Developing single-component epoxy resin (EP) systems that combine fire safety and storage stability is crucial for meeting the stringent demands of advanced engineering applications. In this study, two iron- and phosphorus-containing imidazolium complexes (FA-PPDM and FA-DPCMI) were synthesized via acid-base neutralization from 1,1′-ferrocenedicarboxylic acid and phosphorus-containing bis-imidazoles. These complexes function as latent and flame-retardant curing agents for the EP. At 25 °C, EP/FA-PPDM and EP/FA-DPCMI exhibited a significantly extended shelf life of 15 and 11 days, respectively. Furthermore, both EP systems retained the ability to rapidly cross-link within 30 min at 100 °C. In terms of flame retardancy, both EP/FA-PPDM and EP/FA-DPCMI achieved a UL-94 V-0 rating, with their limiting oxygen index (LOI) values increasing to 30.0% and 34.5%, respectively. Compared to the EP/EMI, the total heat release (THR) of EP/FA-PPDM and EP/FA-DPCMI decreased by 55.4% and 51.9%, respectively. Additionally, total smoke production (TSP) declined by 64.1% and 63.8%, respectively. Therefore, this study demonstrates a highly feasible approach for developing single-component EPs that combine prolonged latency with inherent fire safety.

Vinyl ester resin (VER) is highly flammable, which limits its use in applications requiring high flame resistance. However, conventional flame-retardant approaches often degrade its mechanical properties. To improve both the flame retardancy and mechanical performance, this work developed a grafting strategy incorporating phosphorus-containing flexible side chains. Three DOPO-based diol derivatives with different chain lengths (DOPE, DOPB, and DOPH) were synthesized and then grafted onto the VER backbone through 4,4 '-methylenediphenyl diisocyanate (MDI) to obtain VER/M-DOPx systems. The modified resins showed obvious improvements in both flame retardancy and mechanical properties. VER/M-DOPE and VER/M-DOPB achieved UL-94V-0 ratings, with LOI values of 28.6% and 26.7%, respectively, while VER/M-DOPH reached a V-1 rating and an LOI of 28.9%. VER/M-DOPB showed the best flame-retardant effect, reducing the peak heat release rate (PHRR) and total heat release (THR) by 68.5% and 43.7% compared with VER. The incorporation of flexible DOPO-based diol chains also enhanced toughness and strength; VER/M-DOPE exhibited increases of 4.53%, 26.24%, and 81.35% in tensile, flexural, and impact strengths, respectively. These results indicate the feasibility of enhancing both the flame retardancy and the mechanical properties of VER through grafting phosphorus-containing flexible side chains.

Flexural vibrations frequently occur in engineering pipelines, potentially leading to structural instability and equipment damage. This study proposes a finite periodic pipe, which is structurally simple and easily implementable, designed to suppress flexural vibrations by periodically installing dual-degree-of-freedom flexible dampers on a bare pipe. The damper utilizes Kevlar fibers with both elastic and damping properties to establish a tunable energy dissipation pathway. The vibration attenuation characteristics of the finite periodic pipe were investigated through finite element analysis and experimental studies, which reveal the formation mechanisms and tunable characteristics of the flexural wave stopbands. The results indicate that a transition phenomenon conducive to flexural wave attenuation occurs within the Bragg stopband, whereas energy accumulation in the resonators causes signal delay. Furthermore, the local resonance stopband exhibits vibration-suppression mechanisms associated with rotational resonance and vertical vibration, with the latter effectively impeding the propagation of flexural waves during the initial cycles. Parametric analyses reveal that geometric and material parameters significantly influence vibration attenuation performance. Experimental results verify the feasibility and effectiveness of the proposed flexible damper for suppressing flexural vibrations in pipelines. This study presents a novel strategy for developing tunable vibration-reduction structures and provides valuable insights into vibration control in piping systems.

