We propose and demonstrate a fiber-tip sensor based on an ultra-thin silver diaphragm for highly sensitive and high frequency ultrasonic detection. The diaphragm is prepared by the vacuum thermal deposition method and then transferred to the fiber tip. The sensor demonstrated in this letter has a 300 nm thick diaphragm with an inner diameter of 75 μm, leading to a static pressure sensitivity of 1.6 nm/kPa and a resonant frequency of 1.44 MHz. This sensor has potential applications in many fields such as structural health monitoring and medical ultrasonography.

Powder-based additive manufacturing (AM) technologies have been evaluated for use in different fields of application (aerospace, medical, etc.). Ideally, AM parts should be at least equivalent, or preferably better quality than conventionally produced parts. Manufacturing defects and their effects on the quality and performance of AM parts are a currently a major concern. It is essential to understand the defect types, their generation mechanisms, and the detection methodologies for mechanical properties evaluation and quality control. We consider the various types of microstructural features or defects, their generation mechanisms, their effect on bulk properties and the capability of existing characterisation methodologies for powder based AM parts in this work. Methods of in-situ non-destructive evaluation and the influence of defects on mechanical properties and design considerations are also reviewed. Together, these provide a framework to understand the relevant machine and material parameters, optimise the process and production, and select appropriate characterisation methods.Abstract: Powder-based additive manufacturing (AM) technologies have been evaluated for use in different fields of application (aerospace, medical, etc.). Ideally, AM parts should be at least equivalent, or preferably better quality than conventionally produced parts. Manufacturing defects and their effects on the quality and performance of AM parts are a currently a major concern. It is essential to understand the defect types, their generation mechanisms, and the detection methodologies for mechanical properties evaluation and quality control. We consider the various types of microstructural features or defects, their generation mechanisms, their effect on bulk properties and the capability of existing characterisation methodologies for powder based AM parts in this work. Methods of in-situ non-destructive evaluation and the influence of defects on mechanical properties and design considerations are also reviewed. Together, these provide a framework to understand the relevant machine and material parameters, optimise the process and production, and select appropriate characterisation methods.Reference to this paper should be made as follows: Taheri, H., Shoaib, M.R.M., Koester, L., Bigelow, T.A., Collins, P.C. and Bond, L.J. (2017) 'Powder-based additive manufacturing -a review of types of defects, generation mechanisms, detection, property evaluation and metrology', Int. Institute of Aviation Technology conducting research on UAVs and designing and printing 3D parts for UAVs. His research interest areas include engineering mechanics, additive manufacturing and non-destructive testing.Lucas W. Koester received his PhD from the University of Nebraska at Lincoln studying wave propagation and scattering in polycrystalline media. His current and past research interests include wave propagation in polycrystalline media, defect scattering, and modelling with emphasis on additive manufacturing processes and materials.Powder-based additive...

Diffuse ultrasonic backscatter describes the scattering of elastic waves from interfaces within heterogeneous materials. Previously, theoretical models have been developed for the diffuse backscatter of longitudinal-to-longitudinal (L-L) wave scattering within polycrystalline materials. Following a similar formalism, a mode-conversion scattering model is presented here to quantify the component of an incident longitudinal wave that scatters and is converted to a transverse (shear) wave within a polycrystalline sample. The model is then used to fit experimental measurements associated with a pitch-catch transducer configuration performed using a sample of 1040 steel. From these measurements, an average material correlation length is determined. This value is found to be in agreement with results from L-L scattering measurements and is on the order of the grain size as determined from optical micrographs. Mode-converted ultrasonic backscatter is influenced much less by the front-wall reflection than an L-L measurement and it provides additional microstructural information that is not accessible in any other manner.

Friction stir welding is a method of materials processing that enables the joining of similar and dissimilar materials. The process, as originally designed by The Welding Institute (TWI), provides a unique approach to manufacturing—where materials can be joined in many designs and still retain mechanical properties that are similar to, or greater than, other forms of welding. This process is not free of defects that can alter, limit, and occasionally render the resulting weld unusable. Most common amongst these defects are kissing bonds, wormholes and cracks that are often hidden from visual inspection. To identify these defects, various nondestructive testing methods are being used. This paper presents background to the process of friction stir welding and identifies major process parameters that affect the weld properties, the origin, and types of defects that can occur, and potential nondestructive methods for ex-situ detection and in-situ identification of these potential defects, which can then allow for corrective action to be taken.

Steel cleanliness is of the utmost importance in the production of tapered roller bearings used in the railroad industry. Impurities in the steel can make it vulnerable to fatigue initiation because they act as stress concentration sites in the fabricated parts, especially when these impurities are located in regions of susceptibility for rolling contact fatigue (RCF). Impurities present near the rolling surfaces (e.g., raceways in bearings) are referred to as subsurface inclusions. These subsurface inclusions make the steel susceptible to the initiation of fatigue cracks that can propagate towards the surface leaving a cavity called a "spall". Spalls occurring on the rolling surfaces of bearings can have detrimental effects that may lead to overheated bearings, loss of full service life, and in extreme cases, can lead to derailments if not addressed in-service by early detection methods. The study presented in this paper investigates the effects of subsurface inclusions present beneath the surface of the bearing cup (outer ring) and cone (inner ring) raceways. New bearing components were scanned using a unique ultrasonic technique in order to detect and identify potentially detrimental subsurface inclusions present in the RCF regions of the rolling surfaces. Two service life tests of these components were then carried out: one to examine subsurface inclusions found on cone raceways, and one to explore subsurface inclusions present on cup raceways. The test results indicate that the service life of components containing subsurface inclusions is reduced compared to controls for which no subsurface inclusions were detected. Moreover, subsurface inclusions on bearing cups appear to accelerate spall development relative to those present in bearing cones. This paper summarizes the findings of the experimental study performed on ultrasonically scanned bearing components, and emphasizes the need to establish more refined methods to inspect railroad rolling stock. These results are anticipated to be of great value to fatigue life prediction models relevant to the railroad industry.