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Effect of Inertia in Advanced mass scaling

Altair Forum User

Hi,

What will be the influence of inertia when we are employing advanced mass scaling on the models? I know that there will be no added mass on the model ( even for a higher time step). However the acceleration is dependent on the time step. So there will be a change in acceleration with increase in time step and effects force ( F= M*a). And it is evident in my results ( I'm performing frontal crash simulation of a car with 1 million elements) that there is a change in acceleration curves as I overlay it with no mass scaling / conventional mass scaling. Can you explain me the reason behind it?

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Radioss

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Altair Forum User

Hi Raghav,

As you said Advanced Mass Scaling (AMS) does not modify the global mass. But AMS is not recommended for crash simulations. The conventional mass scaling, that is /DT/NODA/CST (mass error preferably less than 2 percent) is advisable for crash simulations where as AMS is recommended for quasi static simulations, where the velocity is low and the kinetic energy is very small and in such cases the effect is insignificant or negligible.

Andy_20955

Hi Raghav,

There is a lot of good information about AMS under, RADIOSS > User's Guide > Explicit Structural Finite Element Analysis > Time Step> Advanced Mass Scaling (AMS) Guidelines
From the help here are a few more details about how it works.

AMS (Advanced Mass Scaling) saves significant computation time by increasing the time step of the model for an explicit computation. This is similar to traditional mass scaling, except that the added mass does not increase the translational kinetic energy of the system.

A non-diagonal mass matrix is used to increase the time step on each line of the mass matrix. The lumped mass, M0, is increased with some M value compensated with non-diagonal terms such that the total mass to remain constant. Unlike traditional mass scaling, AMS only modifies high frequencies and does not significantly affect low frequencies of the model.

Results Accuracy

The numerical effect of AMS on results is similar to lowering the highest eigen frequencies of the structure. Since AMS affects high frequencies, it is suitable for quasi-static, crash, drop test, and manufacturing (stamping) simulations. It is not recommended to be used with high velocity phenomena like: explosion and ballistics simulation.

Although our help suggests that it can be used for crash analysis it isn't a very common use case because AMS modifies high frequencies in the system and in a dynamic event this can change results. In quasi-static simulation this isn't an issue.

No matter the simulation type, we typically recommend validating AMS results against a non-AMS run. Once you confirm the AMS results are valid for a particular simulation then you can continue to do design iterations with the AMS model making sure to run non-AMS solution on the final design.