Lawrence W. Rehfield

Composite box beam analysis - Theory and experiments

Beam theory is widely used as a first approximation in numerous structural applica tions. When applied to composite beams, the accuracy of beam theory becomes ques tionable because (1) the shearing and warping deformations become significant, as the shearing stiffness of composite laminates is often very low, and (2) several elastic couplings can occur that strongly influence the behavior of composite beams. The tor sional behavior of thin-walled composite beams has important implications for aeronautical structures and is deeply modified by the above non-classical effects. This paper presents two comprehensive analysis methodologies for composite beams and describes experimental results obtained from a thin-walled, rectangular cross-sectional beam. The theoretical predictions are found in good agreement with the observed twist and strain distributions. Out-of-plane torsional warping of the cross-section is found to be the key factor for an accurate modeling of the torsional behavior of such structures.

A theory for stress analysis of composite laminates

Failures in laminated resin matrix composite materials often begin with matrix microcracking and delamination. These modes of damage are three-dimensional in nature and are controlled by interlaminar stresses. One important key to understanding and ultimately predicting the failures in composite materials is an analytical approach that provides reliable stress estimates in critical regions. Conventional laminate theories are inadequate for this purpose as they are based on global displacement assumptions. Moreover, the interlaminar stresses are often neglected in the initial formulations. Therefore, solutions based upon these theories cannot yield realistic stress distributions. Recent theoretical research shows that there are certain nonclassical influences that affect bending-related behavior. They include section warping and its concomitant nonclassical surface-parallel stress contributions and transverse normal strain. The stress prediction capability of a bending theory improves significantly if these nonclassical influences are incorporated. A comprehensive bending theory is developed for arbitrary composite laminates. Its effectiveness is demonstrated in examples for a cross-ply laminate and a quasiisotropic laminate.

Sublaminate analysis of interlaminar fracture in composites. Part I. Analytical model

A simple analysis method based upon a transverse shear deformation theory and a sublaminate approach is utilized to analyze a mixed-mode edge delamination specimen. The analysis provides closed form expressions for the interlaminar shear stresses ahead of the crack, the total strain energy release rate, and the strain energy release rate components. The parameters controlling the behavior are identified. The effect of specimen stacking sequence and delamination interface on the strain energy release rate components is investigated. Results are compared with a finite element simulation for reference. The simple nature of the method makes it suitable for preliminary design analyses which require a large number of configurations to be evaluated quickly and economically. In Part I of this work the analytical model is developed. A comparison of the characteristic roots controlling the behavior in the edge delamination specimen is provided. In Part II an extensive comparison with a finite element solution for 58 test cases is provided in order to validate the analytical model and assess its accuracy.

A comprehensive theory for planar bending of composite laminates

Abstract Modern laminated resin matrix composite structures exhibit more pronounced transverse shear deformation than their conventional monolithic counterparts. This is because transverse shear is resin controlled and benefits little from fiber reinforcement. Recent theoretical research shows that there are additional nonclassical influences that also affect bending-related behavior of their composites. They include the consequences of section warping and its concomitant nonclassical axial stress contribution and transverse normal strain. A comprehensive theory for planar of laminates is developed and applied in this paper which accounts for these influences. It is applied to selected benchmark problems which illustrate the significance of these influences and permit comparison with previous theories.

On the buckling behavior of thin walled laminated composite open section beams

A new thin-walled laminated composite open section beam model and corresporitling h u c k l i ~ ~ g analysis ~nethotlology are presented wliich are created to accurately, but simply, characterize response. Simplicity is achieved 1, ). asswriing the form for the displacerncnt field that is col~sistent witli linear theory. .%rbitrary larniriatecl composite coirstructio~i is considered. T r a n s ~ v r s r shear deformation is accounted for as this effect is far more pronounced for con~posi te rriaterials than for conventional mc,tallic materials. i7alidation of predictions based upon the new nod el with tests and rxtcmsivc finite elerrlent si~nulations are given.

