Small-diameter optical fibre sensor embedment for ambient temperature cure monitoring and residual strain evaluation of CFRP composite laminates produced by vacuum assisted resin infusion

Out of autoclave (OoA) processing techniques, such as liquid composite moulding techniques (LCM) and, particularly, the vacuum assisted resin infusion (VARI) technique, are being used, with increasing success, in replacement of prepreg/autoclave technologies to produce structural aircraft/aerospace polymer composite parts, due to its better cost effectiveness and competitiveness. This work aims to embed Fibre Bragg grating (FBG) sensors to monitor the VARI manufacturing of carbon fibre reinforced polymer (CFRP) composites and evaluate the associated phenomena: ambient curing and post curing reactions and resulting residual strains. The curing kinetics of the epoxy resin system alone was initially studied through isothermal differential scanning calorimetry (DSC) tests and applying the isoconversional Friedman method, and further studied by strain monitoring during ambient curing and post curing resorting to FBG sensors. The FBG sensors in the CFRP laminates were able to detect a subtle increase of strain as infusion of the CFRP started and to measure decreasing strain as resin filled in the dry fabric layers. Subtle strain decrease revealed forming crosslink bonds. Compressive strains measured by the FBG sensors during post curing show that further crosslink takes place. A comparison of resultant residual strains was made between specimens with embedded FBG sensors on small-diameter optical fibres (SDOF) and on large-diameter optical fibres (LDOF).The authors acknowledge the support of the European Regional Development Fund [grant number NORTE-01-0145- FEDER-000015]; and of the European Space Agency through the Network/Partnering Initiative Program

Influence of optical fiber sensor placement on CFRP laminates for process and structural health monitoring

The use of fibre reinforced polymer (FRP) composites has been increasing in the last few decades in the aircraft and aerospace industries to replace metallic structures. The use of FRPs intends to increase the mechanical performance and decrease the weight ratios of structural parts, which allows to reduce fuel consumption, carbon dioxide emissions and, therefore costs. Although composite materials hold promising achievements, their failure is still very difficult to predict, as it may result of a combination of defects, such as fibre breakage and/or misalignment, matrix macro or micro-cracking, material and stress discontinuities, fibre/matrix debonding, delaminations, etc. Fatigue and aging might also lead FRP structures to failure, as they face repeated loading and harsh environmental conditions during their life-time. The eventuality of an unpredicted failure on a composite aircraft structure often requires an over-engineered design to comply with the rigorous and exigent safety rules of aerospace industry and, particularly, of civil aircraft, counteracting the initial purpose of using composites for weight reduction. This paper studies the influence of optical fibre location along the thickness of carbon fibre reinforced polymer (CFRP) laminates to detect BVID, particularly delaminations.Programa Regional do Norte NORTE 2020, ref. NORTE-01-0145-FEDER-000015. European Space Agency - ESA, Projeto Ref. ESA-TEC-QEE-555 "Smart Composite Parts with Multifunctional Properties Ready to Fly

Optimisation of through-thickness embedding location of Fibre Bragg grating sensor in CFRP for impact damage detection

Aerospace composites are susceptible to barely visible impact damage (BVID) produced by low-velocity-impact (LVI) events. Fibre Bragg grating (FBG) sensors can detect BVID, but often FBG sensors are embedded in the mid-plan, where residual strains produced by impact damage are lower, leading to an undervaluation of the damage severity. This study compares the residual strains produced by LVI events measured by FBG embedded at the mid-plan and other through-thickness locations of carbon fibre reinforced polymer (CFRP) composites. The instrumented laminates were subjected to multiple low-velocity impacts while the FBG signals were acquired. The FBG sensor measurements allowed not only for the residual strain after damage to be measured, but also for a strain peak at the time of impact to be detected, which is an important feature to identify the nature and presence of BVID in real-life applications. The results allowed an adequate optical fibre (OF) embedding location to be selected for BVID detection. The effect of small- and large-diameter OF on the impact resistance of the CFRP was compared.This research was funded by the European Regional Development Fund, grant number NORTE-01-0145-FEDER-000015, and the APC was funded by the European Space Agency’s Network/Partnering Initiative Program, with contract number 4000123315

Sensors for process and structural health monitoring of aerospace composites: a review

Structural Health Monitoring (SHM) is a promising approach to overcome the unpredictable failure behaviour of composite materials and further foster their use in aerospace industry with increased confidence. SHM may require a complex system, including sensors, wiring and cabling, data acquisition devices and software, data storage equipment, power equipment and algorithms for signal processing, involving a multidisciplinary team for its adequate development considering the operational environment and requirements of a certain application. This review paper focuses on the most promising type of sensors, laboratory made and commercially available, for SHM of aerospace composites. Sensing principles, characteristics, embedding procedures, sensor/ host materials interactions and acquired sensor data/ material behaviour are discussed. The use of sensors for in-situ process monitoring, specifically for curing and mould filling monitoring in liquid composite moulding processes are discussed. General considerations for the development of SHM systems for the aerospace environment are also briefly mentioned.The authors acknowledge the support of the European Regional Development Fund [grant number NORTE-01-0145-FEDER-000015]; and of the European Space Agency through the Network/Partnering Initiative Program

