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PROJECT #2
MECH 4175/5175 Finite Element Analysis
Fall Semester, 2020
DUE: December 2, 2020
Introduction:
Congratulations! You finally finished your FEA class and graduated. Based on your mad FEA skills, you
landed a job working for an orthopedic device company. Your first project is to analyze the company’s
newest design for a total hip replacement. You need to determine the contact pressure that will occur in
the artificial joint during walking, stair climbing, and getting into a car. You also need to analyze the von
Mises stress throughout the implant to ensure that the implant will not fail during these activities. You
may assume static loading conditions while applying the peak force magnitudes expected during these
activities. After completing the analysis, you must submit a report to the company summarizing and
interpreting your findings, including a recommendation of whether the implant will be safe as designed.
Total Hip Replacement:
In total hip replacement, the hip joint is replaced with an artificial prosthesis. The hip joint is a ball-and-
socket joint. When the hip is replaced, the end of the femur (thigh bone) is removed and replaced with a
stem and round head, and the acetabulum (the hip socket) is replaced with a cup. The most common
design uses a metal replacement for the femur (thigh bone) side of the joint, and a polymer cup encased
in a metal backing for the acetabular side of the joint. The stem and metal backing can be cemented to
the adjacent bone tissue, or they may have a porous coating that allows bone to grow into the pores and
tightly grip the implant components. An x-ray image of a hip replacement is shown in Figure 1 below along
with an exploded view of the implant you will be analyzing.
Stem
Plastic Socket
Metal Backing
Figure 1. X-ray image of a total hip replacement (left) and an exploded view of the prosthesis
to be analyzed (right).
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Solidworks Files:
A Solidworks assembly file called “Stem and Cup Assembly.SLDASM” is
available for download on Canvas. Solidworks parts called “Stem.SLDPRT,”
“Cup.SLDPRT,” and “Cup Outer.SLDPRT” are also available for download. To
open the assembly, make sure that all four files are in the same directory on
your computer. When you open the assembly, the parts will be a
anged in
an exploded view like the one shown in Figure 1. This view will be useful for
setting up the contact set between the ball and plastic socket, and it will also
e useful for viewing the contact pressure on separate sides of the joint. You
can collapse the assembly into its true configuration by clicking the
Configurations button, expanding the Default [Stem and Cup Assembly]
menu, right clicking on ExplView1, and selecting “Collapse” (Figure 2). You can
explode the view again as needed by clicking “Explode” in the same manner.
Implant Materials:
The stem/head component and the metal backing for the acetabular component are to be made of a
titanium alloy (Commercially Pure CP-Ti UNS R50400 (SS)). The acetabular cup will be made of PA Type 6
plastic. Both materials are available in the Solidworks material li
ary.
Hip Joint Load Magnitudes:
The forces applied to hip replacements in patients during a range of different activities have been
measured using instrumented implants (implants with built-in force sensors that can be monitored
wirelessly). The resulting force measurements are available for free at the Orthoload website
(http:
www.orthoload.com/). Use this database to find approximate force magnitudes applied to the hip
during walking, stair climbing, and getting into a car. You will see in the database that many of the forces
are given in terms of BW, which means “body weights.” For the purposes of this project, assume a body
mass of 80 kg, and determine the load magnitudes accordingly.
Hip Joint Force Directions and Other Boundary Conditions:
The ball of the implant is free to rotate inside the plastic socket, so an appropriate contact condition must
e applied. The plastic socket snaps securely into the metal backing, so those two parts can be meshed
together (treated as “bonded” in Solidworks). How you apply the forces and other boundary conditions is
up to you as a modeler. Experiment with different loading directions, and think about what types of
oundary conditions make sense for this analysis. In your project report, clearly explain and justify your
choices. You must explain how you chose your force magnitudes, and how and where they were applied
(for example, distributed over a surface, applied as a point force or forces, or through prescribed
displacements). You must do the same for other boundary conditions, including prescribed displacements.
Convergence Study:
Using a single loading configuration of your choice, use a curvature based mesh with least 5 different
mesh sizes to perform a convergence study. Note that the maximum element size slider under “Mesh
Parameters” in the meshing tool can be adjusted to provide element sizes outside the range provided by
the standard slider that goes from coarse to fine. Use the average von Mises stress on a particular edge
or face of the model for your convergence metric. The goal is to refine your mesh to the point where
further refinement results in less than a 5% change in your convergence metric. Your report will include a
plot of average von Mises stress at that location versus the number of elements in the model, so be sure
to record the number of elements in each mesh.
Figure 2. Menu used to
collapse and explode the
view of the assembly.
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Project Report: Turn in a typed report containing the following:
1. Title page (1 point)
Include your name and a
ief, descriptive title for the project.
2. Introduction and objectives (4 points):
Write a paragraph introducing the project and stating the objectives you will achieve.
3. Methods (15 points)
List the Young’s modulus and Poisson’s ratio for the materials used in the model.
Explain your choice for each force magnitude used in the study. Explain how and where
they were applied on the model.
Describe how other boundary conditions were chosen and applied. Justify your choices.
Explain how contact between the metal head and plastic cup were defined.
Describe your models using figures with appropriate captions.
Describe how you organized your convergence study. What loading configuration was
used? What metric did you use? What mesh sizes were used?
4. Results (15 points)
What contact pressure resulted for each activity?
List the maximum von Mises stress and its location for each activity. Did the von Mises
stress exceed the yield stress?
How did your convergence metric change as your mesh was refined? Provide a plot of
your convergence metric vs. number of elements. List the percent difference in your
metric between each adjacent pair of mesh sizes.
5. Discussion and Conclusions (10 points)
Describe and interpret your results.
Did your answer converge with mesh refinement?
What is your recommendation to the design team? Is the implant safe as designed, or
are there any design revisions you would recommend?