Large Scale Monitoring of CFRP Structures by ...

Large Scale Monitoring of CFRP Structures by Acousto-Ultrasonics—A Flight Test Experience B. ECKSTEIN, M. BACH, C. BOCKENHEIMER, C. CHEUNG, H. CHUNG, D. ZHANG and F. LI

ABSTRACT This paper outlines an Acousto-Ultrasonic Structural Health Monitoring system for flight test purpose, dedicated to damage assessment in large scale areas of structures made of Carbon Fiber Reinforced Plastics. In the second part an overview on a long-term flight test is given and results with special regard to the operational and environmental conditions are presented. BACKGROUND The increasing usage of Carbon Fiber Reinforced Plastics (CFRP) for primary aerospace structures, such as on the A350XWB or the B787, involves to handle the principal susceptibility of composite laminates to impact loads as well as the occurrence of barely visible impact damages. As one consequence, damage tolerance design usually implies that any visible indication of impact damage needs to be assessed by NDI. Increasing the structural robustness in order to decrease maintenance costs and service interruptions, will in turn increase the weight of the structure as well. In this area of conflict, Structural Health Monitoring as an enhancement of conventional NDI offers the potential to decrease the related maintenance cost and increase the availability of the aircraft. The monitoring of areas prone to accidental damages like delamination and stringer debonding involves medium to large areas of several square meters. Especially door surrounding areas are most susceptible to low-velocity impacts by ground vehicles servicing of the aircraft during turn-around. In terms of Guided Ultrasonic Waves propagation these structures have to be regarded as complex. Acousto-Ultrasonic is a well suited technology for this scenario due to its areal coverage and with regard to the required damage assessment performance. Benjamin Eckstein, Martin Bach, EADS Innovation W orks, Airbus-Allee 1, 28199 Bremen, Germany Clemens Bockenheimer, Airbus Operations GmbH, 316 Route de Bayonne, 31060 Toulouse Cedex 9, France Cas Cheung, Howard Chung, David Zhang, Franklin Li, Acellent Technologies Inc, 835 Stewart Dr, Sunnyvale, CA, 94085, U.S.A.

Airbus and Acellent Technologies are conducting a joint development since several years to raise the benefits of SHM with the goal to facilitate the assessment accidental damages in CFRP structures. The flight test presented in the following is one part of this collaborative development. The present paper introduces first the flight test system, followed by the test description and results. ACOUSTO ULTRASONIC SHM FLIGHT TEST SYSTEM DESCRIPTION The system is installed on a CFRP fuselage panel of about 15 m² size, which has been inserted in the fuselage of an A340 Airbus test aircraft [1]. Figure 1 indicates the position of the aircraft in the outside view.

Figure 1. Photo of CFRP panel and test aircraft; monitored area marked by dashed line

System overview An overview on the system components is given in Figure 2. In reference to [2], this architecture is classified as a semi on-board system.

Figure 2. Components of the Acousto-Ultrasonic SHM flight test system

8 SMART layers with 235 piezoelectric transducers create the build-in sensor network. DAQ-hardware comprises a battery-powered ScanGenie II interrogation unit. A Switch-hub and assorted SA-boxes provide up to 320 measurement channels in a modular architecture. The test system is operated via the SMART Composite software running on a Toughbook. SMART layer sensors The SMART (Stanford Multi-Actuator-Receiver Transduction) Layer is a costeffective method for integrating a network of transducers with the structure. The layer is a thin dielectric film with an embedded network of distributed piezoelectric transducers that can be used as either actuators or sensors. The major advantages are: • Signal consistency and sensor reliability, incl. shielding for EMC • Flexibility, lightweight, customizable design • Ease of installation • Durable and reliable under variable environments SENSOR DESIGN AND INTEGRATION A robust and weight efficient design has been realized with a MIL-grade circular connector linking 30 piezoelectric transducers of one SMART layer to one SA-box, see Figure 3 below. This design benefits especially from the possibility to customize the geometry of SMART layers. It is therefore possible to exploit the structural design of the panel in order to route a SMART layer from the point of the connector into the adjacent stringer-frame-bays along the Omega-stringers. As the installation is a retrofit-type installation at a/c, the design needs to reflect the implied boundary conditions in order to ensure the feasibility of installation by secondary bonding. Each SMART layer contains one digital temperature sensor. In total there are 8 SMART layers installed, each typically covering 6 stringer-frame bays.

