PSI - Issue 28

J. Weiland et al. / Procedia Structural Integrity 28 (2020) 1249–1257 Weiland et al. / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction According to DIN EN ISO 9001 and DIN 2304, adhesive bonding is a special process that requires separate standards. This classification is based, among other things, on the fact that adhesively bonded joints cannot be tested completely non-destructively. [1], [2] The motivation of monitoring adhesively bonded joints is based on this fact. The structural condition can be recorded and thus a safe transmission of load can be guaranteed. A further complication is to predict the lifetime of the adhesively bonded joints. This is primarily due to a lack of knowledge about the fatigue behavior of adhesives, especially under varying environmental conditions (temperature, humidity, UV radiation) according Abdel Wahab et al. [3] and Graner Solana et al. [4]. According Zäh et al. [5] at present, most structures are subject to recurring inspections. Visual inspections are state of the art as well as the use of non-destructive testing (NDT) (e.g. ultrasound, lock-in thermography and shearography). Structural Health Monitoring (SHM) means a permanently monitored object by permanently installed sensor technology. Possible systems and their application in practice are investigated in a variety of industries (aircraft construction e.g. Wölcken and Papadopoulos [6], civil engineering e.g. Sigurdardottir [7], wind energy e.g. Cinag et al. [8] and Soman et al. [9] and many more). The methods of SHM for adhesively bonded joints can be divided into intrinsic methods - sensor technology is in the adhesive layer - and extrinsic methods - sensor technology is on the adherent. Both variants have specific advantages and disadvantages. Extrinsic methods have the advantage that they have no influence on the mechanical behavior of the adhesive layer. Takahashi et al. [10] investigates piezoelectric actuators (Lamb Waves) used to check the adhesive joint between the outer skin and the stiffener (stringer) following an aircraft structure. When used at the end of the stringer, damage can be detected via the time delay of the emitted wave signal. Weiland et al. [11], and Sadeghi et al. [12] chooses a structural-mechanical approach in which strain-marking points on the adherend are monitored and compared with the structural-mechanical behavior of an undamaged adhesive bond. For this approach, a detailed knowledge of the structural-mechanical behavior of the adherent and of the adhesive for the types of load involved is necessary. Outside of metals as joining material, the approach could not be confirmed yet. Zockoll and Plagemann [13] monitor the ageing behavior with electrochemical impedance spectroscopy and use both substrate and adhesive-integrated electrodes. There is no direct correlation to the mechanical influence of aging. Intrinsic methods have the advantage of measuring in-situ data from the adhesive layer. This represents an SHM concept independent of the joining material. However, the integrated sensor influences the mechanical behavior of the bond and can lead to considerable stress peaks in the bond. If the mechanical properties of sensor and adhesive show large differences, this problem increases, as is the case with glass fiber-based sensor concepts. Crossley et al. [14] have investigated this approach, among others, who show the principle use of Fiber Bragg Gratings (FBG) for strain measurement within bonded repair patches. Frövel et al [15] use FBG to monitor residual stresses released. These stresses are assigned to a delamination. However, the transfer to a structure demonstrator was not successful. The use of FBG and interferometrically spatially resolved Rayleigh backscattering in glass fibers was the subject of the investigation from Hildebrand et al. [16], [17]. The basic suitability of these methods was successfully demonstrated. However, these sensor principles only detect strain changes in the longitudinal direction of the fiber and therefore, cannot be used in a sensor geometry parallel to the adhesive joint. A detection of strain in radial direction is not possible. Sulejmani et al. [18] used micro-structured optical glass fiber to monitor debonding in adhesively bonded single lap joint. With these kinds of special glass fibers debonds in range of 100 μm can be detected. According Engelbrecht [19] sensors with Polymer Optical Fibers (POF) are still comparatively new in contrast to glass fiber based sensors. Up to now, the focus for SHM is on sensors in the direction of the fiber, for e.g. Djordjevich [20] and Becker at al. [21] did investigations on it. Kuan et al. [22] used special prepared POF to measure the strain of aluminum samples. The POF were mechanically prepared in certain areas to change the light emission under bending significant. These POF were bonded to aluminum samples. By changing the bending radius due to the elongation of the aluminum, the integral light output changed. Further investigations or the integration into the adhesive layer are not known.