비만아 부모의 돌봄 경험: q 방법론

By 27 February 2020, the outbreak of coronavirus disease 2019 (COVID-19) caused 82 623 confirmed cases and 2858 deaths globally, more than severe acute respiratory syndrome (SARS) (8273 cases, 775 deaths) and Middle East respiratory syndrome (MERS) (1139 cases, 431 deaths) caused in 2003 and 2013, respectively. COVID-19 has spread to 46 countries internationally. Total fatality rate of COVID-19 is estimated at 3.46% by far based on published data from the Chinese Center for Disease Control and Prevention (China CDC). Average incubation period of COVID-19 is around 6.4 days, ranges from 0 to 24 days. The basic reproductive number (R0 ) of COVID-19 ranges from 2 to 3.5 at the early phase regardless of different prediction models, which is higher than SARS and MERS. A study from China CDC showed majority of patients (80.9%) were considered asymptomatic or mild pneumonia but released large amounts of viruses at the early phase of infection, which posed enormous challenges for containing the spread of COVID-19. Nosocomial transmission was another severe problem. A total of 3019 health workers were infected by 12 February 2020, which accounted for 3.83% of total number of infections, and extremely burdened the health system, especially in Wuhan. Limited epidemiological and clinical data suggest that the disease spectrum of COVID-19 may differ from SARS or MERS. We summarize latest literatures on genetic, epidemiological, and clinical features of COVID-19 in comparison to SARS and MERS and emphasize special measures on diagnosis and potential interventions. This review will improve our understanding of the unique features of COVID-19 and enhance our control measures in the future.

https://onlinelibrary.wiley.com/doi/epdf/10.1002/jmv.25748

Unique epidemiological and clinical features of the emerging 2019 novel coronavirus pneumonia (COVID-19) implicate special control measures.

A review of techniques, advances and outstanding issues in numerical modelling for rock mechanics and rock engineering

The Mohr–Coulomb (M–C) fracture criterion is revisited with an objective of describing ductile fracture of isotropic crack-free solids. This criterion has been extensively used in rock and soil mechanics as it correctly accounts for the effects of hydrostatic pressure as well as the Lode angle parameter. It turns out that these two parameters, which are critical for characterizing fracture of geo-materials, also control fracture of ductile metals (Bai and Wierzbicki 2008; Xue 2007; Barsoum 2006; Wilkins et al. 1980). The local form of the M–C criterion is transformed/extended to the spherical coordinate system, where the axes are the equivalent strain to fracture $${\bar \varepsilon_f}$$
 , the stress triaxiality η, and the normalized Lode angle parameter $${\bar \theta}$$
 . For a proportional loading, the fracture surface is shown to be an asymmetric function of $${\bar \theta}$$
 . A detailed parametric study is performed to demonstrate the effect of model parameters on the fracture locus. It was found that the M–C fracture locus predicts almost exactly the exponential decay of the material ductility with stress triaxiality, which is in accord with theoretical analysis of Rice and Tracey (1969) and the empirical equation of Hancock and Mackenzie (1976), Johnson and Cook (1985). The M–C criterion also predicts a form of Lode angle dependence which is close to parabolic. Test results of two materials, 2024-T351 aluminum alloy and TRIP RA-K40/70 (TRIP690) high strength steel sheets, are used to calibrate and validate the proposed M–C fracture model. Another advantage of the M–C fracture model is that it predicts uniquely the orientation of the fracture surface. It is shown that the direction cosines of the unit normal vector to the fracture surface are functions of the “friction” coefficient in the M–C criterion. The phenomenological and physical sound M–C criterion has a great potential to be used as an engineering tool for predicting ductile fracture.

Application of extended Mohr–Coulomb criterion to ductile fracture

The purpose of this review is to discuss the development and the state of the art in dynamic testing techniques and dynamic mechanical behaviour of rock materials. The review begins by briefly introducing the history of rock dynamics and explaining the significance of studying these issues. Loading techniques commonly used for both intermediate and high strain rate tests and measurement techniques for dynamic stress and deformation are critically assessed in Sects. 2 and 3. In Sect. 4, methods of dynamic testing and estimation to obtain stress–strain curves at high strain rate are summarized, followed by an in-depth description of various dynamic mechanical properties (e.g. uniaxial and triaxial compressive strength, tensile strength, shear strength and fracture toughness) and corresponding fracture behaviour. Some influencing rock structural features (i.e. microstructure, size and shape) and testing conditions (i.e. confining pressure, temperature and water saturation) are considered, ending with some popular semi-empirical rate-dependent equations for the enhancement of dynamic mechanical properties. Section 5 discusses physical mechanisms of strain rate effects. Section 6 describes phenomenological and mechanically based rate-dependent constitutive models established from the knowledge of the stress–strain behaviour and physical mechanisms. Section 7 presents dynamic fracture criteria for quasi-brittle materials. Finally, a brief summary and some aspects of prospective research are presented.

