Relating the state of cure to the real-time internal strain development in a curing composite using in-fibre Bragg gratings and dielectric sensors

Abstract

The paper reports results of preliminary experiments on a continuous curing carbon fibre reinforced epoxy composite, designed to achieve simultaneous characterizations of the state of cure of the resin and the development of strain in the reinforcing fibres. The measurements were carried out using embedded dielectric microsensors and in-fibre Bragg gratings (IFBGs) to follow changes in the conductivity of the matrix resin and in the internal strain in the embedded optical fibre respectively. This was performed using an automated data acquisition system, part of which uses real-time dielectric measurements to follow the industrially relevant parameters of curing. The system can multiplex several sensors and could be used in a quasi-distributed system. The IFBG sensors were demodulated using a scanning fibre Fabry-Pérot interferometer. The results obtained with this combined cure monitoring system show that it is possible to monitor the strain levels in the optical fibre resulting from the onsets of liquification, gelation and vitrification within the surrounding resin matrix. The combined measurements constitute a first step towards a scientific study of the development of internal residual stresses in a cured composite, which will be crucial to the ultimate elimination of warpage and `spring-back' in large composite structures.

Strain development in a filled epoxy resin curing under constrained and unconstrained conditions as assessed by Fibre Bragg Grating sensors

“…Many authors applied this method for the 'health monitoring' of composite structures [1][2][3][4][5][6][7]. Further studies aim at the use of fibre optical technique to characterize cure of resins [8][9][10][11][12][13][14][15][16][17][18][19]. In our earlier works [8,9] we reported about the use of FBG sensors to monitor the cure-induced strain development in epoxy resins (EP).…”

Field Deployable Fiber Bragg Grating Strain Patch for Long-Term Stable Health Monitoring Applications

“…The effects observed here cannot therefore clearly be separated into temperature load and mechanical forces acting at the sensor. For a description of the curing process, and for the use of these sensors for cure monitoring, it is necessary to develop a specially adapted sensor system based on fundamental work in literature [5][6][7]. Additionally, interferences on spectral phenomena have to be investigated, e.g.…”

Liquid Resin Infusion process monitoring with embedded superimposed long period and short period Bragg grating sensor.

“…The process ends after the cure plateau, with the decrease of temperature (third step), leading to residual cure stresses proportional to the measured residual strains [4,6]. In this case, the Bragg wavelength depends on strain and temperature, as…”

“…However, some well known issues concern for instance the durability evaluation and damage tolerance design [1,2], the characterisation of residual stresses [3,4] as well as the identification of the global mechanical properties of such composite structures [5], that presents obstacles related to the multiplicity of the involved scales, the complexity of the mechanical answers, the ignorance of the material state and the unavoidable properties variability induced by the manufacturing processes. In this way the monitoring of the process can give some answers [6,7]. Among the composite manufacturing processes, the Liquid Resin Infusion (LRI) process is a novel process for polymer matrix composites whose major interests are to lead to quality controlled parts and reduced assembly steps by infusing components and sub-components at the same time.…”