PSI - Issue 25

Angelika Wronkowicz-Katunin et al. / Procedia Structural Integrity 25 (2020) 13–18 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Table 1. Dimensions of the specimens. Type of specimen

a, mm b, mm c, mm d, mm e, mm

Tensile tests (CFRP/GFRP)

280 140 150

25 10 25

1.5/2

65 65 63

1.5/2 1.25

Compressive tests

2 5

Fracture toughness tests

2.5

According to the standards’ requirements , the end-tabs were adhered to the specimens for the tensile and compressive tests in order to reduce stress concentrations in the area of the specimens gripping. Since the decohesion of end-tabs is a common problem during tensile/compressive tests, they need to be properly adhered. For this purpose, the contact surfaces of the specimen and the end-tab were abraded using fine-grain file up to ca. 0.2 mm. After removing dust and degreasing the surface with acetone, the surfaces were adhered with epoxy resin and subjected to static loading for 15 minutes in a clamp. Similarly, hinges need to be properly adhered to the DCB specimens for the fracture toughness tests. Although, forces during such tests are relatively low, one needs to ensure sufficient adherence force, which should be higher than the force of the initial crack propagation. In this case, the epoxy resin was also used as the adhesive between steel hinges and the composite specimens. The surfaces of both the hinges and specimens were abraded and degreased before the adherence. In order to strengthen the adherence, the hinges with holes on their tabs were used, thus, during adhering, the excess of adhesive penetrated the holes. An additional issue for the DCB specimens, which is very important for successful and valid measurements is an appropriate manufacturing of the specimens with nonadhesive inserts that guarantee the initial delamination. The guidelines for the insertion of such nonadhesive inserts are included in (Standard, 2013), however, the method of the insertion is not discussed in detail nor in this standard, neither in the available literature. During manufacturing, two different methods of the insert inclusion were applied for CFRP and GRFP in the DCB specimens. In the case of the CFRP specimens, an efficient method of the introduction of the initial delamination was leaving a piece of peel-ply. After manufacturing, the delaminated ends of a specimen might be slightly bonded. For separation, a thin blade needs to be carefully hammered into the debonded area of the specimen, starting from the edge of debonding. In the case of the GFRP specimens, the initial delamination was introduced by the insertion of a piece of polyethylene foil with the desired dimensions of delamination. The quasi-static tests on the specimens described in section 2.1. were performed on the MTS Criterion ® model 45 Electromechanical Universal Testing Machine at the Silesian Science and Technology Centre for Aviation Industry Ltd. in Czechowice-Dziedzice, Poland (Paw łowski, 2019 ). The quasi-static tests covered the tensile and compressive tests of CFRP and GRFP specimens based on (Standard, 2000; Standard, 2016), and the obtained results were averaged from 10 tests for each material. For the tensile tests two extensometers were used: longitudinal (model 634.31F-24) and transversal (model 632.18F-20) in order to obtain the Young and Kirchhoff moduli of the tested specimens. The displacement rate for this type of tests was assumed as 2 mm/min. The determined average elastic and shear moduli together with the ultimate tensile strength are presented in Table 2. Additionally, the Poisson’s ratios were calculated from the determined moduli according to (Standard, 2017) and their average value is also presented in Table 2. In order to determine the compressive modulus the compressive tests were performed on the same testing machine as the tensile tests, but with the displacement rate of 1 mm/min. Additionally, the foil extensometers were bonded on both sides of the specimen in the measurement area. The resulting average compression modulus and the ultimate compressive strength are presented in Table 2. 2.2. Quasi-static experiments