Search Results (228)

Delamination remains a critical limitation in the structural application of wood, particularly in adhesively bonded joints. This study investigates the use of a cyclic olefin-based hot-melt film adhesive (Zeon^® LS-XU) as a thermoplastic interlayer as a means to delay delamination and enhance joint performance. Single lap joints (SLJs) were tested under quasi-static (1 mm/min) and impact (3 m/s) loading to assess strain-rate effects. Six configurations were examined: two reference, two toughened (with an additional 15 mm of adhesive on each overlap side) and two hybrid configurations combining oak (Quercus alba) and pine (Pinus pinaster Aiton) substrates to improve stress wave propagation. A finite element elastic model was developed to analyse stress distributions and explain the superior performance of hybrid joints. Results revealed that the thermoplastic interlayer delayed delamination onset and increased energy absorption, while hybrid configurations achieved more uniform stress distributions and significantly higher strengths under dynamic loading. The most effective configuration, the hybrid joint under impact conditions, represents a strength improvement of approximately 84% of the peak load compared to the pine reference joints. Overall, introducing a thermoplastic interlayer offers an efficient and lightweight strategy to enhance the toughness and reliability of wood joints exposed to variable loading conditions. Full article

Laser surface microtexturing has emerged as an effective approach for improving the performance of adhesive joints between dissimilar materials. In this study, the influence of laser-generated micrometric surface features on the mechanical behavior of hybrid adhesive joints was investigated for two material systems: structural steel bonded to polyamide (PA66) and structural steel bonded to technical ceramic (Al₂O₃). Single-lap joints were manufactured using a two-component epoxy adhesive with two nominal bond-line thicknesses (0.1 mm and 1.0 mm). Prior to bonding, selected surfaces were modified by ultrashort-pulse laser microtexturing, producing well-defined circular features with characteristic depths on the order of tens of micrometers. The resulting microstructures were characterized using optical and scanning electron microscopy, and their geometric parameters were quantified through profilometric measurements. Mechanical performance was evaluated under shear and bending loading conditions. The results demonstrate a substantial increase in joint strength for laser-microtextured surfaces compared with non-textured references for both material combinations. The effect of surface microtexturing was more pronounced than the influence of adhesive layer thickness within the investigated range. These findings confirm that laser-induced surface microtexturing is a versatile and application-oriented surface preparation method capable of enhancing the reliability of adhesive bonding in hybrid metal–polymer and metal–ceramic assemblies. Full article

The use of carbon fiber-reinforced plastic (CFRP) components has increased significantly in civilian aviation, necessitating effective maintenance and repair strategies to ensure durability and performance. While prior studies have focused on composite repair methods, such as stepped scarf patch and bolted joint repairs, these were limited to specimen and panel levels without addressing full-scale wing models. This study bridges that gap by evaluating stepped-lap repairs on a full-scale composite wing model under realistic loading conditions and exploring various repair scenarios. To reduce computational cost, two-dimensional shell elements were employed to simulate repairs, with results validated using experimental tensile test data from stepped-lap repaired specimens. Numerical models were developed for single regions and two closely located repair regions. For single-region repairs, adding up to two extra layers enhanced mechanical strength, but three extra layers increased strain, diminishing performance. For two closely located repairs, additional layers improved strength, though less effectively than single-region repairs. Square-shaped repairs exhibited higher strain due to stress concentrations at the corners, while circular repairs showed more uniform stress and strain distribution. These findings emphasize the importance of optimizing repair geometry and layer configurations using numerical simulations to ensure optimal structural performance of CFRP components. Full article

