  The mechanical performance of reinforced composites is strongly influenced at
different scales by the stress transferred at the matrix-fibre interfaces and
at any surface where material discontinuity occurs. In particular, the
mechanical response of elastomeric composites where the reinforcement is
composed by cords with helically wound fibres is heavily compromised by fatigue
and delamination phenomena occurring at cord-rubber as well as at the ply
interfaces, since rubber and polymeric matrices are mainly vulnerable to the
accumulation of deviatoric energy due to the shear stresses transferred across
the surfaces. Despite the large diffusion of composites in a vast field of
applications and the mature knowledge of their behaviour, some key mechanical
aspects underlying failure mechanisms are still partially unclear. For example,
stress amplification and strain localization are often difficult to predict by
means of analytical solutions and averaging techniques that usually conceal
stress gradients. In this work, we analyse coupling between torsional and
tensile loads in twisted cords, which are adopted in many cases to reinforce
composites and rubbers in tire applications. We provide a model characterized
by an enriched cord-matrix mechanical interplay able to theoretically explain
and predict actual stress distributions responsible for the onset of
delamination and fatigue-guided phenomena that are experimentally observed in
these composites. In particular, we demonstrate that the assumption of a
monoclinic/trigonal behaviour for the mechanical response of the hierarchical
strands allows to estimate, by means of analytical formulas and a
homogenization approach, hitherto neglected shear stresses at the
matrix-reinforcement interface. These stresses are transferred to the
neighbouring regions, leading to post-elastic behaviour and failure events.
