Tsuyoshi Kubota

Development of Fracture Splitting Method for Case Hardened Connecting Rods

The fracture splitting (hereafter FS method is being increasingly used for manufacturing connecting rods that are made of the powder forging material high‑carbon steel etc. which are comparatively easy to fracture in order to improve the accuracy and productivity. On the other hand case hardened steel (for instance JIS SCM420 which has a high fatigue strength is used for the connecting rod in high performance motorcycles in order to reduce the motorcycles weight. In this report the application of the FS method was examined for manufacturing the connecting rod which used the JIS SCM420 carburized steel. The hardness of this material varies according to the depth from the surface becoming less hard and less brittle toward the center. Therefore a brittle fracture surface over the entire fractured surface area was not obtained by the conventional fracturing method and it was impossible to reassemble the fractured parts. Moreover there was a problem that the fracture starting point was irregular and it caused the breakaway of tiny fractions from the fracture surface. The condition to obtain the brittle fracture surface was then elucidated using a weight falling‑type splitting machine with the temperature and the virtual strain rate as parameters. Moreover as for the fracture starting point measures were taken by analyzing it using the nonlinear simulation of the explicit method. The production of the connecting rod for the Yamaha supersport model YZF‑R1 （Figure 1） using this method was started in 2003.