Effect of Cerium and Magnesium on Surface Microcracks of Al–20Si Alloys Induced by High-Current Pulsed Electron Beam

The effect of Ce and Mg on surface microcracks of Al–20Si alloys induced via high-current pulsed electron beam (HCPEB) was studied. Mg was revealed to refine the primary Si phase in the pristine microstructure by forming a Mg2Si phase, leading to the suppression of microcrack propagation within the brittle phase after HCPEB irradiation. The incorporation of Ce into the Al–Si–Mg alloys further refined the primary Si phase and reduced the local stress concentration in the brittle phase induced by HCPEB irradiation. Ultimately, the surface microcracks were observed to be eliminated by the synergistic effects between the two elements. For Al–20Si–5Mg–0.7Ce alloys, Ce demonstrated a homogeneous distribution in the Al matrix on the HCPEB-irradiated alloy surface, while the Mg and Si exhibited a certain degree of aggregation in the Mg2Si phase. Metastable structures were formed on the HCPEB-irradiated alloy surface, including the nano-primary silicon phase, nano-cellular aluminium structure, and nano-Mg2Si phase. Compared with alloy specimens containing Mg, the Al–20Si–5Mg–0.7Ce alloy specimens exhibited an excellent anticorrosion property after HCPEB irradiation mainly due to the combined effects of the grain refinement and microcrack elimination.

Investigation into Macro- and Microcrack Propagation Mechanism of Red Sandstone under Different Confining Pressures Using 3D Numerical Simulation and CT Verification

The growth and evolvement features of crack are of great significance to study the failure mechanism of rock mass and valuate the stability of the cavity. In this study, in order to obtain the mechanics parameters and external macroscopic crack propagation characteristics of red sandstone, triaxial compression tests were carried out. Based on the experimental results, a numerical model was established through the reasonable parameter calibration by the PFC3D software. The internal and external crack propagation processes of red sandstone under triaxial compression were simulated. Moreover, to verify the simulation results, the CT scanning and three-dimensional reconstruction technologies were used to observe the internal crack state of the specimens. The results showed that the internal crack failures occurred first at the end of the rock specimen. Then, the microcracks continued to accumulate and expand under the combined action of axial stress and confining pressure. The accumulated microcracks finally converged to form a macroscopic oblique shear failure. Based on the homogenizing treatment and reasonable parameter calibration, the internal and external crack expansion and evolution processes of the rock were simulated by the PFC3D model and the simulation results are consistent with the results obtained from the triaxial compression test and the CT scanning. The macro- and microfailure mode of crack propagation of the specimen deepens the understanding of rock failure mechanism. The PFC3D homogenization simulation method provides a new feasible method to study the macro- and microfailure mode of internal and external crack propagation of rock under compression.

A Damage Constitutive Model for a Rock under Compression after Freeze-Thaw Cycles Based on the Micromechanics

The freeze-thaw cycles will cause continuous damage to the rock, which is much related to the microcrack length, rock permeability, and frost heaving pressure. However, the failure mechanism of the rock under compression after freeze-thaw cycles is not very clear; therefore, it is studied with the damage theory here. First of all, according to the hydraulic pressure theory, the relationship between the frost heaving pressure and the microcrack propagation length in one single microcrack is established based on the elastoplastic mechanics and fracture theory. Second, by assuming the total strain of the rock under compression is comprised of the initial damage strain, elastic strain, additional damage strain, and plastic damage strain, a constitutive model for a rock based on the deformation and propagation of the microcrack under compression after freeze-thaw cycles is established. Finally, the proposed model is verified with the test result. In all, the proposed model can perfectly reflect the deterioration of the rock mechanical behavior under compression after the freeze-thaw cycles.

Rapid Consolidation of WC-ZrSiO4 Hard Materials by Spark Plasma Sintering: Microstructure, Densification, and Mechanical Properties

Densely consolidated WC-based hard materials with 5–20 vol% ZrSiO4 was fabricated by spark plasma sintering at 1400 ℃ at a constant heating rate of 70 ℃/min−1. To achieve mechanical alloying of WC-ZrSiO4, planetary ball milling was carried out for 12 h, during which the brittle-brittle components (WC-ZrSiO4) became fragmented and their particles became refined. It was observed that certain, specific, non-isothermal sintering kinetics, such as apparent activation energy, sintering exponents, and densification strain, affected the densification behavior. The evolution of phase structure from powder to compact was found to be related the lattice distortion and micro-strain in the basal planes of WC. By examining the mechanical properties of the samples, it was that the added zircon content leads to enhanced fracture toughness (12.9 MPa m1/2) owing to the presence of WC-ZrSiO4 in the cemented carbide. In fact, the microcrack propagation of the fracture passed through zircon from a transgranular to a ductile component (fcc) where the crack tips could be absorbed. Graphic Abstract

