Stress and strain field distribution in ankle-foot orthosis (AFO) using FEM.
Pascu, Adrian ; Oleksik, Valentin ; Curtu, Ioan 等
1. INTRODUCTION
In last years, computer and video technologies in the form of
motion analysis are used extensively in assessing the complex
configurations of working, they suplling directly valuable information
on the joints movement, muscle activity and force production.
Specialists in orthosis describe three periods for walking phase (Figure
1), which are: heel-strike, mid-stance and push-off. These three periods
are used because of their importance for specialists in the field of
prosthetic and orthosis when evaluating the configuration worked.
Heel-strike is defined as the time of the attack with the heel
until reaching the ground with all foot-flat. Leg begins at the moment
to gain in stability.
Mid-stance is defined as the time of contact on the ground with the
foot-flat until the heel detachment on the ground (heel-off). During
this period, the force vector moves above ankles and posterior knee and
hip and would need to provide optimum stability device (Lee, 2006).
Push-off represents the final time frame from heel-off to toe-off.
During this time, the force vector is in front of the ankle and changing
from anterior to posterior at the knee. During this period, the device
gradually reduces stability, allowing for toe-off. The swing phase
contains the two periods of acceleration and deceleration. The
acceleration period is the time in which the limb is moving forward
rapidly to increase the stride length. The deceleration period is the
time in which the limb is slowed down in preparation for heel contact.
[FIGURE 1 OMITTED]
We can define the orthosis like an external devices applied to one
segment of the body to prevent or correct the disfunctionality of that
segment (limiting mobility, correction or prevention vicious positions
or deformation, axial load-reduction, etc.). There are three types of
orthoses in terms of constructive: rigid orthoses, soft orthoses and
semi-rigid orthoses.
Rigid orthoses are used primarily for walking or standing to the
patient and are usually constructed of plastic or other similar
resilient material. The device is usually made from a type of structure
which takes the form of the patient leg and comprises the entire
foot-flat.
Soft orthoses are used to restore balance, to absorb shocks and
improving pain from certain areas inflamed. A soft orthose is built from
a soft material, pliable and easily adaptable to different cases. This
type of orthose is often used for those with diabetes or those suffering
from a foot malformation
Semi-rigid orthoses are used primarily by those who practice
sports. These orthoses are made of a soft material and a part of plastic
material in the area requested the athletes. For athletes, this type of
orthoses helps the tendons to work efficiently by the time of effort
(Jason, 2008).
The aim of this paper is to determine and as much as possible and
to eliminate possible causes leading to rupture after a certain period
of use of ankle-foot orthosis (AFO), which are part of rigid orthosis. A
first assumption regarding the disposal of the cause could be the
accidental over functional in use. For this reason we wanted to check
the stresses and strains as AFO in the two times in the mid-stance
phase.
2. NUMERICAL SIMULATION
For numerical analysis of the state of stress and deformation that
occurs in AFO during the first two phase of mid-stance period, we used
the package software Ansys v 11.0 (Ansys 2009). After being created the
3D model (Chu, 1995) of orthosis studied, we assign boundary conditions
and external loading (50 N) (Figure 2, a). AFO model was meshing using
solid element type, with free mesh (Figure 2, b).
Type of material items associated with the AFO model is
Polyethylene PE-HWV, a special plastic material with follow
characteristics (EN ISO 587): tensile modulus of elasticity E = 800 GPa,
yield stress [[sigma].sub.y] = 21 MPa, Poisson's coefficient v =
0.22.
Figures 3 and 4 shown distribution of equivalent (von Mises) stress
for the first phase of mid-stance period and respectively for the second
phase of mid-stance period.
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
Analysis of figures with equivalent stress (von Mises)
distributions is observed that the maximum value for the first phase of
mid-stance period is 19.509 MPa and for the second phase of mid-stance
period is 14.323 MPa. From this analysis we can observe the maximum
values of the equivalent von Misses stress are bellow values
corresponding at elastic field.
The focus of the fatigue analysis in Ansys is to provide useful
information to the design engineer, when fatigue failure may be a
concern. Also, the fatigue results can be added before or after a stress
solution has been performed.
Fatigue, by definition, is caused by changing the load on a
component over time. Ansys can perform fatigue calculations for either
constant amplitude loading or proportional non-constant amplitude
loading. A scale factor can be applied to the base loading if desired.
The classic calculation uses the constant amplitude, proportional
loading option. Loading is of constant amplitude because only one set
from the finite element stress results along with a loading ratio are
required to calculate the alternating and mean stresses (Bathe, 2007).
Common types of constant amplitude loading are fully reversed (R = -1)
and zero-based (R = 0). In present analysis we use a fully reversed
cycle.
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
The results of the fatigue analysis are quoted to the number of
life cycle and safety factor for both phase of mid-stance period
(Figures 5 and 6). We can observe in these figures that the safety
factor is bellow one, which means the reason of the AFO failure is
fatigue.
4 SUMMARY AND CONCLUSIONS
Results of this numerical analysis using FEM, shows that the
application material PE-HWV lies in the elastic domain, at some specific
strains and stress that not even approach the limit of a material breach
and in conclusion, the only reason for the break occurrence is the
material fatigue. This was confirmed by the few fatigue tests performed
using specimens made from the same material like AFO. Results showing
that we are breaking the material after several thousand cycles of
request.
The minimum number of life cycles that result from the fatigue
analysis (1,647 cycles for the first phase of mid-stance period and
3,805 cycles for the second phase of mid-stance period) lead to the same
conclusion. There are two alternatives to solve this problem: changing
the AFO material or constructively optimizing the AFO.
As a solution against these transfer function, it can propose the
adoption of introduction of fittings in areas of strong attachment
required, namely in the area of attachment of the outside wall and
foot-flat, respectively in the intersection area of AFO side walls.
5. ACKNOWLEDGMENTS
This work was carried out within the framework of the research
grant named "Theoretical and experimental analysis of the static
and dynamic behavior of the grafts in the autologous ostheocondral
transplantation" support by the Romania Ministry of Education and
Research and National University Research Council.
6 REFERENCES
Bathe, K.J. (2007). Finite Element Procedures, Prentice Hall, ISBN 0-13-301458-4, Engelwood Cliffs.
Chu Tm., Reddy Np., Padovan J., (1995). 3-Dimensional
finite-element stress-analysis of the polypropylene, ankle-foot
orthosis--static analysis, Medical Engineering & Physics, Volume:
17, Issue: 5, Pages: 372-379.
Jason Tak-Man Cheung, Benno M. Nigg, (2008). Clinical applications
of computational simulation of foot and ankle, Sport Orthopadie
Traumatologie, Volume: 23, Issue: 4, Pages: 264-271.
Lee Y.S., Choi Y.J., Kim H.S., Lee H.S., Cho K.H., (2006). A study
on the structural stress analysis of plastic ankle foot orthosis (AFO)
under dorsiflexion and plantarflextion conditions, International Journal
of Modern Physics B, Volume: 20, Issue: 25-27, Special Issue: Part 3 Sp.
Iss. SI, pp: 4559-4564, Part: 3.
*** (2009) www.ansys.com, Accessed on: 2009-03-24
