PSI - Issue 42

Dennis Domladovac et al. / Procedia Structural Integrity 42 (2022) 382–389 Domladovac et al. / Structural Integrity Procedia 00 (2019) 000–000

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theory to describe discontinuities in the adhesive layer. Joints of rubber-like adhesives with gaps have not yet been investigated using a J-Integral evaluation. Considering the measurement of the specimen deformation during the experiment to gain insight into the fracture mechanical behaviour of adhesive layers in pure mode I, di ff erent measurement techniques have been used in subse quent literature: Salem et al. (2013) used strain gauges along the top surface of a Double Cantilever Beam (DCB) specimen to obtain the crack tip position in context of Backface Strain Measurement (BFSM). Lima et al. (2021) measured the backface strain of a structural adhesively bonded DCB specimen with optical fibres to track the crack tip position from the peek fibre strain. Their comparison between Digital Image Correlation (DIC) and visual inspection suggested that the peak in fibre strain can be used to identify the onset of the damage process zone. Furthermore, Truong et al. (2020) used BFSM to calculate the resistance curve of a composite material. DIC was also used by Reiner et al. (2017) and Sun et al. (2020) to obtain the displacement profiles of the specimen and calculate the energy release rate from the obtained displacement data. Moreover, Schrader et al. (2022) used BFSM with optical fibres to measure the J-Integral and traction separation law in DCB tests with a soft, rubber-like adhesive. DIC and BFSM are used in many fracture mechanics evaluation and are well known. To the authors’ knowl edge, there are no studies using shearography for fracture mechanics testing on DCB Specimen with adhesives. The shearography is typically a measurement method used for non-destructively material and component testing as qual itative method in process chain. The gathered images of the with coherent laser light illuminated surfaces are used to calculate the interferogram. For this, on two states (reference and loaded) four images are taken. The demodulated interferogram is the so called shearogram representing a component of the linearised deformation gradient (Steinchen and Yang (2003)). Kryukov et al. (2018) investigated the principal suitability of shearography for testing adhesive bonds. They came to the conclusion that by means of shearography the defects missing adhesive, air entrapment, inhomogeneous mixing, wrong mixing ratio as well as delamination can be detected well in the structure used there with a structural adhesive. With thick-film adhesives, the detectability of the individual defect types was worse. The defect type uneven adhesive thickness could not be detected. The aim of this research is to investigate the localisability of gaps in thick layered rubber-like adhesive joints at finite deformations. One of the focal points of this study is a comparison between the methods of specimen defor mation measurement, i.e. BFSM, DIC and shearograpy, to determine advantages and disadvantages for their use in fracture mechanical testing and gap localisation. For this purpose, DCB specimens with predefined gaps have been manufactured and tested. The measurement methods DIC, BFSM and shearography were selected to evaluate the us ability of these for the future work. The chosen gap width of 40 mm is far from the previously mentioned voids or kissing bonds, but gaps of this size in the adhesive layer are possible for design reasons and can be used deliberately. The DCB tests for this study were performed on the soft rubber-like adhesive Sikaflex ® -521 UV. This adhe sive is a one-component polyurethane-hybrid sealant, which cures under atmospheric humidity. The adherends of the used specimens were made of the aluminium alloy AlZn5.5MgCu (material grade number 3.4365, Young’s modulus E = 70 · 10 6 GPa). The dimensions of the specimen can be seen in Fig. 1 (analytical bending sti ff ness of EI z = 87 . 5 · 10 6 N / mm 2 ). As a pre-treatment, surfaces of the adherends were sandblasted with corundum (grain size of 100-150 µ m) and degreased with isopropyl alcohol. The adhesive was applied with a pneumatic caulking gun and PTFE spacers were placed at the beginning and end of the adhesive layer to define the nominal thickness of 3 mm. PTFE spacers were also used to predefine the gap and screw clamps were used to hold the adherends in position (curing time of 2 weeks at a temperature of (23 ± 2) ◦ C and relative humidity of (35 ± 5) %). The optical fibres were bonded to the adherends backface with M-Bond-200. Shortly before testing a speckle pattern for the DIC measure ment was applied to one surface with an white and black acrylic based spray lacquer using a spray can and the bottom surface was sprayed white for the shearography measurement. The experimental setup is shown in Fig. 2. The DCB-tests were performed with a electro-mechanical testing machine (inspekt table 10 kN, Hegewald & Peschke, Nossen, Germany) with a load cell of ± 5 kN capacity (S9M, Hottinger Bru¨el & Kjaer GmbH, Darmstadt, Germany). In addition, the rotary encoders (BDH 1P.05A320000-L0- 2. Experimental setup and methods 2.1. Specimen manufacturing and Experimental setup