JEC COMPOSITES MAGAZINE - Issue #73 - June 2012 - 53

someter. Six test specimens (250 mm long, 15 mm wide) were tested. The tensile strength of the unidirectional hemp/polyester composites was measured to be 171.3 ± 6.5 MPa. Static compression tests were conducted according to ASTM d3410 using an Instron 5581 testing machine equipped with a compression test fixture. The test specimens were speckle-coated prior to testing. Six test specimens (140 mm long, 15 mm wide) were tested. A gauge length of 12.7 mm was used to prevent the specimen from buckling. The compressive strength of the unidirectional hemp/polyester composites was measured to be 95.1 ± 6.9 MPa. Fatigue testing: Fatigue tests were conducted as per bS-EN13003:2003 using an Instron 8801 servohydraulic testing machine. Sinusoidal loads at a frequency of 10 Hz were applied. The test specimens were tested to failure to generate data for S-N curves. Tests were carried out at 5 different stress ratios: r=0.1, 0.3, 0.5 in tension-tension (TT) mode, r=-1 in tension-compression (TC) mode, and r=2.5 in compression-compression (CC) mode. Tests were conducted for at least 6 different stress levels (% of the static tensile or compressive strength), up to at least 106 cycles. For TT mode, test specimens were 250 mm long and 15 mm wide with a gauge length of 150 mm. For TC and CC modes, test specimens were 120 mm long and 15

mm wide with a gauge length of 11,5 mm. For all specimens, 50 mm long aluminium end-tabs were attached using Araldite 403, to protect the specimen surface from damage from the jaws of the test machine. After plotting the S-N data, regression equations of the form S = ANb were determined, where b is the fatigue strength coefficient. The constant-life diagram was then plotted using data obtained from the regression lines of the various S-N plots. Results and discussion S-N diagrams : The S-N data of hemp/ polyester composites tested under TT mode are presented in Fig. 4. It is observed that the power-law regression curves are a suitable fit to the data. A gradual decline in fatigue strength with increasing number of fatigue cycles is observed. The fatigue strength coefficient, b, is a useful indicator of material fatigue behaviour; the smaller it is, the steeper the slope of the curve and hence the poorer the fatigue performance. Commonly, S-N curves are presented in normalized form. Importantly, the fatigue strength coefficient remains the same, as can be seen in Fig. 4. For r=0.1, the value of b is found to be -0.062 for unidirectional hemp/polyester composites. Mandell and Samborsky find that b≈-0.074 for both chopped-strand and unidirectional E-glass/polyester composites tested at r=0.1 [13]. This suggests that the normalized fatigue performance

(or “fatigue sensitivity”) of hemp fibre composites is better than or comparable to gFrP. Ud carbon/epoxy composites, on the other hand, have a value of b≈-0.0292 (at r=0.1). Hence, they clearly outperform both gFrP and PFCs. Fig. 5 depicts the effect of stress ratio on the load spectrum; as the stress ratio increases the stress amplitude decreases. From Fig. 4 and Fig. 5, it is observed that increasing the stress ratio leads to improved normalised fatigue performance, since the value of b is observed to increase. Hence, it is possible that as r increases, b increases due to reduced crack growth rates; the latter is a result of reduced stress amplitudes and thus reduced stress/strain gradients at the fibre-matrix interface. Fig. 5 also shows the effect of stress ratio on the failure mode. Composites under fatigue and static loading usually fail in similar modes. While composites tested under TT mode fracture in a brittle catastrophic manner, composites tested under CC mode (r=2.5) and TC mode (r=-1) display the typical wedge failure and single-kink failure, respectively.

Fatigue life prediction of the PFC blade
The constant-life diagram constructed for the unidirectional hemp/polyester composites is presented in Fig. 6. The fatigue life for any combination of stress amplitude and mean stress can be determined from this diagram. Clearly, its accuracy can be improved by generating more fatigue

Fig. 4: S-N data for hemp/polyester composites tested under TT mode

Fig. 5: Effect of stress ratio on i) the applied load spectrum (top), ii) the S-N curve (bottom), and iii) the failure mode (right)

No73 June 2012 /

jec composites magazine 53

JEC COMPOSITES MAGAZINE - Issue #73 - June 2012

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #73 - June 2012