The same msh model is used by dividing it into two regions.

[CherryJGit]

I have a tetrahedral mesh of a sphere in an msh model. I hope that the tetrahedral elements in the upper half of the sphere cannot be eliminated, while those in the lower half can be. Moreover, there should be a regional division between the upper half and the lower half, rather than a determination based on a standard. For example, the tetrahedral elements in the region enclosed by the BoxROI should not be eliminated. I'm not sure if this is feasible. I tried to write an scn file with the following content. However, no matter how I modify it, errors will always occur. I will put the content of the file below. Maybe my train of thought is wrong.And after it runs, due to the influence of gravity, it will split into two parts on its own. What I hope for is the whole. Even so, I spent a long time consulting various AIs, such as Grok, GPT, DeepSeek, etc., before determining this method. I hope everyone can help me check where the problem lies, or provide an scn file for other possible approaches. I would be extremely grateful. Thank you in advance.

<?xml version="1.0" ?>
<Node name="root" dt="0.03" showBoundingTree="0" gravity="0 -9.8 0">
    <RequiredPlugin name="Sofa.Component.Collision.Detection.Algorithm"/> 
    <RequiredPlugin name="Sofa.Component.Collision.Detection.Intersection"/> 
    <RequiredPlugin name="Sofa.Component.Collision.Geometry"/> 
    <RequiredPlugin name="Sofa.Component.Collision.Response.Contact"/> 
    <RequiredPlugin name="Sofa.Component.Constraint.Projective"/> 
    <RequiredPlugin name="Sofa.Component.Engine.Select"/> 
    <RequiredPlugin name="Sofa.Component.IO.Mesh"/> 
    <RequiredPlugin name="Sofa.Component.LinearSolver.Iterative"/> 
    <RequiredPlugin name="Sofa.Component.Mapping.Linear"/> 
    <RequiredPlugin name="Sofa.Component.Mass"/> 
    <RequiredPlugin name="Sofa.Component.MechanicalLoad"/> 
    <RequiredPlugin name="Sofa.Component.ODESolver.Backward"/> 
    <RequiredPlugin name="Sofa.Component.SceneUtility"/> 
    <RequiredPlugin name="Sofa.Component.SolidMechanics.FEM.Elastic"/> 
    <RequiredPlugin name="Sofa.Component.StateContainer"/> 
    <RequiredPlugin name="Sofa.Component.Topology.Container.Dynamic"/> 
    <RequiredPlugin name="Sofa.Component.Topology.Mapping"/> 
    <RequiredPlugin name="Sofa.GL.Component.Rendering3D"/> 
    <CollisionPipeline />
    <DefaultVisualManagerLoop />
    <DefaultAnimationLoop />
    <BruteForceBroadPhase/>
    <BVHNarrowPhase/>
    <CollisionResponse response="PenalityContactForceField" />
    <MinProximityIntersection name="Proximity" alarmDistance="0.8" contactDistance="0.5" />
   <MeshGmshLoader name="loader" filename="mesh/test6msh1.msh" scale3d="50 50 50" rotation="0 0 0" translation="0 0 0"/>
<MechanicalObject src="@loader" name="FullVolume" />
<TetrahedronSetTopologyContainer name="FullTetraContainer" src="@loader" />
<SubsetTopology name="Subset"  tetrahedra="@FullTetraContainer.tetrahedra" box="-100 0 -50 100 300 150" localIndices="1" />
    <Node name="ProtectedLiver">
        <MechanicalObject name="Volume" position="@../Subset.pointsInROI" />
        <TetrahedronSetTopologyContainer name="TetraContainer" tetrahedra="@../Subset.tetrahedraInROI" />
        <TetrahedronSetGeometryAlgorithms name="GeomAlgo" template="Vec3d" drawTetrahedra="0" drawScaleTetrahedra="0.7"/>
        <DiagonalMass massDensity="4.0" />
        <TetrahedronFEMForceField name="FEM" youngModulus="3000" poissonRatio="0.3" method="large" computeVonMisesStress="1"/>
        <EulerImplicitSolver name="cg_odesolver" printLog="false" rayleighStiffness="0.01" rayleighMass="1.0" />
        <CGLinearSolver iterations="25" name="linear solver" tolerance="1.0e-9" threshold="1.0e-9" />
        <PlaneForceField normal="0 1 0" d="-5" stiffness="100000" showPlane="1" showPlaneSize="20"/>
        <BoxROI name="FixedROI" box="-1 6 -0.5 1 7 1.5" position="@Volume.rest_position" drawBoxes="1"/>
        <FixedProjectiveConstraint indices="@FixedROI.indices" />
        
        <Node name="LiverSurface">
            <TriangleSetTopologyContainer name="TriContainer" fileTopology="" tags=" " />
            <TriangleSetTopologyModifier name="Modifier" />
            <TriangleSetGeometryAlgorithms name="GeomAlgo" template="Vec3d" />
            <Tetra2TriangleTopologicalMapping input="@../TetraContainer" output="@TriContainer" />
            <TriangleCollisionModel />
            <Node name="LiverVisu">
                <OglModel name="Visual" color="red" />
                <IdentityMapping input="@../../Volume" />
            </Node>
        </Node>
    </Node>

