Fabrication of a Multi-Physics Integral Structural

Transcription

1 Fabrication of a Multi-Physics Integral Structural Diagnostic System Utilizing Nano-Engineered Materials Seth Kessler, Ajay Raghavan and Christopher Dunn Metis Design Corporation Sunny Wicks, Roberto Guzman devilloria and Brian Wardle Massachusetts Institute of Technology 10 Canal Park Cambridge, MA

2 Advanced Composites & CNTs Carbon Nano-Tube (CNT) laminates are a natural progression for aerospace composites due to their superior specific strength & stiffness 2010 Metis Design Corporation 2010 PHM Society Conference 2

3 Present CNT Challenges Several issues have hindered effectiveness of CNT engineering dispersion: uniform distribution of CNTs in epoxy matrix for homogeneity adhesion: CNT-matrix interfacial bonding for effective load transfer orientation: non-random alignment of CNTs to amplify properties extension: building macroscopic structures by linking individual CNTs Most investigators have focused on addressing 1-2 of these issues individually, id yielding marginal improvements Simultaneously achieving all 4 necessary to achieve true potential of CNTs for enhanced composite structures 2010 Metis Design Corporation 2010 PHM Society Conference 3

4 Aligned MIT 500 µm 100 µm 1 µm MIT has developed a novel patented CNT fabrication process CNTs grown radially aligned on fibers or on substrate to be transferred good alignment, dispersion, adhesion & yields high CNT volume fraction Atmospheric pressure chemical vapor deposition (CVD) self-aligned morphology /cm 2 of continuous CNTs (7-10 nm OD) rapid forest growth of > 2 microns/second (up to 5 mm long) 2010 Metis Design Corporation 2010 PHM Society Conference 4

5 MIT Fuzzy Fiber Laminates 2010 Metis Design Corporation 2010 PHM Society Conference 5

6 Motivation For CNT-based SHM SHM improves reliability, safety & reduced costs sensors add weight, power consumption & computational bandwidth cables add weight, complexity, as well as durability & EMI concerns scaling SHM for large-area coverage has presented challenges Advantages of proposed CNT-based SHM methodology sensing elements actually improve specific strength/stiffness of structure conformal direct-write (DW) electrodes lighter & more durable than cable simple to scale over large structure, maintains good local resolution Traditional SHM CNT-based SHM 2010 Metis Design Corporation 2010 PHM Society Conference 6

7 Direct-Write Technology Subject of Phase I NASA SBIR research, (PHM '09 paper) Direct-Write (DW) technology can be used to replace traditional cables silver inks & copper sintering patterned on structure for conductors polymers & ceramic deposited on structure for insulators TRL for DW technologies is at least 8 DW implemented by Boeing on commercial aircraft production line successful durability/longevity y testing, FAA approved technology 75 cm SHM nodes 5cm CAN/power link 2010 Metis Design Corporation 2010 PHM Society Conference 7

8 CNT-Enhanced Composites for SHM Aligned CNTs greatly enhance composite laminate properties mechanically improve impact, delamination & fatigue resistance multifunctional capabilities introduced by conductivity & piezoresistivity Present research exploits CNTs for in-situ damage imaging direct-write (DW) electrode grids applied similar to LCD technology in-plane & through-thickness resistance measurements collected damage breaks CNT links around affected zone, increases resistivity surface & sub-surface damage images produced in post-processing Multiplexing micro-switch Positive electrode columns Ground electrode rows CNTenhanced composite laminate 2010 Metis Design Corporation 2010 PHM Society Conference 8

9 Proof of Concept (POC) Approach FFRP laminate fabricated alumina fiber satin-weave cloth dipped in solution of 50 mm iron nitrate catalyst coats fibers with CNT seeds inside tows of ply CNTs grow radially aligned μm with modified thermal CVD method 2 plies stacked by hand layup West Systems Epoxy system infused for 12-hour RT cure ~50% fiber volume fraction & ~ 2% CNT (115 x 25 x 2 mm) Silver-ink electrodes applied w/masked silk-screening process 14 parallel conductors patterned in short-dimension on top plate surface 4 parallel conductors patterned in long dimension on opposite surface traces were all 1.5 mm wide and spaced 3 mm edge-to-edge 2010 Metis Design Corporation 2010 PHM Society Conference 9

