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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(a)

(b)

Fig. 3. Load-displacement plots for (a) CFRP (b) GFRP DCB specimens.

As one can notice, the results for GFRP specimens are characterized by linearity after reaching the peak load value, while the results for CFRP specimens are more irregular, which can be considered as an additional factor during the evaluation of interlaminar fracture toughness. The obtained results of the experiments on the DCB specimens were averaged from 5 tests for GFRP specimens and 3 tests for CFRP specimens, since the remaining 2 CFRP specimens were incorrectly damaged during the experiment. 3. Calculation of fracture toughness parameters The determination of the critical energy release rate I for the tested composite materials is possible by performing calculations based on the experimental results described in section 2.3. For this purpose, the Modified Beam Theory (one of three variants of the determination of the energy release rate I suggested in (Standard, 2013)) was used, which is based on the following formula: I = 2 ( 3 + | |) , (1) where is the loading force, is the load point displacement, is the specimen width, is the length of initial delamination, and is the correction factor that prevents overestimation of I , determined experimentally from the least-square plot of √ 3 vs. , where is the compliance calculated from the ratio of ⁄ . In order to determine I , considered as the material property, from the load-displacement experimental data (Fig. 3) one needs to extract the maximal value of loading force max , which is the critical force in such a test. Then, by replacing by max in (1) one can determine I . For the tested materials the determined values of I (as averages of the results obtained for a number of the tested specimens) were as follows: for CFRP I = 511.8 J/m 2 , while for GFRP I = 649.4 J/m 2 . 4. Final remarks and conclusions The presented study was focused on the experimental determination of material properties of CFRP and GRFP structures in quasi-static and fracture interlaminar toughness tests, necessary for the preparation of the reversed numerical model for the prediction of structural residual life after low-velocity impact damage. In order to obtain proper values of material properties, the specimens need to be appropriately prepared and tested. This article discussed numerous technological parameters, important from the point of view of the accuracy of the obtained results. Several factors influencing on the quality of the experimental tests need to be highlighted: • End-tabs in quasi-static tests – the presence of end-tabs adhered to the specimens both for the tensile and compressive tests reduces stress concentrations, but an often problem in such tests is their decohesion, which results in specimen and test failure. Before the tests, it is important to abrade the adhered surfaces in order to increase their roughness, and thus, the contact force. • Tests of DCB specimens – in order to ensure proper contact between the hinge and the specimen, the contact surfaces need to be abraded and degreased before the adherence. Although, the driving force during mode I interlaminar fracture toughness is relatively low, decohesion may occur, especially at the very beginning of