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We present, for the first time, an experimental investigation of internal strain monitoring in thermoplastic composites subjected to quasi-static indentation and low-velocity impact using embedded fiber Bragg gratings (FBGs). The goal is to highlight the interest and limitations of the in-core instrumentation of glass fiber-reinforced polypropylene laminates subjected to these two classical loading conditions. We propose an instrumentation strategy utilizing FBGs that is expected to provide a reliable set of internal strain values and strain rates, which can be used for the analysis of the damage behavior and the validation of a numerical mesoscale model of laminates. Based on a specific sensor insertion procedure, monitoring techniques and optical observations, we show how the applied methodology alleviates major issues, such as determining the in-plane and through-thickness position of the embedded FBGs, their influence on the structural integrity or the interpretation of the reflected optical signal.

Matrix behavior is expected to widely influence the impact response of composites, but detailed conclusions in the case of thermoplastic laminates are still needed. In this paper, we investigated the effect of using either ductile homopolymer PP or less-ductile impact copolymer PP matrices on the low-velocity impact responses of continuous glass fiber-reinforced polypropylene (PP) laminate. These PP types represent two variants in the same family of thermoplastic matrix. A thorough experimental campaign was first performed to provide the tensile properties (for PP and glass/PP) and fracture toughness (Mode-I and Mode-II, glass/PP only) of the employed materials. Then, low-velocity impact tests where the energy levels are ranging from 12 to 30 J were performed. Using ductile PP in glass/PP laminates reduces the energy dissipated during impact as well as the impact damage area. The effect of selected stacking sequences on the resistance to impact was also studied as a way to reveal the difference between ductile and less-ductile glass/PP. Stacking sequence with thin plies shows better impact properties than other sequences regardless of the matrix ductility, which can be explained by micromechanics for both grades of material. Finally, as quasi-static indentation (QSI) is usually used to quickly access the resistance of laminates towards out-of-plane impact, we systematically compared our impact results with QSI results. We found that the prospective use of QSI in forecasting impact properties and damage is very limited in glass/PP composite due to strain-rate sensitivity.

Measuring parameters related to each damage mode of composites subjected to impact is very challenging because of the complex damage phenomenology. Here, we developed an experimental methodology for evaluating the micro-scale fracture characteristics of two principal damage modes, i.e., transverse crack and delamination, and providing the corresponding fracture toughness. We demonstrated the capability of the method by comparing and providing additional insights about two materials, namely homopolymer-based (ductile) and copolymer-based (less-ductile) glass/polypropylene thermoplastic composites. We found that (i) transverse crack behavior of both composites is similar as indicated by a small difference in their fracture toughness, (ii) delamination growth in copolymer-based composites is slower than in homopolymer-based composites, (iii) the fibrillation induced by rubber particles in copolymer-based composites is responsible for decelerating the delamination growth and improving its fracture toughness during delamination. This method is deemed useful and quick for determining the micro-scale fracture behavior of composite laminates under impact in order to support the material selection process.

We investigate the influence of process-induced shrinkage and subsequent annealing on the thermomechanical behavior of unidirectional laminates made of continuous glass fiber-reinforced polypropylene (GFPP). We use two different industrial lamination processes: static hot-press (SHP), and double-belt press (DBP) that are characterized by different cooling rates and pressure levels and most importantly, by the use of a closed mold in the case of SHP manufacturing. We measure the longitudinal and transverse shrinkage during the manufacturing and annealing processes using embedded fiber Bragg gratings (FBGs). The SHP molding reveals much lower induced shrinkage in GFPP as compared to the DBP process, although the relatively slow cooling should promote a higher degree of crystallization. We ascribe this to the constraining effect of the metallic mold used with the SHP process. The poor thermal conductivity of the mold is also responsible for a layer-like crystal microstructure in the GFPP matrix, causing a specific relaxation effect during the post-process heating treatment. Annealing generates additional shrinkage that is due to an increased degree of crystallinity and to the partial relaxation of residual stresses. However, the thermal expansion properties remain impacted by the process-induced strain state of the GFPP laminates and are still process-dependent after annealing.

