Mechanical

Uploaded on

22 Oct 2022

Skill-Lync

Below are the frequently asked questions in Crashworthiness Analysis:

MESSAGE ID: 169

** STOP: ZERO OR NEGATIVE VOLUME

The Negative volume error happens when solid elements get deformed to a very large extent and their characteristic length goes to 0. For large strain formulation, the time step of an element goes to 0 when the element is compressed completely.

**Remedy: **

- Use Ismstr=2 in the solid property and use the option /DT/BRICK/CST which will set the time step value tmin at which the solid elements should switch to small strain. This means that when the time step drops below tmin, it automatically switches to a small strain formulation and if it is greater than tmin, it maintains the large strain formulation.
- If the same issue persists even after Ismstr and /DT/BRICK/CST, then use HA8 solid formulation.
- If the solid element is badly warped, that can also lead to negative volume generation.

- One integration point in a shell element leads to the membrane behaviour of that element (a shell element is the combination of plate and membrane elements). They have no bending strength. The reason is bending strength is calculated by integrating the elements over the thickness.
- In the case of elastic behaviour, 3 integration points are enough to obtain the result. The major reason is that stress is linear.
- In the case of plastic behaviour, the stress is not linear, the same bending is not properly integrated throughout the thickness if the element has 3 integration points, to avoid this we go for 5 integration points in the shell elements.

**ERROR IN OPENING INPUT FILE

- This error occurs if the _0000.rad file contains an improper solver version in it
- Even if the engine file and starter file are in different version formats.

**Remedy:**

- Based on the radioss solver version used one can change the keyword /VERS/14 to the version being used.
- Make sure the same is followed in the starter file as well. This will eradicate the opening of the input file error.

- Get density by weighing the specimen, young’s modulus with a compression test, Poisson coefficient is assumed to be 0.2 approx
- Get ‘fc’ from the compression strength using a cube compression test.
- Inputting the following data will ensure the accuracy of results,
- Get ‘ft’ from the tensile test, it is the direct tensile strength
- Get ft, Ht from the splitting tensile test also called as Brazilian test.
- Get ‘fb’ using biaxial test
- Get f2, s0 from the confined test.

- The curve is basically a plastic portion of the stress vs strain rates curve. True stress vs strain curve is inputted rather than the experimental ones (engineering curve).
- One can convert the engineering curve to a true stress-strain curve by using the below-mentioned formulas

- Depending on the material law, the solid elements are not deleted after the criteria EPSmax is reached.
- For material Law 2, Law 4 and Law 22, only the deviatoric part of the stress tensor is set to zero, the internal pressure of the solid is still computed.
- On the other hand, for material law 3, Law 23, Law 28 and Law 36, the solid elements are deleted when EPSmax is reached.

- Actually, it is not mandatory. Only the external surface of the solid should be defined. It is possible to define the surface of an external skin of a solid part.
- This can be clearly seen when creating a type7 of the interface in frontal crash projects where there is a solid crushable component present in the front of the car being selected for self-impact contact.

- It is always recommended to choose the coarse mesh on the master side for better contact detection.
- If the mesh of fine size is chosen on the master side, the contact is not detected at all.

WARNING ID: 94

** WARNING IN INTERFACE GAP

INPUT GAP xxx

HOWEVER, GAP IS RECOMMENDED TO BE LESS THAN xxx

- The recommended value corresponds to the smallest side length of shell elements on the master side, divided by 2.
- A self-impacting interface for which the GAP equals half the side length of an element is shown below,

- If this element is compressed more than 50%, the red node enters into the gap of the neighboring element. A self-contact is then detected which leads to over-stiffening of the structure. So the warning message means that at least one element on the master side has a side length less than twice the GAP and there is a risk of over-stiffening.
- If the side length L of an element is lower than the GAP, a self-contact will be computed from the beginning of the computation.
- Such a situation can be accepted if it is local enough, but not if it is a frequent situation over the self-impacting interface. It is possible to find the source of this message by using a preprocessor and selecting the elements through size criteria.
- In case the interface is not self-impacting, possible consequences of this message will be low performance, but the model behaviour will be correct.

WARNING ID: 147

*** WARNING: INCOMPATIBLE KINEMATIC CONDITIONS

- Two kinematic conditions which apply at the same time along non-orthogonal directions are considered incompatible.
- For instance, a boundary constraint on a node in the X direction of the global system and an imposed velocity in the same direction are not compatible. On the other hand, a boundary constraint on a node in the X direction of the global system and an imposed velocity in the Y or Z direction of the global system are compatible. Since a rigid body controls the movement of its slave nodes, a rigid body slave node cannot use another condition.
- Moreover, no error is written by RADIOSS starter better than these warnings, which would prevent running the computation, since the analysis of the compatibility of the kinematic conditions is sometimes more complex than RADIOSS starter can manage. In such cases, the WARNINGS provided the only clue to error termination of the run and it is the user’s responsibility to check the model.

