Kamran A. Khan

On Inconstant Poisson's Ratios in Non-Homogeneous Elastic Media

For non-homogeneous linear elastic materials, it is demonstrated that even for the simplest loading case, i.e. quasi-static uniaxial, the Poissons ratio (PR) is space dependent and not a constant. Furthermore, the assumption of constant PR values or of separable temporal and spatial PR functions leads to ill-posed overdetermined problems. Additionally, elastic PRs become space and time dependent under time dependent stresses in non-homogeneous elastic media. Under these and more general circumstances, PRs cannot be considered material property descriptors since they now become functions of the spatially changing moduli and stresses, and vary accordingly. The same conclusions are also drawn for nonlinear elastic media with small or large deformations.

Micromechanical modeling of 8-harness satin weave glass fiber-reinforced composites:

This study introduces a unit cell-based finite element micromechanical model that accounts for correct post cure fabric geometry, in situ material properties and void content within the composite to accurately predict the effective elastic orthotropic properties of 8-harness satin weave glass fiber-reinforced phenolic composites. The micromechanical model utilizes a correct post cure internal architecture of weave, which was obtained through X-ray microtomography tests. Moreover, it utilizes an analytical expression to update the input material properties to account for in situ effects of resin distribution within yarn (the yarn volume fraction) and void content on yarn and matrix properties. This is generally not considered in modeling approaches available in literature and in particular, it has not been demonstrated before for finite element micromechanics models of 8-harness satin weave composites. The unit cell method is used to obtain the effective responses by applying periodic boundary conditions. The outcome of the analysis based on the proposed model is validated through experiments. After validation, the micromechanical model was further utilized to predict the unknown effective properties of the same composite.

Crack density and electrical resistance in indium-tin-oxide/polymer thin films under cyclic loading

AbstractHere, we propose a damage model that describes the degradation of the material properties of indium-tin-oxide (ITO) thin films deposited on polymer substrates under cyclic loading. We base this model on our earlier tensile test model and show that the new model is suitable for cyclic loading. After calibration with experimental data, we are able to capture the stress-strain behavior and changes in electrical resistance of ITO thin films. We are also able to predict the crack density using calibrations from our previous model. Finally, we demonstrate the capabilities of our model based on simulations using material properties reported in the literature. Our model is implemented in the commercially available finite element software ABAQUS using a user subroutine UMAT.

Prediction of crack density and electrical resistance changes in indium tin oxide/polymer thin films under tensile loading

We present unified predictions for the crack onset strain, evolution of crack density, and changes in electrical resistance in indium tin oxide/polymer thin films under tensile loading. We propose a damage mechanics model to quantify and predict such changes as an alternative to fracture mechanics formulations. Our predictions are obtained by assuming that there are no flaws at the onset of loading as opposed to the assumptions of fracture mechanics approaches. We calibrate the crack onset strain and the damage model based on experimental data reported in the literature. We predict crack density and changes in electrical resistance as a function of the damage induced in the films. We implement our model in the commercial finite element software ABAQUS using a user subroutine UMAT. We obtain fair to good agreement with experiments.

Modeling the viscoelastic compaction response of 3D woven fabrics for liquid composite molding processes

In liquid composite molding processes, the compaction characterization of fibrous reinforcements plays a key role in determining the thickness, fiber volume content, and part shape. This study presents detailed experimental and modeling work to study the viscoelastic compaction response of three different types of 3D woven carbon fiber reinforcements, namely, orthogonal, angle interlock, and layer-to-layer, each having a different weave style and z-binder yarn pattern. For all reinforcements, single-step, multistep and cyclic compaction experiments were conducted. A nonlinear viscoelastic model is presented that accounts for large deformations and viscous effects, to capture the response of the material under various loading histories. Model verification is also presented to capture each response with separate sets of material parameters. Parametric studies are also performed to analyze the role of model parameters on the response of different types of loadings. X-ray computed tomography analysis showed significant permanent deformation of z-binder yarns through the thickness of the reinforcements. The comparison of modeling results with the experimental data show that the model is able to capture the stress decay after multiple compaction cycles, yet needs further investigations to predict complete cyclic hysteresis. However, model results agree reasonably well with the single and multistep compaction loading.

Identification of an effective nondestructive technique for bond defect determination in laminate composites—A technical review

Laminate composites are commonly used for the production of critical mechanical structures and components such as wind turbine blades, helicopter rotors, unmanned aerial vehicle wings and honeycomb structures for aircraft wings. During the manufacturing process of these composite structures, zones or areas with weak bond strength are always issues, which may affect the strength and performance of components. The identification and quantification of these zones are always challenging and necessary for the mass production. Non-destructive testing methods available, including ultrasonic A, B, and C-Scan, laser shearography, X-ray tomography, and thermography can be useful for the mentioned purposes. A comparison of these techniques concerning their capacity of identification and quantification of bond defects; however, still needs a comprehensive review. In this paper, a detailed comparison of several non-destructive testing techniques is provided. Emphasis is placed to institute a guideline to select the most suitable technique for the identification of zones with bond defects in laminated composites. Experimental tests on different composite based machined components are also discussed in detail. The discussion provides practical evidence about the effectiveness of different non-destructive testing techniques.

