DOUBLE-DOUBLE Simplifying the Design and Manufacture of Composite Laminates Second Edition

Abstract

The concept of 'Double-Double' (DD) laminates facilitates homogenization, which can make mid-plane symmetry unnecessary. Moreover, single ply drops may be placed on exterior surfaces, mitigating interior defects. Laminates may be designed with little or no warpage, are easy to inspect and easy to repair. DD laminates are automatically balanced and dispense with the 'ten percent rule.' A single DD architecture can be optimized to serve an entire component, in contrast to multiple laminate stacking sequences and may be highly tapered, when transitioning to different thicknesses, to save weight and cost. It has recently been shown that the trace of the stiffness matrix is a sufficient single parameter (as opposed to four) to characterize the stiffness of all laminates, so materials can be rated, structures can be scaled without re-calculation, and testing is simplified. The strength of a laminate is determined by a single parameter defined by the area of a failure envelope and can be used to rate the strength of materials and laminates alike. Scaling for strength is similar to scaling for stiffness, with no re-calculation. Testing for design allowables is simplified for DD. Only tensile data is required because each laminate has the same compressive and shear strengths for all materials, and laminate strength can be related to fiber strength directly, like that for stiffness. The one parameter related to the area of the envelope facilitates this approach. Materials and laminates become equivalent and interchangeable, through their single parameter, linked through their transformation and interpolation properties in a compact, elegant, continuous field. This is a paradigm shift from the use of a collection of discrete quad laminates with 0, ±45, and 90 plies which have dominated laminate design approaches for the past 60 years. Laminate ultimate strength is controlled by fiber strength through longitudinal laminate strengths only. Design allowable determination is made simple and predictable. Legacy testing is more for calibration and validation rather than a requirement to characterize the properties of the material. Consequently, a lack of data can no longer derail innovation. Extension to master envelopes for coupons with open holes has been demonstrated. Analogous extensions to other stress raisers and local damage can be expected. Lamination parameter plots can now replace carpet plots. All laminates can be portrayed on a single plot. DD is rational and simple, and a compelling case is made in this book for its acceptance as the default starting point in the design of laminated composite structures. It represents a seismic change from how we have designed laminated composite structures until now and the breadth and depth of investigations presented should convince all structural analysts and designers to explore this approach.