10 POC Experimental Procedure Electrical resistance measured by multimeter initial data showed significant variability due to contact resistance subsequently thick wires were bonded to traces using silver epoxy resulted in < 1% change (< 0.1 Ω for ~10 Ω trace) across 10 repeat trials Centered impacted with 13 mm diameter steel ball at 75 ft-lbs calibrated to just initiate surface micro-cracking with trial impacts no damage visible to unaided eye, but microscopy revealed cracking post-damage resistance measured between pairs & through thickness 3 mm 3 mm 2010 Metis Design Corporation 2010 PHM Society Conference 10

11 POC Results In-plane resistance between parallel pairs of adjacent traces short traces averaged 9 Ω resistance pre-impact > 100% change in middle traces, < 10% change for outermost trace pairs minor changes < 1% measured between long trace pairs appears to be more sensitive to surface cracking Through-thickness resistance at each virtual grid point 56 through-thickness grid points averaged 20 Ω resistance pre-impact > 100% change in middle points, < 10% change for left half points resistance offset introduced by cracks across traces on right half points appears to be more sensitive to delamination pp In-Plane Resistance Change Through-Thickness Resistance Change 2010 Metis Design Corporation 2010 PHM Society Conference 11

12 Further Validation Research Method validation with more comprehensive test matrix 3 FFRP specimens (115 x 25 x 2 mm) observe effects of multiple progressive impact events ft-lbs Process improvements refined silk-screening screening process to reduce trace resistance & variability flexible circuit frame for "overwrite" connection with DW traces 8 x 32 traces 1.5 mm wide, all traces spaced by 1.5 mm (4x resolution) 2010 Metis Design Corporation 2010 PHM Society Conference 12

13 Multiplexing Hardware Original data collected by hand with multimeter some variability introduced by manual connections & settling times fewer trace combinations measured because of testing time ( ½ day) New multiplexing hardware designed dedicated to this research connects directly to flex-frame via FFC cable, PC via RS-232 dual multiplexer banks to select 2 traces for measurement constant current applied through 2 traces, measure 16-bit voltage resistance directly related to V/I Allows many more combinations to be measured quickly 1140 total measurements (including both sides of traces) test t time dependant d on determined d settling time (1 second to 1 hour) 2010 Metis Design Corporation 2010 PHM Society Conference 13

14 In-plane Resistance Changes No visible damage was present in any of these cases nearly linear increase in % resistance change with impact energy < 1% change in resistance away from impact zone Appeared relatively localized to the actual impacted region 15 ft-lbs impact caused ~10-20% changes 30 ft-lbs impact caused ~20-30% changes 45 ft-lbs impact caused ~40-60% changes 2010 Metis Design Corporation 2010 PHM Society Conference 14

15 Through-Thickness Changes Same trends observed ed for in-plane vs through-thickness thickness results witness specimen testing indicated complete fracture at ft-lbs no visible micro-cracking due to decrease in void fraction (2% to ~0.5%) Appeared to effect width in impact region relatively uniformly 15 ft-lbs impact caused ~2-4% changes 30 ft-lbs impact caused ~4-8% changes 45 ft-lbs impact caused ~8-10% changes 2010 Metis Design Corporation 2010 PHM Society Conference 15

16 Alternative Approaches Alternative dynamic piezoresistivity schemes being explored acoustic emission (AE) measuring momentary resistance changes enhanced penetrating thermographic NDE with applied voltage Same hardware & flex frame can be used to measure dynamic resistance changes or self-induce heating initial pencil-tap experiments verifies that AE can be detected initial thermographs demonstrate method feasibility Impact between DW electrodes indicated 100 mm P R O P R I E T A R Y No impact indicated heating away from site 100 mm 2010 Metis Design Corporation 2010 PHM Society Conference 16

17 Future System Vision Sensing system integrated into vehicle CNTs introduced at ply level to be included in laminate manufacturing flex-frames built in standard sizes, installed at component level automated DW methods apply electrodes over flex-frames local hardware to control & digitize data (leverage existing design) Multi-tiered embedded SHM/NDE system AE event detected by real-time monitoring of sparse DW electrodes resistance measurements narrows down affected region CNT-enhanced thermography used for detailed NDE imaging on ground 6 mm 256 MUX channels 50 MHz DAQ 40 mm 2010 Metis Design Corporation 2010 PHM Society Conference 17

