PSI - Issue 47

6

Johannes Wiener/ Structural Integrity Procedia 00 (2019) 000–000

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

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Fig. 5. (a) Ashby-style plot of impact strength versus bending modulus for blends, microlayer composites and the matrix material and (b) the decoupling of stresses due to very compliant ILs (reprinted with permission of (Wiener et al. 2021)).

2.4. Multilayer composites A multitude of layer architectures was investigated and the parameters Ω and E eq were used to assess relative fracture toughness and stiffness of the specimens. An overview of the results is given in Fig. 6a. ILs made of PP-Soft led to increased Ω in all cases, but reductions in stiffness still persisted due to the aforementioned stress decoupling effects. While fracture toughness was improved by up to 105%, stiffness was also reduced to approximately half of the matrix values. Composites with PP-St ILs, which is a considerably stronger and stiffer grade of PP, preserved stiffness much better (ca. 95% of matrix stiffness). However, crack arresting properties could not be achieved in many layer architectures. Crack re-initiation due to plastic deformation of ILs is presumed to be the cause of the ineffectiveness of these specific ILs (Kolednik et al. 2019a). A depiction of the corresponding J-R curve and the evolution of the plastic zone ( r y IL ) throughout SENB testing are shown in Fig. 6b. Therein, r y IL is small in the initial stages, where the crack grows within the brittle matrix (A, B). As soon as the crack tip encounters the soft IL, a larger plastic zone develops inside the IL (C), which grows in size with increased loading (D). If r y IL grows larger than t (E), the stress concentration of the crack tip reaches the intact matrix ligament and causes crack re-initiation (F). In such a scenario, the crack is not arrested effectively and no improvements in terms of J exp are achieved. Although fracture of the IL itself is possible (G), such cases were never observed in the conducted tests. On the other hand, if the plastic zone is contained within the IL (H), the remaining matrix material is protected and large additional loading is required in order to break the specimen. For that reason, t should always be chosen larger than the expected r y IL , which enables strong and tough composites. This could also be seen in the best-case scenario in Fig. 6a, where Ω ൌ 2.81 and 94% of matrix stiffness was preserved.