CURVED COMPOSITE STRUCTURES AND COMPROMISE BETWEEN PROCESS-INDUCED - PDF document

18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS CURVED COMPOSITE STRUCTURES AND COMPROMISE BETWEEN PROCESS-INDUCED DEFORMATIONS AND STRUCTURAL PERFORMANCE H. Ghiasi 1* , M. Rahmat 2 , P. Hubert 3 , L. Lessard 3 1, 2 Department of Mechanical Engineering, McGill University, Montreal, Canada 1 Postdoctoral fellow, 2 PhD Candidate, 3 Associate Professor * Corresponding author (hossein.ghiasi@mail.mcgill.ca) Keywords : Polymer composite, Process-induced deformation, Curved structures, Performance 1 Introduction residual stresses by simultaneous consideration of structural and processing stresses during the stacking Advanced composite materials usually undergo a sequence design and by adjusting processing significant variation in temperature during the parameters, such as tool material and thickness, cure manufacturing process and are generally cured at a cycle and autoclave heat conduction rate. temperature different than their service temperature. Because of anisotropic material properties and the Both structural and processing parameters are mismatch between the thermal properties of the part studied in order to design a curved structure with and the mold, the temperature variations can create maximum dimensional fidelity and minimum significant residual stresses and undesired weight. Deformations and stresses created by deformations. Magnified by the effect of resin different mechanisms such as mechanical loads, volumetric chemical shrinkage during the cure thermal loads, chemical shrinkage during cure and process, the process-induced deformations can tool-part interactions are taken into account. It is produce major changes in geometry and structural shown that with a proper tailoring of the stacking performance of a curved structure. sequence and processing parameters, undesired distortions can be reduced significantly without a Experimental, analytical and numerical studies are considerable deterioration in structural functionality available on modeling of the process-induced of the part. deformations, demonstrating the contribution of several parameters including cure process [1, 2, 3], tool material [4, 5, 6], tool and part geometries [1, 3] 2 Structural Design and Processing Parameters and laminate stacking sequence [5]. To achieve a A common design approach, usually practiced in high level of dimensional fidelity required for design of composite and non-composite structures, aerospace applications, a design procedure is includes several iterations between the structural required to minimize the undesired process-induced design team and the manufacturing team. The deformations and stresses by controlling these moderately weak interconnection between the two parameters. fields in metallic materials results in an acceptably efficient design process that converges to a Thus far, minimizing the process-induced reasonably good design within a small number of re- deformations is realized mainly through mold design iterations; however, in composite structures because and deformation compensation [5]. However, this of anisotropic material properties and significant method can compensate for process-induced effect of processing parameters on internal material deformations; it is not able to reduce process- properties, this link is stronger. In this case induced stresses. Residual process-induced stresses convergence may require larger number of iterations reduce structural performance of the final structure. and yet the final design may not be the optimum In this paper several test cases are studied, possible structure. Case-studies presented in this demonstrating that it is possible to reduce the paper demonstrate examples where the re-iterative undesired process-induced deformations and approach leads to a non-optimal solution because of

a compromise between process-induced deformation 2.1 Simulations and structural performance. The effect of the cure cycle, resin shrinkage and tool-part interaction on process-induced Two design approaches are practiced and compared. deformations are simulated using COMPRO [7], a In “Approach A” , shown in Figure 1, the lamination composite processing analysis and design software. sequence of a composite structure with a given COMPRO simulates the cure process considering geometry is designed such that it realizes a structure full processing parameters such as changes in with minimum weight while satisfying given material stiffness, temperature, viscosity, degree-of- strength and deflection requirements under certain cure, volumetric cure shrinkage, tool-part mechanical loads. The optimum structural design is interaction, etc. then analyzed for process-induced deformations. Processing parameters are adjusted in order to For structural analysis, including the stress and minimize undesired deformations. The remaining strain field due to mechanical, thermal and chemical process-induced stresses are tolerated by shrinkage a semi-analytical code or a commercial strengthening (i.e. adding layers to) the current finite element package is used depending on the laminate until the final design satisfies strength and geometry of the part, which is discussed at the deformation requirements under mechanical and corresponding section. process-induced stresses. 2.2 Optimization In “Approach B” structural analysis and process simulation are used in a single optimization process. Approach A requires separate optimization of Thus the iterative part in the flowchart shown in stacking sequence and processing parameters. For Fig.1 is excluded. This approach considers process- design of the stacking sequence, depending on the induced deformations at the stage of laminate number and continuity of the design variables, a stacking sequence design. Approach B has this constrained Nelder-Mead simplex optimization advantage of using process-induced stresses to technique [8] or a Branch-and-bound method [9] is compensate for some stresses due to mechanical used. For the processing parameters, where the loads and thus can lead to a lighter and stronger objective function is smoother and the choices for structure compared to Approach A . variables are limited, a design of experiments method, i.e. Tagouchi method [10], is used. The Find stacking sequence/geometry under mechanical load results from this method are employed to find the min weight; s.t.{max deformation ≤δ & safety factor ≥α } best combination of processing parameters. Design mold material & thickness and cure cycle 3 Case-Studies and Discussion For minimum process-induced deformations 3.1 Z-shaped bracket Calculate deflection and safety factor under As a first case-study, a Z-shaped composite bracket process-induced and mechanical loads shown in Fig.2 is studied. The design problem consists of finding three geometrical variables, fiber Add a layer to the laminate if: max deformation ≥δ or safety factor ≤ α orientations for a 20-ply composite laminate and four processing parameters as described in Fig.3. The bracket is made of carbon/epoxy, Hexcel Design mold geometry to compensate for remaining AS4/8552, whose properties and cure kinetics model process-induced deformations are shown in Table 1. The bracket is designed for Fig.1. A common design approach includes minimum vertical deflation under the specified load re-iteration between structural and process design, and the minimum spring-in (change in angles after while an integrated approach exclude the iterative demolding) due to the thermal loads and chemical loop in this flowchart by considering both shrinkage during cure. mechanical and processing stresses in the first block