Thick adhesive layers have potential structural application in ship construction for the joining of a composite superstructure to a steel hull. The purpose of this study is to develop a mechanics model for the adhesive fracture of such lap joints under shear loading. Modified Thick-Adherend-Shear-Test (TAST) specimens made from a MMA-based adhesive and steel adherents are designed and fabricated. Crack initiation and growth of these joints is measured and monitored by Digital Image Correlation (DIC). An attempt is made to use a cohesive zone model to predict the magnitude of shear strain across the adhesive layer both at crack initiation and at peak load, and to predict the extent of crack growth as a function of shear strain across the adhesive layer. The ability of a cohesive zone model to predict several features of specimen failure is assessed for the case of an adhesive layer of high shear ductility.