Peter Matic

Elastic wave propagation in locally resonant sonic material: Comparison between local interaction simulation approach and modal analysis

Locally resonant sonic materials exhibit strong sound attenuation bands in the audible frequency range due to resonance scattering of elastic waves. We compare the results of a finite element modal analysis of a single resonant unit with sound attenuation spectra obtained from wave propagation simulations based on the local interaction simulation approach. The modal analysis yields a complete prediction of all resonance modes including information on node locations, mode degeneration, and modes that do not attenuate sound due to geometrical symmetries. Elliptical instead of circular inclusions break the geometric symmetry of the resonators, splitting the attenuation peak of degenerate modes into separate peaks. A small frequency shift is observed between the resonance frequencies and the frequencies of maximum sound attenuation, due to the asymmetric shape of the attenuation peaks and interference between resonance scattering and free propagating waves.

The Relationship of Tensile Specimen Size and Geometry Effects to Unique Constitutive Parameters for Ductile Materials.

Direct observation of solid deformation at all points in a material test specimen is generally not possible. This becomes important when an internal point experiences the maximum effective deformation of the entire specimen. The simple reduction of load-displacement data to material constitutive parameters, as for inelastic tensile specimen necking, is precluded. Although the mechanics of necking has received considerable attention, the complementary issue of constitutive parameter determination from inelastic specimen experimental data has not. This investigation addresses both aspects of the problem for HY-100 steel. An experimental-computational procedure is demonstrated, which generates material constitutive parameters valid at strains beyond those at the onset of necking. Neck deformation, specimen size and specimen geometry effects are used to ensure the uniqueness of the parameters in the sense that a single uniaxial continuum true-stress–true-strain curve predicts the behaviour of laboratory specimens with different geometries. The solution parameters for HY-100 steel are developed, in the context of an incremental elastic–plastic constitutive formulation, by this procedure. These results support the view that tensile specimen necking is the result of the interaction between imperfection-free specimen geometry, material behaviour and applied load for relatively unsophisticated constitutive formulations if the constitutive parameters are accurately determined. In addition, broken symmetry of specimen deformation is computationally predicted and experimentally confirmed for long, slender specimen geometries.

A cellular automaton-based technique for modeling mesoscale damage evolution

A 2D stochastic cellular automaton model was developed to create a framework to examine the evolution of damage on the mesoscale. The model is based on an expanding representative volume element (RVE) in a cellular domain. The state of a cell, which can be either solid material or void, is determined by mapping existing voids through plastic convection and creating new damage through the cellular automaton. The cellular automaton formulation addresses the effect of damage initiation, propagation and coalescence through the iterative evaluation of individual cells based on their current state and the current states of their eight neighboring cells. The amount of damage in each time step is controlled by conservation of mass in the expanding RVE. By modifying the ratio of damage to plastic convection and the probabilities in the cellular automaton rule table, the model can generate microcrack or microvoid damage morphologies at different spatial scales. Very rapid simulations are achieved using this formulation. Thus, a wide range of material damage morphologies can be rapidly modeled in a single simulation architecture based solely on local cell geometry. However, one will be able to characterize cellular automaton rules and parameters to simulate specific constitutive models of material damage and fracture.

Overview of multifunctional materials

Multifunctional design has evolved over the past decade, moving away from discrete unifunctional subsystems with clearly defined boundaries, to produce systems design and materials design methods that blend performance in new and innovative ways. This presentation looks at the development of multifunctional design from a systems and a materials perspecitve. A classification of multifunctional desings is presented, in terms of the decreasing scales at which the boundaries of subsystems, components and material are blurred. Guidelines for identifying multifunctional opportunities at the system and material scale are also discussed.

MULTIFUNCTIONAL STRUCTURE-BATTERY MATERIALS FOR ENHANCED PERFORMANCE IN SMALL UNMANNED AIR VEHICLES

Aircraft design and manufacturing have been in a state of constant technological evolution over the last 100 years. Considerable effort has been focused on improving performance, durability, and reliability, and lowering costs. This is being accomplished today using cutting-edge design methodology that incorporates multidisciplinary design optimization of complex systems in place of older methods that independently optimized local subsystems and iterated between designs to satisfy global design constraints. Air vehicles are designed to move payload between two points, hence increasing the payload capacity or increasing the flight time endurance or range are important system-level goals in the design process. For winged aircraft, a large percentage of total weight is taken up by the structure (~37%) and fuel (~34%) (Thomas et al., 2002). Decreasing the weight of these subsystems or increasing the fuel weight fraction can improve aircraft performance, and this can be accomplished through structure-power multifunctionality. This abstract reports on the design and use of a multifunctional structure- battery (power) material to increase the flight endurance time of a small electric-propelled unmanned air vehicle (UAV). Flight endurance time is related, in Eq. (1), to the available battery energy, subsystem weights, and aerodynamic parameters. As can be seen from this equation, modifications in the available battery energy or sub-system weights (structure and battery) will affect system performance. Increases in the flight time are sought through a reduction of redundancy between the structure and battery subsystem materials and functions (shape and power). We can accomplish this by using a multifunctional structure-battery material that stores electrical energy while a carrying part of the mechanical load.

