Two modifications to an ultrasonic camera system have been performed in an effort to reduce setup time and post
inspection image processing. Current production ultrasonic cameras have image gates that are adjusted manually. The
process to adjust them prior to each inspection consumes large amounts of time and requires a skilled operator. The
authors have overcome this by integrating the A-Scan and image together such that the image gating is automatically
adjusted using the A-Scan data. The system monitors the A-scan signal which is in the center of the camera's field of
view (FOV) and adjusts the image gating accordingly. This integration will allow for defect detection at any depth of the
inspected area. Ultrasonic camera operation requires the inspector to scan the surface manually while observing the
cameras FOV in the monitor. If the monitor image indicates a defect the operator then stores that image manually and
marks an index on the surface as to where the image has been acquired. The second modification automates this effort
by employing a digital encoder and image capture card. The encoder is used to track movement of the camera on the
structures surface, record positions, and trigger the image capture device. The images are stored real time in the buffer
memory rather than on the hard drive. The storing of images in the buffer enables for a more rapid acquisition time
compared to storing the images individually to the hard drive. Once the images are stored, an algorithm tracks the
movement of the camera through the encoder and accordingly displays the image to the inspector. Upon completion of
the scan, an algorithm digitally stitches all the images to create a single full field image. The modifications were tested
on a aerospace composite laminate with known defects and the results are discussed.
Recent work by our research group on the dynamic demodulation of strain-induced wavelength shifts in fiber Bragg grating (FBG) sensors show that these sensors are suitable for the detection of high frequency ultrasonic waves produced by impact loading. A FBG sensor is incorporated into an optical detection system that uses a broadband tunable laser source in the C-band, a two wave-mixing photorefractive interferometer, and a high-speed photodetector. When an ultrasonic wave interacts with the FBG sensor, the wavelength of the reflected light in the fiber is dynamically shifted due to strain-induced perturbation of the index of refraction and/or the period of the grating in the fiber. The wavelength shift is converted into an intensity change by splitting the light into signal and pump beams and interfering the beams in an InP:Fe photorefractive crystal (PRC). The resulting intensity change is measured by a photodetector. The two-wave mixing (TWM) photorefractive interferometer allows for several FBG sensors to be wavelength multiplexed in one PRC and it also actively compensates for low frequency signal drifts associated with unwanted room vibrations and temperature excursions. In this work, we present preliminary experimental results on the detection of impact signals using a low power (1 mW) TWM PRC based demodulation system. The response time of the PRC is optimized by focusing the signal and pump beams into the crystal allowing for adaptivity of the demodulation system to quasi-static strains or temperature drifts. The TWM intensity gain of the system is optimized for efficient wavelength demodulation through resonant enhancement of the space charge electric field formed in the PRC. The low power demodulation system would facilitate significant reduction in the overall cost of the system.
The quantitative evaluation of damage in woven composites using mode selective excitation of Lamb waves is reported
in this paper. PVDF (polyvinylidene fluoride) comb sensors are used to generate and detect a single plate mode. The top
electrode is a single set of equidistant fingers connected in parallel to the same potential while the bottom electrode is
kept at ground. First, a pair of such sensors is used to generate and detect a single plate mode. Group velocity changes of
a wave packet traveling through the damaged area are used for quantitative damage estimation. Second, a new electrode
configuration is used in order to improve the receiver signal. The proposed configuration referred to as continuous
sensors, is used in structural health monitoring (SHM) for detection of growing cracks. Theoretical and experimental
results are presented. In addition, an analog circuitry to actuate the structure at high frequency (~1MHz) based on energy
tapped from a vibrating cantilever beam (~20Hz) is developed, towards a high-frequency energy-harvested SHM.
