Optimization laminate

For laminate optimization, which we examined in Section 7.7, we have some strong temptations. We could include many design variables. We could talk about which fibers we would deal with out of a collection of those offered by various manufacturers. In addition, we could consider which matrix materials, what percentage of fibers and matrix that we deal with, what orientation of each of the fiber directions, and the thicknesses of the various laminae. All of those various factors are potential design variables, and, in order to treat them, you must have a fairly complicated optimization scheme to be able to achieve the objective of actually tailoring a laminate for specific design requirements. 

However, there are some significant problems with using an optimization scheme because you can end up with a design that simply is not practical to make. We might judge that a laminate would need a certain minimum number of layers for various reasons. One of those 

Two keys to the future use of composite materials are (1) achieving lower raw material cost and (2) developing innovative fabrication techniques that are uniquely suited to the characteristics of composite materials. This duality of approaches is leading to considerable success with composite structures right now, but they also hold the key to the even wider use of composite materials in the future. Let s address the two keys individually. 

achieving lower raw material cost than at present is always an important economic factor. When the price for one material comes down relative to another, the point at which we trade-off between the two materials changes because cost is a factor in most designs. That statement is not meant to imply that engineers are not concerned about cost in some designs, but we must emphasize that some particular structures have functional requirements as the most important issue. Can they or can they not do the job Cost is not the primary issue in that case. We would naturally like to have a less-expensive Space Shuttle, but can we do the job that the Space Shuttle is now doing with a lower-cost structure We could use less-expensive materials, but would they be able to hold up, would they survive reentry, and would the astronauts be able to survive If the astronauts would not be able to survive, then clearly you would acknowledge that we must pay the added cost to get the job done, i.e., to ensure their safety. 

Suppose we change our attention from structures in which the driver is functional consideration alone to something like an automobile where cost is also extremely important. We can get the functional job done with other materials, like steel and aluminum and fiberglass in certain places and unreinforced plastic in others. Then, the question becomes can we make a material substitution that will enable us to compete with the cost of these other materials to do a job that with all the other materials we cannot accomplish That is a different kind of question, and then cost becomes an extremely important driver. And, as cost of advanced composite structures goes down, we can expect to see more and more utilization of advanced composite materials. 

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Additional improvements have been incorporated since 1966 with the availabihty of thinner float glass. Glass thickness and interlayer thickness have been studied to optimize the product for occupant retention, occupant injury, and damage to the windshield from external sources (30,31). The thinner float glass windshields are more resistant to stone impacts than the early plate glass windshields. The majority of laminated windshields are made of two pieces of 2—2.5 mm aimealed glass and 0.76 mm of controlled adhesion interlayer. 

By comparison, high performance composite laminates ate not only ctossphed like plywood, but actually have laminae stacked at very specific angles to one another to achieve optimal uniform properties in the x—y plane (2). 

Let s consider one rather restricted structural optimization problem, that of a composite laminate. You have seen claimed as attractive advantages of composite structures the fact that we can orient the laminae in a laminate to our heart s content to try to get the most efficient structure. This characteristic is totally unlike what is possible with metal structures. This laminate orientation flexibility is certainly an advantage, but how do we use it ... 

The method of optimization is a brute-force search technique. All the possible laminates that can be obtained by changing the individual laminae orientations by 5° increments are candidates for the optimization process. We consider RC7 because this program is widely used and because it is representative of the brute-force search technique. The basic question is because we must carry a certain load, what laminate do we need We have no idea how many layers are required, much less their orientation, but we must start someplace. 

The analytical tools to accomplish laminate design are at least twofold. First, the invariant laminate stiffness concepts developed by Tsai and Pagano [7-16 and 7-17] used to vary laminate stiffnesses. Second, structural optimization techniques as described by Schmit [7-12] can be used to provide a decision-making process for variation of iami-nate design parameters. This duo of techniques is particularly well suited to composite structures design because the simultaneous possibility and necessity to tailor the material to meet structural requirements exists to a degree not seen in isotropic materials. 

Some of the essential attributes of the laminate design process with optimization concepts were described in general terms. The process is indeterminate, unlike that for a metal plate. An iterative procedure must be used to guide a design toward satisfaction of the design requirements. 

Curing of Polyimlde Resin. Thermoset processing involves a large number of simultaneous and interacting phenomena, notably transient and coupled heat and mass transfer. This makes an empirical approach to process optimization difficult. For instance, it is often difficult to ascertain the time at which pressure should be applied to consolidate the laminate. If the pressure is applied too early, the low resin viscosity will lead to excessive bleed and flash. But if the pressure is applied too late, the diluent vapor pressure will be too high or the resin molecular mobility too low to prevent void formation. This example will outline the utility of our finite element code in providing an analytical model for these cure processes. 

Rechak, S. and Sun, C.T. (1990). Optimal use of adhesive layer in reducing impact damage in composite laminates. J. Reinforced Plasl. Composites 9, 569 582. 

Any on-line process control model used for computer-aided manufacturing of high-performance composite laminates must include a thorough treatment of void stability and growth as well as resin transport. These two key components, along with a heat transfer model and additional chemorheological information on kinetics and material properties, should permit optimized production of void-free, controlled-thickness parts. A number of advances have been made toward this goal. 

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