Negative and positive Poynting effects in tendon under simple shear

Shear load transfer is crucial for the redistribution of internal tendon loads and to prevent excessive local stress that can lead to severe damage and injury. To better understand this transfer mechanism, it is important to know the stress state. The aim of the present study is to investigate the normal and shear stresses in tendons sheared with the shear force applied parallel to the fascicles and collagen fibers. A key novelty of the paper is the simultaneous measurement of normal and shear forces, as well as the amount of shear of tendon samples under simple shear. For the sake of simplicity, a more straightforward model is employed to describe the normal and shear behavior of tendons. Expressions were simultaneously fitted to the measured normal and shear stresses. The results reveal that the shear behavior did not exhibit any evidence of strain-stiffening, because the shear stress was approximately proportional to the amount of shear. However, compressive and tensile normal stresses, or positive and negative Poynting effects, respectively, were observed in different samples. Each tendon specimen was sheared along the orientation of the longitudinal fascicles and collagen fibers, which were maintained by random fiber networks associated with connective tissue and cross-link structures. Compressive normal stress indicates that random fiber networks did not influence the behavior or were not significant in a certain range, whereas random fiber networks contribution was more pronounced in the case of tensile normal stress. These findings suggest that the effects of random fiber networks, which can manifest over different length scales, play an important role in the state of normal stress in tendons under simple shear. Understanding how random fiber networks influence tendon mechanics could lead to better treatments for tendon injuries and help design biomimetic materials.