LAUSR

Abstract In an analysis presented in 1909, Poynting (page 546 of [1] ) had shown that for finite elastic deformations, the lines of greatest extension and contraction are inclined to the diagonals of the rhombus into which a square is sheared. The implication of this finding was that when slender structures are twisted, they undergo elongation as experimentally verified by Poynting's measurements with wires [1] . While many theoretical analyses have shown the possibility of Poynting and reverse (inverse) Poynting effect [2] , [3] , [4] , [5] , [6] , [7] , measurements of such effects are rather sparse [8] , [9] , [10] , [11] . We present here measurements of a highly nonlinear Poynting effect, including its reversal from positive to negative (elongation to compression) direction during torsion. Such atypical behavior is exhibited by a rather exceptional material system that has a pantographic internal structure. For this material system, the classical Cauchy-type continuum model fails and a 2nd gradient continuum model is necessary to describe many of its deformation behaviors.