1. For example, let us imagine the flat bottom of a kettle on a table. The contact surface between the kettle and the table is a flat plane. The weight of the kettle is dispersed across the plane of contact. The contact pressure at the surface is determined by dividing the kettle weight by the contact area. By contrast, in the case of a kettle having a spherical bottom shape, although the unstable shape is fictitious, the contact portion becomes a geometric point. This applies when a heavy ball like a shot is placed on a table. The load concentrates at a point to cause an extremely high contact pressure. In practice the contact area takes a small circle because the two objects (shot and table) deform elastically. The concentrated stress is called Hertzian stress. It is possible to calculate the stress mathematically by considering the elastic deformation at the contact portion. H. Hertz is the scientist who first calculated it. The results have been given in various contact situations such as a sphere vs. a sphere, a plane vs. a sphere, etc.
2. Automotive Handbook,2000
3. A similar procedure called modification is also carried out to make the Si particle finer in high-Si aluminum alloys (Chapter 2).