Investigation of Microcrack Propagation and Energy Evolution in Brittle Rocks Based on the Voronoi Model

The cracking of rock mass under compression is the main factor causing structural failure. Therefore, it is very crucial to establish a rock damage evolution model to investigate the crack development process and reveal the failure and instability mechanism of rock under load. In this study, four different strength types of rock samples from hard to weak were selected, and the Voronoi method was used to perform and analyze uniaxial compression tests and the fracture process. The change characteristics of the number, angle, and length of cracks in the process of rock failure and instability were obtained. Three laws of crack development, damage evolution, and energy evolution were analyzed. The main conclusions are as follows. (1) The rock’s initial damage is mainly caused by tensile cracks, and the rapid growth of shear cracks after exceeding the damage threshold indicates that the rock is about to be a failure. The development of micro-cracks is mainly concentrated on the diagonal of the rock sample and gradually expands to the middle along the two ends of the diagonal. (2) The identification point of failure precursor information in Acoustic Emission (AE) can effectively provide a safety warning for the development of rock fracture. (3) The uniaxial compression damage constitutive equation of the rock sample with the crack length as the parameter is established, which can better reflect the damage evolution characteristics of the rock sample. (4) Tensile crack requires low energy consumption and energy dispersion is not concentrated. The damage is not apparent. Shear cracks are concentrated and consume a large amount of energy, resulting in strong damage and making it easy to form macro-cracks.

A Method for Evaluation the Fatigue Microcrack Propagation in Human Cortical Bone Using Differential X-ray Computed Tomography

Fatigue initiation and the propagation of microcracks in a cortical bone is an initial phase of damage development that may ultimately lead to the formation of macroscopic fractures and failure of the bone. In this work, a time-resolved high-resolution X-ray micro-computed tomography (CT) was performed to investigate the system of microcracks in a bone sample loaded by a simulated gait cycle. A low-cycle (1000 cycles) fatigue loading in compression with a 900 N peak amplitude and a 0.4 Hz frequency simulating the slow walk for the initialization of the internal damage of the bone was used. An in-house developed laboratory X-ray micro-CT imaging system coupled with a compact loading device were employed for the in situ uni-axial fatigue experiments reaching a μ2μm effective voxel size. To reach a comparable quality of the reconstructed 3D images with the SEM microscopy, projection-level corrections and focal spot drift correction were performed prior to the digital volume correlation and evaluation using differential tomography for the identification of the individual microcracks in the microstructure. The microcracks in the intact bone, the crack formation after loading, and the changes in the topology of the microcracks were identified on a volumetric basis in the microstructure of the bone.

Comparative Study on Mineral-Scale Microcrack Propagation of Shale under Different Loading Methods

Producing a sufficient volume of multiscale crack networks is key to enhancing recovery of shale gas. The formation of crack network largely depends on initiation and propagation of microcracks. To reveal the influence of different loading methods on the propagation of mineral-scale microcracks, this study used the Voronoi tessellation technique to establish a cohesive zone model of shale mineral distribution and applied six different boundary conditions to represent different loading methods. Crack path characteristics, rupture characteristics, continuous crack propagation and turning, and en echelon intermittent crack propagation under different loading methods were compared and analyzed. The essence of different loading methods affecting the length and complexity of cracks was the spreading range of tensile microcracks. The mechanical properties of minerals led to dissimilarities in continuous crack propagation and turning. The formation and propagation of en echelon intermittent fractures of different scales were mainly impacted by the heterogeneity of minerals and mineral aggregates. The spreading direction and connection form of en echelon intermittent fractures were mainly affected by the loading method. Conclusions arising from mineral-scale simulations contribute to understanding the mechanism of microcrack propagation resulting from different loading methods, and these conclusions have a guiding significance to enhanced shale gas recovery.

Brittle-layer-tuned Microcrack Propagation for High-performance Stretchable Strain Sensors

High linearity is important for stretchable strain sensors. Herein, we propose a new strategy of brittle-layer-tuned microcrack propagation to achieve high-linearity resistive-type stretchable strain sensors by using a highly facile...