Article Number: 95 | VC6 | VC5 | VC4 | VC3 | Post Date: April 3, 2020 | Updated: April 3, 2020
Is it possible to model a cable barrier with Virtual CRASH?
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Yes, using primitive shapes connected by joints, you can model cable barriers. Here we see an example impact between a car and cable barrier:
Before beginning, we strongly recommend you read https://www.vcrashusa.com/blog/2018/6/7/building-complex-systems-with-joint-tools to better understand working with joints in Virtual CRASH. You can learn more about joint stiffness and damping coefficients in the Short Glossary.
It is recommended to construct your own cable object first, rather than using the rope tool, as this reduces unneeded spherical joint connections which can slow down the simulation calculations.
We’re going to start by building a cable constructed with cylinder objects. This is shown in the video below:
After we prototype the first 20 ft section cable, we’ll combine the joints and segments into a single group object. This will make it easier to clone and move each 20 ft section.
With our first group created, we can now clone the group object to create 2 more 20 ft sections. Move the pivot point of the group to the end of the section to make it easier to position.
We have the option of simulating posts and tying the cables to the posts, or we can simply tie the cables to a plane that’s been converted to a terrain object. First, place a plane object beneath the region where the cable barrier is set up. Convert the plane to a terrain object (Create > Physics > Make Unyielding / Terrain from Selection).
Set the joint to “Parent Space” mode, select “Restrict to Z” (press 3 on your keyboard). Switch to “Select, Move and Manipulate” (press F3). Now reposition the spherical joint using the translation grips so that it’s tied to the end of the cable.
You can switch to “Child Space” to separate the other end of the joint from the Parent end. This will effectively “pre-load” the joint. By pre-loading the joint, you’ll prevent slack from forming when the simulation starts.
Repeat this process for the next two segments. Repeat the process again for the other end of each segment.
Next, go through the remaining joints in the cable and separate the child from the parent ends. Again, this pre-loads the internal joints within the cable.
Next, we select all the spherical joints. Using the diagram tool, we input a time-dependent spring constant that changes stiffness after impact to simulate the cable material yielding. We also use a time-dependent damping coefficient.
To simulate the effect of hook bolts in the support beams, which allow cables to having sliding motion with respect to the support beams, we use slider joints connecting between intermediate segments and the plane which was converted to a terrain object, with orientation fixed using large spring constant values.
Fix position is disabled, as is joint limits, which allows the joint to freely slide laterally. A breaking force is used to simulate the effect of cables detaching from hook bolts or the support beam bending or breaking free.
Here we show how to create a test bogie for fine-tuning the joint stiffness values for the cable.
See “Development of Advanced Finite Element Material Models for Cable Barrier Wire Rope” for examples of using test bogies to better understand cable tension responses to bogie penetration.
Finally, you’ll need to tune the coefficient of friction for the vehicle-cable interaction. This is controlled with “friction-body” in the contact menus of both the vehicle and the cable segments. Remember, when you convert a shape to a rigid body object, the preferred contact model is automatically set to “default-auto”, which is the multi-point contact impulse model (Kudlich-Slibar is usually used for vehicle-vehicle or vehicle-immovable object type impacts). Kudlich-Slibar is enabled only with both interacting objects have Kudlich-Slibar selected, otherwise default-auto will be used. For two rigid-body objects (not multibodies) undergoing collision under the “default-auto” model, the friction value used for the collision will be the minimum value of the friction-body values specified for either object. For two rigid-body objects (not multibodies) undergoing collision under the “default-auto” model, the restitution value used for the collision will be the maximum value of the restitution-body values specified for either object.
Improving Behavior by Adding to Vehicle Mesh
You may notice, depending on how you set up your simulation, that one or more cables travel laterally through the tires of your vehicle. This happens, of course, because the simulated wheel objects are not solid objects which can interact arbitrarily with the environment. The issue of lateral wheel interaction is discussed in a few other places in the vCRASH Academy. For example, see:
https://www.vcrashusa.com/blog/2017/3/15/simulating-direct-wheel-impacts-and-curb-trips (Note: Method 3: Extending the Vehicle Mesh)
Modified vcm File
Using Method 3 in the above referenced blog post, we can add cylinder polygons within the wheel objects, whose polygons will interact with the cable polygons.
Create a Group Vehicle Object
Another option is to Group the vehicle mesh (take the vehicle and select Create > Physics > Remove Physics) with simple geometrical shapes such as ellipsoids or cylinders. The workflow, where a group object is converted into a simulated vehicle object, is demonstrated in this post:
https://www.vcrashusa.com/kb-vc-article59
Note, using this workflow, it is important to first center the vehicle mesh and ellipsoids or any other objects at x=0, y=0 before making the group object. In the example above, we’re included an ellipsoid in our group to round the bottom portion of the vehicle so that there is smooth and continuous contact with the cable barriers. This also prevents the cables from going through the wheels. Note, a blue ellipsoid was included in the group object to smooth contact between the side mirror and the upper cable. These ellipsoids can be hidden from view.
Tags: cable barrier, cable impact, wire barrier.
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