STRUCTURAL MONITORING SYSTEMS PLC RICARDO PINHEIRO RULLI and FERNANDO DOTTA
ABSTRACT
In order to understand the various aspects of Structural Health Monitoring (SHM), over the years Embraer has investigated many different SHM technologies. Less complex and less time-consuming procedures - when compared to current NDT technologies - allowed by SHM can not only reduce the amount of time and burden of the inspection tasks, but also minimize the effects of "human-factors" when compared to current inspection procedures. Tests were performed in an E-Jets flight tests aircraft with two different SHM technologies which were considered as promising ones for monitoring structural components. Sensors, cables and connectors of Comparative Vacuum Monitoring (CVM) and Lamb Waves (LW) were installed as part of the developments for the qualification of the two systems. Aircraft scheduled maintenance can take advantage of these systems. Scheduled Structural Health Monitoring (S- SHM) application type considers the installation of sensors and cables into the aircraft for periodic scheduled structural inspections, these inspections being performed on- ground with ground support equipment. Additionally to the installations in an E-Jets flight tests aircraft, Embraer developed Service Bulletins for the installation of the S- SHM systems in different areas of an airline's E-Jets aircraft. Over a fixed period of time, Embraer intends to collect data from the installed S-SHM systems at least four times, aiming to identify if they are operating properly in a normal operating environment. For this qualification program, laboratory tests will also be performed with CVM and LW using structural samples that represent the monitored areas of an aircraft. These lab tests will provide data for generating Probability of Detection (POD) curves for both CVM and LW systems. Embraer representatives will participate in all tests conducted. All the information will be compiled and analyzed by Embraer teams and a data package will be created to provide support for the verification of the SHM solutions as a first step in this qualification effort.
Ricardo Pinheiro Rulli, Embraer S.A., Av. Brigadeiro Faria Lima 2170, Sao Jose dos
Campos/SP, 12227-901, Brazil
Fernando Dotta, Embraer S.A., Av. Brigadeiro Faria Lima 2170, Sao Jose dos Campos/SP,
12227-901, Brazil
INTRODUCTION
Focusing on the improvement of design, operation and, especially, aircraft maintenance, Embraer has investigated different Structural Health Monitoring (SHM) technologies for years. Seeking for potential benefits for current and future aircraft such as less time-consuming and less complex maintenance procedures (when compared to traditional Non-Destructive Testing), facilitated damage detection in areas with restricted access and reduction of maintenance costs, the company has selected SHM to be developed within the company's R&D Portfolio.
SHM technologies, such as, Acoustic Emission, Electro-Mechanical Impedance, Fiber Bragg Gratings, Comparative Vacuum Monitoring and Lamb Waves, have been considered in the Embraer's SHM roadmap.
After presenting good results, demonstrated by ground tests performed in the E- Jets Full-Scale Fatigue Test article, Barrel Test article and others structural component tests, two of the SHM technologies were selected to be engaged in a flight test program. These were Comparative Vacuum Monitoring (CVMTM) and Lamb Waves (LW) [1, 2].
Sensors and cables were installed in an E-Jets flight test aircraft for the evaluation of their operation in a "real environment". The regions with installed sensors were periodically inspected on ground with dedicated interrogators, following the Scheduled Structural Health Monitoring (S-SHM) application type - which considers the use of SHM devices for inspections at an interval set at a fixed schedule. Valuable
results demonstrated the technologies' suitability for aircraft operational environment.
The two technologies were, then, promoted to be part of a Qualification Program.
This paper presents a summary of all tests performed with Comparative Vacuum Monitoring and Lamb Waves SHM technologies, it presents an overview of the flight test program with CVMTM and LW, and explores in more detail the first steps towards the qualification of the two SHM technologies. Subjects such as brief description of CVMTM and LW technologies and the description of S-SHM application type will also be presented.
The Comparative Vacuum Monitoring (CVMTM) technology is based on the principle that a vacuum maintained within a small constant volume is extremely sensitive to any leakage [3]. The system designed for metallic structures uses elastomeric polymer sensors that are self-adhesive, passive, inert and lightweight, and can conform to the material surface contours. When those sensors are adhered to the monitored structure, fine channels on the adhesive face of the sensor form a manifold of galleries with the structure itself. The galleries alternate, one containing the steady state vacuum and the other having air at atmospheric pressure (Figure 1) [3, 4].
