<p>  </p> <p>Thermal shrinkage in advanced composite manufacturing causes residual stress in a cylindrical anisotropic segment. The residual stress later induces a spring-in angle when  the temperature change is negative. The superposition method in the finite element method (FEM) by ABAQUS©  proves that only the residual stress in the circumferential direction controls the spring-in angle and induces the radial residual stress. To predict the angle change, the residual stress is firstly determined by using the closed-loop geometry in FEM and then implemented into the cylindrical cross-ply symmetric laminate segment. Consequently, the geometry creates the spring-in angle under the traction-free surface. The angle change is in good agreement with the Radford equation and is found to depend on the coefficient of thermal expansion (CTE) in the circumferential and radial directions rather than other material properties and geometry dimensions. </p> <p>The study found a new limitation of the Radford equation, in that it is accurate when the part is anisotropic symmetric laminate, but not when it is unsymmetric. The accuracy of the Radford equation is further explored with the double curve geometry. Using the superposition method, the circumferential residual stress along the major curve is found to have an influence on the angle change not only of the major curve, but also of the minor curve. The negative temperature change produces the spring-in angle on the major curve, and both spring-in and -off angles on the minor curve, which rely on the radius ratio. In addition, the spring-in angle on the major curve is coincident with the Radford equation. In sum, knowing the spring-in angle is very helpful in designing a tool in advanced composite manufacturing, and the superposition method and the Radford equation are applicable to predict the spring-in angle.</p>

Transverse ply cracking often leads to the loss of stiffness and reduction in thermal expansion coefficients. This paper presents the thermoelastic degradation of general cross-ply laminates, containing transverse ply cracks, subjected to biaxial extension, bending and thermal loading. The stress and displacement fields are calculated by using the state space equation method [Zhang D, Ye JQ, Sheng HY. Free-edge and ply cracking effect in cross-ply laminated composites under uniform extension and thermal loading. Compos Struct [in press].]. By this approach, a laminated plate may be composed of an arbitrary number of orthotropic layers, each of which may have different material properties and thickness. The method takes into account all independent material constants and guarantees continuous fields of all interlaminar stresses across interfaces between material layers. After introducing the concept of the effective thermoelastic properties of a laminate, the degradations of axial elastic moduli, Poisson's ratios, thermal expansion coefficients and flexural moduli are predicted and compared with numerical results from other methods or available test results. It is found that the theory provides good predictions of the stiffness degradation in both symmetric and antisymmetric cross-ply laminates. The predictions of stiffness reduction in nonsymmetric cross-ply laminates can be used as benchmark test for other methods.

No description available.

No description available.

This paper presents a semianalytical method for the prediction of interlaminar stresses and displacements near the free edges and ply cracks in general angle-ply laminates subjected to biaxial extensions and/or in plane shear deformation. The method is based on a state space representation of the three-dimensional equations of elasticity. Numerical solutions are obtained by using layer refinement in the through thickness direction and Fourier series expansion in the other directions. By this approach, an angle-ply laminate may be composed of an arbitrary number of monoclinic layers and each layer may have different material property and thickness. This method guarantees continuous fields of all interlaminar stresses across interfaces between material layers. Numerical results are compared with those obtained from other methods. It is found that the theory provides a satisfactory approximation to the stress singularities near the free edges and ply cracks. Numerical solutions for antisymmetric laminates under extension and general laminates under shearing are new in the literature and can be used as benchmarks for validating new models.

As a new concept for morphing structures, the use of piezoelectric actuators to effect snap-through behavior of simple unsymmetric cross-ply composite laminates is examined. Many unsymmetric laminates have more than one stable room-temperature shape and can be snapped through from one stable shape to another. In this new concept for morphing structures, one or more piezoelectric actuators are bonded to unsymmetric laminates, and are then used to snap the laminate from one shape to another. The actuator would be used to change shape, but would not be required to maintain the shape. Using the Rayleigh-Ritz technique, several models are developed to predict the interaction between the base laminate and the actuator. In particular, the voltage (applied to the actuator) needed to snap the laminate is predicted. The NASA-LaRC Macro-Fiber Composite&174; (MFC&174;) actuator is chosen as the actuator of choice for this work. A laminate is manufactured, an actuator is bonded to the laminate, and experiments are performed. Since the agreement between the initial models and experimental results was not good, the models were revised. Good agreement between the predictions of the revised model and experiment is reached. Suggestions for future research directions are presented. / Ph. D.

Micromechanics theories have been used to develop macro-level constitutive relations for infinitesimal elastoplastic deformations of unidirectional fiber reinforced laminates. The matrix is assumed to be isotropic and deform elasto-plastically and the fibers transversely isotropic and linear elastically. We have analyzed damage initiation, damage progression, and failure of 16-ply unidirectional fiber reinforced laminates impacted at normal incidence by a rigid sphere. The damage is assumed to initiate when at least one of Hashin's failure criteria is satisfied with the evolving damage modeled by an exponential relation. Transient three dimensional impact problems have solved using the finite element method (FEM) by implementing the material damage model as a user defined subroutine in the FE software ABAQUS. From strains supplied by ABAQUS the subroutine uses the free shear traction technique and values of material parameters of the constituents to find average stresses in a FE, and checks for Hashin's failure criteria. If the damage has initiated, the subroutine evaluates the damage developed, computes resulting stresses, and provides them to ABAQUS. The irreversibility of the damage is satisfied by requiring that the damage evolved does not decrease during unloading. The delamination failure mode is simulated by using the cohesive zone model and the degradation of material properties already available in ABAQUS. The computed time histories of the axial load acting on the impactor are found to agree well with the experimental ones available in the literature. The effect of stacking sequence in the laminate upon the impact load has been ascertained. / Ph. D.

