PSI - Issue 5

Martin Kadlec et al. / Procedia Structural Integrity 5 (2017) 1342–1348 Petr Homola / Structural Integrity Procedia 00 (2017) 000 – 000

1345

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2.4. Methods of morphological and fractographical analysis

Fractographical analyses after the testing were performed as follows. Selected specimens were cut to maintain the whole fracture surface using a diamond-coated saw. Pressurised air was used to remove saw debris from the surface. Each sample was then mounted on a pin stub by means of double sided sticky target and coated with 8-12 nm thick Au/Pt layer using a sputter coater. The microfractographic analyses of fracture surfaces were carried out using a TESCAN VEGA 3SBU Scanning Electron Microscope (SEM) set to secondary electron imaging mode. The high vacuum mode with a voltage value of 10 up to 30 kV was used for the evaluation. The adjusted microscopy parameters for each picture are noted below the pictures. Vega TC (TESCAN) microscope operating software was used.

3. Results

3.1. Quasi-static loading and stress field

The tensile strength results are stated in Table 1. The average strength for alternative A was 533 MPa. The alternative B (single tapering) had strength of 551 MPa that is 3 % higher than alternative A (double tapering). However, the difference between the alternatives was not statistically significant with p-value of 0.22. Fig. 3 shows FE model for 40 kN tensile load with the results of longitudinal and shear stress concentrations. Fig. 3a represents longitudinal stress for alternative B with concentration near the end of the tapering caused by the radius in this zone. The shear stress for alternative A in longitudinal plane between 2 nd and 3 rd ply and between 3rd and 4th ply from the top is shown in Fig. 3b. It shows same stress concentrations (in absolute value) in both interlaminar planes. The same absolute values of the two stress concentrations means that the stress is driven by the 0/90 layer without influence of adjacent layers lay-up – upper adjacent layer was +/-45 and bottom adjacent layer was 0/90. Only negligible shear stress concentration near +/-45 ply drop was observed for the tapering far from the gage section.

Table 1. Static strength results for both alternatives. Strength is calculated from the gage section area (11 layers). Alternative Specimen Max. Load (kN) Strength (MPa) Average strength (MPa)

2-5 2-6 1-7 1-8

47.3 48.1 48.9 50.2

528 538 542 560

A

533 ± 5

B

551 ± 9

Fig. 3. FEM model for 40 kN tensile load: (a) longitudinal stress with concentration near the end of tapering (alternative B); (b) shear stress between plies with concentration above and below of 0/90 ply drop (alternative A). Negligible concentration near +/-45 ply drop.