Learning Forum

[Sajiv Kumar]

Hi,

I am trying to create a transient structural analysis study on ANSYS Mechanical Student. Is it possible to have a fixed support only for a specific amount of time. For example: for two timesteps have a fixed support at one location and for the next two timesteps have it at another location. Please do let me know if there are alternative ways to achieve this.

[peteroznewman]

Please insert some images to describe what you want to do. 

You can apply a zero displacement BC at one location for the first time step, then you can Deactivate that BC for the second time step.

The problem is the other location, the nodes will have moved from where they were at the beginning. When you say you want to constrain those nodes, do you mean at their current location (which makes sense) or do you mean at their original location (which makes less sense)?

[Sajiv Kumar]

At their current location. I am using a single step and have split it into 100 substeps because i have data for 100 intervals. Since it is a single step I am not able to deactivate the displacement for a few substeps. The whole step gets deactivated.

[peteroznewman]

Please show the data you are using. Is that data in the form of a time history of a load?  You can put that data in a load and apply it in one step with more or less than 100 substeps. Ansys will interpolate the input data to each time step, whatever the time is.

Please describe in more detail what you want to do and when the loads are applied relative to that change in boundary conditions.

[Sajiv Kumar]

This is what my data looks like. I've added 5 forces to simulate the effect of a varying load thats changing location after a specific duration of time. Each time the force moves to a different location I would like to have a fixed support in a new location. So for steps 1-21 I want to have a fixed support and then for steps 22-41 I want the fixed support removed from that location and moved to a different location.

Context: I am trying to create an FEA on an ankle foot orthotic and I have data from a gait lab that i will be using to simulate the ground reaction forces. Do you think I have approached this the wrong way? I could also model a flat surface and have the AFO do a walking motion but that seemed more complicated for me so I have chosen to do it this way.

 

[peteroznewman]

Is the data from the Gait Lab in the form of a force-time profile for each part of the foot striking the instrumented ground?  The first profile is the heel strike, the next profile is the ball of the foot strike, etc?

Are you then trying to apply those forces to the Ankle Foot Orthotic FEM?  Does the FEM include a model of the bones of the foot and the ligaments and flesh around the bones?

Please reply with images of the AFO and other elements of your FEM. Show how you are supporting the FEM, point out what is the fixed support. I expect it is some point along the tibia/fibula.

I can imagine the inversion of the model. Ground is fixed and the foot must strike the ground and roll forward from heel to toe.  While it may be simple to get something working, it may be difficult to get the contact forces to match the lab data, so I can see the attraction of applying the lab force data directly to the AFO, Foot, Anke FEM.

[Sajiv Kumar]

The data from the gait lab is a force-gait% profile but I've converted the percentages into seconds using the patients average stride time. Yes the first profile is heel strike and etc etc..

My FEM has only the orthosis and no other elements because I do not have data to simulate the muscle forces. Especially in the case of disability it can be very difficult to get consistent data for muscle forces so I have excluded it for now. Ive decided to ground sections of the AFO where the centre of mass will be passing through instead. Does that make sense?

My desired boundary conditions are as follows; Vertical, Anteroposterior and mediolateral forces applied as a force-time profile and applying a fixed support at the point where the centre of mass is at that specific point in gait. 

[Sajiv Kumar]

0

 

[peteroznewman]

What is the purpose of the simulation? What data do you want to get out of it?

I might have more suggestions if I could see what the AFO looked like including a section view so I could see where the foot would contact the device.

[Sajiv Kumar]

Thank you, I would really appreciate some suggestions. I have attached the pictures of the AFO. The purpose of this simulation is to find out points where cracks are likely to occur (stress, strain, Deformation, Safety factor.. etc, etc) and also at what point of the gait cycle does it happen, so the second revision would be a model better suited to the patients gait.

I only have reaction force data that I have split in 3 components which is Vertical, Anteroposterior and Mediolateral. Since I do not have any data for the opposing forces I thought I would ground the areas of the AFO that were in contact with the foot and the floor at the same time. It is my understanding that those sections would be stationairy. I also wasn't able to find any relevant study material or data on the loading on an AFO during the gait cycle. I would appreciate any suggestions you have, to approach this better since adding fixed supports for a specific duration is not an option anymore. If I do not add any fixed supports then I only have one force in each direction (x, y, z) and the AFO would just be in acceleration.

[peteroznewman]

Thank you for the images of the AFO.  Some researchers have developed a multibody dynamics model of the lower body and foot that they use in simulations that correlate to gait lab data.  Here is a link to one example.