Ozden O. Ochoa

A Parametric Study of Variables That Affect Fiber Microbuckling Initiation in Composite Laminates: Part 1-Analyses

The objectives of this research focus on the effects of nonlinear matrix constitutive behavior, initial fiber waviness, and fiber/matrix interfacial bond strength on fiber microbuckling initiation in thermoplastic composites. Nonlinear geometric and non linear material two-dimensional finite element analysis is used to model the initiation of fiber microbuckling of an initially wavy fiber. Results show that reductions in the resin shear tangent modulus, larger amplitudes of initial fiber wavinesses, and debonds each cause increases in the localized matrix shear strains; these increases lead to premature fiber microbuckling initiation. These numerical results are compared with experimental data obtained during this investigation. These experimental results and comparisons are presented in a companion paper [1] .

Analysis of progressive failure in composites

Abstract An incremental failure accumulation technique is presented for angle-ply, cross-ply and uniaxial composite laminates. Piecewise smooth failure criteria that distinguish between the failure modes are implemented in a finite element analysis with shear deformable elements. For each load increment, these criteria are checked within each lamina. As failure is detected, the material properties of the lamina are modified. Equilibrium iterations are performed before incrementing the load to track further damage in the laminate and, finally, the ultimate failure of the laminate. This technique of progressive failure is illustrated for uniaxial tension and four-point bending problems.

Transverse and longitudinal CTE measurements of carbon fibers and their impact on interfacial residual stresses in composites

In situ transmission electron microscopy (TEM) is utilized to evaluate the coefficients of thermal expansion (CTE) of two polyacrylonitrile (PAN) based (T1000 and IM7) and one pitch-based (P55) carbon fiber in the temperature range 20–1100 C. The transverse morphology of the fibers is studied using field emission scanning electron microscope and selected area diffraction (SAD) patterns in a TEM and is co-related to the fiber transverse CTE. The PAN-based IM7 and T1000 fibers revealed a transversely isotropic structure, which was consistent with their transverse CTE measurements. Even though the pitch-based P55 fiber revealed cylindrical orthotropy, it did not translate to orthotropy in the transverse CTE. Finite element models of unidirectional IM7/977 composite, utilizing the present IM7 CTE values, are compared to experimental lamina scale CTE results where good agreement was obtained for cool-down regime. The models also indicate that the Tresca stresses in the epoxy matrix at the fiber–matrix interface exceed the allowable strength at cryogenic regimes suggesting possible fiber–matrix debonding and transverse matrix cracking.

FINITE ELEMENT FORMULATION INCLUDING INTERLAMINAR STRESS CALCULATIONS

Abstract A quadrilateral plate element is developed on the basis of utilizing the compatibility equations to obtain the in-plane stresses, and the equilibrium equations to obtain both transverse shear and normal stresses. A plate as opposed to shell or solid formulation serves to provide efficient solutions for thin to moderately thick laminated composite configurations. The element formulation involves relaxation of the Kirchhoff hypothesis via superposition of a shear rotation upon a midplane rotation. The displacement field is carefully selected to obtain the desired transverse stress variation. Results are compared to both closed form and numerical solutions.

Similitude study for a laminated cylindrical tube under tensile, torsion, bending, internal and external pressure. Part I : governing equations

Abstract A general analytical model is developed for the stresses and displacements of an assembly of several coaxial laminated hollow circular cylinders made of orthotropic layers, and subjected to internal and external pressure, tensile, torsion and bending loads. Slip and friction conditions at the interfaces are not considered in lieu of perfect bonding. The model results are compared to the experimental tensile test of a composite tube. Displacements and stresses are evaluated for different angle-ply layers and radius-tothickness ratios.

