Mark E. Tuttle

Optimal design of large composite panels with varying loads

Abstract This paper presents an optimization formulation for the design of large composite panels when loads vary over the panel. A methodology termed “blending” is introduced and used to ensure that a panel is manufacturable. Two ways of specifying the blending rules in optimal design formulation are set forth and compared. A global optimization algorithm, Improving Hit-and-Run (IHR), is used to find optimal designs. A composite panel is designed with and without using blending rules to demonstrate their effectiveness. The resulting designs show that blending rules are a great assistance in designing large composite panels that are tailored for varying loads in a practical manner.

The Nonlinear Viscoelastic-Viscoplastic Behavior of IM7/5260 Composites Subjected to Cyclic Loading

A constitutive model consisting of a combination of the Schapery nonlinear viscoelastic heredity integral and a nonlinear viscoplastic functional employed by Zapas and Crissman is described. Material constants associated with the constitutive models are measured for graphite-bismaleimide (IM7/5260) composites at elevated temperatures and stress levels. These results are then combined with classical lamination theory, so as to predict the response of a multi-angle laminate to cyclic thermomechanical loadings. Predictions are favorably compared with measurements obtained during a 50-hr test involving ten 5-hr loading cycles.

Controlling thermal stresses in composites by means of fiber prestress

A new method of applying a prestress to the fibers in composite laminates prior to and during cure is described, and several [0 P /90] T laminates produced at varying prestress levels are discussed. Experimental measurements described in this paper confirm that fiber prestressing can reduce or eliminate residual thermal stresses. This is evidenced by a reduction in the thermal warpage of unsymmetric [0 P /90] T laminates. An existing thermal analysis of unsymmetric laminates (developed by Hyer) is modified slightly in this paper to account for fiber prestressing, and is then used to predict out-of-plane deformations of the [0 P /90] T panels. Measurements compare well with theory. Specimens machined from the prestressed panels were also subjected to tensile loadings and inspected for matrix cracks in the 90° ply. It was found that fiber prestressing greatly reduced matrix cracking in [0 P /90] T specimens, but had little effect on [0/90 P ] T specimens.

Designing laminated composites using random search techniques

A computer program called UWCODA is presented. UWCODA is intended to assist in the design, analysis and optimization of composite plates. UWCODA combines a state-of-the-art global optimization algorithm (Improving Hit and Run) with classical lamination theory. Optimization results are presented for simple loading conditions as well as for complex, biaxial load conditions. The computer code proved to be very effective in the design of composite plates.

A Mechanical/Thermal Analysis of Prestressed Composite Laminates

The effects of fiber preloads on the ply stresses induced within continuous-fiber compos ite laminates are considered. A micromechanics analysis based primarily upon the rule-of- mixtures is used to predict the fiber and matrix stresses within a single prestressed ply. These results are integrated with classical lamination theory, allowing calculation of the ply stresses for prestressed composite laminates. The analysis indicates that prestressing will dramatically influence the ply stresses induced within composite laminates.

Time-dependent response of IM7/5260 composites subjected to cyclic thermo-mechanical loading

Abstract In this study the response of IM7/5260 composites subjected to cyclic thermo-mechanical loading for loading times up to six months has been measured. In a previous paper it was shown that Schaperys non-linear theory when integrated with a functional viscoplastic model, allows predictions of the viscoelastic/ viscoplastic response for cyclic loading under isothermal conditions. In the present paper, the ability of these models to make long-term predictions for cyclic thermo-mechanical loading conditions is considered. Reasonable agreement between measured strains and predictions (to within 7%) is obtained for all laminates.

Optimal design of a composite structure

This paper presents a design methodology for a laminated composite stiffened panel, subjected to multiple in-plane loads and bending moments. Design variables include the skin and stiffener ply orientation angles and stiffener geometry variables. Optimum designs are sought which minimize structural weight and satisfy mechanical performance requirements. Two types of mechanical performance requirements are placed on the panel, maximum strain and minimum strength. Minimum weight designs are presented which document that the choice of mechanical performance requirements cause changes in the optimum design. The effects of lay-up constraints which limit the ply angles to user specified values, such as symmetric or quasi-isotropic laminates, are also investigated.

Optimization of composite I-sections using fiber angles as design variables

Abstract A problem formulation and solution methodology for design optimization of laminated composite I-sections is presented. Objective functions and constraints are given in the form of beam stiffnesses. Two different objective functions are considered, maximum beam bending stiffness and maximum beam axial stiffness. Fiber directions in the beam walls are treated as design variables. It is assumed that the beam is constructed using unidirectional tape, and manufacturing issues associated with the use of unidirectional tape are discussed and included as constraints in the problem formulation and solution. The paper demonstrates that the design optimization of composite thin-walled beams is a complex global optimization problem that cannot be solved by means of traditional convex programming. Therefore, the solutions described are found using a global search algorithm, Improving Hit-and-Run, which allows the design variables to be either continuous or discrete with a user-specified discretization interval. Numerical results for two material systems and nine different design families for manufacturing composite I-sections are presented.

Effect of physical aging and variable stress history on the strain response of polymeric composites

Abstract A constitutive equation capable of modeling the effect of physical aging on the non-linear creep response for frequently changing stress levels is presented. Schaperys non-linear theory is used to model the non-linear viscoelastic behavior and the effect of physical aging is incorporated through the use of Struiks effective time theory. The constitutive model is then combined with classical laminate theory to predict the response of multi-angle laminates subjected to cyclic loading. Predictions on the basis of this constitutive model are compared with data from the literature. These comparisons confirm that preditions without including the effect of physical aging grossly overpredict the measured creep strains.

INCORPORATING MANUFACTURING TOLERANCES IN NEAR-OPTIMAL DESIGN OF COMPOSITE STRUCTURES

When a composite structure is manufactured the design variables in the produced structure may vary from the intended design. These variations are called manufacturing tolerances. When optimization is used, the optimal design is often on an active constraint. This may cause the produced structure to fail, even though the variations from the intended design are within tolerances. In this paper a methodology is introduced for finding a near-optimal design that remains feasible for specified manufacturing tolerances. A random search global optimization algorithm, Improving Hit-and-Run, is used to check if a design with specified manufacturing tolerances remains feasible and an algorithm is presented to determine the size of the tolerances for a feasible design. Finally, by changing the right hand side of the constraints and reoptimizing, it is possible to explore near-optimal designs with different tolerances. The methodology is applied to a composites structural design problem.