When designing a connecting rod, one needs to pay attention to the buckling strength of the rod. The buckling strength is heavily affected by the beam section, and Johnson's buckling equation is used to estimate the buckling strength of a given beam section. This approach is acceptable if the beam section geometry is constant from the small end to the big end. But, recent expectations for light weight, low NVH, and low fuel consumption engines require optimizing the connecting rod section geometries to be progressively changing from the small end to the big end.
Finite Element Analysis (FEA) is often used to evaluate the buckling strength of a rod that has complex changes in beam section. There are two primary FEA methods to do this. One is an eigenvalue method and the other is an explicit dynamic method.
The eigenvalue method can obtain stable results without satisfying Courant-Friedrichs-Lewy condition that is required to control the size of the time step in the explicit dynamic method. The eigenvalue method is meant for analyzing the static (or quasi-static) problem, and is comparable to a static load test that can be done in a structural test lab. To get proper analysis results, this method requires geometry, modal analysis, interpretation of results to include a certain number of mode shapes in the buckling analysis, as well as an imperfection value.
In the case of the explicit dynamic method, the time step size must satisfy Courant-Friedrichs-Lewy condition in order to obtain stable calculation results. This method can analyze quasi-static and dynamic problems, and is useful for calculating the in-situ buckling strength of a rod. An advantage of the explicit dynamic method is that simultaneous equations do not have to be solved, so memory size requirements are reduced and less computation time is used than in the eigenvalue method. Furthermore, the explicit dynamic method requires only the geometry and an input force (or velocity) to get proper analysis results.
This paper shows analysis results from the eigenvalue method and the explicit dynamic method, as well as physical test results. Then, the paper discusses the pros and cons of the two methods for rod buckling analysis.