High stress and plastic strain without impact

[Özgür Yalçın]

Hello, I am trying to run an impact analysis something like drop test. I have initial velocity for my whole body, I also defined global z acceleration for gravity in Load keyword. The problem is I observe really high stress and strain values before the impact of rigid body and I observe these stresses in the solid body initial elements just as picture. 

I defined automatic single surface as usual, I used ignore 2 and zero nothing changed, I am also observing hourglass, negative and sliding energy before the impact. What is the reason of these problem and what should I try to solve it

Thanks.

[James Kennedy]

Dear Ozgur,

 

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See if this example is of some help:

 

Drop Test

 

Example showing a drop test load case. A deformable body is dropped on a rigid wall.

 

https://www.dynaexamples.com/show-cases/drop-test

 

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Drop test Challenge | LS-DYNA

 

AIM: To perform a simple cellphone drop test in LS-Dyna by creating a complete simulation file of the model given below.    MODEL SETUP: In LS-Dyna, we will first open the keyword file of the model which is named as Drop_Test_onlymesh. After importing the keyword file, we will see the model as shown above. The…

 

https://skill-lync.com/student-projects/week-3-drop-test-challenge-102

 

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See if this free White Paper is of some help:

 

https://www.ansys.com/resource-center/white-paper/unbreakable-5-best-practices-for-drop-test-simulation-using-ansys-ls-dyna

 

Unbreakable: 5 Best Practices for Drop Test Simulation using Ansys LS-DYNA 

 

When was the last time you dropped your phone? When it comes to consumer products and electronics, losses resulting from accidents and shipping damages can be very expensive: from replacement costs to customer dissatisfaction. Drop testing helps to support product development in many ways, including failure prediction, material deformation, transient shocks, environmental resistance, energy absorption, general testing standards and much more.

 

While drop test simulation with Ansys LS-DYNA is comprehensive and robust, properly setting up the test has its own set of challenges. Users will need to consider many factors: materials, meshing, performance, runtime, and more. In this paper, we will discuss five best practices and key features for drop test simulation using LS-DYNA. This includes:

 

Mesh Type Selection and Generation Methods

Time Step and Mass Scaling

Contacts

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High-performance Computing and Scalability 

 

Download the paper for free today to help improve your simulation accuracy, user performance and time to results for your drop test applications.

 

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Sincerely,

James M. Kennedy

KB S2 Inc.

February 26, 2024

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[James Kennedy]

Dear Ozgur,

 

Make sure you are using a consistent set of units.

If you are looking at a simple drop test, you will usually have a simple initial
velocity simulation [v = sqrt(2gh)] based on the acceleration of gravity (g) and
the drop height (h).

It is usually recommended that you place the dropped object reasonably close
to the impacting surface to avoid excessive (wasted) computations needed if
you place the object some large distance away. If placed a large distance away,
all you are calculating is the rigid body motion of the dropped object (moving
at a constant velocity if gravity ignored). As stated above, you can specify the
contacting initial velocity.

 

If you place the dropped object too close to the impacting surface, you can have

initial penetration issues:

 

Adverse Effects of Initial Penetrations: The presence of penetration during the

first cycle (after problem initialization) can lead to several issues:

 

Large Initial Forces: LS-DYNA attempts to remove penetrations by applying

forces to the involved nodes. However, these initial forces can be substantial,

affecting model stability.

 

Non-Physical Stresses and Strains: The forces may induce localized initial

stresses and strains that are non-physical.

 

Negative Energy: If penetrations persist beyond the first cycle, they can lead

To negative energy conditions, impacting numerical accuracy.

 

Sincerely,
James M. Kennedy
KBS2 Inc.
February 27, 2024

[Özgür Yalçın]

Dear James, thank you for your answer,

My loadcase is a bit different than usual drop test, it is for simulating bottom of car hitting to curb due to that I have specific velocity and earth gravity as an global acceleration.

I run the model actually everything seems reasoneble( the deformation of the impact area, movement of the body etc.) but the only absurb thing is the high stress strains in the solid body before the impact. I tried to change somethings in single surface contact but unfortunately nothing helped the solve the issue.  I still could not find how to solve it. Since it is observing in a single hex body(mesh created by sweep method) how can there is a penetration

27 Şub 2024 Sal 19:10 tarihinde James Kennedy <j...@kbs2.com> şunu yazdı:

[James Kennedy]

Dear Ozgur,

 

Could you please send me a simple plot of your model with the various parts?

 

I would think you would want to use automatic_surface_to_surface contact of

the primary contact ( bottom of car to curb).

 

Sincerely,

James M. Kennedy

KBS2 Inc.

March 3, 3034