Applied Composite Materials最新文献

This paper investigates the effect of exposure of glass fiber reinforced plastic (GFRP) laminates with epoxy resins to air and a synthetic poly-alpha-olefin (PAO) oil at 180 °C for 500 h (h) and 1000 h. The test is intended for prequalifying material systems for the intended use of the laminates as a composite seal of a stator lamination to enable a directly cooled electric motor. Three commercially available amin curing and one non-commercial anhydride curing epoxy resin systems are investigated. The results of the mechanical testing indicate post curing effects for the epoxy resins during exposure. These translate to higher moduli in the laminates and for most systems the strength rises as well. These effects dominate the change in mechanical properties for the commercial systems making these systems well suited for the intended use case. However, for one commercial and the noncommercial system, some properties drop after exposure to cooling fluid (CF) e.g. in the transverse tensile test. Here a decrease of the fiber-matrix adhesion is the identified cause. This in turn is caused by a moderate laminate quality which is evident by visible imperfections of the laminate. Thermal properties are mostly affected by the chosen temperature and time of exposure, not so much by the ambient medium. From our findings we conclude that the exposure to PAO oils of the chosen systems is not as severe as the findings from other authors suggest. Therefore, the investigated laminates are prequalified for the intended use. In addition, through the successful subsequent use of our results, we find, that the identified test is suitable for prequalifying GFRP for the use in directly cooled electric motors.

Graphical Abstract

Carbon fiber reinforced parts are utilized by the transportation industry due to being strong yet lightweight. The highest quality parts are made using carbon fiber (CF) pre-impregnated with resin processed in an autoclave to reduce void content. The autoclave process is expensive and takes up a large amount of space, so resin transfer molding (RTM), a form of liquid composite molding (LCM), is a less expensive alternative. The parts produced using RTM can be manufactured to rival the mechanical performance of parts made with pre-preg in an autoclave, but concerns remain over voids. Most research pertaining to void formation has been done ex-situ by either micro-CT or ultrasound, but valuable information can be obtained by monitoring void formation in-situ using a UV-sensitive dye mixed with resin. The objective of this research is to validate in-situ void formation observation methods for more complex fiber reinforcements beyond unidirectional. While previous work has established the validity of such observation for unidirectional fiber samples, this research focuses on preformed ply stacks alternating between 0 and 90 degrees.

The incremental hole drilling method is widely used to determine residual stresses in materials. For fiber-reinforced composites, the drilling-induced heat can create measurement errors that are not taken into account in the calculations. Even if the drilling parameters (cutting and feed speeds, type of cutter, etc.) are optimized to provide correct machining results and more reliable residual stress measurement, thermal effects remain relatively poorly understood and analyzed for fiber-reinforced composites. The purpose of this work is to investigate the effects of drilling-induced heat on the local chemical properties of unidirectional carbon/epoxy laminates that have undergone different curing cycles and to discuss the consequences on residual stresses. First, the thermal field generated by drilling was characterized using an infra-red camera. The measurements showed that relatively high temperatures were reached depending on the increment depth. Then, Modulated Differential Scanning Calorimetry before and after drilling were performed to investigate the effects of the produced heat on the chemical properties. The results showed a local increase in the degree of cure and the glass transition temperature (Tg) of the matrix. It is assumed that these mechanisms are responsible for the apparent residual stresses obtained when large increment depths are used during the process. The main conclusion of this study is that the heat generated during the drilling step of the incremental hole drilling method causes a progression of the cure reaction in the vicinity of the hole, leading to Tg-dependent residual stresses which are higher for low-Tg composites.

This study focused on steeple-triggered hat-shaped composite structures (HSCS), conducting crashworthiness design and parametric analysis based on stacking sequence design. A finite element model was developed based on the nonlinear progressive damage theory, and the traction–separation model was adopted to characterize inter-laminar damage behavior. Three lamination schemes were proposed, namely single, antisymmetric, and hybrid angle stacking (Sas, Aas, and Has), aiming to explore the influence law of material stacking sequence on structural crashworthiness. Meanwhile, the mechanism of the effect of loading angle on structural crushing modes was systematically investigated. This study reveals the intrinsic correlations between stacking sequence, loading angle, and structural crashworthiness. The results show that the stacking angle has a significant impact on structural crashworthiness; within a specific range of lamination angles, structural crashworthiness tends to improve with the increase of angle. Notably, the structure with the stacking sequence of [30°, -30°]_2s exhibits the optimal specific energy absorption (SEA) and peak load.

