Application of portable modules for fatigue crack characterization

Transcription

1 Application of portable modules for fatigue crack characterization Igor N. Komsky * Northwestern University ABSTRACT This paper is dealing with the ultrasonic imaging techniques and instrumentation that are used for the inspection of aircraft and bridge structures. A concept of a modular inspection system with interchangeable components will be presented. The modular concept provides greater flexibility in the integration of various commercially available sensors and data acquisition units deployed by operators. Miniature scanning and interface modules were developed as components of the integrated system for wide area inspections using ultrasonic sensors. A novel ultrasonic imaging technique has been utilized for characterization of internal fatigue cracks. The technique is based on a linear scan acquisition and processing. Portable ultrasonic flaw detectors or thickness gages can be used to acquire the data. An image-processing module was developed to manipulate the acquired images and to display the inspection results. Several rapid scanning techniques have been developed to satisfy flaw detection and characterization criteria as well as the scanning and data acquisition module capabilities. The techniques and instrumentation were successfully tested on various aircraft and steel bridge structures. Keywords: Ultrasonic imaging, fatigue crack characterization, aircraft inspection, bridge inspection. 1. INTRODUCTION The concept of the modular inspection system is based on the integration of the interchangeable commercially available and custom-designed elements. The generic modular inspection system consists of the hardware (sensor, coupling, scanning, data acquisition, data post-processing and display, calibration specimen) and the software (set-up, imaging, and calibration) modules. It is imperative that both commercially available and custom-made components are incorporated in the modular system. Deployment of the commercially available units will significantly reduce time and cost of the system development and integration. Design of the modular system for a particular application should begin from the proper selection of the basic commercially available elements. NDI equipment that is widely used in industry needs to be identified and classified with respect to capabilities and constrains. The units that satisfy most of the inspection requirements are selected as sensor, data acquisition, and etc. modules of the inspection system. If necessary, a combination of commercial units, for instance sensors, signal generators, and amplifiers can be used instead of a single custom-made component. Next, the generic inspection technique has to be developed to satisfy required probability of detection and other inspection criteria. This technique needs to be compatible with the selected standard equipment units, so that the modular system elements can operate together. Custom-made hardware elements and interfaces should be added to facilitate the integration and operation of the modular system. These elements include sensor holders, coupling units, scanning devices. Sensor holders can be designed to contain a configuration of sensors or sensor arrays suitable for the developed inspection technique. The deployment of the interchangeable coupling units makes it possible to inspect rough surfaces or surfaces with complex profiles and protruding elements. The scanners with linear, dual-axis, or rotational drivers provide capabilities for wide area inspections on components with various spatial orientations. The data transfer protocol and the standard or custom-made software modules should be selected to match the NDI hardware modules that have been chosen for the application. On the other hand, both the format of the procedure for data processing and the format of data presentation/imaging are based on the type of flaws and configuration of the inspected structure. Various formats of the data presentation can be included in the imaging module on-screen menu to allow an adequate selection for a number of applications or hardware modules. * ikomsky@nwu.edu; phone ; fax ; Center for Quality Engineering and Failure Prevention, Northwestern University, 2137 N. Sheridan Rd. Evanston, IL, USA, Advanced Nondestructive Evaluation for Structural and Biological Health Monitoring, Tribikram Kundu, Editor, Proceedings of SPIE Vol (2001) 2001 SPIE X/01/$15.00