The CVMTM equipment basically consists of a stable source of vacuum, a fluid flow meter and the sensor. The vacuum source provides the vacuum (20 kPa below
ambient atmospheric pressure) to the sensor and also acts as the reference for the flow meter [3]. If no crack is present in the structure the vacuum level will be the same in both the sensor and the vacuum source.
Vacuum Galleries Air Galleries
Test
Structure
Crack
Figure 1: Schematic of an installed CVMTM sensor
If a surface crack develops, it will form a leakage path, air will flow through the passage created from the atmospheric to the vacuum galleries, the vacuum level in the sensors will decrease and the vacuum change will be measured [3, 4]. CVMTM offers an easy way to monitor "hot spot" areas.
Embraer has performed laboratory tests with CVMTM, applying the technology in a metallic barrel test and in its E-Jets Full-Scale Fatigue Test [1]. Example of a crack detected in a metallic barrel test is shown in Figure 2. In parallel, Embraer installed CVMTM sensors in the Full-Scale Fatigue Test of the company's E-Jets aircraft. Figure
3 shows examples of CVMTM sensors installed in the Full-Scale Fatigue Test where different regions and components - such as shear clips, splice joints, windows frames and joint holes - have been periodically monitored.
(a) (b)
Figure 2: Crack detected by a CVMTM sensor in a metallic barrel test. (a) Sensors installed around rivets; (b) One of the sensors was removed and dye penetrant testing confirmed the presence of a crack
Figure 3: CVMTM sensors in different regions of the E-Jets Full-Scale Fatigue Test
Figure 4: CVMTM sensors installed on-board the Embraer-190 flight test aircraft
Another type of test performed by Embraer is related to the installation of CVMTM sensors in a flight test aircraft in order to verify if the technology is capable of withstanding the real aircraft in-flight conditions, shown by Figure 4. Since 2010, the company has performed periodic monitoring of these on-board sensors using a CVMTM ground equipment.
LAMB-WAVES (LW) TECHNOLOGY
The global vibration based method looks for the changes in the structural dynamic characteristics due to structural damage. The fundamental of this technique is based on the assumption that a structural damage changes the physical dynamic response of the structure, such as natural frequencies, mode shapes and damping, frequency response, etc. [5]
Lamb waves represent two-dimensional wave propagation in plates or shells, which are described by known mathematical equations originally formulated by Horace Lamb in 1917 [6]. There are two groups of waves, the symmetric waves and the anti-symmetric waves, that satisfy the wave equation and the boundary conditions. The general solutions can then be split into two modes: symmetric (S0) and anti- symmetric (A0) [7]. For symmetric wave modes, each plate surface has a peak or trough at the same in-plane location. For anti-symmetric wave modes, a peak at one surface corresponds to a trough at the other surface, as shown in Figure 5. Structural dynamic response is sensitive to cracks, and, consequently, the Lamb wave modes are affected, based on that effect. Cracks can be identified by comparing the changes in the signal to a baseline (initial LW signal acquisition).
(a) Anti-Symmetric
(b) Symmetric mode
Figure 5: Wave modes: (a) Anti-symmetric, where a peak at one surface corresponds to a trough at the other surface; (b) Symmetric, where the wave peaks or troughs occur simultaneously at the same in- plane location.
(a) (b)
Figure 6: Tests performed in different specimens: (a) Thickness reduction in aeronautical aluminum; (b) Delamination detection in carbon fiber reinforced polymer
Embraer evaluated Lamb Waves technology for detecting damages - such as cracks and corrosion in metallic materials, and delamination in composite materials (2008) - obtaining valuable results [2, 8]. The company has performed laboratory tests with LW applying it in a wide range of specimens, such as coupons, Full-Scale Fatigue Test, barrel tests and others. Figure 6 shows two examples of the lab tests performed.
