Week - 3 Drop test Challenge
Drop test analysis:
Aim: To perform a drop test analysis on a given model of a cell-phone using LS-Dyna to study and understand how to setup an analysis case and assess the results.
Analysis Setup:
Drop test:
Drop testing is one of the impact tests a manufacturer performs as a set of torture test for a mass produced item such as a cell-phone to test durability of the product. In case of a cell phone a real life scenario which the phone will face more often such as getting dropped from a reasonable height is tested out to make sure the build of the electronic components and the body of the phone itself is strong enough to withstand the shocks and impact.
For this example we consider the given cell-phone model being dropped along its shorter side from a height of 4 feet as per the Military Standard 810G determined for military grade electronics devices such as phones, laptops, tablets etc. on a solid surface such as concrete. To save the computation time, instead of dropping the model from 4 feet, we give it a velocity equivalent to the impact velocity, and perform the drop test against a rigid wall.
A [kg kN mm ms GPa] system was used to perform the analysis is the time steps will be very small in case of impact analysis.
Procedure:
1.PART, SECTION, MAT definition: The model file consists of just the meshes and parts which are needed in this analysis – the solid surface, the cell-phone model and a shell-surface which can also be used as the rigid wall. We can select either of the 2, for this case the solid surface is chosen and the other was deleted.
First and foremost, the parts need to be defined; hence, both the section and materials keywords were added and then attached to the respective parts. As both the ‘parts’ are 3D solids, the sections are identical except the name and id’s. To keep things simple the element formulations and ambient element type properties were left as default.
For the material definition however, things change for the 2. The cell-phone needs to have realistic material formulation to show impact results but the ground/concrete surface does not. For the base part defined as ‘Rigid ground’, ‘M20 – Rigid’ material was selected, and the material properties such as ‘E’-young’s modulus, ‘RO’- Density, ‘PR’- Poisson’s ratio etc. are entered and the part is defined.
To get more accurate results, ‘M24 – Piecewise linear plastic’ material card was selected for the ‘cell-phone’ part where the material properties were inserted along with a stress-strain curve as there is a chance the impact might cause plastic deformation. For this, from ‘define’ keyword, the ‘curve’ option was selected, and the ‘x’, ’y’ data that is the stress and strain values were inserted. This curve acts as the ‘effective stress-strain’ data bank for the material.
This way both the parts are defined.
2. Boundary conditions and Velocity definition:
After defining the parts, now, the analysis parameters are defined. The ground mimicking solid surface although rigid, is free to move in the space and needs to be constrained. During the material selection, it can be done by toggling the ‘CON1’ and ‘CON2’ options depicting the constraints but it is better to use ‘SPC’ option and locking all 6 DOFs for the ‘part’ from the ‘Boundary’ keyword. All the nodes from the part are selected and are constrained using the SPC option. ‘Create Entity’ tool comes handy when allotting boundary conditions to nodes. Here a ‘Node set’ is formed consisting all the nodes in the bottom part which are added the constraint to.
The same tool is used to form ‘node segments’ for both the parts which are later used to define contact. Another node set is formed containing cell-phone nodes to be given initial velocity. To give the initial velocity condition, the ‘velocity’ option from ‘initial’ keyword is used where this ‘node set’ is called and the velocity of 4.89 m/s was added in the ‘-z’ direction.
3. Contact definition: Now, to tell the analysis that the phone model and ground part are to be collided as the phone model drops down at the velocity, we need to define a contact interface between the 2 parts such that they interact with each other and some results are formed after the impact.
In this case as both are solid surfaces, we can easily choose the ‘Automatic surface to surface’ option from the ‘contact’ keyword. Using the ‘node segment ID’s’ defined with ‘create entity’ tool the ‘master’ and ‘slave’ component of the contact are defined. In this case, we are interested in the results from the cell-phone post impact hence the node segment associated with it is defined as slave group.
4. Analysis setting definition: Here the actual parameters for the duration of the analysis were defined. In case of impact analysis, the models can get real complicated such as for a vehicle crash analysis, which demands a lot of computation power and time both of which are needed to be spent efficiently. We do not need to analyse the cell-phone model before the impact and say 10 seconds post impact where no tangible results can be observed.
In our case, the phone takes 4 milliseconds to reach the ground, and hence, the time of analysis needs to be just a bit more than that. Using the ‘termination’ option from the ‘control’ keyword, we can define the ‘ENDTIM’- end time for the analysis which for a bit of a buffer, we defined as 6 ms.
5. Outputs definition: To analyse the result from this analysis the output requests were defined in terms of the data. Here the ‘Database’ keyword is used to request a plot data of the various result parameters such as stresses and strains using ‘Binary d3plot’ option. Similarly using the ‘ASCII’ option the nodal and elemental energy data, hourglass and sliding energy data etc. are requested as well. An output time-step -‘dt’ is defined in all these options to request the data after every ‘dt’ milliseconds of time over the total analysis time. Some more options were toggled to improve the accuracy of the results.