2 2. MODULAR SYSTEMS FOR ULTRASONIC AIRCRAFT INSPECTION 2.1 Ultrasonic transducer modules. Spring-loaded transducer modules were designed to hold and direct a variety of contact ultrasonic transducers ranging from to 0.5 in diameters and from 30 to 90 degrees in refracted angles. Special front and back spring loads provide for stable coupling of the transducers to non-uniform and curved surfaces of the aircraft structures. Another important feature of the transducer modules is their flexible link with a scanning unit. Aircraft inspection procedures frequently require sequential application of different ultrasonic transducers. These transducers might have different sizes and configurations. They may be installed in different azimuthal directions. For some inspections, as, for example, the lower spar cap inspection in the nacelle of the Fairchild SA 226, the transducers should be replaced without removal of the scanning unit from the nacelle. Taking also into consideration inspection time constrains; it would be much more effective to exchange, when necessary, the whole transducer modules, and not the transducers by themselves. The miniature transducer modules with the fast-disconnect locks have been developed for the modular inspection system. Figure 1 shows two interchangeable modules (center and right units) for the 0.25 angle-beam contact ultrasonic transducers that were used for scanning inside the nacelle of the Fairchild SA 226. Figure 1: Portable transducer modules for aircraft and bridge inspections. 2.2 Scanning modules Scanning cartridges are portable dual-axis linear motion systems for accurate positioning of the ultrasonic transducers during inspection (Fig. 2). The cartridges can be attached to the aircraft structure using a set of ¾ suction cups interconnected inside the cartridge frame. The cartridges are small enough to be used on the inspection areas with the limited access. For instance, the inspection area on the wing skin of the SA 226 can only be accessed through a 4 diameter hole in the aircraft nacelle. Hence, the scanning cartridges for this application have to fit the opening in the nacelle. Since the scanning cartridges are designed for applications in the hardly accessible areas, their connections to the driving and vacuum units are also very flexible lines and shafts. For wide area inspections of the aircraft components (fuselage, wings, etc.) the cartridges can be attached to the tracks of various lengths (Fig. 3). The tracks are positioned on the aircraft skin using a separate set of suction cups. Special visors were designed to facilitate the track alignment along splices or rows of fasteners. Proc. SPIE Vol

3 Figure 2: Scanning module for ultrasonic inspection. The scanning modules can be used not only for the X-Y linear scans but also for rotational scanning. This scanning procedure should be used for the inspection of the aircraft structures where multidirectional cracks might be developed around the fasteners. Ultrasonic crack indications have the highest signal-to-noise ratios when an ultrasonic beam is normal to the crack plane. Therefore, to detect cracks with various orientations the transducer should be rotated around the fastener. In rotational inspection mode the scanning module has to be attached to the rotational driving module (Fig. 4). The driving module rotates the cartridge with the ultrasonic transducers around the fastener. When a crack is detected, the cartridge should be moved to the position where an ultrasonic beam is normal to the crack plane. Next, the rotational module has to be locked in that position. Then, the flexible shaft is disconnected from the rotational module, and connected to the similar adapter on the scanning module to proceed with the crack characterization through linear scans. This combination of rotational and linear scanning has been successfully applied to detection and imaging of multidirectional cracks in the second layer of the DC-10 spar-cap/strap connection. Figure 3: Adjustable track with suction cups and visors for repositioning of the scanning module. 292 Proc. SPIE Vol. 4335

4 Figure 4: Rotational driving module with the linear scanning cartridge. 2.3 Interface and control modules The interface and control modules contain two independent miniature vacuum units that are connected to the scanning cartridges, rotational modules, and linear tracks through the fast-disconnect adapters with internal protective valves. The modules also contain bi-directional drivers with special scanning envelope limits protection systems. These systems can instantly disconnect the scanning or rotational modules from the driving unit when the driver is overloaded. An overload might occur when the ultrasonic transducer modules reach the scanning areas that are close to the cartridge frame. The driver can also be overloaded while scanning the aircraft structures with rough or non-uniform surfaces. For better protection of the driving units and ultrasonic transducers from an excessive wear it is very important to have such an option as instant disconnect of the driver from the load. Figure 5: Interface and control module for the EPOC III ultrasonic flaw detector. Proc. SPIE Vol

