Repeated bending and indentation response of a bio-based sandwich panel with flax/epoxy skins and a palm-wood core

This study investigates the mechanical behavior of bio-based sandwich panels (BBSP) composed of flax/epoxy skins and a palm wood core (PWC) under monotonic and repeated bending and indentation loading. PWC, sourced from northern and southern regions of Algeria, was prepared along three principal material directions to capture its anisotropic response. Three-point bending and indentation tests, including Repeated Progressive Loading (RPL), were conducted to assess both static and cyclic behavior. The bending results highlight the anisotropic nature of PWC, with fiber-aligned specimens particularly from the southern region, exhibiting superior mechanical performance. RPL tests reveal the onset of permanent deformation from early cycles; however, below a critical load threshold, the material maintains significant elastic recovery, with residual displacement remaining below 10% of the maximum. The main failure mechanisms identified include core crushing, skin cracking, and interfacial delamination. Acoustic emission analysis confirms that low-amplitude signals (Type A, ∼93%) are dominant and associated with microcrack initiation during the elastic phase. Indentation results show progressive plastic deformation and stiff degradation, accompanied by energy dissipation and permanent deformation. Under cyclic indentation, PWC exhibits a reduction in reduced modulus and hardness followed by stabilization around a normalized cycle ratio of 0.2 due to plastic compression. In contrast, BBSP does not reach stabilization, showing continuous degradation throughout the loading cycles, as reflected by the progressive increase in elastic and plastic energy components up to failure.