Julio F. Davalos

Modeling and characterization of fiber-reinforced plastic honeycomb sandwich panels for highway bridge applications

Abstract Fiber-reinforced plastic (FRP) composite decks have been increasingly used in highway bridge applications, both in new construction and rehabilitation and replacement of existing bridge decks. Recent applications have demonstrated that FRP honeycomb panels can be effectively and economically used for highway bridge deck systems. This paper is concerned with design modeling and experimental characterization of a FRP honeycomb panel with sinusoidal core geometry in the plane and extending vertically between face laminates. The analyses of the honeycomb structure and components include: (1) constituent materials and ply properties, (2) face laminates and core wall engineering properties, (3) equivalent core material properties, and (4) apparent stiffness properties for the honeycomb panel and its equivalent orthotropic material properties. A homogenization process is used to obtain the equivalent core material properties for the honeycomb geometry with sinusoidal waves. To verify the accuracy of the analytical solution, several honeycomb sandwich beams with sinusoidal core waves either in the longitudinal or transverse directions are tested in bending. Also, a deck panel is tested under both symmetric and asymmetric patch loading. Finite element (FE) models of the test samples using layered shell elements are further used to correlate results with analytical predictions and experimental values. A brief summary is given of the present and future use of the FRP honeycomb panel for bridge decks. The present simplified analysis procedure can be used in design applications and optimization of efficient honeycomb structures.

Analysis and design of pultruded FRP shapes under bending

Abstract A comprehensive approach for the analysis and design of pultruded FRP beams in bending is presented. It is shown that the material architecture of pultruded FRP shapes can be efficiently modeled as a layered system. Based on the information provided by the material producers, a detailed procedure is presented for the computation of fiber volume fraction (V f ) of the constituents, including fiber bundles or rovings, continuous strand mats, and cross-ply and angle-ply fabrics. Using the computed V f s, the ply stiffnesses are evaluated from selected micromechanics models. The wall or panel laminate engineering constants can be computed from the ply stiffnesses and macromechanics, and it is shown that the predictions correlate well with coupon test results. The bending response of various H and box sections is studied experimentally and analytically. The mechanics of laminated beams (MLB) model used in this study can accurately predict displacements and strains, and it can be used in engineering design and manufacturing optimization of cross-sectional shapes and lay-up configurations. The experimental results agree closely with the MLB predictions and finite element verifications.

On the Mechanics of Thin-Walled Laminated Composite Beams

A formal engineering approach of the mechanics of thin-walled laminated beams based on kinematic assumptions consistent with Timoshenko beam theory is pre sented. Thin-walled composite beams with open or closed cross section subjected to bend ing and axial load are considered. A variational formulation is employed to obtain a com prehensive description of the structural response. Beam stiffness coefficients, which account for the cross section geometry and for the material anisotropy, are obtained. An explicit expression for the static shear correction factor of thin-walled composite beams is derived from energy equivalence. A numerical example involving a laminated I-beam is used to demonstrate the capability of the model for predicting displacements and ply stresses.

Static shear correction factor for laminated rectangular beams

Most practical analyses of laminated composite beams, particularly rectangular sections used in civil structures, are based on first order shear deformation theories (FSDT), which generally require shear correction factors to account for shear stiffness and transverse shear stress. Using an energy equivalence principle, a general expression for the shear correction factor of laminated rectangular beams with arbitrary lay-up configurations is derived in this paper. A convenient algebraic form of the solution is presented and validated with existing results for composite beams and plates. An example is given to illustrate the use of the present formulation, and a parametric study is conducted to evaluate the effect of number of layers, elastic moduli ratio, and fiber-angle orientation on the shear correction factor for various laminates.

A systematic analysis and design approach for single-span FRP deck/stringer bridges

There is a concern with worldwide deterioration of highway bridges, particularly reinforced concrete. The advantages of fiber reinforced plastic (FRP) composites over conventional materials motivate their use in highway bridges for rehabilitation and replacement of structures. In this paper, a systematic approach for analysis and design of all FRP deck/stringer bridges is presented. The analyses of structural components cover: (1) constituent materials and ply properties, (2) laminated panel engineering properties, (3) stringer stiffness properties, and (4) apparent stiffnesses for composite cellular decks and their equivalent orthotropic material properties. To verify the accuracy of orthotropic material properties, an actual deck is experimentally tested and analyzed by a finite element model. For design analysis of FRP deck/stringer bridge systems, an approximate series solution for orthotropic plates, including first-order shear deformation, is applied to develop simplified design equations, which account for load distribution factors under various loading cases. An FRP deck fabricated by bonding side-by-side box beams is transversely attached to FRP wide-flange beams and tested as a deck/stringer bridge system. The bridge systems are tested under static loads for various load conditions, and the experimental results are correlated with those by an approximate series solution and a finite element model. The present simplified design analysis procedures can be used to develop new efficient FRP sections and to design FRP highway bridge decks and deck/stringer systems, as shown by an illustrative design example.

