Robert Haynes

Families of Hygrothermally Stable Asymmetric Laminated Composites

Necessary and sufficient material-independent conditions are derived for hygrothermal stability of a laminated composite plate with plies made of the same specially orthotropic material based on classical lamination theory. The minimum number of plies required to obtain an asymmetric hygrothermally stable stacking sequence is proven to be five, and families of stable laminates are identified for six, seven, and eight plies. A sensitivity analysis demonstrates the robustness of hygrothermally stable solutions to small errors in ply orientation. A finite element analysis of geometric and ply orientation imperfections was performed to verify the robustness of the classical lamination theory results. Hygrothermally stable asymmetric laminate specimens were fabricated to confirm the analytical predictions.

New families of hygrothermally stable composite laminates with optimal extension-twist coupling

The necessary and sufficient conditions for hygrothermal stability of laminated composites are presented. A thorough survey of hygrothermally stable families is performed to identify stacking sequences that produce maximum extension-twist coupling. This is achieved through a constrained optimization routine, where the objective function is derived from the constitutive law given in classical lamination theory and the constraints are the necessary conditions for hygrothermal stability. A representative sample of these optimized laminates are constructed and tested to demonstrate significant improvement in coupling over the previously known optima. Comparisons are made with a nonlinear model and finite element analysis. An investigation of the robustness of the optimal stacking sequences to errors in ply angle typical of hand-layup manufacturing is presented.

Hygrothermally Stable Extension-Twist Coupled Laminates With Bending-Twist Coupling

The necessary and sufficient conditions for hygrothermal curvature stability of laminated composite plates have been derived in a prior publication and shown to be material independent. From within these conditions, various couplings are being investigated to determine any improvements over previously known optima. Extension-twist and bending-twist coupling have been investigated in previous work, and significant improvements over the previous state-of-the-art are demonstrated. In this work, the combined effect of extension-twist and bending-twist couplings is investigated to determine the level of twist achievable with a single laminate. Hygrothermal stability is taken to be a constraint. Results for laminates consisting of five through ten plies are presented. A Monte Carlo simulation investigates the robustness of the six-ply laminate to errors in ply angle stacking sequence.Copyright

Analytical Investigation of the Toughening Potential of a Failure Tailoring Concept

A parametric study has been performed to determine the upper bounds of toughening benefit achievable using an approximate model for the response of one-dimensional, tailored, flexible composite-material tethers with progressive failure. The tailoring concept is achieved through judicious arrangement of redundant load paths with unequal length and strength and has been proposed, modeled, and experimentally verified in previous research. The model is recast in this work using nondimensional material-independent parameters that are varied over ranges of interest in advanced composites. The effects of each parameter on the response of the tailored member are discussed. An increase of nearly two orders of magnitude in toughness over an untailored member is found to be attainable by taking complete advantage of the energy dissipated by the failure of each load path within the bounds of the study. Challenges to implementation of the tailored member as approximated by the model are also discussed.

Design of Optimal Hygrothermally Stable Laminates with Bending-Twist Coupling by Ant Colony Optimization

4The necessary and sufficient conditions for hygrothermal curvature stability of laminated composite plates have been derived in a prior publication and shown to be material independent. From within these conditions, various couplings are being investigated to determine any improvements over previously known optima. Bending-twist coupling has been investigated in previous work based upon a Sequential Quadratic Programming method to demonstrate the achievable level of coupling under the hygrothermal stability conditions. In this work, the Ant Colony Optimization is adopted as an alternative approach to reach global optima. Comparisons of resulting stacking sequences and associated level of coupling are provided. Results for laminates consisting of two through ten plies made of graphite/epoxy material are presented. The results suggest that the solution to the bending-twist coupling problem consist of multiple global optima. ACO was able to provide new global optimal solutions for laminates consisting of 8-10 plies.

Analysis of the extension-twist coupling in hygrothermally stable star beam composites

An approximate analytical model is utilized to examine the extension-twist coupling and torsional stiffness properties of star beams constructed with a new family of hygrothermally stable optimized stacking sequences. The relationship between axial force and the twist rate is used to quantify the results. The results show that there is a decrease in torsional rigidity corresponding to an increase in extension-twist coupling. The new stacking sequences allow for the addition of more substrips while maintaining comparable levels of extension-twist coupling with beams constructed from prior benchmark layups. This results in higher levels of torsional stiffness for comparable levels of extension-twist coupling in star beams made of the new optimized stacking sequences.© 2011 ASME

The Challenge of Achieving Hygrothermal Stability in Aerospace Composite Structures with Couplings

Composite laminate designs typically have incorporated symmetric, quasi-isotropic layups to ensure that stiffness, strength and hygrothermal stability requirements are met. These designs have often produced overly conservative structures or ones that do not take advantage of the benefits of coupling. This work presents a new approach to achieving hygrothermal stability while incorporating various couplings. Families with improved extension-twist and bending-twist coupling are introduced. Comparisons are made with the current state-of-the-art to show improvement in the achievable level of couplings.