I think this is a prime opportunity to use the inertia relief option. I do not have a lot of experience with this option but this is what it is designed to do. If there are others who do not agree please let me know.
When you are setting up your model, do not apply any constraints but keep your load. When you setup your analysis there is a check box for inertia relief. Thats all it should take.
An easy way to find the pressure to cause yielding: Create a parameter called "PRESSURE" and give it a start value. When you create your pressure load, rather than punching in a value, type in PRESSURE. After you run your initial analysis, run a sensitivity study and vary your pressure through a set up values. You will then be able to identify a stress value for yielding much easier than re-running several analyses.
Here is some background information for inertia relief via PTC:
Using this option, Mechanica analyzes your model as if it were floating freely in space, without any constraints, but with the loads applied.
Note: You can also use the Inertia Relief option to analyze an underconstrained model. However, if the model already has some constraint sets, then Mechanica ignores these constraints during the analysis.
During an analysis with the Inertia Relief option selected, Mechanica internally creates a new Cartesian coordinate system (UCS) and defines a constraint set containing three-point constraints with respect to the newly created UCS. Mechanica also automatically applies body loads that balance your applied loads.
The three-point constraints affect the displacement solution but not the stress solution.
An analysis with inertia relief should always run fine, as long as there is non-zero stiffness for all the six degrees of freedom between two bodies.
Before you run an analysis with inertia relief, Mechanica asks you whether you want error detection to be performed. If you do, Mechanica checks for various modeling conditions including the presence of multiple bodies in the model. If Mechanica encounters more than one body, then it displays a message indicating the number of separate or disjoint bodies that it finds. You can use this information to determine whether parts you thought were connected are truly connected. If you see an unexpected number of bodies, you may want to cancel the analysis or study and correct the assembly.
For an analysis with Inertia relief ensure that the model does not have more than one disjoint body. If multiple disjoint bodies exist in your model, then the analysis fails with an underconstrained error. To run an inertia relief analysis with multiple disjoint bodies, ensure that all the disjoint bodies are connected in such a way that there is no relative motion between the bodies. If the connections are such that relative motion exists between any two bodies in the model, then the inertia relief analysis fails with an underconstrained error. For example, if two bodies are connected by bolts, then the inertia relief analysis fails if the bolts have no stiffness for some degrees of freedom.
