Abstract
The results of experimental research on the pullout properties of circumferentially notched z-pins with various notch designs are presented in this paper. Investigations on notched z-pins with four different notch designs—rectangular, circular, triangular, and sinusoidal—inserted into unidirectional (UD) and quasi-isotropic (QI) laminates were carried out in order to assess the influence of notch design and laminate structure on the resulting z-pin pullout properties. It can be shown that the application of circumferential notches at the z-pin surface causes significant increases in pullout forces and consequently, on the resulting pullout energies, regardless of which notch design is considered. The effect of notched z-pins is higher in a quasi-isotropic than in a unidirectional laminate structure. The highest enhancements of the pullout energy were found on quasi-isotropic laminates with circular and sinusoidal notch designs with increases of up to 69%, in comparison to measurements on samples with unnotched z-pins.
Highlights
In the last few decades, the importance of composite materials has increased steadily
The results show that the measured stiffness c in a unidirectional laminate structure is slightly increased by the introduction of notches
Results of the investigations performed on notched z-pins with different notch designs and a constant notch depth of 20 μm follow the findings described by Hoffmann et al [41]
Summary
In the last few decades, the importance of composite materials has increased steadily. The fracture-mechanical properties are crucial, since inhomogeneous cracks and materials of various sizes are always present in any composite structure Under dynamic loads, they may grow and cause an increasing weakness of the composite structure. Impact damage events can cause a significant reduction in the strength and stiffness properties of structural components made of fiber-reinforced plastics. These events may lead to delamination, which in turn leads to lower residual load capacities. The interactions between z-pins and laminate during the z-pin-pullout process can be defined by the characteristic load-displacement curves, as illustrated by Figure 1
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