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  • 1
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    Inverse dynamics of adaptive structures used as space cranes (1990)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: As a precursor to the real-time control of fast moving adaptive structures used as space cranes, a formulation is given for the flexibility induced motion relative to the nominal motion (i.e., the motion that assumes no flexibility) and for obtaining the open loop time varying driving forces. An algorithm is proposed for the computation of the relative motion and driving forces. The governing equations are given in matrix form with explicit functional dependencies. A simulator is developed to implement the algorithm on a digital computer. In the formulations, the distributed mass of the crane is lumped by two schemes, vz., 'trapezoidal' lumping and 'Simpson's rule' lumping. The effects of the mass lumping schemes are shown by simulator runs.
    Keywords: STRUCTURAL MECHANICS
    Type: Journal of Intelligent Material Systems and Structures (ISSN 1045-389X); 1; 50-75
    Format: text
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  • 2
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    Theory of time-dependent rupture in the Earth (1980)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: Fracture mechanics is used to develop a theory of earthquake mechanism which includes the phenomenon of subcritical crack growth. The following phenomena are predicted: slow earthquakes, multiple events, delayed multiple events (doublets), postseismic rupture growth and afterslip, foreshocks, and aftershocks. The theory predicts a nucleation stage prior to an earthquake, and suggests a physical mechanism by which one earthquake may 'trigger' another.
    Keywords: GEOPHYSICS
    Type: NASA-CR-163567
    Format: application/pdf
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  • 3
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    Why large earthquakes do not nucleate at shallow depths (1983)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Spontaneous rupture models were examined to determine if gradients of average stress-drop and average strength relative to ambient stress account for earthquake nucleation near the base of the seismogenic layer. The frictional strength has been proved to increase with normal stress, which (across a fault) increases with depth in concert with the frictional strength. It is assumed that the large earthquakes occur at a time when the applied stress is equal to the frictional strength. A gradient has been found in frictional strength relative to the ambient stress and the stress-drop with depth. Attention is given to two cases where a circular crack appears, and radiates outward. It is shown that nucleation in the lower part of the seismogenic region will permit growth into a large earthquake, while nucleation in the lower stress-drop regime will encapsulate the crack. It is concluded that only cracks propagating in a high stress-drop regime can move easily into a relatively lower strength region.
    Keywords: GEOPHYSICS
    Type: Nature (ISSN 0028-0836); 305; 621-623
    Format: text
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  • 4
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    Stress corrosion cracking in the earth (1980)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Two fundamental concepts of fracture mechanics are used to develop a theory of the earthquake mechanism which specifically predicts observed time-dependent rupture phenomena such as slow earthquakes, postseismic rupture growth and afterslip, multiple events, foreshocks, and aftershocks. The theory also predicts that there must be a nucleation stage prior to an earthquake, and suggests a physical mechanism by which one earthquake may trigger another. Investigations show that all earthquakes must be preceded by a quasi-static slip over a portion of the rupture surfaces, although it may be difficult to detect in practice, and a study of delayed multiple events characterizes the strength of some barriers in the earth as having a stress corrosion index of about 24.
    Keywords: GEOPHYSICS
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  • 5
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    A numerical method for determination of source time functions for general three-dimensional rupture propagation (1979)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: A method to determine the displacement and the stress on the crack plane for a three-dimensional shear crack of arbitrary shape propagating in an infinite, homogeneous medium which is linearly elastic everywhere off the crack plane is presented. The main idea of the method is to use a representation theorem in which the displacement at any given point on the crack plane is written as an integral of the traction over the whole crack plane. As a test of the accuracy of the numerical technique, the results are compared with known solutions for two simple cases.
    Keywords: STRUCTURAL MECHANICS
    Type: NASA-CR-162139
    Format: application/pdf
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  • 6
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    A mathematical basis for the design and design optimization of adaptive trusses in precision control (1991)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: A mathematical basis for the optimal design of adaptive trusses to be used in supporting precision equipment is provided. The general theory of adaptive structures is introduced, and the global optimization problem of placing a limited number, q, of actuators, so as to maximally achieve precision control and provide prestress, is stated. Two serialized optimization problems, namely, optimal actuator placement for prestress and optimal actuator placement for precision control, are addressed. In the case of prestressing, the computation of a 'desired' prestress is discussed, the interaction between actuators and redundants in conveying the prestress is shown in its mathematical form, and a methodology for arriving at the optimal placement of actuators and additional redundants is discussed. With regard to precision control, an optimal placement scheme (for q actuators) for maximum 'authority' over the precision points is suggested. The results of the two serialized optimization problems are combined to give a suboptimal solution to the global optimization problem. A method for improving this suboptimal actuator placement scheme by iteration is presented.
    Keywords: STRUCTURAL MECHANICS
    Type: Joint U.S./Japan Conference on Adaptive Structures; Nov 13, 1990 - Nov 15, 1990; Maui, HI; United States
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  • 7
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    Optimal actuator placement in adaptive precision trusses (1992)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: Actuator placement in adaptive truss structures is to cater to two needs: displacement control of precision points and preloading the elements to overcome joint slackness. Due to technological and financial considerations, the number of actuators available is much less than the degrees of freedom of precision points to be controlled and the degree of redundancy of the structure. An approach for optimal actuator location is outlined. Test cases to demonstrate the effectiveness of the scheme are applied to the Precision Segmented Reflector Truss.
    Keywords: STRUCTURAL MECHANICS
    Type: AIAA PAPER 92-2344 , AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference; Apr 13, 1992 - Apr 15, 1992; Dallas, TX; United States
    Format: text
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