18 Phase I Summary CNT-enhanced laminates were fabricated, patterned with silver ink electrode grid, & subsequently subjected to impact damage in-plane & through-thickness electrical resistance were collected little to no change in values at points away from the damage zone clear changes were observed in both sets of data close to impacted zone high-resolution full-field representation of sub-surface damage presented Demonstrates t the potential ti of using this approach as an SHM solution, with added benefit of CNTs reinforcing the structure proof-of-concepts p for alternative SHM/NDE methods demonstrated Phase I results provide an excellent foundation for Phase II research 2010 Metis Design Corporation 2010 PHM Society Conference 18

19 Phase II Tasks 1 & 2 Optimization optimize method developed in Phase I prior to expanding technology find trade between detection resolution & electrode spacing uncover any limitations of this method alternative approaches to networking & multiplexing to provide best detection capability over largest coverage area with least system impact Maturation mature optimized method by introducing non-ideal factors real aerospace structures are exposed to temperature, humidity & loads experimental data will be collected for a range of realistic conditions develop compensation algorithms to provide meaningful damage characterization results without false positives in true environments 2010 Metis Design Corporation 2010 PHM Society Conference 19

20 Phase II Tasks 3 & 4 Integration integrate additional multifunctional capabilities into the system develop unified infrastructure to facilitate resistance measurements, acoustic emission & provide enhanced thermographic NDE imaging single multi-physics system will be demonstrated that utilizes CNTs to reliably & accurately detect, localize & characterize damage in composite Expansion expand developments of prior tasks to be demonstrated at a larger scale practical system will need broader coverage with comparable capability demonstrate technology using a larger panel size (1/2 m), CFRP demonstrate in representative built-up configuration (integrally stiffened) 2010 Metis Design Corporation 2010 PHM Society Conference 20

21 CNT-Enhanced Composites for Ice Goal to develop a multi-role system for composite rotor blades detection of presence of ice on blades removal of ice from blades and/or prevention of ice (re)formation detection/characterization of structural damage on blades Current solutions provides high false positive & failure rates Novel technologies combine to produce robust/reliable solution carbon nanotubes (CNTs) direct write (DW) micro-instrumentation instrumentation NAVAIR funded SBIR BAMS H-53K 2010 Metis Design Corporation 2010 PHM Society Conference 21

22 Proposed Smart-Blade System 3 main system elements CNTs embedded in composite blades interdigitated DW electrodes & thermocouples thin electrically insulating abrasion layer System benefits heating elements are structural sensing elements are structural no wires required conformal (light & low profile) uniform surface coverage very efficient (conduction only loss) closed-loop feedback possible can improve blade impact resistance solid state (no moving parts) Direct write electrodes Electrically insulating layer over electrodes CNT composite 2010 Metis Design Corporation 2010 PHM Society Conference 22

23 De/Anti-Icing Demonstration ~1 kw/m 2 ~5 kw/m Metis Design Corporation 2010 PHM Society Conference 23

24 Ice-Detection Formulation Points From 15C Slope w/max & min bars No Water 1 mm Water 2 mm Water 3 mm Water Effective heat capacity method constant current applied to sample for fixed temperature recording time exponential temperature rise (linear data fit), compared to no-ice case Very repeatable results shallower slope correlates to more ice on specimen data consistent for multiple detection temperatures 2010 Metis Design Corporation 2010 PHM Society Conference 24

25 Acknowledgments This research was sponsored by SBIR/STTR funding AFOSR FA C-0165 SHM of CNT-Enhanced Composites NAVAIR N C-0227 Composite Self-Monitoring Anti/De-icing NASA NNX09CC57P Cable-Free Sensor-Bus for SHM of Composites FFRP composites developed by MIT s NECST Consortium Nano-Engineered Composite aerospace STructures consortium supported by large aerospace OEMs & composite fabricators Airbus, Boeing, Embraer, Lockheed, Saab, Spirit AeroSystems, Textron, Composite Systems Technology & TohoTenax 2010 Metis Design Corporation 2010 PHM Society Conference 25