Mauro J. Atalla

Damage detection in composite materials using frequency response methods

Cost-effective and reliable damage detection is critical for the utilization of composite materials. This paper presents part of an experimental and analytical survey of candidate methods for the in situ detection of damage in composite materials. The experimental results are presented for the application of modal analysis techniques applied to graphite/epoxy specimens containing representative damage modes. Changes in natural frequencies and modes were found using a laser vibrometer, and 2-D finite element models were created for comparison with the experimental results. The models accurately predicted the response of the specimens at low frequencies, but coalescence of higher frequency modes makes mode-dependant damage detection difficult for structural applications. The frequency response method was found to be reliable for detecting even small amounts of damage in a simple composite structure, however the potentially important information about damage type, size, location and orientation were lost using this method since several combinations of these variables can yield identical response signatures.

Structural health monitoring in composite materials using frequency response methods

Cost effective and reliable damage detection is critical for the utilization of composite materials in structural applications. Non-destructive evaluation techniques (e.g. ultrasound, radiography, infra-red imaging) are available for use during standard repair and maintenance cycles, however by comparison to the techniques used for metals these are relatively expensive and time consuming. This paper presents part of an experimental and analytical survey of candidate methods for the detection of damage in composite materials. The experimental results are presented for the application of modal analysis techniques applied to rectangular laminated graphite/epoxy specimens containing representative damage modes, including delamination, transverse ply cracks and through-holes. Changes in natural frequencies and modes were then found using a scanning laser vibrometer, and 2-D finite element models were created for comparison with the experimental results. The models accurately predicted the response of the specimems at low frequencies, but the local excitation and coalescence of higher frequency modes make mode-dependent damage detection difficult and most likely impractical for structural applications. The frequency response method was found to be reliable for detecting even small amounts of damage in a simple composite structure, however the potentially important information about damage type, size, location and orientation were lost using this method since several combinations of these variables can yield identical response signatures.

Review of Modal Sensing and Actuation Techniques

The linear behavior of continuous structures is characterized by the combined response of infinite individual modal responses. Knowledge of the contribution of each individual mode to the overall dynamic response simplifies many applications, such as vibration control, shape estimation, and structural health monitoring. Unfortunately, most transducers simultaneously measure a combination of modes, and hence extracting the contribution of an individual mode can be difficult. The concept of modal transducers was developed in the late 1970s to overcome this limitation and allow for the simplified design of active control laws. Modal transducers seek to address individual vibration modes on a structure through the intelligent design of transducers, reducing the apparent system complexity. These modal transducers, also known as modal sensors and modal actuators, greatly simplify the problems of vibration control and shape estimation. This paper reviews the literature concerning the design and implementation of spatial modal sensors and actuators. Construction techniques initially focused on large-area sensors physically shaped to provide the desired spatial coupling. Modern techniques use arrays of transducer elements that have their signals weighted to provide the desired filtering. While temporal modal sensors exist, their use in real-time vibration control, shape estimation, and health monitoring is limited due to large time lags resulting from the temporal filtering. This critical review aims to illuminate the main characteristics, advantages, and limitations of the different techniques and provides an extensive list of references on the subject.

Investigation of filtering techniques applied to the dynamic shape estimation problem

This paper investigates the use of filtering techniques, such as the Kalman filter, to perform dynamic shape estimation of structures. Existing dynamic shape estimation techniques use static estimation techniques at each time step. This approach has been shown to be unsatisfactory, since aliasing of the higher modes, which is generally not seen in the static case, occurs strongly in the dynamic case. In many cases aliasing produces signal to noise ratios significantly greater than unity. Two approaches are proposed. The first approach improves upon existing techniques by using low-pass filters that are designed to roll-off after the natural modes that contribute significantly to the deformation of the structure, reducing effect of high-frequency noise and aliasing. The second approach uses a Kalman filter to sift out the desired low-frequency modes, since they contribute most to the displacements, while treating the higher modes as a component of the noise present in the system. Unlike static estimation techniques, the Kalman filter-based technique easily allows consideration of a number of modes larger than the number of sensors and takes into account the measurement errors. Numerical simulations were conducted to compare various dynamic estimation techniques and the results show that the Kalman filtering technique can reduce the error from 1000% down to less than 1% for an ideal cantilever beam. Experimental data, susceptible to modeling and sensing errors, show that the proposed methods result in significant improvement over existing techniques.

