IN ORDER TO UNDERSTAND how various types of single-load fractures are caused, one must understand the forces acting on the metals and also the characteristics of the metals themselves. All fractures are caused by stresses, and a version of the “weakest link” theory applies: fracture will originate wherever the local stress (load per unit of cross-sectional area) first exceeds the local strength. This location will vary depending on first exceeds the local strength. This location will vary depending on the strength gradients within the metal and the stress gradients imposed on the metal by applied and residual stresses. By understanding the ways in which single-load, or monotonic, fractures are caused, one can better understand fatigue fractures, which are the result of many thousands, millions, or billions of load applications at lower load levels. When a force is applied to any member, components of other forces in other directions result, forming a stress system. To understand the forces, it is necessary to understand the stress systems acting on the part. A useful starting point is to study the stress systems acting on cylindrical members, such as rods or shafts. A variety of stresses can be applied to cylinders,  and the same principles hold for noncylindrical parts. Shafts, and shaftlike parts, are very common and are widely used in construction of many assemblies and mechanisms.