Dynamic behaviour of auxetic gradient composite hexagonal honeycombs /L. Boldrin a , S. Hummel a , F. Scarpa a,b, ⇑ , D. Di Maio a,b , C. Lira a , M. Ruzzene c , C.D.L. Remillat d , T.C. Lim e , R. Rajasekaran f , S. Patsias f - Boldrin,L.