Comparison of Spotweld Crash Analysis performed with Beam and Solid Elements in LS-DYNA

Comparison of Spotweld Crash Analysis performed with Beam and Solid Elements in LS-DYNA

Objective: The aim of this project is to model spotwelds with beam and solid elements, perform a crash test and compare both the results for different parameters:

• Axial and Shear forces for beam and solid elements
• Cross sectional forces at the mid plane.
• Acceleration of a node at the midplane.
• Maximum directional stress and strain along the longitudinal axis and
• Energy plots.

The primary conditions for the crash test to be performed are

• The material properties to be considered for the shell elements and the weld elements is 'Steel'.
• The units system is (g,mm,ms,N,MPa,N-mm).
• Assuming an Initial velocity of 50kmph = 13.888 mm/ms.

Procedure:

1. Modelling of Beam Elements:

Select Element Tool>Element Editing>Create>Element Type-Beam >Select node1,2> Accept >Done. Element ID and Part ID definition is upto the user's interest.

2. Material Creation: The two parts that the beam elements are connected to are given MAT_ELASTIC and the material for beam elements is taken as MAT_SPOTWELD.

3. Section creation: The ELFORM for spotweld beam must be 9 and QR/IRID:4 (4x4 Gauss quadrature)with CST(cross section type)=1, thickness values as 2.5mm. Whereas the section for the parts are Shell section with ELFORM-16, NIP-2, and thickness 1.5mm which can be seen in the picture

4. Part creation: Assign the material and section to the shell and beam elements respectively.

5. Boundary Creation: Two Boundaries one with Single Point Contraints (SPC_SET) on one shell and Prescribed motion on other shell with a Load curve ID (LCID) defined.

The LCID is created which can be seen in the picture below.

6. Contact creation: An AUTOMATIC_SINGLE_SURFACE contact has been defined so that the shell parts do not penetrate each other when the beam elements fail and a TIED_SHELL_EDGE_TO_SURFACE contact also has been created for the contact between edges of the beam elements and shell surfaces. In the AUTOMATIC_SINGLE_SURFACE contact both the shell parts acts as master and slave so only one slave id has been imputed. In the TIED_SHELL_EDGE_TO_SURFACE contact a Node set ID(beam elements) was applied as slave type and parts(shell parts) are masters.

7. Control card: The Termination time has been inputed as 10ms and the control energy card with HGEN and SLNTEN set to 2.

8. Database creation: BINARY_D3PLOT, HISTORY_NODE, CROSS_SECTION_SET, ASCII_option have been created in with

• BINARY_D3PLOT with the time interval of out was given as 1ms.
• HISTORY_NODE- nodes of Beam elements are selected as the acceleration of the nodes at the mid plane has to obtained.
• CROSS_SECTION_SET - nodes at the mid cross section and the shell elements where the nodes of the beams have interconnection between them have been selected.
• ASCII_option- GLSTAT, MATSUM, NODOUT, SECFORC, SLEOUT and SWFORC have been assigned a DT-1ms.

9. Modelling of Solid Elements:

• Everything is same as that of Modelling the Beam Elements but, change the Element Type to Hexa(solid) and select the nodes of interest(8 nodes).

• Change the section to solid for the solid element with ELFORM=1

• Take the contact for the solid elements as CONTACT_SPOTWELD and set the slave type for solid elements nodes and assign them as slave, while shell parts as master type and assigning them for the master

• Change the node set and shell elements for the Cross section set to obtain the cross sectional forces.
• Except these changes everything is in place.
• Run both the files in the solver.
• Open the D3PLOT for the Results.

RESULTS:

Animated results of Beam and Solid elements (front view)

Animated results of Beam and Solid elements (side view)

I. Axial Forces of Spot Weld Beam Elements:

The Maximum Axial Force was identified to be at 110016 beam element with a Force of 2.86e+3 N and takes place at 5ms

Axial Forces of Spot Weld Solid Elements:

The maximum Axial force occur at 110011 and 110017 solid elements with 1.08e+3N and takes place at 5ms

II. Shear Forces of spot weld Beam Elements:

The maximum shear force happens to be at 110011 and 110017, more precisely at 110017 with 2.63e+3 N and takes place at 5ms

Shear Forces of spot weld Solid Elements:

Almost all the solid elements have coincided probably at the same point at 5ms recording a Shear Force of 7.08e+3 N.

