Rani Elhajjar

A quantitative thermoelastic stress analysis method for pultruded composites

A non-contact Thermoelastic Stress Analysis (TSA) method is proposed to measure the sum of the direct strain components on the coated surface of thick pultruded composites. Traditionally, TSA methods are used to relate the change of surface temperature to the change of the first invariant of the stress. The proposed method takes advantage of the in-plane transversely isotropic surface layer and relates the measured temperature change to the sum of the surface strains because the latter are directly related to the first invariant of stress. Quantitative strain measurements using the TSA method are verified for multi-axial stress states by comparing the measured in-plane strain invariant in plate samples with a circular hole to those obtained from finite element (FE) simulations. Good comparisons are obtained when compared to the FE strain contours.

Crack propagation analysis of mode-I fracture in pultruded composites using micromechanical constitutive models

Abstract An experimental and analytical study is carried out to characterize the fracture behavior of fiber reinforced plastic pultruded composites. The composite material system used in this study consists of roving and continuous filament mat (CFM) layers with E-glass fiber and polyester matrix. Eccentrically loaded single-edge-notch-tension ESE(T) fracture toughness specimen were cut from a thick pultruded plate, with the roving orientation transverse to the loading direction. Three-dimensional micromechanical constitutive models for the CFM and roving layers are developed. The micromodels can generate the effective nonlinear behavior while recognizing the in situ response at the fiber and matrix levels. The ability of the proposed micromodels to predict the effective elastic properties as well as the nonlinear response under multi-axial stress states is verified and compared to the stress–strain response from a series of off-axis tests. The micromodels are implemented in a finite element code with a cohesive layer to model the nonlinear fracture behavior of a pultruded composite material with different crack configurations. The properties for the cohesive layer were calibrated from one ESE(T) specimen with a crack to width ratio, a / W of 0.5. Good prediction is demonstrated for a range of notch sizes and geometries. The use of cohesive elements with nonlinear micromodels is effective in modeling the transverse mode-I crack growth behavior in pultruded composites.

Nanocellulose-Enabled Electronics, Energy Harvesting Devices, Smart Materials and Sensors: A Review

Cellulose nanomaterials have a number of interesting and unique properties that make them well-suited for use in electronics applications such as energy harvesting devices, actuators and sensors. Cellulose nanofibrils and nanocrystals have good mechanical properties, high transparency, and low coefficient of thermal expansion, among other properties that facilitate both active and inactive roles in electronics and related devices. For example, these nanomaterials have been demonstrated to operate as substrates for flexible electronics and displays, to improve the efficiency of photovoltaics, to work as a component of magnetostrictive composites and to act as a suitable lithium ion battery separator membrane. A discussion and overview of additional potential applications and of previously published research using cellulose nanomaterials for these advanced applications is provided in this article. The concept of using cellulose nanofibrils in stimuli-responsive materials is illustrated with highlights of preliminary results from magnetostrictive nanocellulose membranes actuated using magnetic fields.

Porosity Defect Morphology Effects in Carbon Fiber – Epoxy Composites

This study examines the morphology of porosity defects and the effects on the flexural stiffness of prepreg produced carbon fiber/epoxy matrix composites. In this study, an analytical and experimental effort is presented for assessing the effects of porosity on the flexural rigidity. The analytical approach for predicting the flexural rigidity is based on a transformed cross-section of the original laminate. Specimens of carbon/epoxy multi-layered specimens are manufactured with void contents in the range of 1 to 25% for calibration and validation of the proposed data. Significant improvements in flexural rigidity are correlated with higher porosity and thickness measurements.

3D full field strain analysis of polymerization shrinkage in a dental composite

OBJECTIVE The objective of this research was to study the polymerization shrinkage in a dental composite using 3D digital image correlation (DIC). METHODS Using 2 coupled cameras, digital images were taken of bar-shaped composite (Premise Universal Composite; Kerr) specimens before light curing and after for 10 min. Three-dimensional DIC was used to assess in-plane and out-of-plane deformations associated with polymerization shrinkage. RESULTS The results show the polymerization shrinkage to be highly variable with the peak values occurring 0.6-0.8mm away from the surface. Volumetric shrinkage began to significantly decrease at 3.2mm from the specimen surface and reached a minimum at 4mm within the composite. Approximately 25-30% of the strain registered at 5 min occurred after light-activation. Application of 3D DIC dental applications can be performed without the need for assumptions on the deformation field. SIGNIFICANCE Understanding the local deformations and strain fields from the initial polymerization shrinkage can lead to a better understanding of the composite material and interaction with surrounding tooth structure, aiding in their further development and clinical prognosis.

