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.
Stability and safety of rock structures are easily influenced by the dynamic disturbance, especially when weak joint planes exist. In order to investigate the filling joint effect on the dynamic ...response of rock specimens, a series of impact dynamic tests were conducted by a modified split Hopkinson pressure bar (SHPB) system. A sandwich type of sandstone specimens filling with different thicknesses layer of cemented mortar (filling joint) were tested in the study. The results show that the transmission coefficient, dynamic strength and energy absorption all decrease with increasing joint thickness. However, the reflection coefficient, peak strain and joint closure show an opposite variation trend. In addition, the deformation of the cemented mortar joint is the main reason to cause the deformation and final failure of jointed rock specimens. Tensile cracks dominate the fracturing behavior during the dynamic loading, but they have a slight influence on the final failure mode which transforms from localized slabbing to axial splitting failure with the filling joint becoming thicker.
•The transversal seismic response of rectangular tunnels is studied by means of a numerical parametric study.•A rocking deformation mode coupled with racking distortion is identified during ...shaking.•Numerical R-F relations are developed and compared to the existing literature.•Rocking response is quantified by means of dimensionless relations (θ/γff-F)•The effects of lining rigidity, soil-tunnel interface conditions and soil yielding are highlighted.
A numerical parametric study was conducted on diverse soil-rectangular tunnel systems, aiming to shed light on critical response characteristics of rectangular tunnels subjected to transversal ground shaking. Salient parameters that affect the dynamic response, such as: (i) the soil-tunnel relative stiffness and interface properties, (ii) the shape, dimensions and burial depth of the tunnel section, (iii) the soil deposit characteristics, and (iv) the input motion characteristics, were accounted for in this study. This paper summarizes the key findings of this investigation, focusing on the complex deformation modes of the tunnels during shaking, the dynamic earth pressures and the soil dynamic shear stresses developed around the tunnel, and the dynamic lining forces. The numerical results indicated a combined racking-rocking deformation pattern for the tunnels during shaking, while inward deformations of the slabs and the side-walls were also observed for flexible tunnels, when soil inelasticity was encountered. To quantify the racking deformation of rectangular tunnels, a series of numerical racking ratio - flexibility ratio (R-F) relations were developed and compared with existing analytical and empirical ones. The rocking response of rectangular tunnels was quantified by means of dimensionless relations (θ/γff-F), similar to the R-F relations. The soil-tunnel relative stiffness, the interface characteristics and the soil yielding affected significantly the above relations, as well as the dynamic earth pressures, the soil dynamic shear stresses and the dynamic forces developed on the lining during shaking. The presented results lead to a better understanding of the seismic response of rectangular tunnels in soft soil, while the proposed relations contribute towards the improvement of the R-F analysis method.
•The re-entrant jet induced cloud shedding is confirmed in hot water.•Whole stages of the cavity clouds developing and collapsing are presented.•Shedding dynamics of cavitation over a wide ...tempetature range are identified.•The thermal transition and increase-decrease trend of cavity length are analyzed.
The effects of temperature on hydraulic cavitation dynamics are investigated under various operating conditions, in a close loop cavitation tunnel with a small-scale venturi type section. A systematic study is performed with temperatures varying between 24°C and 85°C, using a high-speed visualization system to observe the cavitating flow. The image processing methods provided in the paper present a quantitative comparison of the cavitation dynamics and structure development. The results show that the increase of the fluid temperature induces the growth of the cavitation volume up to about 55°C, thereafter an additional increase in temperature has the opposite effect. This evolution is interpreted as a competition between a Reynolds effect and the well-known thermal effect. Cavitation is more closely investigated within the temperature range 50°C - 65°C, to analyze the changes in the structure and the cavitation dynamics. For the prediction of thermal suppression head, the thermal effect parameter Σ which can be used empirically, is derived at the maximum cavitation length. This fluid thermodynamic parameter Σ(Ttrans) at the transition peak can be referred to to avoid the maximum cavitation aggressiveness induced vibration or erosion for thermos-fluids around the thermal transition temperature. Finally, the factors influencing cavitation length and shedding frequency are presented and analyzed.