Fiber-reinforced composites with improved dissipation of energy during impact loading have recently been developed based on a polypropylene copolymer commonly called impact polypropylene (IPP). Composites made of IPP reinforced with glass fibers (GF) are particularly attractive to the automotive industry due to their low cost and good impact resistance. In such composites, the cooling rate varies depending on processing techniques and manufacturing choices. Here, we study the effects of the cooling rate of GF-IPP composites on shear behavior, which is critical in impact applications, using [ ± 45 ] s monotonic and cyclic (load/unload) tensile specimens. The specimens were manufactured under a wide range of cooling rates (3 °C/min, 22 °C/min, 500–1000 °C/min). Mainly dominated by the properties of the matrix, the global shear behavior of GF-IPP composites differed considerably with respect to the cooling rate. However, the performance of the fiber-matrix interface (chemically modified) appeared to be unaffected by the range of cooling rates used in this study. We found that the cooling rate has a minor effect on the rate of damage accumulation, while it strongly modifies the shear-activated rate-dependant viscoelastic behavior.

Impact copolymer polypropylene (IPP), a blend of isotactic polypropylene and ethylene-propylene rubber, and its continuous glass fiber composite form (glass fiber-reinforced impact polypropylene, GFIPP) are promising materials for impact-prone automotive structures. However, basic mechanical properties and corresponding damage of IPP and GFIPP at different rates, which are of keen interest in the material development stage and numerical tool validation, have not been reported. Here, we applied monotonic and cyclic tensile loads to IPP and GFIPP at different strain rates (0.001/s, 0.01/s and 0.1/s) to study the mechanical properties, failure modes and the damage parameters. We used monotonic and cyclic tests to obtain mechanical properties and define damage parameters, respectively. We also used scanning electron microscopy (SEM) images to visualize the failure mode. We found that IPP generally exhibits brittle fracture (with relatively low failure strain of 2.69–3.74%) and viscoelastic-viscoplastic behavior. GFIPP [90]8 is generally insensitive to strain rate due to localized damage initiation mostly in the matrix phase leading to catastrophic transverse failure. In contrast, GFIPP [±45]s is sensitive to the strain rate as indicated by the change in shear modulus, shear strength and failure mode.

Conductive fibers with enhanced physical properties and functionalities are needed for a diversity of electronic devices. Here, we report very high performance in the thermal and mechanical response of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) microfibers when subjected to an electrical current. These fibers were made by combining the hot-drawing assisted wetspinning process with ethylene glycol doping/de-doping that can work at a current density as high as 1.8 × 104 A cm−2, which is comparable to that of carbon nanotube fibers. Their electrothermal response was investigated using optical sensors and verified to be as fast as 63 °C s−1 and is comparable with that of metallic heating elements (20–50 °C s−1). We investigated the electromechanical actuation resulted from the reversible sorption/desorption of moisture controlled by electro-induced heating. The results revealed an improvement of several orders of magnitudes compared to other linear conductive polymer-based actuators in air. Specifically, the fibers we designed here have a rapid stress generation rate (>40 MPa s−1) and a wide operating frequency range (up to 40 Hz). These fibers have several characteristics including fast response, low-driven voltage, good repeatability, long cycle life and high energy efficiency, favoring their use as heating elements on wearable textiles and as artificial muscles for robotics.

International audience; Monitoring changes in electrical conductivity is a simple way to assess the water uptake from environmental moisture in polymers. However, the relation between water uptake and changes in conductivity is not fully understood. We monitored changes in the electrical volume conductivity of an anhydride-cured epoxy polymer during moisture sorption-desorption experiments. Gravimetric analysis showed that the polymer exhibits a two-stage sorption behavior resulting from the competition between diffusive and reactive mechanisms. As expected, the macroscopic electrical conductivity increases with the diffusion of water. However, our most surprising observation was severe hysteresis in the relation between water uptake and electrical conductivity during the sorption and desorption experiments. This indicates that change in the electrical conductivity depends on both the water uptake and the competition between the diffusive and reactive mechanisms. We studied samples with various thicknesses to determine the relative effects of the diffusive and reactive mechanisms. This is an important observation as it means that general electrical monitoring techniques should be used cautiously when it comes to measuring the moisture content of polymer or polymer-based composite samples.

