Double-double laminates
Double-double composite laminates
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Double–double (DD) laminates, or DD laminates, are a class of composite laminates composed of repeated four-ply sub-laminates arranged in the form [±Φ/±Ψ], where the ply orientations are defined by two continuous angle parameters, Φ and Ψ. Unlike conventional composite laminates that employ a limited set of discrete ply angles, DD laminates use continuous variation of ply orientation.[1][2][3] The concept is employed in structural engineering applications involving composite materials.
Mechanical behavior
Studies have reported that, when DD laminates are manufactured using thin plies and a sufficiently large number of repeated sub-laminates, the laminate response can be approximated as homogenized.[4][5][6] Under these conditions, the overall mechanical behavior has been described as similar to that of a symmetric laminate, even though the stacking sequence is not explicitly symmetric.[4][5][6] A characteristic feature of this homogenization is the reduction of bending–stretching coupling effects, such that the corresponding terms in the ([B]) stiffness matrix become negligible.[7][8][9]
Applications and manufacturing considerations
DD laminates have been discussed in the literature in the context of lightweight structural applications, particularly where changes in laminate thickness are required.[10] Under homogenized conditions, tapering has been reported to be implemented on a ply-by-ply basis using single-ply drops, without the requirement to maintain mid-plane symmetry.[4] In addition, the use of continuous ply-angle parameters has been presented as allowing a broader range of stiffness tailoring compared with conventional quadriaxial laminates, which typically rely on standard ply orientations of 0°, ±45°, and 90°.[1] DD laminates are typically described as being manufactured by stacking of repeated four-ply sub-laminates, following the prescribed angle parameters.[1]