Customized SMART layer

stringer

MIL-grade circular connector

Figure 3. Photo of a section of a bonded SMART layer

SENSOR INSTALLATION The installation of sensors is performed by secondary bonding using a warmcuring epoxy adhesive (Hysol EA9394), vacuum bagging and heat application for a

robust, reproducible, short application process. Additional surface protection is provided by sealant and topcoat. Figure 4 illustrates some of the necessary process steps.

Figure 4. Sensor installation steps Application of (from left to right): Sensors and adhesive, vacuum and heat, sealant and topcoat

Sensor application by secondary bonding requires sufficient accessibility and takes a reasonable time. The alternative of sensor co-bonding during skin panel production is described in [3], which promises a faster process and compliant integration in the overall manufacturing process. Hardware for Data Acquisition Acellent’s data acquisition system has been specifically developed with regard to large scale SHM applications and the requirements for an a/c installation for flight test purpose. The functions of multiplexing, pre-amplification and signal conditioning are located on the switch-hub and SA-boxes. The ScanGenie II provides signal excitation generation and data acquisition. While Figure 1 previously introduced the schematic of the system, Figure 5 below shows a photo of the installed and portable components. Toughbook

ScanGenie II

Switch Hub

SA-boxes

Figure 5. Installed DAQ equipment

The data acquisition system along with the Smart Composite software has been in detail introduced in [2]. It is noteworthy that in the scope of the flight the software allows the operator to report environmental and operational conditions. The used data acquisition system features: • Modular, scalable architecture • Up to 320 measurement channels and extendable beyond that • BITE-function based on transducer impedance measurement • Up to two temperature sensors per SA-box • Ruggedized, reliable hardware components • Battery-powered ScanGenie Interrogation Unit • 12-bit ADC with up to 48MS/s sampling rate • Ease to use software interface TEST OBJECTIVES The purpose of this test is threefold: • Demonstration of integration feasibility and functioning in a/c environment • Durability of sensors under realistic conditions (i.e. pressurization, vibration, high and low temperature, temperature cycling, lightning strike) • Assessment of influence of environmental and operational conditions (EOC) on guided wave propagation The latter objective is considered most important with respect to validation of system performance including especially the algorithms for environmental compensation under realistic conditions. The following test results are limited to the present environmental conditions and derived generic requirements regarding environmental compensation techniques. Environmental and operational influence factors The EOC and their impact on the performance and reliability of the SHM system have been highlighted in [4]; environmental and operational conditions of importance are: • Temperature field • Moisture uptake in CFRP structure [5] • Stress condition / loading condition • Surface condition (inside and outside) Secondary effects which might be seen as related to EOC are ageing effects of the composite material and of the piezoelectric transducers. It has to be noted, that the mentioned conditions can interact also in more complex ways, e.g. thermo-mechanical stresses, hydro-thermal degradation. The temperature is widely recognized as the most influencing environmental factor on guided wave propagation, leading to the development of dedicated compensation algorithms [6]. Even when considering only an on-ground but in-service situation, the temperature in a CFRP fuselage is depending on various factors. Besides the constant intrinsic heat capacity and conductance, absorbance etc., the inside temperature condition and the

outside environmental conditions are prevalent. Inside temperature conditions generally comprises the ambient air temperature, but could also involve heat radiation from nearby a/c aggregates and components as well as air convection. Typically the fuselage is covered by insulation from the inside. The outside conditions are colloquially referred to as weather, for the case that the a/c is not located in a closed hangar. This comprises mainly insolation intensity and direction, precipitation and wind. In addition, radiation from pavement can take place for certain parts. The operational conditions can come in play in case the point of time when the SHM system is used is taken into account. Aircraft aggregates and systems are potentially in different operating modes. Directly after landing, the structure is not yet in temperature equilibrium. One can quickly draw here the conclusion that the exact prediction of in-service structure temperature can get quite complicated. In the scope of aircraft structural design thermal analysis are conducted to determine temperature distributions [7], however for a limited number of representative cases only. The following test results shall give a guide what temperature range and variation are to be considered in general in research and development. The presented data is of course not complete with regard to qualification of an SHM system. TEST RESULTS The system has been installed in 2010 and maintained on the Airbus test aircraft since then. Test data has been obtained only when the a/c was on-ground. The structure has a thermal insulation blanket, but no lining and cabin interior. Survey on present environmental conditions Figure 6 presents the temperature conditions as measured by the eight temperature sensors of the flight test system in relation to the environmental conditions as reported by the operators conducting the measurements. The figure holds in the top plot the mean temperature at different measurement events. Mean temperature is defined here as the average of all eight sensors values at start and end time of the measurement. The delay between start and end is ca. 15 min. In addition, the approx. outside and cabin ambient air temperature is given as fare as data is available. In the middle plot the start and end temperature values of the eight temperature sensors are stated as the difference to the mean temperature as stated in the top plot. The eight sensor layers are plotted from left to right, according to the designation in Figure 2, for each measurement occasion. In the bottom plot, the scaled magnitude of isolation, cloud cover and wind are given. For measurement #2 snowfall was present while for measurement #4 rain was present. Measurement #16 was conducted inside a hangar. The data presented in Figure 6 allows some rough conclusions as following on the existing effects and their order of magnitude on the temperature distribution and variation during an Acousto Ultrasonic measurement. Spatial temperature distribution: There apparent temperature gradients are mainly oriented in circumferential direction, thus with the height of the structure. In case of higher outside AAT and isolation, the temperature increase towards the ceiling (compare #6 to #14). In case of lower AAT, the temperature decreases towards the ceiling (compare #2, #20, #21). Please remember, that sensor 1 and 2 are the bottom

