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

Abstract

A holistic approach to strain monitoring in fibre-reinforced polymer composites is presented using embedded fibre Bragg grating sensors. Internal strains are monitored in unidirectional E-glass/epoxy laminate beams during vacuum infusion, curing, post-curing and subsequent loading in flexure until failure. The internal process-induced strain development is investigated through use of different cure schedules and tool/part interactions. The fibre Bragg grating sensors successfully monitor resin flow front progression during infusion, and strain development during curing, representative of the different cure temperatures and tool/part interfaces used. Substantial internal process-induced strains develop in the transverse fibre direction, which should be taken into consideration when designing fibre-reinforced polymer laminates. Flexure tests indicate no significant difference in the mechanical properties of the differently cured specimens, despite the large differences in measured residual strains. This indicates that conventional flexure testing may not reveal residual strain or stress effects at small specimen scale levels. The internal stresses are seen to influence the accuracy of the fibre Bragg gratings within the loading regime. This study confirms the effectiveness of composite life cycle strain monitoring for developing consistent manufacturing processes.

Recent advances in fiber/matrix interphase engineering for polymer composites

“…The sensor built in the composite may be used for mapping the stress/strain state under various loading modes. As a consequence, the spray-coated "FibSen" fibers may replace Bragg grated optical fibers (FBG) used for both above tasks [132,133]. Major benefit of the "Fibsen" 1D fibers is that their diameter is smaller or comparable with those of the reinforcing fibers of the composite unlike FBG fibers which are much thicker.…”

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.…”

“…The internal strain development during curing of a thick laminate plate at elevated temperatures using a long cure cycle was analysed by Nielsen et al 175,176 The objective with the study was to evaluate how accurate the aforementioned CHILE constitutive model is at predicting the internal strain development, in what is essentially a viscoelastic problem. 3D numerical model total strain predictions are compared with experimentally determined in situ strains within the laminate part, using embedded optical fibres with fibre Bragg grating (FBG) sensors.…”

“…A tempered glass of 10 mm thickness was employed as a mould. 175,176 Prior to infusion, sensors were embedded within the dry preform; including thermocouples and optical fibres (FBGTop, FBGCenter and FBGBottom), each consisting of three FBG sensors interspaced along the optical fibre, see Figure 18(a). The optical fibres were placed transversely to the main reinforcement fibre direction in order to capture the matrix-driven process strains.…”

A comparative experiment for the analysis of microwave and thermal process induced strains of carbon fiber/bismaleimide composite materials

“…The essential structural characteristics of FBG sensors determine the inescapability of double sensitive on temperature and stress when monitoring the cure-induced strains [17] and [18]. A glass capillary is used to pack the FBG for the measuring of the temperature variation.…”