Further development of thermoplastic composites for advanced structural applications, such as in aerospace, requires tough interfaces at bimaterials junctions such as composite-metal interfaces. Mode I failure being the most critical failure mode of interfaces, surface roughening or patterning techniques are commonly used to improve the mode I interface toughness. Patterning typically involves creating grooves on the surface via laser ablation or 3D printing. However, crack propagation may follow two distinct paths: along the groove pattern (interfacial failure) or through the polymer within the grooves (cohesive failure). Cohesive failure is often the toughest mechanism. However, design criteria linking groove geometry to joint materials are currently lacking. This study investigates the influence of groove dimensions, joint dimensions, and material and interface properties on the resulting failure mechanism using a cohesive zone model. First, a small-scale yielding (SSY) model is developed. The results indicate that the characteristic fracture length of the material filling the grooves plays a critical role in determining the failure mechanism. Specifically, cohesive failure is promoted when the groove depth is at least ten times greater than the characteristic length, and when the groove aspect ratio (depth-to-width) exceeds 10. Additionally, filling the grooves with a more compliant material, such as a polymer, helps to prevent interfacial failure. Finally, a double-cantilever model is developed, indicating that the loading configuration significantly influences the failure mechanisms taking place. For the DCB configuration, crack propagation along the interface is promoted, compared to the SSY case, owing to the bending of the adherends.

Laser heating is the most common method in thermoplastic Automated Fiber Placement (AFP) due to its precision and speed, but it poses safety and cost challenges. The HUMM3 pulsed flashlamp offers a promising broadband, programmable, and relatively safer alternative. This study investigates the thermal response and deconsolidation behavior of unidirectional carbon fiber/LM-PAEK (CF/LM-PAEK™) composite tape during HUMM3 heating in AFP. A static setup replicating AFP conditions was used to investigate the thermal response of the composite tape as a function of the programmable parameters: voltage, pulse width and frequency. Deconsolidation of the tape under HUMM3 heating was assessed from micrographs and surface topography, quantified by thickness change, void content, waviness, and roughness. Results showed that voltage is the dominant parameter influencing the thermal response, whereas pulse width and frequency showed no significant effects when total energy input was held constant. The deconsolidation analysis revealed a strong correlation of thickness change, void content, waviness, and surface roughness with local temperature. These changes are believed to be driven by polymer softening, fiber decompaction, internal gas pressure buildup, and local thermal gradients across the tape width. Compared to laser heating, HUMM3 produces less severe deconsolidation, likely due to its broadband UV–VIS–NIR spectrum enabling partial matrix absorption, reducing temperature gradients across the tape width.

Fiber-metal laminates are a well-known and established material concept featuring an enhanced crack propagation resistance when compared to their metal and fiber reinforced plastic (FRP) constituents. In this paper, this approach is transferred to purely carbon fiber reinforced plastic (CFRP) based laminates made from layers having polyetherimide (PEI) and epoxy matrices in an alternating laminate architecture. The laminates are manufactured via hot pressing. Double-cantilever beam (DCB) tests are performed on standard samples for both the hybrid laminates in different configurations as well for the both constituent materials, i.e. carbon fiber reinforced PEI (CFR-PEI) and carbon fiber reinforced epoxy. As the formation of an interphase is already reported in literature for this matrix combination, microstructural investigations have also been carried out in addition to fractography on crack surfaces. It is shown that the hybrid materials outperform both constituents regarding the crack resistance when crack initiation starts in the tougher CFR-PEI layer and the laminate layup is 0/90°. In the other configurations investigated, there is no significant effect. The energy dissipating mechanisms are crack jumping and the formation of several parallel cracks. Consequently, crack resistance in such hybrids might be controlled in future by adjusting the crack resistance of the constituents as well as the laminate architecture.

