A piezoelectric fiber patch (PFP) is a transducer type that is suitable for guided-wave-based structural health monitoring (SHM) due to its light, thin, and flexible characteristics. In our previous ...work, a PFP-based transducer design for selective excitation of the zero-order shear horizontal wave mode (SH0) was introduced (shear horizontal PFP (SHPFP)). In this work, two modified SH0 wave PFP transducer designs are proposed: the rounded corner design and the dual design. The degree of improvement is determined by a numerical simulation and the dual design-the design with the most promise-is experimentally realized. Laser Vibrometry measured the generated wave field, confirming the results from the simulation. The new designs can generate an almost pure SH0 wave. The dual design has a very strong directivity that is useful for several guided-wave-based SHM applications. The conclusions on the design's properties as a transmitter are also valid for its properties as a sensor due to the reciprocity of piezoelectric transducers.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Interest in carbon-fiber-reinforced polymer (CFRP) composites is increasing owing to their excellent mechanical properties, low weight, and good corrosion and fatigue resistance. Generally, CFRPs are ...not reformable after completing the forming process because they are hard materials. Currently, no simple technique is available to reform CFRPs. In this study, a novel stitching technique was developed for forming a thermoplastic CFRP (CFRTP) composite. Carbon laminates were stitched with electrically conductive threads and heated by applying voltage through the threads. Study results showed that temperature control and matrix cracking during folding are key factors in this process. This new method provides a useful approach for manufacturing CFRTPs without using complex molds or heavy equipments.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
In this study, a fatigue crack detection technique, which detects a fatigue crack without relying on any reference data obtained from the intact condition of a target structure, is developed using ...nonlinear ultrasonic modulation and applied to a real bridge structure. Using two wafer-type lead zirconate titanate (PZT) transducers, ultrasonic excitations at two distinctive frequencies are applied to a target inspection spot and the corresponding ultrasonic response is measured by another PZT transducer. Then, the nonlinear modulation components produced by a breathing-crack are extracted from the measured ultrasonic response, and a statistical classifier, which can determine if the nonlinear modulation components are statistically significant in comparison with the background noise level, is proposed. The effectiveness of the proposed fatigue crack detection technique is experimentally validated using the data obtained from aluminum plates and aircraft fitting-lug specimens under varying temperature and loading conditions, and through a field testing of Yeongjong Grand Bridge in South Korea. The uniqueness of this study lies in that (1) detection of a micro fatigue crack with less than 1 m width and fatigue cracks in the range of 10-20 m in width using nonlinear ultrasonic modulation, (2) automated detection of fatigue crack formation without using reference data obtained from an intact condition, (3) reliable and robust diagnosis under varying temperature and loading conditions, (4) application of a local fatigue crack detection technique to online monitoring of a real bridge.
Increasing the safety demands of various progressively complex structures (e.g., in aerospace, automotive engineering, and wind power technology) and the parallel goal of reducing inspection costs ...set new objectives in the field of ultrasonics and guided elastic wave measurements. The focus is to deliver applicable sensors attached permanently to a particular structure for on-demand inspections. Piezoelectric fiber patches (PFPs) are known as lightweight and structure-conforming transducers for guided elastic wave generation and detection. We propose a new PFP concept combining advantages of different previous designs. With the new approach, a user can switch between a Lamb wave and a shear horizontal wave excitation by partially changing the polarity of the transducer electrodes. After summarizing the existing PFP variants, the new concept is introduced and experimentally verified by a three-dimensional laser Doppler vibrometer visualization of excited waves. Moreover, the first prototype of the new PFP transducer is presented.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Residual stress, a factor affecting the fatigue and fracture characteristics of rails, is formed during the processes of fabrication and heat treatment, and is also generated by shrinkage/tension due ...to the temperature differences and vertical loads on wheels due to the weight of vehicles. Moreover, damage to rails tends to accelerate due to residual stress caused by the continuous increase in the number of passes and the high speed of passing vehicles. Because this can have a direct effect on safety accidents on railroad rails, having a technique to evaluate and analyse the residual stresses in rails accurately is very important. In this paper, firstly, residual stresses applied in rails were evaluated using longitudinal critically refracted (LCR) wave. Non-destructive stress measurement using ultrasonic techniques is based on calculation of the acoustoelastic coefficient obtained from the relationship between material stress and LCR wave velocity. This is because LCR waves exhibit a relatively large change in flight time in relation to a change in stress compared to other ultrasonic modes. Barkhausen noise technique is also very suitable for measuring residual stresses on railway rails, which are ferromagnetic materials. Tensile and compression tests were performed with the specimens extracted from the inspection object, and the difference between the ultrasonic speed and the magnetization amplitude according to the applied stress was measured. Then, the feasibility of calculating the internal stresses in railway rails using the reference data obtained from the extracted specimens was determined, and the reliability of each method was verified. Furthermore, based on these results, the difference in the results for the loads asymmetrically applied according to the wheel shape was analysed by measuring for each part of new and used rails.