- It is not recommended because the local system of the master node is not compatible with options imposing rotations. If some elements having rotational stiffness are connected, an incompatibility occurs.
- Depending on the flag ICoG, which is used in the rigid body definition, a lot of mass and inertia can be added locally onto the node; as well as the node can be moved to the centre of mass of the rigid body.

- This is the error which keeps on printing continuously until the solver stops.
- This happens when exporting and importing between hypermesh and hypercrash.

**Remedy:**

- The remedy is to check for any undefined cards, undefined component collectors, undefined material collectors etc and delete them if there are any.

- In Hyperview, select preferences → Option → Visualization menu in hyper view to display eroded elements as shown below. This will help you to understand the propagation of a fracture.

- This is because energy absorbed due to the numerical damping is output there. This means, in output, the place of hourglass energy has been used to present this viscous energy.
- The energy corresponding to the physical stabilization of the hourglass is counted as internal energy for this formulation

- The stresses SIGX, SIGY in animation files represent the mean stresses through the thickness of the shell element.
- The VONM stress represents the mises criteria applied to these mean stresses SIGX, SIGY. These mean stresses are computed by summation of the stresses at each integration point, averaged by the integration weights. They are used for the calculation of internal forces.

Author

Navin Baskar

Author

Skill-Lync

Continue Reading

**Related Blogs**

Moving Frame of Reference

A Moving Reference Frame (MRF) is a very straightforward, reliable, and effective steady-state Computational Fluid Dynamics (CFD) modeling tool to simulate rotating machinery. A quadcopter's rotors, for instance, can be modeled using MRFs.

Mechanical

12 May 2023

Analysis Settings in Ansys Software

Analysis settings in Ansys are the parameters which determine how the simulation should run.

Mechanical

08 May 2023

Comparing the Explicit and Implicit Methods in FEA

In Ansys, the analysis settings play a very important role in converging the solution and obtaining the results. These involve settings about the timestep size, solver type, energy stabilization etc.

Mechanical

06 May 2023

Tensors, Stress, and 2D Meshing: A Primer for Beginners

A tensor is a mathematical object that describes a geometric relationship between vectors, scalars, and other tensors. They describe physical quantities with both magnitude and direction, such as velocity, force, and stress.

Mechanical

05 May 2023

Reynold's law of Similarity

The Reynolds number represents the ratio of inertial to viscous forces and is a convenient parameter for predicting whether a flow condition will be laminar or turbulent. It is defined as the product of the characteristic length and the characteristic velocity divided by the kinematic viscosity.

Mechanical

04 May 2023

Author

Skill-Lync

Continue Reading

**Related Blogs**

Moving Frame of Reference

A Moving Reference Frame (MRF) is a very straightforward, reliable, and effective steady-state Computational Fluid Dynamics (CFD) modeling tool to simulate rotating machinery. A quadcopter's rotors, for instance, can be modeled using MRFs.

Mechanical

12 May 2023

Analysis Settings in Ansys Software

Analysis settings in Ansys are the parameters which determine how the simulation should run.

Mechanical

08 May 2023

Comparing the Explicit and Implicit Methods in FEA

In Ansys, the analysis settings play a very important role in converging the solution and obtaining the results. These involve settings about the timestep size, solver type, energy stabilization etc.

Mechanical

06 May 2023

Tensors, Stress, and 2D Meshing: A Primer for Beginners

A tensor is a mathematical object that describes a geometric relationship between vectors, scalars, and other tensors. They describe physical quantities with both magnitude and direction, such as velocity, force, and stress.

Mechanical

05 May 2023

Reynold's law of Similarity

The Reynolds number represents the ratio of inertial to viscous forces and is a convenient parameter for predicting whether a flow condition will be laminar or turbulent. It is defined as the product of the characteristic length and the characteristic velocity divided by the kinematic viscosity.

Mechanical

04 May 2023

Book a Free Demo, now!

Related Courses

Recently launched

0 Hours of content

Cae Domain

Recently launched

0 Hours of content

Cae Domain

5

0 Hours of content

Design Domain

Showing 1 of 6 courses

Try our top engineering courses, projects & workshops today!Book a Live Demo