Morphology characterization and in-situ three-dimensional strain field monitor of short carbon fiber-reinforced polymer composites under tension

Abstract

In-situ Micro X-ray computed tomography (μCT) offers a new opportunity to monitor the 3D morphology and damage evolution of short carbon fiber-reinforced polymer (SCFRP) composite. However, the sample size of in-situ μCT is generally limited to achieve high revolution, which resulted in different mechanical behavior compared with that obtained from standard samples. In this study, μCT scans with two resolutions were combined to character the 3D geometrical morphology and monitor the 3D deformation fields of SCFRP composites under tension. High resolution μCT scans with voxel size of 0.68 μm were used to quantify the geometric characteristics of fibers and void defects inside small samples. Low resolution in-situ μCT scans with voxel size of 4 μm and digital volume correlation method were utilized to monitor the 3D deformation fields of standard specimens under tension. The failure behavior of SCFRP was determined by the micro geometric morphology. The fracture surface under tension is oriented along 120° and 240° in the XY plane, which is consistent with the fiber and debonding distribution.

Introduction

Carbon fiber-reinforced polymer (CFRP) composites are light substitutions of metal with comparable specific stiffness, strength and good corrosion resistance [1], [2]. As one kind of CFRP, short carbon fiber-reinforced polymer (SCFRP) has more stable properties under complex loading conditions as a result of randomly oriented short fibers, and this fills the gap between the continuous-fiber composites and unreinforced polymers [3]. Thus, SCFRP is widely used in automotive, aircraft and aerospace industry [4]. In order to further improve the performance of these composite materials, several previous researches have discussed the influence of oxidized fibers, matrix properties, fiber aspect ratio and volume fraction on the mechanical properties of composites [5], [6]. Carbon fibers after gaseous oxidation could increase the flexural modulus and strength of SCFRP by 10% and 21%, respectively [7]. Moreover, the tensile strength of composites can be improved with the increase of short-fiber length until it reaches 80 μm [8]. The mechanical properties of SCFRP composites are highly influenced by the complex meso-morphology. Four mesoscale failure mechanisms appeared during tensile tests, including matrix deformation and fracture, fiber/matrix debonding, fiber pull-out, and fiber fracture [9]. These damage mechanisms can occur successively or simultaneously and result in the failure of the composite structures.

In order to characterize the failure mechanism in composite materials, several testing methods, such as optical imaging [10], scanning electron microscopy (SEM) [11] and acoustic emission [12] have been used to visualize the internal defects and to monitor damage evolution under various loading. These studies show that the composite damage usually originates from internal defects. Investigating the damage evolution and failure process in SCFRP composites, and considering real internal microstructure, is an essential approach to uncover the underlying micro-mechanisms. However, the aforementioned testing methods could not visualize the three-dimensional (3D) evolution of damages in composites. With the development of high spatial resolution micro X-ray computed tomography (μCT) and synchrotron radiation X-ray computed tomography (SR-CT), 3D high-fidelity structures of composites could be reconstructed from CT images and the damage evolution could be monitored by in-situ X-ray CT tests [13], [14], [15]. Hu [3], [16], [17] used SR-CT technique to observe the internal microstructure under various tensile loading and analyzed the failure mechanism of short carbon fiber/epoxy composites. The mechanical properties of oxidized fibers reinforced composites have been compared with that of untreated fibers reinforced composites and they inferred that the oxidation treatment improved the mechanical properties of these composites as a result of the ineffective length reduction of fibers. Rollad [18] monitored the damage evolution of short glass fiber reinforced thermoplastics using in-situ SR-CT tensile tests. The damage parameters were quantified by processing the CT scanned images. Watanabe [19] characterized the crack initiation and propagation in carbon fiber/epoxy composites using nanoscopic SR-CT.

In-situ CT characterization offers a new method to understand the damage evolution and failure mechanism of composites under loading. More characterization methods were developed to measure more internal parameters based on in-situ CT scans. Digital volume correlation (DVC) method was proposed to monitor the 3D deformation fields of composites under loading [20]. Imaged-based finite element method was developed to generate high-fidelity numerical models from CT scans directly, which considered the real void defects and geometry deviation of composites [21], [22]. Madra [21] developed a method based on dual kriging to construct a 3D fiber representation of the fiber architecture and they estimated the local permeability using image-based numerical simulation. Huang [23], [24] proposed Micro-CT Aided Geometric Modeling technique and reconstruct “material twin” geometric models of dry textile preforms. The deformation and displacement of fibers tows under compression are studied [25].

However, in order to scan the composites in high spatial resolution and to characterize the 3D morphology of carbon fibers using CT, the dimensions of specimens were quite small, for example 0.39 mm × 0.36 mm × 10 mm or Ф 60 μm × 1 mm [25]. These samples were much smaller than the standard specimen as is listed in ASTM D3039 [26]. The failure mechanisms of these samples with different dimensions should not be the same. Moreover, 3D displacement and strain fields of SCFRPs under tension have not been characterized yet. In this paper, internal structures and 3D deformation fields of short carbon fiber-reinforced polymer composites under tension were captured and characterized combining μCT scans of two spatial resolution. The short fiber orientations were measured, and 3D morphology was directly reconstructed from high spatial resolution μCT images. Damage evolution of standard tensile specimen under quasi-static tension were monitored by in-situ low spatial resolution μCT tests and 3D deformation fields were calculated by DVC. The damage evolution and failure mechanism of SCFRP were elucidated from SEM and CT testing results.