Analysis of crash tube by varying geometry

Analysis of crash tube by varying geometry under different kinematic conditions

OBJECTIVE:

This project is about the analysis of crash tube by defining different kinematic conditions and compare the base setup simulation results. The different contact interfaces are used to define the simulation.

Pre-processing = hypermesh and hypercrash

solver = RADIOSS

MODEL DESCRIPTION:

Crash tube setup

The Setup involves a crash tube impact against a rigid wall. The rear end of crash tube have defined boundary condition where the nodes are constrained. The rigid body have a master node at mass of 500kg, Which is defined for real time contact scenario for impact.

The material properties are

• Youngs Modulus (E) = 210 GPa
• Density (RH0) = 7.8e-3 Kg/mm3
• Poisson's ratio = 0.3

Hour Glass in the system is zero due to use of QEPH shell element with physical hour glass stabilization, by adding stiffness to the element in material level. Both contact energy and hourglass energy are dissipative.

Type 2 contact interface - Tied contact (seam weld)

• This type of contact is mainly used for welds,rivets,etc.,. This contact keeps surfaces as master and nodes as slaves. so whatever master segment does, slave simply follows the master surface.
• This type uses penalty formulation which will add springs at the contact surface by setting spotflag=25. So we can avoid incompatible kinematic conditions.

Type 7 And Type 11 contact interface (self contact)

• Type 7 is for Node to surface contact. Nodes are the slaves and Surfaces are master segment. The whole crash tube is on self contact for impact using type 7.
• Type 11 is for Edge to Edge contact. The lines of parts are combined to form contact. It is an additional contact for crash tube.
• In association with interfaces Type 7 and Type 11 solves the locking cases and uses the same penalty formulation. The search algorith is same like type 7.

The recommended General contact interface parameters are

Load Cases

• The Rigid wall with search distance for nodes is 100mm. The initial velocity of crash tube is 13.3 mm/ms along translational Z direction. The RUN time is 30 ms.
• The rigid wall serves as master and Crash tube serves as a slave for performing calculations.

Case 1 - Run the model without changing parameters. (default model)

The crash tube opened in hypercrash and modified spotflag = 25 (penalty formulation), so that to avoid Incompatible kinematic condition error. The notches initiate the deformation of crash tube and it begins to crashed against the rigid wall. The crash tube continues its deformation and after the 20ms it gets suppressed against the material so it adds up some contact forces to resist the deformation aginst the rigid wall. The boundary conditions are well defined for translational motion of rigid body along -z direction.

• From the above energy plot it can be seen that, the increase in the internal energy is 41691.23 joules.Initially the system is in equilibrium and the internal energy is zero.The initial kinetic energy of the crash tube is 44400.78 joules. The impact energy absorbed is the increase in the total energy of the system.
• In the Rwall plot, The maximum Rwall force that resists the crash tube is 1363.265 joules. This is caused because after the 20ms the materials gets compressed together due to impact thus increase the contact forces and added mass of rigid body increased the Rwall forces to resist.
• The maximum contact forces of seam weld i.e type 2 is 407.44 joules and type 7(self contact) is 1702.36 joules. Due to material compression the contact forces are high after 20ms.

Elapsed time = 199.22 s

No. of cycles = 82094

Energy error = - 4.8%

Mass error = 0.00%

The negative error indicates energy has been dissipated. Due to contact forces, The self contact (type 7) have some amount of energy increased this causes increase in energy error of -4.8%.

Case 2 - Changed inacti = 6 in type 7 contact and Run

Edited the model in hypercrash and changed Inacti = 6,This is used for treating the initial penetrations. The type 7 parameters are defined to get proper impact results and contact interfaces. The crash tube of contact interface card is edited with recommeded parameters forcrash. Then changed inacti = 6 in type 7 contact and run using RADIOSS.This inacti results in removing the penetrated node from the interface or remove the master segment relating to the penetrated node. Then all the previous parameters remains same.

• From the above energy plot it can be seen that, the increase in the internal energy is 40158.04 joules.Initially the system is in equilibrium and the internal energy is zero.The initial kinetic energy of the crash tube is 44400.78 joules. from comaprision with case 1 internal enerrgy decreases because of sme stiffness taken into account.The impact energy absorbed is the increase in the total energy of the system.
• In the Rwall plot, The maximum Rwall force that resists the crash tube is 1489.98 joules. From comparing case 1 model have some stiffness.Due to inacti = 6 , it increases some stiffness to the material,thus the Rwall forces to resist increases.
• De to change in anacti = 6, intial penetrations are accomplished,that results in increasing the selfcontact = 2653.62 joules and seam weld i.e type 2 = 4176.11. type 2 contact forces increased high because of treating initial penetratios.

Elapsed time = 151.48 s

No. of cycles = 78298

Energy error = - 7.9%

Mass error = 0.00%

from comaring with case 1, The time decreased due to increase in stiffness on model and energy error goes on negative because of energy dissipation. The contact forces are also too high this may causes increase of negative energy error. Energy error is increased due to the increase of contact energy. contact energy is also dissipative

Case 3 - create Type 11 contact (edge - edge) with recommended parameters

The recommended paramaters are changed in type 11 and type 7. In association with the intersection of type 7 and type 11 solves the loading cases and uses the same penalty formulation and search algorithm as type 7 . It actually increases the computation time. Because edge to edge contact also adds up some calculation to be done.

