Composite Repair in Wind Turbine Blades: An Overview

Renewable energy sources such as wind energy—together with energy-efficient technologies—are essential to meet global energy demands and address climate change. Fiber-reinforced polymer composites, with their superior structural properties (e.g., high stiffness-to-weight) that allow lightweight and robust designs, play a significant part in the design and manufacture of modern wind turbines, especially turbine blades, for demanding service conditions. However, with the current global growth in onshore/offshore wind farm installations (with total global capacity of ～282 GW by the end of 2012) and trend in wind turbine design (～7–8 MW turbine capacity with ～70–80 m blade length for offshore installations), one of the challenges that the wind energy industry faces with composite turbine blades is the aspect of structural maintenance and repair. Although wind turbines are typically designed for a service life of about 20 years, robust structural maintenance and repair procedures are essential to ensure the structural integrity of wind turbines and prevent catastrophic failures. Wind blades are damaged due to demanding mechanical loads (e.g., static and fatigue), environmental conditions (e.g., temperature and humidity) and also manufacturing defects. If material damage is not extensive, structural repair is the only viable option to restore strength since replacing the entire blade is not cost-effective, especially for larger blades. Composite repairs (e.g., external and scarf patches) can be used to restore damaged laminate/sandwich regions in wind blades. With composite materials in the spar (～30–80 mm thick glass/carbon fiber laminates) and aerodynamic shells (sandwich sections with thin glass fiber skins and thick foam/wood as core), it is important to have reliable and cost-effective structural repair procedures to restore damaged wind blades. However, compared to aerospace bonded repairs, structural repair procedures in wind blades are not as well developed and thus face several challenges. In this regard, the area of composite repair in wind blades is broadly reviewed to provide an overview as well as identify associated challenges.     

Toward Demystifying the Mohs Hardness Scale

Today, the Mohs scale is used profusely throughout educational systems without any persuasive understanding of the fundamental principles. Why one mineral has a scratch hardness over the next culminating in a scale of 1 (chalk) to 10 (diamond) has no atomistic or structure‐sensitive basis that explains this outcome. With modern computationally based atomistic and multiscale models, there is increasing promise of defining the pressure and rate‐dependent parameters that will allow a fundamental understanding of the Mohs scale. This study principally addresses the combined fracture and plasticity parameters that qualitatively affect fracture at the nanoscale. A physical model wherein the crack tip under a scratch is shielded by dislocations is supported by molecular dynamics (MD) simulations in both ductile aluminum and brittle silicon carbide. Next, this model is applied to nanoindentation data from the literature to produce a ranking of Mohs minerals based on their fundamental properties. As such, what is presented here is a first step to address the flow and fracture parameters ultimately required to provide a figure of merit for scratch hardness and thus the Mohs scale.     

Preparation of dual‐layer acetylated methyl cellulose hollow fiber membranes via co‐extrusion using thermally induced phase separation and non‐solvent induced phase separation methods

Dual‐layer acetylated methyl cellulose (AMC) hollow fiber membranes were prepared by coupling the thermally induced phase separation (TIPS) and non‐solvent induced phase separation (NIPS) methods through a co‐extrusion process. The TIPS layer was optimized by investigating the effects of coagulant composition on morphology and tensile strength. The solvent in the aqueous coagulation bath caused both delayed liquid–liquid demixing and decreased polymer concentration at the membrane surface, leading to porous structure. The addition of an additive (triethylene glycol, (TEG)) to the NIPS solution resolved the adhesion instability problem of the TIPS and NIPS layers, which occurred due to the different phase separation rates. The dual‐layer AMC membrane showed good mechanical strength and performance. Comparison of the fouling resistance of the AMC membranes with dual‐layer polyvinylidene fluoride (PVDF) hollow fiber membranes fabricated with the same method revealed less fouling of the AMC than the PVDF hollow fiber membrane. This study demonstrated that a dual‐layer AMC membrane with good mechanical strength, performance, and fouling resistance can be successfully fabricated by a one‐step process of TIPS and NIPS. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42715.     

The effect of BMP‐2 and VEGF loading of gelatin‐pectin‐BCP scaffolds to enhance osteoblast proliferation

A composite scaffold of gelatine (Gel)‐pectin (Pec)‐biphasic calcium phosphate (BCP) was successfully fabricated. Growth factors such as bone morphogenetic protein‐2 (BMP‐2) and vascular endothelial growth factor (VEGF) were loaded into the Gel‐Pec‐BCP hydrogel scaffolds by freeze‐drying. The surface morphology was investigated by scanning electron microscopy, and BCP dispersion in the hydrogel scaffolds was measured by energy dispersive and X‐ray diffraction spectroscopy. The results obtained from Fourier transform infrared spectroscopy and quantitative measurements showed successfully loading of BMP‐2 and VEGF into the Gel‐Pec‐BCP hydrogel scaffolds. In addition MC3T3‐E1 preosteoblasts were cultivated on the three types of scaffolds to investigate the effects of BMP‐2 and VEGF on cell viability and proliferation. The Gel‐Pec‐BCP scaffolds loaded with VEGF and BMP‐2 demonstrated more cell spreading and proliferation compared to those of the Gel‐Pec‐BCP scaffolds. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41241.     

Preparation and characterization of methoxy‐poly(ethylene glycol) side chain grafted onto chitosan as a wound dressing film

Chitosan has received extensive attention as a biomedical material; however, the poor solubility of chitosan is the major limiting factor in its utilization. In this study, chitosan‐based biomaterials with improved aqueous solubility were synthesized. Two molecular weights (750 Da and 2000 Da) of methoxypoly(ethylene glycol) (mPEG) were grafted onto chitosan (mPEG‐g‐chitosan) to form a ～100‐μm‐thick plastic film as a wound dressing. The chemical structures of the mPEG‐g‐chitosan copolymers were confirmed using Fourier transform infrared spectroscopy (FTIR), and the thermal properties were characterized using thermogravimetry analysis (TGA). Their microstructures were observed using scanning electron microscopy (SEM). The other properties were analyzed via the swelling ratio, tensile strength, elongation, and water vapor transmission rate (WVTR). Biocompatibility evaluations through biodegradability, cytotoxicity, and antimicrobial effect studies were also performed. The obtained mPEG‐g‐chitosan copolymers were soluble in slightly acidic aqueous solutions (pH～6.5) at a concentration of 10 wt %. The optimal mPEG‐g‐chitosan hydrogels had swelling ratios greater than 100% and WVTRs greater than 2000 g/m²/day. Their performance against Staphylococcus aureus will be subjected to further improvements with respect to medical applications. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42340.