5 Figure 6: Interface and control module integrated with the Krautkramer-Branson DMS 2. Encoding units inside the interface and control modules convert the linear or angular positions of the ultrasonic transducers into the encoded signals. These signals are instantaneously transferred to the portable ultrasonic units to generate images. Two models of the interface and control modules have been developed. Figure 5 shows the module for the EPOCH III ultrasonic flaw detector. Another model was developed for integration with the Krautkramer-Branson DMS 2 thickness gage (Fig. 6). Both models are highly portable devices that require neither AC power nor high-pressure air to drive or support the scanning and rotational modules. For instance, the flaw detector s battery powers the module in Fig. 5. The module for the Krautkramer-Branson thickness gage utilizes four AA batteries Figure 7: Rotational ultrasonic scans of radial cracks with various orientations. 294 Proc. SPIE Vol. 4335

6 Figure 8: Rotational ultrasonic scans of non-radial cracks with various orientations. 2.4 Ultrasonic characterization of fatigue cracks The scanning procedure for crack detection and characterization starts from the rotational scans. These scans are mostly used for the inspection of the aircraft structures with multidirectional cracks around fasteners. The rotational scan provides information on the crack azimuthal orientation. Two scans of the radial cracks with different orientations are shown in Fig. 7. The rotational scans can also be used to determine the crack starting point. Information on the starting point of the crack could help to distinguish radial cracks from non-radial (eyebrow) cracks of the same orientations. Indications from the non-radial cracks will have the same positions on the horizontal scan axis but different vertical amplitudes, if compared with the indications from the radial cracks of the same orientations. Figure 8 shows the ultrasonic scans with the indications from the non-radial cracks. To determine the crack length an ultrasonic transducer should be scanned along the crack direction, so that the transducer axis is normal to the crack plane. The scan results in the rectangular 2-D image. Figure 9 shows the ultrasonic image of the 5/16 fastener hole with the 1/8 radial crack emanating from the hole. The horizontal axis of the scan represents the transducer positions along the line of scanning. The vertical axis can be viewed as a depth of the structure. One more ultrasonic image has to be generated to obtain information on the crack depth. This image is created from the scan across the crack plane. Three linear scans (one rotational scan and two straight linear scans) can be subsequently superimposed to produce 3-D image of the crack (Fig. 10). 1/8 long radial crack 5/16 hole Figure 9: B-scan image of 5/16 fastener hole with 1/8 radial crack. Proc. SPIE Vol

7 Figure 10: 3-D image of the radial crack around fastener hole. 3. MODULAR SYSTEMS FOR ULTRASONIC INSPECTION OF PINS IN STEEL BRIDGES Due to the wide variations of pins configurations, dimensions, and retaining methods (threads, cotter pins, center bore holes, etc.) a very flexible ultrasonic inspection system is required for pin inspections. A modular approach was used to design two generations of the portable semi-automatic scanning system for the ultrasonic inspection of pins in pin hanger assemblies. The first generation scanning unit consists of three interchangeable modules: coupling module, transducer module, and control module (Fig. 11). The coupling module provides a reliable interface between the rotating transducer module and the non-uniform surface of the pin. The coupling module is also used as a mounting base for the scanning unit. The module contains an adjustable self-centered clamping unit with the lightweight high-efficiency permanent magnets. The unit can be rapidly repositioned from one pin to another using fast-release adapters that were developed to lock and unlock the permanent magnets. The clamping unit is designed for a wide range of applications on pins from 2" to 4.5" in diameters. Figure 11: First generation scanning module on the calibration pin. 296 Proc. SPIE Vol. 4335

