Bruno Rocha

Embedded fiber optic sensors for monitoring processing, quality and structural health of resin transfer molded components

Due to their small size and flexibility fiber optics can be embedded into composite materials with little negative effect on strength and reliability of the host material. Fiber optic sensors such as Fiber Bragg Gratings (FBG) or Etched Fiber Sensors (EFS) can be used to detect a number of relevant parameters such as flow, degree of cure, quality and structural health throughout the life of a composite component. With a detection algorithm these embedded sensors can be used to detect damage in real time while the component remains in service. This paper presents the research being conducted on the use of fiber optic sensors for process and Structural Health Monitoring (SHM) of Resin Transfer Molded (RTM) composite structures. Fiber optic sensors are used at all life stages of an RTM composite panel. A laboratory scale RTM apparatus was developed with the capability of visually monitoring the resin filling process. A technique for embedding fiber optic sensors with this apparatus has also been developed. Both FBGs and EFSs have been embedded in composite panels using the apparatus. EFSs to monitor the fabrication process, specifically resin flow have been embedded and shown to be capable of detecting the presence of resin at various locations as it is injected into the mold. Simultaneously these sensors were multiplexed on the same fiber with FBGs, which have the ability to measure strain. Since multiple sensors can be multiplexed on a single fiber the number of ingress/egress locations required per sensor can be significantly reduced. To characterize the FBGs for strain detection tensile test specimens with embedded FBG sensors have been produced. These specimens have been instrumented with a resistive strain gauge for benchmarking. Both specimens and embedded sensors were characterized through tensile testing. Furthermore FBGs have been embedded into composite panels in a manner that is conducive to detection of Lamb waves generated with a centrally located PZT. To sense Lamb waves a high speed, high precision sensing technique is required to acquire data from embedded FBGs due to the high velocities and small strain amplitudes of these guided waves. A technique based on a filter consisting of a tunable FBG was developed. Since this filter is not dependant on moving parts, tests executed with this filter concluded with the detection of Lamb waves, removing the influence of temperature and operational strains. A damage detection algorithm was developed to detect and localize cracks and delaminations.

Structural Health Monitoring System Using Piezoelectric Networks with Tuned Lamb Waves

The paper presents a structural health monitoring system based on propagation of tuned Lamb waves and their interference with discontinuities. The dispersion curves are studied to determine the appropriate type and dimension of transducers and to select the optimum scanning frequencies and relevant propagation modes. A piezoelectric sensor network was implemented in an aluminum plate in order to generate and to sense the wave propagation and associated reflections. The algorithm developed for damage detection relies on the comparison of undamaged and damaged responses of the structure. Combinations of filters and statistical methods were applied to detect differences in the sensor signals acquired for the two different states (damaged and undamaged), corresponding to damage reflections. In order to eliminate the false positives due to noise, a probability analysis is performed to obtain the final damage position. The software designed for the current application allows the automatic calculation of dispersion curves, it executes the scans, performs data processing, executes the detection algorithm and presents the probable damages and their positions in a graphical form. Experiments were performed with the introduction of cumulative damages in the plate such as surface and through-the-thickness holes and cuts, ranging from 7 mm to 1 mm in diameter. Additionally, a stringer was attached to the plate by a single rivet line to simulate an aircraft skin structure. Cuts originating from rivet holes and connecting adjacent rivets, as well as loosened rivets were detected by the system. The introduction of the stringer resulted in a loss of precision in the determination of the radial position of the damages near it. Also, the network revealed significant difficulties in the detection of damages beyond the stringer.

PZT Network and Phased Array Lamb Wave Based SHM Systems

With the application of newer materials, such as composite materials, and growing complexity and capacity of current aircraft structures, reliably and completely assess the condition of the total structures in real time is then of growing and utmost importance. PZT Network and Phased Array, Lamb wave based Structural Health Monitoring (SHM) systems were developed to be applied to thin panels. The selection of transducers, their size and selected locations for their installation are described. The development and selection of the signal generation and data acquisition systems is also presented in detail. The requirements conducing to the development and selection of these systems are laid and particularly the selection of the actuation signal applied is justified. The development of a damage detection algorithm based in the comparison of the current structural state to a reference state is described, to detect damage reflected Lamb waves. Such method was implemented in software and integrated in the SHM system developed. Subsequently the detection algorithm, based in discrete signals correlation, was further improved by incorporating statistical methods. For phased arrays, a novel damage location algorithm is presented based on the individual sensors response. A visualization method based concurrently in the statistical methods developed and superposition of the different results obtained from a test set was implemented. These tests conducted to the successful and repeatable detection of 1mm damages in a multiple damaged plate with great confidence. Finally, a brief comparison and a hybrid system implementation is presented.

