Pappu L. N. Murthy

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.

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.

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.

Progressive fracture of polymer matrix composite structures

Abstract An approach independent of stress intensity factors and fracture toughness parameters has been developed and is described for the computational simulation of progressive fracture of polymer matrix composite structures. The damage stages are quantified based on physics via composite mechanics while the degran of the structural behavior is quantified via the finite element method. The approach accounts for all types of composite behavior, structures, load conditions, and fracture processes starting from damage initiation, to unstable propagation and to global structural collapse. Results of structural fracture in composite beams, panels, plates, and shells are presented to demonstrate the effectiveness and versatility of this new approach. Parameters and guidelines are identified which can be used as criteria for structural fracture, inspection intervals and retirement for cause. Generalization to structures made of monolithic metallic materials are outlined and lesson learned in undertaking the development of new approaches, in general, are summarized.

Computational simulation of high temperature metal matrix composites cyclic behavior

The purpose of the present paper is to describe the computational tool CEMCAN which refers to CEramic Matrix Composite ANalyzer. This computer code is the result of an ongoing research activity in analytical modeling of ceramic matrix composites at NASA-Lewis Research Center. The computer code is based on micromechanics approach and a unique fiber substructuring concept. In this new concept the conventional unit cell (the smallest representative volume element of the composite) of micromechanics approach has been modified by substructuring the unit cell into several slices and developing the micromechanics based equations at the slice level. Also incorporated in the code are nonlinearities in material behavior due to temperature and progressive fracture/degradation of the interphase. The important features of the code and its effectiveness are described herein with select examples. Comparisons of CEMCAN predictions with limited experimental data are also provided.

Discontinuously Stiffened Composite Panel under Compressive Loading

The design of composite structures requires an evaluation of their safety and durability under service loads and possible overload conditions. This paper presents a computational tool that has been developed to examine the response of stiffened composite panels via the simulation of damage initiation, growth, accumulation, progression, and propagation to structural fracture or collapse. The structural durability of a composite panel with a discontinuous stiffener is investigated under compressive loading induced by the gradual displacement of an end support. Results indicate damage initiation and progression to have significant effects on structural behavior under loading. Utilization of an integrated computer code for structural durability assessment is demonstrated.

Structural durability of stiffened composite shells

The durability of a stiffened composite cylindrical shell panel is investigated under several loading conditions. An integrated computer code is utilized for the simulation of load induced structural degradation. Damage initiation, growth, and accumulation up to the stage of propagation to fracture are included in the computational simulation. Results indicate significant differences in the degradation paths for different loading cases. The effects of combined loading on structural durability and ultimate structural strength of a stiffened shell are assessed.

MICROMECHANICS FOR PARTICULATE-REINFORCED COMPOSITES

A set of micromechanics equations for the analysis of particulate-reinforced composites is developed using the mechanics of materials approach. Simplified equations are used to compute homogenized or equivalent thermal and mechanical properties of particulate-reinforced composites in terms of the properties of the constituent materials. The microstress equations are also presented here to decompose the applied stresses on the overall composite to the microstresses in the constituent materials. The properties of a “generic” particulate composite as well as those of a particle-reinforced metal matrix composite are predicted and compared with other theories as well as some experimental data. The micromechanics predictions ate in excellent agreement with the measured values.