LS-DYNA - Modelling Spotwelds

LS DYNA – Modelling_Spotwelds

By Enos Leslie

Mechanical Engineer

3^rd August 2021

QUESTION

In this assignment, you will model spotwelds for the given assembly of parts and run a crash test similar to the one in assignment 4. Details about spotweld location is in the image below. The yellow line signifies the spot-weld directions. You need to use 3-7 spotwelds along this line, in a symmetric manner, before running the simulation.

The spotwelds should be modeled using beam elements and solid elements separately. While using solid elements, one spotweld should have one solid element.  The axial and shear force should be compared among beam and solid elements. The same material card using *MAT_SPOTWELD should be used for both beam and solid elements. You can use steel material data for *MAT_SPOTWELD cards and calibrate the failure data according to your results. The best simulation should have a failure at half of the termination time.

Important Warning:

Please proceed with this assignment only when you complete the assignment 4 and the spotweld modeling approach lessons from Module 4 (Part08-11).

Deliverables:

All deliverable exactly similar to assignment 4 but beam and solid spotweld results should be compared in relevant cases.

OBJECTIVE

1. The objective of this assignment is to model spot-welds for the given assembly by using beam and solid elements
2. Compare and analyze the difference in output results of both beam and solid elements.

PROCEDURE

Import

The model is imported into LS-Prepost deck using the keyword file (.kfile). The model consist of two overlapping plates which need to be joined by spotwelds along the yellow marker direction as shown below.

SECTION

A section card is defined for two plates using SECTION_SHELL card. Thickness of sheets = 1.5mm

• ELFORM: Element formulation options = 16 fully integrated shell element (Very Fast)
• SECID: Section ID
• SHRF: Shear correction factor which scales the transverse shear stress = 1
• NIP : Number of integration points = 2

MATERIAL

Unit system ( g mm ms ). Steel material is used for both plates.

• Density = (7850* 10^-6 )gm/mm^3
• Young Modulus = 210 * 10^3 g/(mm*
• Poisson Ratio = 0.3

ASSIGNING MAT AND SECTION

Material made and section is assigned to the two plates using their various specified part ID’s

SPOTWELD MODELING USING BEAM

The spots weld are modelled along the specified path above. ID beam elements are used by selecting

Mesh>Element Generation>Beam>Give part id>Select Beam by two nodes>

Pick node 1 and 2 from top and bottom sheets>Create>Accept>Done

SECTION FOR BEAM

Note that the default section and material card automatically created is deleted and a custom section and material is assigned

Using SECTION_BEAM_CARD

• ELFORM: Formulation 9 is used as only this supports MAT 100 spot-weld material.
• CST : Tubular type
• TS1= 3mm
• TS2 =3mm
• TT1=0mm (inner dia)
• TT2=0mm (inner dia)

MATERIAL

Unit system ( g mm ms ).Material Card 100 Spotweld.

• Density = (7850* 10^-6 )gm/mm^3
• Young Modulus = 210 * 10^3 g/(mm*
• Poisson Ratio = 0.3
• TFAIL= Failure time =6 ms
• EFAIL: Effective Plastic Strain in weld material= 1000
• NRR: Axial Force at failure = 10000
• NRS: Shear Force at failure =10000
• MRR: Torsional moment at failure =90000
• MSS: Moment resultant at failure =90000

Materials and section is assigned to the individual beams just like the shell part.

CONTACT

• CONTACT_TIED_SHELL_EDGE_TO_SURFACE- Using this contact type, SSTYP is 4 for node set ID and MSTYP is 2 for part set ID. The part ID of Two plates is created using create entity and specified as the master. A node set is created by selecting all the nodes of the beam and specified as the slave
• AUTOMATIC_SINGLE_SURFACE_CONTACT- This is a self-contact made to account for self-contact of the plates. The slave is set to Part ID and the two plates are selected.

BOUNDARY CONDITION

• PRESCRIBED MOTION SET is made to define the displacement boundary in the X direction. Using a node Set of the edge nodes of the upper plate, a prescribed motion displacement of rigid body is selected. A load curve is defined below.

DATABASE

• ASCII OPTION

GLSAT: Global statistics: 1ms

NODOUT: Nodal displacement: 1ms

SWFORC: Spot-weld reaction forces: 1ms

• BINARY D3 PLOT

• HISTORY NODE

Using create entity >Database>History>Nodes of beam are selected.

These nodes are selected in order to analyze the behavior of the nodes of the beam.

