Monitoring of the curing process of composite structures by tunnelling junction sensors
Introduction
The quality control of long-fibre epoxy–carbon composite structures represents a technical and economical challenge for many industries such as aeronautics, aerospace and fluids transport. Because these structures are made up by superposing numerous layers with different fibre orientations, one upon each other; they allow placing measurement devices, such as thermocouples or strain gauges between their layers. This monitoring technique provides survey of composite’s properties throughout the manufacturing process and it also can give indications of the mechanical and thermal behaviour during service conditions. This practice is known as in-core instrumentation, and many studies have underlined its value added to survey the through-the-thickness properties of composite materials [1], [2], [3], [4]. However, composites in-core instrumentation still exhibits difficulties which retard its fulltime application in the industry. Sensor’s size, autonomy and wire connections are the major issues to deal with.
Several researchers have shown their interest on the thermal phenomena which occur during composites curing [5], [6], [7]. The autoclave process is widely used to manufacture high performance composite structures in aeronautics. The quality of the composite parts is obtained thanks to the association of three curing parameters which are temperature, pressure and vacuum. Each one of them has an important influence on the initial properties of the material, such as fibre volume ratio, resin volume ratio and porosity density. These initial properties will also have a major impact on the final mechanical properties of the composite.
In the case of thermoset composite systems, the matrix reticulated network structure (MRN) is built during polymerization when heating activates chemical reactions responsible of curing and solidification of the matrix. The MRN provides cross-links that bond one polymer chain to another, making the material more rigid under loading. The expansion phenomena which accompany the curing impose compressive stresses on reinforcement fibres and produce internal stresses in the composite material. Therefore, the heating rate and the curing temperature are two of the main parameters to survey inside the autoclave [8], [9]. Thermocouples are widely used to track the temperature inside the autoclave chamber and they are also employed to know the surface temperature of the composite structure. However, these devices are not quite adapted to measure the in-core temperature of the composite, because its larger thickness (at least 500 μm) compared to the composite’s plies (about 250 μm). In addition, the difference between the coefficient of thermal expansion (CTE) of the metallic wire of the thermocouple and the carbon fibres of the composite could cause additional residual stresses which make them not suitable for this task.
With the aim of improving these drawbacks of composite instrumentation, research has been conducted to develop multi-parameters devices, which could detect many autoclave variables, including temperature. One type of these new devices, which shows strong potential to monitor thermal phenomena, is the “tunnelling junction sensor (TJS)” [10], [11], [12] which takes advantage of the tunnelling effect and its dependency on temperature. By polarizing the TJS with an electrical current, this device returns a voltage that depends on the temperature of the surrounding environment. Furthermore, the TJS dimensions, typically less than 300 μm of thickness, make them better suited to embed inside composite materials with a minimum layout alteration.
In this paper, the goal is to have a deeper comprehension of the capabilities of the TJS to identify temperature variations inside composite structures. First, a general explanation of the TJS functioning principle is provided. Then, the TJS manufacturing procedure is described. After that, the embedding procedure of a TJS inside a composite plate is cited; with the aim of testing the TJS in a composite’s natural curing conditions. Finally, the TJS sensitivity to thermal changes is obtained, proving them comparable to common thermocouples in composite monitoring applications.
Section snippets
Tunnelling junction sensors (TJS)
Tunnelling junction sensors (TJS) are electronic devices which take advantage of inverse polarization of Zener P–N junctions. During the last decade, the LAAS has tested the TJS on micro-fluids applications and as thermal micro-actuators [10], [11]. These sensors have been developed in order to measure different variables such as temperature, strain and pressure with the aim to create a multi-physics device. The TJS consist of a silicon polycrystalline prism doped with phosphorus and boron by
Manufacturing of tunnelling junction sensor
Polycrystalline silicon has been chosen for TJS due to its good dielectric properties. First, a polycrystalline silicon layer of 0.5 μm thickness is deposited on a glass substrate placed by Low Pressure Chemical Vapour Deposition (LPCVD) at 650 °C for 60 min (Fig. 3a). The P+/N− junctions are formed in two steps. The Si is P+ doped with boron (Fig. 3b), then the band design is made by photolithography with a photosensitive resin placed over a SiO2 mask of 0.25 μm of thickness and the thermal
Electrical and thermal response of tunnelling junction sensors
To stimulate the P+/N− junctions, the TJS are polarized continuously; the TJS receives a range of electrical currents and returns a range of electrical voltages. The research conducted by the LAAS has shown that the sensitivity of the TSE is higher when polarizing the TJS with current, instead with voltage [10], [11], [12]. Electrical characterization of a tunnelling junction sensor, with two Symmetric Thresholds Elements (STE) in series connection, shows three functional regimes (Fig. 4).
For a
Monitoring of curing process of a composite plate by TJS
The wide use of composite materials on aircraft structures makes the monitoring of the curing process a key point on the manufacturing process. Most of the composites manufacturing process, such as vacuum bagging in autoclave, resin moulding transfer (RTM) or sheet moulding compound (SMC), point to temperature, as one of the main variables to survey in the curing process [16], [17], [18]. As cited before, temperature is responsible for activating the chemical reactions of curing and
Conclusion
The scope of this article is focused on the ability of tunnelling junction sensors (TJS) to detect temperature changes. The goal is to propose these sensors for monitoring temperature during the curing cycle of composite structures.
