PSI - Issue 33

Girolamo Costanza et al. / Procedia Structural Integrity 33 (2021) 544–555

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Author name / Structural Integrity Procedia 00 (2019) 000 – 000

Fig. 9. Schematic presentation of the three-point bending test setup. The distance dc between the center of a 5 mm long strain gauge and the edge of a web plate was for the static and fatigue specimens 7 and 9 mm, respectively. The latter value allowed an easier installation of the gauges (Karttunen et al. 2017). During fatigue failure mode, cracks were initiated and propagated at the laser-stake welds, consequently, cohesive fracture occurs in the foam. Fig. 10 shows results that are similar to the literature in terms of static and ultimate strength, but the fatigue life improvement is far better than expected; as the load level at 2 million cycles was increased by a factor of 8.5 when the beams are filled with a low-density polyvinylchloride foam (Divinycell H80). This is a clear indication that filling technology is an effective technique to improve the fatigue strength of lightweight sandwich panels. This means that the order of limit states needs to be reconsidered in design. The fatigue problems in laser-welded sandwich panels are due to shear-induced warping and these deformations are often very local in the plate domain. Thus, it is recommended that the filling of a sandwich panel is carried out only at the locations of high out-of-plane shear opposite to the web-plate direction. However, before applying such functional grading an investigation of the underlying physics of crack propagation in three-dimensional plate structures is required.

Fig. 10. Fatigue strength from experiments. The slopes and the scatter range indices related to 10-90% probabilities of survival are given (Karttunen et al. 2017).