A single slave-node-based penetration criterion is used. Fluid will penetrate into the surface between the contacting bodies from one or multiple locations, which are exposed to the fluid, until a point is reached where the contact pressure is greater than the specified critical value, cutting off further penetration of the fluid.

When the fluid pressure penetration criterion is satisfied, the fluid pressure is applied normal to the surfaces. If the full current fluid pressure is applied immediately, the resulting large changes in the strains near the contact surfaces can cause convergence difficulties. For large-strain problems severe mesh distortion can also occur. To ensure a smooth solution, the fluid pressure is ramped up linearly over a time period from zero pressure penetration load to the full current magnitude.

You can specify the time period taken for the fluid pressure penetration load to reach the full current magnitude on newly penetrated surface segments. The penetration time period can be chosen to be a fraction of the initial increment size. If the accumulated increment size, measured immediately after the penetration, is greater than the penetration time, the full current fluid pressure penetration load will be applied; otherwise, the fluid pressure on the newly penetrated surface segments is ramped up linearly to the current magnitude over the penetration time period, possibly over a number of increments. When the penetration time is equal to 0, the current fluid pressure is applied immediately once the fluid pressure penetration criterion is satisfied. The default penetration time is chosen to be 0.001 of the current step time. The penetration time is ignored in a linear perturbation analysis.

*PRESSURE PENETRATION, PENETRATION TIME=n

The fluid can penetrate from either one or multiple locations of the surface. You must identify a node on the slave surface of the contacting bodies that defines where the surface is exposed to the fluid pressure. You must also identify a node on the master surface that defines where the surface is exposed to the fluid pressure if the master surface is not an analytical rigid surface (see “Defining analytical rigid surfaces,” Section 2.3.4). You can specify multiple nodes if multiple locations of the surface are exposed to the fluid. These nodes are always subjected to the pressure penetration load, regardless of their contact status. The fluid then starts to penetrate into the surface between the two contacting bodies from these nodes.

You must define the reference magnitude of the fluid pressure. You can define the variation of the fluid pressure during a step by referring to an amplitude curve. By default, the reference magnitude is applied immediately at the beginning of the step or ramped up linearly over the step, depending on the amplitude variation assigned to the step (see “Procedures: overview,” Section 6.1.1).

The fluid pressure penetration load will be applied to the element surface based on the pressure penetration criterion at the beginning of an increment and will remain constant over that increment even if the fluid penetrates further during that increment. A nodal integration scheme is used to integrate the distributed fluid pressure penetration load over an element; the variation of the distributed fluid pressure over an element will be determined by the load magnitudes at the element's nodes.

*PRESSURE PENETRATION, AMPLITUDE=name

*PRESSURE PENETRATION, OP=MOD (default)

*PRESSURE PENETRATION, OP=NEW

To account for the asperities on the contacting surfaces, a critical contact pressure, below which fluid penetration starts to occur, is introduced. The higher this value, the easier the fluid penetrates. The default value of the critical contact pressure is zero, in which case fluid penetration occurs only if contact is lost.

Linear perturbation analyses can be performed from time to time during a fully nonlinear analysis by including linear perturbation steps between the general analysis steps. Because contact conditions cannot change during a linear perturbation analysis, the fluid will not penetrate further into the surface and it remains as it was defined in the base state. The fluid pressure magnitude applied in the previous general analysis step, however, can be modified during a linear perturbation analysis step. In steady-state dynamic analyses (direct or modal—see “Direct-solution steady-state dynamic analysis,” Section 6.3.4, and “Mode-based steady-state dynamic analysis,” Section 6.3.8) you can specify both the real (in-phase) and imaginary (out-of-phase) parts of the loading.

*PRESSURE PENETRATION, LOAD CASE=1 (default)

*PRESSURE PENETRATION, LOAD CASE=2

Pressure penetration loads are available only for planar or axisymmetric elements. Each slave surface subjected to pressure penetration loading must be continuous and cannot be a closed loop. Pressure penetration loading cannot be used with a node-based slave surface and cannot use a finite-sliding, surface-to-surface formulation. The pressure penetration load applied at any increment is based on the contact status at the beginning of that increment. You should, therefore, be careful in interpreting the results at the end of an increment during which the contact status has changed. Small time increments are recommended to obtain accurate results.

When pressure penetrates into contacting bodies between an analytical rigid surface and a deformable surface, no pressure penetration load will be applied to the analytical rigid surface. The reference node on the analytical rigid surface should, therefore, be constrained in all directions. To account for the effect of fluid pressure penetration loads on the rigid surface, the analytical rigid surface should be replaced with an element-based rigid surface.