Direct observation of microstructural fracture dynamics in carbon fiber reinforced plastics via in situ transmission electron microscopy

Abstract

The tensile deformation process of carbon fiber reinforced plastics (CFRPs) was in situ observed by high-resolution transmission electron microscopy (TEM). CFRPs were prepared with sizing and unsizing treatment on the fiber surfaces. The microstructural evolution of crack propagation along fiber/resin interfaces and inside resin matrices was examined. Fine resin fragments remained on the fiber surfaces after fracture. From the differences in the shape of the residual resin fragments between the two types of CFRPs, the influence of the sizing treatment on fracture was quantitatively evaluated. We demonstrated a new method on the basis of in situ TEM to evaluate the microstructural fracture dynamics and the influence of the sizing treatment on fracture.

Introduction

Remarkable mechanical properties of carbon fiber reinforced plastics (CFRPs) have been demonstrated by various strength and fracture tests to obtain relating various physical quantities, such as specific strength, specific stiffness, and fracture length [[1], [2], [3], [4], [5], [6], [7], [8]]. The fracture of CFRPs, which represents the reliability of CFRPs as structural materials, has been investigated using various methods, and the obtained results have been utilized in their material designs [5,[9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]]. The examples of the methods are X-ray computer tomography [[20], [21], [22]], the digital image correlation [23], and scanning electron microscopy [16]. In these studies, observations were performed before and after fracture with spatial resolutions on the micrometer order of magnitude; the fracture dynamics was not directly observed. As a result, the fracture processes and mechanisms have been speculated on the basis of the observed results before and after fracture. One of the most effective methods to evaluate fracture in CFRPs is to observe in situ the localized microstructural dynamics in crack generation and propagation, although the method is a state-of-the-art technology. In situ transmission electron microscopy (TEM) allows us to observe such fracture dynamics, as demonstrated for that in isolated carbon fibers [24]. In particular, the atomic resolution of TEM provides a clue to the elucidation of the atomistic fracture processes in CFRPs [25]. Since the fracture and the other mechanical properties of CFRPs are affected by the adhesive strength of fiber/resin interfaces, the fiber surfaces are coated by sizing agents [1,15,16,[26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46]]. Thus, it is important to clarify the relationship between fracture features and the variations in the interface structures by sizing treatments. In this study, the fracture of CFRPs during off-axis tensile deformation tests was directly observed by in situ TEM. This observation resulted in the quantitative analyses of the influence of a sizing treatment on fracture.