Week - 4 - Crash Box Simulation
CRASH BOX SIMULATION
AIM:
Aim of the project is setup the crash box using shell elements with elasto_plastic material with initial and imposed velocity of 50kmph and illustrate how buckling happens in the body.
INTRODUCTION:-
- Finite element method is the good wy to approximate the field variable of the governing equation.In above case(structural) we are intrested tin stress and strain developed in the body.
Before starting the simulation we need to think about the certain effective result-
1.what method we should use to achieve proper solutions wheather implict or explicit?
2.how the model the material to get realistic result(elastic,plastic,hyperelastic)?
3.what are the boundary conditions used in the model and how to apply them?
4.time period to run the simulation?
5.what output data we need to fullfill the need of simulation?
6.how to verify our results in terms of accuracy?
These are the questions we should answer in the model,if we answer these questions then we can say our model matches to the real time scenario.
- In the context of the our problem the answers are
1.the crash simulation represents the dynamic situation and for solving the dynamic problem we use explicit method for fast solution.
2.in this model initially i modelled elastic material using young's modulus,possion ratio,density and after verfiying the results change the material to elastoplastic material using (effective plastive strain vs effective stress)curve.
3.For this model i used two cases
- imposed velocity.
- initial velocity.
Rigidwall is defined using rigidwall keyword and self contact is defined.
4.Element formulation is really important in finite element analysis.generally we have element formulation broadly cllasified on the basis of the integration point.
- full integration method.
- reduced integration method.
- selective reduced integration method.
fully integration method is accurate but there is an issue of shear locking and volume locking.we can solve the shear locking and volume locking by reduced integration method but this method leads to hourglass energy.so, hourglass control card must be used.
hourglass can be conroled by two ways:
- introducing excess stifness to the nodes.
- introducing the viscous effect.
5.Time period for simulation is should be calculated.
Time=dist/velocity.
6.we need to enable the global energy data(glstat),material data(matsum),reaction force data(rcforce)
7.we should plot the energies like total energy,internal energy,kinetic energy,hourglass from the model.
UNIT SYSTEM USED:G MM MS
PROCEDURE
1.importing:
- FILE>OPEN>LS DYNA KEYWORD FILE>NEW_CRASH_BOX.
After importing the model it is required to check keyword and then check for the duplicate elements.
2.Define section for shell element:
Here,we will have to run the simulation with 2 different thickness on with 1.2mm and 1.5mm.
3.Define material using "mat_elastic"card:
4.Define section card to the part:
6.Define rigidwall:
- rigidwall should be created for the crash box to hit the wall with some initial velocity provided.
- After creating the rigidwall we should translate the rigidwall to some distance to move the crash box with some initial velocity.
- now we should change the direction of the rigidwall the vector direction should be towards the crash box as shown below,otherwise it will not treat contact between the wall and box.for that we should define vector and apply that to the rigidwall.
7.Define initial velocity:
50kmph=13.889mm/ms
the node set should be defined for which the velocity should be applied.
- Here,the crash box should move in x-direction to hit the rigidwall.so,the velocity should be provided in that direction.
8.Defing the contact:
self contact should be defined for the box surface using AUTOMATIC_SINGLE_SURFACE in contact keyword file.Here,the master and slave part should be crash box because the rigidwall is asuper rigid and it will consider the master node as rigidwalland remaining all are as slave part.so,here in this case it will be crash box.
9.Defining control cards:
we used the reduced integration point formulation method which introduce the hourglass energy in the simulation.to reduce hourglass energy it is necessary to enable the hourglass card in the ENERGY card from control keyword and enable the HOURGLASS card from the control keyword.set IHQ=5,QH=0.03.
- After enabling this we should include that in the HOURGLASS control keyword and include the hourglass energy.
10.Time period for simulation:
The termination time required for the model should be defined in the control card.
11.enabling the output data:
enable the energy output using glstat,matsum
reaction force using rcforce
animation using BINARY_D3PLOT.
now the step is ready to run the simulation then save the file.
- up to now we have done the simulation for elastic material using "MAT_001".this material contains the density,young's modulus,possions ratio but now we should change the material and check the deformation of the crash box in D3PLOT.so for that we have choosen a plastic material .
PLASTIC MATERIAL
After verifying the simulation with elastic material then with plastic behaviour using MAT_24 card.
for this we should define the curve between effective plastic strain and effective srtress.if the curve is linear we can define in ETAN if non-linear we should go for the curve and plot or we can directly give the values in EPSI and ESI.
- After defining the material card we should just change the material card(MAT_24) of the part in keyword manager.
- up to now we have run the simulation with initial velocity now we should run the simulation with imposed velocity.it means we should check how it deform after hitting the rigidwall.
IMPOSED VELOCITY:
The steps are similar as above but differnce is defining the velocity boundary condition.Instead of defining initial velocity we used imposed velocity.
1.define the node set (rigid) at the end of the crash box using constrain nodal rigid body(CNRB) and it make the master node at the centre of the all the nodes of the crash box and make them as slave nodes.here,we can directly apply the imposed velocity to the master node it will distribute equally to all the slave nodes and form a cluster.
2.Define PRESCRIBED_MOTION_SET card from boundary in keyword manager.
- we should first define a curve between vekocity and time to load the curve in LCID as a boundary condition to the crash box.
- now load this curve in the PRESCRIBED_MOTION_SET and define the curve in keyword file.here,the NSID should be the master node to apply the imposed velocity and load the curve.
now using the combination of velocity conditions(initial,imposed) and material type (elastic,elasto_plastic)we can create different simulation setup like:
- elastic with imposed velocity.
- plastic with initial velocity.
compare the results with different setup files and check the results.
- similar way we can check the simulation for different thickness 1.2mm and 1.5mm and plot the graph.
POST PROCESSING
Elastic with initial
- D3plot animation for initial velocity:
Elastic with imposed velocity
- Kinetic Energy is generated due to the motion of an object with velocity.it is defined as the work needed to accelerate a body.
Total energy=Kinetic Energy+Internal Energy
hourglass energy is generated due to reduced integration method used.it should not be more than 10% of the total energy but the % is totally based up on the industrial standards.
here,i have reduced the hourglass energy by changing the QH=0.03 as mentioned in the above case.
plastic with initial
- cross-sectional force generated at the centre of the crash box along its length.
- acceleration of a node along the centre of the crash box.
- effective plastic strain of an element is occured because after yeild stress the material will not come back to its original position its kown as hardening the corrosponding plastic strain is kown as effective plastic strain.
plastic with imposed
- effective plastic starin:
Result:
- The simulation for the given crash box is done using initial velocity and imposed velocity by using different material properties and sections.
- The model was solved sucessfully and the results are compared for both 1.2mm and 1.5mm thick crash box
- The results for 1.5mm crash box showed higher energy absorbing capacity and less deformation compared to 1.2mm thick crash box.
Learning outcome:
- how to create material property and section card and assign them to the part.
- how to create contacts
- how to define rigidwall.
- to define initial velocity and imposed velocity