Transparent laminates are widely used for body armor, goggles, windows and windshields. Improved understanding of their deformations under impact loading and of energy dissipation mechanisms is needed for minimizing their weight. This requires verified and robust computational algorithms and validated mathematical models of the problem. Here we have developed a mathematical model for analyzing the impact response of transparent laminates made of polymeric materials and implemented it in the finite element software LS-DYNA. Materials considered are polymethylmethacrylate (PMMA), polycarbonate (PC) and adhesives. The PMMA and the PC are modeled as elasto-thermo-visco-plastic and adhesives as viscoelastic. Their failure criteria are stated and simulated by the element deletion technique. Values of material parameters of the PMMA and the PC are taken from the literature, and those of adhesives determined from their test data. Constitutive equations are implemented as user-defined subroutines in LS-DYNA which are verified by comparing numerical and analytical solutions of several initial-boundary-value problems. Delamination at interfaces is simulated by using a bilinear traction separation law and the cohesive zone model. We present mathematical and computational models in chapter one and validate them by comparing their predictions with test findings for impacts of monolithic and laminated plates. The principal source of energy dissipation of impacted PMMA/adhesive/PC laminates is plastic deformations of the PC. In chapter two we analyze impact resistance of doubly curved monolithic PC panels and delineate the effect of curvature on the energy dissipated. It is found that the improved performance of curved panels is due to the decrease in the magnitude of stresses near the center of impact. In chapter three we propose constitutive relations for finite deformations of adhesives and find values of material parameters by considering test data for five portions of cyclic loading. Even though these values give different amounts of energy dissipated in the adhesive, their effect on the computed impact response of PMMA/adhesive/PC laminates is found to be minimal. In chapter four we conduct sensitivity analysis to identify critical parameters that significantly affect the energy dissipated. The genetic algorithm is used to optimally design a transparent laminate in chapter five. / Ph. D.

No / This paper presents the details of a methodology for predicting the thermoelastic properties degradation in general symmetric laminates with uniform ply cracks in some or all of the 90° layers. First, a stress transfer method is derived by using the concept of state space equation. The laminate can be subjected to any combination of in-plane biaxial and shear loading, and the uniform thermal loading is also taken into account. The method takes into account all independent material constants and guarantees continuous fields of all interlaminar stresses across interfaces between material layers. By this method, a laminate may be composed of an arbitrary number of monoclinic layers and each layer may have different material property and thickness. Second, the concept of the effective thermoelastic properties of a cracked laminate is introduced. Based on the numerical solutions of specially designed loading cases, the effective thermoelastic constants of a cracked laminate can be obtained. Finally, the applications of the methodology are shown by numerical examples and compared with numerical results from other models and experiment data in the literature. It is found that the theory provides good predictions of the thermoelastic properties degradation in general symmetric laminates.

Les nouveaux renforts NCF (Non Crimp Fabrics) sont adaptés aux procédés RTM (Resin Transfer Moulding) ou RIM (Resin Infusion Moulding) et permettent d’élaborer des structures aéronautiques complexes et de grande taille. Cependant, la présence de la couture peut conduire à une morphologie spécifique hétérogène du matériau avec un réseau 3D de zones riches en résine. Ces dernières, sous cycles de vieillissement hygrothermiques, sont à l’origine d’un état spécifique de fissuration. Ainsi, le présent travail se concentre sur la caractérisation morphologique et la fissuration d’une famille particulière des NCF - NC2 (Non Crimp New Concept), soumis au vieillissement hygrothermique cyclique. Pour cela, des cycles accélérés de vieillissement sont définis, diverses méthodes de caractérisation sont utilisées et différentes variables représentatives sont introduites. Au sujet de la morphologie du matériau, une hétérogénéité multi-échelles a été visualisée en surface et dans l’épaisseur en effectuant des coupes sous microscope 2D et de la reconstruction volumique sous tomographie 3D à RX. En ce qui concerne la fissuration hygrothermique, son initiation et son développement ainsi que sa morphologie ont été étudiés. L’influence de la morphologie et des paramètres de chargement au cours des cycles a été identifiée. De plus, afin de maîtriser le comportement des zones riches en résine, un couplage thermique/hygrothermique-mécanique à différents états de vieillissement du matériau a été décrit finement par des mesures de champs. Enfin, la tenue mécanique du matériau vieilli a été étudiée. / Stitched multiaxial laminates NCF (Non-Crimp Fabric) are potential candidate materials as new high performance preforms for manufacturing complex and large aeronautical composite structures by RTM (Resin Transfer Moulding) or infusion processes. Stitching within the preform leads to a particular morphology including 3D resin-rich regions and to a specific crack network developed in the bulk of the laminate when this is subjected to hygrothermal ageing cycles. The present work focuses on the characterization of the morphology and the crack development in a particular family of NCF - NC2 (Non Crimp New Concept) subjected to hygrothermal cycling. For this purpose, different accelerated thermal/hygrothermal ageing cycles were defined, various characterisation methods were adopted and representative variables were introduced. Regarding the structural morphology, a multi-scale heterogeneity of the NC2 could be visualized on the surface and through the thickness by optical microscopy as well as by the non-destructive volumetric analysis of X-Ray tomography. Regarding hygrothermal cracking, its initiation, its development and its morphology were studied. The influence of the morphology and the role of loading parameters on crack development were identified. Furthermore, for a better control of resin-rich region behaviour, the thermal/hygrothermal-mechanical coupling at different ageing states was investigated by full-field image correlation. Finally, the mechanical strength of the aged material was determined.