In this paper, fatigue life estimation for delamination growth of laminated fibre reinforced polymer (FRP) composite structures in mode I and mode II based on fracture mechanics is presented. The proposed method was applied to delamination of glass/epoxy laminated composite. Both the threshold energy release rate ( ) and the delamination propagation based on Paris' law were studied. The double cantilever beam (DCB) specimen for mode I and 3 points End-Notched Flexure (3ENF) specimen for mode II were used for monotonic fracture tests and the resistance and as a function of delamination length were determined. For DCB tests, the fatigue onset life test was conducted and the threshold energy release rate, , was found for the subcritical region. Constant amplitude, displacement controlled cyclic fatigue test for both modes was conducted and the delamination crack growth rate ( ) as a function of maximum cyclic energy release rate and for DCB and 3ENF specimens were determined, respectively. From curve fitting to the experimental data, the Paris' law material constants C and m for mode I and mode II were obtained. Finally, the SEM fractography studies of delaminated surfaces of 3ENF static and fatigued specimens have been done and the different features observed on these surfaces were discussed.

In this paper, the sensitivity of the structural integrity of wind turbine blades to debonding of the shear web from the spar cap was investigated. In this regard, modal analysis, static and fatigue testing were performed on a 45.7 m blade for three states of the blade: (i) as received blade (ii) when a crack of 200 mm was introduced between the web and the spar cap and (iii) when the crack was extended to 1000 mm. Calibration pull-tests for all three states of the blade were performed to obtain the strain-bending moment relationship of the blade according to the estimated target bending moment (BM) which the blade is expected to experience in its service life. The resultant data was used to apply appropriate load in the fatigue tests. The blade natural frequencies in flapwise and edgewise directions over a range of frequency domain were found by modal testing for all three states of the blade. The blade first natural frequency for each state was used for the flapwise fatigue tests. These were performed in accordance with technical specification IEC TS 61400-23. The fatigue results showed that, for a 200 mm crack between the web and spar cap at 9 m from the blade root, the crack did not propagate at 50% of the target BM up to 62,110 cycles. However, when the load was increased to 70% of target BM, some damages were detected on the pressure side of the blade. When the 200 mm crack was extended to 1000 mm, the crack began to propagate when the applied load exceeded 100% of target BM and the blade experienced delaminations, adhesive joint failure, compression failure and sandwich core failure.

In this work tensile and compressive properties and fatigue performances of laminated glass fibre reinforced polymer (GFRP) composite under constant amplitude sinusoidal waveform load control at frequency of 5 Hz and at room temperature were investigated for three different types of loading: tension-tension at R= 0.1 and 0.5, reverse loading tensioncompression at R= −1 and compression-compression at R= 2 and 10 in fibre and normal to fibre directions. From these series of tests, the corresponding S-N diagrams are obtained.The dynamic stiffness during fatigue loading has shown classical degradation of the GFRP laminates. It is observed that the dynamic modulus decreases with time and the hysteresis loop area changes with some distortion according to the loading condition. Finally hysteresis loops throughout fatigue testing were examined and the variation of energy dissipated per cycle throughout the specimen lifetime was quantified. It is demonstrated that the dissipated energy during the fatigue lifetime is dependent on R-ratio and fibre orientation.However, in majority of the cases, the energy dissipated per cycle near the end of the fatigue lifetime increases as a result of an increase in the area captured by hysteresis loops.

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