Broadband active structural-acoustic control of a fuselage test bed using collocated piezoelectric sensors and actuators

We describe structural-acoustic control experiments on a model fuselage test-bed, using collocated pairs of piezoelectric sensors and actuators. The test-bed is a hybrid-scaled model fuselage designed to be representative of complex aircraft structures with rib and stringer construction, which results in a structure with high modal density and complex behavior. The sensor/actuator pairs consist of PVDF film and PZT ceramic sheets bonded to the surface of the model fuselage. Closed- loop control of the fuselage skin was carried out with 30 collocated sensor/actuator pairs, covering approximately 10% of the surface area of the test-bed. The disturbance source is a PZT patch bonded to an adjacent panel. Rate feedback was applied to each collocated pair simultaneously (independent loop closure). Accelerometers attached to the panels and microphones located inside the test-bed were used as performance sensors. The experimental results show a reduction of as much as 20 dB in structural acceleration and up to 10 dB of attenuation in the interior acoustic pressure levels at resonant peaks, over the frequency range of 100 - 2000 Hz.

Active structural acoustic control of a thick-walled cylindrical shell

We consider the problem of reducing the noise radiation from a thick-walled cylindrical shell by actively controlling the motion of the shells outer surface. Because the shell is very stiff, it is difficult to directly control the shell deflections. Instead, the proposed approach is to cover the shells outer surface with curved active composite panels. Each panel contains several embedded accelerometers mounted to its outer and inner surfaces, which can sense both the motion of the panel base (i.e., the outer motion of the shell) and the outer surface of the panel (i.e., the radiating surface). The accelerometers are used in both feedback and feedforward architectures, in which the accelerometer signals are used to command the panel displacement, in order to reduce the motion of the panel outer surface, reducing the radiated noise. Experimental results show that, in the best case, 10 - 30 dB of surface vibration reduction can be achieved in the frequency range of interest, which is 250 - 2000 Hz.

Mixed-domain traveling-wave motor model with lossy (complex) material properties

A piezoelectric traveling-wave motor model has been developed with parameters entirely related to physical properties. The approach is well-rooted in the formulation suggested earlier by Hagood and McFarland, but several model improvements have been integrated in an effort to realize an accurate model suited for automated design optimization. Additional model considerations include a flexible rotor model and a hysteretic stick-slip friction contact model which replace the previous assumptions of a rigid rotor and pure slip. The most notable contribution has been the use of lossy (complex) material properties to account for inherent material losses, supplanting the use of non-physical damping coefficients. The model is partly formulated in the frequency domain, and by representing the modal states and forces as Fourier series expansions and retaining higher harmonic terms, it has been generalized to account for non-ideal traveling-wave excitation. Needing to simulate the hysteretic contact model in the time domain, a mixed-domain solution procedure has been implemented to maintain some of the computational efficiency of frequency domain analysis. A preliminary validation study has demonstrated excellent correlation between simulation results and experimental data for a commercial motor.

Design of Reduced-Order Controllers on a Representative Aircraft Fuselage

The traditional approach to minimizing structural vibration implements either passive dampers or global model-based active controllers. Unfortunately, passive control becomes massive for control of low frequency disturbances. Model-based active controllers are difficult to apply to complex structures, because the model needs to be of roughly the same order as the system that it describes in order to achieve robust performance. Modeling errors due to mismodeled dynamics, missed dynamics, or time-varying dynamics can be performance degrading and potentially destabilizing [6]. This paper focuses on the lightly damped and modally dense systems where modeling errors are more significant.

Reconfigurable arrays of collocated sensors and actuators for modal isolation

Arrays of sensors and actuators designed to provide robust broadband feedback control with high performance and limited modeling are the subject of this paper. The reconfigurable array technique proposed here enables the design of reduced- order controllers for complex structures and offers the potential to improve closed-loop robustness and to broaden the region of good performance even as the plant changes. The weighted summation of sensor signals senses the modes that are relevant to performance while rejecting the remaining modes; therefore reducing the required complexity of the controller. These weights are obtained from the minimization of a cost function and under certain assumptions; it can be shown that a single optimum solution exists. The use of reconfigurable arrays is motivated by the need to control the vibration of complex structures. A thirty element collocated actuator and sensor array was bonded to a cylinder section. Array weights were computed and successfully applied to isolate target modes. Different methods of computing the weights are implemented and compared. The deleterious effects of spatial aliasing and the performance as a function of the array size are experimentally explored.

New wave-number domain sensing method for active structural acoustic control

A new wavenumber domain sensing method has been developed and applied to feedback controller design for active structural acoustic control. The approach is to minimize the total acoustic power radiated form vibrating structures in the wavenumber domain. If the disturbance spectrum is given, the target wavenumbers in the supersonic domain (i.e., the radiating wavenumbers) can be determined. Then, a state-space model can be found to estimate the magnitude of the supersonic wavenumber components. Once we have a state-space model that can be used for active structural acoustic control, a modern control design paradigm can be applied to minimize the acoustic power radiated from vibrating structures. The new sensing method was numerically validated on a thick-walled cylindrical shell with 55 active composite panels mounted. It is found that the method enables us to systematically find a state-space model for wavenumber components in the supersonic region, and therefore makes it easy to design MIMO LQG controller.