Levon Minnetyan

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.

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.

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.

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.

Structural Durability of a Composite Pressure Vessel

The effect of local ply fiber fracture on the load carrying capability and structural behavior of a composite cylindrical shell under internal pressure is investigated. An integrated computer code is utilized for the simulation of composite structural degradation under loading. Damage initiation, growth, accumulation, and propagation to structural fracture are included in the simulation. Results demonstrate the significance of local defects on the structural durability of pressurized composite cylindrical shells.

Progression of damage and fracture in composites under dynamic loading

A new computational simulation method is presented to evaluate the dynamic aspects of composite structural response and durability that have not been simulated previously. Composite structural behavior under any loading condition, geometry, composite system, laminate configuration, and boundary conditions can now be simulated. Structural degradation, delamination, fracture, and damage propagation are included in the simulation. An angle-plied composite plate structure under normal impact loading is used as an example to demonstrate the versatility of the simulation method.