Low-frequency peaks in the Raman spectra of amorphous poly(ether ether ketone) (PEEK) were investigated. An amorphous sample with zero crystallinity, as confirmed by wide-angle X-ray diffraction, was used in this study. In a previous study, two peaks were observed in the low-frequency Raman spectra of the crystallized samples. Among these, the peaks at 135 cm-1 disappeared for the amorphous sample. Meanwhile, for the first time, the peak at 50 cm-1 was observed in the crystallized sample. Similar to the peak at 135 cm-1, the peak at 50 cm-1 disappeared in the amorphous state, and its intensity increased with increasing crystallinity. The origins of the two peaks were associated with the Ph-CO-Ph-type intermolecular vibrational modes in the simulation. This suggests that the Ph-CO-Ph vibrational mode observed in the low-frequency region of PEEK was strongly influenced by the intermolecular order.

This investigation introduces advanced predictive models for estimating axial strains in Carbon Fiber-Reinforced Polymer (CFRP) confined concrete cylinders, addressing critical aspects of structural integrity in seismic environments. By synthesizing insights from a substantial dataset comprising 708 experimental observations, we harness the power of Artificial Neural Networks (ANNs) and General Regression Analysis (GRA) to refine predictive accuracy and reliability. The enhanced models developed through this research demonstrate superior performance, evidenced by an impressive R-squared value of 0.85 and a Root Mean Square Error (RMSE) of 1.42, and significantly advance our understanding of the behavior of CFRP-confined structures under load. Detailed comparisons with existing predictive models reveal our approaches\' superior capacity to mimic and forecast axial strain behaviors accurately, offering essential benefits for designing and reinforcing concrete structures in earthquake-prone areas. This investigation sets a new benchmark in the field through meticulous analysis and innovative modeling, providing a robust framework for future engineering applications and research.

Carbon fiber reinforced composite structures have been a research hotspot in recent years, with 9 specimens designed for static tests under bending and torsion loads of CFRP concrete filled steel tubes. The torque angle (T-θ) curve was studied from an experimental perspective. Subsequently, a reasonable finite element model was established using ABAQUS software. In addition, the effects of changes in parameters such as the number of steel concrete strength, bending ratio, and steel ratio on C-CF-CFRP-ST bending and torsion specimens were studied through numerical parameter research. Finally, equations of bearing capacity of CFRP-concrete filled steel tube under coupled bending and torsion are proposed.

The gyroid structure is a bio-inspired structure that was discovered in butterfly wings. The geometric design of the gyroid structure in butterfly wings offers a unique combination of strength and flexibility. This study investigated sandwich panels consisting of a 3D-printed gyroid structure core and carbon fiber-reinforced polymer (CFRP) facing skin. A filament fused fabrication 3D printer machine was used to print the gyroid cores with three different relative densities, namely 10%, 15%, and 20%. Polylactic acid (PLA) was used as the printing material for the gyroid. The gyroid structure was then sandwiched and joined by an epoxy resin between CFRP laminates. Polyurethane foam (PUF) was filled into the gyroid core to fill the cavity on the core for another set of samples. Flexural and compression tests were performed on the samples to investigate the mechanical behavior of the sandwiches. Moreover, the two-parameter Weibull distribution was used to evaluate the results statistically. As a result, the sandwich-specific facing stress and core shear strength from the three-point bending test of the composites increased with the increase in sandwich density. Core density controls the flexural characteristics of the sandwich. Adding PUF improves the deflection at the maximum stress and the sustained load after fracture of the sandwich. Compression strength, modulus, and energy absorbed by gyroid core sandwiches and their specific properties are higher than the PUF-filled gyroid core sandwiches at equal sandwich density.

This paper presents a mesoscale damage model for composite materials and its validation at the coupon level by predicting scaling effects in un-notched carbon-fiber reinforced polymer (CFRP) laminates. The proposed material model presents a revised longitudinal damage law that accounts for the effect of complex 3D stress states in the prediction of onset and broadening of longitudinal compressive failure mechanisms. To predict transverse failure mechanisms of unidirectional CFRPs, this model was then combined with a 3D frictional smeared crack model. The complete mesoscale damage model was implemented in ABAQUS®/Explicit. Intralaminar damage onset and propagation were predicted using solid elements, and in-situ properties were included using different material cards according to the position and effective thickness of the plies. Delamination was captured using cohesive elements. To validate the implemented damage model, the analysis of size effects in quasi-isotropic un-notched coupons under tensile and compressive loading was compared with the test data available in the literature. Two types of scaling were addressed: sublaminate-level scaling, obtained by the repetition of the sublaminate stacking sequence, and ply-level scaling, realized by changing the effective thickness of each ply block. Validation was successfully completed as the obtained results were in agreement with the experimental findings, having an acceptable deviation from the mean experimental values.

The primary drawback of concrete lies in its low tensile strength, prompting the development of various solutions to enhance this aspect. A notable approach is the utilization of Prestressed Reinforced Concrete (PRC) with tendons, aimed at bolstering its tensile strength. As the use of diverse tendon types in the PRC continues to surge, a review becomes imperative to delve into this evolution. Therefore, this study delved into the engineering characteristics, performance, and evolution of different tendon varieties, encompassing both steel and composite options. Despite certain drawbacks associated with employing composite materials such as Fiber Reinforced Polymer (FRP) tendons - such as heightened costs, limited availability of composite materials, and intricate manufacturing processes - there are distinct advantages and merits to incorporating FRP composite tendons in the realm of construction. In this respect, Carbon FRP tendons exhibited superior strength, comparable to their steel counterparts. Glass FRP tendons, lacking metallic components, possessed non-magnetic properties, rendering them resistant to corrosion. Additionally, Aramid FRP tendons boasted low flammability and exceptional resistance to elevated temperatures. Lastly, Basalt FRP tendons offered sustainability, rust resistance, and non-corrosiveness. The findings derived from this review study serve as a valuable resource for researchers seeking to advance the applications of steel tendons and FRP composite materials within the construction industry.

