SURVICE Metrology was recently awarded a U.S. government research grant to improve upon the current methods used to measure and repair damage to composite materials associated with next-generation aircraft. The problem is that current ways of measuring and repairing damage (as a result of combat incidents, weather, or mishaps) is inadequate for composite structures and components, with measurements having to be collected manually.

This time-consuming process is prone to human error and it also limits the detail and accuracy of the data being collected. This complicates and slows down the engineering analysis, disposition, and repair process. Identifying the exact location and extent of damage is crucial for determining the type of repair that is suitable. An accurate, portable, and automated method of collecting damage information (to include location and ultrasonic sensor data), electronically storing, and transferring the data, and subsequent damage evaluation/processing is essential to ensure that repair methods and technologies match the advancements made in material science. The goal of the proposed approach is to assemble current state-of-the-art metrology hardware with ultrasonic sensing equipment and customized software to provide an integrated data collection and processing system to meet the Navy requirements.

The patent-pending approach to solving this problem includes integration of ultrasonic sensors and data collection with Metris’s iGPS laser-based metrology system. The iGPS system consists of two or more laser transmitters that can be set up on tripods, or affixed to walls in shop or work areas.  The transmitters flood the vicinity with encoded laser light, which is received by sensors on a vector bar. Through collaboration with the University of Delaware Center for Composite Materials, SURVICE is coupling the Metris iGPS system with portable ultrasonic inspection equipment to identify the extent of nonvisible delamination within composite structure in the vicinity of otherwise-visible combat damage to automate and expedite maintenance and repair activities.

Introduction

SURVICE, in coordination with our team members at Metris and the Center for Composite Materials (CCM) at the University of Delaware, proposed the development of a customized damage collection and evaluation system to meet Department of Defense (DoD) needs for streamlining aircraft maintenance activities. The system heavily leverages commercial state-of-the-art metrology (i.e., laser scanning) equipment as well as some existing and customizable SURVICE-developed 3-D visualization and manipulation software.

SURVICE selected the use, enhancement, and extension of the Metris iGPS large-volume metrology equipment (as seen in figure 1) as part of the solution. The iGPS system is a lightweight and portable laser-based metrology device that is ideally suited for the requirements of the research effort. By having Metris as part of the team, SURVICE has been able to develop customized hardware adaptations to the existing commercial metrology equipment offerings.

Figure 1:  Metris iGPS system

The iGPS system consists of two or more laser transmitters that are set up on tripods or affixed to walls in a shop or work area. The transmitters can run on standard electric power or internal/rechargeable batteries. The transmitters flood the vicinity with encoded laser light, which is received by sensors on a “vector bar,” as seen in figures 1 and 2.  The sensor data are fed to a battery-powered belt pack and sent wirelessly to a laptop (or desktop PC), where the raw sensor data from multiple laser transmitters are triangulated and converted into spatial coordinates. The precise location of the measurement tip is updated continuously as the vector bar is moved around the subject.

Figure 2: iGPS setup

The iGPS system allows multiple operators to roam around a given area and collect 3-D data. The system has a range of approximately 50 meters and can collect spatial data within 100–400 μm. The iGPS system provides a constant stream of xyz coordinate data, and the user clicks a button on the vector bar (as seen in figure 1) to store the current/selected location. SURVICE proposed the integration of ultrasonic sensor technology to the standard vector bar and allowed a second data stream to be fed, collected, and stored with the corresponding spatial information. This new configuration allows the operator to seamlessly collect composite delamination and location information quickly and easily.

Under separate but related research grants as well as internal research and development activities, SURVICE has developed custom 3-D software solutions that integrate with the iGPS system, which streamline data acquisition and subsequent analysis, including superimposing the damage onto existing CAD models, as seen in figure 3.

Figure 3: Superposition of damage onto CAD model

One of the SURVICE-developed tools is graphics package called Archer, which is specifically designed to visualize and manipulate large 3-D data sets (as seen in figure 4).  Archer is built upon the U.S. Army BRL-CAD suite (reference www.brlcad.com) and SURVICE’s proprietary IVAVIEW graphics database. The Archer “core” is freely distributable as part of the BRL-CAD package (with more than 300,000 downloads to date). The code is designed to be extensible, allowing new features to be developed and integrated to support specialized/vertical markets. This software served as the basis for modification under the subject research grant.

