Ciprian Ionuț Morăraș

This study investigates the mechanical and thermomechanical response of quartz-fibre-reinforced polymer (QFRP) sandwich structures incorporating additively manufactured gyroid cores. Three core variants were considered under the same nominal gyroid topology and 28% infill setting: chopped-carbon-fibre-reinforced polyphthalamide (PPA-CF), chopped-carbon-fibre-reinforced ONYX, and ONYX incorporating additional continuous carbon-fibre reinforcement (ONYX-R). Identical QFRP face sheets manufactured from woven quartz-fibre/cyanate-ester prepreg were co-cured directly onto the printed cores without a separate adhesive film. The resulting sandwich structures were characterized by flatwise compression, three-point bending, and dynamic mechanical analysis (DMA). Under flatwise compression, QFRP-ONYX-R exhibited the highest yield strength, 12.45 ± 0.16 MPa, followed by QFRP-PPA-CF at 11.70 ± 0.10 MPa, whereas QFRP-ONYX yielded at 2.00 ± 0.09 MPa. QFRP-PPA-CF exhibited the highest nominal maximum bending stress (131.55 ± 3.50 MPa) and apparent bending modulus (7.32 ± 0.07 GPa), while QFRP-ONYX-R showed an intermediate flexural response and QFRP-ONYX exhibited the greatest deformation capacity before break. DMA revealed a strong dependence of the apparent structural response on core configuration. QFRP-PPA-CF exhibited the highest storage modulus, approximately 8.4 GPa at 30 °C, and retained approximately 1.9–2.0 GPa at 160 °C. QFRP-ONYX-R displayed intermediate stiffness but the highest principal tan δ response, with peaks of 0.135–0.150 at approximately 97–100 °C, whereas QFRP-ONYX exhibited the lowest storage and loss moduli. Overall, the results show that, under a nominally similar gyroid architecture, core material and reinforcement strategy strongly modify compressive yielding, bending behaviour, apparent thermomechanical stiffness, and damping in QFRP-faced sandwich structures.

This study experimentally evaluates how various 3D-printed gyroid core materials influence the mechanical and thermo-mechanical performance of lightweight carbon-fiber-reinforced polymer (CFRP) sandwich structures. Three gyroid core configurations were considered: short-carbon-fiber-reinforced polyphthalamide (PPA-CF), continuous-carbon-fiber-reinforced nylon (ONYX-R), and nylon short-carbon-fiber-reinforced (ONYX), all combined with similar 0/90° carbon-fiber-reinforced polymer composite (CFRP) face sheets. The sandwich specimens were manufactured using a one-step co-curing process and characterized by flatwise compression, three-point bending, and dynamic mechanical analysis (DMA). The PPA-CF-CFRP sandwich configuration exhibited the highest flexural performance, with a maximum flexural stress of 148–156 MPa, approximately 39% higher than that of ONYX-R-CFRP, and the highest mean compression modulus of approximately 87 MPa. However, ONYX-R-CFRP achieved the highest mean compressive strength of 12.84 MPa, compared with 11.85 MPa for PPA-CF-CFRP. These findings indicate that the mechanical response depends on the loading mode and is influenced by the combined effects of the thermoplastic matrix, carbon-fiber reinforcement architecture, and the resulting structural characteristics of the printed gyroid cores. DMA revealed that the PPA-CF-CFRP sandwich achieved a storage modulus of ~9 GPa at 30 °C, outperforming the ONYX-R-CFRP configuration by approximately 70%. Conversely, ONYX-R-CFRP exhibited the highest damping capacity, with a maximum tan δ of approximately 0.16, about 60% higher than the PPA-CF-CFRP configuration. The ONYX-CFRP sandwich configuration displayed the lowest mechanical and thermo-mechanical performance, demonstrating that the stiffness, deformation capacity, damping response, and temperature-dependent mechanical behavior of these sandwich structures can be tailored through the selection of the additively manufactured core configuration.

