Finite-Element Model for Failure Study of Two-Dimensional Triaxially Braided Composite

A new three-dimensional finite-element model of two-dimensional, triaxially braided composites is presented in this paper. This mesoscale modeling technique is used to examine and predict the deformation and damage observed in tests of straight-sided specimens. A unit cell-based approach is used to consider the braiding architecture and the mechanical properties of the fiber tows, the matrix, and the fiber tow-matrix interface. A 0°/±60° braiding configuration has been investigated by conducting static finite-element analyses. Failure initiation and progressive degradation has been simulated in the fiber tows by using the Hashin failure criteria and a damage evolution law. The fiber tow-matrix interface was modeled by using a cohesive zone approach to capture any fiber-matrix debonding. By comparing the analytical results with those obtained experimentally, the applicability of the developed model was assessed and the failure process was investigated.     

Simplified Braiding through Integration Points Model for Triaxially Braided Composites

A simplified methodology has been developed for modeling two-dimensional triaxially braided composite plates impacted by a soft projectile using an explicit nonlinear finite-element analysis code LS-DYNA. The fiber preform architecture is modeled using shell elements by incorporating the fiber preform architecture at the level of integration points. The soft projectile was modeled by an equation of state. An arbitrary Lagrangian–Eulerian formulation is used to resolve numerical problems caused by large deformation of the projectile. The computed results indicate that this numerical model is able to simulate a triaxially braided composite undergoing a ballistic impact effectively and accurately, including the deformation and failure with a reasonable level of computational efficiency.     

Efficient Multiscale Modeling Framework for Triaxially Braided Composites using Generalized Method of Cells

In this paper, a framework for a three-scale analysis, beginning at the constituent response and propagating to the braid repeating unit cell (RUC) level, is presented. At each scale in the analysis, the response of the appropriate RUC is represented by homogenized effective properties determined from the generalized method of cells micromechanics theory. Two different macroscale RUC architectures are considered, one for eventual finite-element implementation and the other for material design, and their differences are compared. Model validation is presented through comparison to both experimental data and detailed finite-element simulations. Results show good correlation within range of experimental scatter and the finite-element simulation. Results are also presented for parametric studies varying both the overall fiber volume fraction and braid angle. These studies are compared to predictions from classical lamination theory for reference. Finally, the multiscale analysis framework is used to predict the onset of failure in a transversely loaded triaxially braided composite. The predicted transverse failure initiation stress value shows excellent correlation and provides the bound for which linear elastic constitutive models are acceptable for implementation.     

Modeling of Multilayer Composite Fabrics for Gas Turbine Engine Containment Systems

An experimental and modeling system for the modeling of multilayer composite fabrics used in a gas turbine engine containment system is developed. Specifically, Kevlar 49 (17×17) and Zylon AS (35×35) fabrics are used in the study. The experimental setup is first used to obtain the material properties of these fabrics. Later, one or more layers of these fabrics is tightly wrapped around a steel cylinder that simulates an engine containment housing. A steel penerator (or a blunt nose) is used in a static test by slowly pushing against the fabric. The resulting load-deflection data are used to compute a variety of parameters, including the energy absorption capacity. The material behavior obtained from the experimental study is then used as the constitutive model in a finite element simulation of the static test. The objective is to develop a procedure for understanding the relative strengths and weaknesses of different fabrics and to aid in the development of finite element modeling of actual fan blade-out events.     

Damage Approach for the Prediction of Debonding Failure on Concrete Elements Strengthened with FRP

In this study, experimental and numerical procedures are proposed to predict the debonding failure of concrete elements strengthened with fiber-reinforced polymers (FRPs). Such debonding is modeled as a damage process, which takes place in a band along the bond line (crack band). Three-point bending tests were designed to obtain the softening curve of the crack band. The numerical simulations are conducted using a plastic-damage model. In this approach, the damage resulting in debonding is defined using the softening curve of the crack band. Numerical results are validated against experimental results obtained from single-lap shear tests. The numerical models were capable of predicting the experimentally observed load versus strain behavior, failure load, and failure mechanism of the single-lap shear specimens. The predictive capabilities of the numerical approach presented here were further investigated by means of a parametric study of the single-lap shear test. Results from this study indicate the applicability of the crack band approach to predict the behavior of concrete–FRP joints; they also indicate that the failure load determined from a single-lap shear test is geometry dependent.     

