UWE Bristol Engineering showcase 2015

Stress (Pa)
Stress (Pa)
Tom Leggett
BEng Mechanical Engineering
Design of a Prosthetic Ankle using Composite Materials
Design Stage 1 – Basic shape and dimensions
This design allowed flexibility in achieving the
defined requirements of the prosthetic ankle. The
main benefit is that there is no complicated pivot
mechanism required to achieve dorsiflexion and
plantar flexion. The curved shape acts as both a
spring and a pivot for energy return and imitation
of ankle mechanics.
Design Stage 2 – Angles and Radii
The basic design has been created, with some
dimensions being constant and others being the
variable. The table below indicated which
dimensions are which:
Constant Dimension
Top Section Length (70mm)
Variable Dimension
Radius of Curve (r)
Bottom Section Length (80mm) Angle of Top Section (θ 1 )
Thickness (3.75mm) Angle of Bottom Section (θ 2 )
Width (80mm)
7.00E+06
6.00E+06
5.00E+06
4.00E+06
3.00E+06
2.00E+06
1.00E+06
0.00E+00
Outside Centreline Stress - Varying r
a45b10r20 a45b10r30 a45b10r35
0 50 100 150 200 250
True Distance Along Path
Design Stage 2 – Results
35° produced less
variation between the
two peaks as well as a
lower maximum value
of 4.6 MPa. An angle of
0° for the lower section
would make it parallel
to the floor
encouraging flat foot to
occur. It was therefore
decided that an angle
of 5° for θ 2 would be
chosen for design stage
2. It was found that the
bending machine could
only bend to a
minimum diameter of
50mm Therefore 25mm
radius was chosen
Design Stage 3 – Composite Layups
Epoxy resin and E-glass were the chosen matrix
and reinforcement for layup testing. 7 layups were
tested with the aim of achieving stresses close as
possible to that of the project objective.
[0 5 /90 2 /90/90 2 /0 5 ] Layup 7 had a slightly lower
stress over the load area due to the increase of 0°
plies, but a much higher stress on the outside and
inside centreline. The peak values were 3.05 MPa
and 3.1 MPa for the outside and inside
respectively. The stresses along the edges
exhibited the same results but with a higher peak
stress of 3.45 MPa for both the inside and outside.
The outside centreline and edge strains mimicked
the previous graph shapes but with a doubled
peak value of 0.95E-03. The increased flexibility of
layup 7 is summarised by the massive increase in
deflection to 4.2mm under loading. The final
results of layup 7 provide a successful design.
4.00E+06
3.50E+06
3.00E+06
2.50E+06
2.00E+06
1.50E+06
1.00E+06
5.00E+05
0.00E+00
Stress for Varying Layups - Outside Edge
[0/0/0/45/-45/45/-45][0][-45/45/-45/45/0/0/0]
[0/90/0/45/-45/0/0][0][0/0/-45/45/0/90/0]
[0/0/0/0/0/90/90][90][90/90/0/0/0/0/0]
0 50 100 150 200 250
True Distance Along Path (mm)
Final Product Manufacture and Testing
The inside edge clearly shows signs of the fibres
buckling, especially on the outside face of the
laminate. The simulations and these results agree
that the maximum compression occurs at the
bottom of the curved section, at half way along
the entire length of the model.
Project Supervisor
Dr Ruth Jones
Project summary
An investigation into the design of a prosthetic ankle
using composite materials to mimic the
characteristics of a normal human was undertaken.
The minimum load and maximum stress experienced
by a normal ankle was found from literature. Current
designs of prosthetic feet were used to create an
initial model on Abaqus, a finite element analysis
software. The best design was chosen and then
manufactured in the University West of England
laboratory. The simulation results were then verified
by testing the completed composite ankle model in a
compression test. The model withstood the minimum
load and hence verified the design of the ankle. The
design is not the complete solution for a prosthetic
foot, but provided a basis by which the other
components of the foot can be designed around.
Project Objectives
Design a prosthetic ankle capable of supporting the
weight of an average human during a normal walking
gait cycle, whilst replicating the stress of a normal
ankle joint.
• Minimum load 3500N
• Maximum stress 3.464 MPa
Project Conclusion
The modelling on Abaqus found the best angles for θ 1
and θ 2 were 35° and 5° respectively. Both were found
to have the lowest peak stress and less variation
between the two peaks on the top and bottom of the
curved section. A smaller radius provided less stress
and strain, but due to manufacturing limitations a
radius of 25mm was selected. A layup of
[0 5 /90 2 /90/90 2 /0 5 ] was the final orientation chosen,
as it gave a maximum stress value of 3.45 MPa,
extremely close to that defined in the specification.
The final model testing was a simplistic compression
test, but it proved successful. The ankle passed the
minimum load requirement of 3500N and began to
show signs of failure at 4700N. The difference
between the simulation and the real world test
highlighted issues with the model simplification, but
despite this a successful product was made that met
the specification and aims of the investigation.