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In this article, the use of additive-manufactured thermoplastics, specifically polylactic acid (PLA), to fabricate segments of wind turbine blades with core sandwich composites was verified through their compressive bucking performance, demonstrating their costeffectivness in manufacturing and transportation. A small wind blade was constructed by joining these segments to demonstrate their application potential in renewable energy technologies. The study’s focus was on the compressive buckling behavior of these fusion joined blades, particularly on the heterogeneity at the resistance welding bond line. An approach was adopted to integrate a hybrid of solid and cohesive elements within the cohesive zone modeling (CZM) framework using the Abaqus–Riks method. This allowed us to insert a thin layer of solid–cohesive elements at the bond line, enhancing the fidelity of our simulations. The validity of our numerical results was examined by comparing them with the surface strain field measured by digital image correlation (DIC) and assessing the compressive response. Furthermore, the applicability of classical Euler and Johnson formulas was evaluated in predicting buckling loads and modes. The Euler formula was found adequate for the first flexural buckling mode in beams with high slenderness ratios (≥12). Our findings demonstrate that the hybrid CZM approach effectively models the buckling behavior of fusion-joined beams, accommodating a range of slenderness ratios (6 to 18) and various buckling modes. This study provides insights into the structural analysis of fusion-joined components for potential applications of additive manufacturing in wind energy. 

Additively manufactured thermoplastic polymers, such as polylactic acid (PLA), hold significant promise for sustainable engineering structures, including wind turbine blades. Upscaling these structures beyond the limitations of 3D printer build volumes is a challenge; fusion joining presents a potential solution. This paper introduces a displacement-controlled resistance welding process for PLA, as an alternative to the typical force controlled methods. We investigated the bonding quality of resistance-welded and adhesive-bonded PLA beams through three-point bending and measured the surface deformations using digital image correlation. Different metal meshes (30 %/0.11 mm Ni–Cu, 34 %/0.07 mm Ni–Cu, and 36 %/0.25 mm Co–Ni) served as heating elements. The process parameters were varied for the 34 %/0.07 mm Ni–Cu mesh to identify an optimum set of parameters. Results showed that this optimized displacement-controlled welding achieved 94 % of the original strength of monolithic samples. This indicates that the new welding process not only ensures high quality bonding and fine surface finishing but also promotes sustainability, recyclability, and economic efficiency in various polymer and composite structural applications. 

Strong, tough, and lightweight composites are increasingly needed for diverse applications, from wind turbines to cars and aircraft. These composites typically contain sheets of strong and high-modulus fibers in a matrix that are joined with other materials to resist fracture. Coupling these dissimilar materials together is challenging to enhance delamination properties at their interface. We herein investigate using a trace amount of carbon nanotube sheets to improve the coupling between composite skins and core in a composite sandwich. Ultra-thin (~100 nm) forest-drawn multi-walled carbon nanotube (MWNT) sheets are interleaved within the skin/core interphase, with MWNTs aligned in the longitudinal direction. The mechanical behavior is characterized by end notched flexural testing (ENF). With two MWNT sheets placed in the skin/core interphase, the following performance enhancements are achieved: 36.8 % increase in flexural strength; 127.3 % and 125.7 % increases in mode I & II fracture toughness values, respectively; and 152.8 % increase in interfacial shear strength (IFSS). These are achieved with negligible weight gain of the composite sandwich (0.084 wt% increase over the baseline sandwich without MWNT sheets). The finite element simulation results show that MWNT sheets enhance the skin/core coupling by reducing stress concentration, enabling the transition from catastrophic brittle failure to a stable ductile failure mode. The MWNT sheets interleaved sandwich composites are thus demonstrated to be stronger and tougher while providing electrical conductivity (4.3 × 104 S/m) at the skin/core interface for potential de-icing, electromagnetic interference shielding, and structural health monitoring. 

Debonding at the core–skin interphase region is one of the primary failure modes in core sandwich composites under shear loads. As a result, the ability to characterize the mechanical properties at the interphase region between the composite skin and core is critical for design analysis. This work intends to use nanoindentation to characterize the viscoelastic properties at the interphase region, which can potentially have mechanical properties changing from the composite skin to the core. A sandwich composite using a polyvinyl chloride foam core covered with glass fiber/resin composite skins was prepared by vacuum-assisted resin transfer molding. Nanoindentation at an array of sites was made by a Berkovich nanoindenter tip. The recorded nanoindentation load and depth as a function of time were analyzed using viscoelastic analysis. Results are reported for the shear creep compliance and Young’s relaxation modulus at various locations of the interphase region. The change of viscoelastic properties from higher values close to the fiber composite skin region to the smaller values close to the foam core was captured. The Young’s modulus at a given strain rate, which is also equal to the time-averaged Young’s modulus across the interphase region was obtained. The interphase Young’s modulus at a loading rate of 1 mN/s was determined to change from 1.4 GPa close to composite skin to 0.8 GPa close to the core. This work demonstrated the feasibility and effectiveness of nanoindentation-based interphase characterizations to be used as an input for the interphase stress distribution calculations, which can eventually enrich the design process of such sandwich composites. 

Abstract Resin uptake plays a critical role in the stiffness‐to‐weight ratio of wind turbine blades in which sandwich composites are used extensively. This work examines the flexural properties of nominally half‐inch thick sandwich composites made with polyvinyl chloride (PVC) foam cores (H60 and H80; PSC and GPC) at several resin uptakes. We found that the specific flexural strength and modulus for the H80 GPC sandwich composites increase from 82.04 to 90.70 kN · m/kg and 6.03 to 7.13 MN · m/kg, respectively, with 11.0% resin uptake reduction, which stands out among the four core sandwich composites. Considering reaching a high stiffness‐to‐weight ratio while preventing resin starvation, 32% to 38% and 40% to 45% resin uptakes are adequate ranges for the H80 PSC and GPC sandwich composites, respectively. The H60 GPC sandwich composites have lower debonding toughness than H60 PSC due to stress concentration in the smooth side skin‐core interphase region. The ailure mode of the sandwich composites depends on the core stiffness and surface texture. The H60 GPC sandwich composites exhibit core shearing and bottom skin‐core debonding failure, while the H80 GPC and PSC sandwich composites show top skin cracking and core crushing failure. The findings indicate that an appropriate range of resin uptake exists for each type of core sandwich composite, and that within the range, a low‐resin uptake leads to lighter blades and thus lower cyclic gravitational loads, beneficial for long blades.