The relative molar yields of the desorbing species as a function of temperature were derived by fitting the mass spectra, and the observed trends are interpreted in light of the results of earlier mechanistic studies on the pyrolysis of phenolic resins. The temperature-dependent product evolution was consistent with earlier descriptions of three stages of pyrolysis, with each stage corresponding to a temperature range. The two main products observed were H2O and CO, with their maximum yields occurring at ∼350 °C and ∼450 °C, respectively. Other significant products were CH4, CO2, and phenol and its methylated derivatives; these products tended to desorb concurrently with H2O and CO, over the range from about 200 to 600 °C. H2 is presumed to be the main product, especially at the highest pyrolysis temperatures used, but the relative molar yield of H2 was not quantified. The observation of a much higher yield of CO than CH4 suggests the presence of significant hydroxyl group substitution on phenol prior to the synthesis of the phenolic resin used in PICA. The detection of CH4 in combination with the methylated derivatives of phenol suggests that the phenol also has some degree of methyl substitution. The methodology developed is suitable for real-time measurements of PICA pyrolysis and should lend itself well to the validation of nonequilibrium models whose aim is to simulate the response of TPS materials during atmospheric entry of spacecraft.

On some aspects of the wheel/rail interaction

Problems in tribology and contact mechanics are becoming increasingly amenable to solution through models and simulation. But in application to the wheel/rail contact, there remain a number of very important features for which either gross simplifications or a lack of understanding or ability severely limits the success of those efforts. Examples include models of friction and material response (including damage functions). This paper examines a rather eclectic mix of wheel/rail factors with the goal of encouraging researchers to begin tackling and eradicating some of the bigger problem areas that remain in wheel–rail interaction modeling and to consider more rigorous implementation of real world conditions in simulations.

Modeling Shape-Memory Behavior of Polymers

Shape-memory behavior in polymers allows the material to undergo large controllable shape changes in response to a specific environmental stimulus. The shape-memory performance depends on the coordination of multiple physical mechanisms, and considerable opportunities exist to tailor the polymer structure and shape-memory programming procedure to achieve the desired performance. Because of the inherent complexity of the material response, theoretical and computational models provide efficient tools to explore different combinations of design options. They also provide an investigative platform to evaluate the importance of various physical mechanisms for the shape-memory response and determine the relationship between polymer properties and shape-memory performance. This article reviews recent advances in constitutive modeling of thermally induced and photo-induced shape-memory behavior in polymers.

Constraints on burial depth and yield of the 25 May 2009 North Korean test from hydrodynamic simulations in a granite medium

[1] Yield : depth of burial (DoB) tradeoff curves (TOCs) based on seismic magnitudes of the 25 May 2009 North Korean test depend strongly on the choice of empirical cavity radius (Rc) scaling model. Ambiguities over Rc scaling, particularly at large scaled DoB (SDoB), translate into unacceptably large systematic errors on yield estimates for this test. Hydrodynamic calculations involving realistic material response models offer a viable alternative to characterize Rc scaling for a range of SDoB where limited data from past nuclear tests exist. Results of such calculations are presented for a granite medium with a material response validated by modeling four phenomenological criteria for past nuclear tests in granite (free field velocity, energy partitioning into the seismic wavefield, velocity attenuation, and measured Rc). These results unambiguously favor the Rc scaling model of Denny and Johnson (DJ91) and the TOC based on that model. Lower bounds on yield and DoB of the North Korean test are constrained by predictions of an SDoB threshold for free surface damage from 2-D simulations since no such reported damage was observed for this test. Constrained by the hydrodynamic simulations, the DJ91 model indicates the minimum yield and DoB for the 25 May 2009 North Korean test is 5.7 kilotons and 375 m.

Multiscale Modelling of Gradual Degradation in Al<sub>2</sub>O<sub>3</sub>/ZrO<sub>2</sub> Ceramic Composites under Tension

Two-phase ceramic composite materials, (CMC, e.g. Al2O3/ZrO2), have a non-linear and complex overall response to applied loads due to: different phases, existence of an inital porosity, development of limited plasticity and internal microdefects. All microdefects act as stress concentrators and locally change the state of stress, leading to the development of mesocracks and finally macrocracks. Experimental results show that defects develop mainly inter-granular and cause inhomogeneity and induced anisotropy of the solid. Modelling of such material response is possible by multiscale approach describing different phenomena occuring at different scales: micro- meso- and macro- ones. The paper presents uniaxial tension process of the Al2O3/ZrO2 composite with the gradual degradation of the material properties due to different defects development.

