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.
Issue link: https://hpc.epubxp.com/i/101340
commercial application by sara black carbon fber fabrics. illustration / karl reque C-PLY material, manufactured by Chomarat (Le Cheylard, France), features two plies of carbon fiber, at 0�� and a shallow angle (shown here at 45��), stitched together. C-PLY reportedly enables Sonoma, Calif.-based American Artisan Bicycles to achieve part quality and performance usually seen only in much more expensive autoclaved parts. duction bicycle frame from American Artisan Bicycles (Sonoma, Calif.). Bye-bye, black aluminum Why bi-angle material? The answer lies in the mathematical prediction of laminate failure, traditionally a dif���cult subject due to the multiple plies in various directions, differences in material properties from ply to ply and even outside in���uences, such as temperature or moisture. In a nutshell, if a quasi-isotropic laminate is subjected to increasing load in the direction of the 0�� ���bers, matrix cracking eventually occurs in the off-axis or transverse plies. This phenomenon, known as ���rst-ply failure, decreases matrix stiffness. Eventually, as loads increase, ultimate, or last-ply, failure occurs. The load levels at the two failure points and the difference between them depend on the layup and the ���ber/resin combination, among other factors. ���You put load on a quasi-isotropic laminate and the off-axis plies fail early and microcrack, due to shear,��� says Bob Skillen, founder and chief engineer at VX Aerospace (Morganton, N.C.), who has tested the material in aerospace parts. ���You���re essentially accepting microcracking in your design,��� he contends, but he points out that ���there���s no reason to do that.��� Because early methods for predicting progressive failure were unsophisticated, says one industry analysis expert, most designers compensated with conservative ���rst-ply-failure solutions. But adding Source: Chomarat Unbalanced fabric finds favor in new bike frame ���insurance��� plies ��� overdesigning ��� sacri���ces the design advantages of composites (see ���Learn More���). Tsai, therefore, pushed non-quasi-isotropic designs over the years and even developed spreadsheet-based failure analysis software, called MIC-MAC (for micro-macromechanical analysis), which quickly calculates optimized layups at any angle and predicts how they will behave, and fail, under load. Fig. 1, p. 48, shows that when the cross-ply angle is reduced in relation to the 0�� direction, the resulting laminate withstands signi���cantly greater stress before ���rst-ply failure and last-ply failure and generally performs better in many load applications, thanks to the reduction in interlaminar forces. The ���gure also shows that the stresses that cause ���rstply failure and last-ply failure equalize at a cross-ply angle of about 20��. Concern that very low-angle cross plies might result in ���potato-chip warping��� of the laminate is alleviated, says Tsai, if enough layers are stacked together. ���The difference between symmetrical and asymmetrical laminates disappears when 16 or more bi-angle layers are stacked,��� he explains. ���Continuous stacking makes the location of plies, and reversing the order of stacking relative to the mid-plane, irrelevant.��� In other words, the large number of repeated layers effectively homogenizes the laminate. The unexpected result is mechanical performance that matches that of autoclaved uni prepreg tapes, via easier vacuum-bag processing. Moreover, the Out-of-autoclave aerospace part Source: Chomarat An H-46 Chinook helicopter tunnel cover prototype, made with vacuum-infused C-PLY by VX Aerospace (Morganton, N.C.). resulting laminate is lighter and thinner than a quasi-isotropic counterpart because ���more, thinner plies make a stronger and tougher part than fewer, thicker plies,��� Skillen explains. Such a layup also has a >30 percent higher ���rst natural frequency, a factor that often dominates laminate design, Tsai adds. With a higher ���rst natural frequency, the operating range of the part in terms of vibration performance is greater. From theory to practice Tsai knew, however, that hand cutting of shallow angles using uni tapes was impractical. The key was to make available a fabric with 0�� and a shallow angle already plied together, which would en- january 2013 | 47