Analytical modeling of local stresses at Balsa/timber core joints of FRP sandwich structures

In order to optimize sandwich structures, core materials of different density and thus different stiffness and strength are often combined. The material discontinuities at joints of dissimilar core materials may lead to abrupt changes of the shear angles and local bending of the face sheets at these locations, both of which may cause significant stress concentrations. Two analytical models have been developed to estimate local stress concentrations at butt and scarf joints of dissimilar sandwich cores. The first model predicts local axial stresses in the face sheets at scarf joints while the second model predicts local shear stresses in cores at both butt and scarf joints. The models are based on an existing closed form model and have been validated by FEM for Balsa–Douglas fir butt and scarf joints, both implemented in a sandwich with glass fiber-reinforced polymer (GFRP) face sheets. The models are applicable for scarf joints with angles of termination between 20° and 60°. They also demonstrate the positive effect of scarf joints compared to butt joints. The axial stress concentrations in the face sheets and shear stress concentrations in the cores almost disappear if a butt joint is changed to a scarf joint configuration.