Structural Health Monitoring of Composite Plates Subjected to Impacts Using the Electromechanical Impedance Technique

The aim of this paper is to evaluate the use of the Structural Health Monitoring (SHM) technique based on the concept of electromechanical impedance for the assessment of low-energy impact damage in laminated carbon-fiber composite plates. The experiments were carried-out by using an especially designed pendulum, and were planned in such a way to accommodate a range of test conditions, such as impact energy and dimension of the impacting piece. Also, it was investigated the influence of the frequency band in which the impedance functions are measured. Additionally, statistical metamodels were built aiming at establishing functional relations between the values of the damage metric and impact energy for single and multiple impacts. The obtained results demonstrate the capability of the monitoring method to identify various damage levels corresponding to different impact conditions.

Development of a Low-Cost Automated Tension Estimation System for Cable-Stayed Bridges

Cable tension force is one of the most important structural parameters to monitor in cable-stayed bridges. For example, cable tension needs to be monitored during construction and maintenance to ensure the bridge is not overloaded. To economically monitor tension forces, this study proposes the use of an automated wireless tension force estimation system (WFTES) developed solely for cable force estimation. The design of the WFTES system can be divided into two parts: low-cost hardware and automated software. The low-cost hardware consists of an integrated platform containing a wireless sensing unit constructed from commercial off-the-shelf components, a low-cost commercial MEMS accelerometer, and a signal conditioning board for signal amplification and filtering. With respect to the automated software, a vibration-based algorithm using estimated modal parameters and information on the cable sag and bending stiffness is embedded into the wireless sensing unit. Since modal parameters are inputs to the algorithm, additional algorithms are necessary to extract modal features from measured cable accelerations. To validate the proposed WFTES, a scaled-down cable model was constructed in the laboratory using steel rope wire. The wire was exposed to broad-band excitations while the WFTES recorded the cable response and embedded algorithms interrogated the measured acceleration to estimate tension force. The results reveal the embedded algorithms properly identify the lower natural frequencies of the cable and make accurate estimates of cable tension. This paper concludes with a summary of the salient research findings and suggestions for future work.

Design and Fabrication of a Bio-Inspired Flapping Flight Micro-Air Vehicle

The main objective of the BATMAV project is the development of a biologically-inspired Micro Aerial Vehicle (MAV) with flexible and foldable wings for flapping flight. While flapping flight in MAV has been previously studied and a number of models were realized they usually had unfoldable wings actuated with DC motors and mechanical transmission to provide the flapping motion, a system that brings the disadvantage of a heavy flight platform. This phase of the BATMAV project presents a flight platform that features bat-inspired wings with a number of flexible joints to allow mimicking the kinematics of the real mammalian flyer. The bat was chosen after an extensive analysis of the flight parameters of small birds, bats and large insects characterized by a superior maneuverability and wind gust rejection. Morphological and aerodynamic parameters were collected from existing literature and compared concluding that bat wing present a suitable platform that can be actuated efficiently using artificial muscles. Due to their wing camber variation, the bat species can operate effectively at a large rage of speeds and allow remarkably maneuverable and agile flight. Bat skeleton measurements were taken and modeled in SolidWorks to accurately reproduce bones and body via rapid prototyping machines. Much attention was paid specifically to achieving the comparable strength, elasticity, and range of motion of a naturally occurring bat. Therefore, a desktop model was designed, fabricated and assembled in order to study and optimize the effect of various flapping patterns on thrust and lift forces. As a whole, the BATMAV project consists of four major stages of development: the current phase — design and fabrication of the skeletal structure of the flight platform, selection and testing different materials for the design of a compliant bat-like membrane, analysis of the kinematics and kinetics of bat flight in order to design a biomechanical muscle system for actuation, and design of the electrical control architecture to coordinate the platform flight.

System Identification and Pseudo-Earthquake Excitation of a Real-Scaled 5 Story Steel Frame Structure

The accurate identification of the dynamic response characteristics of a building structure excited by input signals such as real earthquake or wind load is essential not only for the evaluation of the safety and serviceability of the building structure, but for the verification of an analytical model used in the seismic or wind design. In the field of system identification (SI) which constructs system matrices describing the accurate input/output relationship, it is critical that input should have enough energy to excite fundamental structural modes and a good quality of output containing structural information should be measured. In this study forced vibration testing which is important for correlating the mathematical model of a structure with the real one and for evaluating the performance of the real structure was implemented. There exist various techniques available for evaluating the seismic performance using dynamic and static measurements. In this paper, full scale forced vibration tests simulating earthquake response are implemented by using a hybrid mass damper. The finite element (FE) model of the structure was analytically constructed using ANSYS and the model was updated using the results experimentally measured by the forced vibration test. Pseudo-earthquake excitation tests showed that HMD induced floor responses coincided with the earthquake induced ones which was numerically calculated based on the updated FE model.

