Structural health monitoring of aerospace materials

Structural health monitoring of aerospace materials used in industry using electrical potential mapping methods S. A. Grammatikos, M.-E. Kouli, G. Gkikas, A. S. Paipetis* Dept. of Materials Engineering, University of Ioannina, 45110 Ioannina, Greece *e-mail: [email protected]

ABSTRACT The increasing use of composite materials in aerostructures has prompted the development of an effective structural health monitoring system. A safe and economical way of inspection is needed in order for composite materials to be used more extensively. Critical defects may be induced during the scheduled repair which may degrade severely the mechanical properties of the structure. Low velocity impact LVI damage is one of the most dangerous and very difficult to detect types of structural deterioration as delaminations and flaws are generated and propagated during the life of the structure. In that sense large areas need to be scanned rapidly and efficiently without removal of the particular components. For that purpose, an electrical potential mapping was employed for the identification of damage and the structural degradation of aerospace materials. Electric current was internally injected and the potential difference was measured in order to identify induced damage in Carbon Fiber Reinforced Polymer (CFRP) structures. The experimental results of the method were compared with conventional C-scan imaging and evaluated. KEYWORDS: Composites, Electrical potential mapping, Structural Health Monitoring 1. INTRODUCTION Fibre reinforced polumers (FRPs) are widely employed as structural elements in the aircraft and marine industry. Due the laminate nature of such structures, they are suitable for complex geometrical applications. In addition, FRP’s are distinguishable materials with excellent specific mechanical properties. Depending on the orientation of the layers of the laminate as well as the employed matrix, tailored mechanical properties may be achieved. Despite the benefits of composite materials, special concern should be considered to retaining of their structural integrity during their service life, as they are relatively new materials compared to metals. A qualitative and quantitative non-destructive inspection is essential in order to assess the mechanical properties of a composite laminate [1-7]. A large amount of non-destructive techniques can be found in literature, and are widely employed by the aircraft industry. Infrared thermography, X-rays, Shearography and Ultrasonics are some of the mostly used non-destructive inspection techniques [6, 8-11]. Each technique may provide specific information about the condition of the structure. The great challenge of a non-destructive inspection system is to be time cost and also energy effective. On the other hand, structural degradation caused by low velocity impact damage (LVI) is one of the most dangerous types of damage encountered in aircraft structures. LVI damage is usually invisible or barely visible to the naked eye. Thus, the inspection of impact damage is of primary importance as layered structures are by nature susceptible to this type of damage. Carbon Fibre Reinforced Polymers (CFRPs) are ideal materials for new age aircrafts as they possess improved specific mechanical properties. At the same time, CFRPs have been shown to function both as structural and sensing elements [1, 3, 12-16], via the monitoring of their electrical properties. This minimizes the need of any external sensing device as the material functions as a sensor itself. CFRPs are conductive due to the presence of the reinforcement and as a result they possess higher electrical conductivity in the direction of the carbon fibres. In order to render the electrical properties of CFRPs more isotropic Carbon Nanotubes (CNTs) may be dispersed within the matrix [5, 17-20]. In all cases, the monitoring of electrical properties may be performed either in service or out of service, for bulk or surface measurements or even for damage mapping in 2D or 3D [1, 7, 18, 21-28]. Within the scope of this study an electrical potential mapping technique (EPM) was developed in order to identify induced damage in aircraft materials. Rectangular CFRP plates with a circular notch as well as subjected to low velocity impact damage were investigated using EPM in order to assess the efficiency of the method. In the case of the impacted specimen, conventional C-scan was employed in order to validate the results of the EPM system. Smart Sensor Phenomena, Technology, Networks, and Systems Integration 2012, edited by Theodore E. Matikas, Kara J. Peters, Wolfgang Ecke, Proc. of SPIE Vol. 8346, 83461K · © 2012 SPIE · CCC code: 0277-786X/12/$18 · doi: 10.1117/12.915492 Proc. of SPIE Vol. 8346 83461K-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 09/08/2016 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx

