For aerospace applications, the successful transition and use of integrated structural health monitoring systems will require durable sensors that can perform in their intended environment for many years. For legacy aircraft the primary means of implementing a sensor system will be through surface mounting or bonding of the sensors to the structure. Previous work has shown that the performance of surface-bonded piezo sensors can degrade due to environmental effects such as vibrations, temperature fluctuations, and substrate flexure motions. This performance degradation included sensor cracking, disbonding, and general loss of efficiency over time. In this activity, the bond and piezo material characteristics of a typical surface-bonded piezo sensor system were studied to understand and improve the long-term durability and survivability of the sensor system. Analytic and computational models were developed and used to understand stress-strain relationships for the bonded sensor system, with a special emphasis being place on coefficient of thermal expansion issues. Accelerated environmental testing was accomplished for simple bonded piezo sensor systems, where a displacement-field imaging technique was used to understand the piezo sensor performance. Future activities will focus on identifying the optimal bond conditions and piezo material type, with the ultimate goal of improving the robustness of health monitoring systems through improved sensor system design and packaging.
The external coating systems of nearly all military aircraft are stripped to bare metal during programmed depot maintenance cycles. This paint stripping process has become cost prohibitive in recent years, and is expected to continue to be a major and escalating problem for the sustainment of an aging Air Force fleet. Although a number of competing factors come into play, the key reason behind current paint stripping practices is centered on requirements for visual inspection of the aircraft structure to determine if corrosion and/or fatigue damage is present. In recent years, a number of advancements have been made in the area of nondestructive evaluation (NDE) that provide new inspection capabilities for aircraft skins without the requirement for protective coating removal. In this effort, several advanced imaging methods are evaluated for hidden damage detection and quantification through typical aircraft coating systems. A number of measurement examples are provided for engineered and realistic aircraft reference standards with variations in coating type, coating thickness, hidden damage type, and component complexity being considered. A comparison of measurement sensitivity, resolution, area coverage, ease-of-use, quantitative assessment, data processing requirements, and inspection speed are also made. It is anticipated that the use of one or more of these advanced NDE methods for thru-paint inspections will provide an enabling capability for long-life coating systems and condition based maintenance practices resulting in significant reductions in hazardous waste generation, dramatic cost savings, and enhanced readiness levels for a wide variety of Air Force systems.
The aerospace, automotive, and electronic industries are finding increasing need for components made from silicon carbide (SiC) and silicon nitride (Si3N4). The development and use of miniaturized ceramic parts, in particular, is of significant interest in a variety of critical applications. As these application areas grow, manufacturers are being asked to find new and better solutions for machining and forming ceramic materials with microscopic precision. Recent advances in laser machining technologies are making precision micromachining of ceramics a reality. Questions regarding micromachining accuracy, residual melt region effects, and laser-induced microcracking are of critical concern during the machining process. In this activity, a variety of nondestructive inspection methods have been used to investigate the microscopic features of laser-machined ceramic components. The primary goal was to assess the micromachined areas for machining accuracy and microcracking using laser ultrasound, scanning electron microscopy, and white-light interference microscopic imaging of the machined regions.
KEYWORDS: Sensors, Structural health monitoring, Digital imaging, Environmental sensing, Semiconducting wafers, Active sensors, Aluminum, Imaging systems, Sensor performance, System integration
A key question that needs to be addressed and answered with regard to successfully implementing Structural Health Monitoring technologies in Air Force systems involves the long-term operability, durability, and survivability of integrated sensor systems and their associated hardware. Whether a sensor system is fully integrated within a structural material, or surface-bonded to the structure, a number of environmental and system level influences will tend to degrade the sensor system’s performance and durability over time. In this effort, an initial sensor durability study was undertaken to better understand the performance and degradation of piezo wafer active sensor (PWAS) systems under adverse mechanical, temperature, and moisture conditions. A novel displacement-field imaging approach was utilized to understand the vibration characteristics of PWAS transducers placed in accelerated vibration, temperature-cycling, and moisture-cycling conditions. The results showed damage in the form of PWAS sensor cracking events, bond degradation and failure, as well as indications of performance variation and reduction due to the accelerated exposure levels. Future activities will focus on identifying critical durability and survivability issues through advanced sensor modeling and additional accelerated testing efforts, with the ultimate goal of improving the robustness of health monitoring systems through improved sensor system design and packaging.
Optical generation and detection of surface acoustic waves provides a non-contact, remote means of characterizing microscopic surface-breaking cracks in aerospace and industrial materials. Surface ultrasonic displacement fields generated by a non-destructive, laser induced thermoelastic mechanism and detected with an interferometric probe allow for an all optical and spatially adjustable (spot-size, beam shape, beam separation) utlrasonic NDE system. Location of surface-breaking cracks is achieved through observation of a near-field intensification of the detected ultrasonic signal in the vicinity of the crack. The near-field intensification was found to be optimized by scanning both the source and detection beams with specific spatial variables. This non-contact, laser beam scanning NDE technique, allows for imaging capabilities to further enhance surface-breaking crack characterization.
Laser ultrasonic imaging of surface acoustic waves on a material surface provides a non-contact and sensitive method for detecting and characterizing defects and anomalies in aerospace and industrial materials. It has recently been shown that the surface acoustic wave interactions with sharp material discontinuities such as surface-breaking cracks provides an additional benefit of an intensification of the displacement field in the immediate vicinity of the crack site. This near-field intensification can be exploited by scanning, point-by-point, a laser-ultrasonic system to create detailed microscopic images of the surface breaking cracks. In this effort, a number of laser ultrasonic system parameters were studied to better understand the optimized conditions to imaging surface breaking cracks using ultrasonic generation in the thermoelastic regime, and laser detection of ultrasound fields using heterodyne interferometer. In particular, laser beam sizes, separation distances, and motives were varied. In addition, several different time-gating analysis methods were studied, which had a significant effect on both the characteristics as well as the quality of the resultant crack images.
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