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Aim : To create a complete simulation file from this to perform a drop test. Solution : First we will discuss the basics of ls dyna simulation : There are keywords defined for the following: Part : It tells us that the to which exact part it belongs to using a keyword Mat : It defines the material properties…
abhijeet dhillon
updated on 27 Jul 2021
Aim :
To create a complete simulation file from this to perform a drop test.
Solution :
First we will discuss the basics of ls dyna simulation :
There are keywords defined for the following:
Part : It tells us that the to which exact part it belongs to using a keyword
Mat : It defines the material properties of the component
Control : It tell us into how many small parts is the solution written for ,
Data Output: These keywords basically define what all outputs we will be getting.The data output can be written in binary or normal text depending on the choice we take .Different outputs such as force etc can be requested .Some of the important ones are :
1.GLSTAT :In this command we have recorded the kinetic , internal and total energy of the model.
2.MATSUM:In this command whatever energy is created due to the material is recorded.
3.RCFORCE:It record all the contact forces .
4.D3PLOT:This command makes sure that all the data is written such as animation etc.
Section :Through this we can create shell and solid elements .
Star Include: It is used to include the multiple files into a single input file .It is used to combine a lot of input files into a single input file .
Intial Velocity : This card is used to define intial velocity to the component to give it some velocity.
Now we will define hourglass along with its card :
Hourglass :
It is essentially a spurious deformation mode of a Finite Element Mesh, resulting from the excitation of zero-energy degrees of freedom. It typically manifests as a patchwork of zig-zag or hourglass like element shapes (Fig.1), where individual elements are severely deformed, while the overall mesh section is undeformed. This happens on hexahedral 3D solid reduced integration elements and on the respective tetrahedral 3D shell elements and 2D solid elements.
Consider a 4 noded Rectangular shell element called as S4 element. They may have any number of Integration points (depending upon full or reduced integration). Stresses and strains at this element are captured by integrating the values got at these integration points.
It is to be remembered that values are more accurate at the exact location of the Integration point and not so accurate as we move away from it.
For sake of lesser computational time, we can use S4R element which means Reduced Integration where in the one Integration point is used and it is ideally at the centre of the element.
In the case of these Reduced Integration elements, while post processing results, we can see a false or fake deformation mode (zig zag lines). Such a thing is the Hour Glass phenomenon.
The above is an example of a Single Integration point shell element. The values captured at the single integration point at the centre remains same as the dotted lines have not changed in magnitude or in angle. However, the element as a whole has shown a change in configuration. This is because elements with lesser integration points are less stiffer and as there is no stiffness in this mode, these elements have shown this kind of change in Configuration.
In actual physical scenario or in Experiments, this does not happen. But in FEA this happens. This is just a non physical zero energy mode which leads to excessive element distortions. The more structured and less distorted your elements are, the better the results.
To solve this problem,
Now we will be talking about the material laws as shown below :
In ls dyna MAT 24 is highly used to describe the material properties as shown.In LS dyna we take 100-200 test values which have been conducted according to ISO or ASTME standards and later we fit the curve points with the help of matlab so that we get a smooth curve which determines the numeric stability of the solution.
Now we will be talking about timestep as shown :
Since ls dyna is an explicit solver it will solve the equation in a very small timestep for the whole run time , if we take the example that the total run time is 150 ms and the timestep is 0.1 ms , it will solve each 0.1 ms upto 150 ms .At every node it will calculate the strain by referring to the material such as hyperelastic etc and from the strain it will create the force which will be passed to the next node because of the mesh flow .It is like a wave passing from one node to another and before it passes it calculates the displacement .The time step is calculated according to the formula :
The above gives us the speed of sound and :
Time Step = Characteristic Length of Element / Speed of Sound in material
LS-DYNA calculates your solution by running over all the nodes in the model at each time step. In a very simplified explanation, LS-DYNA will look at the total force on each node from the previous time step, use that to find the acceleration of that node, and then to find the displacement of that node. The node is then moved by this much. The calculation then updates the strain on the connecting element. This then goes through the material constitutive equation to get stress, which is applied as a force on the neighboring node in the next time step.
You can imagine this series of calculations behaving like a wave, propagating through your mesh. The critical thing, however, is you want to calculate all the node positions of your model at the same time, without it being affected by the calculations from the previously calculated node. This basically means that you need to set your time increment such that the code can calculate the displacements of the next node before the wave from the previous node calculation can hit it.
For instance, if my boundary conditions on the mesh shown below were along the left and bottom edges, and I applied a downward load at node 25, the calculations at t=0 would go as follows:
F at N25 = F0.
This gives acceleration (from F=ma, with mass at 1/4th the mass on element 16).
From acceleration, I calculate displacement for my next time step and move N25.
Now, the movement of this node imposes a strain on element 16, which generates a stress in element 16 (from the material law) which will apply a force on N20. However, I still haven't calculated the displacement of N20 at t=0. I must do this before the wave propagating from N25's displacement hits N20.
I would also like to have a little margin for safety. If I set my time step to exactly the time for the wave to hit N20, it would be a little risky and can lead to instability. Therefore, LS-DYNA has a parameter in the *CONTROL_TIMESTEP card called TSSFAC, which is set to a default of 0.9. This is basically a scale factor on the calculated time step to ensure that N20 is calculated well in advance of the wave front.
Now, that makes sense for a regular mesh (I hope). All the elements have the same time-step, and everything works out great. What happens if my mesh is not regular? I have some smaller elements, some larger elements, different materials (affecting wave speeds), etc. This is unavoidable in real parts because of the necessity of capturing the geometry accurately. Then LS-DYNA will use the smallest time step calculated over all the elements to ensure that none of the elements get unstable because of the above-explained phenomenon. That means if you have one element with a tiny time step, the time-step of your entire model is dictated by that one element, necessitating a lot more calculations than are necessary.
Now we will define the problem in ls dyna and discuss the various cards used :
As you can see we have defined the cards for the impact of the phone on the wall ,now we will describe the control cards as shown :
1.Contact Card :
As there will be contact between the elements of the phone as well as of the wall , we need to define card for contact which defines the master elements and slave elements and applies the contact types as type 7 or 11 , in this case the wall is the master elements and the phone is slave elements .
2.Velocity Card
Throught this card we have given an intial velocity of 13 ms to the node set 1 which basically the phone which will impact the wall .
3.Control Termination
The control termination is used to determine how long will the problem run for in our case the problem will run for 4 seconds
4.Control Timestep
Through this card we are able to determine the timestep of the solution for example if the solution is running for 4 seconds , and we give an input time step of 0.1 seconds , than the total no of steps will be 40 for the solution which will be written every 0.1 seconds , if we take the option of 0 in dtinit , ls dyna will calculate the time step on its own for the problem based on the meshing and material.
Now to be on the safer side so that the solution has stability we scale the time by a scale so that we have a slightly higher timestep so that the solution does fail with the card TSSFAC.
4.Binary D3Plot
Because of this card all output which are neccessary for interpretation are recorded at every timestep which is an input by the user as shown above .
5.Material and Shell Card
In this way we have given shell property to the way and solid property to the phone and we have given them a material property as well
Now we will run the keyword file in ls run :
Now we will discuss the results :
Von Mises Stress :
Conclusion
Therefore we have set up the whole problem and got the results
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