Each material behaves in a similar manner when placed under a load. There is a period of elastic deformation, in which the material is stretched, but it returns to its original size when unloaded. The point at which it fails to return to the original specifications is called the yield stress. After the yield stress, another stress point can be reached called the ultimate stress point. At this point, a material has essentially reached the point of no return.
Failure is imminent, and even a decreased amount of stress can cause fracture.
For the type of steel that a connecting rod would likely be created with, the ultimate tensile strength would be about 80 to 180 thousand pounds per in 2 . If aluminum were used, the ultimate tensile strength would be closer to 70 thousand pounds per in 2 . So, you can see that our connecting rod, under a stress of 880 thousand psi, would be in serious trouble. Failure would almost definitely occur, even if incredibly high strength steel were used. It is in calculations like these that automotive engineers are able to predict exactly what materials and specifications can be used in a high-performance engine.