Recent technological advancement has led to the development of fiber reinforced polymers with applications in numerous areas (e.g., aerospace, automotive, naval, sports, infrastructure, etc.) due to their excellent mechanical properties. Unfortunately, they are susceptible to failures when subjected to different loading conditions. Thus, the development of models for accurately predicting failure modes in these materials and structures is highly desirable. However, understanding of the underlying failure mechanisms involved in damage and failure of fiber reinforced polymers is not a trivial task due to their complex microstructures.
Presently, among the important types of fracture in fiber reinforced polymers, the opening fracture mode I (i.e., applied load normal to the crack plane) is widely studied for characterization of toughness and crack propagation. It can be tested using double cantilever beam specimens to allow analysis of the bridging zone developed in the wake of the crack due to inter- or intralaminar fracture of a composite material. The results demonstrate the significant differences of inter- and intralaminar crack propagation and fracture response with different bridging fiber morphology as observed in terms of size and population. For instance, interlaminar fracture contains mainly individual bridging fibers while intralaminar fracture morphology entails larger fiber bundles with irregular cross-sections and as a result much higher fracture resistance. To this end, there is an imperative need to understand the causes of the differences in inter- and intralaminar fracture for the same material.
To this note, École Polytechnique Fédérale de Lausanne (EPFL) scientists: Dr. Montans Fernando Naya, Dr. Georgios Pappas and Prof. John Botsis investigated the effects of microstructure on inter- and intralaminar fracture. Specifically, the initial formation of fiber bridging under mode I failure was studied based on computational micromechanics method.
In brief, the experimental work entailed: double cantilever beam specimens that were used to demonstrate the differences in fracture resistance for inter- and intralaminar fracture in carbon/epoxy composite laminates with different microstructures. Next, they conducted a postmortem analysis on the material’s cross-section to determine the differences in microstructure taking into consideration the fiber dispersion. Furthermore, a representative volume element based computational model was utilized to investigate the effects of microstructure (i.e., fiber dispersion and resin rich zones) on the initial stages of plastic deformation and microcracking development, leading to large-scale bridging development and enhancement of toughness.
The authors observed that the differences in inter- and intralaminar fractures were due to differences in the interaction between the microcracks and the deformation induced by the resin-rich zones in the composite’s plies and their orientation versus the applied load. Consequently, the resin-rich domains branched the microcracks in the fiber rich zones leading to the development of large fiber bundles that could bridge the crack. Furthermore, it is worth noting that the thicknesses of the resin-rich zone and the fiber-rich layer had an important influence on the size of the fiber bundles and the microcrack spreading phenomenon.
In summary, the Swiss scientists presented the significance of the microstructure in the development of the initial stages of the large-scale bridging, e.g., random fiber dispersion and resin reach zones are the major contributors to increased fracture resistance. In general, they reproduced the composite’s behavior based on the actual microstructures.
By revealing the importance of micromechanical parameters, the study paves the way for the development of strategies to develop, predict and control toughening mechanisms in fiber reinforced polymers. Their research is currently published in the research journal, Composite Structures.
Naya, F., Pappas, G., & Botsis, J. (2019). Micromechanical study on the origin of fiber bridging under interlaminar and intralaminar mode I failure. Composite Structures, 210, 877-891.Go To Composite Structures