Nonlinear Analysis of Masonry Arches Strengthened with Composite Materials

The nonlinear behavior of masonry arches strengthened with externally bonded composite materials is investigated. A finite-element (FE) formulation that is specially tailored for the nonlinear analysis of the strengthened arch is developed. The FE formulation takes into account material nonlinearity of the masonry construction and high-order kinematic relations for the layered element. Implementation of the above concept in the FE framework reduces the general problem to a one-dimensional nonlinear formulation in polar coordinates with a closed-form representation of the elemental Jacobian matrix (tangent stiffness). A numerical study that examines the capabilities of the model and highlights various aspects of the nonlinear behavior of the strengthened masonry arch is presented. Emphasis is placed on the unique effects near irregular points and the nonlinear evolution of these effects through the loading process. A comparison with experimental results and a discussion of the correlating aspects and the ones that designate needs of further study are also presented.     

Parametric Study on the Dynamic Instability Behavior of Laminated Composite Stiffened Plate

This paper deals with the study of dynamic or parametric instability behavior of laminated composite stiffened plates with step-uniform and concentrated in-plane harmonic edge loading. The eight-noded isoparametric degenerated shell element and a compatible three-noded curved beam element are used to model the plate and the stiffeners, respectively. The method of Hill’s infinite determinant is applied to analyze the dynamic instability regions. Numerical results are presented through convergence and comparison with the published results from the literature. The effects of parameters like loading type, stiffening scheme, lamination scheme, dynamic load factor, and boundary conditions are considered in the dynamic instability analysis of laminated composite stiffened plate. It has been shown that the type of loading and the width of loading have remarkable effect on the dynamic instability characteristics of the stiffened plate.     

Effect of Localized Bending at Through‐Flaws in Pressurized Composite Cylinders

An analytical and experimental investigation was conducted to determine the effect of localized bending at through‐flaws in pressurized composite cylinders. A finite‐difference solution was formulated to determine the stress, strain, and displacement fields in the vicinity of a slit. Tests were conducted on 305‐mm‐diameter cylinders made from graphite/epoxy fabric in a (0,?45)s configuration, with axial slits. Surface‐strain‐field measurements made in the vicinity of the slit showed a significant bending, which verified the finite‐difference solution. This significant bending near the slit tip results in a large magnification of the stresses there. An average stress criterion was employed to predict the failure response of these cylinders based on data obtained from coupon specimens. The finite‐difference solution provided correction factors to account for the localized bending. The prediction utilizing this methodology was excellent in all cases. A generalized methodology to assess damage tolerance of structural configurations with notches is proposed.     

Study of Stress Intensity Factor of a Cracked Steel Plate with a Single-Side CFRP Composite Patching

Composite fiber patching techniques have been considered as alternatives to traditional methods of strengthening and fatigue crack repair in steel structures. It is known that the fatigue strength of a cracked steel element depends on the stress intensity factor (SIF) at the crack tip which is a function of the stress/strain distribution of the plate. This paper presents an experimental study of tension tests of cracked steel plates repaired by single-side carbon fiber-reinforced polymer (CFRP) patching in investigating the strain distribution in the vicinity of the cracked region. The test parameters included patch length, patch width, tapered end, and axial stiffness ratio of adherend. It is shown from the test results that the single-side CFRP patching applied onto the cracked steel plate decreased the crack tip strains significantly in the patched face and increased the strains in the unpatched face. Finite-element analyses of the specimens using both the three layers model proposed by previous researchers and a modified three layers model proposed in this study were conducted to examine the strain distributions in the vicinity of the crack. In general, the strain distributions of the specimens were predicted well by the finite-element analyses using either model. The finite-element results showed that the SIF at the crack tip through the plate thickness was significantly reduced except on the unpatched side and the modified three layers model was able to capture the nonlinear SIF variation through the thickness of a cracked steel plate with single-side patching. Meanwhile, the three layers model overestimated the SIF on the patched side and underestimated the SIF on the unpatched side by about 10% on average compared to those of the modified three layers model. Based on the finite-element analysis results of the modified three layers model, it is shown that the width and the length of patching had only a marginal effect on the SIF. On the other hand, the effect of the number of layers of patching on the reduction of SIF was more pronounced.     

