A new laminate in double-double

by Professor Stephen W Tsai ( Standford University )

Aeronautics & Astronautics -  Stanford University - USA

Double-double laminates are simplified by 2 single parameters in trace for stiffness and the average strength of failure envelopes.  Materials and laminates can now be ranked and scaled so one test for each material can replace hundreds of test with no expectation now needed to generate design allowable.  DD in based on 4 plies in [±A/±B], and can be homogenized with repeated building blocks as few as 4.  

When used with thin plies pre-plied DD tape and sheets can be as thin as a unitape, and can be laid along one axis for laminated structures without cross-plying.  Such homogenized structures can be tapered to save 50 perent weight.  Quad laminates of 0, ±45, 90 plies can not be so easily homogenized, and tapered.   Other features of DD include having ply drops in singles place on the exterior surface rather than those with 2 or 4 plies with mid-plane symmetry required Quad and placed in the interior.  

Unlike Quad DD laminates have not interior discontinuities.  Homogenized laminates are as easy to design as metals but better than metals because DD is orthotropic and can be tapered.  It turns out DD is much simpler and more rational than Quad.  DD laminates are continuous and can interpolate and zoom (Quad cannot).  Structural properties can be represented by one table because it is based on one family (Quad cannot and the number of plies in the laminates defines one family.  Then there are no limit how many Quad families there are!).  

DD is orthortropic (balanced) while Quad is not.  Another DD adventage is the laminate stiffness and strength can be linked directly to those of fibers without micro mechanics.  To understand DD is no different from Quad or any other laminates.  The only new concept is the invariant which can be easily shown as part of coordinate transformation.  There are master curves and properties from which specific material and laminate can be scaled, without recalculation.  If one material is 20 percent higher than another material, that 20 percent appies to all laminates.  Conversely, if one laminate is 20 percent higher than another laminate, that 20 percent applies to all materials.  

Such simplicity is evident with DD, but not so with Quad because Quad cannot be covered in one family.  In tme DD will replace Quad.  It is inevitable.

Stephen W Tsai

Born in Beijing, he earned his D.Eng at Yale University in 1961.  He started Journal of Composite Materials as its Editor-in-Chief in 1967; discovered Halpin-Tsai equation in 1970, and Tsai-Wu failure criterion in 1975; elected to the US Academy of Engineers in 1995; discovered Tsai’s modulus in 2015 as the single parameter for laminate stiffness; new lamination in double-double in 2020 for weight reduction; ranking of materials and laminates in 2021 that can scale design and testing.

He is the winner if the Guggenheim Medal for 2025 of the American Institute of Aeronautics & Astronautics, and the Spirit of St. Louis Medal for 2025 of the American Society of Mechanical Engineering for their Structures, Structural Dynamics and Materials Conference.