A methodology for flexibility analysis of process piping

Design of piping system requires a systematic consideration of various factors as addressed by the codes and standards. This research paper aims to provide a method for flexibility analysis of a selected area of process piping at an industrial plant. Analysis is done for the purpose of accommodating a spare heat exchanger in the process layout. The analysis follows a systematic procedure, with preparation of a tentative model of the system on CAESAR II software followed by insertion of different pipe supports. The selection and location of these supports is based on the results obtained from displacement, stress, reaction and equipment nozzle analysis of the piping system. The design is in accordance with ASME B31.3, which is the standard code for process piping. The proposed method can be adapted for piping configuration of any industrial plant. With the provision of a systematic procedure, the method ensures time saving and efficient flexibility analysis of any piping system.

Response of Gaussian-modulated guided wave in aluminum: An analytical, numerical, and experimental study

The application of guided-wave ultrasonic testing in structural health monitoring has been widely accepted. Comprehensive experimental works have been performed in the past but their validation with possible analytical and numerical solutions still requires serious efforts. In this paper, behavior and detection of the Gaussian-modulated sinusoidal guided-wave pulse traveling in an aluminum plate are presented. An analytical solution is derived for sensing guided wave at a given distance from the actuator. This solution can predict the primary wave modes separately. Numerical analysis is also carried out in COMSOL® Multiphysics software. An experimental setup comprising piezoelectric transducers is used for the validation. Comparison of experimental results with those obtained from analytical and numerical solutions shows close agreement.

Analyzing the Effect of Energy Dissipation on Thermo-Mechanical Responses of Viscoelastic Fiber Reinforced Composites Using Finite Element Method

Carbon fiber reinforced composites (CFRCs) are preferred materials used in the aerospace industry for high performance load carrying applications. The polymer matrix in CFRCs is a viscoelastic material and its mechanical properties vary with time, temperature and applied external loads. Experimental work suggests that polymers and polymeric composites, subjected to high-frequency cyclic loading, can generate enormous amount of heat from the energy dissipation which softens the polymer and accelerates failure. Current research efforts on the cyclic response in CFRC focus on understanding the macroscopic (overall) performance of composites, i.e., number of cycles to failure for a given frequency and loading amplitude. A systematic understanding on the formation of heat generation and its effect on the mechanical properties of the constituents in composites, and microscopic responses of composite is currently lacking. Changes in the micromechanical field variables (strain, stress, temperature) of CFRCs during cyclic loading can be crucial in understanding failure in composites. This study attempts to provide a detailed understanding on the effect of energy dissipation due to the viscoelastic nature of polymer on the overall mechanical responses of CFRP composites subjected to cyclic loading. Finite element (FE) analyses on the deformations of CFRP composites under various boundary conditions and loading histories are presented. A thermo-mechanical viscoelastic constitutive model is used for the polymer which is defined using a material subroutine, in ABAQUS FE code. In addition, voids are added to the CFRC models to account for manufacturing imperfections and their influence on the field variables and macroscopic behaviors are investigated.

Effective thermal conductivity of two-phase composites containing highly conductive inclusions

Thermal conductivity is one of the key material properties to understand the effective thermo-mechanical behavior of advanced composites. Experimental studies show that when highly conductive inclusions are embedded in a less thermally conductive matrix, the effective thermal conductivity of the composite changes drastically with the increase of volume fraction (Vf) of the inclusions. This study presents a theoretical model to predict the effective thermal conductivity of two-phase particulate composites containing highly conductive inclusions in a polymeric matrix. The probabilistic approach presented by Tsao (1961) has been modified and extended for predicting the effective thermal conductivity of two-phase composites. The expression for the effective thermal conductivity of a unit cube of two-phase composite is derived implicitly in terms of distribution function, Vf and thermal conductivity of the constituents. Different distribution functions of the inclusions are proposed and the optimum function is obtained to describe the effective thermal conductivity of highly conductive particulate composites. Results of the effective thermal conductivity of a cubic unit cell obtained from different distributions of inclusions are compared with published experimental data, and other analytical and numerical models for particulate composites available in the literature. The results show a linear distribution of inclusions gives reasonable estimates of the effective thermal conductivity of the particulate composites. It is anticipated that the proposed approach can be used to develop models for the effective thermal conductivity of advanced composites containing highly conductive inclusions.