18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS MULTI-PHYSICS NANO-ENGINEERED STRUCTURAL DAMAGE DETECTION AND DE-ICING R. Guzman de Villoria 1 *, S.S. Kessler 2 , S. Wicks 1 , A. Miravete 1 , B.L. Wardle 1 1 Department of Aeronautics and Astronautics, Massachusetts Institute of Technology, Cambridge, MA, USA, 2 Metis Design Corporation, Cambridge, MA, USA * Corresponding author (rguzman@mit.edu) Keywords : Hierarchical Structures, non destructive evaluation, de-icing, carbon nanotube 1 General Introduction alumina fiber volume fraction, and ~ 2% CNT volume fraction. [5] . Catastrophic structural failures are the cause of many physical and personal losses, at a worldwide 2.3 Non-Destructive Evaluation by Infrared cost estimated at billions of dollars per year. Non- Camera destructive evaluation (NDE) techniques have been Damaged specimens were heated via electrical pursued and employed for damage detection of current (Joule-effect heating). Temperature was structures to detect cracks and other damage at pre- recorded using an infrared (IR) camera at 160x120 critical levels for remediation [1-3]. To address pixels, with a temperature range of -10 to 250°C and drawbacks with state-of-the-art approaches, a novel a resolution of 0.15 o C. In order to apply the current multi-physics approach is reported that takes to the sample, copper adhesive tape (Compac advantage of the effects that damage has on the Corporation) was placed as electrodes on both ends electrical and thermal transport in a material of the sample (dimensions ≈90x25x2.2 mm 3 ), and containing aligned carbon nanotubes (CNTs) to connected to a nine volts alkaline battery via create a new damage detection technique. Another alligator tips. Thermal images were taken with a application of the same nano-engineered composites thermal camera (PCE-TC 3, PCE Group) 30 seconds is in thermal applications such as de-icing and anti- after applying the voltage to allow temperature icing systems. Icing is a serious problem that has readings to stabilize [7]. caused several aircraft incidents associated with temperatures ranging between -40 ºC to 0 ºC. 2.4 Non-Destructive Evaluation Using a Although some technologies have been developed, Thermochromic Ink improved solutions are desirable in order to obtain Two silver contacts were painted on both ends of the lighter and more efficient technologies [4]. specimen and two alligator clips were attached to the 2. Experimental specimen. A ~4mm diameter and ~3 mm deep groove was drilled carefully in one side of the 2.1 Aligned CNT Synthesis specimen, avoiding damage to the other side, which Aligned CNTs were grown on the surface of was coated with thermochromic ink (Chiral nematic alumina fibers (11 m diameter) contained in tows sprayable liquid crystal, thermal range 25°C - 30°C that were pre-woven into a cloth. Details of the ThermometerSite) using an airbrush. A nine volt process can be found in preceding work [5]. battery was connected to the alligator clips and the Aligned ~30 m long multi-walled CNTs were undamaged site of the specimen was filmed with a grown uniformly on fiber surfaces in a “Mohawk” conventional USB camera. morphology [6]. 2.5 Direct De-icing Experiment 2.2 Composite Fabrication For the de-icing test, two silver contacts were Composite specimens were made by hand lay-up painted on both ends of the specimen and two using a commercial epoxy system (Resin 105 and alligator clips were attached to the specimen. The Hardener 206, West Systems Epoxy) to create 3-ply specimen was buried under a layer of snow 2.5 cm laminates. The resulting composites have ~50% thick. Thermal and optical images were taken every
five minutes during the voltage application (0.74A, amplified because the heat flow was impeded in 30V) for half an hour at an ambient temperature of - areas of damage. These changes of temperature can 10 o C. be locally visualized through a conventional IR thermal camera. Low-power operation (a nine volts 2. 6 Ice-detection and De-Icing/Anti-Icing System standard battery is exemplary, providing a 15 ºC rise Once the feasibility of using nano-enginered at 1 Watt) and high spatial resolution was composites for direct de-icing applications has been demonstrated that is beyond state-of-the-art in non- demonstrated via Joule heating, an ice-detection destructive evaluation [7]. system was developed for controlled de-icing and Multiple applications have been identified and anti-icing applications. Ice-detection here is based investigated using this Nano-Engineered Thermal on effective heat capacity, where power is applied to NDE (NET-NDE) method. In typical engineering a 115×25×2 mm 3 nano-engineered specimen for structures where composite components are joined seconds, and the slope of the temperature rise is by metallic rivets or bolts, cracks and other defects shown to be well correlated to the thickness of ice may form during the structure’s operational life in present. Silver epoxy vertical electrodes (32) on the the vicinity of the attachment. Inspecting such a top face and horizontal electrodes (8) on the back region (5.06 mm diameter hole) using the NET-NDE face were printed and connected to a DC power directly with two electrode tips that apply 12.3 V on supply (see Figure 1). Two thermocouples were the surface of the composite component, allows mounted, one on the center of the specimen and the damaged areas to be identified and later verified by other ~15 cm to the left of the center of the sample. careful visual inspection. A significant drawback of Different amounts of water were poured on top of applying advanced composites in aerospace the sample, cooling it down to -15 ºC to cause icing. components is detecting internal damage, as A vacuum tape frame was assembled around the exemplified by “barely visible impact damage” specimen to prevent water leaks [8]. (BVID) that is caused when the composite structure is damaged internally with little or no visual evidence on the external (visible) surface. Impact damage has been detected by applying a 0.11 A, 0.34V between two surface electrodes in prior work [7]. Fig. 1. Ice-detection sample. The electrodes (perpendicular white lines) were printed by direct writing. 3 Results and Discussion Fig.2. Power and resolution characteristics of nano- engineered non-destructive evaluation (NDE) 3.1 Non Destructive Evaluation by Infrared technique: (a) optical image and (b) thermograph of Camera a composite specimen powered by a 9V battery. When a potential difference was applied to an 3.2 Non Destructive Evaluation Using a impact damaged nano-engineered composite (Figure Thermochromic Ink 2), electric field lines concentrate in the vicinity of cracks as electrons flow around the damage, causing Thermochromic inks are made of substances, in our “hot spots” via Joule heating. This effect was case liquid crystals, that display different colors at
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