Rapid numerical assessment of process-induced dimensional changes and residual stresses in large aerospace composite parts

Process-induced deformations (PIDs) are a major issue in fabricating composite aerostructures, hindering the assembly process. For example, geometrical mismatches between spars and wing skins can lead to considerable assembly delays during the assembly of a composite wing box. In these cases, shims with specific geometries, based on the mismatch of the original parts, are fabricated and used to meet the current tight tolerances in aerospace. Given such parts ‘complex and large geometry, process simulations to assess PIDs are often quite slow. This paper aims to evaluate the effect of geometry and design details on PIDs and residual stresses of composite spars efficiently and robustly. Typical cross-sectional geometries such as the Omega shape are considered. Furthermore, the impact of design details, including ply drop-off, on PIDs such as spring-in angles, warpage, and 3D stress distributions are evaluated along long spars. The structural modeling is handled using 1D higher-order layer-wise theories based on the Carrera Unified Formulation (CUF) to speed up the process simulation of large parts. Such theories are necessary to detect relevant mechanical behaviors: transverse stretching, shear deformation, anisotropy, and layerwise changes of the physical properties. On the other hand, using 1D theories significantly impacts the computational overhead as there are no aspect ratio constraints on the finite elements (FE), thus leading to much fewer degrees of freedom than 2D or 3D models. Although 1D, the model provides the complete 3D strain and stress fields as the primary unknowns – in this paper, pure displacements – are expanded using higher-order Lagrange polynomials to remove the typical assumptions of 1D modes, e.g., rigid cross-sections, null or constant shear distributions. The evolution of material properties, such as the evolution of the degree of cure, viscoelastic moduli, and free strains, are characterized using established DSC and DMA tests. Accordingly, a cure-hardening instantaneously linear elastic (CHILE) constitutive model is adopted for numerical simulations. The results are verified through analytical formulations and published data in the literature. The proposed simulation approach allows for rapid evaluation of residual stresses and PIDs. Due to its numerical efficiency, the effect of various design parameters can be quickly evaluated. Therefore, this tool can explore the design space for large and complexcomposite parts and potentially develop mitigation strategies.