•The data-driven crack quantification and prognosis using an ANN is proposed.•Two novel features extracted from the nonlinear ultrasonic modulation are used.•The ANN is optimized using the ...experimentally obtained training data.•The error of crack quantification and prognosis are less than 2 mm and 3 k cycles.
In this study, an online monitoring technique for continuous fatigue crack quantification and remaining fatigue life estimation is developed for plate-like structures using nonlinear ultrasonic modulation and an artificial neural network (ANN). First, multiple aluminum plates of different thicknesses are subjected to cyclic loading tests at a constant amplitude, and the ultrasonic responses are obtained from the piezoelectric transducers attached to each specimen. Second, an ANN is constructed by defining (1) the specimen thickness; the elapsed fatigue cycles; and two features extracted from the ultrasonic responses, namely, the cumulative increase and decrease in the nonlinear beta parameter, as inputs and (2) the crack length and remaining fatigue life as outputs. Then, the architecture and learning parameter of the ANN are optimized using the data obtained from the specimen tests. Finally, the performance of the trained ANN is examined using the blind test data obtained from additional specimens. The results of the blind tests indicate that the proposed technique can estimate the crack length and remaining fatigue life with a maximum error of 2 mm and 3 k cycles, respectively, for the tested aluminum plates. The uniqueness of this technique lies in (1) the fatigue crack quantification and remaining fatigue life estimation using nonlinear ultrasonic modulation and (2) the data-driven crack quantification and prognosis using an ANN for online monitoring.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
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•An online fatigue crack failure warning system is presented.•Nonlinear ultrasonic modulation is used for the real-time failure warning system.•Aluminum specimens with different ...materials and different thicknesses were examined.•Good agreement between the theoretical and the experimental critical crack length.
This study presents an online fatigue crack failure warning system that can provide a warning of imminent failure of an aluminum plate structure with an initial notch when the plate is subject to constant-amplitude cyclic loading. For real-time warning, three piezoelectric transducers (PZT) are mounted on the surface of the structure for ultrasonic generation and sensing. Two sinusoidal input signals at distinct frequencies are applied respectively to two PZTs, and corresponding responses are measured by the third PZT. Ultrasonic waves cause crack closing and opening at the presence of a fatigue crack, and nonlinear ultrasonic modulation components appear at sum and difference of the input frequencies. The amplitudes of nonlinear ultrasonic modulation components initially increase as a fatigue crack grows. However, when the maximum stress intensity factor at the crack tip reaches the fracture toughness value of material, the modulation amplitudes suddenly decrease. The proposed failure warning system provides a failure warning by studying the patterns of measured nonlinear modulation components with respect to the fatigue crack growth. The performance of the proposed failure warning system is examined using aluminum plate specimens with various thicknesses and material properties.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Fiber–metal laminates (FMLs) are hybrid materials that provide the advantages of both metal and fiber-reinforced polymer composite (FRP) such as improved damage tolerance and impact strength, ...particularly in aerospace and automotive applications. However, the production of FMLs remains expensive because the manufacturing process is performed in an autoclave. In addition, delamination failure due to low interfacial adhesive strength between the metal and fiber layer is a problem that must be solved to improve the mechanical properties of FMLs. In this study, simple stitch-assisted carbon fiber-aluminum laminates were developed using vacuum-assisted resin transfer molding (VARTM). The FMLs produced through vacuum-assisted resin transfer molding were stitched with aramid fibers through holes on Al sheets for the resin flow associated with the VARTM. The mechanical properties of the FMLs were investigated by impact and compression after impact tests as well as double cantilever beam and flexural tests. The stitching effectively improved the interfacial strength and toughness owing to the anchoring between the aluminum and carbon fiber layers, thus preventing delamination crack propagation. As a result, the stitched FML with significantly improved interfacial adhesive and toughness was developed by the VARTM process, which will accelerate the application of FML to various lightweight structures.
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NUK, OILJ, SAZU, UKNU, UL, UM, UPUK