Our aim with this work was to evaluate how the thermoplastic resin used in the composite adherends and on the energy director affected the static ultrasonic welding process in both parallel and misaligned configurations. Polyetheretherketone (PEEK) and low-melt polyaryletherketone (LMPAEK) were the resins used and their thermomechanical properties were characterized via dynamic-mechanical analysis and modulated differential scanning calorimetry. With parallel adherends, neither the welding time nor the through-thickness heating in the adherends vary significantly. This similarity was attributed to a larger heat capacity of the PEEK energy director counterbalancing its higher viscoelastic heating rate. With misaligned adherends, the welding time was larger for PEEK welds than for LMPAEK welds and LMPAEK adherends presented a larger though-thickness heating. These effects were attributed to the larger bulk viscoelastic heating rate of carbon fibre reinforced/LMPAEK adherends adding up to the lower heat capacity of LMPAEK.

Structural reuse is a promising alternative to recycling of composite materials; it preserves material composition while liberating the materials for reuse in secondary applications. Thermoplastic reinforced composite materials have the potential to expand reuse opportunities by adapting their shape, or reversing them to a laminate blank. In this study, we evaluated reverse forming of glass fibre-polypropylene (GF-PP) laminates by developing a processing method, testing material properties and the effect of three design parameters: forming strain, laminate architecture and material type. Forming strain relates to the deformation mechanism of inter-ply slip, and is imposed through varying the contour depth and bending radius. Laminate architecture relates to resin redistribution, and is imposed by using an orthogonal as well as quasi isotropic layup. Finally, the material type affects both Inter-ply slip as well as resin redistribution, and is imposed by using plain and twill weaves. GF-PP blanks were prepared using a heated platen press and subsequently formed and flattened using convection heating (<165 °C) and vacuum pressure in a novel moulding process. The samples had typical values for flexural strength of 91 - 113 MPa and flexural modulus of 9–16 GPa. Using a Design of Experiments analysis the process was deemed robust for the given boundary conditions. These results demonstrate the feasibility of reverse forming for cases where inter-ply slip is the governing deformation mechanism. The presented reverse forming process and design parameters can be used to create new thermoplastic composite parts, anticipating for structural reuse through reverse forming.

The high void content in laser-assisted fiber placement (LAFP)-manufactured thermoplastic (TP) hinders industrial adoption, with tape deconsolidation being a critical yet understudied factor. To address this research gap, this study provides an in-depth investigation into the deconsolidation mechanisms of TP tapes during the LAFP heating phase. A series of comparative experiments were conducted to systematically evaluate the effects of tape residual stress states (fiber-matrix combined, fiber-dominated, and near stress-free) and heating methods (laser vs. oven heating) on deconsolidation behavior. Deformation along the width, voids, thickness variations and surface roughness were identified as key factors to characterize deconsolidation behavior and to elucidate its underlying mechanisms. The results reveal that the matrix residual stress plays a dominant role in exacerbating deformation along the width, nonuniformity in thickness and intralaminar voids. Additionally, fiber decompaction—induced by the recovery of elastic deformation—contributes to surface deformation by generating voids near the surfaces. Furthermore, laser-heated tapes exhibit more pronounced intralaminar voids and higher surface roughness than oven-heated counterparts, underscoring the influence of heating rate on the release of residual stress. This study advances the understanding of deconsolidation mechanisms during the LAFP heating phase, and provides recommendations for optimizing the manufacturing of LAFP-grade TP tapes.

Accelerated laboratory testing is essential to understand the rain erosion behavior of coated samples applied to the leading edge surface of a wind turbine blade. This study investigates the impact of droplet impact frequencies and dry intervals on the incubation time for damage on polyurethane-coated samples using a Pulsating Jet Erosion Tester (PJET). A novel theoretical model for water slug volume is introduced, allowing for a more accurate comparison across different impact velocities and frequencies. The effect of dry intervals on coating performance is quantified, revealing that longer dry intervals and shorter pre-dry rain exposure can significantly increase the number of impacts a coating can withstand before damage. The study challenges the traditional continuous impingement testing by demonstrating that dry intervals can extend incubation time by a factor of three to five. Additionally, this paper proposes a recalibrated approach to PJET testing, which better mimics the cyclic nature of real-world rainfall, leading to improved predictive models for material degradation. The findings emphasize the importance of considering the visco-elastic behavior of coatings and the role of intermittent rain exposure in erosion testing, offering invaluable insights for designing future PJET test parameters.

