As a weak link in large wind turbine blades, adhesively bonded joints have recently received unprecedented attention in the field of wind energy because they are easily damaged and have short lifetimes. This paper deals with the effects of geometrical dimensions and laminate layups on failure load and the associated failure mode of composite bonded joints for wind turbine blades. Single-lap composite-to-composite bonded joints were fabricated and tested to complete failure under tensile load, and the features of a thick adhesive layer, a large overlap length and width, and actual laminate layups were considered. The failure load and mode of bonded joints along with the geometrical dimensions and composite layups were analyzed and compared with previous studies. The results show that failure load decreases overall with adhesive thickness, while it increases rapidly with overlap length and increases slightly faster than is proportional with overlap width. A combination of both adhesive failure and thin-layer cohesive failure is prone to initiate in thick and short bonded joints, while fiber-tear failure is initiated in thin and long bonded joints. Thick adherends can enhance the bonded joint to some extent. Therefore, it is recommended that at least one unidirectional lamina is laid in the middle of the composite layups to avoid the premature failure of adherends for bonded joints in wind turbine blades.

Sectional wind turbine blades, by dividing an intact blade into multiple segments, have the advantage of being easy to handle and transport. To determine a suitable blade division location, this study was performed to clarify some crucial aspects and challenges for sectional blades. This paper proposes a method to estimate the effects of the location of the blade division on structural, manufacturing, and assembling performance of sectional blades. The advantage of this method is the ease of the assessment process, since it can be performed at an early stage of blade design, where only the aerodynamic profile, mass density and stiffness distribution, and service fatigue loads of original blades are essential. A case study with the proposed method was carried out based on a 38-meter commercial blade. Results show that the best position for the division of sectional blades is located 20% from the blade root by balancing the three aspects listed above. The key approaches to reduce additional increases in stiffness and weight of sectional blades are related to improving the fatigue strength and the choice of low-modulus materials for connecting bolts. The effects of the division location on assembling accessibility and natural frequencies of scaled sectional blades are consistent with the basic sectional blade. Unfavorable effects occur when up-scaling the diameter of the bolts; and, harsh external loads on the connections have negative effects on the application of sectional blades with larger wind turbines. In this regard, lightweight design is indispensable to reduce bolt stress.

The paper presented an experimental study on the effect of the resin reinforced core configuration and core thickness on in-plane compressive responses and failure behaviors of composite sandwich specimens. Two resin reinforced core machining configurations were designed with three core thickness along. In-plane compressive load, displacement, strains on both sides, and failure morphology were closely monitored during the loading process. Meanwhile, the theoretical method also was supplementary to forecast the failures of sandwich structures. It was found that the enhancement of grooved, perforated holes and contour cut (GPC) core was better than double-side grooved and perforated hole (DGP) core to improve the in-plane compressive capacity of sandwich specimens for all thick cores. The core fracture or skin/core debonding failure of sandwich specimens resulted in an instant drop of in-plane compressive load, and the global buckling led to a slower reduction. The failure mode changed from global buckling to skin/core debonding at both sides as the core thickness increased for the Plain core sandwich specimen; switched from global buckling to a combined failure of core fracture and skin/core debonding at both sides, and then to skin/core debonding at both sides for the DGP core sandwich specimen; the skin/core debonding at the shallow side occurred for all GPC core specimens. The slight buckling trace of strains before the peak load probably triggered the skin/core debonding of sandwich specimens. The theoretical method could well forecast failure loads and corresponding failure modes of sandwich specimens with the 15 mm thick core, and reasonably predict failure loads for sandwich specimens with 30 mm and 45 mm thick cores.

Power loss due to the aeroelastic effect of the blade is becoming an important problem of large-scale blade design. Prior work has already employed the pretwisting method to deal with this problem and obtained some good results at reference wind speed. The aim of this study was to compensate for the power loss for all of the wind speeds by using the pretwisting method. Therefore, we developed an aeroelastic coupling optimization model, which takes the pretwist angles along the blade as free variables, the maximum AEP (annual energy production) as the optimal object, and the smooth of the twist distribution as one of the constraint conditions. In this optimization model, a PSO (particle swarm optimization) algorithm is used and combined with the BEM-3DFEM (blade element momentum—three-dimensional finite element method) model. Then, the optimization model was compared with an iteration method, which was recently developed by another study and can well compensate the power loss at reference wind speed. By a design test, we found that the power loss can be reduced by pretwisting the origin blade, whether using the optimization model or the iteration method. Moreover, the optimization model has better ability than the iteration method to compensate the power loss with lower thrust coefficient while keeping the twist distribution smooth.

Composite sandwich structures are widely used in the fields of aviation, marine, and energy due to their high specific stiffness and design flexibility. Improving the mechanical properties of the cores is significant to the strength, modulus, and stability of composite sandwich structures. Two kinds of core machining configurations were designed by combining thin grooves, perforated holes, and thick contour cuts as well as non-machining plain cores. The cores and sandwich structures with these configurations were fabricated using a vacuum-assistant infusion process. Static tensile, compressive, shear, and peeling tests were conducted on the infused cores and sandwich structures. The results showed that the tensile, compressive, and shear moduli, and compressive strength of the infused cores can be greatly improved. The tensile strength changed negligibly due to stress concentration induced by irregular foam cell and the shear-lag phenomenon of the resin column/foam interface. The shear strength of the infused cores increased slightly. The thick contour cuts and perforated holes can greatly improve the face sheet/core peel capacity of the sandwich structures, whereas the thin grooves can moderately improve the peel capacity. Both infused cores with the designed machining configurations exhibited positive effects on the compressive, tensile, and shear moduli, and compressive strength, considering the material costs. The study provides a comprehensive and quantitative insight into the effects of core machining configurations on mechanical properties of infused cores and composite sandwich structures.

Significant vibrations of the traction wire rope can impact the efficiency and accuracy of static testing in wind turbine blade assessments. This study focuses on the vibration characteristics of the wire rope under static loading conditions. A simulation model for single-point static tests of wind turbine blades was developed using Adams software and validated through wire rope tension and longitudinal acceleration measurements during static tests on a full-scale 71.5-m blade. The validated model was used to analyze the effects of wire rope span and pulley position on vibration amplitude and tension in single-point loading scenarios. The results show that increasing the wire rope span and the distance between the pulley and blade fixture significantly amplifies vibration. Adjusting the span of the wire rope and the pulley position causes the primary vibration frequency to approach the natural frequency, leading to a substantial increase in vibration near the resonance frequency. To avoid resonance and reduce vibration, it is recommended to use two misaligned ground tracks, ensuring the wire rope span does not exceed 30 m and the distance between the pulley and blade fixture does not exceed 7 m. Specific resonance combinations of wire rope span and pulley position should be avoided to improve the precision and reliability of the testing system.