18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS APPLICATION OF ANOMALOUS WAVE PROPAGATION IMAGING METHOD WITH ADJACENT WAVE SUBTRACTION TO ACTUAL DAMAGES IN COMPOSITE WING Chen Ciang Chia 1 , Jung-Ryul Lee 1 *, Chan-Yik Park 2 1 Department of Aerospace Engineering, Chonbuk National University, Jeonju, Chonbuk, Republic of Korea, 2 Aeronautical Technology Directorate, Agency for Defense Development, Daejeon, Republic of Korea * Corresponding author (leejrr@jbnu.ac.kr) Keywords : field-test, laser ultrasonics, nondestructive evaluation, debonding, impact damage Abstract 2 Field Application of the AWPI Method Anomalous wave propagation imaging method was 2.1 Core Components and Setup applied for the on-site in-situ nondestructive evaluation of actual debonding and impact damages The hardware needed for the application of the in a composite wing with high density of structural AWPI method is the ultrasonic propagation imaging elements. The promising results showed that it has system [2]. A laser ultrasonic generator produces high potential as on-site nondestructive evaluation laser pulses, a galvanometric laser mirror scanner method of other complex engineering structures. deflects the laser pulses toward the inspection structure for ultrasonic wave generation, and the ultrasonic receiver integrated in the structure 1 Introduction captures the ultrasonic waves for result processing. Structural health monitoring (SHM) and The laser adopted in this study was a Q-switched nondestructive evaluation (NDE) technologies can Nd:YAG diode-pumped solid state laser (QL). Its accrue enormous economic and life-safety benefits. wavelength, diameter of the laser beam at the exit Among the many schemes adopted in these port of the laser head, divergence, and pulse duration technologies, the guided acousto-ultrasonic wave was 532 nm, 0.7 mm, 1.6 mrad, and 30 ns, technique is one of the widely accepted schemes. respectively. In practice, the 532 nm visible green Based on the laser ultrasonic scheme, we developed laser beam provides convenience when an inspector the anomalous wave propagation imaging (AWPI) controls the scanning area. The ultrasonic receiver method with adjacent wave subtraction, and the selected for this study was an omni-directional, Variable Time Window Amplitude Mapping amplifier-integrated broadband piezoelectric sensor (VTWAM) in our previous work [1]. The results with a cut-off frequency of 2 MHz and diameter of 4 from laboratory tests were compared favorably with mm. the results of an immersion ultrasonic C-scan. The target of inspection in this study was a real wing However, application to aerospace structures is not testbed of an unmanned aerial vehicle, attached to an straightforward due to the stringent requirements H-beam fixture shown in Fig. 1. The testbed was imposed by the regulatory agencies. Extensive made of carbon fiber reinforced plastic, with laboratory investigations are required, and validation common wing structural elements such as spars, in actual structures for the evaluation of real damage stringers, ribs, and skin. Both the laser and the is emphasized. The field application of the AWPI scanner were put on a two-axis slider above the method in this study was conducted in the endeavor testbed, and the rest of the system components not of seeking wider acceptance of this method by the related to laser scanning were installed in a mobile aerospace industry. It was conducted using a newly rack on the ground level. The two-axis slider could proposed test setup with a built-in laser ultrasonic be easily repositioned to facilitate inspection at an propagation imaging system [2] that can be arbitrary site. Using this configuration, the laser expanded easily for automatic laser-based NDE of pulses could be deflected by the scanner towards the wing structures in a whole hanger.
