Issues on the Durability of Piezoceramic Transducers for in situ SHM Using Acousto Ultrasonics

Transcription

1 2nd International Symposium on NDT in Aerospace Mo.5.A.2 Issues on the Durability of Piezoceramic Transducers for in situ SHM Using Acousto Ultrasonics N. RAJIC, K. A. TSOI AND C. ROSALIE Defence Science and Technology Organisation, 506 Lorimer Street Fishermans Bend, Victoria, 3207, Australia Abstract. It is expected that acousto-ultrasonic techniques applied using structurally integrated low-profile piezoceramic transducers will eventually provide an effective low-cost and reliable means of non-destructive inspection for high value engineering assets like aircraft. One of the key attractions of this form of structural inspection is that broad diagnostic coverage can be achieved with a relatively low sensor density, which in principle should result in a relatively low system footprint and thereby alleviate some of the system certification challenges associated with the application of structural health monitoring technology to aircraft. Another critical aspect from a certification viewpoint is system reliability, which is determined in large part by the stability of piezoceramic materials under environmental and mechanical loading. This paper describes some experimental work addressing the effects on the performance of piezoceramic wafer transducers of exposure to a mechanical loading spectrum derived from a structurally critical location on a military aircraft. It proposes some simple metrics that provide a promising basis for diagnosing certain forms of transducer degradation and also describes a novel means of mitigating load transfer from a host into a bonded transducer allowing piezoceramic materials to be applied in applications where the mechanical loading would normally lead to a rapid deterioration in performance. License: 1

2 Introduction The transition of structural health monitoring (SHM) technology from the laboratory into practice is an ambitious step involving challenges on many levels, including scientific, engineering, and logistical. The complexity of the task leads inevitably to an incremental and often slow rate of development. This certainly applies to acousto-ultrasonics (AU) [1]. Despite its great potential across a wide range of SHM applications the technique remains largely confined to the laboratory, with the occasional technology field demonstration [2], and arguably is still some way from providing a useful alternative to conventional forms of ultrasonic nondestructive inspection (NDI). A key concern relates to the reliability of piezoceramic sensors integrated within structural components exposed to mechanical loads and environmental extremes. The factors that precipitate the very structural damage that SHM techniques are proposed to detect constitute a similar threat to the structural integrity and performance of in situ transducers. Although SHM is recognised as offering large potential advantages over conventional NDI practice, on the issue of transducer reliability it is currently at a disadvantage. Without stable and reliable sensor performance many of the projected advantages of SHM to structural integrity management cannot be realised. Aircraft pose some of the most challenging applications in this respect as the structural loads in an airframe tend to be relatively high. This paper focuses on the issue of piezoceramic transducer durability. For context, it introduces a case study involving the application of AU as a means of SHM for a structural integrity problem in the F-111 aircraft. The case involves a critical location in the lowerwing skin where the flight spectrum loading leads to strain excursions approaching When exposed to that particular loading spectrum piezoceramic transducers are shown to degrade in performance leading to an impairment of diagnostic function. A vital first step in addressing transducer durability is to identify and understand the precise modes of degradation. This paper describes experimental work done by the authors to assess the efficacy of a set of simple metrics in helping to reliably identify certain forms of transducer degradation. The paper also describes a means of improving transducer durability through the addition of a small structural element to a piezoceramic transducer that significantly reduces load transfer from the host without compromising the acoustic performance of the transducer. Cracking in the Lower Wing Skin of the F-111 A Case Study The F-111 is a high-performance strike aircraft and for almost 37 years has been a vital asset for the Royal Australian Air Force (RAAF). However, like most aging aircraft the F- 111 has suffered a range of structural integrity problems that have had to be carefully managed by the RAAF to ensure its continued safe and economic operation. A particularly serious issue was identified in the mid 1990's and involved fatigue cracking in the lower wing skin at the location of a fuel-transfer groove on the inside of the wing, shown in Figure 1a. The problem was eventually solved by applying a boron-composite bonded patch to the external side of the wing skin directly over the fuel transfer groove. The patch reinforces the structure leading to reduced stresses and a decline in the rate of fatigue crack growth [3]. In fact, an assessment conducted on one fleet aircraft 668 flight hours after installation of the patch revealed that no further crack growth had occurred in the repaired structure [4]. 2