CASE_2 (MODELING SOLID SPOT-WELD ELEMENTS)

Solid spot welds are created at the same position of the previous beam element.This is created from Element editing>Create_Hexa. The hexa solid welds can be seen below

SECTION

A section card is defined for the solid weld – SECTION SOLID. ELFORM =1

MATERIAL

Unit system ( g mm ms ). MATERIAL_SPOT_WELD

• Density = (7850* 10^-6 )gm/mm^3
• Young Modulus = 210 * 10^3 g/(mm*
• Poisson Ratio = 0.3

ASSIGNING MAT AND SECTION

Material made and section is assigned to the two plates using their various specified part ID’s

CONTACT

• CONTACT_SPOT-WELD- Using this contact type, SSTYP is 4 for node set ID and MSTYP is 2 for part set ID. The part ID of Two plates is created using create entity and specified as the master. A node set is created by selecting all the nodes of the beam and specified as the slave
• AUTOMATIC_SINGLE_SURFACE_CONTACT- This is a self-contact made to account for self-contact of the plates. The slave is set to Part ID and the two plates are selected.

CONTROL CARD

• TERMINATION- The endtime is specified at 10ms

RESULTS AND DISCUSSION

BEAM

ENERGY

Conservation of energy, principle of physics according to which the energy of interacting bodies or particles in a closed system remains constant. The first kind of energy to be recognized was kinetic energy, or energy of motion. The kinetic energy of the system remain almost zero from 1ms to 6ms. After the welds failed the parts became loose and the body was free to move thereby gaining slight increase in its kinetic energy.

The internal energy is the energy gained to the system due to the load applied to the end of the plate . As the load increases from 0 to 10N the internal energy increases until the weld fails. It then takes a drop as most of the energy applied is lost.

The total energy of the body remained the same as internal energy as majority of the constituents of energy involves internal energy. Very small amount of energy percentage is produced from kinetic energy and even way smaller energy is produced from hourglass effect. Therefore it can be seen that the total energy slightly follows the internal energy curve until the weld failure. After the weld failure , total energy becomes constant at a specific value because the is almost equal amount of drop in internal energy as increase in kinetic energy.

From the display below it can be seen that the displacement of the plates occurred smoothly without any abnormalities. The elements are displaced the way they should showing success in displacement

Stress

The maximum stresses are seen at the points of beam contacts with the surface of the plates as the beams are being failed. The stresses are concentrated mostly at the corners of the plate and the maximum stress is 2342 Mpa. Beam elements disappear after 6ms when they fail.

Shear Force

The numbers in the image below indicates the various beam elements which are being analyzed. The shear force generated on failure is recorded. The maximum Shear force is 4080 N is found averagely on all beam spotwelds. The welds fail at 6ms as the force drops after 6ms.

Axial Force

The numbers in the image below indicates the various beam elements which are being analyzed. The axial force generated on failure is recorded. He force drops to zero after the beam fails. The maximum axial force is 2630 N is highest in beam G, and fails at 6ms.

SOLID

From the display below it can be seen that the displacement of the plates occurred smoothly without any abnormalities. The elements are displaced the way they should showing success in displacement.

Stress

The maximum stresses are seen at the points of weld contacts with the surface of the plates as the beams are being failed. The stresses are concentrated mostly at the corners of the plate and the maximum stress is 1978 Mpa. The solid welds elements disappear

after 6ms when they fail

Shear Force

The numbers in the image below indicates the various beam elements which are being analyzed. The shear force generated on failure is recorded. The maximum Shear force is 28,300 N is found averagely on all solid spot-welds. The welds fail at 6ms as the force drops after 6ms.

ENERGY

Conservation of energy, principle of physics according to which the energy of interacting bodies or particles in a closed system remains constant. The first kind of energy to be recognized was kinetic energy, or energy of motion. The kinetic energy of the system remain almost zero from 1ms to 6ms. After the welds failed the parts became loose and the body was free to move thereby gaining slight increase in its kinetic energy.

The internal energy is the energy gained to the system due to the load applied to the end of the plate . As the load increases from 0 to 10N the internal energy increases until the weld fails. It then takes a drop as most of the energy applied is lost.

The total energy of the body remained the same as internal energy as majority of the constituents of energy involves internal energy. Very small amount of energy percentage is produced from kinetic energy and even way smaller energy is produced from hourglass effect. Therefore it can be seen that the total energy slightly follows the internal energy curve until the weld failure. After the weld failure , total energy becomes constant at a specific value because the is almost equal amount of drop in internal energy as increase in kinetic energy.

Axial Force

The numbers in the image below indicates the various beam elements which are being analyzed. The axial force generated on failure is recorded. He force drops to zero after the beam fails. The maximum axial force is 11,100 N is highest in beam F, and fails at 6ms.

From the above table it can be seen that the solid weld produced very high forces in terms of shear and axial because the hex elements produces mire stiffness compared to the beam elements therefore requires more energy to fail at the requires time of failure of 6ms.

The other observation is that the shear force is generally higher than the axial in both cases. The load applied is in the transverse direction of the beam elements so greater forces of shear would be expierenced in both cases

CONCLUSION

1. The spotwelds were modelled using 1D beam elements and 3d hexagonal solid elements
2. The simulation was carried out using a time dependent weld failure at 6ms which is about half the termination time.
3. The shear force and axial forces in both cases are plotted and compared with each other