Nanoscopic origin of cracks in carbon fibre-reinforced plastic composites

Masao Kimura,Toshiki Watanabe,Yasuo Takeichi,Yasuihiro Niwa

doi:10.1038/s41598-019-55904-2

Abstract

Voids and cracks can fatally degrade structural materials such as metals and ceramics but are tolerated in carbon fibre-reinforced plastic (CFRP) composites if monitored to prevent their growth to a critical size. Thus, the use of CFRPs as aeronautical structural materials requires an understanding of microscopic crack formation. However, this crack-formation mechanism remains unclear because experimental difficulties have hindered studies of relevant phenomena that occur before crack formation. Herein, we report high-resolution (~50 nm) and non-destructive three-dimensional observations of crack initiation and propagation under applied stress. This evaluation reveals that voids and cracks do not simply result from local stresses but instead occur largely through two competing nanoscale mechanisms, namely, fibre/plastic interface debonding and in-plastic crack initiation. Therefore, nanoscopic insights into these heterogeneities are essential for controlling crack initiation and determining reasonable safety margins for CFRP composite use.

Highlights

Voids and cracks can fatally degrade structural materials such as metals and ceramics but are tolerated in carbon fibre-reinforced plastic (CFRP) composites if monitored to prevent their growth to a critical size
When CFRPs are subjected to mechanical loading, different failure mechanisms are induced at the macroscopic scale[8,9,10,11]: (M1) transverse matrix cracking in 90° plies, (M2) longitudinal matrix cracking in 0° plies, (M3) delamination between 0° and 90° plies, and (M4) fibre fractures in 0° plies (Fig. 1b)
A columnar specimen with unidirectional carbon fibres was mechanically loaded with a diamond indenter using a nanomechanical test stage[36] (Fig. 1c), and in situ observations under stress were performed by nanoscopic SR X-ray computed tomography (X-CT)

Summary

Introduction

Voids and cracks can fatally degrade structural materials such as metals and ceramics but are tolerated in carbon fibre-reinforced plastic (CFRP) composites if monitored to prevent their growth to a critical size. Nanoscopic insights into these heterogeneities are essential for controlling crack initiation and determining reasonable safety margins for CFRP composite use Structural materials such as steel, ceramics, and composites are commonly subjected to multiple loads that can cause serious damage. Their failure is always associated with the presence of defects on various scales, and the initiation and propagation mechanisms of these defects vary broadly according to differences in material bonding[1,2,3]. Owing to recent advances in the production of CFRPs through microscopic design, observation with even higher spatial resolution of less than a micrometre is required in order to understand the mechanism of crack initiation and propagation. Modification of the carbon fibre surface has been attempted using various techniques to control the microscopic structure, such as sizing, plasma, or chemical treatments and carbon nanotube or nanoparticle coatings, to increase the wettability and interfacial adhesion with polymeric matrices[21]

Methods

Results

Conclusion