FEA of Complex Bridge System with FRP Composite Deck

Innovative fiber-reinforced polymer (FRP) composite highway bridge deck systems are gradually gaining acceptance in replacing damaged/deteriorated concrete and timber decks. FRP bridge decks can be designed to meet the American Association of State Highway and Transportation Officials (AASHTO) HS-25 load requirements. Because a rather complex sub- and superstructure system is used to support the FRP deck, it is important to include the entire system in analyzing the deck behavior and performance. In this paper, we will present a finite-element analysis (FEA) that is able to consider the structural complexity of the entire bridge system and the material complexity of an FRP sandwich deck. The FEA is constructed using a two-step analysis approach. The first step is to analyze the global behavior of the entire bridge under the AASHTO HS-25 loading. The next step is to analyze the local behavior of the FRP deck with appropriate load and boundary conditions determined from the first step. For the latter, a layered FEA module is proposed to compute the internal stresses and deformations of the FRP sandwich deck. This approach produces predictions that are in good agreement with experimental measurements.     

Asymptotic Approach for Thermoelastic Analysis of Laminated Composite Plates

A thermoelastic model for analyzing laminated composite plates under both mechanical and thermal loadings is constructed by the variational asymptotic method. The original three-dimensional nonlinear thermoelasticity problem is formulated based on a set of intrinsic variables defined on the reference plane and for arbitrary deformation of the normal line. Then the variational asymptotic method is used to rigorously split the three-dimensional problem into two problems: A nonlinear, two-dimensional, plate analysis over the reference plane to obtain the global deformation and a linear analysis through the thickness to provide the two-dimensional generalized constitutive law and the recovering relations to approximate the original three-dimensional results. The nonuniqueness of asymptotic theory correct up to a certain order is used to cast the obtained asymptotically correct second-order free energy into a Reissner–Mindlin type model to account for transverse shear deformation. The present theory is implemented into the computer program, variational asymptotic plate and shell analysis (VAPAS). Results from VAPAS for several cases have been compared with the exact thermoelasticity solutions, classical lamination theory, and first-order shear-deformation theory to demonstrate the accuracy and power of the proposed theory.     

Elastoplastic Mesoscale Homogenization of Composite Materials

Mesoscale homogenization provides a computationally efficient way of capturing some degree of local variation in the behavior of a composite microstructure. In this work, techniques are explored in which the local two-phase microstructure is homogenized using the moving-window generalized method of cells (GMC) technique. Both elastic and plastic material behavior is investigated using GMC-generated anisotropic stress-strain curves. An optimization procedure is used to define Hill’s yield criterion parameters which best fit the GMC-generated data. Two perfectly plastic models are developed based on the GMC results; these are called the subcell initial yield model and the matrix average yield model. A technique is also developed which incorporates hardening behavior. Different windowing techniques are investigated: an overlapping windowing technique which requires more computational time, and a nonoverlapping technique which requires less computational time. It is found that the matrix average model using small nonoverlapping windows is the best technique in the cases studied, combining accuracy and computational efficiency.     

Low-Velocity Impact Dynamic Behavior of Laminated Composite Nonprismatic Folded Plate Structures

In this paper, the higher-order shear deformation theory is used to study the response of graphite/epoxy laminated composite nonprismatic folded plates subjected to impact loads. A finite-element model of the theory is also developed. The modified Hertzian contact law incorporated within the Newton–Raphson method is used to calculate the contact force between the impactor and the laminated plate. For time integration, the Newmark direct integration was adopted. Numerical results are presented to demonstrate the effects of span-to-thickness ratio, fiber angle, stacking sequence, and crank angle on the response of laminated plate subjected to impact. It is demonstrated that the results obtained from the present investigation compare well with those reported in the open literature.     

SMA Actuated Mechanism for an Adaptive Wing

A preliminary study of an adaptive unmanned aerial vehicle (UAV) wing actuated by shape memory alloy (SMA) devices is presented. The wing consists of a sandwich box substructure, flexible ribs, and a flexible laminated skin. The adaptation capability to the changing flight conditions is obtained via airfoil shape adjustments. Torsion SMA tubes are employed for wing camber control, while levers powered by SMA wires are employed for local shape control. A new architecture is proposed: the downward or upward actuation torque is provided by counterrotating concentric tubes connected through a clutch and a positioning piezoelectric motor to the flexible ribs. These actuator tubes are heated one at a time while the other is made free by the clutch in order to obtain any wanted shape without waiting for cooling. The capability of the wing to bear the aerodynamic loads, the power required by the actuators, and their force and torque are assessed by finite-element simulations. An improved version of a recently developed element is employed that accurately and efficiently captures stresses and deformations in the composite structure. The wing requires a peak power of 1,223 W that is compatible with the UAV considered here, i.e., with a maximum take-off weight of 1,000 kg and jet engine. It can smoothly deform with a camber mean rotation of 22° and rotation at the tip of 40° with a load factor of 5, a differential camber rotation of 10°, and a profile variation from 40 to 55% of the chord (4.5% increase and 3.9% decrease of thickness) at cruise speed.     