This study investigates the effect of z-pin through-thickness reinforcement on the Mode I interlaminar fracture toughness of composite laminates. Initially, experimental test data were used to validate multiscale Finite Element Analysis (FEA) models developed to simulate Double Cantilever Beam (DCB) mechanical evaluation of z-pin reinforced composites. The validated models were then employed to explore the enhancement in interlaminar toughness associated with variations in z-pin diameter and areal density. Results indicate that z-pins substantially improve fracture toughness, with smaller diameter z-pins and higher areal densities yielding the greatest enhancements as to be expected. This improvement is attributed to a greater number of active z-pins bridging the crack front and an expanded interfacial surface area. Furthermore, the study finds that the z-pin layout pattern exerts minimal influence on interlaminar performance, with improvements primarily driven by optimizing pin size and density. These findings provide theoretical support for the optimization of z-pinning techniques and their application in advanced composite structures.

This study examines the flexural performance and damage evolution in multiscale graphene nanoplatelet-reinforced fiber metal laminates (FMLs). A multiscale modeling framework was developed to analyze the material’s mechanical response. At the mesoscale, a periodic representative volume element (RVE) approach was applied to carbon fiber/epoxy hybrid regions, enabling the calculation of homogenized mechanical properties for these composite layers. At the macroscale, simulations replicated the FMLs’ layered architecture through a unified framework integrating three material behavior laws: the Johnson-Cook model to describe aluminum sheet plasticity and damage, the three-dimensional Hashin criterion to predict failure in carbon fiber/epoxy layers, and a cohesive zone model to simulate interfacial debonding between the metal and composite phases. Experimental investigations employed quasi-static three-point bending tests to evaluate deformation and failure mechanisms in autoclave-processed FMLs reinforced with graphene. Finite element simulations corroborated experimental observations, elucidating stress distribution and progressive damage across material layers through force-displacement curves and damage propagation maps. Microscopic analysis via scanning electron microscopy (SEM) further revealed the role of graphene nanoplatelets in enhancing resin-aluminum interfacial adhesion and mitigating delamination. The proposed method is also applicable to the modification of FMLs using other types of nanofillers.

Internal thermal expansion molding is a cost-effective composite fabrication technique that enables the integrated formation of complex, enclosed structures. In sandwich structures, interfacial delamination critically affects bearing capacity, making interface performance a key concern. Thus, this study primarily focuses on integrated multilayer sandwich structures to investigate the mechanism of mode I interfacial fracture toughness. The research findings firstly demonstrate that the expansion ratio (ER) systematically influences various parameters, including compliance, delamination length, fracture propagation mode, and final loading. The interface between carbon fiber reinforced polymer (CFRP) and thermal expansion foam (TEF) (C-T interface) exhibits an initial fracture toughness of ~ 0.53 kJ/m², which exceeds the TEF itself by ~ 235.7%. Similarly, the interface between polymethacrylimide (PMI) and TEF (P-T interface) achieves a synergistic enhancement, with interfacial toughness surpassing that of the individual base materials. These improvements are attributed to co-curing-induced bonding, increased interfacial contact, reduced thermal stress, the formation of mechanical interlocks, and mitigation of stress concentrations. The results demonstrate that the integrated multilayer sandwich composite structures fabricated via the internal thermal expansion technique not only enable efficient, integrated manufacturing, but also achieve superior interfacial properties relative to traditional three-layer structures.

