Connecting rods (conrods) are the automotive components that connect the crankshaft to the piston, allowing the transformation of the alternative linear motion of the pistons into the rotation movement of the crankshaft. The motion of the conrods is plane and can be viewed as the superposition of a rotation around an instantaneous centre and a translation. Connecting rods are constrained with hinges both to crankshaft and to the piston pin: this is the reason why they are subjected to axial internal forces, at least if the speed of the engine is low enough to neglect the inertial forces. But the conrod, due to the transversal acceleration of its movement with respect of its longitudinal axis, presents also a bending action, when the rotation speed of the engine is high. And it could be trivial to underline that in those cases bending is the most dangerous action as regards the problem of fatigue failure.
This is the reason why car manufacturers developed conrods made with material with good stiffness (due to possible buckling problems), with minor density (to reduce inertia effect) and, for sure, with good fatigue strength. This is true especially for sports car manufacturers, where limitation of budget is not the first constraint for the engineers and the primary goal is to win and to go faster and faster. On the other side, there are cars for mass production, and for everyday cars, where cost is one of the most important factors (maybe the most important one) in the development process of a car.
In this case the traditional technological choice for conrods was to used forging and steels, able to guarantee satisfactory mechanical strength and adequate stiffness. But forging is expensive and car manufacturers passed to nodular cast iron, a cheaper material with cheaper manufacturing technologies associated…but with a lower mechanical strength, especially as regards fatigue strength.
In summary, there are different engines, different goals, different materials, and different cars but a common factor among them is the use of shot peening to improve the fatigue strength.
In fact, shot peening is used to improve the fatigue strength of cast iron connecting rods, and shot peening is used to improve the performance of titanium conrods and even of steel ones.
The way shot peening is done changes by changing the materials, of course, and the optimal use of shot peening is related to the execution of experimental tests, to the definition of design approaches able to take shot peening effects into account or to the ability to use reference data. Unfortunately car manufacturers are really jealous of their results and it is not easy to find data about tests carried out on real conrods and not on specimens. I like to recall the work done by Walter Shütz, recently passed to a better life, who spent his professional time dealing with fatigue of mechanical components and with the ability of shot peening to improve fatigue strength and who published experimental data related to real components.
As regards conrods it is important to notice that by shot peening cast iron conrods it is possible to obtain a fatigue limit greater than with unpeened forged steels, allowing for remarkable savings in costs and a reduced scatter in fatigue data.
It is interesting to note that experimental public data shows that if forged rods are considered and the Almen intensity is varied, negligible advantage in terms of fatigue limit is obtained, at least if the coverage value is equal to 300%.
A last remark about conrods of sports race cars. These components are mainly subjected to severe tensile fatigue loading and the major benefit in term of fatigue strength that can be obtained by shot peening is to do it while the conrod is mechanically in-tension loaded, that is to say that shot peening is performed with a prestress able to induce a deeper and larger compressive residual stress in the sub-surface layer. In this case the gain is more marked with fatigue cycles with a ratio R=0 than with R=-1 (that is to say that the effect is more pronounced with positive fatigue cycles with a minimum stress equal to zero than with alternate fatigue cycles).
All in all there is not much other data about fatigue strength of shot peened conrods in the public domain. Anyway, the conclusion is that shot peening conrods is useful independent of the material used, but that full advantage of using shot peening can be obtained only by focusing research and development on the case of interest, with dedicated experimental tests, since the known data are limited and only refers to the optimal treatment parameter variations case by case.

Shot Peening in the Automotive Industry
by Mario Guagliano
Contributing Editor MFN and
Associate Professor of Technical University of Milan
20156 Milan, Italy
E-mail: mario@mfn.li

Author: Mario Guagliano