Experimental Study on the Dilatancy and Energy Evolution Behaviors of Red-Bed Rocks under Unloading Conditions.

Surrounding rock deformation and consequent support failure are the most prominent issues in red-bed rock tunnel engineering and are mainly caused by the effects of unloading, rheology, and swelling. This study investigated the mechanical responses of two kinds of red-bed mudstone and sandstone under unloading conditions via laboratory observation. Volume dilation was observed on the rocks during unloading, and the dilatancy stress was linear with the initial confining pressure. However, the ratios of dilatancy stress to peak stress of the two rocks kept at a range from 0.8 to 0.9, regardless of confining pressures. Both the elastic strain energy and the dissipated energy evolved synchronously with the stress-strain curve and exhibited conspicuous confining pressure dependence. Special attention was paid to the evolution behavior of the dilatancy angle. The dilatancy angle changed linearly during unloading. When the confining pressure was 10 MPa, the dilatancy angle of mudstone decreased from 26.8° to 12.5° whereas the dilatancy angle of sandstone increased from 34.6° to 51.1°; when the confining pressure rose to 25 MPa, the dilatancy angle of mudstone and sandstone decreased from 45.8° to 17.4° and increased from 21.7° to 39.5°, respectively. To further understand the evolution of the dilatancy angle, we discussed the links between the variable dilatancy angle and the processes of rock deformation and energy dissipation.

A Three-Parameter Weibull Distribution Method to Determine the Fracture Property of PMMA Bone Cement.

Poly (methyl methacrylate) (PMMA) bone cement is an excellent biological material for anchoring joint replacements. Tensile strength ft and fracture toughness KIC have a considerable impact on its application and service life. Considering the variability of PMMA bone cement, a three-parameter Weibull distribution method is suggested in the current study to evaluate its tensile strength and fracture toughness distribution. The coefficients of variation for tensile strength and fracture toughness were the minimum when the characteristic crack of PMMA bone cement was αch∗=8dav. Using the simple equation αch∗=8dav and fictitious crack length Δαfic=1.0dav, the mean value µ (= 43.23 MPa), minimum value ftmin (= 26.29 MPa), standard deviation σ (= 6.42 MPa) of tensile strength, and these values of fracture toughness (µ = 1.77 MPa⋅m1/2, KICmin = 1.02 MPa⋅m1/2, σ = 0.2644 MPa⋅m1/2) were determined simultaneously through experimental data from a wedge splitting test. Based on the statistical analysis, the prediction line between peak load Pmax and equivalent area Ae1Ae2 was obtained with 95% reliability. Nearly all experimental data are located within the scope of a 95% confidence interval. Furthermore, relationships were established between tensile strength, fracture toughness, and peak load Pmax. Consequently, it was revealed that peak load might be used to easily obtain PMMA bone cement fracture characteristics. Finally, the critical geometric dimension value of the PMMA bone cement sample with a linear elastic fracture was estimated.

Compaction Behavior and Damage Constitutive Model for Porous Cement Mortar under Uniaxial Cyclic Loads.

The void compression stage causes porous cement mortar to present special mechanical properties. In order to study the compaction behavior and the damage evolution of the porous material, cement mortar specimens with an average porosity of 26.8% were created and cyclic uniaxial compression tests were carried out. The irreversible strain accumulated in the tests was obtained by cyclic loading and unloading. As the secant modulus of the porous cement mortar increases with stress in the pre-peak deformation stage, its damage variable is defined according to the accumulated irreversible strain instead of modulus degradation. The strain-based damage indicator fitted with the damage evolution law is characterized by linear accumulation at the beginning and has an acceleration rate of about 0.3 in the pre-peak deformation stage, and the damage value converges to 1 at failure. Based on the Weibull distribution, a constitutive damage model of porous cement mortar is improved by considering both the damage evolution during the plastic deformation stage and the mechanical behavior in the compaction stage. The theoretical envelope curves obtained by the constitutive model are in good agreement with the experimental envelope curves of cyclic uniaxial compression in the compaction and pre-peak stages, and the average absolute error is about 0.54 MPa in the entire pre-peak stage, so the proposed damage constitutive model can characterize the damage-induced mechanical properties of porous cement mortar in the compaction and pre-peak stages.

[Dandelion clock-like sign on CT for diagnose of COVID-19].

OBJECTIVE: To explore the value of the "dandelion clock-like" sign on chest CT for diagnosis of SARS-CoV-2-associated pneumonia. METHODS: This retrospective analysis was conducted based on the data of 119 cases from the Department of Fever and the Department of Infection undergoing chest high-resolution CT examinations in Sanshui District People's Hospital between January, 24 and February 10, 2020. The cases with no abnormality on chest CT were excluded. Twenty-three patients were diagnosed to have pneumonia, including 9 with SARS-CoV-2-associated pneumonia and 14 with other types of pneumonia. We comparatively analyzed the CT signs, location of the lesions and the dandelion clock-like sign among different types of pneumonia. RESULTS: Among the 23 patients with pneumonia, 9 (39.1%) had common or severe SARS-CoV-2- associated pneumonia with a positive epidemiological history and corresponding respiratory symptoms. Seven of the SARSCoV-2-associated pneumonia patients had multiple lesions in bilateral lungs, compromising mainly the lung field and the subpleural area and showing patchy, lumpy, and umbrella-shaped ground glass opacity, often accompanied by pulmonary vascular thickening and increased microvessels, interlobular septal thickening and fibrosis and lined with grid-like and small-bubble-like "crazy-paving" patterns. The dandelion clock-like sign was found in all the 9 patients with SARSCoV-2-associated pneumonia, with a total of 46 lesions (60.5%, 76 total lesions); 9 of the lesions presented with a "dandelion clek-like" sign and 37 with a "dandelion seed sign". These signs were not found in the 14 patients with other types of pneumonia. CONCLUSIONS: The dandelion clock-like sign is a common and characteristic chest CT finding in patients with SARS-CoV-2-associated pneumonia, and can help to distinguish SARS-CoV-2-associated pneumonia from other types of pneumonia.