The aim of this study is to experimentally evaluate the damage mechanisms occurring in the adhesive-bonded regions of glass fiber-reinforced polymer (GFRP) and carbon fiber-reinforced polymer (CFRP) composites, which are widely used in marine and offshore wind turbine applications, under environmental conditions. In particular, this study focuses on the degradation caused by long-term seawater exposure and its effects on the bending behavior and load-carrying capacity of adhesive joints. For this purpose, the specimens were prepared in accordance with ASTM D5868-01, using 7-layer GFRP and 8-layer CFRP laminates. Single-lap adhesive joints were fabricated. To simulate marine environmental conditions, the single-lap adhesive joints were immersed in natural seawater obtained from the Aegean Sea (22 °C temperature and 3.3–3.7% salinity) for 1, 2, and 3 months in separate containers. Three-point bending (3PB) tests were performed on specimens representing marine applications, while four-point bending (4PB) tests were conducted on specimens representing offshore wind turbine blade structures. The results quantitatively revealed the influence of seawater on adhesive-bonded composite joints. In 3PB tests, the reductions in the Young’s modulus of GFRP specimens after 1, 2, and 3 months of exposure were measured as 5.94%, 8.90%, and 12.98%, respectively. For CFRP specimens, degradation was more limited, with corresponding reductions of 1.28%, 3.39%, and 3.74%. A similar trend was observed in 4PB tests representing offshore wind turbine applications, where GFRP joints exhibited modulus reductions of 3.15%, 6.42%, and 9.45%, while CFRP joints showed reductions of 1.29%, 2.62%, and 3.48% for the same exposure durations. Overall, the findings demonstrate that CFRP composites exhibit more stable mechanical behavior under environmental exposure, whereas GFRP structures undergo more pronounced performance losses, particularly in moisture- and salt-rich environments. These results highlight the critical importance of material selection for long-term durability in offshore composite structures. The outcomes of this study contribute to a better understanding of the damage processes occurring in composite adhesive joints under environmental conditions and provide a scientific basis for developing more reliable design and material selection strategies in both the marine and wind energy sectors. Full article

The search for innovative solutions in the field of construction materials used in aircraft manufacturing has led to the development of composite materials, particularly CFRP polymer composites. Composite airframe components, which are required to have high strength, are joined using mechanical fasteners. Considering that the composite consists of a polymer matrix, which is a material susceptible to rheological phenomena occurring rapidly at elevated temperature, there is a high probability of significant changes in the strength and performance properties. Coupled thermal and mechanical loads on composite material joints occur in everyday aircraft operation. Experimental tests were conducted using a quasi-isotropic CFRP on an epoxy resin matrix with aerospace certification. The assessment of changes in the strength parameters of the material itself showed a decrease of approx. 40% in its short-term strength at 80 °C compared to the ambient temperature and a decrease in the load-bearing capacity of single-lap bolted joints of over 25%. Even more rapid changes were observed when assessing the fatigue life of the joints assessed at ambient and elevated temperature. In addition, the actual glass transition temperature of the resin was determined using the DSC technique. Analysis of the damage mechanisms showed that at 80 °C, the main degradation mechanisms of the material are accelerated creep processes of the CFRP and softening of the matrix, increasing its susceptibility to damage in the joint area. Full article

Additive manufacturing via LCD vat photopolymerisation enables direct bonding of photopolymer to textile substrates, but optimal processing parameters remain unclear. A 3 × 3 factorial design investigated the effects of layer thickness (0.01, 0.025, 0.05 mm) and UV exposure time (40, 80, 120 s) on the single-lap shear strength of woven fabric-photopolymer joints (65% polyester/35% cotton) using a novel pause-and-bond methodology, following the EN ISO 4587:2003 standard. Five replicate specimens per condition yielded 45 samples for mechanical testing. All specimens (45/45) exhibited adherend-limited failure within the textile substrate rather than at the polymer-textile interface, yielding consistent shear strengths of 1.38 ± 0.04 MPa (range: 1.30–1.45 MPa). Two-way ANOVA revealed no significant parametric effects (p > 0.05), indicating that interfacial bond strength consistently exceeded textile cohesive strength across all parameter combinations. The minimum resource-efficient condition (0.01 mm/40 s) achieves equivalent performance to higher-parameter combinations, enabling substantial process optimisation for textile-integrated photopolymer sandwich structures while reducing material and processing time requirements. Full article

