Christos C. Chamis

Mechanics of Composite Materials: Past, Present and Future

Composite mechanics disciplines are presented and described at their various levels of sophistication and attendant scales of application. Correlation with experimental data is used as the prime discriminator between alternative methods and level of sophistication. Major emphasis is placed on (1) where composite mechanics has been; (2) what it has accomplished; (3) where it is headed, based on present research activities; and (4) at the risk of being presumptuous, where it should be headed. The discussion is developed using selected, but typical, examples of each composite mechanics discipline identifying degree of success, with respect to correlation with experimental data, and problems remaining. The discussion is centered about fiber/resin composites drawn mainly from the authors research activities and experience spanning two decades at National Aeronautics and Space Administration (NASA) Lewis Research Center.

Prediction of composite laminate fracture: micromechanics and progressive fracture

An investigation is described on the prediction of first-ply failure and fracture in selected composite laminates. The laminates are made from glass fibers and graphite fibers in epoxy matrices. Failure envelopes are generated for combined loading of these laminates on the basis of first-ply failure and laminate fracture. The evaluation is performed by a micromechanics-based theory and progressive fracture.

Structural Behavior of Composites with Progressive Fracture

Structural characteristics such as natural frequencies and buckling loads with corresponding mode shapes were investigated during progressive fracture of mul tilayer, angle-plied polymer matrix composites. A computer program was used to generate the numerical results for overall mechanical response of damaged composites. Variations in structural characteristics as a function of the previously applied loading were studied. Results indicate that free-vibration and buckling stability properties were preserved throughout a significant proportion of the ultimate fracture load. For the cases studied, changes in structural behavior begin to occur after 70 percent of the ultimate fracture load had been applied. However, the individual nature of the structural change was rather varied depending on the laminate configuration, fiber orientation, and the boundary condi tions.

Composite structure global fracture toughness via computational simulation

Abstract A computational method for the simulation of damage and fracture propagation in laminated composites is presented. A quantitative evaluation of the global fracture toughness of composites is shown as a tool for monitoring the fracture stability of composites under sustained loading. Changes in overall structural properties such as natural frequencies and the fundamental buckling load are also computed with increasing load-induced damage. Structural degradation, delamination, fracture, and damage propagation are included in the simulation. An angle-plied composite plate structure subjected to inplane tensile loading is used as an example to demonstrate some of the features of the computational method.

Simplified Composite Micromechanics For Predicting Microstresses

A unified set of composite micromechanics equations is summarized and described. This unified set is for predicting the ply microstresses when the ply stresses are known. The set consists of equations of simple form for predicting three-dimensional stresses (six each) in the matrix, fiber, and interface. Several numerical examples are included to illus trate use and computational effectiveness of the equations in this unified set. Numberical results from these examples are discussed with respect to their significance on microcrack formation and, therefore, damage initiation in fiber composites.

Fiber Composite Sandwich Thermostructural Behavior - Computational Simulation

Several computational levels of progressive sophistication/simplification are described to computationally simulate composite sandwich hygral, thermal, and structural behavior. The several computational levels of sophistication include (1) three-dimensional detailed finite element modeling of the honeycomb, the adhesive, and the composite faces; (2) three-dimensional finite element modeling of the honeycomb assumed to be an equivalent continuous, homogeneous medium, the adhesive, and the composite faces; (3) laminate theory simulation where the honeycomb (metal or composite) is assumed to consist of plies with equivalent properties; and (4) derivations of approximate, simplified equations for thermal and mechanical properties by simulating the honeycomb as an equivalent homogeneous medium. The approximate equations are combined with composite hygrothermomechanical and laminate theories to provide a simple and effective computational procedure for simulating the thermomechanical/thermostructural behavior of fiber composite sandwich structures.

Progressive Fracture in Composites Subjected to Hygrothermal Environment

The influence of hygrothermal environmental conditions on the load carry ing ability and response of composite structures are investigated via computational simula tion. An integrated computer code is utilized for the simulation of composite structural degradation under loading. Damage initiation, damage growth, fracture progression, and global structural fracture are included in the simulation. Results demonstrate the signifi cance of hygrothermal effects on composite structural response, toughness, and durability.

Application of progressive fracture analysis for predicting failure envelopes and stress–strain behaviors of composite laminates: a comparison with experimental results

Abstract The theoretical predictions, published in Part A of the failure exercise, are compared with experimental results provided by the organizers of the exercise. Two computer codes, ICAN and CODSTRAN, developed at NASA (Glenn Research Center at Lewis Field), were applied to predict the damage initiation, damage growth and global structural fracture in a wide range of multidirectional laminates. CODSTRAN was employed to predict (I) seven biaxial failure envelopes of [0°] unidirectional and [0°/±45°/90°]s, [±30°/90°]s and [±55°]s multi-layered composite laminates and (II) seven stress–strain curves for [0°/±45°/90°]s, [±55°]s, [0°/90°]s and [±45°]s laminates under uniaxial and biaxial loadings. In general, CODSTRAN gave reasonable predictions for cases where final failure was dominated by fibre fracture. There was, however, large discrepancy between the predicted and measured failure strengths and strains for cases where failure was dominated by matrix failure. Some of the discrepancy is attributed to (a) the effect of residual matrix stiffness that is discounted in simulations and (b) sensitivity of the specimens to the presence of biaxial stress state in certain cases.

Free-Edge Delamination: Laminate Width and Loading Conditions Effects

The width and loading conditions effects on free-edge stress fields in composite laminates are investigated by using a three-dimensional finite element analysis. The analysis includes a special free-edge region refinement or superelement with progressive substructuring (mesh refinement) and finite thickness interply layers. The different loading conditions include in-plane and out-of-plane bending, combined axial tension and in-plane shear, twisting, uniform temperature, and uniform moisture. Results obtained indicate that axial tension causes the smallest magnitude of interlaminar free edge stress compared to other loading conditions; laminates with practical dimensions may not delaminate because of free edge stresses alone since the magnitude of these stresses are found to be quite insignificant.

Micromechanics for Ceramic Matrix Composites via Fiber Substructuring

A generic unit cell model which includes a unique fiber substructuring concept is proposed for the development of micromechanics equations for continuous fiber reinforced ceramic composites. The unit cell consists of three constituents: fiber, matrix and an interphase. In the present approach, the unit cell is further subdivided into several slices and the equations of micromechanics are derived for each slice. These are subsequently integrated to obtain ply level properties. A stand-alone computer code containing the micromechanics model as a module is currently being developed specifically for the analysis of ceramic matrix composites. Towards this development, equivalent ply property results for a SiC (silicon carbide fiber) /Ti-15-3 (titanium matrix) composite with a 0.5 fiber volume ratio are presented and compared with those obtained from customary micromechanics models to illustrate the concept. Also, comparisons with limited experimental data for the ceramic matrix composite, SiC/RBSN (Reaction Bonded Silicon Nitride) with a 0.3 fiber volume ratio are given to validate the concepts.