Similitude study for a laminated cylindrical tube under tension, torsion, bending, internal and external pressure Part II: scale models

The aim of this study is to demonstrate the validity of scale model development testing and predict the laminated cylindrical tube behavior under tensile, torsion, bending, internal and external pressure load. Similitude theory is used to develop the necessary similarity conditions. In the amplitude approach, the coefficients of the governing differential equation for the prototype and the model of the prototype are compared to develop scaling laws. For composites, these scaling laws depend not just on geometry, but also on constituent properties and the laminate stacking sequences. The model tubes were loaded under scaled test conditions until catastrophic failure. Data acquired included load, strain measurements and non destructive evaluation of damage mechanisms.

Modeling progressive damage in composites: a shear deformable element for ABAQUS®

The motivation for the present work is to enable a designer to model composite structures with plate/shell elements that capture a three-dimensional state of stress for damage progression. This is accomplished by incorporating a shear deformable composite element with built-in progressive damage capability into a commercial finite element program, ABAQUS®, as a user element. The user elements ability to capture a three-dimensional state of stress with damage progression is displayed in two example problems of composite plates.

Carbon Foam Core Composite Sandwich Beams: Flexure Response

The possibility of utilizing an open-cell carbon foam in structural applications is assessed through integrated testing and computational analysis. The structural elements considered are sandwich beams with carbon-epoxy laminate face sheets and carbon foam core subjected to bending loads. The primary damage mode observed is the formation of shear cracks in the carbon foam core at a measured axial strain of 2473 me on the face sheet. Subsequently, three-dimensional FEA models are developed to identify the damage modes and the progressive damage pattern for an isotropic carbon foam core. Results of the computational simulations – displacements, axial strains, and damage modes – correlated well with the test outcome.

Design and analysis of test coupons for composite blade repairs

Abstract Advancements in composite material technology have spurred an increase in the use of composites in modem rotorcraft structures. In addition to the desirable high strength, low-weight features of these material systems, the enhanced fatigue strength and damage tolerance has made composites particularly attractive for advanced structural applications. Even though these structures are often damaged during service and need repair, composites can be repaired consistently. In order to assure successful repair, it is crucial to be able to predict the strength, life and possible damage modes of a repaired component for reliability and design issues. This paper focuses on the first phase of a composite repair methodology development program where the goal is to develop appropriate test specimens to evaluate computationally and experimentally the effects of static and cyclic loads on a composite blade with a repair site. The first phase addresses the design of the test coupons representative of overlap and scarf precured-patch repairs and their analysis for static loading conditions. A global blade model is used to determine the strain distribution for a given set of loads. A local finite element model which represents the test coupon geometry, is then created with the appropriate boundary conditions. From this local model, overlap and scarf patch models are generated. These models are analyzed under uniaxial compressive and tensile loads. A user subroutine, based on the ultimate stress allowable is incorporated into the analysis to detect the initiation and progression of damage in the adhesive.

The effects of fastener hole defects

The influence of drilling-induced defects, such as delamination, on the fa tigue life of a pin bearing joint in a toughened carbon epoxy system was investigated. In a pin bearing joint, the parameters of interest are the bearing stress and the pin position or tolerance. Tension bearing by-pass tests were conducted on coupons under ambient and elevated temperature wet conditions. Specimens were tested in a bearing tension frame to static failure in order to measure the failure load and to calculate pin bearing stress. From static test results, a fatigue load was selected as 66% of the static pin bearing failure load. Coupons were then tested under pin bearing fatigue loading at ambient and elevated tem perature wet conditions. Two-dimensional finite element models were used to simulate the pin and tension loads applied to the coupons. These models simulated the stress distribu tion around the loaded pin. The loads in the finite element models represented the failure load incurred in the static pin bearing tests as well as the maximum load during a selected cycle of the pin bearing fatigue tests. Experimental results showed the pin movement to vary with respect to delamination diameter, specimen thickness, and configuration. Numerical results compared with the static failure levels and indicated the form of the delaminated regions. Results give an indication of the effects of environment and hole quality under pin bearing load. Large variations in drilling-induced damage tended to cause variations in the pin bearing response of like specimens.