Mechanical, thermal and morphological characteristics of poly(methyl methacrylate) (PMMA) nanocomposites reinforced with Cu-Cr layered double hydroxide

The current work deals with the influence of Cu-Cr layered double hydroxide (LDH) on the properties of PMMA nanocomposites. The Cu-Cr LDH was organically modified with sodium dodecyl sulfate (SDS) in order to obtain LDH layers more compatible with polymer chains. PMMA with various quantities of Cu-Cr LDH nanofiller (1-5 wt%) and 5 wt% of polystyrene-grafted maleic anhydride were first melted and extruded in a twin screw extruder and, finally, processed by injection moulding machine to prepare testing specimens. The nanocomposites were examined by X-Ray diffraction (XRD), transmission electron microscope (TEM) and submitted to thermogravimetric (TGA), tensile and hardness testing. The XRD analysis showed an exfoliated structure of the nanocomposites whereas TEM exhibited the intercalated morphology at higher LDH contents. The TGA results revealed that the thermal degradation temperature was enhanced by the incorporation of Cu-Cr LDH. When 50% of the initial sample weight is established as point of comparison, the decomposition temperature of the nanocomposites was increased about 26 - 32 oC with respect to neat PMMA. From the mechanical analysis, it was also observed that PMMA nanocomposite with 1 wt% LDH presented higher tensile strength and hardness than neat PMMA.Portugal Programa Operacional Regional do Norte de Portugal -NORTE 2020, projeto ref. NORTE-01-0145-FEDER-00015 “TSSiPRO-Technologies for Sustainable and Smart Innovative Products

FBG sensors for process and structural health monitoring of a small type III composite overwrapped pressure vessel for unmanned aerial vehicle

In this work, fibre Bragg grating (FBG) sensors were embedded in a 1-litre type III composite overwrapped pressure vessel (COPV) for an unmanned aerial vehicle (UAV), during its manufacturing for process and structural health monitoring (SHM). A single optical fibre having eight FBG sensors was fixed around an aluminium liner to form a grid network. The carbon fibre/epoxy composite layers were wound on top of it with a winding machine. The FBG sensors were able to follow the developed strain during winding, making it an important quality control tool. The same sensors were used to monitor impact tests, where the distances between real and predicted locations varied between 17 and 56 mm.ANI -H2020, projeto AC

Damage localization on CFRP composites by electrical impedance tomography

This work exploited the use of electrical impedance tomography (EIT) with one-step difference Gaussian-Newton (GN) algorithm to detect different types of damage on unidirectional carbon fibre/epoxy composite laminates. The major challenge concerning the implementation of EIT on composite materials has to do with their anisotropy. To assess this issue, this study was conducted on carbon fibre composites having different layup configurations with different degrees of anisotropy: a quasi-isotropic layup, to approximate as much as possible these layered materials to an isotropic material, and an unbalanced layup, with further degree of anisotropy. Damage detection in the highly anisotropic unbalanced laminates is a major challenge for EIT technique, which has not been assessed before in the literature. Severe damage, in the form of through-thickness holes, was created in the laminates with different diameters and at two locations of the specimen to evaluate the sensitivity of this technique to damage size and its capacity to detect multiple damages. EIT showed progressive decrease of electrical conductivity as the diameter of through-thickness holes increased. Impact damages of different severities were also created. The EIT technique was able to distinguish different damage shapes in the laminates with different anisotropy. EIT identified elongated shaped damages, produced by impact events of different impact energies, on unbalanced laminates. However, the EIT images overestimate the damaged area, as compared to non-destructive ultrasonic inspections. The EIT images of the quasi-isotropic laminates revealed damage in the central area of the specimens, but a well-defined damage shape could not be distinguished.ANI, H2020, Contrato NORTE-01-0145-FEDER-00001

Electrical impedance tomography for damage detection and localization on carbon fibre reinforced polymer composites

