Double-double structures offer simplified design, testing and manufacturing

JEC Composites Magazine: You developed the Double-Double concept more than a decade ago. Can you remind us how Double-Double is defined?

Stephen Tsai: The traditional ply schedule, called the Quad, features a layup of 0°/45°/-45°/90°, with several variants of six, eight, or 10, or 15 or more plies. This is a generally accepted ply schedule, it’s highly qualified, and one that the entire composites industry has become used to. But it has problems. First, it produces a heterogeneous laminate whose mechanical properties can only be determined by rigorous testing. Second, the Quad is difficult to use in tapered structures. Double-Double solves these problems. It’s a 0°/-60°/60°/0° ply schedule without variants, which means there is a repeating and predictable building block structure. Because of this, Double-Double structures are homogeneous, like steel, easy to use in tapered structures and offer predictable, linear mechanical perfor­mance properties.

Explain further about how a Double-Double laminate is better suited for tapering.

S.T.: The key is homogenisation. Whenever you taper, you have to drop a ply. With Double-Double, because of its repeatable, predicta­ble building block structure, dropping a ply does not impact mechanical performance. With a Quad, however, you don’t know which ply you will drop, and because it’s non-homogenous, loss of a ply has unpredictable impact on laminate properties.

Another key component of Double-Double is the use of thin plies. Can you explain that?

S.T.: Yes. As you know, interlaminar failure is a major concern for the performance of any composite structure. The use of thin plies in a Double-Double laminate does an excellent job of suppressing interlam­inar failure. The Double-Double we provide uses 40-gsm plies, and such a structure readily mitigates crack propagation. Because of this, Double-Double does not require a cross-ply. From a manufacturing perspective, this allows you to increase fibre placement speeds by up to 6X.

You mentioned that Double-Double makes laminate properties easier to predict than Quad laminate properties. What does this mean?

S.T.: We promote the use of Trace values with Double-Double. Trace the sum of the three principal elastic constants: the longitudinal, the transverse and the shear. Trace is a constant, invariant for each combi­nation of fibre and resin matrix. So, the only variable in a laminate is the number of plies, and because Double-Double is homogenous, adding or subtracting plies increases or decreases properties proportionally. In this way, they become predictable. Testing, then, becomes a way to validate and calibrate properties. We have many years of testing data that shows the predictability of Double-Double laminates and how they compare to the predictability of Quad laminates. Once you see this data, the value of Double-Double becomes easy to understand.

Other than your presentation at JEC World 2026, what new Double-Double developments have you featured at the show?

S.T.: We had samples of pre-plied Double-Double prepregs, property data we developed in cooperation with the National Institute for Aviation Research (NIAR), and possibly some new information on the use of Double-Double in negative thermal expansion applica­tions – shrink fitting. This could have application in aerostructures manufacturing to help minimise use of fasteners. It can also be used in pressure vessel fabrication for storage of cryogenic hydrogen.

Cover photo: Stephen Tsai presenting its book Double-double