Menu

Executive Programs

Workshops

Projects

Blogs

Careers

Student Reviews



More

Academic Training

Informative Articles

Find Jobs

We are Hiring!


All Courses

Choose a category

Loading...

All Courses

All Courses

logo

Mechanical

Modified on

17 Oct 2022 08:32 pm

What is Strain Formulation?

logo

Skill-Lync

Strain formulation is sought-after in nonlinear analysis as the model/component of interest that undergoes plastic deformation. We have already studied that the explicit solver follows courant stability i.e critical time must be less than that of the critical length of the element. Under such situations, when plastic deformation happens, there are a lot of chances of large deformation happening in the model. When large deformation happens, the element's lengths tend to reduce, which in turn puts the stress on the solver to take care of such elements too.  If the solver considers those elements then the stability of solving further is going to be a big question.  

Now, basically, our solver is going to take the deformation in the previous case as a reference and it is going to calculate the same strain and deformation for the next step. As mentioned earlier, if this stability is reduced then solving for the next step is going to be a difficult task. This particular issue is taken care of by what is called strain formulation. 

Strain Formulation

Strain formulation can be divided into two: Large strain formulation and Small strain formulation. The explicit solver follows a large strain formulation. 

Large strain formulation is by default used in the RADIOSS calculation. Now let’s see what is a large deformation. Large deformation theory or large strain theory is defined as the deformation of a solid body in which the displacements created by the force are assumed to be larger and creates permanent deformation. Basically, the term ‘large’ is considered if the deformation is higher than the component’s thickness.

The phenomena like negative volume generation, large distortion, and excessive displacement come under large deformations. Large strain formulation is best suited for nonlinear, elastoplastic behavior and small strain formulation is suited for linear elastic behavior.

Small Strain Options

Small strain options are used in shell property definition in RADIOSS. In the same shell property, we select Ismstr=2 which is ‘full geometric nonlinearities with possible small strain formulation activation in Radioss Engine’, this is recommended property in a crash and also if not explicitly given, radioss takes this option only. As the term means, whenever there is a situation of very large deformation, a small strain flag can be activated. Now, to understand how this small strain works, let's get into a stress-strain relationship.

We know the difference between engineering stress-strain and true stress-strain. To give a quick answer, the engineering stress-strain curve depends on the original cross-section and the gauge length of the specimen and the true stress-strain curve depends on the instantaneous cross-section and gauge length of the specimen.

The large strain formulation results from incremental strain computation i.e stress and strains in large strain formulation are true stress and true strains. When a very large deformation happens in the case of negative volume, it leads to a very high decrease in the time step, and the stability of the solution reduces. At those times, the solver automatically takes that kind of deformation as a small strain formulation where it converts the true stress-strain curve to engineering stress-strain values which are going to be very less than the true stress-strain values. By doing this, the large deformations are reduced to a larger extent and the stability of the solution is confirmed. 

To make it very simple, explicit analysis follows large strain formulations. In large strain formulation, the material data are taken as true stress-strain values by the solver. Whenever, a very large deformation happens which results in negative volumes, it affects the solver’s stability condition resulting in a reduction in the time step. To avoid these problems, a small strain formulation is important. The small strain formulation, at large deformations, converts the material values as engineering stress-strain values, which results in better solving of the solver engine. Hence, we give freedom to RADIOSS to jump between large strain and small strain formulation at times of crisis. 

Things to Remember

It is important to note that a small strain formulation is not recommended for crash analysis tools for shell elements. The reason is very simple: shell elements during the crash undergo bending. Small strain formulations do not cover the bending of elements. If we look into the Ismstr option in the /PROP/SHELL card we get a better understanding: 

  • If Ismstr=1 or 3, then the whole simulation will be carried out in small strain formulation but the credibility of the results is not ensured. 
  • In the same way, if Ismstr=1 or 3 then the material data is taken as engineering stress-strain. 
  • If Ismstr=4, RADIOSS will not switch to a small strain formulation whenever required. 
  • If Ismstr=10, then it takes only a small strain formulation for hyperelastic materials

To conclude, the small strain formulation helps to keep the stability of the model under control during times of duress. 


