Applied Composite Materials最新文献

This study investigates the impact of ultrasonic welding amplitudes and time on the properties of carbon fiber reinforced polyaryletherketone (CF/PAEK) composite joints. To enhance the performance of CF/PAEK ultrasonic welded joints, a hybrid energy director (ED) was proposed, which was composed of the interfacial microgroove and resin film or metal mesh. This study investigated the effect of different types of ED on the forming quality, shear failure load, and fracture interface microstructure of single lap joints made of CF/PAEK. The results indicated that the hybrid ED with resin film offers a distinct effect on enhancing the strength of ultrasonic welded joints. The hybrid ED with resin film essentially improves the tensile properties of the joint, with the strength and toughness increased by 35.8% and 174.3%, respectively. This strengthening effect is primarily attributed to the added resin film providing adequate interfacial resin. Sufficient resin is filled into the interior of the microgroove, ultimately forming a mechanical anchoring structure to strengthen the joint strength.

Auxetic materials differ from typical materials in that they expand in the transverse direction when stretched longitudinally, giving them special features. It is possible to weave auxetic fabrics using both auxetic and non-auxetic threads. This study exhibits the semi-empirical modeling of the auxetic woven fabric followed by computational modeling for the prediction of Poisson's ratio. Further, woven fabrics have been developed to test the geometry's potential for producing an auxetic fabric, which can be used for maternity and children's clothing, wound dressing, and protective clothing, providing better comfort and longevity of application. Poisson’s ratio of the developed auxetic samples is measured and compared with experimental results. The effect of thread density and float length on the auxeticity of the fabric based on waveform geometry is also investigated in this study. It was observed that the increase in thread density increases the auxeticity of the fabric, whereas the increase in float length decreases the auxeticity. Auxetic composites were successfully developed using silicon rubber gel as the matrix system.

This paper introduces a novel multi-die pultrusion system for producing small-diameter phenolic-based CFRP rods. The system consists of multiple short heating dies arranged in series, facilitating the escape of vapor from the die cavities and improving the product quality. The results demonstrate that compared to the traditional dies, the rods produced using the multi-die pultrusion system exhibit higher dimensional stability, and their interlaminar shear strength is mostly above 35 MPa, reaching up to 52 MPa. Compared to the traditional mold, in one instance, its interlaminar shear strength value increased by nearly 71.5%, but in another case, it was only 14.72%. Due to relying solely on one control sample, the results are inconclusive. SEM indicates that the rods produced using the multi-die pultrusion system have fewer voids and better fiber-resin bonding compared to the traditional dies. Additionally, cross-sectional optical microscopy shows that when the pultrusion speed is at or below 0.6 m/min, the impregnation of carbon fibers by phenolic resin is more effective. The proposed multi-die pultrusion system provides a new idea for the production of small-diameter rods.

Graphical Abstract

Taking crustacean organisms in nature as prototypes helps improve the design of protective gears. Drawing inspiration from the high-damage-tolerance helical-structured cuticle of the American crayfish, we conduct an optimization of processing parameters for Fused Filament Fabrication 3D printing products. Various values of in-plane raster angle and interlayer thickness are employed to replicate the damage-resistant feature mimicked from nature. The effect of flexural resistances on 3D-printed three-point bending specimens is being investigated using a combination of four helical printing raster angles at four different layer thicknesses. Acrylonitrile-butadiene-styrene (ABS) and glass fiber-reinforced ABS (ABS-GF) are employed as material models. A Dino-lite handheld microscope and a Keyence VHX-7000 optical microscope are used to characterize the microstructure of the samples’ fracture resistance after the three-point bending test. Explanations of the mechanism of fracture resistance for helical structures are given. The results show that the specimen with a layer thickness of 0.04 mm and a spiral angle of 30° has the highest bending strength and bending elastic modulus among all the tested specimens. When compared with the layer thickness of 0.16 mm, the bending strength and bending elastic modulus of the ABS helix specimen with a layer thickness of 0.04 mm are increased by 6.45% and 2.67%, and those of the ABS-GF helix specimen are increased by 21.21% and 10.03%, respectively. The microstructural observation of the samples reveals that the spiral specimens with a helix angle of 11.25° have a greater displacement of crack propagation to resist the damage extending inside when resisting fracture. Our bio-inspired study presents an alternative approach to comprehensively optimize FFF printing parameters for enhanced mechanical performance.

Graphical Abstract

In this study, the mechanical properties and failure characteristics of bolted and hybrid bonded-bolted GFRP/Al joints under different loading speeds were investigated. The failure process and strain evolution were recorded using high-speed cameras and digital image correlation (DIC) techniques. The micro-morphology of the fracture was also investigated to explore the effect of loading speed on the fracture mode. The results showed that the peak load, failure displacement, and energy absorption for all joints were sensitive to the loading speed. The peak load and energy absorption of the hybrid joints were higher than that of the bolted joints under both static and dynamic loading. The loading speed had no significant effect on the failure mode of GFRP material in bolted joints, which were all shear-out failures. While for the hybrid joint, the addition of the adhesive layer changed the failure mode of GFRP material from shear-out failures to tension failures. As the loading speed increased, the final failure area of GFRP in hybrid joints gradually decreased. In hybrid joints, a greater amount of bearing damage preceded a final tension failure in GFRP material with the increase in loading speed. The fracture surface became flatter and the pulled-out fiber bundles were more integral due to the fact that cracks within the material could not extend sufficiently at high loading rates.

