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Analysis of deformation of skin due to a probe

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Akarsh

Akarsh

Mechanical
Dec 9, 2016
8
Hi,
I have problem of finding the deformation of skin using a probe. I have identified the different layers of skin and made the model accordingly along with a probe in contact with the top layer of skin. The model is basically 15x15mm model with extrudes of different thickness for each layer.
The layers are stratum corneum, viable epidermis, dermis, hypodermis and muscle respectively. Each layer has different mechanical properties and is given isotropic elasticity condition.
The force of 0.1N is given to the probe and the bottom and the sides of the skin model are given fixed support condition.
The deformation in the first four layers are acceptable but the muscle layer acts like a cube of immense stiffness in a sense that no stress, no deformation, no strain occurs in the muscle layer. I cannot seem to identify the cause of this problem and i need some HELP figuring this out!

PS: I have attached the picture of the model, the sectional view of the results and the material properties of each layer
The model was created on SOLIDWORKS 2015

The normal strain in y-axis
Normal_strain_Y-axis_j2nbhd.png


Material properties
Material_properties_ddynr4.png


Sectional view of deformation
Sectional_view_of_deformation_jryi20.png


Skin model with probe
Entire_model_with_5_layers_and_1_probe_fre85c.png


Closeup of top 3 layers of the skin
Layers_of_skin_xfadug.png
 
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why is the muscle (the lower 3 rows) so rigid ?

isn't it amazing what FEA is being used to solve these days ?

another day in paradise, or is paradise one day closer ?
 
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I would have thought a stress plot would be revealing. Assuming that your layers are in order stating from the top I suspect that your super soft hypodermis is spreading the load out across the entire face of the muscle. Also what boundary constraints do you have? a Poisson's ratio of 0.5 implies no volume change under strain, which has significant implications.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376
 
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well i got those values of young's modulus and poisson's ratio from literature.
@greglocock the boundary conditions are that all sides of the model except for the top side is fixed and a force of 0.1N is applied to the top of the probe.

I had also assumed that because muscle is much more stiffer than the hypodermis, this problem happens. But my mentor says that something is wrong in the model and that this problem should not occur. So i tried redoing the entire thing again (many times) and the problem still persists.

From the above replies i don't know whether my problem is actually a problem or just a solution.
 
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Give us a stress plot. ideally three stress plots, von mises, y and z. You have locked your muscle slab because it is of fixed volume, it can't go up, it can't go out, it is stuck. release the y coordinate.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376
 
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Hi
It may be so that from the perspective of the hypodermis the muscle is rigid. Try by making the hypodermis 100 times stiffer. Just to see what happens.

Thomas
 
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All the below pics are sectional view of the results

Von mises stress
equivalent_stress_nuu8gf.png


Normal stress (y axis)
normal_stress_y_axis_zvogur.png


Normal stress (z axis)
normal_stress_z_axis_naoaay.png
 
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The pics below are results when the y coordinate is free,i.e, the bottom face of the model is not fixed

Deformation
deformation_qmgiex.png


Von mises stress
equivalent_stress_winrgh.png


Normal stress(y axis)
normal_stress_y_axis_uonckg.png


Normal stress (z axis)
normal_stress_z_axis_dlxz2d.png
 
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i have kept the bottom fixed and let the sides free

Deformation
Deformation_pjlhhf.png


Von mises
Von_mises_ddrklf.png


Normal stress (y axis)
Normal_stress_y_axis_oiqw6r.png


Normal stress (z axis)
Normal_stress_z_axis_vqdjdm.png
 
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Your stress plots are still useless. Anyway you have failed to cosntrain the sides correctly, how can they move sideways if x is constrained?

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376
 
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the x coordinate is not constrained but the y coordinate is, when the bottom side is only fixed.
 
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So far all I can tell is that you have a rubbish model and can't produce the plots to help me, at least, figure out the issue. I do not believe your 9 Dec 16 08:22 plots of deformation could be right if the sides were not constrained in x.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376
 
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The muscle layer is the block that does not get deformed at all,i.e, the blue cuboid at the bottom of the deformation results.

Every time i post the results i state the constraints i use.

The 1st set of results shown are when the x and the y coordinates are fixed (the sides and the bottom are given fixed support condition)
2nd set of results are also when the x and y coordinates are fixed (the sides and the bottom are given fixed support condition)
3rd set is when the x coordinate is fixed (bottom of the muscle layer (muscle layer is the last layer) is NOT fixed)
4th set is when the y coordinate is fixed (bottom of the muscle layer (muscle layer is last layer) is fixed).

The issue is that the muscle layer does not show any deformation or stress or strain. Even though i gave a fixed support condition to the bottom of the muscle layer, the interpretation for results shows that a fixed support condition is given to the bottom of the hypodermis layer (the layer prior to the muscle layer)
 
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@greglock any ideas on how to improve the model? (all suggestions are welcome)
 
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Hi again
like I sauid before, I think that from the perspective of the hypodermis the muscle layer is "rigid". It is 125 times stiffer. I would make the hypodermis 100 times stiffer as a test.

If that doesn't work I would remake the entire model to reduce the the stiffness variation. Just to ensure that the probes action is reasonable. You can also try by reducing the Poissons ration to 0.4 or something. 0.49 can result in numerical issues.

Thomas
 
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it looks to me as though your model allows only one layer to change volume. The mesh for the first couple of layers isn't deflecting (ie squishing), only the layer above the bone ? that looks "odd".

I assume you're modelling this as a flat "plate", not as a cylinder ??



another day in paradise, or is paradise one day closer ?
 
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