Why the reduction of total energy?
Altair Employee
Answers
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Hi,
Total energy is the sum of internal energy, kinetic energy, contact energy, hourglass energy..etc in the model. For a typical crash analysis energy plot based on the external works, the total energy must remain constant or can decrease slightly.
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Dear Georage
Thanks for answer.
What is the difference between external works and total energy?
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Hi,
During a crash scenario due to the external forces applied there will be change in internal energy, kinetic energy, contact energy....etc and all these constitutes to the total energy of the model. When work is done by external forces , the total energy of the object may be altered.
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Dear Georage
Thanks for answer.
So, the energy loss resulting from conversion of external forces?
Why does the loss of energy happen?
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Hi,
Towards the end of simulation the energy absorbed during crash will be less and this can be reflected in total energy of the simulation.
Please refer Online Help where a topic on energy balance is available.
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Hi,
In your case, external force is the normal reaction from the barrier which causes deformations in the structure leading to energy absorption (I am not considering gravity now)The work done by this force is the external force work here. Can you show the plot for other energies like hourglass, contact etc to understand where the energy is decreasing and also a brief schematic of your problem.
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Thanks for answer
I show you others energy.And, what is difference between hourglass energy and internal energy?
Also, this model barrier is rigid wall.
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Hi,
Internal energy is the energy absorbed by the elements during deformation. This is also referred to as strain energy.
Hourglass energy is the energy associated with the hourglass mode of element where the element stiffness become zero and tries to come out of simulation. In such scenarios, a force is applied to that particular element to displacement and the work done by that force gives the hourglass energy. This should be not more than 10-15 % of the total energy. This is purely based on shell element formulation. Like if you go to property and edit it you will see, IShell which defines the element formulation. More details for the same is also given in help.
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Thanks for answer.
I want to know what is contact energy. And this model barrier is rigid body. So, I think barrier can't absorb energy. Why does total energy decrease?
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Hi,
Yes if the model barrier is rigid, it will have zero internal energy as deformation is not possible.
Contact energy is not a physical energy of the system, it is the basically the energy spent by enabling a contact force to avoid penetration.
Contact energy = contact force*distance moved to avoid penetration.
This should be minimal and can be around 0-5 % of total energy.
Any springs also modelled in simulation, if yes how the spring energy varies?
Also is the internal energy became equal to the input energy or it is less?
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Thanks for your answer.
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sir, can i know why the total energy is decreasing during full frontal impact with rigid wall? what is the main reason contributing to decrease in Total energy? although by energy error is acceptable contact energies and spring internal energies are acceptable ive observed that it is decling! and what is DTE- Total energy external work force energy ? what we should validate T.E or D.T.E?
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Hi,
Is your graph similar to the above graph in this query. If not ,provide your energy graph.
TE is mainly the sum of IE, KE, RKE, HE, CE,SE
As per the above graph, the HE, CE are increasing with time and we have a constant input KE which is set to decrease and IE is set to increase. Considering complete KE goes into IE also at the same rate , we would have difference in total energy of HE ,CE and SE on increasing side.
The important is rate at which IE is increasing and KE is decreasing. You can infer from it and also through plot of IE and KE for parts.
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yes sir the graph is same but the increase in HG and CE are in acceptable rangle <10%
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Hi Ramakiran,
Can you please upload a snapshot of the global energy (TE, KE, IE,HE and CE) plot.
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sir this is my graph and i want to make total energy as constant
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Hi Ramakiran,
Please review the interface defined in the model. Check for any penetrations in the contact and review the contact parameters and try running the model.
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yes sir iw ill check them, thank you.
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Hi!
I´m running a Rigid Wall crash simulation
For my material, i´m using a max value for Failure plastic strain
So, when my frame crashes, some shell elements rupture...
Does this affect energy?
Because i have run both analysis, with and without EPS_max
And the Total Energy are very different?
If the element gets in rupture, the internal energy that this one absorbed dissapears from the system or what happens?
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Hi @misa_roll,
the toughness (ability to absorb mechanical energy up to the point of failure) is less with failure plastic strain (EPS_max) defined because the area under the stress-strain curve is reduced.
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Send some pictures to give example about what i´m talking about.
In the first one, my material doesn´t have a value for EPS_max
Then in this one, my material has an EPS_max value
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These graphs are expected:
-the model without EPS_max defined (Default = 1030) will absorb more energy, because it will continue to absorb energy even at large strains, which is less realistic.
-the model with EPS_max defined will absorb less energy, because it will only absorb energy up to the plastic strain at failure defined by this parameter, after which the ruptured elements will not contribute stiffness.
Also, if both models have the same boundary conditions the initial kinetic energy of the system should be the same. However, in your examples, the initial KE is different (3.3E+7 vs. 8.5E+6).
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