Train-track-bridge dynamic interaction is a fundamental concern in the field of railway engineering, which plays an extremely important role in the optimal design of railway bridges, especially in ...high-speed railways and heavy-haul railways. This paper systematically presents a state-of-the-art review of train-track-bridge dynamic interaction. The evolution process of train-bridge dynamic interaction model is described briefly, from the simplest moving constant force model to the sophisticated train-track-bridge dynamic interaction model (TTBDIM). The modelling methodology of the key elements in the TTBDIM is systematically reviewed, including the train, the track, the bridge, the wheel-rail contact, the track-bridge interaction, the system excitation and the solution algorithm. The significance of detailed track modelling in the whole system is highlighted. The experimental research and filed test focusing on modelling validation, safety assessment and long-term performance investigation of the train-track-bridge system are briefly presented. The practical applications of train-track-bridge dynamic interaction theory are comprehensively discussed in terms of the system dynamic performance evaluation, the system safety assessment and train-induced environmental vibration and noise prediction. The guidance is provided on further improvement of the train-track-bridge dynamic interaction model and the challenging research topics in the future.
The dynamic recrystallization (DRX) phenomena occurring in different thermo-mechanical processing (TMP) conditions for various metallic materials are reviewed. Several types of DRX are described: ...discontinuous dynamic recrystallization (DDRX), continuous dynamic recrystallization (CDRX) and geometric dynamic recrystallization (GDRX). The terminologies used in this field are summarized, together with the key factors influencing the DRX processes including stacking fault energy, initial grain size, TMP conditions and second-phase particles. Both standard and advanced experimental techniques used to characterize DRX processes are examined. The focus is placed on the mechanisms of these three types of DRX, and the related numerical models.
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•Three types of dynamic recrystallization processes occurring during hot deformation are reviewed.•The mechanisms of these three types of dynamic recrystallization processes are discussed in detail.•Physically based numerical models for all the three dynamic recrystallization process are reviewed.•Topics for further investigation on dynamic recrystallization are recommended.
To estimate the dynamic effects of an absorbing treatment, researchers often use two-way fixed effects regressions that include leads and lags of the treatment. We show that in settings with ...variation in treatment timing across units, the coefficient on a given lead or lag can be contaminated by effects from other periods, and apparent pretrends can arise solely from treatment effects heterogeneity. We propose an alternative estimator that is free of contamination, and illustrate the relative shortcomings of two-way fixed effects regressions with leads and lags through an empirical application.
This article reviews the recent development of adaptive dynamic programming (ADP) with applications in control. First, its applications in optimal regulation are introduced, and some skilled and ...efficient algorithms are presented. Next, the use of ADP to solve game problems, mainly nonzero-sum game problems, is elaborated. It is followed by applications in large-scale systems. Note that although the functions presented in this article are based on continuous-time systems, various applications of ADP in discrete-time systems are also analyzed. Moreover, in each section, not only some existing techniques are discussed, but also possible directions for future work are pointed out. Finally, some overall prospects for the future are given, followed by conclusions of this article. Through a comprehensive and complete investigation of its applications in many existing fields, this article fully demonstrates that the ADP intelligent control method is promising in today's artificial intelligence era. Furthermore, it also plays a significant role in promoting economic and social development.
Probabilistic forecasting, i.e., estimating a time series’ future probability distribution given its past, is a key enabler for optimizing business processes. In retail businesses, for example, ...probabilistic demand forecasts are crucial for having the right inventory available at the right time and in the right place. This paper proposes DeepAR, a methodology for producing accurate probabilistic forecasts, based on training an autoregressive recurrent neural network model on a large number of related time series. We demonstrate how the application of deep learning techniques to forecasting can overcome many of the challenges that are faced by widely-used classical approaches to the problem. By means of extensive empirical evaluations on several real-world forecasting datasets, we show that our methodology produces more accurate forecasts than other state-of-the-art methods, while requiring minimal manual work.
We present the first experimental observation of a topological transition in a non-Hermitian system. In contrast to standard methods for examining topological properties, which involve probing edge ...(or surface) states, we monitor the topological transition by employing bulk dynamics only. The system is composed of a lattice of evanescently coupled optical waveguides, and non-Hermitian behavior is engineered by inducing bending loss by spatially "wiggling" every second waveguide.
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