Fractal Characterization of Shot-Peened Surfaces

Author:  Y.Oshida,T.Seno,T.Nishihara,A.Itoh
Source:  The Shot Peener magazine, Vol 08/Issue 2, Summer 1994
Doc ID:  1994046
Year of Publication:  1994
Abstract:  
Presented at the joint Europe-USA seminar on Shot Peening 29-30 Sept. 1992, Cincinnati, Ohio, USA - 14 Oct. 1992, Grenoble, France. Abstract There are several methods to determine the coverage of shot-peening; (1) direct methods (visual methods, the Straub method, the Peenscan method), (2) indirect method (the Valentine method), or (3) simply the percent coverage is estimated from the curve of Almen arc height against the duration of shot exposure. In this study, a novel method is proposed. Shot-peened surfaces of quench-tempered steel were subject to the Fractal Dimension Analysis to determine the degree of the coverage. Using steel shots and controlling the impingement time, four levels of coverage were pre-determined; namely, 60%, 110%, 225%, and 450%. Surface roughnesses were measured on samples shot-peened with four different pre-determined coverages. The "box counting method" was employed on surface roughness profiles to determine the Fractal Dimension (DF). It was found that DF decreased linearly from the asmachined condition up to the pre-determined coverage of 225%, and once DF reached 1.22 (= log4/log3 in the Koch curve), it remains constant. Therefore, it is suggested that the optimum coverage value (i.e., about 200%) can be determined by monitoring changes in DF parameter on shot-peened surfaces.