A large number of sensors are required to perform real-time structural health monitoring (SHM) to detect acoustic emissions (AE) produced by damage growth on large complicated structures. This requires a large number of high sampling rate data acquisition channels to analyze high frequency signals. To overcome the cost and complexity of having such a large data acquisition system, a structural neural system (SNS) was developed. The SNS reduces the required number of data acquisition channels and predicts the location of damage within a sensor grid. The sensor grid uses interconnected sensor nodes to form continuous sensors. The combination of continuous sensors and the biomimetic parallel processing of the SNS tremendously reduce the complexity of SHM. A wave simulation algorithm (WSA) was developed to understand the flexural wave propagation in composite structures and to utilize the code for developing the SNS. Simulation of AE responses in a plate and comparison with experimental results are shown in the paper. The SNS was recently tested by a team of researchers from University of Cincinnati and North Carolina A&T State University during a quasi-static proof test of a 9 meter long wind turbine blade at the National Renewable Energy Laboratory (NREL) test facility in Golden, Colorado. Twelve piezoelectric sensor nodes were used to form four continuous sensors to monitor the condition of the blade during the test. The four continuous sensors are used as inputs to the SNS. There are only two analog output channels of the SNS, and these signals are digitized and analyzed in a computer to detect damage. In the test of the wind turbine blade, multiple damages were identified and later verified by sectioning of the blade. The results of damage identification using the SNS during this proof test will be shown in this paper. Overall, the SNS is very sensitive and can detect damage on complex structures with ribs, joints, and different materials, and the system relatively inexpensive and simple to implement on large structures.
A method for impact and damage detection on a plate using strain responses from long continuous sensors and analysis by a neural network technique was presented and verified by numerical simulation. The response characteristics of continuous sensors, which are long ribbon-like sensors, were studied by simulation of wave propagation in a panel. The advantage of the continuous sensor is to improve damage detection by having a large coverage of sensors on the structure using a small number of channels of data acquisition. Strain responses from the continuous sensors were used to estimate the damage location using the neural network technique. Several numerical wave propagation simulation runs for a plate were carried out to train the neural network and verify the proposed method for damage localization. The identified damage locations agreed reasonably well with the exact damage locations. Overall, the approach presented is meant to simplify the instrumentation needed for damage detection by using continuous sensors, a small number of channels of data acquisition, and training a neural network to do the work of locating the damage source.
KEYWORDS: Sensors, Composites, Neurons, Data acquisition, Analog electronics, Wave propagation, Diagnostics, Structural health monitoring, Signal processing, Prototyping
A small size prototype of a Structural Neural System (SNS) was tested in real time for damage detection in a
laboratory setting and the results are presented in this paper. The SNS is a passive online structural health
monitoring (SHM) system that can detect small propagating damages in real time before the overall failure of the
structure is realized. The passive SHM method is based on the concept of detecting acoustic emissions (AE) due to
damage propagating. Propagating cracks were identified near the vicinity of a sensor in a composite specimen
during fatigue testing. In the composite specimen, in additions to a propagating crack, fretting occurred because of
slipping contact between the load points and the composite specimen. The SNS was able to predict the location of
damage due to crack propagation and also detect signals from fretting simultaneously in real time.
Structural Health Monitoring ideally would check the health of the structure in real time all the time. Simplifying the sensor system and the data acquisition equipment plays a very important role in achieving this goal. This paper discusses a practical technique that uses long continuous sensors and biomimetic signal processing to simplify health monitoring. The testing of a structural neural system with an updated analog processor module is discussed in this paper. A neuron is formed by connecting sensor elements to an analog processor. The structural neural system is formed by connecting multiple neurons to mimic the signal processing architecture of the neural system of the human body. This approach reduces the required number of data acquisition channels and still predicts the location of damage within a grid of miniature neurons. Different types of sensors can also be used. A piezoelectric ribbon sensor can sense damage due to impacts or crack growth because these damages generate Lamb waves that are detected by the neural system. The neuron can also receive diagnostic waves generated to check the structure on demand and when it is not in operation. In addition, new continuous multi-wall carbon nanotube sensors are being used as strain and crack detection neurons that operate during both static and dynamic loading. In general, the Structural Neural System may provide an advantage for the continuous monitoring of most large sensor systems in which anomalous events must be detected, and where it is impractical to have a separate channel of data acquisition for each sensor. Moreover, the data reduction technique and damage detection algorithm are easy to understand, simple to implement, reliable, and many sensor types can be used.
