Composite Multiaxial Mechanics: Laminate Design Optimization of Taper-Less Wind Turbine Blades with Ramie Fiber-Reinforced Polylactic Acid

As the research on composite materials based on natural resources proliferates further, ramie fiber and polylactic acid (PLA), which are fully biodegradable composite materials, are expected to be used for mechanical application due to their excellent strength and degradability. Various natural fibers have been applied to a wind turbine blade composite structure, as reinforcement material. However, none of them are fully biodegradable, as the matrix still uses synthetic resins. Hence, this study aims to theoretically optimize the fully biodegradable ramie/PLA laminate design using its lamina orientation on a taper-less blade shell of a wind turbine, as the operating structure experienced multiaxial loading through bending and torsional moment derived by the wind. The selection of taper-less blades was made due to their congruence with the wind speed categorization in southeastern Indonesian territory. The optimization was carried out using the nonlinear Generalized Reduced Gradient (GRG) method on Microsoft Excel. The optimized laminate result is in a stacking sequence of [-4°, 24°, 47°, 65°, 74°, 79°]_S that delivers the factor of safety, which is the ratio between the allowable stress and the actual stress, of 7.296 and 18.057 on the longitudinal axis and the laminate shear-plane, respectively, This renders the composite laminate highly safe, both theoretically and numerically. 

Composite laminate optimization; Multiaxial loading; Polylactic acid; Ramie fiber; Taper-less blade

Fiber-reinforced composites have been used as an alternative material in many mechanical applications because their specific strength and stiffness are superior to other engineering materials in general. The development of fiber-reinforced composites in Indonesia has reached an advanced stage, especially in relation to natural fibers (Shieddieque et al., 2021). The use of natural fibers in reinforcing polymer composite materials offers several advantages, since they have low density and are biodegradable, inexpensive, and renewable (Pickering et al., 2016; Rohan et al., 2018).

One of the natural fibers that can be utilized as a reinforcement of polymer composites is ramie fiber. Ramie fiber reinforcement in composites has been utilized in various applications, such as LPG tanks and bulletproof panels (Saidah, 2004; Suryaneta, 2007). Meanwhile, the use of polylactic acid (PLA) as a composite matrix is being intensively pursued in an effort to implement the Green Composite campaign.

PLA is also considered to be able to compete with synthetic (conventional) matrices because of its relatively good strength, having a tensile strength of 50.75 MPa and a tensile modulus of 3.5 GPa (Sawpan et al., 2007). With this potential, PLA and ramie fiber should be used to subvert the domination of synthetic material in the composite field for mechanical applications. However, PLA has experienced a critical decline in its mechanical properties (strength, modulus, toughness) during weather degradation. For example, PLA underwent a 92% decrease in tensile strength (Varsavas and Kaynak, 2018). Additionally, the water absorption will increase, and the microorganisms will be attracted by the addition of natural fibers in PLA composites, which will assist in the hydrolysis of polymers and enhance the degradation rate of the composites (Surip et al., 2018). This indicates that PLA has good biodegradability. PLA is also commercially attractive, especially in developing countries and high-plastic-consumption countries such as Indonesia. PLA has a low melting point of 150 ?C, which makes the energy requirements and greenhouse gas emissions low during preparation. Hence, the low melting point indicates good manufacturability, which will enable many preparation methods, such as extrusion, injection molding, hot pressing, film stacking, and pultrusion (Rajeshkumar et al., 2021).

PLA and ramie fiber have also been considered to be the most common natural or bio-composite material pair. The research on these natural material pairs has been conducted via various experimental studies, which have also indicated that most bio-composite, especially ramie/PLA, requires pre-treatment to enhance their mechanical behavior, whether through chemical compound pre-treatment (Yu et al., 2015; Fatra et al., 2016) or pre-loading treatment (Zhou et al., 2013). Ramie/PLA composites also exhibit good water absorption but lower moisture absorption when pre-impregnated in hybrid-woven yarn with different weaving patterns (Baghaei et al., 2015).

