D. J. Lekou Centre for Renewable Energy Sources (CRES) T. T. Assimakopoulou, T. P. Philippidis

1. Estimation of the Uncertainty in Measurement of Composite Material Mechanical Properties during Static Testing D. J. Lekou Centre for Renewable Energy Sources (CRES) T. T. Assimakopoulou, T. P. Philippidis University of Patras

2. Blade Structural Design European Wind Energy Conference 2009

3. Lamina Characterization European Wind Energy Conference 2009

4. The uncertainty • Parameter associated with the result of a measurement • Characterizing the dispersion of values, reasonable attributed to the measurand • e.g. standard deviation • Describes the level of knowledge of the measurand • Increases confidence in the validity of a measurement result European Wind Energy Conference 2009

5. The Tests European Wind Energy Conference 2009

6. The Measurands European Wind Energy Conference 2009

7. Uncertainty analysis • Test instruments • Load Cell, Strain gauges, Micrometer • Specimen • Dimensions, Shape & Tolerances • Test procedure • Zeroing, Sampling, Alignment • Test environment • Temperature, Humidity European Wind Energy Conference 2009

8. Uncertainty of Coupon Strength • Cross-sectional area • Instrument, Coupon shape • Load • Load cell • Specimen alignment • Manufacturing & Test procedure • Recording of Ultimate Stress • Sample rate & Equipment European Wind Energy Conference 2009

9. Uncertainty of Coupon Elastic Property • Stress (measurand) • Strain (measured) • k-factor, etc. • Misalignment • Calculation procedure • Range • Linear regression • Averaging • Test procedure • Zeroing European Wind Energy Conference 2009

10. Uncertainty of Reported Properties • Arithmetic mean • Standard deviation • Correlated measurements • Same equipment • Coupons from same batch European Wind Energy Conference 2009

11. Properties of UD on-axis • Unc. in Cross-Section • uA = 0.84% • Unc. in Strength • u = 1.10% • Unc. in Ult. Strain • u = 0.90% • Unc. in E-modulus • uE = 1.65% European Wind Energy Conference 2009

12. Misalignment effects on properties European Wind Energy Conference 2009

13. E-modulus European Wind Energy Conference 2009

14. Mean value of E1 • Experiments: • 29 tests, Mean= 39.04 GPa, COV= 2.64% • Usual approach (95% conf. level): • E1 = 39.04 ± 0.63 GPa (1.6%) • Current analysis: • E1 = 39.04 ± 1.28 GPa (3.3%) • 2% lower Stiffness (95% Conf.) European Wind Energy Conference 2009

15. Mean value of XT • Experiments: • 29 tests, Mean= 776.50 MPa, COV= 4.65% • Usual approach: • Rk,XT = 695.55 MPa (89.6% of mean value) • Current analysis: • Rk,XT = 646.92 MPa (83.3% of mean value) • 7% lower characteristic strength value European Wind Energy Conference 2009

16. Conclusions – Recommendations • Uncertainty in Measurements • Distinguish between uncertainty in measurement and inherent property variability • Report Uncertainty of measurements • As input for designers & certification bodies • For improvement on test procedures • Use Analysis on Uncertainty • For estimation of number of tests • Extension of presented analysis • Fatigue results European Wind Energy Conference 2009

17. Thank you for your attention European Wind Energy Conference 2009