Guowei Ma

Bio: Guowei Ma is an academic researcher from Hebei University of Technology. The author has contributed to research in topics: Ultimate tensile strength & Rock mass classification. The author has an hindex of 54, co-authored 401 publications receiving 10194 citations. Previous affiliations of Guowei Ma include University of Delaware & Beijing Institute of Technology.

Suggested Methods for Determining the Dynamic Strength Parameters and Mode-I Fracture Toughness of Rock Materials

Abstract: The properties of rocks under dynamic loading are important for the study of a whole range of rock mechanics and rock engineering problems, including blasting, protective design, explosives storage, rock bursts and seismic events. The propagation of dynamic stress waves in the ground, response of rock tunnels to dynamic load, dynamic support design and damage assessment all require a good understanding of the behavior of rocks under dynamic loading. Due to the transient nature of dynamic loading, the dynamic tests of rock material are very different from static tests.

Printable properties of cementitious material containing copper tailings for extrusion based 3D printing

Abstract: 3D printing for cementitious material is an innovative and promising construction method that is rapidly gaining ground in recent years. Utilizing waste or recyclable materials as the primary raw material to produce cementitious material for 3D printing can greatly promote the 3D printing to reach its maximum cost-effective potentials. This paper proposes an environmental friendly cementitious mixture that is compatible with an extrusion based printing process. In this study, six replacement ratio of tailing to sand from 0% to 50% are investigated. A single nozzle printing system is developed and the operational process is illustrated. Experimental tests are performed to determine the printable properties of mixtures containing various content of tailings, including the extrudability, buildability, flowability, open time, fresh and hardened properties, etc. Based on the measurements, the optimal mixture is determined as substituting natural sand with 30% mass ratio of mining tailings, which enables structures achieve a favorable buildability and a relatively high mechanical strength. In particular, the critical value of controlling parameters to achieve sufficient printability are specified. And the compressive and flexural strength of the printed and the casted samples are measured and compared. To conclude the present research, extrudability and buildability coefficients are proposed for optimizing design.

Numerical simulation of blasting-induced rock fractures

Abstract: In the present study, the Johnson–Holmquist (J–H) material model is implemented into the commercial software LS-DYNA through user-subroutines to simulate the blasting-induced rock fractures. The J–H model consists of strength models for both intact and fully fractured materials, a polynomial equation of state, and a damage model that represents the material from an intact state to a fully fractured state. Influences of the key parameters in smooth blasting, viz., loading rate, distance from a free face, earth stress, and pre-existing joint planes, etc., on fracture patterns are explored. According to the simulation results, the rock fracture pattern is significantly influenced by the loading rate. Fracture control techniques, namely, notched borehole and charge holder with slits are also simulated. Effectiveness of the fracture control techniques is demonstrated. The numerical simulation in the present study reproduces some of the well-known phenomena observed by other researchers. It has the potential to be applied in practical blast control and gas and hydraulic fracturing engineering.

Mechanical anisotropy of aligned fiber reinforced composite for extrusion-based 3D printing

Abstract: 3D printing techniques are being researched extensively in the construction sector. However, the key issue lies in the development of cementitious materials with both favorable printability and enough mechanical capability by means of high strength and ductility. In this study, an optimal basalt fiber content was determined basing firstly on suitable printability and then on mechanical performance. A self-developed 3D printer was used for extrusion of the cementitious material and also for mechanical enhancement of fiber alignment along the print direction by keeping the nozzle diameter smaller than the length of the basalt fiber. The printing process deposits directional filaments, intrinsically resulting in laminated structures and mechanical anisotropy. Anisotropic performances of the printed material were evaluated by direction-based mechanical performance testing and confirmed by ultrasonic pulse velocity testing. The mechanical behaviors of 3D printed samples exposed to compressive, tensile, flexural and shearing loadings were experimentally investigated. The mesoscale structures of printed samples were detected through the advanced CT scanning technique. Both mechanical and acoustic indexes were proposed to evaluate the anisotropic properties of printed materials. In particular, empirical relationships between the mechanical anisotropic properties and ultrasonic signals were established. On the microstructural level, mechanical enhancement of fiber alignment, fiber pullout and fiber fracture were all probed through scanning electron microscope (SEM) imaging.

Modeling complex crack problems using the numerical manifold method

Abstract: In the numerical manifold method, there are two kinds of covers, namely mathematical cover and physical cover. Mathematical covers are independent of the physical domain of the problem, over which weight functions are defined. Physical covers are the intersection of the mathematical covers and the physical domain, over which cover functions with unknowns to be determined are defined. With these two kinds of covers, the method is quite suitable for modeling discontinuous problems. In this paper, complex crack problems such as multiple branched and intersecting cracks are studied to exhibit the advantageous features of the numerical manifold method. Complex displacement discontinuities across crack surfaces are modeled by different cover functions in a natural and straightforward manner. For the crack tip singularity, the asymptotic near tip field is incorporated to the cover function of the singular physical cover. By virtue of the domain form of the interaction integral, the mixed mode stress intensity factors are evaluated for three typical examples. The excellent results show that the numerical manifold method is prominent in modeling the complex crack problems.

