The response of embedded FBG sensors to non-uniform strains in CFRP composites during processing and delamination

From airplanes to sailboats to bridges, composite materials have become a significant part of our everyday structures. With increasing demand, these materials are pushed to their limits to improve structural efficiency. As a consequence, research and development must continually improve products and provide support for the end user who will need to know the characteristics of their new material. Progress made in the area of optical fibre sensing has opened new avenues for measuring and monitoring fibre-reinforced polymer (FRP) composites, since they can be embedded directly into the composite during manufacturing. These globally noninvasive sensors can provide internal strain and temperature measurements from the moment processing starts until the final failure of the part. The goal of this research is to develop and demonstrate fibre optic sensing techniques that can characterize the internal strain state of FRP composites. In particular, this work focuses on measuring three-dimensional, non-uniform strain fields in carbon fibre-reinforced polymers (CFRP) using fibre Bragg grating (FBG) sensors. Although FBG sensors are becoming widespread for simple uniaxial strain measurements, their response to complex, non-homogeneous strain fields is still difficult to interpret. To illustrate advances in both experimental techniques and the interpretation of measured FBG data, two main areas of composite monitoring are addressed. They include the study of residual strain evolution and of delamination cracking, which both produce non-homogeneous strain fields. Unidirectional carbon fibre-reinforced polyphenylene sulphide (AS4/PPS) laminates are observed during processing to measure residual strain progression, and then later subjected to Mode I double cantilever beam delamination tests. These thermoplastic composite specimens are also produced in a cross-ply configuration, for the purpose of residual strain monitoring. In each laminate, a long-gauge length (20-35 mm) FBG is embedded parallel to the reinforcing fibres, and centred along the length of the plate. Results of polarization sensitive FBG monitoring indicate characteristic material state changes such as the glass-transition and the melting temperatures. These measurements take advantage of both the transverse and longitudinal strain sensitivity of the FBG. When transverse strains are unequal they induce birefringence in the FBG (defining a fast and a slow axis), which results in a split of the normally bell-shaped reflected spectrum. An evolution of this birefringence is monitored during cooling, culminating in average residual transverse strain differences in the embedded FBGs of 230 με and 410 με for unidirectional and cross-ply specimens respectively. Based on the wavelengths measured along the fast polarization axis of the fibre, (observed to be less sensitive to transverse strains) cross-ply specimens exhibit absolute longitudinal residual strains in the order of -350 με. Small longitudinal strain values are the result of the low coefficient of thermal expansion of the carbon reinforcing fibres. An important step forward in FBG monitoring is taken by measuring the absolute values of the three unequal principal strains in a composite material without making assumptions about the state of strain in the FBG (i.e. diametric loads, plane stress, axisymmetry, etc.). For this purpose, a polarization controlled, hybrid FBG-Fabry Pérot optical sensing technique is developed to measure residual strain evolution. The Fabry Pérot sensor used in this hybrid method is only sensitive to longitudinal strains, thus providing the additional data required to solve the three-dimensional strain state directly. To better understand the state of residual strain in the composite material, a temperature dependent thermoelastic finite element model is employed to investigate the strain accumulation during cooling. By comparing modelled results to the data from the optical fibre, it is shown that the mould influences the residual strain development during cooling, and that some of these strains are released after demoulding. Examination of the simulated and experimental curves indicates that the final residual strain state observed with the FBG is close to that of a freely cooling composite plate. Since the embedded optical fibre is a local inclusion, its strain state is not necessarily that of its host material. In this work, finite element models are used to determine the stresses and strains developed in the surrounding composite material. Near the optical fibre, there is a perturbation of the strain field that extends to a distance of three fibre diameters. In the far-field of cross-ply specimens, tensile transverse stresses reach half the matrix fracture strength. This may help to explain matrix cracking observed on the surface of these specimens. The second portion of this study is aimed at the measurement of non-uniform longitudinal strains superimposed on an already three-dimensional residual strain state. A polarization adapted optical low coherence reflectometry (OLCR) technique takes distributed measurements of the local Bragg wavelengths for a given polarization axis. For a constant state of birefringence, one can relate the distributed wavelengths to the non-uniform longitudinal strains along the length of the FBG sensor. Delamination cracking in double cantilever beam specimens creates a non-uniform strain field ideally suited to illustrate this type of measurement. At increasing crack lengths, the distributed wavelengths (proportional to axial strain) are measured by an FBG embedded parallel to the delamination plane. The long gauge length of the sensor provides a sufficiently large set of data so that the crack position and growth direction can be distinguished. The strains retrieved from these experiments are further employed to determine the stress distribution caused by the fibres bridging the delamination crack. The combination of FBG measurements with inverse identification via finite element modelling is a new technique for determining bridging laws from static delamination specimens. Results of this work indicate that the maximum bridging stress is approximately 2.5 MPa and that the fibre bridging zone length ranges from 20-50 mm. Comparisons of bridging laws determined using this method and a J-integral approach are made using a second finite element model that includes cohesive elements. Simulations of advancing delamination cracks highlight the sensitivity of the force-displacement response of the specimen to differences in bridging laws. Through the advances in FBG-based methods outlined in this thesis, significant progress is made in the area of non-homogeneous strain detection in fibre-reinforced composites. This allows for improved characterization of three-dimensional residual strain states and the non-uniform strain distributions caused by delamination cracking.

Botsis, Ioannis
Lausanne, EPFL
Other identifiers:
urn: urn:nbn:ch:bel-epfl-thesis3710-7

 Record created 2006-11-24, last modified 2018-03-17

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