Non-Darcian Behavior of Pyrolysis Gas in a Thermal Protection System

DOI: 10.2514/1.44103 To improve heat and ablation rate modeling of the thermal protection system for reentry vehicles, a material response model with surface ablation and pyrolysis is developed. To accurately model the effects of the pyrolysis gas within the ablator, Darcy’s law is replaced by Forchheimer’s law for flow through porous media. The use of Forchheimer’s law accounts for the inertial effects of the gas and removes any dependency on microscopic parameters,suchasporesize.Tocharacterizethe flow,theForchheimernumberisproposedbecauseitdependsonly on macroscopic quantities. To verify and validate the model, comparisons to experimental data and to prior computational results are presented. Applying Ergun’s equation to evaluate the inertial parameter of the Forchheimer number, a simple test case is run. For the case of a generic carbon-phenolic ablator subjected to a typical reentry trajectory, conditions for non-Darcian behavior are investigated by way of a parametric study. Finally, the necessary conditions required for gas kinetic energy to be relevant are highlighted.

Mechanical Response of an Alumina-filled Epoxy at Various Strain Rates

The compressive response of an alumina-filled epoxy composite material was characterized at various strain rates using a Kolsky bar and universal testing frame (MTS). In the Kolsky-bar experiments, dynamic stress equilibrium and constant strain-rate deformation in specimens were achieved using pulse-shaping techniques. The effects of specimen aspect ratio and interfacial friction on the dynamic response of the material were examined. Compressive stress-strain curves for the composite were obtained at strain rates ranging from 9.4 × 10-4 to 1.35 × 103 s-1 which exhibited strong strain-rate sensitivity. A material response model, which agrees well with the experimental measurements for the strain rates examined in this study, is also described.

Mechanical Response of an Alumina-Filled Epoxy at Various Strain Rates

The compressive response of an alumina-filled epoxy composite material was characterized at various strain rates using a Kolsky bar and universal testing frame (MTS). In the Kolsky-bar experiments, dynamic stress equilibrium and constant strain-rate deformation in specimens were achieved using pulse-shaping techniques. The effects of specimen aspect ratio and interfacial friction on the dynamic response of the material were examined. Compressive stress-strain curves for the composite were obtained at strain rates ranging from 9.4 × 10-4 to 1.35 × 103 s-1 which exhibited strong strain-rate sensitivity. A material response model, which agrees well with the experimental measurements for the strain rates examined in this study, is also described.

Incubation Time and Cavitation Erosion Rate of Work-Hardening Materials

A phenomenological analysis of the cavitation erosion process of ductile materials is proposed. On the material side, the main parameters are the thickness of the hardened layer together with the conventional yield strength and ultimate strength. On the fluid side, the erosive potential of the cavitating flow is described in a simplified way using three integral parameters: rate, mean amplitude, and mean size of hydrodynamic impact loads. Explicit equations are derived for the computation of the incubation time and the steady-state erosion rate. They point out two characteristic scales. The time scale, which is relevant to the erosion phenomenon, is the covering time—the time necessary for the impacts to cover the material surface—whereas the pertinent length scale for ductile materials is the thickness of the hardened layer. The incubation time is proportional to the covering time with a multiplicative factor, which strongly depends on flow aggressiveness in terms of the mean amplitude of impact loads. As for the erosion rate under steady-state conditions, it is scaled by the ratio of the thickness of hardened layers to the covering time with an additional dependence on flow aggressiveness, too. The approach is supported by erosion tests conducted in a cavitation tunnel at a velocity of 65 m/s on stainless steel 316 L. Flow aggressiveness is inferred from pitting tests. The same model of material response that was used for mass loss prediction is applied to derive the original hydrodynamic impact loads due to bubble collapses from the geometric features of the pits. Long duration tests are performed in order to determine experimentally the incubation time and the mean depth of penetration rate and to validate the theoretical approach.

Simple classical model for Fano statistics in radiation detectors

A simple classical model that captures the essential statistics of energy partitioning processes involved in the creation of information carriers (ICs) in radiation detectors is presented. The model pictures IC formation from a fixed amount of deposited energy in terms of the statistically analogous process of successively sampling water from a large, finite-volume container (“bathtub”) with a small dipping implement (“shot or whiskey glass”). The model exhibits sub-Poisson variance in the distribution of the number of ICs generated (the “Fano effect”). Elementary statistical analysis of the model clarifies the role of energy conservation in producing the Fano effect and yields Fano's prescription for computing the relative variance of the IC number distribution in terms of the mean and variance of the underlying, single-IC energy distribution. The partitioning model is applied to the development of the impact ionization cascade in semiconductor radiation detectors. It is shown that, in tandem with simple assumptions regarding the distribution of energies required to create an (electron, hole) pair, the model yields an energy-independent Fano factor of 0.083, in accord with the lower end of the range of literature values reported for silicon and high-purity germanium. The utility of this simple picture as a diagnostic tool for guiding or constraining more detailed, “microscopic” physical models of detector material response to ionizing radiation is discussed.

