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/mm 2 Compressive = - 0.134 N/mm 2 Area B Tensile = + 341 N/mm 2 Compressive = - 0.00151 N/mm 2 Area C Tensile = + 0.165 N/mm 2 Compressive = - 449 N/mm 2 Area D Tensile = + 33.7 N/mm 2 Compressive = - 322 N/mm 2 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/mm 2 In the compressive situation This shows that the compressive stress in the same element and node is 0.134 N/mm 2 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/mm 2 & a tensile stress of + 607 N/mm 2. 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 stress in element 3525, node 5527 is + 341 N/mm 2 in the compressive situation. 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/mm 2 and a compressive stress of + 341N/mm 2. 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/mm 2 in the compression loading situation. The same node shows a tensile stress of 0.165 N/mm 2 in the tensile loading situation. In area C the stress alternates between a compressive stress of 449 N/mm 2 and a tensile stress of 0.165 N/mm 2. 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/mm 2 in the compression situation. This shows that the same node has a tensile stress of + 33.7 N/mm 2 in the tension situation. In area D the stress fluctuates between -332 N/mm 2 compressive and + 33.2 N/mm 2 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.