The inherent flammability of vinyl ester resin (VER) limits its use in high-performance applications, and conventional flame-retardant approaches often compromise mechanical and thermal properties. In this work, a grafting strategy was adopted in which three diisocyanates were used as bridging agents to covalently introduce diphenylphosphine oxide (DPPO) units into the crosslinked VER network. After modification, the resins still exhibited low viscosity (<800 mPa & centerdot;s) and high curing reactivity, with lower apparent activation energies than the control. The resulting thermosets (VEHD, VETD and VEMD) maintained crosslink densities similar to neat VER, while their mechanical and thermomechanical properties were significantly improved. The grafted samples showed maximum increases of 37.0% in tensile strength, 56.1% in flexural strength and 283.0% in unnotched impact strength. The room-temperature storage modulus and dynamic mechanical glass transition temperature (T-alpha) increased by 26.1% and 16.1 degrees C, respectively. Incorporation of phosphorus-containing urethanes altered the thermal decomposition behavior and imparted excellent flame retardancy. Among the grafted resins, VEMD exhibited the best performance, attaining a limiting oxygen index (LOI) of 31.3%, a V-0 rating in the UL-94 vertical burning test, and reductions of 64.0% in peak heat release rate and 41.7% in total heat release relative to neat VER. Moreover, all grafted resins retained the transparency and chemical resistance of the matrix, while VEMD and VETD provided additional UV-shielding performance. Thus, the proposed grafting strategy offers a facile yet effective route for fabricating intrinsically flame-retardant, high-performance vinyl ester resins.

The US faces a nursing shortage, particularly in senior care, highlighting the need for enhanced gerontological nursing education. Human-centered immersive Virtual Reality shows promise in improving nursing training, especially when combined with AI technologies. This study explores the use of Intelligent Immersive Virtual Reality (IIVR) in gerontological nursing through six simulated scenarios. A two-stage validation process assessed the tool. Stage one focused on efficacy, including simulation sickness, system usability, and user experience, with fifteen college students. Stage two evaluated motivation, cognitive workload, and efficacy with forty-five end-users in a senior care facility. The results demonstrated that the developed IIVR tool exceeds "acceptable" levels of efficacy and participants can use the tool without user experience issues. The assessments also suggest that participants are more willing to adopt this tool to enhance their daily gerontological nursing skills. Additionally, the cognitive load of the utilizing developed IIVR tool is less than conventional learning approach.

Single-component epoxy resin (EP) is the premix of EP and latent curing agent, which is highly demanded in industries. However, current single-component EPs struggle to balance storage stability with mechanical properties and flame retardancy. To address this issue, three phosphorus-derived imidazolium salts (MPOx, x = 2, 3, 4) were synthesized using 2-ethyl-4-methylimidazole (EMI) and phosphorus-containing acids with different oxidation states (diphenylphosphinic acid, phenyl hydrogen phenylphosphonate, and diphenyl phosphate). The oxidation state of phosphorus significantly influenced thermal latency of MPOx, with higher oxidation states leading to improved latency. EP/MPO4 achieved the longest shelf life of 42 d at 25 degrees C. EP/MPO3 and EP/MPO4 exhibited enhanced tensile strength, modulus, and impact resistance compared to EP/EMI, but EP/MPO2 showed poor mechanical properties due to phase separation. All EP/MPOx achieved limiting oxygen index (LOI) exceeding 30 %, with EP/MPO3 showing the highest LOI of 34.0 % and significant reductions in heat release and smoke production. Flame-retardant mechanistic studies revealed a shift from gaseous-phase flame inhibition to condensed-phase promoting carbonization with increasing phosphorus oxidation state. Obviously, MPOx provides a tailored balance of latency, mechanical strength, and flame retardancy, making it a promising solution for advanced single-component EPs in aerospace, electronics, and optical applications.

Acoustic black hole (ABH) is a passive vibration damping technique that can efficiently mitigate vibrations in beam structures. However, the bandgap performance in most conventional periodic ABH structures is constrained to the flexural direction, and the unit cell possesses a single symmetric configuration. Therefore, this paper proposes a bidirectional asymmetric ABH metamaterial beam to investigate its bandgap characteristics in both the flexural and longitudinal directions. The finite element method was employed to predict the bandgap structure of the bidirectional asymmetric ABH metamaterial beam in various directions, along with the attenuation of evanescent waves within the bandgaps, which was subsequently validated by experimental results. Additionally, the influence of geometric parameters and material damping on the bandgap performance of this metamaterial beam was also examined. The results suggest that, compared to conventional unidirectional metamaterial beams, this metamaterial beam exhibits a wider unidirectional bandgap, several comparatively broader complete bandgaps, and a higher evanescent wave attenuation factor. The formation of flexural bandgaps can be attributed to the combined effects of the ABH effect and Bragg scattering, whereas the formation of longitudinal bandgaps results solely from the Bragg scattering effect. It was further observed that the formation of complete bandgaps arises from the coupling and decoupling of these two effects. Parametric analysis demonstrates that tunable bandgaps in various directions can be realized through uniform changes in geometric parameters. Material damping has a negligible effect on bandwidth but significantly improves the attenuation of flexural waves, while having only a minor influence on the attenuation of longitudinal waves. Numerical analysis and experimental results demonstrate the superior vibration damping performance of this metamaterial beam in both the flexural and longitudinal directions. The proposed structure could provide a novel perspective on the design of multi-directional vibration beams.