Detecting and locating cracks in structural components and joints that have high feature densities is a challenging problem in the field of Structural Health Monitoring. There have been advances in piezoelectric sensors, actuators, wave propagation, MEMS, and optical fiber sensors. However, few sensor-signal processing techniques have been applied to the monitoring of joints and complex structural geometries. This is in part because maintaining and analyzing a large amount of data obtained from a large number of sensors that may be needed to monitor joints for cracks is difficult. Reliable low cost assessment of the health of structures is crucial to maintain operational availability and productivity, reduce maintenance cost, and prevent catastrophic failure of large structures such as wind turbines, aircraft, and civil infrastructure. Recently, there have also been advances in development of simple passive techniques for health monitoring including a technique based on mimicking the biological neural system using electronic logic circuits. This technique aids in reducing the required number of data acquisition channels by a factor of ten or more and is able to predict the location of a crack within a rectangular grid or within an arbitrarily arranged network of continuous sensors or neurons. The current paper shows results obtained by implementing this method on an aluminum plate and joint. The plates were tested using simulated acoustic emissions and also loading via an MTS machine. The testing indicates that the neural system can monitor complex joints and detect acoustic emissions due to propagating cracks. High sensitivity of the neural system is needed, and further sensor development and testing on different types of joints is required. Also indicated is that sensor geometry, sensor location, signal filtering, and logic parameters of the neural system will be specific to the particular type of joint (material, thickness, geometry) being monitored. Also, a novel piezoresistive carbon nanotube nerve crack sensor is presented that can become a neuron and respond to local crack growth.
Conventional finite element approaches for modeling delaminations in laminated composite structures use the Heaviside unit step function at the interfacial nodes in the delaminated zone of the structure to model the possible jumps in the displacement field during “breathing” of the delaminated layers. In quantum mechanics, the Fermi-Dirac distribution applies to Fermion particles whose characteristics are half-integer spins. The present paper uses the Fermi-Dirac distribution function to model a smoother transition in the displacement and the strain fields of the delaminated interfaces during the opening and closing of the delaminated layers under vibratory loads. This paper successfully shows that the Fermi-Dirac distribution function can be used to more accurately model the dynamic effects of delaminations in laminated composite structures. Optimizing the parameters in the Fermi-Dirac distribution function can lead to more accurate modeling of the dynamic and transient behavior of the delaminated zones in laminated composite structures. Further applications of the Fermi-Dirac distribution function in other physics based dynamic models are suggested. This paper also effectively demonstrates how hybrid sensors comprising of out of plane displacement sensors and in plane strain sensors can effectively map a composite structure to detect and locate the delaminated zones. It also shows how simple mode shapes can be used to determine the locations of single and multiple delaminations in laminated composite structures.
This paper explores concepts for new smart materials that have extraordinary properties based on nanotechnology. Carbon and boron nitride nanotubes in theory can be used to manufacture fibers that have piezoelectric, pyroelectric, piezoresistive, and electrochemical field properties. Smart nanocomposites designed using these fibers will sense and respond to elastic, thermal, and chemical fields in a positive human-like way to improve the performance of structures, devices, and possibly humans. Remarkable strength, morphing, cooling, energy harvesting, strain and temperature sensing, chemical sensing and filtering, and high natural frequencies and damping will be the properties of these new materials. Synthesis of these unique atomically precise nanotubes, fibers, and nanocomposites is at present challenging and expensive, however, there is the possibility that we can synthesize the strongest and lightest actuators and most efficient sensors man has ever made. A particular advantage of nanotube transducers is their very high load bearing capability. Carbon nanotube electrochemical actuators have a predicted energy density at low frequencies that is thirty times greater than typical piezoceramic materials while boron nitride nanotubes are insulators and can operate at high temperatures, but they have a predicted piezoelectric induced stress constant that is about twenty times smaller than piezoceramic materials. Carbon nanotube fibers and composites exhibit a change in electrical conductivity due to strain that can be used for sensing. Some concepts for nanocomposite material sensors are presented and initial efforts to fabricate carbon nanocomposite load sensors are discussed.
This paper examines the use of continuous sensors to detect damage in composite materials. Continuous sensors contain multiple interconnected sensor nodes that can be integrated into an artificial neural system as an array of sensor nerves. The advantage of this passive health monitoring approach is that the sensor system is highly distributed and uses parallel processing allowing large structures to be monitored for damage using a small number of channels of data acquisition. In the paper, the continuous sensor system is modeled and simulated by solving the elastic response of a plate and the coupled piezoelectric constitutive equations. The model and simulation allow the sensor system to be optimized for a particular material and plate size. The simulation predicts that acoustic waves representative of damage growth can be detected anywhere in the plate using a simple artificial neural system. To improve the sensitivity of the continuous sensor, unidirectional active fiber composite sensors were built from piezoceramic ribbon preforms. Manufacturing of the active fiber composite sensors is also discussed in the paper. The continuous sensors were evaluated in a realistic test to show their ability detect acoustic emissions caused by damage to a composite material. The sensors were mounted on narrow glass fiber plates and tested to failure in a mechanical test machine. Results from the experiments are presented.
