Strain Measurements of Composite Laminates with Embedded Fibre Bragg Gratings: Criticism and Opportunities for Research

Embedded optical fibre sensors are considered for structural health monitoring purposes in numerous applications. In fibre reinforced plastics, embedded fibre Bragg gratings are found to be one of the most popular and reliable solutions for strain monitoring. Despite of their growing popularity, users should keep in mind their shortcomings, many of which are associated with the embedding process. This review paper starts with an overview of some of the technical issues to be considered when embedding fibre optics in fibrous composite materials. Next, a monitoring scheme is introduced which shows the different steps necessary to relate the output of an embedded FBG to the strain of the structure in which it is embedded. Each step of the process has already been addressed separately in literature without considering the complete cycle, from embedding of the sensor to the internal strain measurement of the structure. This review paper summarizes the work reported in literature and tries to fit it into the big picture of internal strain measurements with embedded fibre Bragg gratings. The last part of the paper focuses on temperature compensation methods which should not be ignored in terms of in-situ measurement of strains with fibre Bragg gratings. Throughout the paper criticism is given where appropriate, which should be regarded as opportunities for future research

Multi-axial strain monitoring of fibre reinforced thermosetting plastics using embedded highly birefringent optical fibre Bragg sensors

There is a growing interest in the use of fibre reinforced plastics (FRPs) as high-grade construction material for variouw applications that need to be lightweight, yet strong in sometimes harsh loading conditions. Despite the growing popularity of structural composite materials, one has to realize that their mechanical behaviour is significantly different compared to conventional isotropic construction materials. Strain monitoring of an in-service structure should greatly enhance the insight and confidence in the (long-term) behaviour of high performance composite structures. Structural health monitoring necessitates the possibility of measuring multi-axials strain fields. High birefringent optical fibres (HiBi-fibres) with Bragg grating can become a solution in this matter. Designing a multi-axial strain sensor based on optical FBGs should meet several basic requirements which are discussed in this dissertation

Novel applications of pulse pre-pump Brillouin Optical Time Domain Analysis for behavior evaluation of structures under thermal and mechanical loading

This study aims to: (1) develop an analytical model for the strain transfer effect of distributed fiber optic sensors in a uniform or non-uniform stress field; (2) develop a measurement approach to monitor strains in concrete and detect damage (e.g. crack and delamination) in bonded and unbonded concrete overlays; (3) characterize the strain and temperature sensitivities of distributed fiber optic sensors at elevated temperatures; (4) develop a thermal annealing approach to enhance the thermal stability and temperature sensitivity of the distributed sensors; and (5) apply the distributed sensors to assess structural behaviors of concrete and steel structures exposed to fire. The pulse pre-pump Brillouin Optical Time Domain Analysis (PPP-BOTDA) was employed to measure strain and temperature distributions along a fused silica single-mode optical fiber. Strain distributions in concrete were measured from the distributed fiber optic sensors embedded in bonded and unbonded concrete overlays. Peaks of the strain distributions represent the effect of concrete cracks and delamination. The strain sensitivity coefficient of distributed sensors was reduced from 0.054 MHz/µε to 0.042 MHz/µε when temperature increased from 22 ⁰C to 750 ⁰C. The temperature sensitivity coefficient of distributed sensors was reduced from 1.349x10-3 GHz/⁰C to 0.419x10-3 GHz/⁰C when temperature increased from 22 ⁰C to 1000 ⁰C. The distributed sensors embedded in concrete beams measured non-uniform temperature distributions with local peaks representing a sudden increase of temperature through concrete cracks. Temperature distributions measured from the distributed sensors attached on steel beams enabled an enhanced thermo-mechanical analysis to understand the structural behaviors of steel beams subjected to fire --Abstract, page iii

In-situ deformation monitoring of aerospace qualified composites with embedded improved draw tower fibre Bragg gratings

Aerospace certified fibre reinforced plastics (FRPs) are extreme performing construction materials, which today are increasingly applied in primary structures of the new generation aircrafts (e.g. Boeing 787, Airbus 350, Bombardier C-Series), such as the fuselage, the wings and the fin. An interesting aspect on the technological point of view of sensing is that airplane manufacturers such as Airbus and Boeing are looking at incorporating health-monitoring systems (such as optical fibre sensors, especially fibre Bragg gratings) that will allow the airplane to self-monitor and report maintenance requirements to ground-based computer systems. However, one has to realize that the mechanical behaviour of anisotropic FRPs is significantly different compared to conventional isotropic construction materials. In this dissertation, the author focuses on monitoring the strain and (permanent) deformation in carbon reinforced plastic laminates with embedded fibre Bragg gratings. The research is divided in two main parts. In the first part of this research, the existing fibre draw tower technology is utilized, to manufacture an improved version of the existing in-line high quality, draw tower fibre Bragg gratings (DTG®s). With respect to accurate measurements and structural integrity, the research focuses on reducing the total diameter of the optical fibre, so the incorporation in the reinforcement fibres is enhanced and the distortion in the composite is reduced. The author elaborates in detail the methods of strain and temperature calibrations and the different setups which are applied. Additionally, with respect to the high temperatures during the composite manufacturing process, the thermal stability of the DTG®s is studied at elevated temperatures (>300°C). In the second part, the author embeds the DTG®s in specific types of thermoset and thermoplastic carbon reinforced plastic laminates. The author applies the embedded DTG®s in several stages of the composite lifetime. Starting with the monitoring of the composite manufacturing process and ending with fatigue testing until failure of the composite laminates. During the different experiments, the sensors are subjected to high temperatures, high pressures, extreme longitudinal strains and transverse strains and in the mean time, they are employed to very accurately measure (multi-axial) strains inside composites at microstrain level (~10 6)

Strain state detection in composite structures: Review and new challenges

Developing an advanced monitoring system for strain measurements on structural components represents a significant task, both in relation to testing of in-service parameters and early identification of structural problems. This paper aims to provide a state-of-the-art review on strain detection techniques in composite structures. The review represented a good opportunity for direct comparison of different novel strain measurement techniques. Fibers Bragg grating (FBG) was discussed as well as non-contact techniques together with semiconductor strain gauges (SGs), specifically infrared (IR) thermography and the digital image correlation (DIC) applied in order to detect strain and failure growth during the tests. The challenges of the research community are finally discussed by opening the current scenario to new objectives and industrial applications

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