Double-Double (DD) laminates have attracted growing interest because of their design flexibility and the potential to reduce ply count while maintaining stiffness; however, their performance under repeated impacts and the ensuing residual compressive strength (RCS) remain insufficiently understood. In this work, a DD laminate with the stacking sequence (:{left[52/-32/-52/32right]}_{4T}) was configured to match the bending stiffness of a Quadriaxial (Quad) laminate, (:{left[{-45}_{2}/{45}_{2}/{0}_{2}/{90}_{2}right]}_{s}). A finite element framework—previously validated against low-velocity impact (LVI) experiments—was employed to simulate two successive same-site impacts with energies of 5, 15, and 25 J, followed by compression after impact (CAI) loading to determine RCS. Results show that the DD laminate better preserves global structural integrity; however, once damage is initiated, its severity exceeds that of the Quad laminate at the same impact energy. Under otherwise identical conditions, the delamination damage projected area (DDPA) in the DD laminate is 30.01–53.45% smaller than that of the Quad, and during the second impact, the delamination preferentially propagates through the thickness rather than in-plane. CAI analyses further indicate that the DD laminate accumulates less matrix compressive damage than the Quad. For the pristine DD laminate, compressive failure is governed by the combined action of delamination initiated under load and fiber compression damage, whereas in the impacted DD laminate it is dominated by delamination. The second impact does not alter the failure mode. Under the same conditions, the DD laminate achieves an RCS 6.17–23.80% higher than that of the Quad, and the second impact reduces RCS by less than 10%. These findings provide a reference for evaluating secondary impact response and RCS of DD laminates.

This study presents a novel methodology for the multi-objective parametric optimization of a non-symmetrical filament-wound composite vessel subjected to thermo-mechanical loading. Due to the vessel’s unequal polar openings, a nonuniform fiber angle distribution is required along the cylindrical section. The proposed approach incorporates both geodesic and non-geodesic fiber paths, accommodating the asymmetry in polar openings through cross-linear modeling of the helical layers across the domes and the cylindrical region. To achieve an innovative and weight-efficient design, a multi-objective optimization problem was formulated. The optimal radii of the polar openings and the composite layer thicknesses were determined to enhance the vessel’s thermomechanical performance. Thermal loading, particularly at high operating temperatures, is identified as a critical factor due to its potential to induce structural degradation. A specialized iterative algorithm was developed to perform the optimization, integrating an improved thermomechanical module, an optimization module, and a custom Python script. The Multi-Island Genetic Algorithm (MIGA) was employed to solve the optimization problem. Test cases were evaluated using two commercially available composite materials. Compared to purely mechanical models, the thermo-mechanical model can optimize material usage by approximately 10%.

This study investigates the mechanical properties and failure characteristics of CFRPs fabricated with thermoplastic resins—polypropylene (PP) and polyamide 6 (PA6)—through a newly proposed experimental approach, in which the failure process was systematically examined from initial loading to the final failure mode. The CFRPs, composed of carbon fibers embedded in the respective resin matrices, were produced under varying molding pressures. The mechanical properties of CFRP-PP and CFRP-PA6 show clear distinctions. The bending strength of CFRP-PA6 reached approximately 1,200 MPa, which is about 20% higher than that of CFRP-PP (1,000 MPa). Moreover, their post-peak responses exhibited distinct differences. At the ultimate bending strength point (σ_b), the bending stress experienced a sharp drop (σ_d), followed by a recovery phase that varied depending on the resin type. Specifically, CFRP-PA6 demonstrated a noticeable stress recovery, whereas CFRP-PP showed a continued decline. Moreover, CFRPs molded under higher pressures exhibited more pronounced stress recovery in both resin systems. SEM analyses revealed distinct failure mechanisms. At the σ_b point, CFRP-PA6 showed no clear evidence of failure, while CFRP-PP displayed a combination of fiber breakage and delamination. At the σ_d and the final fracture points, CFRP-PA6 exhibited relatively weak failure, primarily characterized by fiber breakage and localized strain distortion, whereas CFRP-PP suffered severe fiber breakage and extensive delamination. These differences in failure modes are attributed to variations in interfacial bonding strength between the carbon fibers and the resin matrices. Bonding tests confirmed that CFRP-PA6 possesses superior adhesion and wettability to carbon fibers compared to CFRP-PP.