Search Results (41)

Advanced composite materials have been widely applied in aerospace structures owing to their low weight, high strength, excellent fatigue resistance, and corrosion resistance. However, during the curing and molding processes of carbon-fiber composites, significant residual stresses are introduced within the components due to factors such as thermal contraction, resin curing shrinkage, and the mismatch in thermal expansion coefficients between the fibers and the matrix. This consequently compromises the performance and reliability of the composites, potentially leading to deformation, warpage, and even cracking, which in severe cases may result in component rejection. This paper aims to investigate the mechanisms by which different forms of prestress influence curing-induced residual stress and strain states, and to analyze their effects on the mechanical properties and microstructural evolution of the materials. In this study, embedded fiber Bragg grating sensors were used to monitor strain evolution during the heating, isothermal curing, and cooling stages of unidirectional carbon-fiber-reinforced polymer laminates subjected to no prestressing, uniaxial through-thickness compressive prestressing, or biaxial prestressing. Tensile tests and scanning electron microscopy were subsequently conducted to evaluate the mechanical response and fracture morphology. The fiber Bragg grating results showed that the strain response varied markedly with the prestressing mode and force level. The findings are expected to provide theoretical guidance and experimental references for mitigating curing-induced residual stresses and improving the molding quality of composite materials. Full article

The UHV dry-type bushing plays a critical role in power transmission by enabling electrical connection, electrical insulation, and mechanical support, making it a core component for ensuring the safe and stable operation of UHV direct current (DC) transmission projects. Epoxy resin, serving as the fundamental insulating material for the bushing core, undergoes significant residual strain during high-temperature curing due to chemical shrinkage and thermal strain, which directly affects the molding quality and service reliability of the component. This paper investigates the curing process of a large-thickness epoxy material, which is on the same scale as a UHV bushing. An in situ monitoring system combining fiber Bragg grating (FBG) sensors and thermocouples, together with COMSOL Multiphysics simulations, is employed to systematically study the evolution of the temperature field and residual strain throughout the entire curing process, considering the demolding effect. The results show that during the curing stage, the internal temperature distribution is non-uniform, with a maximum temperature difference of 65 °C between the center and the edge. The residual strain is dominated by chemical shrinkage (accounting for 73.25%) and exhibits a pronounced radial gradient. Mold constraint and demolding cause abrupt changes in the strain. The developed thermo-chemo-mechanical coupled model shows good agreement between simulations and experimental measurements. Thermal cycling relaxes the residual stress, achieving a reduction of 3.89–5.77%. This study provides support for process optimization and defect prevention in large-scale epoxy insulation components. Full article

Interfacial stress concentration induced by curing shrinkage during the manufacturing of epoxy resin is a primary trigger for micro-nano defect formation and electrical performance degradation in power equipment. To address the computational complexity of traditional viscoelastic models and the thermoelastic behavior wherein the stiffness of the epoxy resin varies with temperature during curing, this paper proposes an improved viscoelastic constitutive model incorporating a thermo-elastic factor. By coupling curing kinetics, heat conduction, chemical shrinkage, and mechanical effects, a multi-physics simulation framework is constructed to describe the complete epoxy curing process, thereby revealing the spatiotemporal evolution of curing stress deformation. To verify the model’s accuracy, an in situ monitoring system based on Fiber Bragg Grating (FBG) sensors was established. A temperature compensation method was utilized to effectively decouple temperature and stress within the complex exothermic curing environment. This study reveals a significant strain gradient effect during the resin curing process. Experimental measurements indicate strains of 21,609 με and 5800 με at the interface and surface, respectively, with numerical simulations exhibiting high agreement with the experimental data. This research not only provides an efficient simulation approach for predicting curing stress but also offers a theoretical basis for the crack-resistant structural design of high-performance epoxy-based power equipment. Full article