Enhancing the bonding strength between composites and metals is one of the urgent challenges that needs to be addressed. One of the methods to improve the strength of adhesive joints is to change the geometry of the joints. In this paper, four different types of aluminum alloy sleeves were developed by designing grooves with varying angles and shapes. Glass fiber reinforced polymer rods were adhesively bonded to these sleeves, resulting in the preparation of five different types of adhesive joints, including a conventional structure. The mechanical performance and failure mechanisms of these joints were analyzed experimentally and numerically. The results indicate that, compared to conventional adhesive joints, the load-bearing capacity of the four designed adhesive joints has been significantly improved, with a maximum increase of 49.6% and a minimum increase of 35.6%. The axial angle of the groove structures designed within the joint is a factor that influences the ultimate load capacity of the joint. Furthermore, the shear stress in the adhesive layer is identified as the primary cause of adhesive layer failure. The designed mechanical interlocking structures can not only increase the interfacial bonding force between the adhesive layer and the substrate but also delay the complete failure of the adhesive layer, thereby improving the load-bearing strength of the joint. This work is expected to provide new insights for the design of composite and metal joints.

To address the flammability, dense smoke emission, and fire safety concerns of epoxy resin (EP) in automotive components, this study develops a sulfur-free, multi-element reactive flame retardant (FA) integrating phosphorus, nitrogen, boron, and silicon. Synthesized from 4-formylphenylboronic acid, 3-aminopropyltriethoxysilane, and DOPO, FA enables a synergistic flame-retardant mechanism in EP. At a 2.5 wt

In response to the increasing need for fire-resistant single-component epoxy resin (EP) systems, this research introduces five metal-based phosphorus/imidazole-containing complexes (M-DA) as latent curing agents. These complexes, synthesized by coordinating Fe3+, Co2+, Ni2+, Cu2+, or Zn2+ into a phosphorus-containing imidazole derivative (DA), were designed to improve flame retardancy, smoke suppression, and latency of single-component EPs. EP/M-DA mixtures exhibited prolonged shelf life and rapid gel times at moderate temperatures, with latency improvements following the trend: Cu2+ > Ni2+ > Co2+ > Zn2+ > Fe3+. Notably, EP/Cu-DA achieved a storage life of 43 days. In addition, it demonstrated improved thermal stability as well as superior mechanical strength and toughness. All EP/M-DA thermosets achieved a UL-94 V-0 rating and high limiting oxygen index (LOI) values exceeding 29.0 %, with EP/Cu-DA showing the highest LOI of 37.5 %. EP/Cu-DA also achieved significant reductions (46.5 % and 21.1 %) in peak heat release rate and total smoke production, highlighting its superior flame retardancy and smoke suppression. These improvements were attributed to synergistic effects between transition metal ions and phosphorus, which promote condensed-phase carbonization and gaseous-phase combustion inhibition. Among the single-component EPs, EP/Cu-DA exhibited the best combination of latency, mechanical strength, fire safety, and smoke suppression, providing a promising strategy for developing high-performance single-component EPs.

The static compression capacity of composite helical springs limits their industrial applications. In this study, a new "shell-core" composite structure helical spring (CSHS) was fabricated using the protrusion molding process. The shell is a metal tube, and the core is a fiber-reinforced polymer (FRP). By varying the ratio of metal to composite materials, the static and fatigue compressive properties of the composite helical springs were investigated. The results indicate that as the metal shell enveloping the composite core increases, the spring constant of the composite structure helical spring gradually rises from 5.557 to 12.260 N/mm, thereby enhancing the spring's compression performance. The specific stiffness exhibits an initial rapid increase followed by a plateau. Additionally, the finite element method was employed to further analyze the stress evolution of "shell-core" composite structure helical springs during compression and to predict the spring constant and specific stiffness through curve fitting, which aids in the design of "shell-core" composite structure helical springs.

