In the case of the curved composite sections, the difference between the in-plane and the through-thickness thermal expansion coefficient as well as chemical shrinkage of the epoxy matrix leads to a reduction of the enclosed angle, commonly referred to as spring-in.
Numerical simulation of the autoclave curing process can predict this phenomenon and enable reduction of process development costs and increase product quality. Currently tool designers account for process induced deformations based on their experience and often approach the problem using trial-and-error. Although this can give good results for parts with relatively simple geometry, complex shaped composite parts require sophisticated models to capture the interactions between different geometrical features with the aim to predict the tooling geometry required to consistently produce structures of high-quality within tight dimensional tolerances. ANSYS COMPOSITE CURE SIMULATION (ACCS) was developed by LMAT in response to the growing market demand for a reliable simulation platform to support manufacturing and tooling engineers throughout the process design cycle. The platform has been comprehensively validated and subsequently used to compensate millions of pounds worth of rib, spar and wing-skin tools across the European aerospace and wind energy industries, where previously the only method of achieving parts of sufficiently high tolerance to meet strict aerospace assembly rules was to re-machine finished tooling after molding the first part.
Challenges in modelling polimerisation
During curing of thermosetting composites the epoxy resin undergoes cross-linking reactions that lead to an increase of material density and reduction in volume. A schematic representation of the curing of a thermoset is shown below.
It traces cure from chain formation and linear growth, through branching and finally to a cross-linked, infinite network. There are two main transitions that can be distinguished during the curing process of a thermosetting resin. The first one is gelation occurring at approximately 40% – 50 % degree of conversion and the second one is vitrification occurring when the glass transition temperature (Tg) crosses the instantaneous cure temperature. Gelation is an irreversible process and corresponds to the formation of a 3-D infinite network of polymer chains. Vitrification occurs once the glass transition temperature reaches the curing temperature and the resin transforms from the rubbery to the glassy, solid state. Both thermal and resin shrinkage induced stresses which develop throughout cure can subsequently cause distortion and premature cracking of the composite mouldings. This sophisticated behaviour was captured with the ACCS material curing model and is now made available to the end user within ANSYS WORKBENCH simulation workflow.
Simulation work flow
The ACCS is fully integrated into the ANSYS Workbench environment. The composite material data necessary for the cure simulation are stored in the engineering data module. Connectivity to Ansys Composite Pre-Post (ACP) as well as other ANSYS products enabled seamless exchange of simulation and process design data. The connection with the ANSYS DesignXplorer enables advanced design optimization of material and manufacturing process parameters. Seamless transfer of the lay-up information from the existing CATIA and FIBERSIM models is also available through the HDF5 format.
The ACCS chemical solver is embedded within transient thermal module and simulates development of polimerisation, glass transition temperature as well as internal heat generation related to exothermic cross-linking reactions. Subsequently thermal and cure data is passed into the structural data module where ACCS cure material model calculates development of residual stresses and process induced distortions. For relatively thin laminates (<5 mm thick) where a uniform temperature distribution can be assumed ACCS offers a quick three step simulation approach. This quick solution can be used in the early design stages for quick assessment of the process parameters. Once the simulation process is completed the distorted geometry of the finite element model can be used to generate compensated tooling geometry. A recent example of the use of ACCS was in the UK's Composite Innovation Cluster project called RITAA. The RITAA project draws together a partnership of material supplier Cytec Industrial Materials, equipment supplier Kraus Maffei, tooling supplier Formaplex, reinforcement supplier Formax and simulation experts LMAT under collaborative effort to implement and commission an operational facility and then to embark upon a process of characterisation, simulation and refinement of materials and process technologies. Within this project ACCS was used to analyse the severity of the exothermic reaction in the inlet of the HP-RTM tooling system and its impact on the quality of the composite part. Findings of these simulations will be used for further optimisation of the processing parameters as well as modification of the inlet manifolds to allow high volume part production with no compromise on the final product quality as shown in the figure below. [caption id="attachment_269" align="aligncenter" width="598"] High pressure RTM tooling courtesy of FORMAPLEX and RITAA project.[/caption]
Employing ACCS in composite manufacturing process design allows for manufacture of composite components with tight geometrical tolerance, reduces scrap and part cost, and shortens the entire product development cycle.