2. THEORETICAL BACKROUND 2.1 Electrical potential mapping The electrical potential mapping (EPM) technique is a relatively novel technique although it has been widely investigated the last few years. The electrical potential / resistance mapping techniques have been also employed in systems different than those of the aerospace environment. EPM lies on the exploitation of the intrinsic electrical properties of composite materials which change when the material is damaged [25, 29-34]. For CFRPs, the conductive reinforcement within an insulating matrix increases the challenges posed for the application of the technique. Electrical conduction is mainly accomplished by the carbon fibre reinforcement, which renders the material highly electrically anisotropic. This Electrical anisotropy may be reduced by modifying the matrix with a conductive nanophase, such as Carbon nanotubes (CNTs). More analytically, in the case of plain CFRPs, the electrically conductive path is only the reinforcement. In other directions such as in the transverse or the through thickness direction, conduction is achieved via the random fibre contacts, which depend directly on the fibre volume fraction. When the conductive path within a CFRP is interrupted, the electrical properties of the material change. For example, upon delamination and fibre breakage the through thickness and the longitudinal electrical resistance respectively increase in a monotonic irreversible manner. By mapping the electrical properties i.e. electrical potential / resistance of the reinforced material, information about the damage state might be obtained. Since, the EPM does not require expensive external devices, fast and low-cost inspection is feasible. More analytically the requirements of the method are simply i) an electrical current supplier and ii) a digital multimeter [25, 29-34]. 2.2 Impact damage Low velocity impact damage is one of the most dangerous types of damage in composite structures. Impact damage instigates the catastrophic failure of a structure which may take place at an unknown time i.e. during a flight of an airplane. The orientation of the different layers of a composite laminate as well as the nature of the matrix makes the composite laminate a susceptible to load structure [35]. It is found that low velocity impact mostly initiates delaminations. In general, composite damage is categorized into four main groups, matrix cracking, interlaminar failure (delamination), interfacial failure (fibre-matrix debonding) and fibre failure [36]. Fig.3 depicts intralaminar cracking and delaminations observed after a low velocity impact. In most circumstances the surface of the damaged structure is shown to be intact [36-39].

Figure 1. Schematic representation of low velocity impact damage; invisible impact damaged.

3. EXPERIMENTAL 3.1 Material For the purposes of the current study, 10-ply quasi-isotropic CFRP specimens were manufactured using the hand lay-up method. The laminae orientation was (0,+45,90,-45,0)2 (Fig.2). A unidirectional carbon fabric G0947 1040 by Hexcel (France) was employed for the reinforcement. The epoxy matrix used was the Araldite LY 5052/Aradur 5052 by Huntsman International LLC (Switzerland). The employed curing cycle was 24h at room temperature conditions

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followed by 4h at 100 0C. The tested CFRPs were square laminates in dimensions 60x60x1mm3 as is clearly shown in Fig.2.

Figure 2. (a) CFRP specimen configuration, (b) quasi-isotropic CFRP laminate.

3.2 Experimental setup; EPM system implementation As aforementioned, rectangular CFRP specimens were cut from laminates. The two interrogated types of damage were (i) a 5mm drilled open hole and (ii) a 5J impact damage. The impact damage was imposed using a drop-weight round impactor, as shown in Fig.3.

Figure 3. Drop-weight impact setup configuration.

For the purposes of the electrical potential mapping, 16 electrical copper contacts were electrochemically plated onto the surface of the coupons (Fig.4). The adjacent strategy was adopted for all measurements, employing a simple algorithm for data processing and image construction in MatLab environment. In Fig.4 the EPM experimental setup and the measurement protocol are depicted. 16 electrodes were attached on the surface of the rectangle-shaped CFRP specimens. 100mA DC current was injected through an electrical contact pair. The voltage between all the remaining electrical contact pairs was subsequently measured using the digital multimeter. The same measurement sequence was followed by moving the pair of the injecting electrodes and measuring the voltage at the remaining electrical contacts. A simple algorithm was built for the data acquisition. As is depicted, the electrical contacts on the surface of the investigated coupons (Fig.4b) draw a circle which is divided in separated cells. Every cell represents an average of measurements. It was decided that when the figurative lines cover over than the 1/3 of the respective cell, then this cell is assumed to the total average.