3 Figure 1. (a) Structural detail at a fatigue prone location in the F-111 lower wing skin, and (b) strain histogram at an optimal transducer location to inspect this location, corresponding to a 500 hour block of F- 111 flight spectrum loading. Although a highly cost-effective means of repair the technique is not presently certified for application to flight-critical structure. Its use on the F-111 problem was permitted only on the basis of an exhaustive experimental substantiation of the repair and the application of a rigorous in-service NDI regime to ensure the integrity of the bond-line. Such conditions obviously add considerable expense and restrict the scope of application of bonded repair technology. SHM provides a potential solution. Using a network of piezoceramic elements embedded within the patch it is possible in principle to assess autonomously and continuously both the integrity of the bond-line as well as the repaired structure. The concept, termed the Active Smart Patch, is currently under development by DSTO [5]. As part of this development effort fatigue tests were conducted on structurally detailed specimens (Figure 1a) in the absence of a repair patch to help identify optimal transducer locations for maximum sensitivity to crack growth in the fuel transfer groove. The use of an F-111 flight spectrum loading sequence (see Figure 1b ) was vital to ensure that crack growth in the specimen reflected as much as possible the situation in actual fleet aircraft. Despite the relatively benign median strain level (approx. 800 ) of the sequence, the transducer response signal showed anomalous behaviour that could only be related to degradation of the transducer. This is demonstrated by Figure 2 which shows the signal from two transducers located adjacent to the fuel transfer groove, spanning a test duration equivalent to almost 2000 simulated flight hours (SFH). Prior to the confirmed initiation of a crack in the fuel transfer groove at approximately 1500 SFH the response signal declined in strength by almost 20%. This trend reflects a deterioration in the performance of the transducer and in relation to the second transducer trace (solid line) is almost indistinguishable from the crack signature. It is also important to bear in mind that in an SHM scenario decisions about the presence or absence of structural defects are made largely on a continuous basis and will not have the benefit of the complete trace shown here. As a typical SHM application, the example serves to demonstrate the importance of identifying, understanding, and if possible preventing transducer degradation. 3

4 Figure 2. Response signal from transducers located near the fuel transfer groove, measured periodically during fatigue testing under flight spectrum loading. The presence of a crack in the fuel transfer groove was confirmed at approximately 1500 simulated flight hours. The primary causes of piezoceramic transducer deterioration under mechanical loading are stress depoling, fatigue and structural disbonding of the transducer from the host and of the transducer package itself. The latter two are in principle the simplest to address and the following section discusses some experimental techniques that can be applied in practice to identify these modes of degradation. Transducer Disbonding Piezoelectric transducers developed for SHM applications normally comprise a thin piezoceramic wafer sandwiched between a metallised polyimide film [6,7] which provides a structurally robust electrical connection between the transducer and the interrogation system, as well as ensuring that the transducer is electrically isolated from the host. The transducer package is then bonded to or embedded within the structure under investigation. Consequently, at least three material interfaces are involved in a typical piezoceramic transducer installation, and two of these form the essential load transfer path from the host to the piezoceramic wafer and are vulnerable to deterioration when the strain levels in the host are relatively high [7-11]. The electromechanical impedance spectrum of a structurally integrated piezoceramic transducer is largely determined by the electrical properties of the transducer and the mechanical properties of the transducer/host system which includes the adhesive bondline. This is shown in Figure 3a which reveals that the attachment of a piezoceramic element to an aluminium substrate produces a large change in response at the system resonances. Most notably, the first lateral mode increases in frequency which reflects an effective stiffening of the transducer, and the strength or quality of the resonance declines. The free and fully bonded cases represent extremes and in practice a disbond is likely to involve only part of the transducer bondline, so the case of a partial disbond is of greater practical interest. Accordingly, a study was done to characterise the variation in impedance spectrum for disbonds of varying size and in different parts of the transducer assembly, 4