The TJS manufacture process is optimized in order to obtain several sensors from a single one glass wafer, improving cost effectiveness. Additionally, due to its dimensions and materials, TJS are better suited to imbedding inside composite materials than classic
Acknowledgments
The present work is part of the research project “Multi-sensor Instrumentation for Composite Materials and Structures (I2MC)” financially supported by the Thematic Advanced Research Network for Aeronautic and Space Sciences & Technologies of Toulouse (RTRA STAE). The I2MC project combines the expertise of seven research teams with the goal to study the in-core instrumentation of composites structures by applying the Multi-Instrumented Technological Evaluator toolbox (MITE toolbox) [23].
M.
M. Torres received his Ph.D degree in 2012 in Mechanical Engineering from the University of Toulouse, France. He is CONACYT Research Fellow at the Engineering and Industrial Development Center (CIDESI) in Mexico. His research is based on multi-instrumentation of composite structures for aeronautics, automotive and oil transportation.
References (28)
- et al.
Entire life time monitoring of filament wound composite cylinders using Bragg grating sensors: II. process monitoring
Appl. Compos. Mater.
(2009) - et al.
Through-the-thickness material properties identification in a technological specimen using 3D-DIC and embedded FBG measurements
Exp. Mech.
(2008) - et al.
Development of life extension strategies for Australian military aircrafts using structural health monitoring of composite repair joints
Compos. Struct.
(2004) - et al.
Disbond monitoring in adhesive joints using shear stress optical fiber sensors
Smart Mater. Struct.
(2014) - et al.
Study on the curing process for carbon/epoxy composites to reduce thermal residual stress
Compos. Part A: Appl. Sci. Manuf.
(2012) - et al.
Correlation between physical properties, microstructure and thermo-mechanical behavior of PPS-based composites processed by stamping
J. Reinf. Plast. Compos.
(2014) - et al.
Thermal expansion of carbon–epoxy laminates measured with embedded FBGS—comparison with other experimental techniques and numerical simulation
Compos. Part A
(2007) - et al.
Residual stress build-up in thermoset films cured below their ultimate glass transition temperature
Polymer
(1997) A note upon the development of residual curing strains in carbon/epoxy laminates: study by thermomechanical analysis
Compos. Part A
(2006)- et al.
Development of polysilicon devices for microfluidic thermal instrumentation
Sensors Actuators A: Phys.
(2013)
New polysilicon sensor and actuator technology for the development of a thermal platform
Proceedings of the Solid-State Sensors, Actuators and Microsystems International Conference
Déploiement d'un réseau de capteurs enfouis dans des multi-plis carbone-époxy pour l’instrumentation in-situ de structures composites pour l’avionique
PhD Thesis (in French). Laboratory for Analysis and Architecture of Systems (LAAS)
Solution générique pour l’adressage matriciel de micro-actionneurs thermiques et optimisation de micro-sources thermiques
PhD Thesis (in French). Laboratory for Analysis and Architecture of Systems (LAAS)
Conception, réalisation et mise en œuvre d'une plateforme d'instrumentation thermique pour des applications microfluidiques
PhD Thesis (in French). Laboratory for Analysis and Architecture of Systems (LAAS)
Cited by (0)
M. Torres received his Ph.D degree in 2012 in Mechanical Engineering from the University of Toulouse, France. He is CONACYT Research Fellow at the Engineering and Industrial Development Center (CIDESI) in Mexico. His research is based on multi-instrumentation of composite structures for aeronautics, automotive and oil transportation.
F. Collombet received his Ph.D. degree in Civil Engineering from the École Nationale Supérieur—Cachan in Paris, France. He is Tenure Professor at Institut Clemet Ader of University of Toulouse. He is the author of the Multi-Instrumented Technological Evaluator toolbox (MITE toolbox) conceived for structural health monitoring (SHM) of composite structures.
B. Douchin received his Ph.D. degree in 2000 in Mechanical Engineering from the École Nationale Supérieur—Cachan in Paris, France. He is Associate Professor at the University of Toulouse. His research concerns filament winding structures for oil transportation, as well as, numerical simulation of composite structures by finite element method (FEM).
L. Crouzeix received his Ph.D degree in 2008 in Mechanical Engineering from the University of Toulouse, France. He is Associate Professor at the University of Toulouse. His research concerns numerical simulation of composite structures by finite element method (FEM) and calculation of mechanical properties of composite materials by Digital Image Correlation (DIC).
Y-H. Grunevald is the founder and CEO of Composite, Expertise and Solutions (CES). His company works on design, fabrication and characterization of composite structures for aircraft structures, automotive and oil transportation. He, in collaboration with Pr. F. Collombet, has been promoted over the last decade the Multi-Instrumented Technological Evaluator toolbox (MITE toolbox) for structural health monitoring (SHM) of composite structures.
J. Lubin received his Ph.D in Microelectronics in 2012 from University of Toulouse, France. His research concerns health monitoring of carbon composite structures using Tunnelling Junction Sensors.
T. Camps received his Ph.D. degree in 1991 in Microelectronics from the University of Toulouse, France. He is Tenure Professor at LAAS-CNRS. He has worked on GaAs HF power HBT’s. His research concerns design, fabrication and characterization of microsystems for microfluidic instrumentation and also the development of new solutions for structural health monitoring (SHM) both for avionics structures, bridges or buildings.