This paper discusses the development of continuous Active Fiber Composite sensors to detect damage in composite materials. Continuous sensors contain multiple interconnected sensor nodes that can be integrated into an artificial neural system as an array of sensor nerves. Continuous sensors have demonstrated a possibility of damage detection in large structures when used as a part of Artificial Neural System. The advantage of this passive health monitoring approach is that the sensor system is highly distributed and uses parallel processing allowing large structures to be monitored for damage using a small number of channels of data acquisition. In the paper, the continuous sensor system is modeled and simulated by solving the elastic response of a plate and the coupled piezoelectric constitutive equations. The model and simulation allow the sensor system to be optimized for a particular material and plate size. The simulation predicts that acoustic waves representative of damage growth can be detected anywhere in the plate using a simple artificial neural system. To improve the sensitivity of the continuous sensor, unidirectional active fiber composite sensors were built from piezoceramic ribbon preforms. Manufacturing of the active fiber composite sensors is discussed in the paper. The continuous sensors were evaluated in a realistic test to show their ability detect acoustic emissions caused by damage to a composite material. The sensors were mounted on narrow glass fiber plates and tested to failure in a mechanical test machine. Results from the experiments are also presented.
This paper discusses recent advances in modeling and simulation of an artificial neural system and simulation of wave propagation for designing structural health monitoring systems. An artificial neural system was modeled using piezoceramic nerves and electronic components. Wave propagation in a panel is modeled using classical plate theory and a closed-form solution of wave propagation and reflection is obtained. Equations representing a half sine input similar to a projectile impact or a tone burst excitation were added to the existing algorithm that predicts the response of the artificial neural system due to impulse inputs. Firing switches have been modeled in the simulation to predict the sequential firing of the neurons as the waves pass over them. Also, new active fiber sensors have been designed for use in the artificial neural system. Simulated responses of the artificial neural system are shown in this paper and indicate that large neural systems can be formed with hundreds of sensor nodes. Experiments were performed to study a small neural system on a glass fiber panel. Waves were induced in the panel due to a lead break to simulate a crack and due to an impact from an impact hammer. Testing showed the location of a crack could be determined within the unit cell of the neural system for an orthotropic panel.
KEYWORDS: Sensors, Neurons, Acoustic emission, Signal processing, Composites, Signal detection, Biomimetics, Structural health monitoring, Digital signal processing, Structural design
A new approach for the Health Monitoring of structural systems is described in this paper. This technique is based on detecting the acoustic emission signals from damage progression in structures using an array of sensory nodes. Two different sensor configurations that could be used for monitoring wide areas on a structure are discussed. An reliable and cost effective health monitoring system can be an enabling technology for the widespread use of newly discovered high performance materials and design concepts in structural applications. Without a reliable health monitoring system, the lack of service experience and the susceptibility of new classes of materials to unexpected and unknown failure modes will likely delay their acceptance into actual structures. The proposed sensory system mimics biological neurons in its architecture and such an architecture can reduces the cost and complexity of the monitoring system. It is potentially scalable to large and complex structures and could be integrated into the structural materials. The paper summarizes recent work related to this sensory system and provides some new results.
This paper discusses the potential for using Piezoceramic and Nanotube materials to develop an artificial neural system for structural health monitoring. An artificial neural system array was modeled using piezoceramic nerves and electronic components. The neural system was simulated using one hundred dual-output sensor nodes on a four-foot square composite panel. The nodal outputs were combined into twenty neuron firing signals, one row time signal, and one column time signal. This system was able to detect and locate acoustic waves and large strains in the panel. Also discussed, is the potential for using nanotubes for building the artificial neural system. In carbon nanotubes, an electrochemical process can be used to achieve low voltage actuation at high strain, but the process velocity is slow and a structural polymer electrolyte must be used for ion exchange. Carbon and boron nitride nanotubes can be piezoelectric, and piezonanotechnolgy may be useful for building high bandwidth neural systems. The operating temperature of boron nitride is high and the amount of material needed to build artificial nerves is small, but the piezoelectric coefficients appear to be small. Nanotube molecular electronics and the change in conductance of nanotubes might also be used to develop artificial nerves.
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