Henry T. Y. Yang

A survey of recent shell finite elements

Since the mid-1960s when the forms of curved shell finite elements were originated, including those pioneered by Professor Gallagher, the published literature on the subject has grown extensively. The first two present authors and Liaw presented a survey of such literature in 1990 in this journal. Professor Gallagher maintained an active interest in this subject during his entire academic career, publishing milestone research works and providing periodic reviews of the literature. In this paper, we endeavor to summarize the important literature on shell finite elements over the past 15 years. It is hoped that this will be a befitting tribute to the pioneering achievements and sustained legacy of our beloved Professor Gallagher in the area of shell finite elements. This survey includes: the degenerated shell approach; stress-resultant-based formulations and Cosserat surface approach; reduced integration with stabilization; incompatible modes approach; enhanced strain formulations; 3-D elasticity elements; drilling d.o.f. elements; co-rotational approach; and higher-order theories for composites. Copyright

Semi-inverse method for predicting stress-strain relationship from cone indentations

A method for determining the stress–strain relationship of a material from hardness values H obtained from cone indentation tests with various apical angles is presented. The materials studied were assumed to exhibit power-law hardening. As a result, the properties of importance are the Youngs modulus E , yield strength Y , and the work-hardening exponent n . Previous work [W.C. Oliver and G.M. Pharr, J. Mater. Res. 7, 1564 (1992)] showed that E can be determined from initial force–displacement data collected while unloading the indenter from the material. Consequently, the properties that need to be determined are Y and n . Dimensional analysis was used to generalize H/E so that it was a function of Y/E and n [Y-T. Cheng and C-M. Cheng, J. Appl. Phys. 84, 1284 (1999); Philos. Mag. Lett. 77, 39 (1998)]. A parametric study of Y/E and n was conducted using the finite element method to model material behavior. Regression analysis was used to correlate the H/E findings from the simulations to Y/E and n . With the a priori knowledge of E , this correlation was used to estimate Y and n .

Panel Flutter Detection and Control Using the Eigenvector Orientation Method and Piezoelectric Layers

A basic eigenvector orientation approach has been used to evaluate the possibility of controlling the onset of panel flutter using a simple flat panel (wide beam) as an illustrative example. The use of the eigenvector orientation method for prediction of the flutter boundary (indicated by a gradual loss of orthogonality between two eigenvectors) was developed in a previous study and can thus provide a lead time for possible flutter control. As a first step, piezoelectric layers are assumed to be bonded to the top and bottom surfaces of the panel in order to provide counterbending moments at joints between elements. The standard linear quadratic control theory is used for controller design and full state feedback is considered for simplicity. The controllers are designed to restabilize the system at the onset of flutter; as a result, flutter occurrence can be offset to a higher flutter speed. To illustrate the applicability and effectiveness of the developed method, several simple wide-beam examples are studied and presented. The effects of control moment locations are studied so as to fulfill the objective of adjusting the flutter speed to be within a desirable range. Potential applications of this basic method may be straightforwardly applied to plate and shell structures of laminated composites using the versatile finite element method.

Integration of Health Monitoring and Control of Building Structures during Earthquakes

AbstractA hybrid real-time structural health monitoring and control system for building structures is presented in this study. A model-reference adaptive control algorithm for the designed substructures was developed and integrated with a previously developed interstory drift–based acceleration feedback method for health monitoring. A virtual healthy model, installed with health monitors, is used to generate proper control forces to obtain the desired response for the controlled substructure during a disastrous event such as an earthquake. An adaptive controller and an adaptation mechanism are designed using the Lyapunov theory to calculate the real-time adaptive control force. The local feedback control actuates the actual substructures to track the desired response signals. The results obtained by numerical simulations for the illustrative example in this study are further validated by experimental investigations using a 3-story aluminum frame structure. The asymptotical tracking of the state of the sub...

Structural Damage Diagnosis Using Interstory Drift–Based Acceleration Feedback with Test Validation

AbstractThis study presents a numerical structural damage detection method using interstory drift–based structural acceleration measurements in the time domain. The coupling effect of the damage at different locations in the multiple-degree-of-freedom building system is eliminated by projecting the measured accelerations onto specific independent subspaces. The damage in a region will only affect the output of the designed monitor observing the substructure within the region. The severity of the damage is estimated numerically using a model-based prediction curve of stiffness change. Results obtained by the present numerical simulations for the illustrative examples are validated by experimental investigations using a 3-story aluminum frame structure and a 12-story concrete frame structure, and the numerical simulation results are compared with some representative experimental data with favorable correlations. Incorporation of the incomplete measurement, different structural materials, and different excit...