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A Review of Dynamic Experimental Techniques and Mechanical Behaviour of Rock Materials

Construction and utilization of rock caverns in Singapore Part B: Development costs and utilization

The distinct lattice spring model (DLSM) is a newly developed numerical tool for modeling rock dynamics problems, i.e. dynamic failure and wave propagation. In this paper, parallelization of DLSM is presented. With the development of parallel computing technologies in both hardware and software, parallelization of a code is becoming easier than before. There are many available choices now. In this paper, Open Multi-Processing (OpenMP) with multicore personal computer (PC) and message passing interface (MPI) with cluster are selected as the environments to parallelize DLSM. Performances of these parallel DLSM codes are tested on different computers. It is found that the parallel DLSM code with OpenMP can reach a maximum speed-up of 4.68x on a quad-core PC. The parallel DLSM code with MPI can achieve a speed-up of 40.886x when 256 CPUs are used on a cluster. At the end of this paper, a high-resolution model with four million particles, which is too big to handle by the serial code, is simulated by using the parallel DLSM code on a cluster. It is concluded that the parallelization of DLSM is successful.

Parallelization of the distinct lattice spring model

Underground cavern development in the Jurong sedimentary rock formation

Investigation of rock progressive damage under static confinement and strain rates facilitates the generation mechanism of natural fault damage zones. A triaxial Hopkinson bar apparatus is used to perform dynamic triaxial compression tests to examine the damage and degradation process of rocks subjected to multiple impacts. Dynamic mechanical properties are determined under a static triaxial pre-stress of (30, 20, 10) MPa and multiple dynamic loadings, with the repetitive impact velocity of 27 m/s and strain rates from 50 to 150/s. The acoustic characteristics are identified by ultrasonic measurement to qualify the damage values. The micro-crack parameters, including crack area and volumes are detected using synchrotron X-ray micro-computed tomography (μCT) to characterize the progressive damage. In addition, the microcrack orientation, density and fractal dimension are analysed from thin section. Experimental results show that dynamic stress-strain curves can be divided to elastic, nonlinear deformation and unloading phases. Dynamic peak stress, Young’s modulus and ultrasonic wave velocity decrease with increasing impact times. The high frequency of ultrasonic wave is filtered by the induced microcracks. The progressive damage and evolution of fracture networks are associated highly with microcrack initiation, propagation, branching and coalescence. Shear bands are commonly generated in granite, and tensile cracks are dominant in marble, while sandstone is mainly failed by compaction and deformation band. The absorbed energy of rock increases nonlinearly with increasing crack surface and volume. Besides, microcracks propagate primarily along the maximum principal stress; the density and fractal dimension exhibit an anisotropic distribution controlled by true triaxial confinement and dynamic impacts.

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Progressive Damage Behaviours of Triaxially Confined Rocks under Multiple Dynamic Loads

The crack initiation and propagation induced by loading finally lead to the failure of rocks. An identification of crack evolution has been widely investigated in quasi-static tests (Hoek and Martin 2014), which is essential to prevent the catastrophic rock engineering disaster. A consensus in those studies is that the crack process has several stages corresponding to several stress thresholds. The validity of these thresholds was majorly verified by the interpretation of acoustic emission (AE) method (Eberhardt et al. 1998; Moradian et al. 2016; Zhao et al. 2015). Xue et al. (2014) summarised the studies of uniaxial compression tests to evaluate similarities and differences of stress thresholds among igneous, metamorphic and sedimentary rocks. Using the volumetric strain–stress and petrographic technology, Nicksiar and Martin (2013) discussed the relationship between the mineral composition and crack initiation stress. The formation and propagation of microcracks, and fracturing mechanisms are also dependent on Poisson’s ratio of geomaterials (Gercek 2007). With the increased accuracy in computational input for numerical analyses, a precise knowledge of the Poisson’s ratio is of importance. In quasi-static tests, however, the lateral strain is substantially challenging to determine (Swamy 1971). The strain gauge is widely used, but large strain will generate when the stress is above its peak value. Due to the influences of localised failure, it is desirable to measure the lateral strain as an average over a part instead of one point of the specimen. Such measurements include chain extensometer or fluid-field vessel by the fluid level change, but these contact methods to some extent lead to a confinement. Therefore, non-contact optical techniques have substantial advantages. The digital image correlation (DIC) has been proved robust in dealing with multiple deformation conditions (Xing et al. 2017). Cui et al. (2016), Pan et al. (2015) and Pritchard et al. (2013) used 2D-DIC to investigate the axial and transverse strain of composite material or alloy with a thin-section shape in static tests. Munoz et al. (2016) applied the 3D-DIC to determine the crack evolution stages of sandstone in quasi-static uniaxial compression tests. In addition to the static condition, rock engineering may encounter high-strain-rate loadings from explosion, impact or seismic activity (Barla and Zhao 2010; Zhou and Zhao 2011). To the best of our knowledge, there is no report on the crack thresholds or direct measurement of dynamic Poisson’s ratio of rock under high-strain-rate conditions. The primary challenge is that the lateral strain cannot be accurately acquired by traditional methods such as the strain gauge and extensometer under dynamic loading conditions. In this study, stress and strain thresholds of crack development of sandstone under different high strain rates with a split Hopkinson pressure bar (SHPB) were characterised by the high-speed 3D-DIC technique. The real-time evolution of Poisson’s ratio was extracted together with its strain-rate effect according to strain fields in 3D-DIC.

Jian Zhao

Papers

Construction and utilization of rock caverns in Singapore Part B: Development costs and utilization

Parallelization of the distinct lattice spring model

Underground cavern development in the Jurong sedimentary rock formation

Progressive Damage Behaviours of Triaxially Confined Rocks under Multiple Dynamic Loads

Stress Thresholds of Crack Development and Poisson’s Ratio of Rock Material at High Strain Rate