5 specifically the adhesive bondline between the transducer and host, as well as at the interface between the piezoceramic and the polyimide layer adjacent to the adhesive bondline. The transducers were disc-shaped, 6.35 mm in diameter with a total thickness including the polyimide film and the various intermediate layers of approximately 650 m. The piezoceramic disc was 500 m thick. The transducers were selected from a larger collection with the aim of ensuring the minimum possible variance in electrical capacitance within the chosen set - the maximum variance was 2.8%. Partial disbonds in the polyimide layer involving 25%, 50% and 75% of the transducer area were created by carefully peeling the appropriate length of polyimide film from the piezoceramic wafer. These elements were then adhered to a 3 mm thick aluminium plate using epoxy adhesive with the disbonded side of the transducer facing the plate. Identically sized partial disbonds in the adhesive bondline were produced by masking the appropriate area of the transducer from the epoxy adhesive using a thin layer of Teflon. Figure 3b shows the impedance spectra for the three transducers with partial disbonds in the adhesive layer. The results generally follow expectation. They indicate a slightly higher quality resonance and a downward shift in resonant frequency with increasing disbond size, consistent with a reduced level of mechanical constraint. It seems likely that the nominal target disbond sizes of 25%, 50% and 75% were not actually achieved. This is supported by the fact that the spectrum for the 25% partial disbond appears similar to the fully bonded case shown in Figure 3a, and that little difference is observed between the spectra for the 50% and 75% disbond cases. Notwithstanding these experimental limitations, the results confirm that a measurement of the impedance spectrum offers a very powerful tool for the assessment of bondline integrity. The results for the polyimide disbond group (Figure 3c) show a more erratic pattern of behaviour. This is largely explained by the fact that when the polyimide layer is peeled from the piezoceramic wafer to produce a disbond, separation can occur at either of two interfaces: between the piezoceramic wafer and its metallic electrode layer or within the metallisation layers between the polyimide film and the piezoceramic wafer. The effects on the impedance spectrum are very different. The first scenario leads to a loss of electrode area and consequently an increase in the capacitive impedance of the transducer. Such a shift is observed for the 50% partial disbond case shown in Figure 3c and visual inspection of the disbonded interface verified that a large section of electrode had peeled away from the bare pieoceramic. The second scenario is largely equivalent to a disbond in the adhesive layer. Both the 25% and 75% partial disbond cases have an impedance magnitude at low frequency (away from resonance) broadly consistent with that of the equivalent adhesive disbond cases (Figure 3b) suggesting that the disbond was mostly confined to the polyimide layer. If we assume that the mechanical impedance of the host structure is much greater than that of the PZT transducer and if we ignore dielectric losses in the PZT, the electrical admittance for a bonded PZT transducer can be expressed as, [12] A T 2 E Yb ( w) iw ( 33 d31y p ) tc A 2 E Yfree( w) iw ( d31y p ) t c 5

6 where Y free (w) is the admittance of an unbonded element and is given by, T iw 33 A t c iwc T and A, t c, 33 and C are the surface area, thickness, dielectric constant and E capacitance of the transducer, respectively. Y p is the complex Young's modulus of the transducer at zero electric field and d 31 is the piezoelectric coupling constant. The second term in the expression describes the loss in admittance caused by the structural attachment of an element to a host and should in theory vanish when an element becomes detached. Park et al have shown that this produces a change in the gradient of the imaginary component of the admittance spectrum, and yields a potentially useful basis for the detection of transducer disbonding. It was a relatively straightforward extension of the previous exercise to produce imaginary admittance spectra for all of the transducer conditions described previously. These are shown alongside the impedance magnitude spectra as subfigures Figure 3d, e and f. Figure 3d confirms the presence of a measurable effect however it also reveals a noticeable level of variation in the gradient amongst nominally identical elements, amounting to about 5% of the difference between a free and fully bonded element. Although the traces for the different partial epoxy disbonds are difficult to separate at the scale used in Figure 3e, a close examination confirms that the trends are consistent with those observed in the impedance magnitude spectra. That is the spectra for the 50% and 75% partial disbond cases are almost identical, while the 25% case yields an admittance spectrum similar to that of a fully bonded element, reinforcing the doubt expressed earlier about the precise size of the disbonds. Far greater variation in admittance occurs for the transducers with a disbond involving the polyimide layer. The 50% case shows a particularly large decline in admittance which is consistent with earlier remarks about the drop in capacitance of this transducer caused by the loss of a large part of the 6

7 piezoceramic electrode layer. Figure 3. Measured impedance magnitude spectra for transducers in various states of bonding to a structural host (a) fully bonded and free, (b) partial disbonds in the adhesive layer and (c) partial disbonds within the transducer package. The corresponding imaginary admittance spectra are shown in (d)-(f), respectively. Solid lines denote the response of a free element and dashed lines a bonded element. Given percentages define the relative disbond area. Transducer Load Mitigation Since mechanical loading is the primary cause for degradation of piezoceramic materials used in AU applications any measures that result in a reduction in load transfer to a piezoceramic element will result in more reliable performance. A simple method was recently described [13] that produces substantial reductions in the amount of load transfer to surface bonded transducers. The essential idea is to introduce local 7

