Article Number: 88 | VC4 | VC3 | Post Date: July 11, 2019 | Last Updated: July 11, 2019
I’m trying to simulate a motorcycle t-bone crash, but my motorcycle seems to go through my vehicle even though there shouldn’t be so much crush damage in my case. I’ve also sometimes noticed that my multibody model seems to go inside of the vehicle body panels in certain pedestrian impacts. What’s going on?
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In each case, you’re seeing behavior that’s expected. Let’s start with the motorcycle t-bone case. For vehicle versus vehicle impacts in which the default Kudlich-Slibar impulse-momentum collision model is used, the inter-penetration of two colliding vehicles is a function of the “depth of penetration” parameter and the integration time-step size. The depth of penetration parameter is an attribute of the ees object and has been written about extensively in this blog post. It’s also referenced in various other places in the vCRASH Academy. See the following to learn more about depth of penetration:
For example, in the simulated motorcycle crash based on the 2017 CA2RS motorcycle crash test below,
we see our impulse centroid is placed near the center of the front wheel and the depth of penetration was set to 22 msec, thereby reducing the amount of inter-penetration between the two vehicles before the initial impulse exchange when compared to the default value of 30 msec. The inter-penetration can be further reduced by lowering the depth of penetration time.
Another factor that can come into play with vehicle penetration is the integration time-step size. In the extreme example below, we have an SUV traveling at high speed into a stationary movable barrier.
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In the example, the depth of penetration was set to 37 msec. The integration time-step size is 5 msec.
Looking at a graph of the x-coordinate of the front-most point on the SUV and the front-most surface of the barrier, we can see the effect of both the depth of penetration and the integration time-step. Since Virtual CRASH is a numerical integration engine, it seeks the next available integration-time step to associate with first contact between polygons of interacting objects. Virtual CRASH then exchanges impulses between vehicles N more time steps, where
N = integer( depth-of-penetration / time-step )
This tells us that in extreme cases—such as this one—the integration time-step size can also play a role in inter-vehicle penetration.
In the extreme case below, for example, our car is moving forward at about 215 ft/s. This implies that in a single integration time-step (5 msec), the vehicle travels over 1 foot; this is nearly the distance covered in the example pedestrian impact below from time-step = 0.025 seconds to time-step = 0.030 seconds.
So while 5 milliseconds is a good integration time-step size for most cases, in extreme cases with high closing-speeds—including the example here—it makes sense to reduce the integration time-step size to a lower value in order to minimize inter-object penetration.
But why do I see hands going through the car?
Remember, the multibody is actually composed of ellipsoids covered with a skin-like polygon mesh (see Chapter 13 of the User’s Guide). It is the underlying ellipsoids that make contact with other bodies in your scene, not the skin. According, in some instances it may appear as if the hand or head is penetrating deep within a vehicle’s hood or some other panel, but if you look underneath the skin polygon mesh, you’ll see the underlying ellipsoids engaged in an expected way, where the ellipsoidal penetration is determined by the integration time-step and closing-speed at the particular point of contact as discussed above.
Tags: depth of penetration, integration time-step, objects moving through each other.
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