• From the above energy plot it can be seen that, the increase in the internal energy is 39851.86 joules.Initially the system is in equilibrium and the internal energy is zero.The initial kinetic energy of the crash tube is 44400.78 joules. From comparing case 2 there is drop in internal energy due to type 11 contact. The impact energy absorbed is the increase in the total energy of the system.
• In the Rwall plot, The maximum Rwall force that resists the crash tube is 1470.20 joules. This is caused because after the 20ms the materials gets compressed due to impact and added mass of rigid body increased the Rwall forces to resist. It can clearly see that the model adds up some stiffness.
• The maximum contact forces of seam weld i.e type 2 is 5578 joules and type 7(self contact) is 911.18 joules and added type 11 is 3851.88 joules. Due to material compression the contact forces are high after 20ms. The increase in contact forces is happened by defined parameters of contact interfaces for typical crash scenario.

Elapsed time = 257.44 s

No. of cycles = 91373

Energy error = - 8.6%

Mass error = 0.00%

The time step increases because of type 11 contacts which adds some computation to get better results but comparitively type 7 gives the same expected results. The negative energy error occurs because of energy dissipation due to loss some amount of internal energy when compare to case 2 and slight increase in contact forces also accounted for energy error.

Case 4- notches are aligned and boundary conditions on rigid body

The notches are alighned by using align nodes in hypermesh. The boundary condition is removed by using hypercrash. This effects in the slave nodes of rigid body doesn't follow translational motion, it gets deviated from the translation axis along z-direction.

• From the above energy plot it can be seen that, the increase in the internal energy is 40397.98 joules.Initially the system is in equilibrium and the internal energy is zero.The initial kinetic energy of the crash tube is 44400.78 joules. From comparing case 3, the internal energy have slight increase. The impact energy absorbed is the increase in the total energy of the system.
• In the Rwall plot, The maximum Rwall force that resists the crash tube is 1048.26 joules. The Rwall force to resist have decreases because of no boundary condition.
• The maximum contact forces of seam weld i.e type 2 is 5596.77 joules and type 7(self contact) is 301.55 joules and type 11 contat force is 3130.05 joules. There is no boundary condition to get proper deformation for crash. The deformation is improper due to which type 7 contact have lsses its contact energy for impact because of no boundary condition and improper deformation.

Elapsed time = 266.92 s

No. of cycles = 100586

Energy error = - 7.6%

Mass error = 0.00%

The simulation time increases because of less stiffness due to loss of type 7 contact force. The energy is negative because energy dissipation and no boundary conditions. Energy error is slightly reduced while rebounding because of the decrease of self contact type 7 interface even though if there is a friction.

Case 5 - Notch in the middle with Boundary condition

The notch is created in the middle of crash tube by using edge offset command in hypermesh and created a boundary condition in hypercrash. The initial deformation occurs at the middle of notch and to get proper deformation i created a boundary condition in rigid body. And the deformation is proper because of boundary condition.

• From the above energy plot it can be seen that, the increase in the internal energy is 40445.45 joules.Initially the system is in equilibrium and the internal energy is zero.The initial kinetic energy of the crash tube is 44400.78 joules. Which is nearly same like previous cases.The impact energy absorbed is the increase in the total energy of the system.
• In the Rwall plot, The maximum Rwall force that resists the crash tube is 1437.96 joules. The Rwall force resist during impact is hig, because not in middle creates some stiffness.
• The maximum contact forces of seam weld i.e type 2 is 5059.62 joules and type 7(self contact) is 764.56 joules and Type 11 contact have 3444.07 joules Because of added boundary condition we regain the contact forces.

Elapsed time = 355.04 s

No. of cycles = 102502

Energy error = - 7.3%

Mass error = 0.00%

The time step increases because of added stiffness created by notch in middle. when compared to previous case, The dissipated negative energy error have slight decrease because of added boundary condition.

Case 6 - New notch on opposite faces

The new notch is created by aligning 2 faces using align nodes in hypermesh. Initially the offset face gets deformed and creates buckling from the faces, then the model gets deformed.

• From the above energy plot it can be seen that, the increase in the internal energy is 40036.70 joules.Initially the system is in equilibrium and the internal energy is zero.The initial kinetic energy of the crash tube is 44400.78 joules. The impact energy absorbed is the increase in the total energy of the system.
• In the Rwall plot, The maximum Rwall force that resists the crash tube is 1297.72 joules. This is caused because after the 20ms the materials gets compressed due to impact and added mass of rigid body increased the Rwall forces to resist.
• The maximum contact forces of seam weld i.e type 2 is 5371.98 joules and type 7(self contact) is 1616.14 joules and type 11 contact force is 3212.08 joules. when compared to previous case, notch in the face creates increase in the contact energy of type 7 contact interface.

Elapsed time = 225.85 s

No. of cycles = 84911

Energy error = - 8.5%

Mass error = 0.00%

The stiffness of the material is high because of notch in face reduces the time step. The energy error is dissipative , when compared to previous case there is an increase in contact energy which increase the energy error. Due to stiffness the cycle time reduces.

Conclusion:

The analysis of crash tube is compared as six cases with some small changes in setup (notches) and different kinematic conditions.

• The case 1 have high amount of internal energy (energy absorbed) but that model did't that much strong when compared to case 2 because of resistive Rwall force is high in case 2. Comparively case 2 absorbed nearly same energy like case 1 and Rwall force is high which defined high resistance ensures high strength.
• The case 3 have high computation time and it gives nearly same result as case 2. case 4 creates loss of contact in type 7 when compared to all cases. case 5 and 6 also have same results but it have less stiffness thus increase the run time.
• From comparing all the cases, Case 2 (Type7 contact) is suggested because of less computation time giving required result. if there is an locking behaviour of material we can associate type 7 and type 11 to get good results but comparitively the CPU cost is high.

Google drive link:

https://drive.google.com/drive/folders/1h6XS2hasYuOYDnJwYQC8Xd4IZN7qY2H1?usp=sharing