8 Figure 12: First generation portable scanning system on the calibration pin-hanger assembly. The transducer module includes two adjustable transducer holders with precise X-Y positioners. The transducer holders are designed to make adjustments of three variables (two linear and one angular) of the ultrasonic transducer position. The control module contains a dual motorized driver for the angular and radial movements of the transducer module. The motorized driver includes a digital encoding system. All systems of the portable scanning module are powered by the batteries from the ultrasonic portable units. Figure 12 shows the first generation portable scanner that was integrated with the EPOCH III ultrasonic flaw detector. The portable inspection system was tested at the Portage Bridge in Portage, IN. Twenty four 3 diameter pins made from Nitronic 60 stainless steel have been inspected for the radial fatigue cracks as small as in depth. The angular and radial scans were acquired with resolutions of 1 degree in the angular direction and in the radial direction. The scans have been subsequently superimposed to produce 3-D ultrasonic images. Figure 13 shows the portable scanner installed on the lower pin of the pin-hanger assembly at the time of the field tests in Indiana. Figure 13: Field test of the first generation portable scanner at the Portage Bridge in Portage, IN. Proc. SPIE Vol

9 The field tests in Indiana have demonstrated high effectiveness of the portable scanning system for pin inspections on the bridges with the standard configurations of the pin-hanger assemblies. However, the scanning unit was not suitable for applications on the old railroad bridges with the limited access to the pin-eyebar assemblies (less than 6 clearance from the top of the pin). The scanning unit was modified using interchangeable features of the modular components. In the new design the rotational driving module and the dual-axis scanning cartridge (Fig. 4) were integrated with the coupling module (Fig. 11). The transducer holder was also redesigned (Fig. 1, left). The new scanning unit with the substantially reduced weight and dimensions (Fig. 14) was successfully tested on the representative assemblies and at the bridge sites. Figure 14: Modified portable scanning system on the calibration pin. 4. CONCLUSION Several portable ultrasonic inspection systems have been developed for quantitative characterization of fatigue cracks in the aircraft and bridge structures. The systems are based on the integration of the rotational and linear scanning modules with the portable ultrasonic data acquisition units. The prototype scanning modules have successfully been tested on the calibration specimens and the actual aircraft and bridge components. The modular concept of the inspection systems makes it possible to inspect the aircraft and bridge structural components regardless of types of the ultrasonic equipment available at the inspection or maintenance facilities. This approach will allow maintenance cost reduction, and, ultimately, will extend life and enhance safety of the airplane and bridge structures. ACKNOWLEDGEMENTS The work on the aircraft structures was supported by the Federal Aviation Administration under Contract # DTFA03-98-D-00008, Delivery Order # 98-IA007 and performed at Northwestern University as part of the Center for Aviation Systems Reliability program through the Airworthiness Center of Excellence. The Infrastructure Technology Institute of Northwestern University supported the work on the pin inspection. REFERENCES 1. I. N. Komsky, J. D. Achenbach, G. A. Andrew, R. H. Grills, J. Register, and G. Linkert, Ultrasonic Imaging of Defects in Multilayered Airplane Structures, in ASNT 1995 Spring Conference Proceedings, pp , ASNT, Columbus, OH, Proc. SPIE Vol. 4335

10 2. K. I. McRae and R. W. Nolan, Ultrasonic Imaging of Cracks under Installed Fasteners, in Review of Progress in QNDE, eds. D. O. Thompson and D. E. Chimenti, Vol.17B, pp , Plenum, New York, I. N. Komsky, Ultrasonic Scanning of Airplane and Bridge Structures, in Paper Summaries of The First US-Japan Symposium on Advances in NDT, pp , ASNT, Columbus, OH, I. N. Komsky and J. D. Achenbach, Ultrasonic Imaging of Corrosion and Fatigue Cracks in Multilayered Airplane Structures, in Nondestructive Evaluation of Aging Aircraft, Airports, and Aerospace Hardware, edited by R.D. Rempt and A.L. Broz, SPIE Conference Proceedings 2945, pp , I. N. Komsky and J. D. Achenbach, A Computerized Imaging System for Ultrasonic Inspection of Steel Bridge Structures, in Structural Materials Technology An NDT Conference, pp , Technomic Publishing Co., Lancaster, PA, Proc. SPIE Vol