R. K. Miller

Identification of Nonlinear Vibrating Structures: Part I—Formulation

A self-starting multistage, time-domain procedure is presented for the identification of nonlinear, multi-degree-of-freedom systems undergoing free oscillations or subjected to arbitrary direct force excitations and/or nonuniform support motions. Recursive least-squares parameter estimation methods combined with nonparametric identification techniques are used to represent, with sufficient accuracy, the identified system in a form that allows the convenient prediction of its transient response under excitations that differ from the test signals. The utility of this procedure is demonstrated in a companion paper.

Finite element analysis of partly wrinkled membranes

Abstract An evaluation of the accuracy and efficiency of a numerical algorithm for the stressstrain behavior of partly wrinkled membranes is presented. The evaluation is based on a comparison of analytical and numerical results for stresses and displacements in two “benchmark” problems involving a partly wrinkled flat membrane. The comparisons reveal a high degree of accuracy for the finite element algorithm. Furthermore, convergence of the required iterative procedure in this nonlinear problem was achieved without excessive computation.

The Olin curriculum: thinking toward the future

In 1997, the F. W. Olin Foundation of New York established the Franklin W. Olin College of Engineering, Needham, MA, with the mission of creating an engineering school for the 21st century. Over the last five years, the college has transformed from an idea to a functioning entity that admitted its first freshman class in fall 2002. This paper describes the broad outlines of the Olin curriculum with some emphasis on the electrical and computer engineering degree. The curriculum incorporates the best practices from many other institutions as well as new ideas and approaches in an attempt to address the future of engineering education.

Malignant lymphoma in a warthin's tumor report of a case

A case is reported of a Warthins tumor with a lymphomatous component. The tumor was uniformly involved by neoplastic follicles which were composed of predominantly small cleaved follicular center cells. A subsequent lymph node biopsy showed the same subtype of malignant lymphoma. This uniform involvement of a Warthins tumor by malignant lymphoma has not been previously reported.

Development of bearing friction models from experimental measurements

Abstract For a better understanding of the underlying physics involved in friction phenomena, a carefully controlled experimental study is presented in which the frictional forces and dynamic response of a shaft oscillating within a pair of sleeve bearings were monitored and analyzed in order to gain further insight into the basic phenomenological features of bearing friction forces. Through a qualitative review of the data, it is shown that the trajectory of the bearing force versus slip velocity exhibits an hysteretic-type loop super-imposed on Coulomb and viscous actions. Parametric identification techniques are used to develop a simplified mathematical model incorporating an idealized Coulomb friction element. A comprehensive application of non-linear system identification teniques, to extract more quantitative and less obvious characteristics of the measured frictional behavior, is presented. Processing of the data through the use of a time-domain procedure for the identification of non-linear vibrating structures shows that an optimum (in the least squares sense) non-linear mathematical model can be developed to match, with reasonable accuracy, all of the measured response time histories. Furthermore, the mathematical representation of the model allows convenient separation of the contribution of the equivalent linear and non-linear internal forces developed in the physical system.

Active Parameter Control of Nonlinear Vibrating Structures

The procedure uses nonlinear auxiliary mass dampers with adjustable motion-limiting stops located at selected positions throughout a given nonlinear system. The degree of the primary structure oscillation near each vibration damper determines the dampers actively-controlled gap size and activation time. By using control energy to adjust the damper parameters instead of directly attenuating the motion of the primary system, a significant improvements is achieved in the total amount of energy expended to accomplish a given level of vibration control. In a related paper, the direct method of Lyapunov is used to establish that the response of the controlled nonlinear primary structure is Lagrange stable

Structural concepts for large solar concentrators

Abstract This paper deals with the types of structural arrangements that are appropriate for large solar concentrator systems for use in space. Past concepts originating in the 1960s are discussed. These include stiff sandwich panels and aluminum dishes as well as inflated and umbrella-type membrane configurations. The Sunflower concentrator, developed in the early 1970s, is described as a salient example of a high-efficiency collector. Existing concepts for high-efficiency reflector surfaces are examined with attention to accuracy needs for concentration ratios of 1000 to 3000. It is concluded that concepts using stiff reflector panels are most likely to exhibit the long-term consistent accuracy necessary for low-orbit operation, particularly for the higher concentration ratios. Quantitative results are shown relating the allowable error to the desired concentration and focal-length-diameter ratios. A new configuration is presented that addresses both of these problems. It consists of a deployable Pactruss backup structure with identical panels installed on the deployed structure after deployment in space. Analytical results show that with reasonable pointing errors, this new concept is capable of concentration ratios greater than 2000.

Orthogonal decomposition and transmission of nonstationary random processes

Abstract A relatively simple and straightforward procedure is given for representing analytically defined or data-based covariance kernels of arbitrary random processes in a compact form that allows its convenient use in later analytical random vibration response studies. The method is based on the spectral decomposition of the random process by the orthogonal Karhuen Loeve expansion and the subsequent use of least-squares approaches to develop an approximating analytical fit for the eigenvectors of the underlying random process. The resulting compact analytical representation of the random process is then used to derive a closed-form solution for the nonstationary response of a damped SDOF harmonic oscillator. The utility of the method for representing the excitation and calculating the mean square response is illustrated by the use of an analytically-defined covariance kernel widely employed in random vibration studies. It is shown that the method offers the potential of being a useful tool for feature extraction of experimentally measured covariance kernels of nonstationary random processes.

A System Identification Approach to the Detection of Changes in Structural Parameters

Nondestructive evaluation (NDE) methods for the detection of damage in structural systems have been receiving increasing attention in the recent past. Among the promising NDE methods are those based on the analysis of structural dynamic response measurements to identify a suitable mathematical model corresponding to the (changing) state of the physical structure.

The buckling of lattice columns with stochastic imperfections

Abstract An analysis is presented for determining the buckling load of triangular lattice columns with combined local and overall imperfections. For the case where the imperfections are deterministic and uniform, the nonlinear problem is solved in terms of quadratures. The resulting buckling loads are shown to compare favorably with the predictions of a straightforward single-term Ritz approximation. The Ritz approach is used to derive estimators for the mean and standard deviation of the buckling load for the situation where the local imperfections are stochastic. The resulting estimators are shown to be valid by comparing their results with those obtained by a Monte Carlo simulation.