Use of Phased Array Ultrasonics in Aerospace Engine Component Inspections: Transition from Conventional Transducers

The University of Dayton Research Institute (UDRI), under contract by the US Air Force, has designed and constructed a fully automated ultrasonic inspection system for the detection of embedded defects in rotating gas turbine engine components. The automated inspection system, designed and developed under the Turbine Engine Sustainment Initiative (TESI), makes use of phased array ultrasonic technology for increased throughput and detection capability. Current inspection requirements for aerospace applications, however, are based on the use of conventional transducers and time honored methodologies. The implementation of phased array technology into existing routine ultrasonic inspections requires a reinterpretation of inspection procedures and results from conventional transducers. This paper will discuss current progress in a study conducted to directly compare ultrasonic setups for phased array and conventional transducers. The inspection results obtained from both conventional and phased array systems under several setup conditions will be presented. Introduction: The use of ultrasonic inspections for the detection of hidden defects has been used extensively throughout the aerospace industry for structures, pre-production and serviced engine components, as well as material processing quality control [1,2]. Procedures for the use of conventional single element transducers for the inspection of pre-production and serviced engine components are well established, and the processes are routine for many components. Many of these procedures were developed based on antiquated electronic devices however, and have not been updated to reflect changes in ultrasonic technology over the years. Implementation of newer technology that provides increased detection capability, as well as data storage and processing opportunities, will allow for improvements in reliability and efficiency of the inspection process. These technologies will require new approaches to embedded defect inspections and necessary changes to the time honored techniques that are commonly employed. The challenge to the inspection developer is to incorporate the new technology into the inspection process, without any degradation in coverage or sensitivity from that under which the original inspection procedures were developed. Under the Turbine Engine Sustainment Initiative (TESI), the University of Dayton Research Institute (UDRI) has designed and constructed a fully automated ultrasonic inspection system for the detection of embedded defects in rotating gas turbine engine components. The TESI inspection system makes use of advanced technologies in many areas, including: probe positioning (robotics), part recognition (vision systems) and automated scan plan execution (software) [3]. Improvements in ultrasonic inspection capability and efficiency are provided in the system by the incorporation of phased array ultrasound technology. Use of the phased-array ultrasonic instrument and probes allows for optimization of both the sensitivity and resolution for each inspection through electronic beam-steering, scanning, and focusing processes. However, direct implementation of phased array transducers into existing inspection procedures developed for conventional transducers is not straightforward. Specifications for conventional probe acceptance and qualification are not applicable to multidimensional array probes and must be rewritten accordingly. Similarly, routine surface focusing procedures commonly used with single element probes are inappropriate for array transducers. The focusing and steering conditions used in an inspection can be optimized for the specific component geometry and inspection requirements, since there are many possible probe configurations that simply achieve the same focal depth. Characterization of the beam profile for each possible probe configuration is not possible, or practical for a routine setup. However characterization of the beam profile throughout the interrogation volume is required for developing an inspection procedure that ensures coverage and sensitivity throughout the inspection. Therefore, standardized processes for characterizing the beam requirements and focusing conditions for array transducers are needed, but have not yet been developed. Under the TESI program inspection procedures are being developed for engine disk bore geometries, which include many flat surfaces. These relatively simple geometries allow for the efficient use of the linear array transducers for much of the inspection. Currently, 5 and 10 MHz linear array transducers are being characterized for use in the disk inspections. Since creation of the focused beam relies on refraction at the water/material interface, proper characterization of the array transducer beam profile must be performed using reflectors inside a specimen, preferably of the same material. Characterization blocks which contain closely spaced side drilled hole (SDH) and flat bottom hole (FBH) targets from 0.125-3.0 in. depth have been designed and produced by UDRI for beam profile measurements. In order to insure that the beam profile and performance of the array transducer compares closely with that of the specified conventional transducers currently being used, the beam profiles of both transducers have been characterized in the same way, and the results compared. The following discussion will focus on the characterization of a 10 MHz, 64 element (0.45 X 5.0 mm element size) linear array transducer (R/D Tech Ultrasound Transducers, 60 Decibel Rd., State College PA) and a 10 MHz, 3” focus, 0.375” diameter single element transducer, (UTX, Inc. 112 Milltown Road, Holmes, New York). Results: The experimental setup used in this study consists of the R/D Tech Tomoscan III pulser/receiver and Tomoview 2.2R9 software for both data acquisition and hardware control for all inspections using both the linear array and conventional single element transducer. The gantry x-y scanning system provided transducer positioning with a scan and index resolution of 0.5mm. For the linear array transducer, electronic scanning in the index direction was combined with mechanical stepping, where needed, to acquire data over the entire inspection volume. Characterization of the ultrasonic beam was performed using a Rene 95 calibration block which contained 0.020 inch diameter SDH’s located at 0.125, 0.250, 0.500, 0.750, 1.00, 1.250, 1.500, 1.750 and 2.000 inches depth. The use of SDH targets allows for characterization of the ultrasonic beam in both a normal incidence or shear beam configuration, however this paper will discuss only normal incidence beam characterization. The beam width for an unfocused transducer 