JEC COMPOSITES MAGAZINE - Issue #73 - June 2012 - 52

wind energy

cycle at up to 200 rpm, which amasses to ~2.1x109 cycles over a 20-year design life. Over this period, the blade materials need to sustain large “normal operation” design loads which are fatigue loads. A fatigue assessment is thus a prerequisite for the design of a cost-effective blade. However, a noticeable lack of fatigue data on PFCs [8-11] limits their prospective use in fatigue-critical components (like SWTbs) as fatigue life prediction of the components is not possible. This study includes three aspects: i) evaluating the fatigue loads on a 3.5-metre study blade, as per bS-EN61400-2:2006, ii) constructing S-N diagrams and constant-life diagrams of PFCs for varying parameters, and iii) predicting the design life of a hemp/polyester composite blade. In prediction of the fatigue life, it is confirmed that the PFC blade satisfies the 20-year design life criteria, inclusive of a 1.50 safety factor.

Estimation of design fatigue loads The fatigue loads on the blade are estimated using the “Simplified load model” (Load case A) in bSEN61400-2:2006 [12]. The model states that the design load for “normal operation” is a fatigue load and that fatigue loading is considered to occur at the airfoil-root junction or at the root-hub junction (whichever has the lowest strength). From static testing of a full-scale prototype gFrP blade, static failure is found to occur at the root-hub junction. The PFC blade is assumed to fail similarly. Under Class II wind conditions (design wind speed = 11.9 m/s) and at a design rotor speed of 170 rpm, the fatigue loads at the root of a 25-kg hemp/polyester blade are found to be: - Centrifugal load, ΔFz = 25.3 kN - Edgewise bending moment, ΔMx = 1.03 kNm - Flapwise bending moment, ΔMy = 1.26 kNm The loads include a material partial safety factor of 1.25. The resultant tension-tension and compressioncompression fatigue stresses on the different constituents of the blade are then determined, as per bSEN61400-2:2006 [12]. These are presented in Table 1, showing that unidirectional (Ud) hemp/polyester experiences the most severe stresses.

Table 1: Fatigue stresses on different constituents at the blade root

Tensiontension
Maximum [MPa] Minimum [MPa] Mean [MPa] Amplitude [MPa] Stress ratio

Foam
1.3 0.4 0.9 0.5 0.29

Triax
21.1 6.2 14.9 7.5 0.29

UD
57.6 16.5 41.1 20.6 0.29

Compressioncompression
Maximum [MPa] Minimum [MPa] Mean [MPa] Amplitude [MPa] Stress ratio

Foam
-1.1 -0.4 -0.7 0.3 2.6

Triax
18.6 -7.1 -11.6 5.8 2.6

UD
-44.1 -17.4 -26.7 13.3 2.5

Blade fatigue load evaluation
Design summary of the PFC wind turbine blade The study blade is a 3.5-metre hemp/ polyester blade that is to be set-up on an 11-kW 3-bladed horizontal axis “Class II” small wind turbine at the University of Nottingham. The profile and lay-up of the blade are depicted in Fig. 3. The blade will be manufactured through a single-shot VArTM process in an aluminium mould tool.

Constant-life diagram for unidirectional hemp/polyester composites
Experimental Manufacture of composites: 3-3.3 mm thick unidirectional hemp yarn composites were manufactured through vacuum infusion in an aluminium mould tool. Hemp yarn (250 tex, 180 tpm) was sourced from Safilin (Poland). An orthophthalic unsaturated polyester (reichhold Norpol type 420-100) matrix was used. The resin was mixed with 0.25 wt% NL49P accelerator and 1 wt% butanox M50 initiator. Post-cure was carried out at 55°C for 6 hours after ambient cure for 16 hours. The density of the fibre, cured resin and composite were measured using helium pycnometry. The fibre and void volume fraction were then determined to be 35.6 ± 3.3 % and 1.4 ± 0.4 %, respectively. Static testing: Static tensile tests were conducted according to bSEN527-4:1997 using an Instron 5985 testing machine equipped with an exten-

Fig. 3: Profile and lay-up of the PFC blade

52 jec composites magazine / No73 June 2012

JEC COMPOSITES MAGAZINE - Issue #73 - June 2012

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