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

In underground rock engineering, water-bearing faults may be subjected to dynamic loading, resulting in the coupling of hydraulic and dynamic hazards. Understanding the interaction mechanism between the stress waves induced by dynamic loadings and liquid-filled rock joints is therefore crucial. In this study, an auxiliary device for simulating the liquid-filled layer was developed to analyze the dynamic response characteristics of liquid-filled rock joints in laboratory. Granite and polymethyl methacrylate (PMMA) specimens were chosen for testing, and high-amplitude shock waves induced by a split Hopkinson pressure bar (SHPB) were used to produce dynamic loadings. Impact loading tests were conducted on liquid-filled rock joints with different joint inclinations. The energy propagation coefficient and peak liquid pressure were proposed to investigate the energy propagation and attenuation of waves propagating across the joints, as well as the dynamic response characteristics of the liquid in the liquid-filled rock joints. For the inclination angle range considered herein, the experimental results showed that the energy propagation coefficient gently diminished with increasing joint inclination, and smaller coefficient values were obtained for granite specimens compared with PMMA specimens. The peak liquid pressure exhibited a gradually decreasing trend with increasing joint inclination, and the peak pressure for granite specimens was slightly higher than that for PMMA specimens. Overall, this paper may provide a considerably better method for studying liquid-filled rock joints at the laboratory scale, and serves as a guide for interpreting the underlying mechanisms for interactions between stress waves and liquid-filled rock joints.

Laboratory-scale investigation of response characteristics of liquid-filled rock joints with different joint inclinations under dynamic loading

Based on viscoelastic theory,an equivalent one-dimensional dynamic continuum model is proposed for rock mass with parallel joints The model includes two portions,one is the viscoelastic medium model and the other is the concept of"virtual wave source(VWS)" The time retardation and attenuation of longitudinal wave propagation across jointed rock mass are considered in the viscoelastic medium model;and the concept of VWS is adopted to account for the wave reflections between joints The parameters in the equivalent viscoelastic model are derived analytically based on existing solution for longitudinal wave propagation across a single rock joint The proposed model is then verified by applying identical incident waves to the discontinuous and equivalent dynamic media at one end and receiving output waves at the other end,ie by comparing two results from the displacement discontinuity method and the present equivalent medium method The results show that the proposed model can simplify the simulation of stress propagation effectively

Jian Zhao

Papers

Laboratory-scale investigation of response characteristics of liquid-filled rock joints with different joint inclinations under dynamic loading

An equivalent 1D dynamic continuum model for rock mass with parallel joints

Advances and challenges in underground space use in Singapore

Estimating the dynamic strength of rock using Mohr-Coulomb and Hoek-Brown criteria

Revisiting statistical correlation between Mohr-Coulomb shear strength parameters of Hoek-Brown rock masses