Machine learning

Abstract: Machine Learning is the study of methods for programming computers to learn. Computers are applied to a wide range of tasks, and for most of these it is relatively easy for programmers to design and implement the necessary software. However, there are many tasks for which this is difficult or impossible. These can be divided into four general categories. First, there are problems for which there exist no human experts. For example, in modern automated manufacturing facilities, there is a need to predict machine failures before they occur by analyzing sensor readings. Because the machines are new, there are no human experts who can be interviewed by a programmer to provide the knowledge necessary to build a computer system. A machine learning system can study recorded data and subsequent machine failures and learn prediction rules. Second, there are problems where human experts exist, but where they are unable to explain their expertise. This is the case in many perceptual tasks, such as speech recognition, hand-writing recognition, and natural language understanding. Virtually all humans exhibit expert-level abilities on these tasks, but none of them can describe the detailed steps that they follow as they perform them. Fortunately, humans can provide machines with examples of the inputs and correct outputs for these tasks, so machine learning algorithms can learn to map the inputs to the outputs. Third, there are problems where phenomena are changing rapidly. In finance, for example, people would like to predict the future behavior of the stock market, of consumer purchases, or of exchange rates. These behaviors change frequently, so that even if a programmer could construct a good predictive computer program, it would need to be rewritten frequently. A learning program can relieve the programmer of this burden by constantly modifying and tuning a set of learned prediction rules. Fourth, there are applications that need to be customized for each computer user separately. Consider, for example, a program to filter unwanted electronic mail messages. Different users will need different filters. It is unreasonable to expect each user to program his or her own rules, and it is infeasible to provide every user with a software engineer to keep the rules up-to-date. A machine learning system can learn which mail messages the user rejects and maintain the filtering rules automatically. Machine learning addresses many of the same research questions as the fields of statistics, data mining, and psychology, but with differences of emphasis. Statistics focuses on understanding the phenomena that have generated the data, often with the goal of testing different hypotheses about those phenomena. Data mining seeks to find patterns in the data that are understandable by people. Psychological studies of human learning aspire to understand the mechanisms underlying the various learning behaviors exhibited by people (concept learning, skill acquisition, strategy change, etc.).

VALIDITY OF CUBIC LAW FOR FLUID FLOW IN A DEFORMABLE ROCK FRACTURE - eScholarship

Abstract: The validity of the cubic law for laminar flow of fluids through open fractures consisting of parallel planar plates has been established by others over a wide range of conditions with apertures ranging down to a minimum of 0.2 µm. The law may be given in simplified form by Q/Δh = C(2b)3, where Q is the flow rate, Δh is the difference in hydraulic head, C is a constant that depends on the flow geometry and fluid properties, and 2b is the fracture aperture. The validity of this law for flow in a closed fracture where the surfaces are in contact and the aperture is being decreased under stress has been investigated at room temperature by using homogeneous samples of granite, basalt, and marble. Tension fractures were artificially induced, and the laboratory setup used radial as well as straight flow geometries. Apertures ranged from 250 down to 4µm, which was the minimum size that could be attained under a normal stress of 20 MPa. The cubic law was found to be valid whether the fracture surfaces were held open or were being closed under stress, and the results are not dependent on rock type. Permeability was uniquely defined by fracture aperture and was independent of the stress history used in these investigations. The effects of deviations from the ideal parallel plate concept only cause an apparent reduction in flow and may be incorporated into the cubic law by replacing C by C/ƒ. The factor ƒ varied from 1.04 to 1.65 in these investigations. The model of a fracture that is being closed under normal stress is visualized as being controlled by the strength of the asperities that are in contact. These contact areas are able to withstand significant stresses while maintaining space for fluids to continue to flow as the fracture aperture decreases. The controlling factor is the magnitude of the aperture, and since flow depends on (2b)3, a slight change in aperture evidently can easily dominate any other change in the geometry of the flow field. Thus one does not see any noticeable shift in the correlations of our experimental results in passing from a condition where the fracture surfaces were held open to one where the surfaces were being closed under stress.

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

Abstract: The purpose of this review paper is to present the techniques, advances, problems and likely future developments in numerical modelling for rock mechanics. Such modelling is essential for studying the fundamental processes occurring in rocks and for rock engineering design. The review begins by explaining the special nature of rock masses and the consequential difficulties when attempting to model their inherent characteristics of discontinuousness, anisotropy, inhomogeneity and inelasticity. The rock engineering design backdrop to the review is also presented. The different types of numerical models are outlined in Section 2, together with a discussion on how to obtain the necessary parameters for the models. There is also discussion on the value that is obtained from the modelling, especially the enhanced understanding of those mechanisms initiated by engineering perturbations. In Section 3, the largest section, states-of-the-art and advances associated with the main methods are presented in detail. In many cases, for the model to adequately represent the rock reality, it is necessary to incorporate couplings between the thermal, hydraulic and mechanical processes. The physical processes and the equations characterizing the coupled behaviour are included in Section 4, with an illustrative example and discussion on the likely future development of coupled models. Finally, in Section 5, the advances and outstanding issues in the subject are listed and in Section 6 there are specific recommendations concerning quality control, enhancing confidence in the models, and the potential future developments.

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

Abstract: 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.