Multiple holes presented in composite structures due to design requirements or accidental impacts can reduce the structural strength, thus posing potential safety risks for aircraft structures. For the random hole damages caused by the impacts, hole numbers, hole-to-hole distance and position angles are part of the parameters that affect the structural strength. Effects of the position angle on tensile behavior, including stress concentration factors, stress distributions, damage initiation and propagation, and tensile strength of CFRP laminates are investigated experimentally and numerically based on progressive damage analysis. The stress concentration factor reaches its lowest and highest value when the angle is 0° and 60°, respectively. The tensile strength shows an opposite trend. Interferences of the stresses around the holes lead to a slight offset of the damage initiation point and a change of tangential stress pattern at the hole boundary. For the case of 60°, the matrix, fiber and delamination damages appear mostly in between the holes, causing a large reduction of its loading-carrying capacity.

Structural adhesives characterized a turning point in the post-connection of structural elements due to their excellent performances and ability to transfer stress without losing their integrity. These materials are typically particle-reinforced composites made by a thermoset polymer matrix and fillers. During the in-situ application of this material, the thermal activation of the polymer is typically not possible, leading to an undefined degree of cure and therefore to a variation of the mechanical performance over time. This altering means that after applying a sustained load on a bonded anchor system installed at regular temperature, the adhesive changes material properties. Ample studies convince that the progressive increase of the degree of cure of the thermosetting polymer leads to higher strength and stiffness. However, limited studies have been dedicated to the post-curing effects on the long-term behavior. The main goal of this work is to investigate the tensile and shear creep behavior of two commercially available structural adhesives and the influence of curing conditions on their long-term performances. An extensive experimental campaign comprising short and long-term characterizations has been carried out on specimens subjected to three different curing and post-curing protocols, with the scope of imitating relevant in-situ conditions. The results demonstrate that structural adhesives cured at higher temperatures are less subjected to creep deformations. As a material equation, the generalized Kelvin model is utilized to fit the tensile and shear creep data, and two continuous creep spectra have been selected to represent the creep behavior and facilitate extrapolations to the long-term behavior.

The experimental and numerical investigations on the dynamic responses and failure mechanisms of honeycomb panels under low-velocity impact were carried out in the present work. The carbon fiber composite hexagonal honeycomb panels were fabricated using the hot press molding method. Then, low-velocity drop-weight impact tests on the composite honeycomb panels were conducted under impact energy levels of 5J, 10J, 30J, 50J, 60J, 70J, and 100J to study the deformation mechanisms and damage modes. The VUMAT was developed to model the behavior of sandwich panels, in which a progressive damage model based on the strain-based failure criterion of composite fabric and Yeh delamination failure criteria was implemented in ABAQUS/Explicit. Two-dimensional topological honeycomb configurations with the same relative density were established. The energy absorption and load-bearing capacity of hexagonal, square, triangular, Kagome, and two kinds of circular (CS and CH types) honeycombs under 100J impact energy were discussed. The results showed that the circular honeycomb (CH type) had the largest first peak force of 6.714 kN, while the hexagonal honeycomb had the smallest first peak force of 3.715 kN. Compared with hexagonal honeycomb, the energy absorption of the triangle, Kagome, and circular honeycombs (CH type) were increased by 37.15%, 38.18%, and 47.06%, respectively. This study provided a series of experimental and numerical results, which could provide a reference for selecting suitable honeycomb configurations in the protection field.

Binders are known to influence the different steps in liquid composite molding process chains. Most reseach focuses on powder binder and veils. Spray binders are rarely studied. Therefore, the effect of an epoxy spray binder on the infusion and cure steps of liquid composite molding processes is studied in this paper. Permeability measurements, solubility tests and measurements of glass transition temperature, resin cure time and resin viscosity show a complex interaction between binder and resin depending on the process conditions. The binder mostly increased the permeability of the tested preforms. This effect increases with increasing binder loading. At low fiber volume fractions the binder had a positive effect on preform permeability, most likely by to delaying the closure of macro flow channels or textile inhomogeneity. The binder did not dissolve in the resin until temperatures of around 120 °C, while remaining as a separate phase at 22 °C, 40 and 80 °C. This has to be considered when defining the cure profile of the resin, as the binder is expected to participate in the cure reaction. The glass transition temperature remained unchanged for the binder-resin combination used. An increase in viscosity by a factor of 1.5-2 was observed when the binder was dissolved in the resin. Samples with dissolved binder cured slightly faster than pure resin. The experimental results were theoretically transferred to real infusion processes.

Binder/tackifier materials are commonly used in preforming processes to preserve the structural integrity of the preform during processing. In the following resin infusion or injection process, this additional material will influence the resin flow. While the influence on fabric permeability is thoroughly examined in scientific literature, only few studies investigate the capillary behavior. By thermal activation of the binder, the material melts and spreads across the layer’s surface or is imbibed by the rovings.

In this study, capillary rise experiments in planar direction with four different carbon fiber fabrics were performed. The tested stacks were activated at different temperature levels and compressed in a vacuum bag, one of them with additional external pressure in an autoclave. In case of no external pressure, the processing and testing conditions showed a larger influence than binder activation temperature, while autoclave-conditioned specimens showed a decreased capillary rise velocity for all levels of activation temperature. Digital microscopy images of the specimens show that molten binder can create a thin film between the layers, which prevents peripheral flow and thus forces the fluid to rise in the (angulated) capillary tubes inside the rovings.