PSI - Issue 28

1592 M.Z. Sadeghi et al. / Procedia Structural Integrity 28 (2020) 1590–1600 M.Z. Sadeghi et al./ Structural Integrity Procedia 00 (2020) 000–000 in three rows on one of the adherents and positioned them based on the greatest of peaks obtained from BFS curves, where the crack is believed to have initiated by using the Finite Element Analysis (FEA). Even though many researchers have worked on BFS measurement using particular strain gauge positions, that being said however, the investigation into the effect of geometrical tolerances (existing in the assembly process) on strain measurement by strain gauges has not received much attention. In the present work, a location for positioning the strain gauge in single lap joint adhesively bonded joints was proposed. The location of mounting the strain gauge which was determined by FE model is based on a point on the adherent in which the strain is initially zero. The so called zero strain point (ZSP) on the adherent remains with zero strain as long as the bonded joint has no damage. Once the damage appears on the joint, the former so-called ZSP gains strain. The influence of joint geometrical tolerances on the recorded strains was investigated in this work. 2. Experimental work In the present work, SLJs with steel S700MC adherents (with the yield stress of ≥ 750 MPa) were manufactured. 3M Scotch-Weld DP 490 was chosen for bonding the adherents. This adhesive is commonly used in the automotive industry where the combination of toughness and strength is needed. Representative stress-strain (engineering) curve of the adhesive tested based on DIN EN ISO 527-1 is shown in Fig. 1. The properties of the adhesive used in the present study is given in Table 1. The dimensions of the adhesively bonded joints tested are depicted in Fig. 2. As it can be seen in Fig. 2, PTFE strips with the length of 10 mm were used in the assembly process to avoid adhesives surplus from free edges (i.e. right and left sides of the bonded joint). This arrangement provides the possibility to achieve the desired thickness of the adhesive by choosing the corresponding thickness of the PTFE strips. For the surface treatment of steel adherents, Corundum blasting was used (grain size of 150 µm to 210 µm and a pressure of 6 bar). The metallic adherents were cleaned with Isopropanol prior and after of the surface treatment by using an ultrasonic bath. The metallic adherents were bonded together with the adhesive in the designed mold for the SLJs. After the assembly, the joints were left at the room temperature for 24 hours and for another 3 hours in the oven with the temperature of 60 0 C. For both adherents, the nominal bond-line thickness of 0.7 mm was achieved after the assembly.

Fig. 1. Representative stress-strain (engineering) curve for 3M Scotch-Weld DP 490.

The displacement-control tensile tests on SLJs were carried out with the electric Instron 7075 (with the capacity of 30 kN) at the rate of 1 mm/min. To record the real displacement of the joint during the tensile test, Digital Image Correlation (DIC- GOM Correlate Software-Germany) system to measure the local deformation of the joint. Two strain gauges were mounted on the middle of the adherents on the location of ZSP which was determined by FE