PSI - Issue 13

Simon Bard et al. / Procedia Structural Integrity 13 (2018) 1442–1446 Simon Bard / Structural Integrity Procedia 00 (2018) 000–000

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The laminate with graphite particles in the epoxy matrix are characterized by an energy release rate of 321 ± 32 J/m², which is already 27% higher than the reference. As no matrix-modification of prepreg laminates can be found in literature so far, the results can only be explained by modified epoxy resins without fibers. Herein the stress intensity factor (K IC ) increased by the incorporation of macro-sized particles, resulting from crack deviation [9]. Higher energy release rates of toughened epoxy resins generally lead to higher energy release rates in carbon-fiber reinforced laminates. [10] So it can be expected that the toughening mechanism can be transferred from the filled matrix to the laminate. Is can be well seen from Figure 4, left , that the crack growth undulating and not straight as in Figure 3. Figure 4, right , shows the crack deflection at graphitic particles at higher magnification.

Figure 4: left : Undulation crack growth in laminate, right : Crack deflection at graphite particle in laminate

Figure 5: left: Crack transition in thermoplastic interleaved laminate, right : evolution of a second crack in laminate

Thermoplastic interleaves are well known from the literature to improve the fracture toughness of fibre reinforced laminates. [5–7, 11–14] The G IC increased from 252 ± 2 J/m² in the unmodified laminate to 626 ± 62 J/m² in the interleaved laminate, which corresponds to a relative increase of 248%. Although in the literature different crack mechanism have been suggested, they are poorly proven. From the R-curves, Beckerman [7] concluded that the crack grows with the path of the least resistance. He came to the conclusion that the crack can transition from the interlayer into other regions of the laminate, which seems one toughening mechanism. Figure shows that several cracks were observed in SEM. The crack is transitioned from one layer of carbon fibre to another layer and a new crack starts to grow. The magnification of the crack tip shows that Crack 1 stops growing (crack pinning) at the lower part of the interleave region and then a second crack (marked as Crack 2) starts at the upper part of the interleave. As explained by Beckerman [7], the crack takes the path of the least resistance and the interface between interleave region and carbon fibres seems to show low adhesion.