Valued for their lightweight, high strength-to-weight ratio, and energy absorption, sandwich composites were adopted across multiple industries. This paper evaluates the static and dynamic mechanical performances of sandwich configurations exploiting carbon fiber-reinforced polymer (CFRP) and quartz fiber-reinforced polymer (QFRP) composite skins bonded to hexagonal Nomex^® or compliant Flex-Core^® cores. Skin thickness varied negligibly; therefore, this study focused on skin type and particularly core architecture influence on mechanical behavior. Three-point bending and flatwise compression tests evaluated flexural stiffness, core shear strength, and core compressive strength. DMA analysis was used to characterize their temperature-dependent viscoelastic response through the storage modulus, loss modulus, and damping factor. QFRP/Nomex emerged as the optimal configuration achieving the highest flexural stress, outperforming CFRP/Nomex by 14%, while restricting strain to 2.4% (compared to 7% for CFRP). DMA (Dynamic Mechanical Analyzer) analysis showed that QFRP/Nomex exhibits the highest storage and loss moduli. Observing the energy at break, CFRP/Nomex^® sandwiches stand out in their ability to absorb 60% more energy than QFRP/Nomex^® before total failure occurs, showing an overall superior energy absorption of CFRP skins. Conversely, flatwise compression tests revealed that QFRP/Flex-Core^® excelled in yield and compressive strengths, outperforming CFRP/Nomex by 10% and 8%, respectively, due to superior elastic matching. DMA damping profiles confirmed that the geometric compliance of curved Flex-Core^® cell walls in conjunction with QFRP skins accelerates structural yielding under shear and viscoelastic energy dissipation prior to chemical softening. This work highlights that sandwich structure design critically depends on managing core architecture and skin-to-core stiffness mismatch.

The present study investigates the combined influence of manufacturing route and material configuration on the mechanical, elastic, viscoelastic, and impact behavior of Kevlar–carbon hybrid laminates. Three eight-ply laminate configurations (V1, V2, and V3) were manufactured through distinct technological routes: fully prepreg-based hot pressing, Kevlar-prepreg/dry-carbon hand lay-up followed by vacuum curing, and multi-stage hybrid consolidation combining repeated hot pressing with subsequent vacuum curing. The experimental characterization included tensile tests according to ASTM D3039, compression tests according to ASTM D695, determination of Young’s modulus from extensometer measurements and Poisson’s ratio using strain-gauge instrumentation, dynamic mechanical analysis (DMA), and low-velocity impact tests under controlled energy conditions. The novelty of this work consists in the integrated process–configuration–property comparison of these Kevlar–carbon hybrid routes within the same experimental framework, rather than in a generic demonstration that manufacturing affects composite laminates. The V1 laminate exhibited the highest strength-related performance, reaching an average tensile strength of 335.88 MPa and a compressive strength of 165.85 MPa, and it also showed the highest DMA storage modulus at 30 °C, E’ = 53.42 GPa. The V2 laminate presented lower tensile performance but the most pronounced damping response, with the highest tanδ peak value. The Young’s modulus determined from the extensometer measurements was 29.26 ± 1.45 GPa for V1, 26.10 ± 0.22 GPa for V2, and 29.52 ± 1.27 GPa for V3, indicating comparable longitudinal stiffness for the V1 and V3 laminates. The results indicate that the measured behavior is governed by the combined effects of reinforcement form, matrix/resin arrangement, consolidation route, and laminate architecture. Direct quantification of laminate compaction, fiber volume fraction, and void content was outside the scope of the present experimental campaign and is identified as a necessary step for future validation.

Most components fail under complex states of stress and for this reason the study of materials failure under these conditions is an important topic. The article presents the experimental study of the failure of a CFRP material, with a 0/90° cross-ply configuration, subjected to both simple loading conditions (tension, compression, and shear) and combined loading (tension–shear), using a modified Arcan testing method. The Arcan device and specimen geometry were redesigned to reduce experimental errors and the dispersion of results. It was found that there are significant differences between the strength values obtained for simple loads performed by the standardized methods and by the Arcan method, respectively. For this reason, it is recommended to use the Arcan method only for mixed loading modes. Specimens with steel tabs were used to reduce both hole ovality during testing and the number of clamping screws to only four. It was found that the experimental results under complex stress states are well described by the Tsai–Hill failure criterion and the failure envelope for the material studied was plotted. Recommendations are provided regarding the appropriate use of the Arcan method in order to obtain precise results for CFRP composites under multiaxial loading.

