Abstract
The paper presents the use of micro-X-ray computed tomography (CT) system and associated automatic loading device in visualizing and analyzing the propagation of penny-shaped flaw in gypsum-like 3D printing specimen. During the loading process, a micro-X-ray computed tomography (CT) system was used to scan the specimen with a resolution of 30 × 30 μm2. The volumetric images of specimen were reconstructed based on two-dimensional images. Thus, the propagation of penny-shaped flaw in gypsum-like 3D printing specimen in spatial was observed. The device can record the evolution of the internal penny-shaped flaw by X-ray CT scanning and the evolution of the surface crack by digital radiography at the same time. Fractal analysis was employed to quantify the cracking process. Two- and three-dimensional box-counting methods were applied to analyze slice images and volumetric images, respectively. Comparison between fractal dimensions calculated from two- and three-dimensional box-counting method was carried out. The results show that the fractal dimension increases with the propagation of cracks. Moreover, the common approach to obtain the 3D fractal dimension of a self-similar fractal object by adding one to its corresponding 2D fractal dimension is found to be inappropriate.
Highlights
Crack propagation and fracture of materials are directly related to the failure and catastrophic safety of structures in aerospace, transportation, chemical industry, machinery, energy, civil engineering, and other fields from the explosion of aerospace caused by low-stress brittle fracture of steel to the earthquake and tunnel instability caused by fracture of underground rock at various scales
Adams and Sines [11] investigated three-dimensional elliptical-shaped flaw by PMMA, which is a kind of Plexiglas, and the experiment revealed that the principle of three-dimensional flaw propagation was more complex than predicted, under uniaxial compression. e tip of elliptical-shaped flaw is extended firstly and generated huge fin-shaped crack on the two crack tip line
It can be seen that the uniaxial compressive strength of samples with internal coin cracks is lower than that of standard samples. e uniaxial compression strengths of the samples with built-in coin-shaped cracks and the standard samples are 2.89 MPa and 4.67 MPa, respectively. e stress and strain curves of the two types of samples under uniaxial compression are smooth, and the stress-strain curves of the sample with coin-shaped cracks do not appear to be caused by the stress mutation caused by the crack propagation
Summary
Crack propagation and fracture of materials are directly related to the failure and catastrophic safety of structures in aerospace, transportation, chemical industry, machinery, energy, civil engineering, and other fields from the explosion of aerospace caused by low-stress brittle fracture of steel to the earthquake and tunnel instability caused by fracture of underground rock at various scales. Fu and Zhu [12] used a new type of transparent resin which has a higher degree of brittleness to study the propagation and coalescence process of internal 3D penny-shaped flaw under uniaxial compression, and the test results showed that the fracture of internal 3D flaw produced a variety of cracks such as petal-shaped crack and finshaped crack. A micro-X-ray CT system and the matching automatic loading device were used to investigate the internal penny-shaped flaw propagation of gypsum-like 3D printing sample to determine and quantify the damage and failure features accompanying with uniaxial compressive loading. From the observation results of the scanning electron microscope (Figure 2), it can be seen that there are micrometer scale holes in the 3D printing plaster sample and the aggregation particles of gypsum crystals with different sizes from ten microns to dozens of microns. It is further proved the lack of compactness of 3D-printed gypsum samples and shows that 3D-printed gypsum samples exhibit heterogeneity at the micron scale
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