Applied Composite Materials最新文献

The hygrothermal aging of vinylester resin and its carbon fiber fabric-reinforced composite structures is examined here, focusing on moisture absorption and the consequent degradation of mechanical properties. Specifically, resin casting and CFRP (carbon fiber reinforced polymer) specimens were prepared and immersed into the deionized water and artificial seawater, respectively, at a temperature of 70 °C. Regular weight measurements were taken, accompanied by surface morphology observations using scanning electron microscopy (SEM) and identification of variations in functional groups through Fourier-transform infrared (FTIR) spectroscopy. Meanwhile, the mechanical properties of resin and CFRP were periodically checked. The gravimetric analysis results indicate that resin immersed in deionized water exhibits non-Fickian diffusion due to strong hydrolysis, while CFRP obeys approximately Fickian diffusion because of the embedded carbon fiber inhibiting the hydrolysis. The examination of mechanical properties for CFRP reveals that moisture absorption significantly influences interlaminar shear strength, resulting in a maximum reduction of 13.5%.

Abstract

The effects of service temperatures on the mechanical properties of self-piercing riveting (SPR) joints of carbon fiber reinforced polymer (CFRP) sheets and AA5754 aluminum alloy sheets were investigated in this study. Three different thicknesses of 0°/90° lay-up sequences of CFRP sheets and aluminum alloy sheets were selected for the SPR joints, and these three joints were subjected to static tensile tests at four different temperatures of 25 °C, 50 °C, 80 °C and 125 °C. A noncontact strain measurement DIC-3D system was used to record changes in the strain field and scanning electron microscopy (SEM) was used to observe the failure area at the rivet hole of the CFRP sheet to study the damage forms and mechanisms of the joints. The results of the tests showed an average reduction of 35.4% in maximum load and an average degradation of 21.9% in energy absorption for the three joints at 125 °C compared to room temperature conditions.

This paper investigates the effect of intra-laminar fibre hybridisation, i.e., primary and secondary fibres within a matrix, on the homogenised properties and micro-stress fields in uni-directional polymer composite laminae. The study is focused on S-glass/epoxy laminae which are hybridised with secondary fibres (e.g., polypropylene). Two-dimensional repeating unit cells (2D RUCs) with periodic microstructures are developed to conduct the micro-mechanical analyses under transverse tensile and transverse shear loading conditions. Uni-directional fibre-hybrid S-glass/epoxy laminae with different secondary fibres are studied by varying (a) the periodic microstructure and (b) the material properties of the constituent fibres to assess the effect of such geometric and material variations on the homogenised elastic lamina properties and intra-lamina micro-stress fields. The results show that intra-laminar fibre hybridisation significantly affects the elastic lamina properties and micro-stress fields. Notably, the presence of the secondary fibres significantly increases or reduces the stress fields in the matrix and at the fibre-matrix interfaces (i.e. normal and shears stress components)–depending on the microstructure and the stiffness of the secondary fibres–which could be explored to manipulate the damage modes and thus energy dissipation mechanisms.

Abstract

In this paper, the effects of interference-fit sizes and service environment temperature on the preload and relaxation of CFRP bolted joints are investigated based on an ultrasonic monitoring method. Specimens of different interference-fit sizes were subjected to insertion, preloading and preloading force monitoring for up to 200 h. To describe the preloading relaxation response of CFRP bolted joints, a comprehensive relaxation mechanics model is proposed. Experimental results demonstrate that this model accurately describe the variations in bolted preloading force under interference-fit conditions and thermal-oxygen environments. During the preloading process, a portion the axial force in interference-fit bolted joints is dissipated by interfacial frictional force and the magnitude of the frictional force is influenced by the interference-fit sizes. The interference-fit will lead to a tightly coupled interface, causing interface friction between the bolt-shank and the joint-holes, which can lead to a weakening transformation ability from tightening torque to axial force. Compared to clearance-fit condition, interference-fit can suppress the preloading relaxation effect of CFRP bolted joints to a certain extent. With an increase in interference-fit percentage (from 0% to 1.2%), the preloading relaxation coefficient rises from 94.4% to 95.7%. The additional interfacial friction effectively suppresses the creep deformation of composites. However, with an increase in the service temperature, the relaxation behavior of preloading forces in CFRP bolted joint significantly intensifies. As the environmental temperature rises from 25 ℃ to 150 ℃, the preloading relaxation coefficient decreases from 95.0% to 79.8%. High-temperature environments can lead changes in the material properties of composite and interface friction characteristics, even potentially leading to damage.

Internal delamination damage in carbon fibre reinforced polymer (CFRP) composites occurs easily after a fastener is installed. To determine the internal delamination damage in CFRPs with a fastener under lightning strike conditions, an experiment was conducted with different lightning channel gaps, fastener diameters, and fitting conditions. On the basis of the experimental findings and the results of a coupled thermal–electrical simulation model, a circuit model for CFRPs with a fastener was established to explore the current path, delamination properties and internal damage in CFRP specimens. The factors influencing the correlations between the generation and development of internal delamination damage under multiple conditions were proposed to clarify the lightning damage mechanism of the CFRP composites. The results indicated that internal delamination damage was mainly caused by resin pyrolysis and pyrolysis gas expansion; in addition, the thermal–electrical coupling effect of the contact interface had a significant impact on the internal delamination damage. For example, at approximately 50 kA, the damage area of the specimen with a 6 mm diameter fastener was 28.5% smaller than that of the specimen with a 4 mm diameter fastener. This work provides a basis for understanding the propagation of delamination in CFRP composites with a fastener and for reducing the delamination damage under lightning strike environment.

