High-Performance Composites

JUL 2014

High-Performance Composites is read by qualified composites industry professionals in the fields of continuous carbon fiber and other high-performance composites as well as the associated end-markets of aerospace, military, and automotive.

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J U L Y 2 0 1 4 | 1 1 SPEAKING OUT SPEAKING OUT Stephen W. Tsai is professor research e m e r i t u s i n t h e D e p a r t m e n t o f Aeronautics & Astro- nautics at Stanford University. He holds a BE and D.Eng from Yale University. Tsai is a lifetime member of the National Acad- emy of Engineering and the International Committee on Composite Materials (ICCM), a life fellow of the American Society of Mechanical Engineers (ASME) and fellow of the Society for the Advancement of Material and Process Engineering (SAMPE). He has trained thousands of engineers in composite materials and is a coauthor, with the late Edward M. Wu, of the Tsai-Wu failure criterion for aniso- tropic composites. O OVERNIGHT DESIGN ALLOWABLES? AN INVARIANT-BASED METHOD FOR ACCELERATING AEROSPACE CERTIFICATION TESTING ne of the problems with compos- ite materials, as we all know, is the number of properties that must be measured and reported. At a minimum, each unidirectional ply has four stiffness properties: longitudinal and transverse Young's moduli, plus one Poisson's ra- tio and one in-plane shear modulus. In addition, it has fve strength properties: longitudinal tensile and compressive strengths, transverse tensile and com- pressive strengths and in-plane shear strength. Then, when multidirectional laminates are made of unidirectional plies, the possible variations in stiffness and strength properties are limitless. These static stiffness and strength properties of plies and laminates are cur- rently also tested under different temper- ature and moisture combinations, such as room-temperature-dry, cold-dry, hot- dry and hot-wet. Strengths are measured for smooth, plain (unnotched) coupons as well as those with open or flled holes, and additional loading conditions of fa- tigue and compression-after-impact also are tested. Thus, the combination of tests easily totals 1,000 specimens, requiring months of testing and a huge budget. This reality has limited the adoption of new materials and processes for com- posite structures, because allowables data are not promptly available. It is my purpose here, and in an upcoming work- shop (described below), to put forth a different approach that will accelerate design allowables generation. It was de- veloped with my co-investigators: José Daniel Diniz Melo, a consulting profes- sor; Alan Nettles, a NASA (Huntsville Ala.) composite materials engineer and visiting scholar, both in the Department of Aeronautics & Astronautics at Stan- ford University (Palo Alto, Calif.); Dr. Waruna P. Seneviratne, technical direc- tor/scientist at the National Institute for Aviation Research (NIAR, Wichita, Kan.); Yasushi Miyano, professor of engineer- ing at Kanazawa Institute of Technology in Japan (Ishikawa, Japan); and Jared Nel- son, a doctoral research assistant from Montana State University (Bozeman, Mont.). We are proposing a radically new approach that will reduce the physical testing required for material evaluation, thereby accelerating the development of design allowables for new composite materials and processing. The frst step in this process is recogni- tion that trace of the stiffness matrix of an orthotropic material is invariant and em- bodies the entire stiffness relationship of the material. (Trace is the material factor derived from the mathematical matrix that is generated for a composite lami- nate, considering the four stiffness prop- erties mentioned above, transformed to a single, all-inclusive property.) For the laminate stiffness, trace [A] = A 11 + A 22 + 2A 66 . In fact, working with NIAR, we found that for nearly 20 different carbon fber/epoxy materials, the coeffcient of variation (CV) for longitudinal stiffness is 1.5 percent (see the table below) — far smaller than the experimental variation. We propose to use the median value of these normalized factors from many different materials and call it the master ply. We assert that the trace is the single value that can represent all the stiffness components of a composite material in both uni- and multidirectional laminates. Thus, if a given material can be rep- Trace Normalized Factors Material [0] Qxx* Qyy* Qss*, Es Tr, GPa Trace* Ex* Ey* nu/x* Ey/Ex IM7/977-3 0.88 0.046 0.036 218 1.00 0.88 0.046 0.35 0.052 T800/Cytec 0.90 0.050 0.027 183 1.00 0.89 0.049 0.40 0.056 T7 C-PLY-55 0.88 0.057 0.034 139 1.00 0.87 0.058 0.30 0.066 T7 C-PLY-64 0.87 0.057 0.036 163 1.00 0.86 0.057 0.30 0.066 AS4/3501 0.86 0.056 0.044 162 1.00 0.85 0.055 0.30 0.065 IM6/epoxy 0.88 0.049 0.036 232 1.00 0.88 0.048 0.32 0.055 AS4/F937 0.89 0.058 0.027 168 1.00 0.88 0.057 0.30 0.065 T300/N5208 0.88 0.050 0.035 206 1.00 0.88 0.050 0.28 0.057 IM7/8552 0.90 0.048 0.028 192 1.00 0.89 0.047 0.31 0.053 IM&/MTM45 0.90 0.042 0.028 195 1.00 0.90 0.042 0.33 0.047 Master ply 0.883 0.050 0.034 187 1.00 0.877 0.050 0.305 0.0609 Std dev 0.013 0.005 0.005 28.468 0.001 0.014 0.006 0.034 0.5% CV 1.5% 10.9% 15.8% - 0.1% 1.5% 11.1% 11.3% 9.0% Table: Trace-normalized factors for various carbon fiber/epoxy composites and the proposed "master ply." Note: CV = coefficient of variation. All values other than Tr and percentages are dimensionless.. 0714HPC SpeakingOut-OK.indd 11 6/17/2014 10:10:58 AM

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