Dynamic response of two strain-hardened aluminum alloys

Despite their common usage in armor applications such as lightweight armored vehicles, the dynamic material response of 5083-H131 and 5083-H32 strain-hardened aluminum alloys has not been previously reported in the open literature. Measurement of the dynamic material properties, including the shock Hugoniot equation of state (EOS), provides hydrocode modelers with critical information required for accurate modeling of material response to intense loading. In the work reported here we investigate the Hugoniot EOS and Hugoniot elastic limit over the stress range of 1.5–8.0GPa. All experiments were performed on the Army Research Laboratory 102mm bore single-stage light gas gun. Impact conditions were uniaxial and planar to within 1mrad of tilt. Both direct-impact- and shock-transmission-type experiments were performed using velocity interferometry diagnostics to record particle velocity histories with 0.5ns temporal resolution. The shock Hugoniot for 5083-H131 is extrapolated to 50GPa and compared to the previous high pressure results of Hauver and Melani (1973) [Ballistic Research Laboratory December Technical Report No. BRL 2345, 1973] and to prior shock studies of 5083-O aluminum alloy.

Advanced constitutive modelling of bituminous materials

Currently in the UK, the mechanical properties of bituminous (asphalt) mixtures are determined by means of a suite of tests (stiffness modulus, resistance to permanent deformation and resistance to fatigue cracking) performed by means of a pneumatic apparatus known as the Nottingham Asphalt Tester (NAT). This paper presents a methodology for an alternative experimental characterisation and modelling procedure for bituminous (asphalt) mixtures using a dynamic plasticity based constitutive model. The paper briefly describes the material response model and details the monotonic, uniaxial compression and tension testing at various temperatures and strain rates that are necessary to determine the basic model parameters. Two standard UK wearing course mixtures have been included in the testing programme comprising a continuously graded, Dense Bitumen Macadam (DBM), and a gap graded, Hot Rolled Asphalt (HRA). The uniaxial test data has been used to determine mathematical functions for the compressive and tensile strengths of the asphalt mixtures as well as functions describing their hardening and softening behaviour. Plots of the flow surface (or yield surface) have been produced for both mixtures over a range of temperatures and strain rates and the model has been successfully used to simulate the compressive response of asphalt mixtures. Once implemented in a finite element code, the constitutive model has been able to simulate the damage occurring in a flexible bituminous pavement under traffic loading.

Maximum recoverable work in linear thermoelectromagnetism

We give a general closed expression for the minimum free energy in terms of Fourier-transformed quantities for a thermoelectromagnetic conductor with memory effects for the electric current density and the heat flux in the case where the integrated histories of the electric field and of the temperature gradient are chosen to characterize the states of the material. An equivalent formulation is obtained and applied to the discrete spectrum model material response.

The minimum free energy for incompressible viscoelastic fluids

A general closed expression is given in the frequency domain for the isothermal minimum free energy of an incompressible viscoelastic fluid, whose constitutive equation is expressed by a linear functional of the history of strain. Another equivalent form of the minimum free energy is also derived and used to study the particular case of a discrete model material response. Copyright © 2006 John Wiley & Sons, Ltd.

The minimum free energy for an electromagnetic conductor

We consider the linear theory of the electromagnetism characterized by a constitutive equation for the current density with memory effects. We find in the frequency domain a first expression of the minimum free energy, which is the maximum recoverable work we can obtain from a given state of the material. By using another equivalent formulation of the minimum free energy, we give the explicit formulae for the particular case of a discrete spectrum model material response.

Coarse-grained models of materials with non-convex free-energy: two case studies

Bridging across length scales is one of the fundamental challenges in the computational modelling of material systems whose mechanical response is driven by rough energy landscapes. The typical feature of such systems is that of exhibiting fine scale microstructures. Two case studies, namely, nematic elastomers and ferromagnetic shape memory alloys, are presented to illustrate the use of modern techniques from (non-convex) calculus of variations in developing coarse-grained models of microstructure-driven material response.

A study of localized deformation pattern in granular media

Shear localization in granular materials are studied numerically and compared with experimental results. General agreements are shown for the localization pattern in granular body behind a retaining wall in active and passive limit state and under a strip footing. Quantitative agreement is also shown for the limit load in strip footing. Numerical simulations are based on a micro-polar (Cosserat) continuum approach. An extended hypoplastic model is used for constitutive description of granular media, which includes rotational degrees of freedom for material particles and takes into account the effects of grain size and inter-granular friction. For a more realistic modeling of material response, fluctuation in state variables are introduced based on physical argument. It is shown through numerical examples that realistic localized deformation pattern can be initiated with a fluctuation in initial density.