The orientation of fractures with transpressional and transtensional wrenches in pre-existing faults has not been quantitatively determined. Based on Coulomb failure criterion and Byerlee’s frictional sliding criteri...The orientation of fractures with transpressional and transtensional wrenches in pre-existing faults has not been quantitatively determined. Based on Coulomb failure criterion and Byerlee’s frictional sliding criterion, this paper has indicated quantitative geometric relationships between the pre-existing fault and the local induced principal stress axes caused by the rejuvenation of the pre-existing fault. For a hidden pre-existing fault with some cohesion, the angles between the local induced principal stress axes and the pre-existing fault quantitatively vary with the applied stress and the cohesion coefficient, the ratio of the thickness of the cover layer to the thickness of the whole wrench body, whether transpressional or transtensional wrenches occur. For a surface pre-existing fault with zero cohesion, the angles between the pre-existing fault and the local induced principal stress axes are related to the rock inner frictional angle regardless of both the applied stress and the cohesion coefficient where transpressional wrenches occur, and the local induced maximum principal stress axis is identical with the applied maximum principal stress axis where transtensional wrenches occur. Therefore, the geometric relationships between the pre-existing faults and their related fractures are defined, because the local induced principal stress axes determine the directions of the related fractures. The results can be applied to pre-existing weak fabrics. They can help to understand and analyze wrench structures in outcrops or subsurface areas. They are of significance in petroleum exploration.展开更多
The formation conditions and time sequences for various types of wrench-related fractures are not clear. Based on a parabola-type failure criterion, this paper has gotten new insights on those questions. In a simple s...The formation conditions and time sequences for various types of wrench-related fractures are not clear. Based on a parabola-type failure criterion, this paper has gotten new insights on those questions. In a simple shear, the occurrence of either tensional fractures or Riedel shears is controlled by the ratio (Rtc) of tensile strength to cohesion. In a pure shear, the occurrence of either second order tensional fractures or second order Riedel shears is controlled by the ratio (Rtci) of tensile strength to cohesion, given a constant inner frictional coefficient. Where the Rtc or the Rtci is less than a certain value, the en echelon tensional fractures will occur first. Where the Rtc or the Rtci is bigger than the certain value, the Riedel shears will occur first. Where the Rtc or the Rtci is equal to the certain value, the en echelon tensional fractures and the Riedel shears will occur simultaneously. The understandings will enhance the research on wrench related fractures and will be of significance in petroleum exploration and development, because fractures are both important accumulation spaces and key migration paths for oil and gas.展开更多
From the point of view of real application of multifingered hand, the problem of estimating the gravity of a grasped object and the wrench exerted on the object by the external environment, using multifingered hand eq...From the point of view of real application of multifingered hand, the problem of estimating the gravity of a grasped object and the wrench exerted on the object by the external environment, using multifingered hand equipped with force/torque sensors, is addressed. At first the problem is modeled mathematically by the concept of wrench, then gravitational effect of fingertips is taken into accounted to eliminate the biasing effect in real implementation. On experimental system HKUST HAND, some experiments are carried out to implement and verify the proposed approach. This is an introduction to real application of multifingered hand, such as assembly of parts.展开更多
Upper limb exoskeletal rehabilitation robots are required to assist patients＇ arms to perform activities of daily living according to their motion intentions. In this paper, we address two questions： how to design a...Upper limb exoskeletal rehabilitation robots are required to assist patients＇ arms to perform activities of daily living according to their motion intentions. In this paper, we address two questions： how to design an exoskeletal robot which can mechanically reconstruct functional movements using only a few actuators and how to establish wrench-based assistive control. We first show that the mechanism replicating the synergic feature of the human upper limb can be designed in a recursive manner, meaning that the entire robot can be constructed from two basic mechanical units. Next, we illustrate that the assistive control for the synergetic exoskeletal robot can be transformed into an optimization problem and a Riemannian metric is proposed to generate anthropomorphic reaching movements according to contact forces and torques. Finally, experiments are carried out to verify the functionality of the proposed theory.展开更多
