# 16. Cohesive Material Models

This chapter describes the theory and usage of cohesive models in development. There are typically two different types of cohesive models, intrinsic and extrinsic. Intrinsic models are used for cohesive surfaces that are known a priori and are included in the model from the beginning. These models by definition produce zero traction for zero cohesive separation and have a loading region before failure. Extrinsic models are used when cohesive surfaces are dynamically inserted based on some material criteria. These models typically are initialized to produce an equilibrium traction at zero separation based on the cohesive zone insertion criteria. Section 16.1 describes the intrinsic cohesive zone models in development, whereas Section 16.2 describes the extrinsic models.