(a) (b)

Figure 4. (a) EPM experimental setup, (b) EPM measurement protocol.

3.3 Results and discussion Fig.5 depicts optical images of the impacted specimen. In Fig.5c a C-scan image is manifested which points the induced damage by the round impactor. As is shown from Figs.5a and 5b the 5J energy level did not lead to penetration of the coupon. However, induced delaminations were imposed and are clearly visible in Fig.5a and 5b. In the case of the drilled-hole CFRP coupon, C-scan imaging was not required as the damage was an open circular notch. In Fig.6 an optical snapshot of the drilled specimen presents the artificial damage of the coupon. Figs.7 depicts the images which resulted from the electrical potential mapping technique for the two interrogated damage concepts; (a) CFRP specimen with the central 5mm drilled hole (Fig.7a) and (b) CFRP specimen including a central 5J impact damage (Fig.7b).

(a) (b) (c) Figure 5. Impact damaged coupon; (a) Top side of the coupon, (b) bottom side of the coupon, (c) C-scan imaging.

Figure 6. Circular drilled notch configuration.


(a)

(b)

Figure 7. ERM images; (a) 5mm drilled hole, (b) 5J impact damage.

In Fig.7a the differentiation in color in the centre of the image represents the drilled hole of the CFRP specimen The diversion from the normal color, in Fig.7b, around the centre of the image, depicts the damage between the layers and the damage of the surface, observed by the impact enforcement. As expected the impact damage resulted in a larger deteriorated area as manifested by Fig.7a and 7b. It is noteworthy to say that the orientation of the top layer of the composite coupon designates the direction of the high electrical conductivity values. The through thickness and the transverse electrical conductivity possess always lower values than the longitudinal conductivity or that of the reinforcement. Hence, the electrical current ‘prefers’ to pass through direction of the reinforcement. In other words, the electrical potential measurements are always affected by the fibre direction of the surface laminae. Fig.8 shows a schematic representation of the electrical behavior of a CFRP. Every fibre-layer may resemble an array of electrical resistances. R

Figure 8. Schematic representation of the CFRP electrical behavior.

4. CONCLUSIONS The scope of this work was to develop an electrical potential mapping (EPM) system with a view to identify induced damage in composite laminates. The preliminary results on this task proved the ability of the technique to identify induced damage. For the purposes of the study, rectangle-shaped CFRPs were manufactured. A 5mm drilled open hole and a 5J impact damage were the two interrogated types of damage. With respect to the EPM system, 16 electrically conductive contacts were electrochemically plated onto the surface of the employed coupons. In all cases, 16 electrodes were attached on the surface of the specimens. The adjacent strategy was employed for all measurements. 100mA direct current was injected through a pair of electrodes and the electrical potential was measured between the remaining contacts. This process was performed for all the remaining pairs. The measured data were through a simple algorithm imported in Matlab environment for the image construction. The employed damages were identified and compared each other. The ERM technique resulted in a different color in the centre of the image for the drilled-hole damage case whereas for the impact damage case in a wider deteriorated area. This is due to the presence of delaminations imposed


by the low velocity impact. It should be noted that the electrical measurements are highly affected by the surface characteristics and the fibre direction of the inspected components. Summarizing the above, the implementation of the ERM technique proved its reliability and effectiveness in identified induced damage in composite laminates. The preliminary experimental results provided a strong motivation to further investigate the potential of the technique. ACKNOWLEDGEMENTS Authors would like to thank Associate Prof. Stefanos Zaoutsos for providing the impact damage facilities located at the Dept. of Mechanical Engineering of the Technological Educational Institute of Larissa, Greece. REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

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