Advanced modelling makes better composites

Tom Shelley reports on software that should ensure composite constructions get manufactured right first time.

Advances in software that allow the modelling of resin penetration and the effects of curing on thick, complex fabrications should ensure that they come out of the mould right first time, without the need for repeated trial and error in manufacturing operations.

'Prepreg' composite moulding (PCM) is becoming increasingly common as a means of producing flexbeams, blades and spars in aerospace manufacturing and other thick laminate composite parts. The process involves placing prepreg into a mould cavity prior to consolidation under heat and pressure to produce thick composite parts.

Unfortunately, curing leads to residual stresses, and these can make the shape distort badly when is comes out of the mould, which may require modifying either the shape of the mould and/or the process conditions in some way, so that the end result is what is wanted. Until now, this has often led to many unsuccessful trial mouldings, as was the case with the carbon fibre 'MonoCell' for the McLaren MP4-12C road car, which took 4 years and 94 attempts to get right.

However, developers at MSC have now produced software based on MSC MARC, which is an FEA package specially tailored to deal with large deformations, to address mechanical problems caused by uneven composite curing. According to a paper presented to a conference in Venice, the software has to perform a coupled thermal-curing-mechanical analysis and the simulation of the coupled transport phenomena of heat, fluid and resin cure kinetics.

The first stage is to analyse the fibre bed consolidation part of the process. This requires using coupled soil models to model resin flow under pressure through the pores in the fibre mats.

The authors of the paper showed the results of modelling the consolidation of a tapered fibre block, and how this led to distortion of the fibre bed, before curing has even started. On it has started, there is a curing pass, a thermal pass and a mechanical behaviour pass in each computing iteration.

Cure rate evaluation is based in temperature and curing kinetics models. A curing heat flux term resulting from the change in the degree of cure is added to the right hand side of the heat transfer equation during each heat transfer calculation pass. The degree of cure and calculated temperatures are inserted during each mechanical calculation pass.

Volumetric resin shrinkage is also incorporated during each mechanical calculation pass. The degree of shrinkage is a function of both degree of cure and temperature. Curing shrinkage strain components are calculated from volumetric resin shrinkage. The software supports various curing kinetic and curing shrinkage modules and post processing data includes: degree of strain, curing heat flux, and resin shrinkage strains.

It is possible to directly input curing exotherms that have been experimentally determined. While helping, nobody is pretending that this is a panacea for all problems relating to designing and manufacturing composite parts.

For example, Andy Woodward, technical consultant with MSC MARC vendors Desktop Engineering points out that if designing, say a composite radar radome for the front of an aeroplane, structural engineering takes second place to electromagnetic requirements. Nor is the module described here anything like the end of the story. Research and development continues.

Design Pointers
• Software is able to make realistic predictions of distortions in composite parts resulting from curing
• It should then be possible to reduce problems by redesigning parts and/or modifying processing in order that parts come out right to purpose

Tom Shelley

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