Structural Health Evaluation of Composite Materials using Lamb Wave Methods NRO APD BAA for 2002 Kick-off Meeting June 27 th, 2002 Technology Laboratory for Advanced Composites Department of Aeronautics and Astronautics Massachusetts Institute of Technology Composites Engineering? SHM? Mechanical Design http://www.metisdesign.com
BAA Team Members Metis Design Corporation Seth S. Kessler, Ph.D. President Kristin Jugenheimer Chief Engineer Massachusetts Institute of Technology Kim B. Blair, Ph.D. Principal Researcher S. Mark Spearing, Ph.D. Associate Professor Peter W. Young, USAF Col. retired Senior Lecturer NRO BAA Kickoff 2
Background Metis Design Corporation young company of MIT graduates based in Cambridge, MA originally HDAS, split into 2 to consolidate resources (defense related) MDC focus areas introduction of advanced composite materials into nontraditional areas development of custom structural health monitoring systems design, modeling and analysis of complex mechanisms Engineering tools Computer Aided Design: SolidWorks and Pro-E Finite Element Modeling: NASTRAN and ABAQUS customized codes: MATLAB, Mathematica, FORTRAN, C++ NRO BAA Kickoff 3
Current Contracts Design, CAD and FEA work for civil utility companies (electrical & phone) Design, CAD and FEA work for companies in shipping FEA and dynamic and failure analysis for cryogenic structures Composite design for automotive applications Custom SHM solutions for Boeing (work just beginning) NRO BAA Kickoff 4
MIT - TELAC Areas of Research failure mechanics and damage resistance longevity and durability, environmental degradation design of thermal structures, multi-functional layered materials, adhesive joints and repair methodologies active control of composite structures damage detection and SHM by various methods Personnel 4 Faculty 11 Graduate and 8 undergraduate students Funding FY 2000, Funding at $1.2 Million NASA, Boeing, Rockwell, Sikorsky, NSF 20? m Hackles NRO BAA Kickoff 5
Facilities Manufacturing 3 foot diameter by 5 foot length autoclave Post-cure oven Layup room with 4 material freezers Composite machine shop Testing Four servohydraulic load frames Computer-based data acquisition equipment Blast chamber High temperature and humidity chamber Cryogenic chamber Optical and digital microscopes X-Ray device Photoelasticity equipment High performance computing NRO BAA Kickoff 6
SHM Motivations Structural Health Monitoring (SHM) denotes a system with the ability to detect and interpret adverse changes in a structure in order to reduce life-cycle costs and improve reliability Applicable to any field highest payoff in air/spacecraft Inspection and maintenance expenses could be reduced by SHM use CBM over damage tolerant design reduce weight up to 25% eliminate scheduled inspections and operational down-time improve efficiency and accuracy of maintenance Reliability of damage detection and failure prediction increased retrofit SHM systems into existing systems to monitor damage growth integrate SHM networks into new systems to guide inspections and dictate maintenance and repair based upon need most inspection is currently visible, forms of damage can be overlooked NRO BAA Kickoff 7
Lamb Wave Methods Form of elastic perturbation that propagates in a solid medium actuation parameters determined from governing equations excite A o wave for long travel distances and to minimize clutter Damage can be identified in several ways group velocity approximately? (E/?) 1/2, damage slows down waves reflected wave from damage can be used to determine locations Research uses piezoelectric sensors to detect energy present in transmitted waves, and reflections using self-sensing actuators NRO BAA Kickoff 8
Previous Research: Building Block Approach Narrow coupon laminates same specimen used for FRM several types of damage Narrow sandwich beams various types of cores tested disbonds between laminate and core 25 cm Stiffened plate various types of bonded ribs disbonds between laminate and rib 25 cm Composite sandwich cylinder 0.4m diameter cylinder with core low velocity impacted region 1 m 2 cm NRO BAA Kickoff 9
Previous Research: Parameter Optimization Actuation parameters determined from governing equations from material properties dispersion curves are calculated from group velocity dispersion curve, operating frequency selected from operating wavelength, actuator size is selected number of pulses to be sent determined by distance between features Excite A o wave for long travel distances and to minimize clutter Experimental procedure for present work used these equations frequencies between 15-50 khz utilizes 3.5 sine waves under a Hanning window Piezoceramic Sensors Piezoceramic Actuator Sent Signal NRO BAA Kickoff 10