Hybrid composites containing carbon fibers and ramie fibers in an epoxy polymer matrix were prepared (denoted as CRFRP), after which the composites were immersed in distilled water at three different temperatures (20, 40, and 60 °C) for a period up to 2 months. Water absorption tests and static (tensile and flexural) and dynamic (low-velocity impact) mechanical tests were then conducted on the hygrothermally-treated composites to explore their hydrothermal aging mechanism. Results show that water uptake by CRFRP composites was enhanced by increasing the hygrothermal treatment temperature or aging time, with the water uptake obeying a Fickian diffusion model. Hygrothermal aging decreased the tensile strength, tensile modulus, flexural strength and flexural modulus of the CRFRP composites, though enhanced the impact absorption energy since the ramie fibers had greater plasticity and deformability after aging. Based on the experimental findings, a plausible mechanism was developed for the hydrothermal aging of the hybrid composites. Importantly, CRFRP composites were lighter than carbon fiber-reinforced composites (CFRP), whilst offering similar all-round performance, suggesting CRFRP composites may be useful in applications where CFRP composites have traditionally been used.

The primary objective of this study is to conduct a comprehensive experimental investigation into the impact of yarn reduction on the damage mechanisms and progression of 3D woven composites under bending loads, utilizing a combination of micro-XCT and digital image correlation (DIC) techniques. Typical bending behaviors of 3D woven composites have been discussed through load–displacement curves combining camera photography techniques. The influence of yarn reduction on strain distribution during bending deformation can be obtained by utilizing DIC techniques. Additionally, the final failure mode analysis of three-dimensional woven composite materials was conducted using micro-XCT techniques.

Luffa cylindrical (LC) has an exceptionally multipartite architecture, a hierarchical and light structure, and a low density. Such a structure is potentially suitable to replace conventional porous-type composites for low-energy absorption and material reinforcement applications. This paper presents an experimental study of the impact behavior of four different luffa/epoxy composites, named (A), (B), (C), and (D) subjected to low-velocity impact (LVI) at energies ranging from barely visible impact damage (BVID) to perforation (5,15, and 20J). Acoustic emission (AE), scanning electron microscopy (SEM), and digital image correlation (DIC) were introduced to the indentation test to offer additional information on damage mechanisms and on strain and displacement fields since the LVI test has a short duration and real-time damage monitoring is not always achievable. The results showed that the values of the peak force of laminates (A), (B), and (D) are relatively lower compared to laminates (C). In the case of perforation impact energy (20J), the Coefficients of Restitution (CoR) of composites (A), (B), and (D) are equal to 0, which indicates that the nature of the impact is completely plastic, except for composite (C) had a value of 0.11, and a lower degree of damage at all impact energies. Composites (C) exhibit the highest impact resistance, followed by composites (A), while composites (D) display the highest energy absorption, followed by composites (B). Multivariable statistical analysis of the AE signals identified four classes of damage: matrix cracking, fiber-matrix debonding, delamination, and fiber breakage. The damage modes found by AE are well presented and proven by SEM analysis. The luffa fiber-reinforced composite has better impact properties than other natural fiber-reinforced composites.

Abstract

A novel 3D printing method for continuous carbon fiber-reinforced thermosetting epoxy resin composites (CCFRTC) was proposed, including CCFRTC prepreg filament manufacturing, secondary impregnation, printing and curing stages. Through the addition of an impregnation stage before printing, this method ensures a close interface bond and uniform distribution of fibers and resin. After testing, the average tensile strength and tensile modulus of the uncured pre-impregnated continuous filaments were found to be 968 MPa and 58.6 GPa, respectively. Mechanical testing of the specimens revealed that the maximum tensile strength and flexural strength of the CCFRTC specimens reached 825 MPa and 557 MPa, with tensile and flexural modulus measuring 157 GPa and 185 GPa. Furthermore, scanning electron microscopy (SEM) examination of the cross-sections indicated a highly uniform impregnation of both the filaments and printed specimens. In conclusion, the method proposed in this study enables the preparation and printing of continuous fiber-reinforced thermosetting resin composite materials, addressing the issues of inadequate impregnation and poor interfacial bonding performance in continuous carbon fiber-reinforced thermosetting resin composite materials. These findings may broaden the potential applications of 3D printing CCFRTC in the aerospace, defense, and automotive industries.

Trans-laminar fracture is an important topic for engineering composites. In this study, trans-laminar fracture initiation in quasi-isotropic carbon/epoxy laminates made of non-crimp fabrics was examined using in situ fast synchrotron X-ray radiography and ex situ X-ray computed tomography. The maximum split lengths were measured by in situ radiography and were compared with the predicted values in a detailed FE model using cohesive elements. Ex situ computed tomography scans were also conducted to confirm that no fibre breakage occurs before the final load drop in the experiments. In situ and ex situ observations are complementary for the understanding of damage initiation.