Mid-to-low frequency vibration reduction in pipelines is crucial for ensuring the stealth of naval vessels. Although traditional absorbers are effective in suppressing vibrations, they predominantly function at higher frequencies and exhibit limited bandwidth. Consequently, this paper proposes a radiative two-degree-of-freedom damper composed of Kevlar fibers, termed a flexible damper, for vibration attenuation in pipelines. Initially, the relationship between force and displacement in a cantilevered pipeline with a series of concentrated masses and damped dampers was derived using the transfer matrix method. Subsequently, a finite element model of a singlecoupled cantilevered pipeline incorporating a flexible damper was developed to investigate its vibration characteristics. The results indicate that the vibration transmission curve exhibits two distinct stopbands: the localized resonance stopband attributed to the resonance of the flexible damper, and the Bragg stopband resulting from the mass and anti-resonance effects of the flexible damper. Furthermore, a parametric study was conducted to evaluate how variations in geometric structures and material properties influence vibration absorption performance. Experimental samples of the flexible damper were fabricated, and evaluations of dynamic responses demonstrated a strong correlation with numerical analyses. This study provides structural design guidelines for controlling mid-to-low frequency vibration propagation in naval piping systems.

Large-tow carbon fiber fabrics generate considerable structured background texture noise in infrared thermographic inspections due to their nonuniform thermal properties. The presence of such noise poses notable challenges to the reliable and precise detection of defects. To address this issue, this article proposes a defect detection method that integrates the frame averaging difference (FAD) algorithm and principal component analysis (PCA). The method utilizes FAD to remove nontime-varying noise, improving the contrast-to-noise ratio of defect signals. It then employs PCA to separate background texture noise. Building upon this foundation, a region growing segmentation (RGS) algorithm based on fuzzy affinity (FA) was introduced. The algorithm integrates a spatial distance function based on both pixel intensity and spatial proximity, significantly enhancing the accuracy of defect region segmentation. To validate the effectiveness of the proposed method, artificial delamination defects were simulated by embedding Teflon films between layers. Large-tow carbon fiber fabric composite laminates were then fabricated as test samples. The experimental results demonstrate that the algorithm effectively reduces the error rate in defect detection for large-tow carbon fiber composite laminates. This study provides a practical technical solution for the nondestructive testing of large-tow carbon fiber composites and shows strong applicability and robustness under complex background noise conditions.

In recent years, thermoplastic acrylate resins have been widely used due to their excellent properties, but the thermoplastic resin has a high melt viscosity, which makes it difficult to achieve a good impregnation effect with fibers. In this paper, methacrylic acid (MAA) copolymer-modified acrylate resin composites are prepared by an in-situ polymerization/reactive chain growth impregnation process. This process improves the intermolecular forces and the interfacial bonding between resin and fiber by introducing strong polar groups of carboxylate, thus significantly improving the overall performance of the resin matrix and its glass-fiber (GF)-reinforced composites. Compared to the unmodified resin polymethyl methacrylate (PMMA), the tensile strength was increased by 14.54%, tensile modulus by 23.36%, flexural strength by 20.05%, flexural modulus by 21.62%, impact toughness by 35.98%, and Tg by 15.42%. Compared with the unmodified composite GF/PMMA, tensile strength increased by 53.68%, tensile modulus by 38.94%, flexural strength by 24.75%, flexural modulus by 7.68%, and tensile shear strength increased by 21.44%.

The high conductivity of commercial carbon fibers leads to a noticeable skinning effect and weak microwave impedance matching. This results in insufficient effective absorption bandwidth and less than ideal overall wave absorption performance, mainly reflecting electromagnetic waves. These limitations restrict the application of carbon fibers. In this study, we successfully prepared carbon fiber/epoxy composites with adjustable electrical conductivity by precisely controlling the temperature and time of pre-oxidized polyacrylonitrile (PAN) fibers during the low-temperature carbonization stage. This allowed us to regulate the components and microstructure of conductive carbon fibers. The carbon fiber/epoxy composites demonstrated exceptional microwave absorption performance due to their various loss mechanisms. The impedance matching conditions and microwave attenuation capability were also quantitatively evaluated. For 850-9 samples, an effective bandwidth of approximately 7 GHz (11 GHz-18 GHz) was achieved with a thickness of 2.5 mm. For 900-9 samples, the best reflection loss achieved is -39.9 dB at 5.8GHz, which corresponds to a thickness of 3 mm. The carbon fibers with different electrical conductivity can be obtained by modulating the microstructure of carbon fibers, which can improve the impedance matching performance of conductive carbon fibers and reduce their reflection of electromagnetic waves to improve the microwave absorption performance, achieving the transformation of traditional carbon fibers from electromagnetic shielding to effective microwave absorption. This simple and efficient preparation method has yielded promising results and offers a new avenue for the advancement of high-performance carbon fiber microwave absorbing materials.

