Athanasios Iliopoulos

A Computational Workbench for Remote Full Field 3D Displacement and Strain Measurements

The present paper reports on the progress towards the development of a computational workbench infrastructure that implements the Meshless Random Grid (MRG) method for the remote (non contact) measurement of displacement and strain fields in 3D space. The method is applicable to structures bounded by flat surfaces that deform under various mechanical and generalized loading conditions in and out of plane. A brief description of the 3D MRG method is followed by the description of the current prototype of a software workbench developed for the computational implementation of the algorithms involved with the analysis display and export of the experimental results associated with any specific applications of the method.Copyright

Data-Driven Design Optimization for Composite Material Characterization

The main goal of the present paper is to demonstrate the value of design optimization beyond its use for structural shape determination in the realm of the constitutive characterization of anisotropic material systems such as polymer matrix composites with or without damage. The approaches discussed are based on the availability of massive experimental data representing the excitation and response behavior of specimens tested by automated mechatronic material testing systems capable of applying multiaxial loading. Material constitutive characterization is achieved by minimizing the difference between experimentally measured and analytically computed system responses as described by surface strain and strain energy density fields. Small and large strain formulations based on additive strain energy density decompositions are introduced and utilized for constructing the necessary objective functions and their subsequent minimization. Numerical examples based on both synthetic (for one-dimensional systems) and actual data (for realistic 3D material systems) demonstrate the successful application of design optimization for constitutive characterization.

Accuracy of Inverse Composite Laminate Characterization via the Mesh Free Random Grid Method

The present paper reports on the progress towards the evaluation of the Mesh Free Random Grid Method (MFRGM) for the inverse constitutive characterization of composite materials. The method provides the capability for the remote (non contact) measurement of displacement and strain fields of structures bounded by flat surfaces that deform under various mechanical and generalized loading conditions. The known forward solution of an anisotropic plate with an open hole, loaded at infinity, is used to generate synthetic images MFRG. The inverse problem for determining the constitutive parameters formulated directly on the generalized constitutive law. Performance of the technique is evaluated by the usage of just one frame corresponding to one set of strain state for various amounts of noise. The evaluation is repeated by utilizing frames corresponding to different rotations of the laminate relative to the loading direction. Finally the exceedingly accurate behavior of the methodology is discussed.Copyright

First Industrial Strength Multi-Axial Robotic Testing Campaign for Composite Material Characterization

In this paper we are reporting on the first successful campaign of systematic, automated and massive multiaxial tests for composite material constitutive characterization. The 6 degrees of freedom system developed at the Naval Research Laboratory (NRL) called NRL66.3, was used for this task. This was the in-augural run that served as the validation of the proposed overall constitutive characterization methodology. It involved accomplishing performing 1152 tests in 12 business days reaching a peak throughput of 212 tests per day. We describe the context of the effort in terms of the reasoning and the actual methods behind it. Finally, we present representative experimental data and associated constitutive characterization results for representative loading paths.© 2012 ASME

Four Parameter Inverse Characterization of Fractal Surfaces

Motivated by the need to determine the mechanical, electrical and thermal properties of contact surfaces between deformable materials that conduct electricity and heat, we are presenting here a method for characterizing certain topological characteristics of rough surfaces. The inverse identification of a set of parameters associated with the parametric representation of any rough surface based on profilometric data is described in contrast with the standard one parameter approaches. The description of the surface topography parametrization is first given in terms of a function that enables the generation of synthetic data. Objective functions are created based on both the profilometric evaluations of the parametric representation of the surface as well as its power spectrum. A statistical Monte Carlo based optimization method is implemented for determining the characteristic parameters needed for further analysis that leads to the determination of other physical properties of the surface. Numerical application of the method validates the efficiency and the accuracy of the proposed approach.Copyright

Towards a Recursive Hexapod for the Multidimensional Mechanical Testing of Composites

Automated inverse methods for material constitutive characterization under multidimensional loading conditions has motivated the custom design, manufacturing and utilization of mechatronic loading machines. This present paper reports on the architecture of a mechatronic system capable of enforcing 6-DoF kinematic boundary conditions on deformable material specimens under testing, while at the same time measuring both the imposed kinematics and the corresponding reaction forces in a fully automated manner. This system has a recursive nature as it consists of a hexapod configuration that repeats itself six times. In addition to the architecture, we also present the historical evolution, and current status of its manufacturing implementation and the initial fielding of our system for composite material testing and characterization.Copyright