A small experimental setup for in-situ measurement of the load-transferring strains during the curing process of thermosets is proposed. Combining the output from an unconstrained and a kinematically constrained setup, it is possible to design a cure profile for the first time, lowering the residual stresses in the final product while keeping the cure time short based on the output from a few simple experiments. The stress relaxation during the curing process under a kinetically constrained condition is accounted for by comparing the final cure-induced strain during a kinetically unconstrained and constrained cure experiment. The constrained polymer is curing between two laminates where the constraining layer is removed after finalizing the cure profile, making it possible to measure the final cure-induced strain for that case as well. The temperature at which the load-transferring point is reached is found to be a key process parameter from which the final cure-induced strains can be predicted for the unconstrained case. From the corresponding constrained cure experiments, the final residual stresses can be measured. Full article

The curing process of hair-pin motor stator insulation is critical, as residual stress increases the risk of partial discharge and shortens a motor’s lifespan. However, studies on the stress-induced defects during insulation varnish curing remain limited. This research integrates three-dimensional numerical simulations and experimental analysis to develop a curing model based on unsaturated polyester imide resin, aiming to explore the mechanisms of residual stress formation and optimization strategies. A dual fiber Bragg grating (FBG) sensor system is employed for simultaneous temperature and strain monitoring, while curing kinetics tests confirm the self-catalytic nature of the process and yield the corresponding kinetic equations. The multi-physics simulation model demonstrates strong agreement with the experimental data. The results show that optimizing the curing process reduces the maximum stress from 45.1 MPa to 38.6 MPa, effectively alleviating the stress concentration. These findings highlight the significant influence of the post-curing temperature phase on residual stress. The proposed model offers a reliable tool for stress prediction and process optimization in various insulating materials, providing valuable insights for motor insulation system design. Full article

Smart concrete is a structural element that can combine both sensing and structural capabilities. In addition, smart concrete can monitor the curing of concrete, positively impacting design and construction approaches. In concrete, if the curing process is not well developed, the structural element may develop cracks in this early stage due to shrinkage, decreasing structural mechanical strength. In this paper, a system of measurement using fiber Bragg grating (FBG) sensors for monitoring the curing of concrete was developed to evaluate autogenous shrinkage strain, temperature, and relative humidity (RH) in a single system. Furthermore, K-type thermocouples were used as reference temperature sensors. The results presented maximum autogenous shrinkage strains of 213.64 $\mu \epsilon$, 125.44 $\mu \epsilon$, and 173.33 $\mu \epsilon$ for FBG4, FBG5, and FBG6, respectively. Regarding humidity, the measured maximum relative humidity was 98.20 %RH, which was reached before 10 h. In this case, the recorded maximum temperature was 63.65 °C and 61.85 °C by FBG2 and the thermocouple, respectively. Subsequently, the concrete specimen with the FBG strain sensor embedded underwent a bend test simulating beam behavior. The measurement system can transform a simple structure like a beam into a smart concrete structure, in which the FBG sensors’ signal was maintained by the entire applied load cycles and compared with FBG strain sensors superficially positioned. In this test, the maximum strain measurements were 85.65 $\mu \epsilon$, 123.71 $\mu \epsilon$, and 56.38 $\mu \epsilon$ on FBG7, FBG8, and FBG3, respectively, with FBG3 also monitoring autogenous shrinkage strain. Therefore, the results confirm that the proposed system of measurement can monitor the cited parameters throughout the entire process of curing concrete. Full article