Previous Research: Thin Laminate TOF Results Specimen labeled on plot Superimposed control specimen Time-trace of voltage signal from PZT sensor 20 cm from actuator driving at 15 khz High degree of consistency between all control traces All damaged traces show a delay in time of arrival, and smaller amplitude responses Since these are short specimens, many reflections combine quickly While TOF is easily reproduced, difficult to measure accurately NRO BAA Kickoff 11
Previous Research: Thin Laminate Wavelet Results Wavelet decomposition performed using Morlet signal, similar to FFT Compare received signal s energy content at dominant frequency Control specimen clearly has the most energy transmitted Appears that as damage becomes more severe, more energy is lost Differences seem obvious enough for process to be automated Still not much information about damage type and location Demonstrates ability to detect presence of damage and judge extent NRO BAA Kickoff 12
Previous Research: Finite Element Results Figure on left shows FEA results for coupon without damage Figure on right shows FEA results for coupon with 25 mm disbond Movie files show z-displacement at 100 microsecond intervals Can use to measure time-of-flight and observe reflections NRO BAA Kickoff 13
Previous Research: Advanced Results Wavelet coefficient plot for beam blind test compares energy content for 50 khz Three control specimens with Al core, one has an unknown delamination Compared to a damaged specimen Top two clearly have more energy Bottom two with little energy present are debonded specimens Two composite plates with stiffening ribs compared, one with disbond Disbond yields fringe pattern in both reflected and transmitted wave Indicates viability of wavelet method for use in at least simple structures NRO BAA Kickoff 14
Strengths Lamb Wave Method Conclusions shows great sensitivity to local presence of many types of damage potential for damage location calculation with self-sensing actuators Limitations method must be tailored for particular material and application patch size and location depends upon material, thickness, curvature complex results by comparison to other methods results may be localized to straight line paths SHM implementation potential can integrate and compare transmitted and reflected energy groups of sensors to be placed in areas of concern for triangulation NRO BAA Kickoff 15
Current Program Goals Motivations for SHM within NRO OSL hidden damage possible due to handling, overloading or dropped tools detect and interpret damage in composite primary spacecraft structure, pressure vessels, thin-wall shells, tubes, and support structures challenging to make quantitative measurements due to lack of access SHM systems requirements for space launch detect/map extent of damage in manufacturing and integration facilities, or on the launch pad retrofitable, generic patches to sense state of structure facilitate launch/no-launch decisions low cost, simple to interpret, automated Three areas of research sensor development (MIT led) testing (joint venture) analysis and system design (MDC led) NRO BAA Kickoff 16
Work Plan Design Initiative Determine exact inspection requirements vehicles and critical components of interest material systems (metals, composites, or combinations) characteristic flaw type and size, damage threshold flight loads and other environmental factors Using requirements and collection of equations calculate actuator dimensions build optimal waveform for actuation determine characteristic detection range for sensor spacing NRO BAA Kickoff 17
Work Plan Sensors Initiative Actuator/sensor materials materials (PZT vs PVDF, etc.) backing material selection robust packaging Electronics self-sensing actuators phased/guided/interdigitated electrodes robust wiring NRO BAA Kickoff 18
Work Plan Testing Initiative Specimen types narrow composite coupons bonded to metal flat plates bonded to metal curved plates bonded to metal (both large and small radius) COPV (or other NRO desired target application) Tests transmission through damage regions reflection with self-sensing actuators triangulation to determine damage location NRO BAA Kickoff 19
Work Plan Analysis Initiative Filtering of data gathered during previous research further wavelet decomposition work sideband frequency research differentiation between damage types Same procedure for newly collected data determine best filter to resolve presence of damage differentiation between damage types Automation software Design analysis suitable for launch vehicles NRO BAA Kickoff 20
Gantt Chart Design Initiative Requirements Calculations Sensors Initiative Materials Backing Packaging Self-sense Electrodes Wiring Testing Initiative Narrow Plate Curved COPV Triangulation Analysis Initiative Old data New data Automation Software Kick-off Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Mid-term Final review NRO BAA Kickoff 21