Adhesively bonded joints are extensively utilized in structural assemblies involving metals, composites, and hybrid materials due to their favorable mechanical and manufacturing characteristics. However, their performance under high-velocity impacts—common in aerospace, automotive, and defense applications—remains insufficiently understood. This work investigates the high-velocity performance and subsequent tensile response of adhesively bonded single-lap joints (SLJs) by integrating experimental testing with numerical simulations. High-velocity impacts were applied to SLJs fabricated from 4 mm aluminum adherends with overlap lengths of 15 mm and 25 mm, using a 1.25 g projectile at 288 m/s, followed by quasi-static tensile assessment. Experimental findings revealed substantial degradation in tensile strength for the 15 mm overlap configuration (reduced the load-bearing capacity by about 33% (from ~12 kN to ~8 kN)), while the 25 mm overlap retained its structural integrity. Finite element simulations conducted in ABAQUS 2021 employed the Johnson–Cook constitutive model for the adherends and a cohesive zone model for the adhesive layer, successfully replicating damage evolution and stress distributions. The results highlight the critical role of geometric parameters—particularly overlap length and adherend thickness—in determining the damage tolerance and residual load-bearing capacity of SLJs subjected to high-velocity impacts. These insights contribute to the development of more robust bonded joint designs for impact-prone environments. Full article

Additive manufacturing technologies are no longer limited to rapid prototyping but are increasingly used for low-volume production of functional end-use components. Among advanced AM techniques, HP Multi-Jet Fusion (MJF) stands out for its high precision and efficiency. Polyamides, thanks to their balanced mechanical and thermal properties, are commonly used as building materials in this technology. However, these materials are notoriously difficult to bond with conventional adhesives. This study investigates the shear strength of bonded joints made from two frequently used MJF materials—PA12 and glass-bead-filled PA12—using four different industrial adhesives. Experimental procedures were conducted according to ASTM standards. Specimens for single-lap-shear tests were fabricated on an HP MJF 4200 series printer, bonded using a custom jig, and tested on a Zwick-Roell Z250 electro-mechanical testing machine. Surface roughness of the adherends was measured with a 3D optical microscope to assess its influence on bonding performance. The polyurethane-based adhesive (3M Scotch-Weld DP620NS) demonstrated superior performance with maximum shear strengths of 5.0 ± 0.35 MPa for PA12 and 4.4 ± 0.03 MPa for PA12GB, representing 30% and 17% higher strength, respectively, compared to epoxy-based alternatives. The hybrid cyanoacrylate–epoxy adhesive (Loctite HY4090) was the only system showing improved performance with glass-bead-reinforced substrate (16.5% increase from PA12 to PA12GB). Statistical analysis confirmed significant differences between adhesive types (F_3,24 = 31.37, p < 0.001), with adhesive selection accounting for 65.7% of total performance variance. In addition to the experimental work, a finite element-based numerical simulation was performed to analyze the distribution of shear and peel stresses across the adhesive layer using Siemens Simcenter 3D 2406 software with the NX Nastran solver. The numerical results were compared with analytical predictions from the Volkersen and Goland–Reissner models. Full article

Adhesively bonded joints between galvanized steel and carbon fiber-reinforced polymers (CFRPs) are critical in modern lightweight structures, but their performance is often limited by failure at the zinc–adhesive interface. This study presents a parametric analysis to investigate the influence of key geometric parameters on interfacial cracking in a single-lap joint (SLJ) configuration, employing a simplified analytical methodology based on Interface Fracture Mechanics (IFM). The model combines the Goland–Reissner approach for estimating crack-tip loads with highly simplified, constant shape functions to calculate the energy release rate ($G_{int}$) and phase angle ($\psi$). To provide a practical reference, experimental data from shear tests on S350 GD galvanized steel bonded to CFRP were used to estimate the range of interfacial fracture toughness for this material system. The parametric results demonstrate that, for a constant load, increasing the overlap length reduces the crack driving force ($G_{int}$), while increasing the adhesive thickness raises it. Crucially, the model indicates that a thicker adhesive layer shifts the fracture mode from shear- to opening-dominated, a trend consistent with the established mechanics of SLJs, where increased joint rotation amplifies peel stresses. The study concludes that while the use of constant shape functions limits the model’s quantitative accuracy, this simplified analytical framework effectively captures the qualitative influence of key geometric parameters on the joint’s fracture behavior. It serves as a valuable and resource-efficient tool for preliminary design explorations and for interpreting experimentally observed failure trends in galvanized steel–CFRP joints. Full article

