Carbon fibre composites components made using prepreg technology are generally of better quality than those produced by other techniques and are thus favoured by the aerospace sector. They are nevertheless still prone to the formation of fibre path defects such as wrinkles. These defects can be detrimental for the components’ integrity (e.g. through thickness strength reduction greater than 50% has been observed and, in general, the reduction of the tensile and compressive strength can be as great as 30% [
]) and are therefore of great concern. Wrinkles can either originate from defects already present in the as received reinforcement/prepreg or be inherent to the design and manufacture of the component (e.g. the component geometry, the stacking sequence, the cure and pressure cycle etc) [
]. It is the wrinkles arising from this second factor that are the most concerning as they are reproducible and are formed every time the component is manufactured. In the case where such a wrinkle is formed, the entire part has to be scraped or re-worked, adding considerable extra cost to the part. Indeed, 80% of the total cost of a composite component occurs during the part development. Having a numerical tool able to simulate the manufacturing process and able to flag up cases where the part geometry and manufacturing conditions will lead to the formation of wrinkles has the potential to save both time and money, allowing right-first-time design (which not only takes account of the part final mechanical properties but also the manufacturing constraints).
A number of authors have shown that advanced numerical techniques can accurately predict the knockdown effects associated with the presence of a single wrinkle embedded in a composite laminates under both static [
] and fatigue [
] loading providing that an accurate representation of the internal ply architecture is explicitly represented in the models. A recent study by Wilhelmsson et al. [
] showed that even when a number of wrinkles are present in the laminate and interact with each other, failure predictions fall with the 5% error margin compared to the experimentally measured data. Studies that followed on from [
], by Bender et al. [
], showed that all the geometrical characteristics of a wrinkle (e.g. angle, amplitude, wavelength, wash out, etc) have an influence on a component’s strength. This sets the bar for the accuracy of the process models relatively high as prepreg-based composites manufacturing involves a number of physical phenomena (i.e. consolidation, temperature change, chemical transformation etc) that interact with each other and that each need to be modelled accurately for good quantitative predictions of the wrinkle geometrical shape.
Simulating the manufacturing of a composite is a complex task as the physical manufacturing process often involves more than one step (e.g. draping and then infusion) and different physical phenomena (i.e. significant changes of viscosities, thermo-conductivity and volume fraction and/or chemical transformation) that are coupled with each other. Therefore, reliable process models have only started to emerge in the last 20 years or so with the simulation of draping and infusion processes being the most mature. A comprehensive review on the simulation of draping and infusion was provided by Pierce and Falzon [
] and a more specific look out into drape models (with a particular focus on forming induced wrinkles) was provided by Boisse et al. [
]. Although the idea of a virtual process chain for the manufacturing and design of composites has been formulated for some time now [
], it has only more recently started to produce interesting results. Pierce et al. [
], for example, successfully simulated the manufacture of resin-infused composite aerostructures performing draping simulations and then modelling the flow of resin through the obtained formed preform. A number of recent publications demonstrated the feasibility to take this one step further and simulate the structural performance of the obtained parts, thus building a fully integrated virtual process chain. A non-exhaustive list of examples includes references [
]. The simulation of infusion processes is now such that it can specifically design the weaving pattern of a fabric to reduce the porosity in the final part [
] and study complex stochastic phenomena [
] that are of great concern for industry. The simulation of prepreg-based manufacturing has also matured in recent years and phenomena such as curing (of thermosets), viscosity, chemical shrinkage and residual stress development are now better understood [
]. A comprehensive review of the state-of-the-art modelling technique of manufacturing processes is provided in [
One area where more work has been needed is the understanding and modelling of prepreg consolidation. Most commercial consolidation simulation tools available to the designer are based around Darcy’s law and assume [
] that any point within a piece of prepreg will behave the same as a point situated at the middle of a large flat panel (i.e. any edge effects are neglected). This is a perfectly valid assumption for a wide variety of applications, but as discussed by Hubert [
] and Hubert and Poursartip [
], wrinkles in composite processing are often formed in tapered and shape transition regions that are characterised by the existence of gaps and overlaps, shorter plies etc. and where edges effect play an important role in the deformation process. This suggest that, in order to capture wrinkles, state-of-the-art modelling techniques for pre-preg consolidation need to be improved. Understanding consolidation and how to model it has been the focus of recent research at the École centrale de Nantes [
] and Bristol Composites Institute (ACCIS) who both pointed out the existence of strong size effects when prepreg strips of large thickness to width ratio are considered. Particular efforts at Bristol have focused on the understanding of how the consolidation of thick composite sections influence the formation of out-of-plane wrinkles in a laminate. Hence, Belnoue et al. [
] have proposed a new modelling framework for the consolidation of toughened prepreg under processing conditions. Considering the compaction of cruciform shaped samples of different sizes and lay-up sequences [
], they have been able to formulate and extract a set of material parameters (evolving with temperature) that permit accurate prediction of the thickness and width evolution with time of laminates subjected to complex pressure and pressure rate cycles. The model was verified and validated for two material systems: IM7/8552 [
] and IMA/M21 [
]. The proposed analytical model was then adapted as a transversely isotropic hyper-viscoelastic model and implemented within a material subroutine (UMAT) for the commercial finite element (FE) package Abaqus/Standard. The ability of the model to predict the occurrence of consolidation-induced fibre path defects in a variety of industrially relevant cases such as Automated Fibre Placement (AFP) gaps and overlaps laminates [
] or L-, C- and tapered-sections [
] parts was demonstrated in subsequent publications. These publications, however, fell short of providing a full quantitative validation for key geometrical characteristics of wrinkles that play an important role in the failure mechanisms of composites.
In the present contribution, the industrially relevant case of a stepped laminate, as a simplification of a stringer foot run-out, is studied both numerically and experimentally. The mechanism by which the wrinkle forms here is slightly different from previous studies that used the same constitutive models [
] as it is generated through a lack of pressure in certain areas of the part (due to the bridging of the vacuum bag) rather than by the creation of an excess length stemming from a distortion of the prepreg stack that follows the application of the autoclave consolidation pressure. An in-depth quantitative comparison between geometric wrinkles characteristics (such as angle, height and wavelength) predicted by the simulation and measured on micrographs of real samples is performed. Finally, a numerical study of the impact of the observed geometrical differences between the ‘as-manufactured’ and the ‘as-predicted’ samples on the strength of the laminate is performed. Conclusions on the feasibility to integrate process simulations in the traditional design loop of composite parts to decrease their cost through a drastic reduction of the number of manufacturing trials necessary to release a part into the market are drawn.
In this paper, a framework for predicting wrinkle defects in thick autoclave-consolidated composite and their effects on the structural performance of components has been proposed, taking the industrially relevant case of a stepped laminate as an example. It was demonstrated that use of process models can help reduce fibre path defects that form in every part made, if the design process does not take account of manufacturing constraints. The limitations of using only pristine part information for design of composite parts to meet strength requirements has been highlighted. A difference of 25% between the predicted ‘as-designed’ and experimental ‘as-manufactured’ failure stress was reduced to 12% by using process models alone. Further improvements to the predicted strength can be reduced to less than 5% by using the ‘as-manufactured’ fibre paths in the FE models. Finally, it was shown, that even when a defect cannot be completely supressed, taking account of its existence in failure analysis can help greatly improve the predictive capabilities of the failure models and therefore help reduce the safety factors currently at play in the industry. This would, in turn, lead the way towards lighter designs, more fuel-efficient vehicles and a reduced usage of materials.
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