This study investigates the manufacturing, testing, and analysis of ultra-thick laminated polymer matrix composite (PMC) beams with the aim of developing high-performance PMC leaf springs for automotive applications. An innovative aspect of this study is the integration of Fiber Bragg Grating (FBG) sensors and thermocouples (TCs) to monitor residual strain and exothermic reactions in composite structures during curing and post-curing manufacturing cycles. Additionally, the Calibration Coefficients (CCs) are calculated using Strain Gauge measurement results under static three-point bending tests. A major part of the study focuses on developing a properly correlated Finite Element (FE) model with large deflection (LD) effects using geometrical nonlinear analysis (GNA) to understand the deformation behavior of ultra thick composite beam (ComBeam) samples, advancing the understanding of large deformation behavior and filling critical research gaps in composite materials. This model will help assess the internal strain distribution, which is verified by correlating data from FBG sensors, Strain Gauges (SGs), and FE analysis. In addition, this research focuses on the application of FBG sensors in structural health monitoring (SHM) in fatigue tests under three-point bending with the support of load-deflection sensors: a new approach for composites at this scale. This study revealed that the fatigue performance of ComBeam samples drastically decreased with increasing displacement ranges, even at the same maximum level, underscoring the potential of FBG sensors to enhance SHM capabilities linked to smart maintenance. Full article

The temperature and strain fields monitoring during the preparation process of buoyancy materials, as well as the health status after molding, are important for mastering the mechanical properties of buoyancy materials and ensuring the safety of operators and equipment. This paper proposes a short and high-density femtosecond fiber Bragg grating (fs-FBG) array based on different temperature coefficients fibers. By optimizing the parameters of femtosecond laser point-by-point writing technology, high-performance fs-FBG arrays with millimeter level gating length and millimeter level spatial resolution were prepared on two types of fibers. These were successfully embedded in buoyancy materials to achieve in-situ online monitoring of the curing process and after molding. The experimental results show that the fs-FBG array sensor has good anti-chirp performance and achieves online monitoring of millimeter-level spatial resolution. Intelligent buoyancy materials can provide real-time feedback on the health status of equipment in harsh underwater environments. The system can achieve temperature monitoring with an accuracy of 0.56 °C and deformation monitoring with sub-millimeter accuracy; the error is in the order of micrometers, which is of great significance in the field of deep-sea exploration. Full article

This article discloses the activity developed in the framework of the research project “GRID” aiming at the feasibility demonstration of a fiber optic sensing system (FOS), based on fiber Bragg gratings (FGB), embedded in the ribs of a conical grid structure demonstrator in composite material (CFRP), manufactured by means of dry robotic winding, liquid resin infusion and oven curing. This structure represents an optimized and highly efficient conical adapter for satellite applications that was designed under the same requirements of a conventional CFRP benchmark solution in order to evaluate possible mass savings. Specific interfaces were conceived in order to facilitate the insertion of the fiber optics in the center of helical ribs—pausing the automated deposition phase of the dry preform—and secure them to the structure. Representative grid articles were produced and tested to select the materials and evaluate the preliminary feasibility of the integrated system in conjunction with the infusion process. The proper functioning and use of the sensing system were finally proven during the various phases of the mechanical testing campaign of the demonstrator. Such a campaign included stiffness and strength evaluations and culminated with the catastrophic failure of the structure. The significant amount of data collected from several sensors embedded in the ribs and from conventional sensors glued outside the ribs helped us to better understand the structural behavior and to validate the design and analysis models. The main steps of the design, manufacturing and tests of this project are here addressed. Full article

