In situmeasurement using FBGs of process‐induced strains during curing of thick glass/epoxy laminate plate: experimental results and numerical modelling

Abstract

For large composite structures, such as wind turbine blades, thick laminates are required to withstand large in-service loads. During the manufacture of thick laminates, one of the challenges met is avoiding process-induced shape distortions and residual stresses. In this paper, embedded fibre Bragg grating sensors are used to monitor process-induced strains during vacuum infusion of a thick glass/epoxy laminate. The measured strains are compared with predictions from a cure hardening instantaneous linear elastic (CHILE) thermomechanical numerical model where different mechanical boundary conditions are employed. The accuracy of the CHILE model in predicting process-induced internal strains, in what is essentially a viscoelastic boundary value problem, is investigated. A parametric study is furthermore performed to reveal the effect of increasing the laminate thickness. The numerical model predicts the experimental transverse strains well when a tied boundary condition at the tool/part interface is used and the tool thermal expansion is taken into account. However, the CHILE approach is shown to overestimate residual strains after demoulding because of the shortcomings of the model in considering viscoelastic effects. The process-induced strain magnitude furthermore increases when the laminate thickness was increased, owing mainly to a decrease in through-thickness internal transverse stresses.In situ process-induced strains during curing of thick laminate M. W. Nielsen et al.In situ process-induced strains during curing of thick laminate M. W. Nielsen et al.

Life cycle strain monitoring in glass fibre reinforced polymer laminates using embedded fibre Bragg grating sensors from manufacturing to failure

“…Studies found that good predictions of process strains in neat resin and composite laminates were achievable using a similar setup. 13,16,19 …”

Multiphysics modelling of manufacturing processes: A review

“…Johnston 174 proposed a modified linear elastic model incorporating temperature dependency which also allows thermal softening. 171,175,176 This model which is often denoted the model cure hardening instantaneous linear elastic (CHILE) model is used in the present work. This indicates that with an increase in degree of cure, the modulus increases monotonically.…”

“…In the model, it was assumed that the preform is perfectly impregnated through the use of a constant fibre volume fraction resembling the final state of the part after processing. 171 Demoulding is modelled after curing at ambient temperature by simply suppressing all mechanical constraints between tool and part. Figure 19 presents the experimentally determined centre plane laminate temperature (T2), compared with model predictions of temperature and corresponding cure degree model predictions.…”

Process Induced Defects in Liquid Molding Processes of Composites

“…While selecting process parameters can be straightforward for thin composite laminates, thick laminates often require more sophisticated methods. A possible approach has been to use numerical simulations based on experimentally validated constitutive models to optimize the cure process parameters (Alexandric et al, 2016;Baran et al, 2017;Bogetti and Gillespie, 1991;Khoun and Hubert, 2010;Michaud et al, 2002;Nielsen et al, 2013;Rai and Pichumani 1997a;Rai and Pichumani, 1997b;Rai and Pichumani, 1997c).…”

“…In addition to optimizing cure, researchers aim to reduce residual stresses and limit the occurrence of cure induced defects (Baran et al, 2017;Dong et al, 2004;Khoun and Hubert, 2010;Mostafa et al, 2017;Nielsen et al, 2013;Ruiz and Trochu, 2006). For instance, Ruiz and Trochu (2006) developed a multi-criteria optimization algorithm to optimize an objective function that incorporates conflicting goals such as minimization of residual stresses, maximization of the final degree of cure and reduction of cycle time, as well as avoiding thermal degradation of the matrix.…”