Electrical impedance tomography (EIT) is being developed as promising non-intrusive technology for damage detection in conductive fibre reinforced polymers (FRP) composites. This work assessed EIT and one-step difference Gaussian-Newton algorithm to detect different damages in CFRP laminates, including through-thickness holes and impact damage of different severities. Two layup laminates were studied: quasi-isotropic [0/45/90/-45]s and unbalanced [0/0/45/90/-45]s. Each laminate configuration was subjected to three levels of impact energy.Through-thickness holes with diameters as small as 2 mm were detected. The LVI on unbalanced specimens created elongated shaped damages, which were observed through EIT and ultrasonic C-scan. Differently, the ultrasonic C-scan inspections revealed circular shaped damages on the centre of the quasi-isotropic specimens, while EIT could not reveal a well-defined damage shape Yet, the presence of damage was observed in the centre of the specimens by EIT. Although EIT overestimated the damaged area, it was highly sensitive to the imposed damages.The authors would like to acknowledge the support of the European Regional Development Fund [grant number NORTE-01-0145-FEDER-000015]; and of the European Space Agency [Network/Partnering Initiative Program - ESA Contract 4000123315]

Building up of a nested granite intrusion: magnetic fabric, gravity modelling and fluid inclusion planes studies in Santa Eulália Plutonic Complex (Ossa Morena Zone, Portugal)

The Santa Eulália Plutonic Complex (SEPC), located in the Ossa Morena Zone (south Portugal), is composed of a medium- to coarse-grained pink granite (G0-type) and a central grey medium-grained biotite granite (G1-type). Available Rb–Sr data indicates an age of 290 Ma. An emplacement model for the SEPC is proposed, taking into account magnetic fabric, 2D gravity modelling and fluid inclusion planes studies. The G0 and G1 types demonstrate different magnetic behaviour: G0 is considered a magnetite-type granite and G1 is an ilmenite-type granite. The formation of G0 required oxidized conditions related to the interaction of mafic rocks with a felsic magma. The 2D gravity modelling and subvertical magnetic lineations show that the feeder zone of the SEPC is located in the eastern part of the pluton, confirming the role of the Assumar and Messejana Variscan faults in the process of ascent and emplacement. The magma emplacement was controlled by ENE–WSW planar anisotropies related to the final brittle stages of the Variscan Orogeny. The emplacement of the two granites was almost synchronous as shown by their gradational contacts in the field. The magnetic fabric however suggests emplacement of the G0-type first, closely followed by emplacement of the G1-type, pushing the G0 laterally which becomes more anisotropic towards the margin. The G1-type became flattened, acquiring a dome-like structure. The SEPC is a nested pluton with G0-type granite assuming a tabular flat shape and G1-type forming a rooted dome-like structure. After emplacement, SEPC recorded increments of the late Variscan stress field documented by fluid inclusion planes in quartz

The ITRAX core scanner, a useful tool to distinguish anthropic vs. climatic influences in lagoon of Aveiro (N Portugal)

The main goal of this work is to distinguish anthropic and climatic influences in sediments from the lagoon of Aveiro (Portugal). This study is based on a core (240-cm long) collected in Murtosa Channel. Optical and X-radiographic images and high-resolution elemental profiles were acquired with ITRAX micro-X-ray fluorescence (XRF) core scanner. Samples collected at each ≈3 cm along the core were analysed for grain size and total organic carbon. Furthermore, the fine fraction of selected layers was subjected to geochemical analysis by ICP-MS, after total acid digestion of the sediments, and mineralogical analysis, by XRD techniques. A radiocarbon age was determined by AMS, using molluscs shells collected at a depth of 90 cm. Sediments along the core are composed by fine and medium sand, with several mud layers. Sediments composing the first 100-cm may have been deposited after 1950, as it is indicated by the radiocarbon data, the increasing trend of Zn/Al, Pb/Al and Cu/Al and total concentrations of Zn, Pb, Cu, V, Cr, As and Ni in this interval that therefore might be linked with industrial activities of Chemical Complex of Estarreja. The progressive increase of Si/Al, Cl/Al, Rb/Al, K/Al and Br/Al and reduced Al concentrations, from the base to the top of this core, are interpreted as being related to higher marine influence and greater differences in tidal currents with longer exposition to air of the sediments with the consequent formation of brines favouring minerals precipitation in the area (e.g. anhydrite). These results seem to be a consequence of several works developed over time like: i) dredging to improve the navigation access to the harbour, located in the external sector of the lagoon; ii) the control of the course of some rivers influencing the supply of sediments. The tendency of sea level rise may have also emphasized the gradual increase of marine influence in this area. Fine-grained sections, related to an increase in Al, phyllosilicates, organic matter, pyrite and siderite contents would be attributed to phases of greater supply of fine-sediments during heavy rainfall events by the nearby Antuã river and other streams during negative phases of North Atlantic Oscillation. Higher deposition of organic matter enhanced diagenetic changes with pyrite and siderite formation. In the bottom of the core another record of pollution was unveiled to mining activities at the beginning of 20th century.FCT de Portugal - C/CTE/UI4035/2011CNPq - 401803/2010-