Fiber-Reinforced Composite and Wood Bonded Interfaces: Part 1. Durability and Shear Strength

Fiber-reinforced plastic (FRP) composites have shown the potential for reinforcement of wood structures (e.g., bonding of FRP strips or fabrics to wood members). Although significant increases in stiffness and strength are achieved by this reinforcing technique, there is a concern about the reliable performance of the FRP-wood adhesive bond, which is susceptible to delamination, The overall objective of this two-part paper is to develop a qualification program to evaluate the service performance and fracture of composite/wood bonded interfaces. Two types of FRP-wood interfaces are studied: phenolic FRP-wood and epoxy FRP-wood bonds. In the present paper, Part 1, the durability and shear strength of FRP-wood bonds are evaluated by modified ASTM tests. First, the service performance and durability of FRP-wood interface bond is evaluated using a modified ASTM delamination test. Then, the apparent shear strengths of bonded interfaces under both dry and wet conditions are obtained from modified ASTM block-shear tests. It is shown that the modified ASTM D 2559 standard test can be successfully used to study the effect of several parameters (e.g., bonding pressure, assembling time, and coupling agents) on bondline performance under wet-dry exposure cycles. Then for the best combination of parameters, the average interface shear strengths can be obtained from block-shear tests of ASTM D 905, modified for hybrid laminates. Mode I fracture of FRP-wood bonded interfaces and guidelines for FRP-wood bond performance evaluation are presented in the companion Part 2 paper.

Flexural-torsional buckling of fiber-reinforced plastic composite cantilever I-beams

Abstract A combined analytical and experimental study of flexural–torsional buckling of pultruded fiber-reinforced plastic (FRP) composite cantilever I-beams is presented. An energy method based on nonlinear plate theory is developed for instability of FRP I-beam, and the formulation includes shear effect and bending–twisting coupling. Three different types of buckling mode shape functions of transcendental function, polynomial function, and half simply supported beam function, which all satisfy the cantilever beam boundary conditions, are used to obtain the eigenvalue solution, and their accuracy in the analysis are investigated in relation to finite element results. Four different geometries of FRP I-beams with cantilever beam configurations and with varying span lengths are experimentally tested under tip loads to evaluate their flexural–torsional buckling response. The loads are applied at the centroid of the tip cross-sections, and the critical buckling loads are obtained by gradually adding weight onto a loading platform. A good agreement among the proposed analytical solutions, experimental testing, and finite element method is obtained, and simplified explicit formulas for flexural–torsional buckling of cantilever beams with applied load at the centroid of the cross-section are developed. The effects of vertical load position through the cross-section, fiber orientation and fiber volume fraction on buckling behavior are also studied. The proposed analytical solutions can be used to predict the flexural–torsional buckling loads of FRP cantilever beams and to formulate simplified design equations.

Flexural-torsional buckling of pultruded fiber reinforced plastic composite I-beams: experimental and analytical evaluations

Abstract In this paper a comprehensive experimental and analytical approach is presented to study flexural-torsional buckling behavior of full-size pultruded fiber-reinforced plastic (FRP) I-beams. Two full-size FRP I-beams with distinct material architectures are tested under midspan-concentrated loads to evaluate their flexural-torsional buckling responses. To monitor rotations of the cross-section and the onset of critical buckling loads, transverse bars are attached to the beam crosssection and are subsequently connected to LVDTs; strain gages bonded at the edges of the top flange are also used. The analysis is based on energy principles, and the total potential energy equations for the instability of FRP I-beams are derived using nonlinear elastic theory. The equilibrium equation in terms of the total potential energy is solved by the Rayleigh-Ritz method, and simplified engineering equations for predicting the critical flexural-torsional buckling loads are formulated. A good agreement is obtained between the experimental results, proposed analytical solutions and finite-element analyses. Through the combined experimental and analytical evaluations reported in this study, it is shown that the testing setup used can be efficiently implemented in the characterization of flexural-torsional buckling of FRP shapes and the proposed analytical design equations can be adopted to predict flexural-torsional buckling loads.

Analysis of tapered ENF specimen and characterization of bonded interface fracture under Mode-II loading

Abstract An engineering approach for evaluating the shear-mode (Mode-II) fracture toughness of wood–wood and wood-composite bonded interfaces is presented. A tapered beam on elastic foundation model is developed to analyze and design a linear tapered end-notched flexure (TENF) specimen for fracture tests of bonded interfaces. The elastic foundation model is verified numerically by finite element analysis and experimentally by compliance calibration tests, which demonstrate that the present model can accurately predict the compliance and compliance rate-change of the specimen, and with proper design, an approximate constant rate of compliance change with respect to crack length can be achieved. The proposed TENF specimen can be used for Mode-II fracture toughness evaluations with reasonable confidence in the linearity of compliance crack-length relationship. The fracture of wood–wood and wood-composite bonded interfaces under Mode-II loading is experimentally evaluated using the proposed TENF specimen, and the corresponding values of critical strain energy release rate are obtained. The modeling technique and testing method presented can be efficiently used for characterization of Mode-II fracture of bonded bimaterial interfaces.

Analysis of laminated beams with a layer-wise constant shear theory

Abstract Based on generalized laminate plate theory, the formulation of a one-dimensional beam finite element with layer-wise constant shear (BLCS) is presented. The linear layer-wise representation of in-plane displacements permit accurate computation of normal stresses and transverse shear stresses on each layer for laminated beams with dissimilar ply stiffnesses. The BLCS formulation is equivalent to a first-order shear deformation beam theory (Timoshenko beam theory) on each layer. For the accurate computation of interlaminar shear stresses, the layer-wise constant shear stresses obtained from constitutive relations are transformed into parabolic shear stress distributions in a post-processing operation described in detail. The accuracy of the BLCS element is demonstrated by solving several numerical examples reported in the literature. While retaining the simplicity of a laminated beam theory, the element predicts results as accurate as much more complex elasticity analyses, and it is suitable to model frame-type structures.