Feasibility Assessment of Deployable Composite Telescope

Volume constraints in existing launch vehicles require large space-based sensors to be folded during launch and subsequently deployed in space. This paper outlines the development of a prototype deployable astronomical telescope that would maintain both structural stability and optical alignment for potential space-based deployment. To achieve this goal, the structure must possess adequate stiffness and maintain its positional accuracy after a deployment cycle has ensued. The development and testing were based on a consumer-astronomy Newtonian telescope. A foldable carbon-epoxy composite replacement structure was integrated to replace the aluminum-truss assembly provided by the manufacturer. The composite telescope’s structure and optical output were evaluated using computational (finite-element analyses and closed-form equations) and experimental methods. The structure was subjected to postdeployment displacement tests to quantify alignment accuracy. The stresses introduced in folding the tape springs were evaluated for both magnitude and mode of failure using the Tsai-Wu failure criterion.     

Numerical Modeling of Coupled Hygromechanical Degradation of Cementitious Materials

A coupled hygromechanical model for finite-element analyses of structures made of cementitious materials such as concrete or plaster is formulated within the framework of thermomechanics of partially saturated porous media. A multisurface elastoplastic-damage model, formulated in the space of plastic effective stresses, is employed to describe the nonlinear pre- and postfailure material behavior of concrete, taking the degradation of stiffness as well as the growth of inelastic strains as a consequence of the opening of microcracks into account. From relating stress and strain quantities defined on the mesolevel to respective homogenized quantities on the macrolevel, the hygromechanical coupling coefficients are identified. The effect of cracking on the isothermal liquid permeability is also accounted for. As a representative example, a two-dimensional simulation of a base restrained concrete wall subjected to both uniform drying and to rewetting at the foundation is described in the paper.     

Three-Dimensional Micromechanical Finite-Element Network Model for Elastic Damage Behavior of Idealized Stone-Based Composite Materials

This paper presents a three-dimensional (3D) micromechanical finite-element (FE) network model for predicting elastic damage behavior of the idealized stone-based materials. Stone-based composite materials have multiphase structures: an aggregate (or stone) skeleton, a binding medium, fillers, and air voids. Numerical simulation of the micromechanical behavior of the idealized stone-based materials was accomplished by using a microframe element network model that incorporated the mechanical load transfer between adjacent particles. The elastic stiffness matrix of this special element was obtained from an approximate elastic stress-strain analysis of straight cement between particle pairs. A damage-coupled microframe element was then formulated with bilinear damage laws, including elastic and softening behavior based on the equivalent fracture release energy. Indirect tension and compression simulations were conducted with developed FE models on the idealized digital samples of the stone-based materials. These simulations predicted the internal microdamage distribution and global fracture behavior of these samples, which qualitatively agree with the laboratory observations. The results indicate that the developed FE models have the capability to predict the typical loading-related damage behavior observed from the stone-based materials.     

Transverse Shear Effect on Flutter of Composite Panels

The effect of transverse shear deformation on the supersonic flutter of composite panels has been investigated using the finite element method. First‐order shear‐deformation laminated‐plate theory and quasi‐steady aerodynamic theory are employed for the analysis. The total displacement of the plate is expressed as the sum of the displacement due to bending and the displacement due to shear deformation. Thus, the aerodynamic pressure induced by the plate motion is also the sum of the pressure induced by bending deformation and the pressure induced by shear deformation. Numerical results show that the transverse shear deformation may have a significant effect on the flutter boundary if aerodynamic damping were small or neglected in the determination of flutter boundary.     

Nonlinear Transient Response of Laminated Composite Shells

This paper first compares the writers’ results of static and dynamic analyses of plates, cylindrical and spherical shells employing four-, eight-, and nine-noded elements with different integration rules with those of earlier investigators and including some of the recent composite theories. Thereafter, the nonlinear transient responses of laminated composite cylindrical and spherical shell panels with cutouts are investigated taking up additional examples that are yet to appear in the published literature. For these, the finite-element model is employed using eight-noded C0 continuity, an isoparametric quadrilateral element considering von Karman large deflection assumptions. In the time integration, the Newmark average acceleration method is used in conjunction with a modified Newton–Raphson iteration scheme. Important conclusions with respect to nonlinear transient responses are summarized for cylindrical and spherical shells with and without cutouts.