Colloquia

Summary

Continuous Fiber Reinforced Polymers (CFRPs) are layered structures and made of highly anisotropic thin plies, leading to complex, often interconnected damage mechanisms. One of the early damage mechanisms concerns the interlayer cohesion, leading to so-called delamination. It is recognised that these cracks are initiated by matrix cracks, running parallel to the fibres in the adjacent layer. This failure mechanism, related to the delamination failure, is longitudinal intralaminar cracking (also known as transverse cracking) which usually follows the extent of delamination.

Though the mechanical characterisation of delamination is well described and even supported by standard testing methods, that of the longitudinal intralaminar cracking is not. The objective of this research is to propose a testing method and protocol for the experimental evaluation of Mode I longitudinal intralaminar fracture toughness and crack propagation resistance of CFRPs under quasi-static loading. This, similarly to, and partly based on the existing testing standards for the fracture toughness of polymers (ISO 13586) and the standard for determining interlaminar fracture toughness of CFRPs (ISO 15024). The addition of this new test would add on these existing methods, necessary to, for example, model damage evolution in composite structures and determine/predict premature failure.

Several test setups were analysed for their suitability, accuracy, and repeatability where after a Single Edge Notched Bending (SENB) setup was chosen. Furthermore, the applicability of a Linear Elastic Fracture Mechanics (LEFM) approach has been researched, where after ISO 13586 was taken as guideline. The dimension parameters of these SENB specimens have been experimentally determined by data-, literature- and FEM analysis. Regarding the preparation of these specimens, experimental methods have resulted in recommendations for: specimen machining, pre-crack cutting using a microtome, pre-crack depth, and microscopic analysis by constantly iterating between testing the setup and designing the setup. Furthermore, an algorithm using a camera to track the crack tip position in LabVIEW was designed, where after the determination of crack propagation resistance was done using an energy analysis method.

10 specimens were tested on fracture toughness as part of a Round Robin on crack propagation resistance. The results, having a low variance, show a good repeatability of the test methodology used. The used method for pre-crack cutting and chosen pre-crack depth have resulted in sharp crack tips and a seemingly low fracture toughness, more closely resembling a naturally occurring crack.