Keywords defined:
Analysis :
License option : check local license only
Date: 08/01/2020 Time: 04:36:35
Executing with LSTC local workstation license
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|_________________________________________________|
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| LS-DYNA, A Program for Nonlinear Dynamic |
| Analysis of Structures in Three Dimensions |
| Version : smp s R11.0.0 Date: 08/27/2018 |
| Revision: 129956 Time: 13:04:10 |
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| Features enabled in this version: |
| Shared Memory Parallel |
| FFTW (multi-dimensional FFTW Library) |
| Interactive Graphics |
| ANSYS Database format |
| ANSYS License (ANSYS150) |
| MADYMO indirect coupling (7.2 or higher) |
| BigID option |
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| Licensed to: |
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| Platform : WINDOWS X64 (SSE2) |
| OS Level : Windows 7/8/10 & Srv 2008/2012 R2 |
| Compiler : Intel Fortran 13.1 & MSVC++ 2010 |
| Hostname : SANDESH-JOGDAND |
| Precision : Single precision (I4R4) |
| SVN Version: 129958 |
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| Unauthorized use infringes LSTC copyrights |
|_________________________________________________|
Input file: D:\project_utvs\Skill_Lync_LS_Dyna\Drop_test\solid_solid\Drop_Test_edited1.k
The native file format : 32-bit small endian
Memory size from default : 20000000
JOBID : Cell_phone_drop_test
on UNIX computers note the following change:
ctrl-c interrupts ls-dyna and prompts for a sense switch.
type the desired sense switch: sw1., sw2., etc. to continue
the execution. ls-dyna will respond as explained in the users manual
type response
----- ------------------------------------------------------------
quit ls-dyna terminates.
stop ls-dyna terminates.
sw1. a restart file is written and ls-dyna terminates.
sw2. ls-dyna responds with time and cycle numbers.
sw3. a restart file is written and ls-dyna continues calculations.
sw4. a plot state is written and ls-dyna continues calculations.
lpri toggle implicit lin. alg. solver output on/off.
nlpr toggle implicit nonlinear solver output on/off.
iter toggle implicit output to d3iter database on/off.
prof output timing data to messag and continue.
conv force implicit nonlinear convergence for current time step.
ttrm terminate implicit time step, reduce time step, retry time step.
rtrm terminate implicit at end of current time step.
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* Copyright (C) 1990-2018 *
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Beginning of keyword reader 08/01/20 04:36:36
Memory required to process keyword : 715198
input of data is completed
initial kinetic energy = 0.11177543E+02
The LS-DYNA time step size should not exceed 6.828E-04
to avoid contact instabilities. If the step size is
bigger then scale the penalty of the offending surface.
Memory required to begin solution : 716K
Additional dynamically allocated memory: 627K
Total: 1342K
initialization completed
1 t 0.0000E+00 dt 4.96E-04 flush i/o buffers 08/01/20 04:36:37
1 t 0.0000E+00 dt 4.96E-04 write d3plot file 08/01/20 04:36:37
cpu time per zone cycle............ 0 nanoseconds
average cpu time per zone cycle.... 0 nanoseconds
average clock time per zone cycle.. 13013 nanoseconds
estimated total cpu time = 1 sec ( 0 hrs 0 mins)
estimated cpu time to complete = 0 sec ( 0 hrs 0 mins)
estimated total clock time = 414 sec ( 0 hrs 6 mins)
estimated clock time to complete = 413 sec ( 0 hrs 6 mins)
termination time = 8.000E+00
101 t 4.9586E-02 dt 4.96E-04 write d3plot file 08/01/20 04:36:39
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5000 t 2.4788E+00 dt 4.96E-04 flush i/o buffers 08/01/20 04:39:09
5042 t 2.4996E+00 dt 4.96E-04 write d3plot file 08/01/20 04:39:11
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8565 t 4.2496E+00 dt 4.96E-04 write d3plot file 08/01/20 04:41:03
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10000 t 4.9712E+00 dt 4.97E-04 flush i/o buffers 08/01/20 04:41:58
10057 t 4.9999E+00 dt 4.97E-04 write d3plot file 08/01/20 04:42:00
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14146 t 7.0496E+00 dt 5.27E-04 write d3plot file 08/01/20 04:44:01
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14646 t 7.2997E+00 dt 5.27E-04 write d3plot file 08/01/20 04:44:18
14746 t 7.3495E+00 dt 4.97E-04 write d3plot file 08/01/20 04:44:21
14846 t 7.3998E+00 dt 4.97E-04 write d3plot file 08/01/20 04:44:25