Author

author

Navin Baskar


Author

blogdetails

Skill-Lync

Subscribe to Our Free Newsletter

img

Continue Reading

Related Blogs

Shock tube simulation

Learn how to render a shock-tube-simulation and how to work on similar projects after enrolling into anyone of Skill-Lync's CAE courses.

Mechanical

10 May 2020


Design of Frontal BIW enclosure of a car (Bonnet)

In this blog, read how to design the frontal BIW enclosure of a car (Bonnet) and learn how Skill-Lync Master's Program in Automotive Design using CATIA V5 will help you get employed as a design engineer.

Mechanical

10 May 2020


What is Tetra Meshing?

Tetrahedral is a four- nodded solid element that can be generated through the tria element by creating a volume and also through the existing volume of the geometry. These elements are used where the geometry has high thickness and complexity. The image attached below is a representation of a Tetra element. The Tetra element will have 4 triangular faces with four nodes joining them together

Mechanical

02 Aug 2022


Realizing Connectors In HyperMesh

A connector is a mechanism that specifies how an object (vertex, edge, or face) is connected to another object or the ground. By often simulating the desired behaviour without having to build the precise shape or specify contact circumstances, connectors make modeling simpler.

Mechanical

03 Aug 2022


Mesh Sizing In Ansys Workbench

One of the most crucial processes in carrying out an accurate simulation using FEA is meshing. A mesh is composed of elements that have nodes—coordinate positions in space that might change depending on the element type—that symbolise the geometry's shape.

Mechanical

04 Aug 2022



Author

blogdetails

Skill-Lync

Subscribe to Our Free Newsletter

img

Continue Reading

Related Blogs

Shock tube simulation

Learn how to render a shock-tube-simulation and how to work on similar projects after enrolling into anyone of Skill-Lync's CAE courses.

Mechanical

10 May 2020


Design of Frontal BIW enclosure of a car (Bonnet)

In this blog, read how to design the frontal BIW enclosure of a car (Bonnet) and learn how Skill-Lync Master's Program in Automotive Design using CATIA V5 will help you get employed as a design engineer.

Mechanical

10 May 2020


What is Tetra Meshing?

Tetrahedral is a four- nodded solid element that can be generated through the tria element by creating a volume and also through the existing volume of the geometry. These elements are used where the geometry has high thickness and complexity. The image attached below is a representation of a Tetra element. The Tetra element will have 4 triangular faces with four nodes joining them together

Mechanical

02 Aug 2022


Realizing Connectors In HyperMesh

A connector is a mechanism that specifies how an object (vertex, edge, or face) is connected to another object or the ground. By often simulating the desired behaviour without having to build the precise shape or specify contact circumstances, connectors make modeling simpler.

Mechanical

03 Aug 2022


Mesh Sizing In Ansys Workbench

One of the most crucial processes in carrying out an accurate simulation using FEA is meshing. A mesh is composed of elements that have nodes—coordinate positions in space that might change depending on the element type—that symbolise the geometry's shape.

Mechanical

04 Aug 2022


Book a Free Demo, now!

Related Courses

https://d28ljev2bhqcfz.cloudfront.net/mainproject/thumb/mid-surface-extraction-and-meshing-of-an-ip-substrate_1616574368.jpg
Modelling of an Automotive IP Trim component by using ANSA
Recently launched
0 Hours of content
Cae Domain
Know more
https://d28ljev2bhqcfz.cloudfront.net/maincourse/thumb/preprocessor-structural-analysis-ansa_1636604676.jpg
4.7
18 Hours of content
Cae Domain
https://d28ljev2bhqcfz.cloudfront.net/maincourse/thumb/advanced-cfd-meshing-ansa_1612263286.jpg
4.9
12 Hours of content
Cfd Domain
Showing 1 of 6 courses