Need a help with Grav load case in Optistruct

Kg123

Hello Team , good afternoon!

I am trying to apply 5G gravity load case on fuel tank cross member with inertia relief .  But looking into animation I am not observing any mode or movement with fuel tank cross member .

Want to weather all the boundary conditions are met and applying correctly here, can you please help?

Animation01.gif

Xinxing_Zhang

I understand you are applying a 5G gravity load case on the fuel tank cross member using inertia relief, but you are not observing any movement or deformation in the animation. Let me explain what is likely happening.

The core concept of inertia relief is that the solver calculates a set of artificial inertial forces (accelerations) to exactly balance the applied external loads. This allows the structure to be analyzed in a "free-free" condition (without fixed supports) by ensuring the net force and moment on the system are zero.

In your case, you are applying a gravity load (which is an inertial body force). When you activate inertia relief, the solver does the following:

1. It reads the applied gravity load as an external force.
2. It then computes a balancing inertial force (a rigid-body acceleration) that exactly opposes the applied gravity load.
3. The result is that the net force on the entire system becomes zero.

The problem: Since the gravity load and the balancing inertial force are equal in magnitude and opposite in direction, they effectively cancel each other out. The structure experiences no net load. This is why you see no movement or deformation in the animation – the solver has perfectly nullified the gravity field you intended to apply.

In short: Applying a gravity load (an inertial force) within an inertia relief analysis is not appropriate. The inertia relief method is designed to handle non-inertial external loads (e.g., point forces, pressures, moments) and then automatically generates the necessary inertial reactions to keep the system in equilibrium. By applying gravity, you are essentially asking the solver to balance an inertial force with another inertial force, leading to a zero net effect.

Recommendation:

• If you want to simulate the effect of a 5G acceleration on the fuel tank cross member, you should apply it as a body force or acceleration field (e.g., using an acceleration load card like GRAV in Nastran or ACCEL in Abaqus) without using inertia relief. Instead, use appropriate boundary conditions (supports) to constrain the model.
• Alternatively, if you must use inertia relief (e.g., for a free-body analysis), apply the 5G load as an external force (e.g., a distributed force on the cross member representing the inertial effect of the fuel mass) and let the inertia relief algorithm balance it.

Please check your load setup and consider removing the gravity load from the inertia relief case.