Quasi-Isotropic Triaxially Braided Cellulose-Reinforced Composites

In this article, we investigate experimentally and analytically the mechanical properties of a natural fiber quasi-isotropic triaxially braided composite. The composite is prepared from triaxially braided regenerated cellulose fibers and a high-bio-content epoxy resin system using a resin infusion process. Simultaneous mechanical loading, digital image correlation, and acoustic emission tests were performed on notched and unnotched specimens to understand the tensile behavior of the composites and the initiation and propagation of damage. Experimental results were compared with the effective tensile properties determined using an analytical model. The model is a discrete three-layer analytical representation based on a mechanics transformation-based representation of the quasi-isotropic braided layers. The model is used to determine the elastic stiffness and Poisson effects based on the constituent properties such as the fiber volume fractions, the waviness of the bias tows, and the relative thickness of the braided preform. The experimental results show the analytical models ability in predicting the composites elastic properties. The unique fabric architecture is found to have a large influence on the strength properties across the different specimen geometries investigated.

An Infrared Thermoelastic Stress Analysis Investigation for Detecting Fiber Waviness in Composite Structures

A full-field thermoelastic stress analysis infrared (TSA-IR) method is used to investigate the possibility of inspecting for out of plane fiber waviness in composite structures. Two different waviness profiles are generated and compared to control specimen having no anomalies. Tension and compression loading schemes are investigated and the TSA-IR emissions are compared to cross-sectional investigations. A subset of the specimens is also notched to investigate the thermal emissions from holes when compared to the waviness. The interaction of the hole and waviness shows the errors that can be introduced when using TSA-IR for quantitative analysis.

Modeling and characterization of the moisture-dependent bilinear behavior of regenerated cellulose composites

In this study, we investigate the tensile mechanical properties of unidirectional Lyocell/epoxy composites under wet and dry conditions using a multi-scale analysis approach. Characterization of the samples shows a bilinear stress–strain behavior of the fibers and composites under tension loading. The bilinear elastic–plastic stress–strain response of the Lyocell fibers is incorporated into a p-version finite element model for presenting a methodology for structural analysis of this composite system. The proposed finite element models were successfully able to relate the micro to macro-mechanical behavior enabling an approach for determining the 3D orthotropic elastic–plastic constants of regenerated cellulose/epoxy composites.

Characteristics of a Magnetostrictive Composite Stress Sensor

A magnetostrictive composite material (MCM) of a giant magnetostrictive alloy, Terfenol-D (Tb0.3Dy0.7Fe2), and epoxy resin has been used to fabricate mechanical stress sensors that are electrically isolated and based on the Villari effect (or inverse magnetostriction). Under an external mechanical stress, magnetic domains in the MCM rotate and expand proportionally, modifying the magnetic properties due to the Villari effect. We designed a probe consisting of a toroidal, laminated, air-gapped silicon steel core with a tightly wound coil that facilitated monitoring the change in the magnetic susceptibility of the MCM. Since the coil is wound around the core and the air gap is partially filled with the MCM, the change in magnetic susceptibility of the MCM can be estimated by performing inductance measurements. We investigated the effect of the angle between the stress and magnetic susceptibility measurement axes on the response of MCM sensors mounted at different angles onto 6061-T6 aluminum substrates. This sensing technique can be used for non-destructive evaluation and monitoring the integrity of various mechanical and civil engineering structures. This wireless technique might also be extendable to measure the stress vector.

Porosity Content Evaluation in Carbon-Fiber/Epoxy Composites Using X-ray Computed Tomography

In this study we investigate the X-ray computed tomography method for assessment of porosity levels in carbon-fiber/epoxy composites. High resolution computed tomography is used to investigate the distribution, size, and morphology of porosity in composite specimens. The specimens are manufactured with porosity values in ranges that are encountered in the manufacturing of composite structures. Porosity measurement from the optical scans is found to be highly dependent on the number of cross-sections investigated. The stacking sequence is found to have a high effect on the measurements made especially at higher porosity levels due to the preferential alignment of the porosity microstructure.