ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

Table 2 Thickness and mechanical properties of human skin.
Layer Thickness
of layer
[mm]
5
Mooney-Rivlin [MPa]
C10 C01 C11
Stratum corneum 0.1 0.084 0 0.92
Epidermis 0.2 0.3 0.6 0.2
Dermis 0.6 0.3 0.06 0.02
Hypodermis 2 0.005 0.05 0.02
Muscle 4 0.06 0.02 0.02
The plane strain element type 11 was used to construct the skin layers (T. Maeno et al., 1998). The FE model consists of
about 1300 elements and 1460 nodes. The probe was located over the skin above the centre of the model. The probe was
created by a rigid curve. This because the real probe was made from steel and is many times more rigid than skin. Boundary
conditions have been applied to the bottom and both sides of the model. The first one blocks movement in the vertical
direction and the second one blocks movement in the horizontal direction.
As in the real test, the simulation consists of three parts: indentation, move to “zero” position and cyclical movement left
and right. All three steps were carried out for two different probes (R=2.5mm and R=1mm) and two loads (F n1=0.19N and
F n2=0.5N). Table 3 shows additional information used in the FEM model.
Table 3 Data used in FEM simulations.
Probe ball diameter [mm] 2 5
Normal force Fn [N] 0.19 0.5 0.19 0.5
Stroke length [mm] 17.6
Sliding velocity [mm/s] 16
Coefficient
(dynamic)
of friction 0.9 0.7 1.1 0.8
5. RESULTS
The results obtained by FE modelling were compared with results from the experimental study (M. Kwiatkowska et al.,
2009). Figures 7(a) to 7(d) show the displacement for the reference point during the test. The results from the FE model are
quite similar to the real test results. The best fit was observed for the horizontal displacement of the reference point. The fit
was less good for the perpendicular displacement, where there is only a good fit at the extreme parts of the test. In both cases
the fit was not perfect. The 2D model does not describe the complete behaviour of the human skin and also the constant
friction coefficient has an influence on the test results. The constant average coefficient of friction was used during the whole
simulation, but in the real test the coefficient of friction varies during a cycle. Nonetheless, the mechanical properties used in
this work can be useful for both probes and loads. Worthwhile mentioning is the fact that the differences between real test
and the FE simulation are approximately the same for different test parameters. It should be noted that two effects influence
the skin deformation behaviour during the experiment: the horizontal motion of the probe, which induces lateral displacement
of the skin surface, and the probe indentation into the skin, which deforms layers in the skin. There are some issues with the
method used to monitor the reference point position in the experiments. Video cameras and LVDT sensors were used to track
the reference point. The camera was put between the outermost indenter positions. This could have influenced the results.
Moreover, the position between the camera and forearm has a big influence on the results, particularly when displacement
ranges from 0 to 3 mm.
During the simulation the “stick” and “slip” part between probe and skin is very similar to the experiment. This effect is
clearly visible on the figures from 7(a) to 7(d). As a consequence in front of the ball a “bow-wave” is formed (Figure 6).
Figure 6 Stages in the skin deformation - relaxation cycle: a) real test (4); b) FE simulation.