III. Cross sectional Forces at the Mid plane for Beam Elements

The Maximum cross sectional forces occurs at 7ms with a force of 1.85e+3 N and at 9ms it is 1.51e+3N.

Cross sectional Forces at the Mid plane for Solid Elements

The Maximum cross sectional forces occurs at 2ms with a force of 2.1e+4 N and but it is of the interest that at 5ms what is the sectional force. It is noted as 1.88e+4 N.

III. Acceleration of the node at mid section of Beam Elements:

The maximum acceleration of the mid section nodes is 262 mm/ms². At 5ms it is 9.28mm/ms².

Acceleration of the node at mid section of Solid Elements:

Two nodes have been chosen as done in the Beam elements to check the acceleration at the mid section for beam and solid elements and the maximum acceleration occurs for the solid elements at 9ms which is 886mm/ms², for the node 502720.

Acceleration of the node at mid section of Beam Elements:

The maximum acceleration of the mid section nodes at 6ms is 270 mm/ms². At 5ms it is 2.84mm/ms².

Acceleration of the node at mid section of Solid Elements:

The maximum acceleration of the mid section nodes at 10ms is 945mm/ms². At 5ms it is 24.8mm/ms².

IV. Stress along the longitudinal axis

The maximum stress that occurs at the time of failure (5ms) is 1.536e+3MPa.

Stress along the longitudinal axis:

The maximum stress that occurs at the time of failure (5ms) is 2.167e+3MPa.

Strain along the longitudinal axis for beam elements:

The Maximum Strain that is recorded at the time of failure is 4.395e-5.

Strain along the longitudinal axis for solid elements:

The Maximum Strain that is recorded at the time of failure is 6.394e-5.

V. Energies (Internal, Kinetic, Total, Hourglass, Sliding)

The Kinetic energy at 10ms is 4.44e+3 N-mm or 4.44 N-m(J), All of the kinetic energy was being applied to tear (axial force) the beam elements and there by the beam elements absorbing more amount of energy. At the point of Failure the Total energy recorded was 6.23e+4 N-mm and it reached a maximum of 6.31e+4 N-mm. The internal enery was 5.86e+4 N-mm at the end of Simulation.

For solid elements:

The Kinetic energy at 10ms is 1.18e+4 N-mm or 11.8N-m(J), All of the kinetic energy was being applied to shear off the solid elements and there by the solid elements absorbing more amount of energy. At the point of Failure the Total energy recorded was 1.50e+5 N-mm and it reached a maximum of 1.51e+5 N-mm. The internal enery was 1.39e+5 N-mm at the end of Simulation.

Element failure Time:

Conclusion:

SNo	Parameters	Beam elements	Solid elements
1	Axial Force	2.86e+3 N	1.08e+3 N
2	Shear Force	2.63e+3 N	7.08e+3 N

• The beam here is analogous with a Hair Fibre as it is very thin member (Height to diameter ratio). In here beam has been given some thickness which is less when compared to it's thickness. So, it tensile resistance is maximum. That is why, the Axial force of beam elements in more than solid elements.
• It can also be compared to the plane stress and plane strain criteria.
• The ductile material is weak in shear, but it not the problem as beam and solid elements are set to 'steel'. It is the thickness that matters. The width and thickness of the solid elements are observed to be 3.5mm(approx.) which the Beam element lacks which indicates that the Solid elements are more preferable when there is any uncertainty of which element to be considered.
• There is no Sliding or Hourglass energy as there was no contact between the two parts throughout the simulation for both beam and solid elements.
• The control energy card was created to take care of Hourglass energy, more over the ELFORM-9& 16 for beam and solid element section is set to spotweld beam and fully integrated shell element respectively which has no Hourglass deformation.
• The Failure of all the elements both beam elements and the solid elements at half the termination time i.e,(5ms)