Friction stir-based joining techniques offer a promising route for the integration of highly dissimilar materials into single structures, with potential applications in safety-critical sectors such as hydrogen storage and lightweight mobility systems. Ensuring structural integrity under dynamic loading is crucial for their industrial adoption, particularly given the strong inhomogeneity of metal–polymer interfaces. This study investigates the strain rate sensitivity of lap joints between an AA6082-T6 aluminum alloy, and a glass-fiber-reinforced polymer (Noryl™ GFN2) produced using a friction stir process. Quasi-static and intermediate strain rate (≈3 s⁻¹) tensile tests were performed on the joints, while both base materials were additionally characterized at quasi-static, and intermediate strain rate conditions using a custom accelerated electromechanical testing device. Digital image correlation was employed to monitor deformation. The results reveal that the joints exhibit clear strain rate sensitivity, with ultimate remote stress and bending angle stiffness increasing by approximately 30% and 23%, respectively, from quasi-static to intermediate strain rate loading. Fracture consistently initiated in the polymer, indicating that the joints mechanical performance is limited by the polymeric constituent, although the polymer strain rate hardening impacts the peel/shear mix in the loading scenario of intermediate strain rate loading. Overall, the findings highlight that while friction stir metal–polymer joints benefit from strain rate hardening, their performance envelope remains governed by the polymer base material. Full article

Thermoplastic composites are growing in popularity in the aerospace and automotive industries; they enable weldable and recyclable structures. Resistance welded hybrid thermoplastic and metal joints are attractive for rapid assembly, but the thermal mismatch between metals and polymers introduces residual stresses, which can drive edge debonding and compromise durability. This study presents fabricated single-lap PC/CF–Al7075 coupons with measured mid-span bow resulting from welding, evaluated bond quality by step-heating thermography, and an evaluated framework for residual stress prediction using Ansys complemented by a bimetal analytical check. Three thermal cycles were examined with different temperature gradients (200, 220, 240 °C): the measured bow was 16.5 mm and remained constant, whereas analytical calculation increased with ΔT similarly to the FEM prediction. The current FEM under predicted the bow (Mean Absolute Percentage Error is 21%), showing stress contours that decay with distance from the bond and revealing pronounced peaks in both σ_xx and σ_zz components at weld edges, consistent with shear-lag theory. FEM returned edge-peaked peel rising from 43 to −64 MPa and σ_xx was up to 12% more compressive than analytical calculation; an effective CF/PC CTE of 1.5 × 10⁻⁶ K⁻¹ reconciled curvature with test better than catalogue values. The temperature insensitive bow is attributed to polycarbonate flow/viscoelastic relaxation above Tg and hot relaxation in aluminum, with effects not represented in the elastic models. Edge peel and shear govern initiation risk. Full article

Basalt fiber-reinforced polymer (BFRP) composites are increasingly utilized in photovoltaic mounting systems due to their excellent mechanical properties and durability. Bolted connections, valued for their simplicity, ease of installation, and effective load transfer, are widely employed for joining composite components. An orthogonal experimental design was adopted to investigate the effects of key parameters—including bolt end distance, number of bolts, bolt material, bolt diameter, preload, and connection length—on the load-bearing performance of three bolted BFRP plate configurations: lap joint (DJ), single lap joint (DP), and double lap joint (SP). Test results showed that the DJ connection exhibited the highest average tensile load capacity, exceeding those of the SP and DP connections by 45.3% and 50.2%, respectively. This superiority is attributed to the DJ specimen’s longer effective shear length and greater number of load-bearing bolts. Conversely, the SP connection demonstrated the largest average peak displacement, with increases of 29.7% and 52.9% compared to the DP and DJ connections. The double-sided constraint in the SP configuration promotes more uniform preload distribution and enhances shear deformation capacity. Orthogonal sensitivity analysis further revealed that the number of bolts and preload magnitude significantly influenced the ultimate tensile load capacity across all connection types. Finally, a calculation model for the tensile load capacity of bolted BFRP connections was established, incorporating a friction decay coefficient (α) and shear strength (τ). This model yields calculated errors under 15% and is applicable to shear slip-dominated failure modes, thereby providing a parametric basis for optimizing the tensile design of bolted BFRP joints. Full article