This study considered adhesion between thermoplastic and thermoset laminates through interdiffusion at the interface. The influence of the degree of cure of the thermoset at the start of the process was investigated through mechanical testing and microscopy. Increasing the initial degree of cure decreased both interlaminar fracture toughness and interphase thickness. Fracture toughness decreased disproportionately to interphase thickness, attributed to changes in interphase morphology and decreasing surface contact at the interface. A simplified model was developed using gel layer thickness measurement data to predict the level of interdiffusion with increasing initial degree of cure. Compared to thermoset-thermoset co-curing, there was superior bond strength at low initial degrees of cure and a predicted increased sensitivity to the initial degree of cure, suggesting a greater influence of process variability. Hence, for specific property critical applications, the trade-off between the potential manufacturing efficiency gains from semi-curing and the reduced performance would be an important consideration.

This article discusses results from an international contest, open for university student teams (bachelor and master), involving the design, construction, and testing of small wind turbines in a large wind tunnel. The wind tunnel has an outlet of 2.85 x 2.85 m allowing a maximum rotor swept area of 2 m2 without significant tunnel effects. Both horizontal and vertical axis wind turbines are part of the competition. The turbines are evaluated by an external jury of industry experts based on criteria such as Annual Energy Production, cut-in wind speed, innovations, design, and sustainability. Although the contest has been initiated in 2013 with an educational focus, it has also evolved into a valuable database for scientific purposes by providing a decade worth of performance measurements for roughly 9-10 various turbine concepts each year. The collected data may serve as a unique validation resource for assessing the accuracy of design codes in modelling diverse turbine concepts thanks to detailed design reports with model descriptions accompanying each turbine (such turbine descriptions are often considered confidential for field measurements). The paper aims to explore the scientific value of this database by comparing calculations with measurements, offering explanations where possible, and reporting intriguing findings on unconventional concepts' performance. Even though not all observations could be explained fully they provide food for thought. Recommendations are provided for both students to enhance their designs and for contest organizers to elevate the scientific value of the measurements in future contests.

This paper evaluates the potential of changing the welding force and the compliance of the energy director (ED) to reduce the effects caused by misaligned adherends, which were: increased through-thickness heating, reduced size of welded area and increased heating time. In the methodology that was followed, we welded adherends misaligned by approximately 4.5° in different scenarios: with higher welding force; with increased ED compliance by the use of a thicker ED and; with increased ED compliance by the use of a discontinuous ED. The most significant reduction of the effects caused by misaligned adherends was obtained when combining the use of both increased welding force and discontinuous ED. Such improvement derives from the imposed parallelism caused by the use of a higher welding force and from a more efficient concentration of heat generation at the weld line that occurs when a discontinuous ED is used.

A fiber-reinforced composite wind turbine blade (WTB) is exposed to numerous impact threats during its service life causing damages that can be detrimental to its structural integrity. Currently, impact loads are not considered during blade design, so high safety factors are introduced, which result in a conservative design. However, as wind turbine blades become stiffer and lighter and health monitoring systems become more sophisticated, the design process is shifting toward damage-tolerant approaches. The design philosophy accepts damages to the structure, but it also requires that the damaged blade still meet structural and functional requirements. This design procedure requires a comprehensive understanding of different impact threats and their characteristics, which is currently unavailable in the public domain. This paper is a first attempt to review the impact loads on composite wind turbine blades. The aim of the current paper is to (a) identify different sources of impact threats on wind turbine blades during different stages of their service life, (b) describe their qualitative (causes and vulnerable regions) as well as quantitative characteristics (size, mass, and velocity of impactor), and to (c) provide modeling guidelines by comparing these impact threats using five different criteria - (i) relative deformability of projectile and wind turbine blade, (ii) impact velocity, (iii) kinetic energy of impact, (iv) repeatability of impacts and (v) nature of the impact. The review paper will be of special interest to researchers working on wind turbine blades and will serve as a baseline report for designing damage-tolerant blades. Recommendations are also provided for future research.