PSI - Issue 28

M.Z. Sadeghi et al. / Procedia Structural Integrity 28 (2020) 1601–1620 M.Z. Sadeghi et al./ Structural Integrity Procedia 00 (2019) 000–000

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2. Experimental In this work, DCB and MMB tests (with three different mixed-mode ratios) were carried out to achieve the fracture energy envelope. DCB and MMB tests (including manufacturing the tests setup) were based on ASTMD5528 and ASTMD6671. Although such tests are proposed for the measurement of interlaminar fracture toughness of composites, however in many previous works such tests have been successfully applied for the determination of fracture toughness of adhesives (Khoramishad et al., 2010; Moura et al., 2012; Stamoulis et al., 2014). The adherents for the adhesively bonded specimens used for the DCB and MMB tests were strips manufactured from S700MC steel with a yield stress of R p0.2 ൒ 700 MPa. To make sure such steel does not experience plasticity regime under DCB and MMB tests (with different mixed-mode ratios), FE analysis was carried out in advance. The strips had the dimension of 200 × 27 × 3 mm 3 . A ductile two-component epoxy adhesive Araldite 2015 was used for bonding the steel adherents. As for surface treatment, the adherents were first cleaned with acetone to remove invisible oils and fats. Depending on the desired adhesive thickness, PTFE strips were used in the assembly. The nominal thickness of the adhesive 0.35 and 1.0 mm was achieved for DCB and MMB tests. The adherents were sandblasted with corundum and cleaned again with compressed air and acetone to remove all the remaining dust and fat on the surface. To guarantee the desired thickness of 0.25mm for the adhesive layer, Polytetrafluoroethylene (PTFE) strips were used. The PTFE strips also guaranteed an initial crack length of a 0 = 35 mm. The adhesive was applied with a cartridge gun and a static mixer. After assembly, the specimens were left to be cured in the room temperature for 24 hours. Fig. 1 shows the dimensions of the specimens used in the present work.

Fig. 1. Specimen with load introduction blocks used for DCB and MMB tests.

All the tests were carried out by an electrical load and compression test machine (Instron 5567) with a maximum load capacity of 30 kN. The tests were under displacement control of 1.5 mm/min (based on the standards used). Majority of the tests were repeated three times. Two load introduction blocks were attached to the specimens with the cyanoacrylate adhesive Z70. Due to the low shear strength of such an adhesive, the blocks could be detached easily after the tests. The load blocks were connected to the testing machine by small steel shafts. In the load introduction points, two inclinometers were attached to record the angular rotation to measure the fracture energy based on the J integral. The lower base of this set-up is the same as the DCB setup and is attached to the specimen by using a load block. The upper level of the MMB setup includes a bearing (Point D in Fig. 2) which is exactly located in the middle of the distance between the loading block (Point C) and the base bearing (Point B). A schematic MMB test setup is shown in Fig. 2. The mixed-mode ratio can be adjusted by changing the position of the lever in which the load is applied. The lever distance c is shown in Fig. 2, it is calculated according to Equation (4).

Fig. 2. Schematic MMB test setup.