Focusing on the practical application requirements of adhesive-bonded structures in aerospace engineering, this study aims to investigate the mechanical performance and failure mechanisms of adhesive interfaces. Adhesive bonding, valued for its uniform load distribution, low stress concentration, superior sealing, and lightweight properties, serves as a critical joining technology in aerospace engineering. However, its reliable application is constrained by complex multimode failure issues, such as cohesive failure, interfacial debonding, and matrix damage. To address these challenges, a comprehensive evaluation of the novel high-strength epoxy adhesive Dq622JD-136 (Adhesive III) was conducted through systematic tests, including bulk tension, butt joint tension, single lap shear, compressive shear, and fracture toughness (TDCB/ENF) tests. These tests characterized its mechanical properties and fracture behavior under mode-I and mode-II loading, with comparative analyses against conventional adhesives HYJ-16 (Adhesive I) and HYJ-29 (Adhesive II). Key findings reveal that Adhesive III exhibits outstanding elastic modulus, significantly outperforming the comparative adhesives. While its normal and shear strengths are slightly lower than Adhesive I, they surpass Adhesive II. A common characteristic across all adhesives is that normal strength exceeds shear strength. In terms of fracture toughness, Adhesive III demonstrates superior mode-II toughness but relatively lower mode-I toughness. These results elucidate the brittle characteristics of such adhesives, mixed failure modes under normal loading, and cohesive failure behavior under shear loading. The innovation of this work lies in systematically correlating the macroscopic performance of adhesives with failure mechanisms through multi-dimensional testing. Its findings provide critical technical support for multiscale performance evaluation and adhesive selection in aerospace joints subjected to extreme thermomechanical loads. Full article

Adhesive joints provide an effective and lightweight solution for the assembly of structures and offer both mechanical and operational advantages over conventional mechanical fastening systems. In this study, a new single-layer adhesive joint is investigated in which a thin, pretensioned textile reinforcement is inserted into the adhesive layer. In the first part, a simplified analytical model is proposed to describe the distribution of axial stresses in the adhesives and the reinforcement as well as the shear stresses in the adhesive layer. In the second part, the effects of geometric, mechanical and loading variables are investigated in a parametric analysis, focussing on the role of the initial pre-compression on the tensile response of the joint. The third part of this study compares the theoretical results with experimental data obtained with static tests on specimens made of unreinforced GFRP and epoxy resin. The results show small deviations (3–8%) between model and test. Finally, a simplified method for estimating the load-bearing capacity of brittle joints, both conventional and reinforced, is proposed. It is shown how the introduction of reinforcement and prestressing can modulate the stiffness and improve the stability of the joint without significantly affecting the load-bearing capacity. Full article

This study investigates the bond behavior of fabric-reinforced cementitious matrix (FRCM) composites with three common masonry substrates—solid clay bricks (SBs), perforated bricks (PBs), and concrete hollow blocks (HBs)—using knitted polyester grille (KPG) fabric. Through uniaxial tensile tests of the KPG fabric and FRCM system, along with single-lap and double-lap shear tests, the interfacial debonding modes, load-slip responses, and composite utilization ratio were evaluated. Key findings reveal that (i) SB and HB substrates predominantly exhibited fabric slippage (FS) or matrix–fabric (MF) debonding, while PB substrates consistently failed at the matrix–substrate (MS) interface, due to their smooth surface texture. (ii) Prism specimens with mortar joints showed enhanced interfacial friction, leading to higher load fluctuations compared to brick units. PB substrates demonstrated the lowest peak stress (69.64–74.33 MPa), while SB and HB achieved comparable peak stresses (133.91–155.95 MPa). (iii) The FRCM system only achieved a utilization rate of 12–30% in fabric and reinforcement systems. The debonding failure at the matrix–substrate interface is one of the reasons that cannot be ignored, and exploring methods to improve the bonding performance between the matrix–substrate interface is the next research direction. HB bricks have excellent bonding properties, and it is recommended to prioritize their use in retrofit applications, followed by SB bricks. These findings provide insights into optimizing the application of FRCM reinforcement systems in masonry structures. Full article