The use of metal strain gauges and ultrasonic transducers have long been studied in the field of Nondestructive Evaluation (NDE) as a part of structural health monitoring (SHM). Strain gauges use electrical resistance to monitor strains during the loading of a component. Ultrasonic transducers are piezo devices that use a crystal-like sensing element with very low excitation energy that can monitor small strains such as acoustic emissions (AEs). These types of devices have been used to locate the sources of AEs from artificial sources, such as Hsu-Nielsen pencil lead break (PLB) tests, or natural sources such as quasi-static fracture or crack propagation. This type of evaluation has significant advantages over other types of damage inspections such as liquid die penetrant, Blue Light, Eddy Current, or X-ray inspections where visual inspections, large defects, and high levels of user experience are required. The ultrathin silicon membrane (USM) sensor developed by NanoSonic Inc. is a piezoresistive sensor, incorporating the best aspects of a conventional strain gauge and ultrasonic transducer. The sensor can measure both the strain of a component, as well as any acoustic emission that is emitted on the component. To the author's knowledge this is the only sensor capable of simultaneous measurement of these two data types. This paper presents the sensor's ability to be used for quasi-static fracture monitoring. The sensor is first compared to commercial ultrasonic transducers in an unloaded pencil lead break (PLB) test for determining the ability in measuring lamb waves for source location estimation. The NanoSonic USM sensor is further compared to commercial strain gauges and ultrasonic transducers during a PLB test under a tensile load where it is demonstrated the USM sensor yields similar measurements to both commercial sensors. The final test was a quasi-static fracture test, where the NanoSonic USM sensor was able to detect substantially lower energy AEs than the previous test and record the strain history during fracture. This duality of the USM sensor demonstrates an inherent usefulness to NDE and SHM fields. The sensor offers sensing capabilities comparable to commercially available sensors in a smaller package, with less power consumption, at a lower cost. / Master of Science / Nondestructive evaluation (NDE) is a field within structural health monitoring (SHM) that refers to determining any defects within a component that would hinder its performance without modifying its existing condition. This has historically been done by visual inspection by which experienced personnel examine the part for defects. This is inherently flawed as cracking below the surface of a component is common and would not be detected and extensive experience is required to successfully complete this task. Components in hard-to-reach places, with coatings, or that are small also prove challenges to visual inspection. Engineers have developed several new testing methods to combat these flaws. The use of acoustic emission (AE) testing allows sensors, called ultrasonic transducers, to receive and emit sounds at high frequencies to conduct the inspection. This can be done by emitting a sound which is then propagated as a wave along the surface of the component, if the wave hits a defect, it is scattered. A receiving sensor would then receive an unexpected signal, indicating that there is a problem. Furthermore, an array of these sensors can be employed to 'listen' for these surfaces waves that may be emitted during the standard operation of the components. Things like high loads, cracking, and impacts will all be able to be detected. The use of an array of sensors will allow the location of these events. This paper will discuss a new type of sensor, an ultrathin silicon membrane (USM) sensor developed by NanoSonic Inc. This type of sensor can detect high frequencies similar to an ultrasonic transducer, as well as measure large loads that would deform the part, resulting in an event known as strain. The novelty of the NanoSonic USM sensor is its ability to monitor both pieces of information simultaneously, which is believed to be the first to do so in the field. The ability to obtain information on strain and locations of acoustic events within a component during standard operation would be a valuable prospect for the aerospace, civil, and automotive industries.

The relationship between crack path and test method was examined by comparing the performance of adhesive-adherend combinations (six) in quasi-static fracture, mixed-mode fatigue, and creep crack growth. Crack paths in creep and quasi-static fracture were similar due to similar crack-tip plastic zone sizes in the epoxy adhesive even though the crack growth rates in creep were much smaller. Under condensed moisture and mixed-mode, creep and threshold fatigue tests produced interfacial failure. Under room-temperature dry environment, near threshold mixed-mode fatigue was interfacial, but was not in creep or quasi-static fracture. Smaller plastic zone size and crack path proximity to the interface that followed increased the sensitivity of near threshold, mixed-mode fatigue to surface properties. Therefore, the interfacial or cohesive failure of an adhesive system, which may judge the quality of the bond, can be a function of the test being conducted and may not be an absolute indicator of joint quality.

The relationship between crack path and test method was examined by comparing the performance of adhesive-adherend combinations (six) in quasi-static fracture, mixed-mode fatigue, and creep crack growth. Crack paths in creep and quasi-static fracture were similar due to similar crack-tip plastic zone sizes in the epoxy adhesive even though the crack growth rates in creep were much smaller. Under condensed moisture and mixed-mode, creep and threshold fatigue tests produced interfacial failure. Under room-temperature dry environment, near threshold mixed-mode fatigue was interfacial, but was not in creep or quasi-static fracture. Smaller plastic zone size and crack path proximity to the interface that followed increased the sensitivity of near threshold, mixed-mode fatigue to surface properties. Therefore, the interfacial or cohesive failure of an adhesive system, which may judge the quality of the bond, can be a function of the test being conducted and may not be an absolute indicator of joint quality.