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 2. Averaged values of material properties determined in quasi-static tests for CFRP and GFRP. Material property CFRP GFRP Elastic modulus at tension, GPa 55.017 24.69 Shear modulus at compression, GPa 25.482 10.76 Ultimate tensile strength, MPa 542.91 434.28 Poisson ratio, – 0.08 0.16 Elastic modulus at compression, MPa 58.99 29.19 Ultimate compressive strength, MPa 446.03 347.97

2.3. Fracture toughness experiments

The mode I interlaminar fracture toughness tests were performed on the same testing machine as the quasi-static tests according to (Standard, 2013). The hinges of the DCB specimens were fixed in the grips for tensile/compressive testing and subjected to opening loading with the displacement rate of 5 mm/min. During the tests, the displacement and the opening force were registered by the testing machine and the dedicated software. According to the requirements of (Standard, 2013), the marks on a specimen’s side were introduced and highlighted by using white paint as a background and pencil for marking (see Fig. 2(a)). According to (Standard, 2013), the delamination propagation from the initial length should be monitored with the magnifying device (like optical microscope or magnifying lens), however, such an approach seems to be impractical, since rapid and unstable delamination propagation as well as inaccuracies in visual onset of the delamination movement determination may significantly overestimate the results and bias the determined fracture toughness parameters. In the following study, two cameras synchronized with the universal testing machine were used in order to monitor the delamination propagation (Fig. 2(b)). It should be noted that it is difficult to precisely determine the exact moment of the delamination propagation up to the length of marks in the specimens due to a difficulty in visual detection of the real delamination front. However, a careful observation of the recorded videos in a zoomed view, frame by frame, allowed for the reduction of the detection errors.

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Fig. 2. A view of GFRP DCB specimen under loading (a) and experimental setup (b).

The tests were performed, according to the requirements of (Standard, 2013), in two steps. Firstly, the specimen was initially loaded until the initial delamination length increased by 3 to 5 mm. Then, the specimen was unloaded with the displacement rate of 25 mm/min, and reloaded with the same loading parameters until the delamination length reached the last mark. It should be noted that for the CFRP specimens the delamination propagation was of the step-wise character, which made it impossible to follow the standard requirements. For these specimens the values were recorded for delamination lengths different than the multiplicities of 5 mm marked in the specimens. The results of the tests on DCB specimens are presented in Fig. 3.