Publications

System behaviors can be accurately simulated using artificial neural networks (ANNs), and one that performs well in simulation of structural response is the radial basis function network. A specific implementation of this is the connectionist normalized linear spline (CNLS) network, investigated in this study. A useful framework for ANN simulation of structural response is the recurrent network. This framework simulates the response of a structure one step at a time. It requires as inputs some measures of the excitation, and the response at previous times. On output, the recurrent ANN yields the response at some time in the future. This framework is practical to implement because every ANN requires training, and this is executed by showing the ANN examples of correct input/output behavior (exemplars), and requiring the ANN to simulate this behavior. In practical applications, hundreds or, perhaps, thousands, of exemplars are required for ANN training. The usual laboratory and non-neural numerical applications to be simulated by ANNs produce these amounts of information. Once the recurrent ANN is trained, it can be provided with excitation information, and used to propagate structural response, simulating the response it was trained to approximate. The structural characteristics, parameters in the CNLS network, and degree of training influence the accuracy of approximation. This investigation studies the accuracy of structural response simulation for a single-degree-of-freedom (SDF), nonlinear system excited by random vibration loading. The ANN used to simulate structural response is a recurrent CNLS network. We investigate the error in structural system simulation.

This paper proposes a framework for the comprehensive analysis of complex problems in probabilistic structural mechanics. Tools that can be used to accurately estimate the probabilistic behavior of mechanical systems are discussed, and some of the techniques proposed in the paper are developed and used in the solution of a problem in nonlinear structural dynamics.

System dynamicists frequently encounter signals they interpret as realizations of normal random processes. To simulate these analytically and in the laboratory they use methods that yield approximately normal random signals. The traditional digital methods for generating such signals have been developed during the past 25 years. During the same period of time much development has been done in the theory of chaotic processes. The conditions under which chaos occurs have been studied, and several measures of the nature of chaotic processes have been developed. Some of the measures used to characterize the nature of dynamic system motions are common to the study of both random vibrations and chaotic processes. This paper considers chaotic processes and random vibrations. It shows contrasts between the two and situations where they are indistinguishable. The applicability of the Central Limit Theorem to chaotic processes is demonstrated. 12 refs., 8 figs.

This paper and a companion paper show that the traditional limits on amplitude and frequency that can be generated in a laboratory test on a vibration exciter can be substantially extended. This is accomplished by attaching a device to the shaker that permits controlled metal to metal impacts that generate a high acceleration, high frequency environment on a test surface. A companion paper derives some of the mechanical relations for the system. This paper shows that a sinusoidal shaker input can be used to excite deterministic chaotic dynamics of the system yielding a random vibration environment on the test surface, or a random motion of the shaker can be used to generate a random vibration environment on the test surface. Numerical examples are presented to show the kind of environments that can be generated in this system. 9 refs., 9 figs.

This paper presents some simple concepts for fixtures that can be used in two and three-axis vibration testing. Two, two-axis fixtures were built and tested in the laboratory. Test results are shown, and serve to confirm the validity of the concept. Simple methods for extending the concepts for three-axis testing are discussed. 6 refs., 9 figs.

This paper presents a combined analytical and experimental method for establishing a set of equations to evaluate the equivalent forces acting on a structure. The method requires that a finite element model of the structure be established. It further requires that the accelerator responses to the external forces be measured at a number of points on the structure. The equivalent forces established in the analysis are a representation of the actual forces. The equivalent forces concentrate the effects of the external forces at the degrees of freedom where the acceleration responses are measured. 6 refs., 4 figs., 1 tab.