PSI - Issue 47

Johannes Wiener et al. / Procedia Structural Integrity 47 (2023) 253–260 Johannes Wiener/ Structural Integrity Procedia 00 (2019) 000–000

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4. Results and discussion The investigation focused on the trade-offs between stiffness and toughness in two different branches of layer architectures, namely microlayer composites and multilayer composites. 2.3. Microlayer composites Regarding microlayer composites, a strong relation between impact strength and matrix layer thickness was observed. To be more precise, a large increase of a c from 10 up to 45 kJ/m 2 could be seen as soon as layer thickness falls below a certain threshold (Fig. 4a). Upon closer investigation, the critical size was found to coincide with the extension of talcum particles in the matrix. One explanation is, that these particles were aligned by force during processing, thus limiting inherent defect size for smaller matrix layers (Fig. 4b). As soon as all particles were aligned properly, a plateau of a c was reached. As for the soft ILs, no restrictions to thickness were found, thus suggesting that ILs should be chosen as thin as processing allows for in order to save material and preserve overall stiffness.

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Fig. 4. (a) Unnotched and notched impact strength in relation to layer thickness and (b) the orientation of talcum particles as a result of the processing of small layers (reprinted with permission of (Wiener et al. 2021)).

Despite large increases in a c , the introduction of the soft phase also turned out to be detrimental to stiffness. Fig. 5a depicts a c versus E b in the style of an Ashby plot. In addition to the microlayer composite (ML_512), a blend of the same material composition (Blend) and the matrix material (PP-HR) are shown. In order to include the influence of the microlayer processing, the blend as well as the matrix material were also subjected to repeated melt strand folding and included in the comparison (Blend_512L and PP-HR_512L, respectively). The blends and matrix material form a trend line, which represents the rule of mixture, where increased toughness comes at the cost of stiffness. However, ML_512L shows drastically reduced stiffness despite having the same amount of soft phase as the blends. Presumably, the stress transfer between matrix layers is strongly limited by the continuous soft layers and separate bending stress distributions are formed for individual layers (Fig. 5b). This stress decoupling contributes to the flaw tolerance of the composite, but also reduces the area moment of inertia of the entire structure, resulting in a bending stiffness as low as 10% of the matrix material. Nonetheless, ML_512 still lies well above the trend line, meaning the stiffness loss is outweighed by the toughness gain. As a countermeasure to this stiffness loss, larger layers should be used, which was investigated in the form of multilayer composites.