Cohesive surface interaction

The following usages of surface-based cohesive behavior are verified in these tests:

• Cohesive behavior properties

• Cohesive behavior restricted to initially contacting nodes

• Cohesive behavior allowing repeated contacts

• Coupled traction-separation behavior

This test verifies damage modeling with cohesive surfaces using different damage initiation criteria and damage evolution laws to simulate the failure of cohesive layers.

The maximum separation and quadratic stress damage initiation criteria are used. Damage evolution is defined based on either effective displacement or energy dissipated. Linear, exponential, and tabular softening laws are defined to specify the nature of the evolution of the damage variable. Each damage model is verified for damage in pure normal and two pure shear modes (one shear mode for two-dimensional and axisymmetric elements). The dependence of damage evolution on the mode mix measure specified in tabular, power law, or Benzeggagh-Kenane form is also considered in this test.

This test verifies modeling “breakable ties” using cohesive behavior and progressive damage. A box and its lid, both modeled with solid elements, are tied together via cohesive behavior at the interface. Default cohesive behavior options are used. The bottom of the box is fixed using prescribed boundary conditions, while the lid is pulled apart via prescribed displacements applied through a kinematic coupling acting on the top surface of the lid.

The maximum stress damage initiation criterion is used. Damage evolution is defined using effective displacement with a linear softening law.

This test verifies modeling “sticky contact” using cohesive behavior and progressive damage. A box, modeled as a rigid body, contains three balls that are modeled using shell elements. The box is completely fixed; the balls, initially suspended in the gap between the top and bottom walls of the rigid box, are given identical initial velocities resulting in their simultaneous impact with the bottom wall of the box. The behavior of each of the balls (Ball A, Ball B, and Ball C) is described below.

Ball A has cohesive behavior without progressive damage defined between its surface and the top wall of the box. No cohesive stiffness is specified, and the default values are used. When this ball impacts the bottom wall, it does not experience any cohesive forces, since no cohesive behavior is prescribed for the interaction between this ball and the bottom wall. The ball rebounds and strikes the top wall of the box, where cohesive forces act to prevent it from rebounding again and ensure that it remains stuck to the top wall for the rest of the analysis.

Ball B has cohesive behavior with progressive damage defined between its surface and the bottom wall of the box. No cohesive stiffness is specified, and the default values are used. The damage model uses the maximum stress damage initiation criteria and has damage evolution defined based on effective displacement with a linear softening law. In addition, postfailure cohesive behavior is enforced for recurrent contacts at nodes on the secondary surface. When this ball impacts the bottom wall and tries to rebound, the cohesive forces act to restrain it from rebounding. However, since the elastic energy of the collision is high, eventually damage initiates, ultimate failure occurs, and the ball breaks free. It then goes on to hit the top wall. There is no cohesive behavior defined with the top wall, so Ball B does not experience any cohesive forces and bounces back and impacts the bottom wall again. Since postfailure cohesive behavior is allowed, cohesive forces reactivate when the ball attempts to rebound again. However, on second impact, the momentum and kinetic energy of the ball is considerably less than during first impact, owing to the dissipation that occurred due to the damage work done during first impact. The cohesive forces this time are sufficiently high to restrain it from rebounding again, and the ball remains stuck to the bottom wall for the rest of the analysis.

Ball C has exactly the same cohesive behavior and progressive damage defined between its surface and the bottom wall as Ball B. As with Ball B, when this ball impacts the bottom wall and tries to rebound, the cohesive forces act to restrain it from rebounding. However, since the elastic energy of the collision is high, eventually damage initiates, ultimate failure occurs, and the ball breaks free. It then goes on to hit the top wall. There is no cohesive behavior defined with the top wall, so Ball C does not experience any cohesive forces and bounces back and impacts the bottom wall again. Since no postfailure cohesive behavior is allowed, cohesive forces are not activated when the ball attempts to rebound following the second impact with the bottom wall. The ball rebounds again and keeps bouncing back and forth between the top and bottom walls throughout the rest of the analysis.