Despite their excellent characteristics, composite is susceptible to fatigue and fracture phenomena when subjected to specific cyclic loads (static or dynamic) and environmental factors (temperature and corrosive media). Therefore, an understanding and prediction of the further propagation of such defects are of paramount importance. Furthermore, the failure mechanism of fiber-reinforced composites is more complicated when subjected to multiaxial (tension torsion) loading than when subjected to uniaxial loading, signifying that there is a solid interaction between axial stress and shear stress when the failure occurs (Lee et al., 1999). A multiaxial fatigue strain energy density has been contributed by the stresses and strains on the critical or fracture plane under various mean stress levels and loading combinations (Glinka et al., 1995). Consequently, experimental research under various complex loading conditions is mandatory to generate testing conditions which approximate reality to apply the damage criteria appropriately (Bathias et al., 1992; Quaresimin et al., 2015). Hence, the prediction of the multiaxial behavior of ramie/PLA composite has been estimated semi-empirically before on thin-walled tube laminate with a load of uniaxial tension-compression, torsion, and internal pressure. The results semi-empirically demonstrate that with a 26% reinforcing volume fraction, the composite laminate can retain a maximum longitudinal stress of 120.5 MPa and a maximum in-plane shear stress of 13.03 MPa in the failure criteria envelope (Lololau, 2021).

On the other hand, Indonesia is a country with high wind energy potential, especially in the southeastern territory, which devotes an average windspeed of 7.5-8 m/s and a maximum of 12 m/s to wind farms (Satwika et al., 2019; Hesty et al., 2021). The use of wind energy in Indonesia is arguably still lacking the technology to achieve what is desired, but in reality, only a handful of people have used wind energy. One method to capture wind energy is to use wind turbines. Wind turbines generally used in Indonesia are horizontal axis wind turbines with three propellers (Yohana et al., 2020). In wind turbines, the first component interacting directly with wind energy is the blades. Designing a wind turbine blade with a good power coefficient is strongly influenced by the blade's geometry. One significant geometric parameter is the blade’s width. A blade with a taper-less type is a blade with a uniform blade width from the base to the blade's tip. This type of blade is suitable for wind turbines with regional manifestations with medium wind speeds of 5-8 m/s (National Weather Service Portland, nd), which applies to many regions in Indonesia.

What then becomes a challenge is applying composite materials to these taper-less blades so that the blades obtained are lighter and have high strength according to the desired design. Composite materials have been used in wind turbine blade application for decades but mainly consist of conventional synthetic constituents, such as glass fiber, carbon fiber, and epoxy (Mishnaevsky et al., 2017). Therefore, it is necessary to alter it with biodegradable ones to address the disposal problem that has been a source of disruption for years. Several natural fibers, such as flax, jute, coir, and sisal, have been applied experimentally on a wind turbine blade structure in combination with glass fiber and epoxy resin matrix (Kalagi et al., 2018; Li et al., 2020). Hence, to the author’s knowledge, no fully natural or bio materials used as composite are applied to the wind turbine blade structure.

On the other hand, when converting wind energy into electrical energy, the blades on a horizontal axis wind turbine generally experience two main loads, namely bending and torsional moment (Piggott, 1997; Ghasemi and Mohandes, 2016). This multiaxial load must be understood and considered before applying these bio-composite materials to the taper-less blade structure. Therefore, it is at least necessary to optimize both theoretically and numerically as a first step to the composite laminate design so that the bio-composite has an effective performance when receiving multiaxial loads that occur in the taper-less blade structure.

Against this background, the research has been undertaken to theoretically or empirically optimize the ramie-reinforced PLA bio-composite laminate design based on their factor of safety applied on multiaxial-loaded taper-less wind turbine blades. It suggests that this optimization would produce a safer design for novel ramie-reinforced PLA taper-less wind turbine blade composite laminate to be manufactured in future projects. In the long term, this research will establish the potential of ramie fiber-reinforcing material in PLA composites as a material used for mechanical products. This study was also a part of the author’s Ramie Fiber-Reinforced PLA (RFRPLA) prepregs development research to determine the mechanical multiaxial characteristics of applying it to automotive, aeronautic, and power plant structure components with 1:1 component realization or slightly smaller.

    The optimization of lamina orientation on taper-less shell blade laminate has been done. The optimized laminate stacking sequence delivers safety factors of 7.296 and 18.057 on the longitudinal axis and the laminate plane, respectively, when experiencing a bending moment of 17.15 Nm and torque of 25.5 Nm. With a constant allowable transverse stress of -1.57 MPa, maximum allowable stress of 57.8 MPa on the longitudinal axis, and a 3.187 MPa of shear in-plane, the composite laminate is safe, both theoretically and numerically (finite element). Hence, future projects can apply the novel ramie-reinforced PLA-optimized laminate for taper-less wind turbine blades preparations. However, there is still an errors value between the two methods that have been employed. This indicates that it requires further optimization of the theoretical computational model and equalizes complex assumptions to achieve robust computational results.

    Ministry of Research, Technology, and Higher Education of Republic Indonesia has funded this research under PMDSU (Pendidikan Magister menuju Doktor untuk Sarjana Unggul) Program through NKB-373/UN2.RST/HKP.05.00/2021 contract number.

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