Applied Composite Materials最新文献

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.

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.

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.

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.

Graphical Abstract

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.