Improved Decentralized Method for Control of Building Structures under Seismic Excitation

A decentralized control method with improved robustness and design flexibility is proposed for reducing vibrations of seismically excited building structures. In a previous study, a control scheme was developed for multistory building models using nonlinear, decentralized control theory. This control method has now been improved in this study in that less information about material properties and geometrical parameters of the building is needed and the selection of control design parameters is more flexible. The nonlinear behavior of the proposed control system is studied and its stability property is proven mathematically. To evaluate the effectiveness and robustness of the proposed method, three illustrative structural models, i.e., an eight-story elastic shear beam model, a two-story nonlinear elastic shear beam model, and a 20-story elastic benchmark model are studied. The 1940 El Centro and the 1995 Kobe earthquakes are used in these examples. The performance of the current control design, as applied to these examples, has shown to be more effective in reducing structural responses and improving robustness.

Bio-Inspired Passive Optimized Base-Isolation System for Seismic Mitigation of Building Structures

AbstractAn energy dissipation mechanism of abalone shells, called sacrificial bonds and hidden length, is simulated and proposed to develop new strategies for base isolation. The concept of integrating this novel energy dispersion mechanism into a conventional linear isolator is proposed. A systematic parametric study is performed to evaluate the influences of the properties of both the isolation layer and the structure on the structural seismic responses to a series of earthquake records. Using the insights gained from the parametric study, an optimization procedure for designing such a bio-inspired isolator is presented based on the multiobjective optimization approach. To demonstrate the advantages of this idea, the optimized bio-inspired isolation system is numerically investigated by first comparing it with the passive isolators such as a high-damping linear isolator and a lead rubber bearing system. The proposed isolator is found to have superior performance to conventional passive isolation systems...

Developing hybrid structural health monitoring via integrated global sensing and local infrared imaging

Sensing technology and sensor development have received increased attention in the recent years, and a number of types of sensors have been developed for various applications for materials and structures. In this paper, we will discuss the concept of combining sensing of global vibration and local infrared imaging techniques. The global vibration-based techniques determine the health condition of structures by the changes in their dynamic properties or responses to external disturbs or excitations. Infrared Imaging is introduced here to detect local defects or problems so that to provide more direct and accurate assessment about the severity and extent of the damage. The progress on developing a hybrid structural health monitoring system is presented through the results on both the global sensing algorithm study and local infrared imaging investigation on a steel C channel.

Structural damage detection and assessment using acceleration feedback

This paper presents a method for structural health monitoring using acceleration measurements. In a previous study a method for detecting, locating, and quantifying structural damages has been developed by directly using the time domain structural vibration measurements. However, only displacement and velocity measurements were used in that study. In this paper, acceleration measurements are used as feedback. Because it is more practical to measure acceleration using accelerometers, it is preferable to use acceleration rather than displacement and velocity measurements for the purpose of structural damage detection and assessment. However, using acceleration measurements is more difficult since the effects of different damages can not be decoupled completely as in the cases of displacement and velocity measurements. One approach of circumventing this difficulty is presented and it involves increasing the order of time derivatives of the linear system. The effectiveness of the proposed method using acceleration feedback is evaluated with illustrative examples of a three and an eight-story model. Results obtained are found to be comparable with results from simulations using displacement measurements as feedback.

Analyzing the effect of hydration on the wedge indentation fracture behavior of cortical bone

Hydration directly affects the mechanical properties of bone. An initial and basic procedure shows both wedge indentation fracture experiments under plane strain conditions in cortical bone and numerical simulation with finite elements agree that dry bone fractures much more easily than fully hydrated bone submerged in an aqueous environment, such as in the body of an animal. The wedge indentation experiments were performed with high speed video microscopy, under dry and fully hydrated (submerged) conditions. The numerical simulation, specifically finite element analysis using cohesive elements to simulate fracture, was utilized to capture plasticity, fracture initiation and propagation, and to study the applicability of brittle material based indentation fracture theory. Experiment and theory give similar results for the dependence of depth of fracture initiation, and size of plastic zone, on hydration state. Comparison of fracture propagation characteristics between wet and dry bone are examined and discussed. This research demonstrates the ability to quantitatively assess the effect of hydration on the fracture initiation, propagation, and plastic zone size of cortical bone, through an approach using simple wedge indentation, with important implications for efforts in developing methods to understand clinical diagnostic testing and general fracture behavior of living bone in the ultimate interest of health care purposes.