8 out-of-plane or secondary bending in the host through a structural augmentation called the stand-off element (SoE). The concept is shown schematically in Figure 4 where the structural host is a simple plate under a far-field tensile stress of. The SoE influences the stress in the transducer through a combined effect on the membrane forces and the bending moments in the layered structure. The load in the piezoelectric element can of course be made arbitrarily low by simply making the SoE thickness sufficiently large, that is by reducing the membrane stress, however it can be shown that this approach requires a large SoE thickness which impacts on the acoustic efficiency of the transducer. It turns out that significant reductions in stress can be achieved with thinner elements through the action of out of plane or secondary bending induced by the load eccentricity caused by the combined transducer/soe assembly. Figure 4. Three layer composite structure representing a piezoceramic wafer element (3) rigidly attached to a stand-off element (2), itself rigidly attached to a plate loaded in tension (1). The approach is described in detail elsewhere [13]. It suffices here to summarise a few of the key points. Figure 5 traces the maximum stress in a 0.5 mm thick piezoceramic wafer for a range of SoE thicknesses. In this hypothetical example the SoE is an aluminium strip and is rigidly attached to an aluminium plate loaded in tension at four stress levels. Produced from an analytical model the results show that a minimum transducer stress state is achieved for certain unique values of SoE thickness. For the highest load case, the benefit amounts to a four-fold reduction in peak transducer stress. An experimental investigation was also done. In that work a carbon-epoxy laminate sample, approximately 1 mm thick, was manufactured with three SoE's made of the same bi-directional pre-preg but using a different number of plies in order to produce three different thicknesses. Only one of these was designed to optimally reduce load transfer. Figure 6 shows the voltage signal recorded from four transducers, three mounted to the SoE's and one bonded to the bare surface, as the sample was exposed to a periodic (fully tensile) load. The traces to note are T2 and T3. T2 is the optimal case and corresponds to an SoE thickness of 0.72 mm. It has the smallest signal amplitude which is consistent with the achievement of an almost ideal transducer stress state under the proposed configuration. T3 represents the standard method of transducer attachment (no SoE) and shows a vastly higher piezoelectric response voltage suggesting higher absolute stress levels in the transducer. 8

9 Figure 5. Stress transfer ratio as a function of SoE thickness t 2. Figure 6. Measured piezoelectric response voltage for a 1 Hz cyclic loading. Conclusions This paper has summarised aspects of a broader experimental work program aimed at developing strategies to deal with the issue of piezoceramic transducer durability. The work underpins an effort to develop a useful and reliable acousto-ultrasonic SHM capability for airframe and other applications where structurally integrated transducers are likely to experience relatively high operational loads. Experimental results were shown that demonstrate how the electromechanical impedance spectrum of an installed transducer provides valuable diagnostic information about the structural integrity of its bondline. In addition, a brief account was given of an approach that substantially reduces the amount of 9

10 load transfer to a surface mounted transducer through the simple insertion of an optimally designed intermediate layer called the stand-off element. References [1] A. Vary. The Acousto-Ultrasonic Approach. Technical Memorandum 89843, NASA, [2] M. Malkin, X.D. Qing, M. Leonard, and M. Derriso. Flight Demonstration: Health Monitoring for Bonded Structural Repairs. In Alfredo Guemes, Editor, Structural Health Monitoring: Proceedings of the Third European Workshop, [3] R. Boykett and K.F. Walker. F-111C Lower Wing Skin Bonded Repair Substrantiation Testing. Technical Report DSTO-TR-0480, Defence Science and Technology Organisation, Australia, [4] A. Baker. Structural Health Monitoring of a Bonded Composite Repair Patch on a Fatigue-Cracked F- 111C Wing. Research Report DSTO-RR-0335, Defence Science and Technology Organisation, Australia, [5] N. Rajic. Development of an Active Smart Patch for Aircraft Repair. In C. Boller, F.K. Chang, and Y. Fujino, Editors, Encyclopedia of Structural Health Monitoring. John Wiley and Sons Limited, Chichester, U.K [6] K. Tsoi and N. Rajic. Durability and Acoustic Performance of Integrated Piezoceramic Transducer Elements Under Cyclic Loading. Materials Forum, 33: ,2009. [7] G. Lanzara, Y. Yoon, Y. Kim and F.K. Chang. Influence of Interface Degradation on the Performance of Piezoelectric Actuators. Journal of Intelligent Material Systems and Structures, 20: , [8] C.A. Paget, K. Levin and C. Delebarre. Actuation Performance of Embedded Piezoceramic Transducer in Mechanically Loaded Composites. Smart Materials and Structures, 11: ,2002. [9] Sung J. Kim and J. D. Jones. Effects of Piezo-Actuator Delamination on the Performance of Active Noise and Vibration Control Systems. Journal of Intelligent Material Systems and Structures, 7: , [10] C. A. Paget and K. Levin. Structural Integrity of Composite with Embedded Piezoelectric Ceramic Transducer. In Proceedings of SPIE, Smart Structures and Integrated Systems, Newport Beach, CA, [11] V. Giurguitiu. In-situ Fabricated Smart Material Active Sensors for Structural Health Monitoring. In Proceedings of SPIE The International Society for Optical Engineering, 5648:68-78, [12] G. Park, C.R. Farrar, F.L. Di Scalea and S. Coccia. Performance Assessment and Validation of Piezoelectric Active-Sensors in Structural Health Monitoring. Smart Materials and Structures, 15(6): , [13] N. Rajic. A Strategy for Achieving Improved Piezoceramic Transducer Durability Under High Structural Loading. Smart Materials and Structures, 19,