Wearable pressure sensing solutions have promising future for practical applications in health monitoring and human/machine interfaces. Here, a highly sensitive, low-cost, wearable pressure sensor based on conductive single-walled carbon nanotube (SWCNT)/alginate hydrogel spheres is reported. Conductive and piezoresistive spheres are embedded between conductive electrodes (indium tin oxide-coated polyethylene terephthalate films) and subjected to environmental pressure. The detection mechanism is based on the piezoresistivity of the SWCNT/alginate conductive spheres and on the sphere-electrode contact. Step-by-step, we optimized the design parameters to maximize the sensitivity of the sensor. The optimized hydrogel sensor exhibited a satisfactory sensitivity (0.176 ΔR/R0/kPa(-1)) and a low detectable limit (10 Pa). Moreover, a brief response time (a few milliseconds) and successful repeatability were also demonstrated. Finally, the efficiency of this strategy was verified through a series of practical tests such as monitoring human wrist pulse, detecting throat muscle motion or identifying the location and the distribution of an external pressure using an array sensor (4 × 4).

International audience; Fiber Bragg gratings (FBG) may be used to monitor strain over the surface of a structure as an alternative technology to conventional strain gauges. However, FBG bonding techniques have still not been established to yield satisfactory surface measurements. Here, two adhesives were investigated, one with low viscosity and the other with high viscosity for bonding FBGs on glass/epoxy sandwich skins. First, instrumented elementary specimens were tested under tension. FBG strain results were analyzed together with digital image correlation (DIC) measurements. The influence of the bonding layer on the measured strain and on the integrity of the sensor was investigated by considering different regions of interest. Next, an instrumented structural sandwich beam was tested under four-point bending. FBG rosettes were compared to conventional strain gauge rosettes. The high viscosity adhesive demonstrated behaviors that affected FBG accuracy. Brittleness of the bonding layer and poor interface adhesion were observed using DIC and X-ray tomography. By contrast, the low viscosity adhesive demonstrated satisfactory results. The FBG strain measurements appeared to be consistent with those of DIC. The accuracy is also adequate as the FBGs and the conventional strain gauges had similar results in three directions, under tension and under compression.

The main topic of this paper deals with the stress analyses—tests dialogue in composite structures considering the influence of the manufacturing conditions of composite material within the structure. The scientific interest of embedded instrumentation exploited through a multiscale approach featuring optical fibers with Bragg gratings is discussed. The multiscale material requests multiscale data for calculation. The addressable scales are thus macro (or structural) scale and meso (ply) scale concerning specimens representative of aeronautical structures (whether instrumented or not). Discrepancies on both meso and macro scales obtained between the measurements and the calculations are evaluated regarding the variability of the numerical input data (i.e., elastic characteristics on average of the composite material).

The manufacturing of a thermoset matrix composite part is quite complex. The curing of this kind of composites is commonly controlled by curing duration, temperature, pressure, and vacuum. On one hand, in order to produce fully cured and compacted and high quality parts, a reasoned and optimum choice must be done. On the other hand, and all the more so as regards thermoset matrix composites, the curing parameters may induce some irreversible effects on several parameters of the ensuing laminated part. For all these reasons, since numerous years, cure monitoring has been paid of a great attention. In this contribution, the reader will be first provided with a condensed survey (including more than 100 references) of the main methods developed from more than 40 years to monitor the changes occurring in a composite material during its curing. In the broad field of real-time cure monitoring, a focus on optical fibers Bragg grating (FBG) and their use for cure monitoring purposes will be made in this article. Some original results are given after a brief introduction about FBG. The selection of a sensing technique depends on the desired results. Nevertheless, the advantages and the drawback of each technique will not be discussed here. The reader will easily find detailed comments in the numerous references proposed at the end of this article. Keywords: cure cycle; cure monitoring; optical fibers Bragg grating; laminated part; in situ strains

International audience; This paper is a continuation of our previous study [Mulle M, Collombet F, Olivier P, Grunevald Y-H. Assessment of cure residual strains through the thickness of carbon-epoxy laminates using FBGs, part I: elementary specimen. Compos Part A 2008. doi:10.1016/j.compositesa.2008.10.008] pertaining to the assessment of autoclave cure-induced strains through the thickness of carbon-epoxy laminates. In this first part, postulates and measurement procedures were established for cure of elementary specimens. Based on these, this study undertakes investigation on what are called technological specimens. These specimens are of the beam type and contain geometrical specificities which represent typical structural issues. In-plane process-induced strains were studied through the thickness of a thick reinforced zone using several optical fibre Bragg gratings (FBGs) sensors embedded at different levels of the ply stack. A non-uniform distribution of residual strains was detected. Once cured, the technological specimen was subjected to a heating test whose cycle was comparable to the cure cycle. Thermally induced strains were measured with the embedded FBGs. The values recorded were compared with those of cure-induced residual strains and FEM simulation. Discrepancies were observed that strongly suggest the possible influence of environmental effects and the need for the calculation to take into account the through-the-thickness variability of thermal properties.

