PSI - Issue 5

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

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3.2. Fatigue loading

The fatigue test results are graphically presented in maximum stress vs. fatigue life graphs. Fig. 4a shows effect of edge surface quality where milling improved fatigue life 5 times. There were different fracture modes for these series. Fracture at a ply drop occurred more often for milled specimens and fracture out-of-ply drop occurred for non-milled specimens. The fatigue lives of the retested run-out specimens with more than 3 million cycles were up to 1000 times lower than new specimens cycled on the same load level (Fig. 4a)). Fig. 4b shows the effect of ply drop alternative. Alternative A with double tapering had in average 10 times longer fatigue life than the alternative with single tapering.

Fig. 4. Fatigue life at f = 0.5 - 7 Hz and R = 0.1: (a) Effect of specimen edge machining and retesting of run out specimens on fatigue life; (b) effect of ply drop alternative.

3.3. Fractography

Fractographic analysis of fracture surfaces was used to confirm the damage mechanisms. Fig. 5a shows rounded cusps typical for mode II fatigue fracture surface observed on for a ply drop fracture (spec. 3-9 – alt. A - milled). Fig. 5b presents a roller typical for mode II fatigue loading observed also on a ply drop fracture surface (spec 4-2 – alt. B – milled). Fig. 5c shows a scarp, typical for mode I loading, in a resin rich zone for out of ply drop fracture that proves opening of the laminate in the lower part of tapered zone (spec 3-6 – alt A - milled).

Fig. 5. (a) Rounded cusps typical for mode II fatigue fracture surface (ply drop fracture – spec 3-9); (b) a roller typical for mode II observed on ply drop fracture surface (ply drop fracture – spec. 4-2); (c) a scarp in a resin rich zone, typical for mode I (out-of-ply drop fracture – spec 3-6).