Epoxy resin (EP) possesses a high flammability risk, but the flame retardants applied often reduce other properties. For the sake of large-scale industrial manufacturing, EP and curing agent are increasingly being premixed as one -component system rather than blended immediately prior to application. However, current designs fail to break the trade-off between flame retardancy and mechanical/thermal/optical properties of one -component EPs. Herein, we developed a series of phosphorus -containing imidazolium compounds, named BADM-X (X = 1, 2, 3, and 4), which served as multifunctional latent hardeners for one -component EPs. BADM-1, BADM-2, BADM-3, and BADM-4 were synthesized in ethanol through the neutralization between bisphenol A diphosphate (BAD) and 2-ethyl-4-methylimidazole (EMI) at molar ratios of 1:1, 1:2, 1:3, and 1:4, respectively, which were easy to industrialize. Varying the imidazole content in BADM-X allowed for adjusting the curing reactivity and thermal latency of EP/BADM-X, making it adaptable for various applications. EP/BADM-2, EP/BADM-3 and EP/BADM-4 not only retained excellent optical properties, but also demonstrated increased glass transition temperatures (Tgs) and mechanical performances. The Tg of EP/BADM-2 reached 162.7 celcius, and its tensile and impact strengths increased by 34.8 % and 61.4 % relative to EP/EMI. EP/BADM-X exhibited enhanced fire safety, and the limiting oxygen index (LOI) and vertical burning (UL -94) classification of EP/BADM-2 reached 28.2 % and V-0, respectively. Besides, EP/BADM-2 demonstrated enduring flame-retardant and mechanical performances, thus showing great application potential. Overall, this study introduces a new class of multifunctional latent curing agents for the fabrication of durable fire -safe, high-performance, one -component EPs.

This work reported a flame-retardant single-component epoxy resin via the integration of two bisphenol-A-type epoxy resins (E20 and E53) and benzimidazolyl-substituted cyclotriphosphazene (BICP). The differential scanning calorimetry (DSC) tests showed that the onset curing temperature of E53/E20/BICP was 30°C lower than that of E53/BICP due to the catalytic decomposition effect of E20 towards BICP under heating. Besides, the shelf life of E53/E20/BICP at room temperature reached ∼40 days, demonstrating good storage stability. E53/E20/BICP maintained satisfactory thermal properties and showed outstanding flame retardancy with a UL-94 V-0 classification. Compared with the unmodified EP sample, the peak heat release rate (PHRR) and total heat release (THR) of E53/E20/BICP were decreased by ∼64.1% and ∼39.0%, respectively. Hence, this flame-retardant single-component EP with satisfactory comprehensive properties exhibited great potential for versatile industrial applications.

Carbonyl iron powder (CIP) is a common microwave absorber frequently utilized to prepare microwave-absorbing materials. However, it exhibits only magnetic loss and no dielectric loss, which inevitably affects its wave-absorbing performance. Here, porous carbonyl iron powder (p-CIP) modified with conductive polypyrrole (PPy) was successfully prepared by one-step chemical etching and in-situ polymerization and then utilized to fabricate epoxy composites with high microwave absorption intensity. The scanning electron microscopy images revealed that conductive PPy was encapsulated on the surface of p-CIP. The reflection loss of the P-200 sample is -41.2dB at 2.2mm thickness. Reduction of the sample thickness to 1.8 mm resulted in a still acceptable reflection loss of -35.5 dB. Moreover, the effective absorption bandwidth widened to approximately 6.75 GHz (10.55–17.3 GHz). This provides a valuable approach for the development of microwave-absorbing materials with high absorption capacity.

In this work, 2-(6-oxido-6H-dibenz<1,2>oxaphosphorin-6-yl)-methanol (ODOPM) was synthesized, and covalently introduced into vinyl ester resin (VER) by combining with 4,4 ' -diphenylmethane diisocyanate (MDI). The influences of ODOPM and MDI on the thermal properties, flame retardancy, and flame-retardant mode-of-action of the resin were investigated. When the phosphorus addition reaches 1.3 wt%, the limiting oxygen index (LOI) of VER/ODOPM-1.3 boosts to 30.3% and vertical burning (UL-94) rating increases to V-0. Compared with the virgin VER, the peak heat release rate (PHRR) and total heat release (THR) of VER/ODOPM-1.3 are significantly reduced by 64.2% and 35.1%, respectively. The introduction of ODOPM and MDI significantly enhances the flame retardancy of VER. ODOPM and MDI are chemically linked to the polymer network of VER, which increases the glass transition temperature (T-g) and rigidity. This work offers a facile method for improving both thermal and flame-retardant properties of vinyl ester resin.