A Structural Health Monitoring Approach based on a PZT Network Using a Tuned Wave Propagation Method

Presently, aircraft structures are maintained and revised on a scheduled basis. Each aircraft manufacturer establishes, for each model, a strict inspection and revision calendar that must be followed firmly that leads to high operation costs. Parts can be replaced just because they exceeded their predicted lifetime, while others are inspected without revealing any kind of damage. Related disassembling and assembling processes are also time-consuming, and sometimes redundant. Structural Health Monitoring is an emerging study field that seeks to address these issues. The advantages of having SHM are the possible reduction of safety factors during the components design process, which will lead to higher payloads and lower fuel consumptions, thus turning aviation more environmentally friendly. There are several approaches being study, all with the same objective in mind: to develop a system capable of monitoring the structural integrity in real time by detecting damage presence, shape, size, growth study, etc. It is possible to achieve accurate component available life-time in real-time by using well established predictive algorithms for damage growth. Another requirement is that such systems must be reliable and avoid false warnings. The method presented here is based on Lamb waves, which are particularly susceptible to interferences caused by damages or boundaries. Several actuation wave functions were investigated. The algorithm for damage detection relies on the assumption that if one is able to compare the response for the undamaged structure with the one presenting damage, when subtracting one from the other, a difference will emerge at a specific time. Such propagation is strongly influenced to interferences caused by damages or boundaries. The actuation system and wave generation requires a careful and detailed approach. The algorithm for damage detection relies on the assumption that if one is able to compare both undamaged and damaged response of the structure, by subtracting one from the other, a difference will emerge at a specific time. The damage is then assumed to work as a source of reflections. After detecting a damage reflection, since the propagation velocity is known for the host medium, a distance for each actuator can be determined and three probability curves can be defined. The point at which the three curves intersect, pinpoints the damage location. To improve the accuracy of the detection method, test runs are repeatedly performed, filters are applied to sensor data and a statistical scheme is put in place. Test runs were successfully performed with the plate undamaged and with different types of damage (surface holes and cracks, through holes and cracks) of different sizes, ranging from 5 mm to as small as 1mm.

Design of an Embedded Sensor Network for Manufacturing Process Monitoring, Quality Control Management and Structural Health Assessment of Advanced Composite Structures

This paper proposes the use of an embedded network of fiber optic sensors for process and Structural Health Monitoring (SHM) of Resin Transfer Molded (RTM) composite structures. A single sensor network is used at each stage of life of a RTM composite panel: flow monitoring, cure monitoring and health monitoring. A laboratory scale RTM apparatus was designed and built with the capability of visually monitoring the resin filling process. A technique for embedding fiber optic sensors into the mold has also been developed. Both Fiber Bragg Gratings (FBG) and Etched Fiber Sensors (EFS) have been embedded in composite panels using the apparatus. Etched Fiber Sensors have the capability of detecting the presence of resin. The sensors have proven to be capable of detecting the presence of resin at various locations as it is injected into the mold and have the capability of being multiplexed with FBGs thus reducing the number of ingress/egress locations required per sensor. Two FBGs and three EFSs were embedded on a single optical fiber. Tensile test specimens that contain embedded FBG sensors have also been produced with this apparatus. These specimen and embedded sensors have been characterized using a strain gage and a material testing machine. FBG sensors have been embedded into composite panels also in a manner that is conducive to detecting Lamb waves generated with a centrally located PZT. To detect Lamb waves a high speed, high precision sensing technique is required for embedded FBGs, since these guided waves travel through the material at very high velocities, presenting relatively small strain amplitudes. A technique based in a filter consisting of a second FBG was developed. Since this filter is not dependant on moving parts, it does not limit the velocity or frequency at which the tests can be performed. Preliminary tests performed using this filter showed that it is possible to detect Lamb waves with amplitudes smaller than 1 microstrain. A damage detection algorithm has been developed and is applied to this system in an attempt to detect and localize damages (cracks and delaminations) in the composite structure.Copyright

Design of a PZT Sensor Network Based on Guided Lamb Waves for Structural Health Monitoring of Metallic Structures

A Structural Health Monitoring (SHM) system of metallic structures based on guided Lamb waves is presented. Lamb waves are reflected on discontinuities in material properties and geometries such as damage. Lamb waves present advantages when applied on thin structures due to their low amplitude damping which enables them to travel longer distances. The selection of transducers, their size and selected locations in the structure are described. Additionally, the design, development and implementation of a new signal generation and data acquisition systems is presented in detail. The requirements leading to the development and selection of these systems are explained and particularly the selection of the actuation signal is discussed. A damage detection algorithm based on the comparison between the damaged structural state and a healthy reference state is used to detect damage based on reflected Lamb waves. Subsequently, the detection algorithm based on discrete signals correlation was further improved by incorporating statistical methods. Tests performed on a plate with multiple surface cuts, through the thickness cuts, loosened rivets and cuts originating from rivets resulted in repeatable detections of 1 mm damages with a probability of detection greater that 95%. New tests are currently being performed on composite panels with embedded Fiber Bragg Grating (FBG) optical sensor network to detect the fast propagating Lamb waves.© 2010 ASME

A Metamodelling Optimization Approach to a Wing Spar Design

Optimization is present in almost every engineering activity. There are several different approaches depending on the type of problem and its related complexity. A search for the optimum solution might reveal itself hard. When the output cannot be expressed by a function of the inputs, analytical optimization methods are not suitable. Another problem arises when there is the possibility of local minimums existence. The search for a global minimum can be pursued using global optimization approaches. On this paper a metamodeling global optimization approach is explained and applied to a real engineering designing problem. The algorithm is programmed in MATLAB and calls for an FEM software to run the models. The solution is compared to one attained from a FEM commercial software package. Final results show that the metamodeling method outputs a better solution and increases the guarantee of having achieved a global minimum. The algorithm here developed has direct application to problems that deal with FEM analyses with all design variables perfectly bounded. The optimization procedure is fully autonomous.