3 Field Inspections H-beam test fixture 3.1 Case I: Suitability for Smart Structures Case I was conducted to show the suitability of the laser head AWPI for the inspection of smart structures with integrated transducers. Hence, the testbed was inspected when it was in intact condition, and the scanner inspection area was selected to include three 18 mm PZT (lead zirconate titanate) elements bonded between the stringers, as shown in Fig. 2. These PZT elements did not participated in the waves sensing, but were merely a type of material discontinuity in loading forces this study. The ultrasonic sensor used for this inspection was temporarily bonded on the internal surface of the wing skin at the position shown in Fig. 2. The size of the inspection area, the fluence of the Fig. 1. Field test setup for real wing inspection. QL, the bandpass, and the sampling time interval were set to 300×300 mm 2 , 37 mJ/cm 2 , 240-290 kHz, scanning grid points on the upper surface of the and T =0.4 µs, respectively. testbed easily. Optimum scanning grid with a pitch access door (a) leading edge of Δ =1.0 mm and a laser pulse repetition frequency of 1 kHz were used throughout this study after considering the inspection speed, the resolution of stringers the results, and the reverberation effect. All signals were sampled using 500 data points. 2.2 Algorithms Case I Algorithms tested in this study were the AWPI and the VTWAM. The AWPI can enhance the visibility temporary of the anomalous waves, which finally works as an sensor enhanced damage evaluation algorithm for the ultrasonic propagation imaging system. In this context, the anomalous waves are the waves related rib to the structural anomalies, including the scattering waves and the confining waves. The AWPI was (b) developed based on the feature that the time domain ultrasonic signals acquired from two adjacent laser embedded scanning grid points are highly similar. Subtraction transducers temporary of the adjacent wave hence suppresses the incident for other sensor used waves and exaggerates the anomalous waves. The application for this VTWAM was developed based on the time between inspection difference of the incident waves and the anomalous stringers waves. The anomalous wave appears after the incident wave sweeps over the structural anomaly, and the confining wave with features similar to the standing wave exhibits a longer duration time within the damage. Using the VTWAM after the AWPI processing can generate an amplitude map that shows the location, size, and shape of the damages Fig. 2. Inspection Case I. (a) Inspection area. (b) resemble to the actual damages. Internal view of the wing testbed.
APPLICATION OF AWPI METHOD WITH ADJACENT WAVE SUBTRACTION TO ACTUAL DAMAGES IN COMPOSITE WING 3.2 Case II: Evaluation of Debonding Damages 3.3 Case III: Evaluation of Impact Damages Case II was conducted after the wing underwent a The testbed was impacted in order to find the critical series of destructive bending tests. The inspection impact energy of the testbed, and to show the area was selected at a damage hot spot, as shown in capability of the developed AWPI method for real Fig. 3. This inspection area was selected because it impact damage evaluation. The impact site was contained many other structural elements including selected near one of the most complex areas of the the boundary of the inspection window, rib flange, testbed, as shown in Fig. 4. Three impact events and the lug, and an integrated ultrasonic sensor (IS1) tabulated in Table 1 were made between the rib and was available at the spar web. It was selected stringers using an indenter of 4 mm diameter. Two intentionally to show that the high density of integrated sensors were available for the inspection structural elements would not interfere with the of the impact damages. The IS2 and IS3 were normal AWPI inspection. The size of the inspection integrated at the web of the rib, 30 mm from the area, the fluence of the QL, the bandpass frequency, upper skin, as shown in Fig. 4. The redundancy of and the sampling time interval were set to 200×200 the sensor within a small region of the wing was mm 2 , 34 mJ/cm 2 , 60-200 kHz, and T =0.6 µs, designed to crosscheck the results obtained from two respectively. sensors. The size of the inspection area, the fluence of the QL, the bandpass filtering, and the sampling inspection (a) time interval were set to 300×300 mm 2 , 75 mJ/cm 2 , window 60-200 kHz, and T =0.8 µs, respectively. Table 1. Details of impact events. Event Energy Visual inspection E1 15 J Invisible E2 15 J Invisible E3 25 J 13 mm diameter puncture rivet IS3 (b) E1 E3 E2 stringer rib rib flange area IS2 rivet 300 x 300 (a) All markings were removed before scan. IS2 & IS3 were located 30 mm from the top skin IS2 IS3 rivet lug Landing gear door opening IS1 (b) Fig. 3. Inspection Case II. (a), (b) high density of Fig. 4. Inspection Case III. structural elements within the inspection area. 3
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