Defect Analysis for Additively Manufactured Materials in Fatigue from the Viewpoint of Quality Control and Statistics of Extremes

Abstract

Additive manufacturing (AM) is expected to be a promising new manufacturing process for components having complex geometry. However, the disadvantage or challenge of AM is the presence of defects and surface roughness which are inevitably produced by the manufacturing process. Without strict and reliable quality control of components regarding defects and surface roughness, we cannot positively admire the advantages of AM as the new technology.In this paper, the method of quantitative evaluation of defects with complicated shapes and configurations, is explained from the viewpoint of small 3D cracks. First of all, the ideal fatigue strength (goal) to be attained by AM is discussed. Use of Fatigue-Grades from 5 (ideal strength) to 1 (lowest grade) is proposed for mutual comparison of fatigue performance of AM materials. The factors which decrease fatigue strength and degrade tensile properties are quantitatively analyzed. It is verified that the fatigue limit of AM materials is determined by the presence of nonpropagating crack emanating from defects. The reasons are made clear for fatigue fracture from surface defects even by tension-compression fatigue test as due to the difference of population of defects and increase in stress intensity factor for surface crack compared to internal cracks. Practical guides will be presented for the fatigue design and development of high quality AM materials, based on the combination of the 3D defect analysis, the statistics of extremes on defects, and the √area parameter model. It is shown that fatigue notch effect in AM materials is influenced by probability of presence of defects at notch root.