Glass fiber-reinforced polymer (GFRP) composites are widely used in structural applications due to their high specific strength and durability; however, their mechanical performance strongly depends on fiber architecture and environmental exposure. This study evaluates the mechanical behavior and moisture-induced degradation of GFRP laminates through tensile tests, impact tests, dynamic mechanical analysis (DMA), and thermomechanical analysis (TMA) performed on a bi-directional glass–epoxy GFRP laminate ([0°/90°]). Tensile tests revealed a maximum longitudinal strength of 369 MPa in dry specimens, while water immersion for up to 21 days led to a significant reduction in tensile strength, from 207 MPa to 63 MPa, in diagonally cut specimens. Impact tests conducted at 12 J showed larger displacements in specimens cut along directions not aligned with the fibers, indicating matrix-dominated behavior. Dynamic mechanical analysis demonstrated strong dependence of stiffness on fiber orientation, with storage modulus values decreasing by approximately 45% in 45° specimens compared with the principal directions, while the glass transition temperature remained within 59–62 °C. Thermomechanical analysis confirmed an increase in the coefficient of thermal expansion after aging, from 205.6 to 291.65 µm/(m·°C) below Tg. These results provide insights into the structure–property–environment relationships governing the durability of GFRP composites and support the optimization of their design for long-term polymer-based applications.

This study investigates the mechanical behavior of various plastic materials through tensile and scratch testing. Three polypropylene-based composites—PP-GB30GF10, PP-TD40, and PP-GF20—were subjected to uniaxial tensile tests in accordance with standard protocols to assess their strength, stiffness, and elongation characteristics. The results highlight notable differences in the tensile performance depending on the type and percentage of reinforcing fillers, such as glass fibers and talc. In parallel, the scratch resistance was evaluated for specimens produced via stereolithography (SLA) using Formlabs Black V4 resin, a common photopolymer used in prototyping applications. The scratch test aimed to characterize the surface durability under localized mechanical stress. The findings contribute to a better understanding of the mechanical performance of these materials and their potential applications in fields requiring both structural integrity and surface resilience, such as automotive components and functional prototyping.

Glass fiber reinforced (GFRP) polymer composites have been prepared by various manufacturing technologies and are widely used for various applications. Fiber glass composites possess good properties such as high strength, flexibility, rigidity, durability, etc. Compared to composite materials reinforced with unidirectional distributed fibers, composites reinforced with fabric have the advantage of the balance achieved between the mechanical and elastic properties in the 0° and 90° directions, although in one of the directions the modulus of elasticity and tensile strength are lower, due to the presence of fiber undulations when reinforcing with fabrics. In this work, a comparison is made between the elastic characteristics of GFRP, determined by tension and by 4 - point bending test.

Compressive testing of specimens taken from relatively thin composite plates is difficult, especially due to the occurrence of buckling. To prevent buckling, the central portion of the specimens used for the compression test has smaller dimensions, and the specimens can be guided along their entire length. For these reasons, optical methods, such as digital image correlation (DIC), cannot be used for the compression test and strain rosettes cannot be glued onto the samples to determine Poisson’s ratio. In this study, compression tests of a glass fiber-reinforced polymer (GFRP) were conducted using both the ASTM D695 (Boeing version) and a newly proposed method. The new method involves using special specimens that allow T-type rosettes to be bonded to determine Poisson’s ratio, whose value of 0.14 was thus determined. SEM images of the failure surfaces were presented and interpreted. A finite element analysis (FEA) of the specimens tested in compression is also presented. The first analyzed case considers the homogeneous and orthotropic composite, loaded with a uniformly distributed force. The normal stress in the central section of the specimen, determined with FEA, has an error of 6.52% compared to that determined experimentally. Additionally, the strain in the center of the strain gauge, determined with FEA, has an error of 4.76% compared to the measured one. In the second case studied with FEA, the sample is loaded with a quasi-concentrated force, which can move in the direction of the symmetry axes of the cross-section, to study the effect of the eccentricity of the compression force on the state of stress. It was shown that the eccentricity of the force has a great influence: the stress distribution in the section of the specimen becomes strongly non-uniform. For a force eccentricity of 0.4 mm in the direction of the OX axis, the minimum stress decreases by 53.7%, and the maximum stress increases by 55.4%. In order to analyze the influence of some manufacturing defects, two other cases were analyzed by FEA, in which it was assumed that the thicknesses of the outer resin layers were modified, making them asymmetrical. For this final FEA, the specimen was considered to be composed of laminates. These results demonstrate the special attention that must be paid to the centric application of force in compression testing.