Ductile alloy constitutive response by correlation of iterative finite element simulation with laboratory video images

Abstract The accurate description of material response consists of a material constitutive formulation and material parameter values. The reduction of material test specimen load versus displacement data to constitutive parameters is often precluded by inelastic material response and deformation inhomogeneity within the specimen. For ductile engineering alloys, these effects are influenced by specimen geometry and must be uncoupled from specimen geometry to characterize the large strain material response. The accuracy of material parameters at such strains should be demonstrated for subsequent applications to design and analysis. Iterative solution for material constitutive parameters is discussed in the investigation. The use of video processing of laboratory tensile test specimens is combined with successive computational simulations of the specimen responses. The solution for HSLA-80 steel constitutive parameters, in the context of incremental plasticity theory, is presented. The material response is treated as the unknown in the computational simulations. It is iteratively modified to achieve correlation with the laboratory experiments. Two different specimen length-to-diameter aspect ratios are utilized to ensure the geometry independence of the material solution and to facilitate efficient solution. The constitutive iteration sequence illustrates the sensitivity of specimen response to material nonlinearity.

A mesoscale computer simulation of multiaxial yield in gasar porous copper

Abstract The mesoscale biaxial plastic flow of a porous copper material produced by the ‘gasar’ gas-eutectic solidification process is examined in this study. The pore morphology is characterized by elongated, oriented and partially ordered pores. Tensile and compressive testing suggested a very anisotropic yield response, weak coupling between stresses generated by deformations applied in the directions parallel to and transverse to the longitudinal pore axes, and bulk density decreases associated with compression in the longitudinal direction. A two dimensional explicit finite element model, based on an image of the microstructure, was subjected to combinations of tensile, zero and compressive displacement loads applied to the model boundaries. The simulations provided insight into the evolution of local micro yielding, plastic connectivity across the microstructures and buckling of the pores to explain the experimental observations.

A Computational investigation of local material strength and toughness on crack growth

Abstract Computational simulation of stable crack growth is an important aspect of structural integrity prediction. Modern alloy strength and ductility increase local material fracture toughness but simultaneously complicate stable crack growth predictions. Material modeling, material parameter identification, fracture criterion and numerical crack growth algorithms are issues which must be addressed for robust crack growth prediction. In this investigation, a series of finite element simulations were undertaken to investigate two-dimensional mode I crack growth in a modified compact tension specimen geometry. Two different crack lengths were considered. HY-100 steel parameters, previously characterized for large strain deformation, were utilized. Additional material responses, based on the HY-100 nonlinear response but with different yield strengths and ductilities, were also considered to assess parametrically material effects on crack growth. A debonding algorithm was employed to produce crack growth by nodal release when local material conditions, satisfying a specified local fracture criterion, were met. Material fracture and crack growth were treated as dependent variables of the analysis, generating crack growth in discrete increments. The results of this parametric computational study demonstrated crack growth of up to 67% of the initial crack length over the total number of load increments allowed for each combination of geometry, material strength and material toughness. The current crack tip energy density histories exhibited piecewise smooth behavior, consistent with the changing crack tip position produced by discrete crack growth across finite elements. The relative loads and crack growth sustained by each specimen were observed to depend on the elastic stress and strain response of the material in addition to the local fracture toughness of the material.

Defect, Constitutive Behavior, and Continuum Toughness Considerations for Weld Integrity Analysis

Abstract : The failure load of a butt welded T-frame with a lack of fusion defect is predicted and compared with experimental data. Energy density concepts are used to quantitatively describe the material toughness at the continuum scale. Particular attention is given to the determination of base metal and weld metal uniaxial stress-strain curves valid for large deformation. Finite element results are used to predict the location where the energy density first reaches a critical value, and the corresponding applied load. The predicted initiation site and load compares favorably with the experiment. Keywords: Weld defects, Fracture, Energy density, True stress-true strain curve.

Structure-battery multifunctional composite design

In multifunctional material design, two or more functions performed by distinct system components or materials are incorporated into a single component or material system to improve system performance. The aim of this paper is to present a framework for the design of structure-battery (power) multifunctional composite materials for unmanned air vehicle (UAV) applications. The design methodology is based on optimization of composite material performance indices and the use of material design selection charts introduced by Ashby and coworkers in a series of papers for homogeneous and two-phase composite materials. Performance indices are derived for prismatic structure-battery composites under various loading conditions. The development of simple design tools in the form of spreadsheet templates is also discussed. Finally, results based on the above-mentioned framework and actual material properties will be presented for structure-battery circular and square struts.