    <Node name="ErasableLiver">
        <MechanicalObject name="Volume" position="@../Subset.pointsOutROI" />
        <TetrahedronSetTopologyContainer name="TetraContainer" tetrahedra="@../Subset.tetrahedraOutROI" />
        <TetrahedronSetTopologyModifier name="Modifier" />
        <TetrahedronSetGeometryAlgorithms name="GeomAlgo" template="Vec3d" drawTetrahedra="0" drawScaleTetrahedra="0.7"/>
        <DiagonalMass massDensity="4.0" />
        <TetrahedronFEMForceField name="FEM" youngModulus="3000" poissonRatio="0.3" method="large" computeVonMisesStress="1"/>
        <EulerImplicitSolver name="cg_odesolver" printLog="false" rayleighStiffness="0.01" rayleighMass="1.0" />
        <CGLinearSolver iterations="25" name="linear solver" tolerance="1.0e-9" threshold="1.0e-9" />
        <PlaneForceField normal="0 1 0" d="-5" stiffness="100000" showPlane="1" showPlaneSize="20"/>
        
        <Node name="LiverSurface">
            <TriangleSetTopologyContainer name="TriContainer" fileTopology="" tags=" " />
            <TriangleSetTopologyModifier name="Modifier" />
            <TriangleSetGeometryAlgorithms name="GeomAlgo" template="Vec3d" />
            <Tetra2TriangleTopologicalMapping input="@../TetraContainer" output="@TriContainer" />
            <TriangleCollisionModel />
            <Node name="LiverVisu">
                <OglModel name="Visual" color="red" />
                <IdentityMapping input="@../../Volume" />
            </Node>
        </Node>
    </Node>
</Node>

The error log brought by this code is：

########## SIG 11 - SIGSEGV: segfault ##########
  sofa::helper::BackTrace::dump
  sofa::helper::BackTrace::sig
  seh_filter_exe
  seh_filter_exe
  _C_specific_handler
  _chkstk
  RtlRaiseException
  KiUserExceptionDispatcher
  sofa::component::solidmechanics::fem::elastic::TetrahedralCorotationalFEMForceField<sofa::defaulttype::StdVectorTypes<sofa::type::Vec<3,double>,sofa::type::Vec<3,double>,double> >::computeRotationLarge
  sofa::component::solidmechanics::fem::elastic::TetrahedronFEMForceField<sofa::defaulttype::StdVectorTypes<sofa::type::Vec<3,double>,sofa::type::Vec<3,double>,double> >::computeVonMisesStress
  sofa::component::solidmechanics::fem::elastic::TetrahedronFEMForceField<sofa::defaulttype::StdVectorTypes<sofa::type::Vec<3,double>,sofa::type::Vec<3,double>,double> >::handleEvent
  sofa::simulation::PropagateEventVisitor::processObject
  sofa::simulation::PrintVisitor::treeTraversal
  sofa::simulation::PrintVisitor::treeTraversal
  sofa::simulation::PropagateEventVisitor::processNodeTopDown
  sofa::simulation::graph::DAGNode::executeVisitorTopDown
  sofa::simulation::graph::DAGNode::executeVisitorTopDown
  sofa::simulation::graph::DAGNode::doExecuteVisitor
  sofa::simulation::Node::executeVisitor
  sofa::simulation::DefaultAnimationLoop::propagateAnimateEndEvent
  sofa::simulation::DefaultAnimationLoop::step
  sofa::simulation::node::animate
  sofa::gui::qt::RealGUI::step
  QMetaObject::activate
  QTimer::timerEvent
  QObject::event
  QApplicationPrivate::notify_helper
  QApplication::notify
  QCoreApplication::notifyInternal2
  QEventDispatcherWin32::event
  QApplicationPrivate::notify_helper
  QApplication::notify
  QCoreApplication::notifyInternal2
  QCoreApplicationPrivate::sendPostedEvents
  qt_plugin_query_metadata
  QEventDispatcherWin32::processEvents
  CallWindowProcW
  DispatchMessageW
  QEventDispatcherWin32::processEvents
  qt_plugin_query_metadata
  QEventLoop::exec
  QCoreApplication::exec
  sofa::gui::qt::RealGUI::mainLoop
  sofa::gui::common::GUIManager::MainLoop
  sofa::gui::common::GUIManager::MainLoop
  sofa::gui::common::GUIManager::MainLoop
  BaseThreadInitThunk
  RtlUserThreadStart
[WARNING] [Sofa.Simulation.Graph] the library has not been cleaned up (sofa::simulation::graph::cleanup() has never been called, see sofa/helper/init.h)
[WARNING] [SofaSimulationCommon] the library has not been cleaned up (sofa::simulation::common::cleanup() has never been called, see sofa/helper/init.h)

[hugtalbot]

Hi @CherryJGit

You seem to want to preserve elements in the upper half while allowing those in the lower half to be removed.
Have you tried to define it as a singe object and use topological changes as you could notice in the examples : RemovingTetra2TriangleProcess.scn

Best

[github-actions[bot]]

⚠️ ⚠️ ⚠️
@CherryJGit
Feedback has been given to you by the project reviewers, however we have not received a response from you. Without further news in the coming weeks, this discussion will be automatically closed in order to keep this forum clean and fresh 🌱 Thank you for your understanding

[github-actions[bot]]

⚠️ ⚠️ ⚠️
@CherryJGit
In accordance with our forum management policy, the last reply is more than 4 months old and is therefore closed. Our objective is to keep the forum up to date and offer the best support experience.

Please feel free to reopen it if the topic is still active by providing us with an update. Please feel free to open a new thread at any time - we'll be happy to help where and when we can.