Local strain measurements are considered as an effective method for structural health monitoring of high-temperature components, which require accurate, reliable and durable sensors. To develop strain sensors that can be used in higher temperature environments, an improved metal-packaged strain sensor based on a regenerated fiber Bragg grating (RFBG) fabricated in hydrogen (H₂)-loaded boron–germanium (B–Ge) co-doped photosensitive fiber is developed using the process of combining magnetron sputtering and electroplating, addressing the limitation of mechanical strength degradation of silica optical fibers after annealing at a high temperature for regeneration. The regeneration characteristics of the RFBGs and the strain characteristics of the sensor are evaluated. Numerical simulation of the sensor is conducted using a three-dimensional finite element model. Anomalous decay behavior of two regeneration regimes is observed for the FBGs written in H₂-loaded B–Ge co-doped fiber. The strain sensor exhibits good linearity, stability and repeatability when exposed to constant high temperatures of up to 540 °C. A satisfactory agreement is obtained between the experimental and numerical results in strain sensitivity. The results demonstrate that the improved metal-packaged strain sensors based on RFBGs in H₂-loaded B–Ge co-doped fiber provide great potential for high-temperature applications by addressing the issues of mechanical integrity and packaging.

Strain measurements are the fundamental task for structural health monitoring of critical equipment for cryogenic applications. However, the failure of strain sensors has been inevitable due to cyclic loading and/or over-loading. Therefore, a durable metal-packaged FBG sensor for strain measurements at cryogenic temperatures is successfully developed by combining magnetron sputtering and electroplating processes. Tensile and fatigue loading effects on the performance of the sensors are evaluated for the first time by uniaxial tensile testing at constant cryogenic temperatures down to 80 K and tension-tension fatigue testing at 133 K to verify their durability and reliability. The sensors which have high-quality multilayer metallic layers and good interfacial bonding exhibit markedly higher sensitivity than that of the bare FBG, with good linearity, stability, and repeatability and without obvious hysteresis. Fatigue failure of the sensors may occur due to fatigue fracture of the optical fibers with the gratings at a certain high strain level and due to zero shift caused by interface debonding or cracking at a relatively lower strain level. As the strain level is lowered, the sensor fatigue life is increased. There is at least one substantially lower strain level at which the sensors with good linearity, repeatability, and stability do not fail at the maximum number of test cycles. These results indicate that the metal-packaged FBG sensors are promising candidates for long-term measurement of strains, and thus monitoring structural health of critical equipment at cryogenic temperatures.

By integrating fiber-reinforced composites (FRCs) with Three-dimensional (3D) printing, the flexibility of lightweight structures was promoted while eliminating the mold’s limitations. The design of the I-beam configuration was performed according to the equal-strength philosophy. Then, a multi-objective optimization analysis was conducted based on the NSGA-II algorithm. 3D printing was utilized to fabricate I-beams in three kinds of configurations and seven distinct materials. The flexural properties of the primitive (P-type), the designed (D-type), and the optimized (O-type) configurations were verified via three-point bending testing at a speed of 2 mm/min. Further, by combining different reinforcements, including continuous carbon fibers (CCFs), short carbon fibers (SCFs), and short glass fibers (SGFs) and distinct matrices, including polyamides (PAs), and polylactides (PLAs), the 3D-printed I-beams were studied experimentally. The results indicate that designed and optimized I-beams exhibit a 14.46% and 30.05% increase in the stiffness-to-mass ratio and a 7.83% and 40.59% increment in the load-to-mass ratio, respectively. The CCFs and SCFs result in an outstanding accretion in the flexural properties of 3D-printed I-beams, while the accretion is 2926% and 1070% in the stiffness-to-mass ratio and 656.7% and 344.4% in the load-to-mass ratio, respectively. For the matrix, PAs are a superior choice compared to PLAs for enhancing the positive impact of reinforcements.

Temperature monitoring is an essential task in cryogenic engineering to ensure the safety of equipment, where highly sensitive and reliable sensors are required to provide real-time and accurate information about temperature. Titanium (Ti)-copper (Cu)-coated fiber Bragg grating (FBG) temperature sensors with a gradient of material properties to minimize thermal stresses are hence fabricated by a combination of Ti/Cu magnetron sputtering and Cu electroplating. The thermal characteristics and thermal fatigue behavior of the sensors are investigated. The sensors exhibit significantly higher sensitivity than that of the bare FBGs at cryogenic temperatures down to 79 K, with good repeatability and stability. Their response to the temperature changes can be precisely described by a quadratic-polynomial equation. No obvious variations in the zero-point, the reflection spectrum, and the temperature sensitivity of the sensors exposed to thermal fatigue from 77 K to room temperature are observed before fatigue failure which could be attributed to the formation and growth of subcritical cracks within the fiber rather than that of fatigue cracks within the metallic coatings, as a result of the cyclic thermal stresses induced by the large differences in coefficient of thermal expansion between the silica optical fiber and the metallic coating materials. The results demonstrate that the multilayer metal-coated FBG sensors provide great potential for temperature measurements or temperature compensation in cryogenic engineering by proper selection of metallic coatings with a gradient of material properties.

Guided wave technique could be a possible method for monitoring components working in high temperature above 350 °C. However, this would require the design of an appropriate waveguide bar to transmit the wave, so that its sensing part is not influenced by the high temperature. In the present study, the shape of waveguide bars is designed based on the analysis of wave source characteristics. The critical frequency-width and frequency-thickness products of waveguide bars are analyzed theoretically and numerically to transmit the zeroth shear horizontal wave SH0* in bars. The results show that waveguide bars can cut off all the other wave modes when their frequency-thickness products are smaller than the critical value fd*, and frequency-width products are not smaller than the critical value fw*. Six waveguide bars are designed and fabricated based on the design criteria, and an experiment system is set up to check their work performance. The testing results indicate that the wave signals of the SH0* mode propagate clearly in waveguide bars, and cut off all the other modes when the frequency-thickness products and frequency-width products of the bars meet the design criteria. It is also demonstrated that the dependency of the experimental group velocity of each waveguide bar on frequency is in good agreement with the numerical result. High-temperature experiments also validate the reliability of the designed waveguide bars. Therefore, the critical frequency-thickness product and frequency-width product can be the basis for the design of the waveguide bars.

The safety of critical pressure equipment in elevated temperature is increasingly important. Moreover, the on-line monitoring method is potentially useful to improve their safety. A waveguide bar system can enable monitoring of critical equipment working in elevated temperature using reliable ultrasonic technology. Among the waveguide bar system, the matching mechanism of the transducer and the waveguide bar is crucial to propagate the pure fundamental quasi-shear mode (shorten for SH0*) wave. In the present research, the loading line sources that can excite pure SH0* wave are investigated and the anti-plane shear loading source is selected. The critical values about the geometric dimensions of the junctions between the piezoelectric transducer and the waveguide bar are explored by simulation and experiments. On the condition that the excitation sources satisfy the critical values, the loading can be approximated to an anti-plane shear one to excite the pure SH0* wave. Some waveguide bar systems are designed based on the simulated critical values and some experiments at high temperature are carried out. The experimental results verify that the designed waveguide bar systems can excite the pure SH0* wave at elevated temperatures, which verify the reliability of the simulated critical results.