文摘The orientation of fractures with transpressional and transtensional wrenches in pre-existing faults has not been quantitatively determined. Based on Coulomb failure criterion and Byerlee’s frictional sliding criterion, this paper has indicated quantitative geometric relationships between the pre-existing fault and the local induced principal stress axes caused by the rejuvenation of the pre-existing fault. For a hidden pre-existing fault with some cohesion, the angles between the local induced principal stress axes and the pre-existing fault quantitatively vary with the applied stress and the cohesion coefficient, the ratio of the thickness of the cover layer to the thickness of the whole wrench body, whether transpressional or transtensional wrenches occur. For a surface pre-existing fault with zero cohesion, the angles between the pre-existing fault and the local induced principal stress axes are related to the rock inner frictional angle regardless of both the applied stress and the cohesion coefficient where transpressional wrenches occur, and the local induced maximum principal stress axis is identical with the applied maximum principal stress axis where transtensional wrenches occur. Therefore, the geometric relationships between the pre-existing faults and their related fractures are defined, because the local induced principal stress axes determine the directions of the related fractures. The results can be applied to pre-existing weak fabrics. They can help to understand and analyze wrench structures in outcrops or subsurface areas. They are of significance in petroleum exploration.
文摘The formation conditions and time sequences for various types of wrench-related fractures are not clear. Based on a parabola-type failure criterion, this paper has gotten new insights on those questions. In a simple shear, the occurrence of either tensional fractures or Riedel shears is controlled by the ratio (Rtc) of tensile strength to cohesion. In a pure shear, the occurrence of either second order tensional fractures or second order Riedel shears is controlled by the ratio (Rtci) of tensile strength to cohesion, given a constant inner frictional coefficient. Where the Rtc or the Rtci is less than a certain value, the en echelon tensional fractures will occur first. Where the Rtc or the Rtci is bigger than the certain value, the Riedel shears will occur first. Where the Rtc or the Rtci is equal to the certain value, the en echelon tensional fractures and the Riedel shears will occur simultaneously. The understandings will enhance the research on wrench related fractures and will be of significance in petroleum exploration and development, because fractures are both important accumulation spaces and key migration paths for oil and gas.
文摘From the point of view of real application of multifingered hand, the problem of estimating the gravity of a grasped object and the wrench exerted on the object by the external environment, using multifingered hand equipped with force/torque sensors, is addressed. At first the problem is modeled mathematically by the concept of wrench, then gravitational effect of fingertips is taken into accounted to eliminate the biasing effect in real implementation. On experimental system HKUST HAND, some experiments are carried out to implement and verify the proposed approach. This is an introduction to real application of multifingered hand, such as assembly of parts.
基金Acknowledgment This work was partially supported by the National Natural Science Foundation of China （Grant Nos. 91648203 and 51335004）, the International Science ＆ Technology Cooperation Program of China （Grant No. 2016YFE0113600）, and the Science Foundation for Innovative Group of Hubei Province （Grant No. 2015CFA004）. The authors would like to thank Ting Wang, Xiaowei Cheng, and Xuan Wu for their contributions to the development of the experimental platform.
文摘Upper limb exoskeletal rehabilitation robots are required to assist patients＇ arms to perform activities of daily living according to their motion intentions. In this paper, we address two questions： how to design an exoskeletal robot which can mechanically reconstruct functional movements using only a few actuators and how to establish wrench-based assistive control. We first show that the mechanism replicating the synergic feature of the human upper limb can be designed in a recursive manner, meaning that the entire robot can be constructed from two basic mechanical units. Next, we illustrate that the assistive control for the synergetic exoskeletal robot can be transformed into an optimization problem and a Riemannian metric is proposed to generate anthropomorphic reaching movements according to contact forces and torques. Finally, experiments are carried out to verify the functionality of the proposed theory.
