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Inertia Relief
Use the Inertia Relief option on the Static Analysis Definition dialog box to analyze an unconstrained model. You can use this option only for a linear static analysis. Using this option, Creo Simulate analyzes your model as if it were floating freely in space, without any constraints, but with the loads applied.
 You can also use the Inertia Relief option to analyze an underconstrained model. However, if the model already has some constraint sets, then Creo Simulate ignores these constraints during the analysis.
During an analysis with the Inertia Relief option selected, Creo Simulate internally creates a new Cartesian coordinate system (UCS) and defines a constraint set containing three-point constraints with respect to the newly created UCS. Creo Simulate 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, Creo Simulate asks you whether you want error detection to be performed. If you do, Creo Simulate checks for various modeling conditions including the presence of multiple bodies in the model. If Creo Simulate 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.