Introduction
This assignment deals with the Finite Element Analysis (FEA) of the top half of a
Connecting rod. By using the ‘Ideas’ computer software I am to analyse and
determine the position and magnitude of the stresses within a section of the conrod.
Assignment Aims
1. To produce an accurate and detailed parametric model of the conrod. That is to
produce a detailed drawing that can be easily modified by altering dimensions
or constraints within the history access.
2. To produce a good Finite Element Model (FEM) of the conrod, applying
accurate forces that the rod experiences within the engine to the model. This
FEM must contain a good mesh, accurate boundary conditions and use the
correct forces. Once these factors are accurate the model must be solved and
the results obtained must be interpreted correctly to analyse the fatigue within
the conrod section.
My conrod model can be found on the CD enclosed with this assignment.
Conrod Selection
I have chosen to model a conrod form the Ford Sigma engine. The engine is also
known as the Zetec SE and has the following specification:
1.6 Litre
4 Cylinder
16 valve DOHC
Max gas force @ 5500 RPM = 73 Bar
The conrod has the following dimensions:
15mm
18mm
137mm
mm
47.1mm
24.9mm
I-
Section
3.8mm
The Sigma Conrod is produced with an I-Shaped shank. The dimensions for the shank
at its thinnest section are as follows:
12.7mm
15.2mm
2.7mm
8.7mm
Analysis
I have analysed 4 separate areas of the conrod for both compressive and tensile stress.
I have called each area A, B, C & D. In order to accurately compare the compressive
and tensile stresses in each area I analysed the same node within each section. This
ensured that the stresses shown allowed the most accurate comparison. The stresses
in each area were as follows
A Goodman diagram illustrating these stress fluctuations can be seen on the following
page.
Area A
Tensile
= + 607 N/mm2
Compressive = - 0.134 N/mm2
Area B
Tensile
= + 341 N/mm2
Compressive = - 0.00151 N/mm2
Area C
Tensile
= + 0.165 N/mm2
Compressive = - 449 N/mm2
Area D
Tensile
= + 33.7 N/mm2
Compressive = - 322 N/mm2
Analysis Of Individual Areas
As I mentioned above, I analysed four key areas of the conrod in order to establish
what the stresses were. I looked at both compression and tension stresses in the exact
same nodes of the model. This gave me the most accurate comparison between
tension and compression, and allowed me to see how the stress fluctuated within the
exact same region of the rod.
By selecting the query function within the post processing section of Ideas it allows
the stress within each element and node to be seen for both the compression and
tension scenarios.
The analysis for all four areas can be seen below:
Area A
This shows that the tensile stress
within Element 2118, node 71 is
607 N/mm2 In the compressive
situation
This shows that the compressive
stress in the same element and
node is – 0.134 N/mm2 in the
tensile situation
From the analysis of area A it can be seen that the stress within that node alternates between a
compressive stress of – 0.134 N/mm2 & a tensile stress of + 607 N/mm2. This is as expected
as the force applied was compressive and the +607 represents a tensile force in the bearing
area of the conrod. Having said this, +607 is slightly higher than I would have expected for
standard gas forces. By looking at the Goodman diagram it can be seen that the stresses
within area A are just within the safety limit and failure would be unlikely.
Area B
This shows that the tensile
in element 3525, node 5527
341 N/mm2 in the
compressive situation.
stress
is +
The compressive stress
in the same node
is –0.00151 N/mm2 in the tension
situation.
In area B the stress alternates between a tensile stress of – 0.00151 N/mm2 and a compressive
stress of + 341N/mm2 . Once again, this is as expected as I would not expect any compressive
stress in this area of the conrod under a tensile loading. The compressive stress showed is so
small it can be considered as zero compressive stress. The tensile stress is not inconsiderable,
but is well within the safety zone and means that there is little risk of fatigue problems
developing in his area. The Goodman diagram shows that the stress in area B is well within
the safety zone in the centre of the diagram confirming my analysis above. The Goodman
diagram shows that area B is one of the least likely areas to develop any fatigue problems.
Area C
This shows that element 3292 node
4198 has a compressive stress of –
449 N/mm2 in the compression
loading situation.
The same node shows a tensile stress of
0.165 N/mm2 in the tensile loading
situation.
In area C the stress alternates between a compressive stress of –449 N/mm2 and a tensile
stress of 0.165 N/mm2 . I would expect a negligible tensile stress in this area under the tensile
loading of the rod. My results confirm that this is the case and there is in fact very little
tensile stress in this area. I did not expect such a high compressive stress in this area of the
rod under compression of the rod. The – 449 represents a relatively high compressive stress
in this area the rod during compression due to the gas force. The Goodman diagram shows
that this compressive stress is still well within the safety zone and should not present a fatigue
problem within the rod.
Area D
This shows that element 3357, node 1032 has a compressive stress of –332 N/mm2 in the
compression situation.
This shows that the same node has a
tensile stress of + 33.7 N/mm2 in the
tension situation.
In area D the stress fluctuates between -332 N/mm2 compressive and + 33.2 N/mm2 tension.
The tension does not pose a problem from a fatigue point of view as it is well within the
safety limit on the Goodman diagram, the compression of the rod doesn’t pose a fatigue
problem either. The Goodman diagram shows that the stresses in this area should not present
any fatigue problems whatsoever.
Final Analysis
From my Goodman diagram I can determine that there are no stresses within the Conrod that
will pose a definite fatigue problem. The one area that does come close to the safety limit is
area A and this gives rise to concern.
This does show that if the situation was made any worse, there could be a fatigue problem in
this area. By the situation being worse I mean one or more of the following occurring:

Poor material quality with a lower UTS and endurance limit could lead to the Conrod
developing a fatigue problem with the stresses calculated.

My FEA could be inaccurate and the stresses I calculated could actually be greater
than shown here.

The actual input forces may be higher than calculated, giving rise to higher stresses
within the rod.
If one or more of these scenarios were to happen I believe that area A would be the first to
develop a fatigue problem and in turn be the first area of the top section of the rod to fail in
practice.
Final Opinion
It is my opinion that the Ford Sigma conrod has no serious fatigue problems. It seems more
feasible to me that my analysis was not perfect and that is why there seems that there may be
a problem in area A. I believe that some of the stresses I arrived at through my FEA analysis
were higher than the actual conrod experiences I practice. I can only assume that my analysis
was not as accurate as it should have been to arrive at more precise stresses. Possibly my
actual drawing of the conrod was slightly inaccurate or maybe my technique for meshing and
setting boundary conditions was not perfect. Maybe my analysis was accurate and Ford do
use rods that are close to developing fatigue problems. I have learnt that FEA can be used
effectively to analyse components with accuracy if used correctly.