Most ionizing radiation produces δ-rays of ≈1 keV that can impart MGy doses to 100 nm³ volumes of DNA. These events can produce severe dual double-strand breaks (DDSBs) on nucleosomes, particularly in dense heterochromatic DNA. This is the most common multiply damaged site, and their probabilities determine the biological effectiveness of different types of radiation. We discuss their frequency, effect on cell survival, DNA repair, and imaging by gold nanoparticle tracers and electron microscopy. This new and valuable nanometer resolution information can be used for determining the optimal tumor cure by maximizing therapeutic effects on tumors and minimizing therapeutic effects on normal tissues. The production of DDSBs makes it important to deliver a rather high dose and LET to the tumor (>2.5 Gy/Fr) and at the same time reach approximately 1.8–2.3 Gy of the lowest possible LET per fraction in TP53 intact normal tissues at risk. Therefore, their intrinsic low-dose hyper-sensitivity (LDHS)-related optimal daily fractionation window is utilized. Before full p53 activation of NHEJ and HR repair at ≈½ Gy, the low-dose apoptosis (LDA) and LDHS minimize normal tissue mutation probabilities. Ion therapy should thus ideally produce the lowest possible LET in normal tissues to avoid elevated DDSBs. Helium to boron ions can achieve this with higher-LET Bragg peaks, producing increased tumor DDSB densities. Interestingly, the highest probability of complication-free cure with boron or heavier ions requires a low LET round-up for the last 10–15 GyE, thereby steepening the dose response and further minimizing normal tissue damage. In conclusion, the new high-resolution DSB and DDSB diagnostic methods, and the new more accurate DNA-repair-based radiation biology, have been combined to increase our understanding of what is clinically important in curative radiation therapy. In fact, we must understand that we already passed the region of optimal LET and need to go back one step rather than forward, with oxygen being contemplated. As seen by the high overkill and severely high LET in the distal tumor and the increased LET to normal tissues (reminding of neutrons or neon ions), it is therefore preferable to use lithium–boron ions or combine carbon with an optimal 10–15 GyE photon, electron, or perhaps even a proton round-up, thus allowing optimized, fractionated, curative, almost complication-free treatments with photons, electrons, and light ions, introducing a real paradigm shift in curative radiation therapy with a potential 5 GyE tumor boost, 25% increase in complication-free cure and apoptotic–senescent Bragg Peak molecular light ion radiation therapy. Full article

The unique physical properties of heavy ion beams, particularly their distinctive depth–dose distribution and sharp lateral dose reduction profiles, have led to their widespread adoption in tumor therapy worldwide. However, the physical properties of heavy ion beams must be investigated to deliver a sufficient dose to tumors without damaging organs at risk. These studies should be performed on phantoms made of biomaterials that closely mimic human tissue. Polymers can serve as soft tissue substitutes and are suitable materials for building radiological phantoms due to their physical, mechanical, biological, and chemical properties. Extensive research, development, and applications of polymeric biomaterials have been encouraged due to these properties. In this study, we investigated the ionization, recoils, phonon release, collision events, and lateral straggle properties of polymeric biomaterials that closely resemble soft tissue using lithium-ion beams and Monte Carlo Transport of Ions in Matter simulation. The results indicated that the Bragg peak position closest to soft tissue was achieved with a 7.3% difference in polymethylmethacrylate, with an average recoils value of 10.5%. Additionally, average values of 33% were observed in collision events and 22.6% in lateral straggle. A significant contribution of this study to the existing literature lies in the exploration of secondary interactions alongside the assessment of linear energy transfer induced by the ⁷Li beam used for treatment. Furthermore, we analyzed the tissue-equivalent properties of polymer biomaterials using heavy ion beams, taking into account phonon release resulting from ionization, recoils, lateral straggle, and all other interactions. This approach allows for the evaluation of the most suitable polymeric biomaterials for heavy ion therapy while considering the full range of interactions involved. Full article

Segmental stress during the construction process plays a pivotal role in assessing the safety and quality of shield tunnels. Fiber Bragg grating (FBG) sensing technology has been proposed for tunnel segment stress monitoring. A laboratory test was conducted to validate the reliable strain measurement of FBG sensors. The field in situ monitoring of a sewerage shield tunnel was carried out to examine the longitudinal and circumferential stresses experienced by the segments throughout the construction phase. The cyclic fluctuations in stress were found to be synchronized with the variations in shield thrust. A comparison was made between the longitudinal and circumferential stress variations observed during the shield-driving and segment-assembly processes. Additionally, the time required for the grouting to reach its full curing strength was estimated, revealing its impact on the stress levels and range of the pipe segment. The findings of this study offer an enhanced understanding of the stress state and health condition of small-diameter shield tunnels, which can help in optimizing the design and construction process of tunnel segments, as well. Full article