Besides the laboratory tests, Lamb Waves sensors were installed in the Embraer-
190 flight test aircraft (Figure 7) in 2010. In this study, only the sensors (Figure 8) and the cables were installed in the aircraft and the inspections were performed periodically using ground support equipment [2].
Figure 7: Flight Test Aircraft where CVMTM and LW sensors were installed
Figure 8: LW sensors applied to the Embraer-190 flight test aircraft
SHM DAMAGE DETECTION TECHNOLOGIES APPLICATION TYPES
According to the defined by the Airlines for America (A4A) MSG-3 document [9], and to the ARP-6461 document from SAE International [10], there are two different types of application for SHM technologies: S-SHM for Scheduled Structural Health Monitoring which means the use of SHM devices for inspections at an interval set at a fixed schedule; and, A-SHM for Automated Structural Health Monitoring that relies on the SHM system to inform maintenance personnel that action must take place [9, 10].
In the Embraer perspective, S-SHM application type considers the installation of SHM sensors, cables and connectors into the aircraft with periodic scheduled structural inspections being performed on-ground with ground support equipment. And, the A-SHM application type will be the installation of not only sensors, cables and connectors, but also SHM interrogators and data recorders into the aircraft. In such a case, the structural inspections would be performed automatically at any time in smaller time intervals (or continuously), what would require to the system to have a power supply during the regular aircraft operation.
These two types of SHM application have different requirements depending on the presence of components, such as interrogators, installed in the aircraft, eventually powered up during flight.
In order to improve the conditions for SHM to be an effective part of current and future commercial aviation maintenance programs, these points must be properly addressed and Embraer is committed to it [11].
Due to the relevance of the structural inspections for the continuous airworthiness of any aircraft, the road to the qualification of SHM systems is not an easy task and requires the involvement of several Embraer teams.
As part of the damage detection systems' qualification effort, a program for generating laboratory test data and systems' operational test data is under way.
Laboratory test data will cover Probability of Detection (POD) and some Environmental Tests with specimen configurations prepared for both Comparative Vacuum Monitoring and Lamb Waves.
The POD method can be used to compare the probability of detection between different types of inspections. Also, it provides information about the corresponding level of confidence that can be statistically verified for each measurement. The target is to confirm that CVMTM and LW solutions have at least 90% probability of detection with 95% of confidence, for specific scenarios. The laboratory tests will use structural specimens that are representative of regions from an Embraer aircraft structure.
An approach for POD determination is the method described in the document
MIL-HDBK-1823A [12, 13], which was originally created for traditional NDT equipment. Another approach for assessing the system's detection capability in terms of POD is the One-Sided Tolerance Interval methodology [14]. Apparently, both methods are suitable for SHM; however, further discussions are still required.
Figure 9: Sensors and Cables installed in an operator's aircraft
For the portion of the program which considers SHM systems' operational test data obtained during aircraft operation, periodic measurements will be performed with ground support equipment, on airports or on maintenance facilities.
In order to foster the installation of sensors and cables into the operator's aircraft,
Embraer developed Service Bulletins presenting all related instructions.
Its main goal is to verify the behavior and survivability of such components (sensors and cables) in a real operating environment of a commercial airplane. This initiative will count on the installation of sensors and cables into a number of operator's aircraft (Figure 9).
Following the installation, measurements will be collected periodically, and will be compiled and analyzed.
In the end of the program, a data package will be created including both the
laboratory test data and the systems' operational test data.
An overview of the laboratory and the on-board tests performed by Embraer with two SHM damage detection technologies, Comparative Vacuum Monitoring (CVMTM) and Lamb Waves (LW), was presented.
As part of the damage detection systems' qualification effort, a program for
generating laboratory test data and systems' operational test data is under way. The installation of sensors and cables in an operator's aircraft was performed. Further results will be generated in the future, as the process moves on.
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presented at the 8th International Workshop on SHM, September 13-15, 2011.
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9. Airlines for America. 2009. MSG-3 Operator/Manufacturer Scheduled Maintenance - Volume 1 - Fixed Wing Aircraft - Revision 2009.1.
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Monitoring on Fixed Wing Aircraft.
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Commercial Aviation Programs", presented at the 8th International Workshop on SHM, September
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