Fracture in the complex balsa cores of fiber-reinforced polymer (FRP) sandwich beams was analyzed. The cores were composed of high- and low-density balsa layers separated by a circular adhesive interface or FRP arch. The balsa layers were cut from panels which consisted of balsa blocks adhesively bonded together. Failure in the beams was initiated by cracks propagating through the balsa core thickness. The crack locations could be predicted using the Tsai-Wu failure criterion. Cracks initiated in the lowest density blocks due to their low fracture toughness. In mixed-mode fracture, crack propagation in the radial longitudinal (RL) plane prevailed due to the low fracture toughness in RL fracture of Mode I. In pure Mode II, propagation occurred in the RL and TL (transverse longitudinal) planes to the same extent since the toughness in RL and TL fracture is similar. Cracks were not able to propagate through the transverse adhesive joints between blocks if the bonding was good. If however the bonding was poor, interface failure occurred and cracks could propagate through the adhesive layer. (C) 2014 Elsevier Ltd. All rights reserved.