Based on the decorrelation calculation of diffusion ultrasound in time-frequency domain, this paper discusses the repeatability and potential significance of Disturbance Sensitive Zone (DSZ) in time-frequency domain. The experimental study of Barely Visible Impact Damage (BVID) on Carbon Fiber Reinforced Polymer (CFRP) is carried out. The decorrelation coefficients of time, frequency, and time-frequency domains and DSZ are calculated and compared. It has been observed that the sensitivity of the scattered wave disturbance caused by impact damage is non-uniformly distributed in both the time and frequency domains. This is evident from the non-uniform distribution of the decorrelation coefficient in time-domain and frequency-domain decorrelation calculations. Further, the decorrelation calculation in the time-frequency domain can show the distribution of the sensitivity of the scattered wave disturbance in the time domain and frequency domain. The decorrelation coefficients in time, frequency, and time-frequency domains increase monotonically with the number of impacts. In addition, in the time-frequency domain decorrelation calculation results, stable and repetitive DSZ are observed, which means that the specific frequency component of the scattered wave is extremely sensitive to the damage evolution of the impact region at a specific time. Finally, the DSZ obtained from the first 15 impacts is used to improve the decorrelation calculation in the 16-th to 20-th impact. The results show that the increment rate of the improved decorrelation coefficient is 10.22%. This study reveals that the diffusion ultrasonic decorrelation calculation improved by DSZ makes it feasible to evaluate early-stage damage caused by BVID.

Composite laminates utilizing autoclave-grade carbon fiber-reinforced plastic (CFRP) prepreg were manufactured using a polymer nanoporous network (NPN) interlayer that generates capillary pressure in lieu of pressure from an autoclave. The polymer nanofiber NPN film is integrated into the interlaminar region and is shown to eliminate voids in a vacuum-bag-only (VBO) curing process. After a preliminary investigation of the effect of NPN thickness on the interlaminar region and performance, an 8 μm thick polymer NPN was selected for a scaled manufacturing demonstration. Combining the polymer NPN with \"out-of-oven\" (OoO) electrothermal heating of a carbon nanotube (CNT)-heated tool, a 0.6 × 0.6 m void-free plate is successfully manufactured. OoO cure enables an accelerated cure cycle, which reduces the cure time by 35% compared to the manufacturer-recommended cure cycle (MRCC). X-ray microcomputed tomography (μ-CT) reveals that the laminates are void-free and of identical quality to autoclave-cured specimens. An array of mechanical tests including tension, compression, open-hole compression (OHC), tension-bearing (bolt-bearing), and compression after impact, reveal that the accelerated NPN-cured composites were broadly equivalent, with some instances of improved properties, relative to the autoclave-cured parts, e.g., OHC strength increased by 5%. With reduced capital costs, energy consumption, and increased throughput, the facile polymer NPN-enabled out-of-autoclave (OoA) fabrication method is shown to be a practical and attractive alternative to conventional autoclave fabrication.

The demand for lightweight aircraft structures has shifted from traditional metals like aluminum to composite materials such as carbon fiber reinforced polymers (CFRP) to achieve weight reduction. However, this transition has led to decreased lightning strike protection efficiency due to the dielectric nature of CFRP. To address this problem, two CFRP samples-one unprotected and the other shielded with carbon nanotube (CNT) sheets-were subjected to artificial lightning strike testing. The research employed a coupled thermal-electrical finite element analysis method to investigate the lightning strike\'s impact and damage mechanisms on both samples. The numerical results closely aligned with published experimental data, validating the simulation. Unprotected CFRP sustained damage through the thickness direction up to 8 composite plies and in-plane direction over a length of 110 mm. In contrast, the sample protected with CNT sheets exhibited damage limited to the surface of the first 4 plies, with in-plane damage reduced to 24 mm. Notably, the damage area in the CFRP protected with CNT sheets showed a substantial 78.1 % reduction compared to the unprotected CFRP sample. This suggests that CNT can enhance the electrical conductivity of CFRP when incorporated between interlayers in both in-plane and thickness directions. The study enhances understanding of CFRP damage behavior and failure modes under lightning strike conditions, emphasizing CNT sheets as an improved and viable lightning strike protection system for aerospace applications, warranting further investigation.

The corrosion behavior of a hybrid laminate consisting of laser-structured aluminum EN AW-6082 ∪ carbon fiber-reinforced polymer was investigated. Specimens were corroded in aqueous NaCl electrolyte (0.1 mol/L) over a period of up to 31 days and characterized continuously by means of scanning electron and light microscopy, supplemented by energy dispersive X-ray spectroscopy. Comparative linear sweep voltammetry was employed on the first and seventh day of the corrosion experiment. The influence of different laser morphologies and production process parameters on corrosion behavior was compared. The corrosion reaction mainly arises from the aluminum component and shows distinct differences in long-term corrosion morphology between pure EN AW-6082 and the hybrid laminate. Compared to short-term investigations, a strong influence of galvanic corrosion on the interface is assumed. No distinct influences of different laser structuring and process parameters on the corrosion behavior were detected. Weight measurements suggest a continuous loss of mass attributed to the detachment of corrosion products.