Consider a balloon (with slightly thicker walls and possibly not rubber).
It contains air, a sealed volume.
Squash it. The air pressure will change.
ignoring the isothermal/adiabatic debate, can someone suggest an approach in mechanica such that the pressure applied to the internal surfaces is correct for each loadstep?
That's a difficult one... My first choice would be to analyze the structure's deformation at different internal pressures, and compute the deformed volume. The combination of a certain pressure and deformed volume corresponds to a certain mass of air. Find the combinations of applied load and internal pressures that results in a constant air mass. That should give you an indication of load-deformation path...
How do you compute the deformed volume? If the wall is "thin" is guess it can be approximated with the external volume. It it's rotationally symmetric, it can possibly be approximated using Pappus theorem: http://en.wikipedia.org/wiki/Pappus%27s_centroid_theorem. Or one can export a vrml, import that into Creo and measure the volume. A lot of work though...
Approach no 2: model the air as a part, with a soft material with material properties corresponding to air. I'm not 100% sure air behaves like this though... (*googling*) ... I think poisson's ratio should be -0.999 (can't be -1) and Young's modulus so that th material corresponds to the ideal gas law...
/Mats Lindqvist/Econocap AB/Sweden/
Pressure could be made a parameter based on volume (linked by BMX?) to a simple calculation.
The first problem with this incarnation of the thought experiment I encountered is how does one calculate the internal volume of the deformed shape at any time step.
I have come across that question from customers several times before. A measure that tracks the volume of a deformed part, or volume region. Another enhancement that I would like to see, is the ability to track the coordinate of a point, or a distance in the model. This can be accomplished using computed measures, (original location+displacement) but it should be easy to implement as a pre-defined measure. For example, I have tried to model and optimize an archery bow, using a large displacement analysis (LDA). I can't use a beam to model the bow string, nor a rigid link - it doesn't work in an LDA, not even in Creo2. It seems Creo can't run models containing entities with zero rotational stiffness even though it is deformed in a stable manner. It is a bit of a hassle to create measures to calculate the length of the imaginary bow string, and to create an optimization that ensures that the bow is deformed to a specified string length. This functionality would be desireable when designing/optimizing non-linear springs, such as an archery bow. In an LDA, being able to use beam elements that can undergo large rotation as a rigid body with beam releases in both ends would solve this. It would behave like a cord.