The increase of device lifetime and reliability of THz photomixers will play an essential role in their possible future application. Therefore, their optimal work conditions/operation range, i.e., the maximal incident optical power should be experimentally estimated. We fabricated and tested THz photomixer devices based on nitrogen-implanted GaAs integrated with a Bragg reflector. Raman spectroscopy was applied to investigate the material properties and to disclose any reversible or irreversible material changes. The results indicate that degradation effects in the photomixer structures/material could be avoided if the total optical power density does not exceed levels of about 0.7 mW/µm² for 100 min of operation. Furthermore, the investigations performed during 1000 min of optical exposure on the photomixer devices’ central region comprising interdigitated metal-semiconductor-metal (MSM) structures suggest a reversible “curing” mechanism if the power density level of ~0.58 mW/µm² is not exceeded. Long-term operation (up to 1000 h) reveals that the photomixer structures can withstand an average optical power density of up to ~0.4 mW/µm² without degradation when biased at 10 V. Besides the decrease of the position of the A_1g (LO) Raman mode from ~291 cm⁻¹ down to ~288 cm⁻¹ with increasing optical power density and operation time, broad Raman modes evolve at about 210 cm⁻¹, which can be attributed to degradation effects in the active photomixer/MSM area. In addition, the performed carrier lifetime and photomixer experiments demonstrated that these structures generated continuous wave sub-THz radiation efficiently as long as their optimal work conditions/operation range were within the limits established by our Raman studies. Full article

In recent decades, the rapid development of radiotherapy has dramatically increased the cure rate of malignant tumors. Heavy-ion radiotherapy, which is characterized by the “Bragg Peak” because of its excellent physical properties, induces extensive unrepairable DNA damage in tumor tissues, while normal tissues in the path of ion beams suffer less damage. However, there are few prognostic molecular biomarkers that can be used to assess the efficacy of heavy ion radiotherapy. In this study, we focus on non-small cell lung cancer (NSCLC) radiotherapy and use RNA sequencing and bioinformatic analysis to investigate the gene expression profiles of A549 cells exposed to X-ray or carbon ion irradiation to screen the key genes involved in the stronger tumor-killing effect induced by carbon ions. The potential ceRNA network was predicted and verified by polymerase chain amplification, western blotting analysis, colony formation assay, and apoptosis assay. The results of the experiments indicated that lncRNA EBLN3P plays a critical role in inhibiting carbon ion-induced cell proliferation and inducing apoptosis of NSCLC cells. These functions were achieved by the EBLN3P/miR-144-3p/TNPO1 (transportin-1) ceRNA network. In summary, the lncRNA EBLN3P functions as a ceRNA to mediate lung cancer inhibition induced by carbon ion irradiation by sponging miR-144-3p to regulate TNPO1 expression, indicating that EBLN3P may be a promising target for increasing the treatment efficacy of conventional radiotherapy for NSCLC. Full article

Through the merits of the arbitrary three-dimensional (3D) fabrication ability and nanoscale resolution of two-photon polymerization, we demonstrated a fully encircled polymerized microfiber Bragg grating using 3D femtosecond laser nanofabrication. In order to generate strong enough polymer Bragg grating units around the microfiber surface, and to possess a possible smaller unit pitch and structure size, the composition of photoresist and grating dimensions were both experimentally optimized. A fast-curing, high-adhesion, great-heat-resistant acrylate monomer EQ4PETA was chosen as the cross-linking element, and a high-efficiency photoinitiator DETC was used. Along the tapered microfiber with a diameter of 2 microns, dozens of grating units of 300 nm thickness were successively fabricated. The resonance wavelength was approximately 1420 nm, with a unit pitch of 1 μm, slightly different with varying unit pitches. The refractive index sensitivity reached up to ~440 nm/RIU, which is much higher than other microfiber grating sensors. We also measured the temperature and strain sensitivity of this fully encircled microfiber Bragg grating, and this was estimated at 88 pm/°C and 6.3 pm/µε. It is foreseeable that with the continuous progress of fabrication technology, more highly integrated functional optical devices will emerge in the future. Full article