Update spring stiffness with corresponding time step

[vpetrov]

Hello.

I'm working on soft robotics and for checking the results of the work I've been doing, I need to create a model inside Abaqus CAE. Tvhis model has to represent a spring. Stiffness of the spring changes with time, so I have two vectors, one of which represents stiffness value, meanwhile the second one represents time step. I saw that similar question has been asked before (thread799-489881), but the details weren't discussed.

Part 1
So I wrote the test_spring_one.inp (attached to the thread) file where I pointed out the desired stiffness and time steps. Here is the text inside the test_spring_one.inp file.

*Heading
** Job name: SpringStiffnessVariation Model name: Model-1
** Generated by: Abaqus/CAE
**
** PARTS
**
*Part, name=SpringPart
*Node
1, 0., 0., 0.
2, 1., 0., 0.
*Element, type=SPRINGA
1, 1, 2
*Elset, elset=SPRINGA
1
*Nset, nset=ALLNODES
1, 2
*End Part
**
** ASSEMBLY
**
*Assembly, name=Assembly
*Instance, name=SpringPart-1, part=SpringPart
*Spring, elset=SpringPart-1.SPRINGA
SpringMaterial, 1.0
*End Instance
*End Assembly
**
** MATERIALS
**
*Material, name=SpringMaterial
*Elastic, dependencies=1
1.0, 0.0
**
** AMPLITUDE
**
*Amplitude, name=StiffnessAmplitude
0.0, 1.0
1.0, 1.5
2.0, 2.0
**
** INITIAL CONDITIONS
**
*Initial Conditions, type=FIELD, variable=1
SpringPart-1.ALLNODES, 1.0
**
** STEP: Apply Load
**
*Step, name=ApplyLoad, nlgeom=NO
*Static
0.1, 1.0, 1e-05, 0.1
**
*Boundary
SpringPart-1.1, 1, 1, 0.
**
*Boundary
SpringPart-1.2, 1, 1, 0.1
**
*Output, field, variable=PRESELECT
**
*End Step

what I do is:
1) Import model to Abaqus, that gives an adequate response:

The model "ABQdummy" has been created.
The model "test_spring_one" has been created.
The part "SPRINGPART" has been imported from the input file.
The model "test_spring_one" has been imported from an input file. 
Please scroll up to check for error and warning messages.

2) Then I create a job, the response is:

The job "Job-1" has been created.

3) Then I submit the job and I get:

The job input file "Job-1.inp" has been submitted for analysis.
Error in job Job-1: THIS MODEL HAS EITHER INCORRECT OR NO ELEMENT DEFINITIONS.
Job Job-1: Analysis Input File Processor aborted due to errors.
Error in job Job-1: Analysis Input File Processor exited with an error - Please see the  Job-1.dat file for possible error messages if the file exists.
Job Job-1 aborted due to errors.

_________________________________________________
Part 2

There is another thing I've done. I created MassSpring.cae file (attached). Inside I defined amplitude curve and I tried to assign it to spring stiffness in the input file. Then I again create a job and submit it and I get:

The model "Model-1" from model database "C:/PhD/abaqus/MassSpring.cae" has been imported as "Model-1".
The job "Job-1" has been created.
The job input file "Job-1.inp" has been submitted for analysis.
Job Job-1: Analysis Input File Processor completed successfully.
Job Job-1: Abaqus/Explicit Packager completed successfully.
Job Job-1: Abaqus/Explicit completed successfully.
Job Job-1 completed successfully.

But the problem is that the amplitude curve is not assigned to the stiffness, the place where I defined stiffness for the spring it just remains a constant. I can assign it to the boundary condition part but not to the stiffness for some reasons. How we can assign amplitude curve to the stiffness, is there any way we can add this tabular table? I wouldn't want to deal with creating subroutines for now, seems like I'm trying to deal with pretty simple issue, but I can't still figure this out.

What am I doing in a wrong way? Can you help me out? Maybe could you provide me with an exemplary .inp file that would represent a spring with two different stiffnesses and two corresponding time steps? I've been dealing with this issue for a while, can't figure things out.
Appreciate your time and efforts!

 

[vpetrov]

Uploaded the file for the 2nd problem

 

[FEA way]

When working directly with input files outside of Abaqus/CAE, it's better to submit them from the command line instead of importing them to CAE.

Spring elements don't use material assignments, you define the stiffness directly. It can be nonlinear but not controlled by an amplitude. Check the *SPRING keyword in the documentation. Nonlinear spring stiffness can't be defined in Abaqus/CAE but you can use connector elements instead.

 

[vpetrov]

I think I was able to deal with this issue by introducing Field Variables