Approximation of pre-twisted Achilles sub-tendons with continuum-based beam elements

Achilles sub-tendons are materially and geometrically challenging structures that can nearly undergo around 15% elongation from their pre-twisted initial states during physical activities. Sub-tendons’ cross-sectional shapes are subject-specific, varying from simple to complicated. Therefore, the Achilles sub-tendons are often described by three-dimensional elements that lead to a remarkable number of degrees of freedom. On the other hand, the continuum-based beam elements in the framework of the absolute nodal coordinate formulation have already been shown to be a reliable and efficient replacement for the three-dimensional continuum elements in some special problems. So far, that element type has been applied only to structures with a simple cross-section geometry. To computationally efficiently describe a pre-twisted Achilles sub-tendon with a complicated cross-section shape, this study will develop a continuum-based beam element based on the absolute nodal coordinate formulation with an arbitrary cross-section description. To demonstrate the applicability of the developed beam element to the Achilles sub-tendons, 16 numerical examples are considered. During these numerical tests, the implemented cross-section descriptions agreed well with the reference solutions and led to faster convergence rates in comparison with the solutions provided by commercial finite element codes. Furthermore, it is demonstrated that in the cases of very complicated cross-sectional forms, the commercial software ANSYS provides inflated values for the elongation deformation in comparison with ABAQUS (about 6.2%) and ANCF (about 9.4%). Additionally, the numerical results reveal a possibility to model the whole sub-tendons via coarse discretization with high accuracy under uniaxial loading. This demonstrates the huge potential for use in biomechanics and also in multibody applications, where the arbitrary cross-section of beam-like structures needs to be taken into account.