Self-Healing Bilayer Lipid Membranes Formed Over Synthetic Substrates

Biological systems demonstrate autonomous healing of damage and are an inspiration for developing self-healing materials. Our recent experimental study has demonstrated that a bilayer lipid membrane (BLM), also called a black lipid membrane, has the ability to self-heal after mechanical failure. These molecules have a unique property that they spontaneously self assembly into organized structures in an aqueous medium. The BLM forms an impervious barrier to ions and fluid between two volumes and strength of the barrier is dependent on the pressure and electrical field applied to the membrane. A BLM formed over an aperture on a silicon substrate is shown to self-heal for 5 pressurization failure cycles.

Robust Autonomous Damage Detection and Assessment in Polymeric Composite Structures

This paper introduces the some of the experimental and analytical work behind the autonomous damage detection technique. The research study conducted here resulted in the development of a Structural Health Monitoring (SHM) system for a 2-D polymeric composite T-joint, used in maritime structures. Two methods of damage detection are discussed — A statistics-based outlier technique and one using Artificial Neural Networks (ANNs). The SHM using ANNs system was found to be capable of not only detecting the presence of multiple delaminations in a composite structure, but also capable of determining the location and extent of all the delaminations present in the T-joint structure, regardless of the load (angle and magnitude) acting on the structure. The system developed relies on the examination of the strain distribution of the structure under operational loading. Finally, on testing the SHM system developed with strain signatures of composite T-joint structures, subjected to variable loading, embedded with all possible damage configurations (including multiple damage scenarios), an overall damage (location & extent) prediction accuracy of 94.1% was achieved. These results are presented and discussed in detail in this paper.

Characterization of Optical Fiber Bragg Gratings as Strain Sensors Considering Load Direction

This study investigates the influence of lay-up and load direction on embedded optical fiber Bragg gratings (FBGs) used as strain sensors. FBGs have shown great promise for application to structural health monitoring with advantages of small size and cylindrical geometry readily allowing for embedment within fiber reinforced composites. Characterization of the embedded FBGs is necessary to develop a rugged and reliable strain sensor. This paper specifically explores the effects of loading direction on the FBG strain outputs. A well behaved baseline case is established with results for gratings loaded parallel to the optical fiber direction while embedded parallel to the adjacent structural fibers in a quasi-isotropic composite. Results and analysis are also presented for a case involving a composite fabricated with the optical and structural fibers parallel to each other but perpendicular to the loading direction. Extremely good results are obtained relating FBG strain measurements with that of surface mounted resistance strain gauges.

Optimal Sensor Placement for Active Sensing

We present a novel approach for optimal actuator and sensor placement for active sensing-based structural health monitoring (SHM). Of particular interest is the optimization of actuator-sensor arrays making use of Lamb wave propagation for detecting damage in thin plate-like structures. Using a detection theory framework, we establish the optimum configuration as the minimization of the expected percentage of the structure to show type I or type II error during the damage detection process. The detector incorporates a statistical model of the active sensing process which implements both pulse-echo and pitch-catch actuation schemes and takes into account line of site and non-uniform damage probabilities. The optimization space was searched using a genetic algorithm with a time varying mutation rate. We provide four example actuator/sensor placement scenarios and the optimal solutions as generated by the algorithm.

On Superharmonic Resonances of Nonlinear Nonuniform Beams

Superharmonic resonances of nonlinear forced bending vibrations of moderately large curvature of nonuniform cantilever beams of rectangular cross section and a sharp end are reported. Cantilevers of constant width and parabolic thickness variation are considered in this paper. Method of multiple scales is directly applied to the governing partial-differential equation of motion and boundary conditions. Two problems, zero- and first-order problem, result. Solving the zero-order problem, the linear modes are obtained in terms of hypergeometric functions by using the factorization method. The first-order problem provides the amplitude and phase evolution equation and consequently the superharmonic frequency response of the nonlinear system.

Nonlinear Position Control of Smart Actuators Using Model Predictive Sliding Mode Control

This paper investigates a novel nonlinear positioning control methodology for piezoelectric stack actuators. Piezoelectric devices become very common recently for precise positioning, primarily due to the fact that they are solid state and can be accurately controlled by a voltage or current input. However, hysteresis decreases positioning accuracy and could lead to instability. The ultimate goal is to reduce it so that the piezoelectric device has a nearly linear relationship between the input field and output strain. The main purpose of this research is the reduction of the hysteresis utilizing a hysteresis model and a nonlinear model-based controller. A novel control method called model predictive sliding mode control (MPSMC) will be utilized on an actuator using a nonlinear energy-based hysteresis model. The idea of MPSMC is to implement model predictive control techniques to improve sliding mode control by forcing the system to reach the sliding surface in an optimal manner. Simulations and experiments were conducted to verify the technique.