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1 2 | H I G H - P E R F O R M A N C E C O M P O S I T E S SPEAKING OUT resented by the master ply, only one test needs to be conducted for the determi- nation of all stiffness parameters. This is similar to metals testing, where only one measurement is required — usually the Young's modulus, assuming that Pois- son's ratio is 0.3, from which shear mod- ulus can be calculated. For composites, it can be the same: If we know the value of trace (Tr), we can fnd all stiffness val- ues just by multiplying the factors by the trace, i.e., longitudinal stiffness of unidi- rectional ply E x * = 0.88Tr; for [0/90] E 1 ° = 0.47Tr, and so on. Now, we see that trace of a carbon/ep- oxy material is the one and only number that can defne all laminate stiffness com- ponents: The frst component, Q xx , is f- ber-dominated and contributes to 88 per- cent of trace, while the other two, Q yy and Q ss , are matrix-dominated and contribute only 12 percent. This may be obvious, but with trace we can defne their contribu- tions quantitatively. Trace is mathemati- cally exact and precise, therefore, no ad- ditional assumption is required. Its value can be determined from one uniaxial test, avoiding the more diffcult shear test. Another beneft of trace is that lami- nated, rather than unidirectional, cou- pons can be used. Laminate coupons are closer to laminated structures and have the same uncertainties, such as processing defects. We further recom- mend the use of coupons with holes in place of smooth, plain ones, to repre- sent manufacturing defects or damage from operation. Coupons with holes have less data variation, lower loads and allow for easier detection of defect and damage. This approach also can simplify test- ing for environmental conditions, such as hot-wet. Work by a variety of research- ers (e.g., Tay, Miyano, Nagada, Ha, Net- tles and Rhead) has shown that stiffness changes due to environmental condi- tions can be regarded as small and can be approximated with an error range of 1 to 2 percent, and use of the time-temper- ature superposition principle is capable of simulating temperature and moisture environmental effects on mechanical properties. Shift factors have been de- veloped to approximate these effects on stiffness and strength. In conclusion, reliable design allow- ables can be very quickly generated with this proposed approach, reducing the number of test coupons required from more than 1,000 to fewer than 100, and testing costs from hundreds of thou- sands of dollars to less than $20,000. Tests actually conducted will be for validation of prediction, not data gen- eration. One quasi-isotropic laminate sample can completely defne not only the stiffness of all laminates, but also the failure stress and strain of open-hole tension and compression. Therefore, with this single panel, the entire design allowables set can be defned, and select- ed tests on critical loading conditions can be conducted in a matter of hours — ready for next-day delivery. I hope that readers of this column will join me on site or online at the upcom- ing Composites Durability workshop, July 27-29 at Stanford University (here's a link for more information: www.stanford. edu/group/composites/cdw19/) to learn more about this exciting approach and to see presentations of work that verifes its accuracy. 0714HPC SpeakingOut-OK.indd 12 6/17/2014 10:11:29 AM