Rotating-bending fatigue damage measurements in SAE 8620 steel

Defining a variable for damage characterization is a complex problem. In macroscopic terms, there are no differences between damage and a non-damaged volume element of a material. Examples of variables used for damage characterization are the amount of micro cracks and cavities in the materials microstructure and the physical and mechanical properties such as electrical resistivity, density, elasticity, plasticity and viscoplasticity. The quantitative damage evaluation involves the macroscopic and microscopic characteristics of the material. The methods used for this purpose can be classified as direct and indirect methods. Measurements of the microdefects density by optical microscopy, measurements of porosity by changes in density and x-ray diffraction are examples of direct methods. Measurements of the changes occurring in the physical and mechanical properties of the material, caused by the damage level induced in it, are examples of indirect methods. This paper presents a study about the quantitative damage evaluation of AISI 8629 steel specimens submitted to a a high cycle fatigue in a rotating-bending fatigue machine. The results obtained from the methods used are discussed and compared. (author)