Learning Forum

[Ahmed Taher]

Hello, 

So I have been trying to apply axial load on a column that is subjected to a blast load from a contact explosion charge, the column is subjected to axial load of 40 tons, having a cross section of 30x30 cm2. in which, I tried to apply said axial load through "bourndary" tap, picked "stress (constant)" and entered the value of the stress that is conducted from the 40 tons load (devided 40 tons by the area of the cross section of the column = 4.44e3). However, I found that the stress of the load doesn't behave as I anticipated, I found that the axial stress transmits through the column as wave, here are some photos explain what I mean:

 

What I was trying to make is, the charge when the column is fully stressed. I want to value of the stress to be there before starting the blasting stage. Is there a way to do it?

Also, if the value of the stress was 4.44e3, why does the stress from contour plots doesn't reach that value?

[Chris Quan]

First of all, what you observed is the expected behavior and physically meaningful.

Stress BC in Autodyn is basically Pressure BC. Pressure is applied to the selected surfaces of a solid structure. Please be aware that the stress inside the solid structure is not necessarily equal to the pressure on the surfaces. This is because every material in Autodyn has Equation of State (EOS). In other words, every material in Autodyn, except the materials have Rigid EOS, is compressible. The stress in the structure is calculated from the strain generated from the pressure load through the material constitutive relations.

Autodyn program is based upon Explicit method and is primarily used to model highly dynamic events in a very short time of period. In such events, the major deformation mode is stress wave progression. 

When a pressure is applied to the end of a cylinder in your model, a stress wave is created at the end and is propagating inside the cylinder towards the other end. From the pictures in your post, you can see that stress wave travels farthur and farthur away from the pressure BC with time. A stable stress state could be reached after the stress wave has traveled back and forth along the cylinder many times.

If the stress from the pressure BC is the pre-stress in static equilibrium, you need to extend the End Time of your simulation to make sure the stress wave has traveled back and forth for several times along the cylinder length. At the same time, you need to apply Static Damping under Controls to damp out the dynamic oscillations of stress (or kinetic energy). Once the kinetic energy has approached zero, the material is in the state of static equilibirum and thus the stress state is static. Then you can conduct the subsequent dynamic analysis in the next load step.

You can find the instructions on how to determine the value of static damping from ANSYS Online Help at 

10.2.2. Damping Controls (ansys.com)

[Ahmed Taher]

would I be able to apply a charge detonation after damping out the dynamic oscillation? if so, would that affect the behaviour of the simulation? What I mean is, I have never applied static damping, all my previous trials were undamped. so would applying a static damping affect the behaviour of column when subjected to the charge detonation?

[Sani Jemis]

Autodyn is a commercial software package developed by Ansys Inc. for simulating dynamic events like explosions, impacts, and other complex physical phenomena. It is primarily used in the field of finite element analysis (FEA) and computational fluid dynamics (CFD) geometry dash scratch. However, Autodyn does not have a direct capability for applying axial loads, as it is not specifically designed for static structural analysis.

If you want to simulate axial loading in Autodyn, you would need to approximate the effect by defining the loading conditions in terms of dynamic events, such as an explosion or an impact, depending on the specific scenario you are trying to model.