Figure 4: SURVICE-developed graphics package

Phase I research and results

The first effort under the research grant was to integrate a selected ultrasonic sensor into the iGPS hardware configuration. Figure 5 shows design drawings developed to house the ultrasonic sensor. Unlike laboratory systems that use dual sensors in a pulse-receive mode (with one sensor transmitting on one side of the test article and another sensor receiving the signal on the opposite side of the test article), the proposed system uses a single sensor in “pulse-echo” mode, requiring the same sensor to perform both duties of signal generator as well as signal receiver, and do so from the same side of the test article.

Figure 5: Vector bar modifications

This design was then fabricated and integrated into prototype hardware, as seen in figure 6.

Figure 6: Hardware prototype

Next, the core of the existing Archer graphics package was modified and enhanced to process multiple data streams; one from the iGPS hardware and one from the ultrasonic sensor (and its associated hardware). This new software application, dubbed Sherlock (for its inspection capabilities), is seen in figure 7. In addition to storing ultrasonic measurement information as metadata to 3-D data points, the software uses heuristics about the data being collected to automatically assign damage into associated groups (based upon aspects such as point proximity, time, etc.), allowing data to be collected with minimal user interaction with the computer.

Figure 7: Sherlock software prototype

With functioning hardware and software prototypes in place, the research effort then focused on the feasibility of the system to accurately collect and identify composite delamination in a portable system.

The University of Delaware (UD) Center for Composite Materials (CCM) fabricated a composite laminate for testing of the ultrasonic sensor and pulse-echo hardware.  Induced voids were added to the panel by using azodicarbonamide (a blowing agent) and Teflon inserts placed within the layers of the composite to simulate debonding. Finally, the panel was impacted with a steel ball at multiple locations. The majority of these defects were not visually observable. Figure 8 shows the test panel used for the tests with the fabricated defects at various locations.

Figure 8: Test panel scan

The panel was evaluated using the prototype system. The probe allows introduction of the ultrasonic pulse into the sample using a coupling agent. In addition, a polymer-based coupler was used to extend/delay the signal in a range (> 9 µs) where the transducer ringing is negligible. Figure 9 shows two specific locations that were evaluated, including one area containing a Teflon insert where a debonding reflection would be expected, and in another area (no voids, Teflon, or damage) where an average signal is expected. This result is similar to the dual-sensor laboratory-based baseline signals at the same locations.  At the defect location, there is virtually a complete reflection at the surface with almost no reflection afterward, and the average panel signal shows the back surface return signal after approximately 1.5 µs and subsequent reflections (as expected).

Figure 9: Ultrasonic scan comparisons

This test verified the feasibility of the prototype system, clearly being able to distinguish between normal and delaminated (or defective) composite material in a portable, pulse-echo ultrasonic sensor configuration integrated into the iGPS hardware.

Conclusions

The goal of the subject research grant was to develop a modern, integrated data collection and sensor system capable of assessing damage and then evaluating and processing repairs to advanced composite materials on fixed- and rotary-wing aircraft. SURVICE’s approach was to assemble current, state-of-the-art data collection hardware and sensing equipment with customized software to provide an integrated data collection and processing system, ensuring that repair methods and technologies match the advancements made in material science.

All of the key objectives were met under the Phase I effort, demonstrating key technologies required to proceed to the Phase II effort. SURVICE is currently awaiting formal notification to proceed to Phase II, where the prototype system developed under the Phase I effort will be fully developed and realized.

Acknowledgements

The author would like to acknowledge the contributions of several key personnel involved with successful completion of this Phase I research effort:

Dr. Dirk Heider of the University of Delaware’s Center for Composite Materials was instrumental in the selection, integration, and verification of the prototype system.

From Metris, Mr. Anderson Sheng was instrumental is supporting the hardware/software interface to the iGPS system.

From SURVICE, Mr. Doug Howard worked on the software modifications, Mr. Kyle Herr and Mr. Jason Duvall developed and fabricated the necessary prototype hardware, and Mr. Michael Hardin and Mr. Dave Turner supported the validation testing and documentation of the results.