Interlaminar analysis of laminated composites using a sublaminate approach

A simple analysis method based upon a transverse shear deformation theory and a sublaminate approach is utilized to analyze a Mixed-Mode edge delamination specimen. The analysis provides a closed f orm distribution of the interlaminar shear stresses ahead of the crack, and the parameters controlling the behavior are identified. The effect o f specimen stacking sequence and delamination interface on the strain energy release rate components is investigated. Results are compared with a finite element simulation for reference. The simple nature of the method makes

Design Analysis and Testing for Mixed-Mode and Mode II Interlaminar Fracture of Composites

This paper presents the design analysis and results of an experimental program that was conducted to evaluate mixed-mode and Mode II interlaminar fracture behavior of a resin matrix composite material system. A double cracked-lap-shear specimen was designed utilizing a simple, new analysis method. The specimen was made of AS4/3502 graphite/epoxy material with a [′45, 0, 90] 6 s quasi-isotropic balanced symmetric layup. The lap interface studied was at ′45° orientations to the loading direction. A fundamental feature of the designed specimen is its ability to be tested under net tensile and compressive loadings. The specimen exhibits mixed-mode or Mode II behavior depending on the loading direction. This paper summarizes the tension testing only. The crack growth during testing was monitored by observing the isochromatic fringes in photoelastic coatings mounted on the two lap surfaces. Preliminary results indicate that the fracture behavior follows a resistance curve. In order to assess the double cracked-lap-shear specimen data and test, a comparison was made with results obtained with single cracked-lap-shear specimens. Less scatter in the data from the double cracked-lap-shear specimens was found.

Interlaminar Fracture Analysis of Composite Laminates Under Bending and Combined Bending and Extension

Interlaminar fracture or delamination is a primary damage mode in laminated composites. It is caused by high interlaminar stresses which are produced by local stress raisers such as holes, free edges, ply drops, and otherdefects and discontinuities which may be manufacturing related or service induced. Delaminations alter internal load paths and usually contribute to the ultimate failure of the structure. The present work is concerned with the development of a simple analytical model which permits the rapidly varying interlaminar stresses and energy release rate to be estimated by elementary means. Extensive numerical computations are avoided, and the results are obtained in closed form. The model is applied to the edge delamination specimen subjected to uniform bending and combined bending and extension loadings. Interlaminar stresses, total energy release rate, and energy release rate components are estimated.

Sublaminate analysis of interlaminar fracture in composites. Part II. Applications

A validation of the sublaminate analysis method developed earlier is provided in this work by comparison of the predictions with a quasi-three-dimensional finite element analysis. A comparison of the total strain energy release rate with a classical plate model is also provided. Closed-form expressions for the total strain energy rate and the strain energy release rate components are developed. The sublaminate analysis developed in this work is simple and the results are generated using a desktop computer.

A New Ply Model for Interlaminar Stress Analysis

An accurate estimate of interlaminar stresses is crucial to understanding, as well as predicting, many delamination-related failures in composite materials. A new model for ply-level sublaminate analysis is presented and applied. The homogeneous plate theory developed earlier by the authors (Valisetty and Rehfield, 1983) is further refined, and the equations are reduced appropriately for the classical finite-width free-edge laminate elasticity problem and a related delamination crack growth problem. It is applied to the laminate on a ply-by-ply basis. This theory incorporates all the essential physical effects and appears to be an adequate model for predicting the behavior of individual layers in equilibrium. On the basis of the number of equations and boundary conditions required for the implementation of layer equilibrium, this theory also appears to be the simplest of its kind presented so far. The stress induced in the free-edge region of a (0,90,90,0) laminate in uniform extension and the energy release rates for the delamination between the -30 deg and 90 deg plies of a (+, -30,+, -30, 90,90)s laminate are computed using the new analysis. The results are in excellent agreement with the existing numerical solutions. The new ply behavioral model appears to be very promising; it yields stresses and displacements that are statically and kinematically compatible at interlaminar surfaces.