One of the dominant characteristics of Arctic ice is the development and persistence of leads. Leads are critically important for climate modeling since they are a source for new ice and an avenue for heat exchange between the atmosphere and ocean. Therefore a specific representation for leads in a constitutive model for pack ice is considered essential. Presented here is an elastic-decohesive constitutive model whose purpose is to specifically indicate when a lead is initiated, to provide the orientation of the lead, to give the mode of failure, and to allow for the numerical determination of the width of leads. Both laboratory and in situ data of sea ice provide the motivation for the basic structure of the model. Sample paths in stress and strain space are used to illustrate how the model can simulate failure. The model can predict multiple failure planes, a feature that is necessary to simulate crack branching. Preexisting planes of weakness such as those formed from previous leads can also be accommodated.