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Multidirectional prepregs and fabrics are not new but pre-plying thin plies can produce unique tapes with unmatched properties and simplicity, and significant reduction in lay-up time. With superior resistance to micro cracking and delamination, pre-plied thin ply tapes offer a possibility to decease simultaneously risk, weight and cost.
(Published on April 2006 – JEC Magazine #24)
STEPHEN W. TSAI, STANFORD UNIVERSITY YASUSHI KIYOBAYASHI, ITOCHU CORPORATION, TOKYO KICHIRO ISHIDA, MITSUYA COMPANY, FUKUI, JAPAN
Multidirectional prepregs and fabrics have been available for a long time. Orthotropic woven fabrics can range from highly unidirectional, balanced, biased to tri-axial. Cross ply and pi/4 quasi-isotropic prepreg are also available. With thin ply laminates, premature failure modes in micro cracking and delamination could be suppressed. Pre-plied thin ply tapes significantly reduce lay-up composite structures. Thus risk and weight could be reduced by having higher design allowable; cost reduced by faster and simpler tape lay-up process and less scrap when [+/-45] plies are pre-plied with .
Thin ply composites
Thin plies of 0.04 mm thick, 1/3 of the conventional 0.12mm, was made by a tow spreading process reported in JEC Composites, #18, June-July 2005, pp 31-33. Resistance to micro cracking and delamination, and a method of pre-plying were also reported in the same paper. The improved toughness is derived from a higher dispersion of thin plies as compared with thick plies; i.e., 3 times more plies when thin plies are used. Pre-plying was in part driven by an attempt to offset the increased number of thin plies in the lay-up process.
Pre-plying conventional 0.12mm plies leads to thick tapes. For a pre-plied [0/+45/-45] tape, the total thickness will be 0.36mm. Such thick laminated tapes are not suitable for tape laying: too rigid to lay up and to compact. To obtain higher percentage of  in a laminate, it becomes prone to delamination. If 50 percent  is desired, five  regular plies with thickness as much as 0.60mm will have to be grouped.
The ply grouping issue can be reduced when thin plies are used. Ply composition in a laminate can also be made more flexible and can better meet minimum gage requirement. These design options are added advantages of thin plies over and beyond higher inherent toughness. A pi/4 quasi-isotropic tape can be as thin as 0.16mm if that is desired. Plies as thin as 0.02mm have been made. Then the thinnest pi/4 laminate could be 0.08mm. Many combinations of pre-plied tapes are possible that can achieve a balance between desired percentage of , the total tape thickness that would be amenable for tape laying and satisfying minimum gage.
Engineering pre-plied tapes
The stiffness and strength relative to the axis of a pre-plied tape can be calculated using laminated plate theory and failure criteria. Although there are many combinations of off-axis ply angles, for most common aircraft wings and fuselage, combinations of  and [+/-45] are practical basic building blocks. However, pre-plied tape may have  plies if the loading and geometry call for it. This would be the case of a narrow dimension in the  degree direction where tape laying would be prohibitively difficult to do.
In the figure below, we show the normalized strength and stiffness along the tape axis as functions of number of  plies combined with one set of [+/-45]. For example, when three plies of  are combined with one [+/-45], the percent  in this pre-plied tape would be 60 percent. This example of the rule of mixtures relation is shown in figure 1 in green arrows; when three  plies are matched with one [+/-45] there will be 60 percent  in this pre-plied tape.
Both the strength and stiffness values are higher than that of the rule of mixtures. The off-axis plies of [+/-45] must have contributed to this synergism. The normalized values were derived from a specific CFRP (T300/5208) but other common CFRP’s were found to have the same trends. The synergism is shown in figure 2 for the same range of  plies. For the case of three  and one [+/-45] the stiffness increase is by a factor of 1.10 (in pink arrows); the strength, 1.16 (in blue) over the rule of mixtures relation (RoM).
If the axial strength and stiffness of the unidirectional ply are 1 500 MPa and 181 GPa, a pre-plied tape of [0/0/0/+45/-45] will have values of 1 500 x 0.6 x 1.16 = 1 044 MPa and 181 x 0.6 x 1.10 = 120 GPa, respectively. The strength is more than twice that of aluminum and woven fabrics. If higher strength CFRP is used a proportionally higher tape strength can be expected. These superior values however must be tempered with some reduction due to the gaps between courses in tape laying and the discontinuous off-axis plies.
Machine lay-up of pre-plied tapes
Cincinnati Machine, a pioneer in tape laying machines, showed the advantages of pre-plied tapes. Instead of unidirectional tapes for four directional courses of tape laying of a quasi-isotropic laminated composite panel, pre-plied tape can result in significant savings in time required.
In an example shown in figure 3, a flat panel with length/height ratio of 3.33, the number of courses for  ply was 9 as compared with at least 28 for the off-axis plies, shown in column B. For a panel of rectangular shape the course parallel to the long side would be fewer in number and simple and fast to lay-up. The time required per course was 3.7 minutes are compared with at least 7.6 minutes, shown in column E. Again off-axis courses consume more time for starting and stopping and produce more scrap. Thus when a pre-plied tape is used in one course along some  axis, the time savings would be nearly a factor of 7. The  direction is still time consuming because of the narrow width of the panel. The [+/-45] directions are difficult to lay up regardless of the aspect ratio of the panel.
A similar comparison shown in figure 4 was made by Vistagy, using their pioneering FiberSim software, between two directional courses along  and  without [+/-45] versus the conventional four courses. The time savings achieved with pre-plied tapes was also above 50 percent.
If lay-up one course along the horizontal axis of this curved panel and two courses along the vertical axis using a pre-plied tape of [0/0/0/+45/-45], the resulting panel will have 20 percent , 40 percent [+/-45] and 40 percent . The time savings from not laying +45 and –45 degree plies would be at least 40 percent. Actual savings in time and materials would be much higher than 40 percent because these off-axis plies are difficult to lay-up particularly near and at the corners of the panel and at ply drops. Cincinnati data shown earlier affirmed this difficulty. If improved laminate toughness from using thin plies is included in the consideration the risk, weight and cost of the panel can be reduced even more.
Thin plies are unique in making pre-plied tapes that can be used directly in tape laying machines. Instead of four directional courses, one direction may suffice for wings; two directions, for fuselage. Time consuming operation associated with laying-up off-axis plies such as [+/-45] can be eliminated. The processing advantages in simplicity, speed, and scrap mean less cycle time, less material, and higher yield from the same tool. Thin plies make tougher laminates; that means less risk, less weight and/or less cost.
Many issues are being investigated. Costs of thin plies and preplying are expected to be nominal because most processes are or will be automated. There are no intrinsic limits on tape dimensions and mass producibility. Other issues included the laps and gaps, scrap in tape laying, discontinuities in [+/-45] plies, and ply drop by pre-plied tapes. Results have been positive, thus far. Handling of tapes and their quality will improve with time. Pre-plied tapes for fiber placement would be a natural extension. The effects of gaps, tape tapering and discontinuous off-axis plies for narrow tapes needs to be determined. Engineering pre-plied tapes would require design and manufacture engineers and machine manufacturers to work together. This will increase cost-effectiveness and minimize rework of composite components. Such components made with pre-plied tapes will be lighter, tougher, more reliable and more affordable than those with fabrics and other material systems.