This study concerns the central reinforced zone of a composite structural specimen. In order to estimate the strain distribution during a series of three and four-point bending tests, several optical fiber Bragg grating sensors have been embedded in various levels of the ply stack. Simultaneously, surface strain field measurements by 3-D digital image correlation are undertaken. Both techniques show a general linear distribution of longitudinal strains through the thickness of the thick zone but values are slightly different. A numerical model is developed and a test-calculation dialogue is carried out. The complementary information given by the two optical techniques for in-core and surface measurements reveals the importance of considering structural and edge effects.

The present paper deals with the use of optical fibers with Bragg gratings for the identification of process-properties relations for composite structures. This study relates a 7 year-experience in the field of process and mechanical characterization of composite parts. It highlights some major topics in terms of technological issues such as sensor integration, information discrimination and measurement accessibility. Results of process monitoring concerning autoclave cure and filament winding are shown through which initial states are identified. Finally, structural tests of instrumented parts are presented, showing the efficiency of the embedded sensors to allow test-calculation dialogues and thus properties identification.

Advanced optical measurement techniques are used to analyze the mechanical behavior of a composite structure submitted to bending tests. This is carried out by considering a beam type specimen which presents design singularities such as important thicknesses, ply drop off zones and a reinforced zone. This study concerns the central reinforced zone of the specimen. In order to estimate the strain distribution during a series of 3 and 4-point bending tests, several optical fiber Bragg grating sensors have been embedded in various levels of the ply stack. Simultaneously, surface strain field measurements by three dimensional digital image correlations are undertaken. Both techniques show a general linear distribution of longitudinal strains through the thickness of the thick zone but values are slightly different. A numerical model is developed and a test-calculation dialogue is carried out. The complementary information gathered by the two optical techniques for in-core and surface measurements reveals the importance of considering structural and edge effects.

International audience; Variability of thereto-mechanical properties within a composite part is a well known issue. It is problematic at the early stage of structural design. This variability being generated during the processing phases, the characterisation of the initial state of a structure becomes a crucial operation. A first step to this objective is presented in this paper. It deals with the assessment of autoclave cure induced strains in carbon epoxy laminates. Distribution and amplitude are considered through the thickness of carbon/epoxy elementary and technological specimens thanks to a series of FBGs embedded at different levels of the ply stack. Part I of this work concerns the analysis of unidirectional reinforced specimens. It begins with all overview of the various phenomena that Cause the presence of residual stresses after cure and how they can be measured in experiments. Their development being closely related to the rheology of the matrix, an analysis of the M21 resin is given and assumptions are made to justify the following assessment procedures. A discussion leads to select the onset of gelation as the key point to start measuring residual cure stresses. The autoclave cure monitoring of the [O(8)] specimen is then carried out and analyzed. Depending on their location through the laminate thickness, the FBG sensors give different information highlighting the effects of interaction between the laminate and the tool/plate

Highly conductive polymer fibers are key components in the design of multifunctional textiles. Measuring the voltage/temperature relationships of these fibers is very challenging due to their very small diameters, making it impossible to rely on classical temperature sensing techniques. These fibers are also so fragile that they cannot withstand any perturbation from external measurement systems. We propose here, a non-contact temperature measurement technique based on fiber Bragg gratings (FBGs). The heat exchange is carefully controlled between the probed fibers and the sensing FBG by promoting radiation and convective heat transfer rather than conduction, which is known to be poorly controlled. We demonstrate our technique on a highly conductive Poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS)-based fiber. A non-phenomenological model of the sensing system based on meaningful physical parameters is validated towards experimental observations. The technique reliably measures the temperature of the polymer fibers when subjected to electrical loading.

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Extrait de : Photomécanique / sous la dir. de J.J ORTEU et D. BALAGEAS. Dans cette revue ont été reprises et développées une sélection des contributions présentées au colloque Photomécanique 2004, Albi, France, 4-6 mai 2004; International audience