0.375 inches in diameter would be roughly 0.094 inches, while the wavelength of sound in Rene 95 at 10 MHz is 0.024 inches. Thus, the 0.020 inch SDH is small enough to properly characterize the beam width, but is also large enough to be easily detected by the 10 MHz wave. Both transducers were aligned so that the active area was parallel to the top surface of the characterization block. Further details of the setup configuration used for each transducer are described below. All inspections conducted using the conventional single element transducer used a 3.0 in. water path. This probe configuration is commonly used for aerospace embedded defect inspections. Placing the focal point of the transducer at the part surface also serves to optimize the inspection for the near surface sensitivity. Sensitivity deeper in the part is obtained with the use of a depth (or time) amplitude correction (DAC). Typically, reflections from side drilled hole (SDH) or flat bottom hole (FBH) targets at various depths are used to determine the gain values for the DAC curve. Although focusing the transducer at the surface of the part does not make efficient use of the transducer focusing power over the inspection volume, surface focusing does allow for the use of a simple linear depth gain correction. Use of a linear array transducer provides the flexibility to focus the beam optimally within the part. However, focusing at many different depths is not necessarily required for all inspections. Design of an efficient ultrasonic inspection process will optimize both sensitivity and inspection speed by minimizing the number and complexity of the focal laws used. In the current study, the primary inspection volume is roughly 2.0 inches in depth. Therefore, the array transducer was set up to focus at 3 different points within this volume, 5 mm, 19 mm and 35 mm. The optimum focal law will be obtained by a comparison of the array beam profiles for each focusing depth, with that of the conventional transducer. Discussion: In order to compare the beam profiles from both transducers, the same setup procedure was used for each. Typical inspection requirements for embedded defects specify that the transducer gains be set to achieve an 80% full scale height (FSH) peak amplitude from a target located at a particular depth. For the current study, the SDH located at 0.250 inches was used as the setup target. The setup process was performed manually, and the instrument gains adjusted to achieve a minimum peak amplitude of 80% FSH from the SDH. The transducer was then scanned across the entire SDH block and the A-scan and C-scan data recorded. The C-scan data obtained with the conventional transducer shows visually that the peak amplitude of the reflection decreases with depth (Figure 1). Analysis of the C-scan results included peak amplitudes and –6dB beam width for each of the SDHs. A plot of peak amplitude versus SDH depth was used to determine beam attenuation, while the –6 dB beam width was used as a measure of beam spreading. Table 1 lists the peak amplitudes and –6 dB beam widths for the conventional transducer. The data show that the amplitude of the SDH reflection does decrease as expected with depth in the part, but there is a significant spreading of the beam diameter with increasing depth also. Application of a DAC which incorporates increasing gain with depth can correct for beam attenuation, but will not compensate for beam spreading. In effect, the wider beam at larger depths amounts to over sampling as well as a decrease in detection resolution. The effect of beam spreading is particularly apparent when viewing the B-scan in Figure 1b. The B-scan view shows the cross section of the characterization block through each of the holes. The tightly focused beam shows the SDH as a small circular feature near the top of the block, but the deeper holes become progressively more spread out. A-scan from 0.250” SDH top C-scan view end D-scan view side B-scan view hole depths 0.250” 0.500” 0.750” 1.000” 1.250” 1.500” 1.750” 2.000” • 0.250 in. deep SDH (a) (b) Figure 1. Beam profile characterization for a 10MHz conventional transducer. (a) A-scan reflection from 0.020” SDH and top view C-scan from ultrasonic beam profile characterization block. (b) simulated side and end views of the block cross-section created from A-scan data. Table 1. Beam Profile Analysis-Conventional Transducer conventional transducer (33.0 dB total gain) SDH depth (mm) peak reflection amplitude/ peak noise amplitude -6 dB beam width (mm) 3.2 90/2 2.0 6.4 86/2 2.5 12.7 62/2 2.5 19.0 47/2 2.5 25.4 32/2 3.0 31.8 24/2 4.0 38.1 19/2 4.0 44.4 17/2 4.0 50.8 9/2 5.5 Beam profile characterization for the linear array transducer was conducted using the same basic procedure as for the conventional probe but with appropriate modifications for the array transducer geometry. Due to the variable focusing capability of the linear array, water path can be selected somewhat independent of the focal depth. Therefore, a 25 mm water path was selected which provides sufficient water path for efficient focal law creation, but also minimizes the variability that can result from the use of a long water path. Another difference between the linear array transducers and conventional spherical focused transducer is the presence of a non-focused beam direction. The conventional transducer is focused to a cylindrically symmetric spot by design. However, without the use of a curved array or lens, the 1-dimensional linear array is inherently a line-focused transducer. Therefore, beam profile characterization of a linear array transducer must be performed for both probe directions. Figure 2 shows schematically the orientation of the array probe relative to the SDH targets and the resulting beam focusing direction. The figure also illustrates that interrogating an asymmetric target, such as a SDH, with an asymmetric beam, such as the line focus from a linear array, the beam area that intersects the target is different for each orientation. Thus, the gain settings required to achieve an 80% FSH reflection from the SDH are expected to also be different for each orientation. This paper will discuss characterization of the beam profile for both probe orientations. The orientation shown in Figure 2a with the focusing direction of the probe parallel to the SDH long axis will be referred to in this text as the lateral scanning mode. The probe orientation shown in Figure 2b, with the probe rotated 90 degrees from that shown in 2a, will be referred to as the normal scanning mode. Lateral scanning mode will be discussed first. beam characterization specimen SDH targets line focused beam z y x ele ctr on ic sc an dir ec tio n mechanical scan direction linear array probe linear array probe beam characterization specimen SDH targets array elements