Figure 6. Environmental Conditions

and 7 and 8 are the top of the array. The temperature difference can easily be 2 °C, or also much more. Temperature variation over time: The variation within a relatively short time window of 15 min are significant and in the range of about 2 °C (compare #9, #11,

#13) up to more than 10 °C (#21). A high difference in inside and outside AAT or a reasonable isolation is facilitating a fast change in temperature. Effect of precipitation on temperature distribution: In case of rain (#4), the observable temporal and spatial temperature difference is much lower than for any other measurement. Internal air convection: During the first 9 measurements, the air condition system was –when powered – providing an air stream on the area of mainly sensor layer 2 and 4. Even though the structure has insulation on the inside, the effect on temperature distribution is visible, especially in #2 and #3. Consequences on environmental compensation techniques Temperature compensation techniques have to account for temperature fields rather than uniform temperature. The variety of temperature field and the variation over time brings up the practical issue to ensure the acquisition of sufficient reference measurement data from a real-life structure. Baseline depending damage assessment techniques need generally to consider also the range of loading conditions and surface conditions. SUMMARY This paper presented an Acousto Ultrasonic flight test system for large scale damage assessment in CFRP aircraft structures. Secondly, the paper provided environmental data from a conducted long-term flight test in order to highlight influence factors and effects and thus aid the development of suitable compensation techniques. The authors gratefully acknowledge the support of the colleagues at the Airbus flight test center who conducted most of the measurements throughout the last two years. REFERENCES 1.

2.

3.

4.

5.

6. 7.

“Flight trials begin to help optimise A350 XWB cabin sound insulation”, Airbus press release 14th of September 2010 (http://www.airbus.com/presscentre/pressreleases/press-releasedetail/detail/flight-trials-begin-to-help-optimise-a350-xwb-cabin-sound-insulation/) Zhang, D.C., Narayanan, V., Zheng, X. B., Chung H., Banerjee, S., Beard, S., Li, I., “Large Sensor Network Architectures for Monitoring Large-Scale Structures”, 2011, Proceedings of the 8th International Workshop on Structural Health Monitoring 2011, pp.421-431. Bach, M., Eckstein, B., Jacquel, N., Bertrand, R., Stolz, C., “Co-Bonding of Piezoelectric Sensors on CFRP Structures”, 2011, Proceedings of the 8th International Workshop on Structural Health Monitoring 2011, pp.1384-1390. Eckstein, B., Fritzen, C.-P., Bach, M., “Considerations on the Reliability of Guided Ultrasonic Wave-Based SHM Systems for CFRP Aerospace Structures”, 2012, Proceedings of the 6th European Workshop on Structural Health Monitoring 2012, pp.957-964. K. Schubert, A. Stieglitz, M. Christ and A. Herrmann, “Analytical and Experimental Investigation of Environmental Influences on Lamb Wave Propagation and Damping Measured with a PiezoBased System”, 2012, Proceedings of the 6th European Workshop on Structural Health Monitoring 2012, pp. 805-813. Croxford, A., Moll, J., Wilcox P., Michaels, J., “Efficient temperature compensation strategies for guided wave structural health monitoring”, Ultrasonics 50 (2010) 517-528 Oehler, Bettina, “Modeling and Simulation of Global Thermal and Fluid Effects in an Aircraft Fuselage”, 2005, 4th International Modelica Conference, Hamburg, pp. 497-506