Towards Static Contact Multiphysics of Rough Surfaces

This paper is describing the current status of ongoing work on developing a comprehensive modeling and simulation infrastructure capable of addressing the multiphysics behavior aspects of rough surfaces in contact. The electrical and thermal response of bodies in contact under the influence of mechanical load electric currents and thermal fluxes, is a topic of interest for many application areas. We are presenting a multiscale theory leading to derivations of expressions of electric and thermal conductivities for the case of static contact. The associated model contains both an asperity based comprehensive model as well as its continuum level coupling. The mechanical pressure and the repulsion effect from electric current through the micro-contacts as well as temperature and strain rate dependence of the plastic behavior of the asperity are accounted for as well. This formalism enables the derivation of physical properties from surface topography and bulk material properties for the interface between two rough surfaces in contact. Numerical analysis results present the dependence of the derived properties from the surface characteristics applied external load and the electric current.Copyright

Complete High Dimensional Inverse Characterization of Fractal Surfaces

The present paper describes a methodology for the inverse identification of the complete set of parameters associated with the Weirstrass-Mandelbrot (W-M) function that can describe any rough surface known by its profilometric or topographic data. Our effort is motivated by the need to determine the mechanical, electrical and thermal properties of contact surfaces between deformable materials that conduct electricity and heat and require an analytical representation of the surfaces involved. Our method involves utilizing a refactoring of the W-M function that permits defining the characterization problem as a high dimensional singular value decomposition problem for the determination of the so-called phases of the function. Coupled with this process is a second level exhaustive search that enables the determination of the density of the frequencies involved in defining the trigonometric functions involved in the definition of the W-M function. Our approach proves that this is the only additional parameter that needs to be determined for full characterization of the W-M function as the rest can be selected arbitrarily. Numerical applications of the proposed method on both synthetic and actual elevation data, validate the efficiency and the accuracy of the proposed approach. This approach constitutes a radical departure from the traditional fractal dimension characterization studies and opens the road for a very large number of applications.Copyright

A Multiphysics Theory for the Static Contact of Deformable Conductors With Fractal Rough Surfaces

In this paper, we present a multifield and multiscale theory leading to derivations of electric and thermal conductivities for the interface between two rough surfaces in contact, activated by mechanical load and electric current pulses. At the macroscale, the proposed approach involves multifield coupling of conduction and induction currents, with heat conduction induced by joule heating. The structural mechanics of the conducting materials are also considered. At the mesoscale and microscale, the theory contains a Weierstrass-Mandelbrot description of the rough contact surface profilometry and an asperity-based comprehensive model, respectively. They are both combined to derive homogenized macroscale properties for the interface boundary. The mechanical pressure and the repulsion effect from electric current through the microcontacts are accounted for as well. The results of the numerical analysis illustrate the dependence of the derived properties on the surface characteristics, external load, and electric current. Finally, the entire framework is applied to an actual conductor configuration of hollow cylinders under compression and a high current pulse to demonstrate the feasibility of the entire approach. In addition to providing typical simulation results for all selected fields present during the experiment, we also provide a comparison between the experimentally acquired resistance and the numerically derived resistance to validate the contact theory.

Multiphysics Challenges for Controlling Layered Manufacturing Processes Targeting Thermomechanical Performance

In an effort to enable on-demand process control of additive manufacturing processes for achieving component performance by design from a modeling and simulation perspective and context, we introduce a method for identifying relevant modeling and simulation challenges for the purpose of motivating research that addresses this problem. We first present the abstraction of the multiscale modeling processes connecting process control with functional performance both from the forward and inverse perspectives. We subsequently introduce a brief ontology describing the ordering of dependency and membership of all components of a model in order to isolate the potential areas where challenges can be exposed. We subsequently select some features that are usually ignored by the community during modeling. In particular, we demonstrate using a simple problem of mass and heat transfer, which is relevant to layered additive manufacturing, the implications and dangers related to ignoring process dependence on deposition path history.Copyright