14946 t 7.4499E+00 dt 4.97E-04 write d3plot file 08/01/20 04:44:28
15000 t 7.4770E+00 dt 4.97E-04 flush i/o buffers 08/01/20 04:44:30
15046 t 7.4998E+00 dt 4.97E-04 write d3plot file 08/01/20 04:44:31
15146 t 7.5497E+00 dt 5.27E-04 write d3plot file 08/01/20 04:44:34
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15446 t 7.6997E+00 dt 4.96E-04 write d3plot file 08/01/20 04:44:43
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15747 t 7.8500E+00 dt 4.97E-04 write d3plot file 08/01/20 04:44:51
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16046 t 7.9996E+00 dt 4.96E-04 write d3plot file 08/01/20 04:45:02
*** termination time reached ***
16046 t 8.0000E+00 dt 4.96E-04 write d3dump01 file 08/01/20 04:45:02
16046 t 8.0000E+00 dt 4.96E-04 write d3plot file 08/01/20 04:45:02
N o r m a l t e r m i n a t i o n 08/01/20 04:45:02
Memory required to complete solution : 716K
Additional dynamically allocated memory: 627K
Total: 1342K
T i m i n g i n f o r m a t i o n
CPU(seconds) %CPU Clock(seconds) %Clock
----------------------------------------------------------------
Keyword Processing ... 0.0000E+00 0.00 4.6600E-01 0.09
KW Reading ......... 0.0000E+00 0.00 7.7000E-02 0.02
KW Writing ......... 0.0000E+00 0.00 1.2700E-01 0.03
Initialization ....... 1.0000E+00 0.20 5.6800E-01 0.11
Init Proc Phase 1 .. 0.0000E+00 0.00 3.8700E-01 0.08
Init Proc Phase 2 .. 1.0000E+00 0.20 4.8000E-02 0.01
Element processing ... 1.5000E+01 2.96 1.5036E+01 2.97
Solids ............. 1.5000E+01 2.96 1.4725E+01 2.91
Binary databases ..... 1.0000E+00 0.20 1.0820E+00 0.21
ASCII database ....... 4.7300E+02 93.48 4.7725E+02 94.17
Contact algorithm .... 8.0000E+00 1.58 5.5880E+00 1.10
Interf. ID 1 8.0000E+00 1.58 5.2570E+00 1.04
Rigid Bodies ......... 0.0000E+00 0.00 9.0400E-01 0.18
Time step size ....... 1.0000E+00 0.20 9.5000E-02 0.02
Group force file ..... 0.0000E+00 0.00 3.0000E-02 0.01
Others ............... 2.0000E+00 0.40 6.1600E-01 0.12
Misc. 1 .............. 2.0000E+00 0.40 2.9110E+00 0.57
Misc. 2 .............. 2.0000E+00 0.40 1.4920E+00 0.29
Misc. 3 .............. 0.0000E+00 0.00 1.9100E-01 0.04
Misc. 4 .............. 1.0000E+00 0.20 5.6100E-01 0.11
----------------------------------------------------------------
T o t a l s 5.0600E+02 100.00 5.0679E+02 100.00
Problem time = 8.0000E+00
Problem cycle = 16046
Total CPU time = 506 seconds ( 0 hours 8 minutes 26 seconds)
CPU time per zone cycle = 15975.642 nanoseconds
Clock time per zone cycle= 15999.593 nanoseconds
Number of CPU's 1
NLQ used/max 136/ 136
Start time 08/01/2020 04:36:36
End time 08/01/2020 04:45:02
Elapsed time 506 seconds for 16046 cycles using 1 SMP thread
( 0 hour 8 minutes 26 seconds)
N o r m a l t e r m i n a t i o n 08/01/20 04:45:02
Click close button or execute exit command to quit the Window
D:\project_utvs\Skill_Lync_LS_Dyna\Drop_test\solid_solid>Results:
- Von-mises stress:
A maximum von-mises stress of 250 MPa is recorded at the impact face at the instance the phone hits the ground. The stress wave then propagates as the stress is the distributed throughout the body of the phone with varying intensity along its length.
- Effective strain:
The stress results are confirmed by the strain distribution with very negligible value of maximum strain but results are at the right location. The strain figures show that the impact was not very severe and the stresses generated were well within the elastic limit of the material as zero plastic deformation is observed.
- Energy plots:
The energy plots are good indicators of right execution of the analysis. As the moving/dropping phone with Kinetic energy due to inertia, comes in contact with a rigid ground comes to a halt for a fraction of a second, the internal energy which represents the strain energy spikes up and an overlap of the two occurs. This phenomenon can be seen around the 4 ms mark as the phone impacts the ground. As there is no significant plastic deformation, the phone acts as an elastic material and bounces off the ground after the impact and as a result the energy plots return to their initial states.
A small drop in K.E. is seen post impact equivalent to the gain in I.E. suggesting the small strain and shockwave propagation. A minute amount of hourglass energy is also recorded which is negligible compared to the total energy and hence the analysis can be considered as a success.
Conclusion:
From the acquired results it can be concluded that the given model of the phone for the assumed material formulation should survive such a drop test without failure.