PSI - Issue 53
Rainer Wagener et al. / Procedia Structural Integrity 53 (2024) 161–171 Author name / Structural Integrity Procedia 00 (2019) 000–000
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It is widely believed that the surface has a major impact on the fatigue life. In case of WAAM produced components in the as-built state the surface roughness is quite high and cannot be neglected. Due to the challenge of defining the right cross-section and in order to get a first impression of the influence of the roughness, strain-controlled Incremental Step Tests are carried out. Compared to the results of polished specimens the fatigue life for the as-built specimens is lower and seems to be anisotropic, Fig. 5.
Due to the high peak-to-valley altitude the conventional roughness parameters like R t , R max , R z , R a , R p , R mr , etc. reach their limits. It should be noted that these parameters are developed to evaluate the surface quality of technical surfaces and not their influence on the fatigue, even though numerous studies have been done on this subject. Average values or maximal altitudes of a profile are nice to know, but they do not cover the essential information for a fatigue approach. In case of fatigue the crack will initiate at the weakest link, which means the highest stressed point. Furthermore, it should be kept in mind that the stress concentration depends on the dimensions of a notch geometry and its orientation to the load direction. Consequently, it is not satisfying to know the point with the maximum altitude. It is rather crucial to know also the distance needed to complete the height difference. That means the length from peak to valley. Applying this to the surface profile, which can be taken as a sequence of geometrical notches, requires to identify the highest altitude change Δ h within the shortest measuring distance, which means peak-to-valley length Δ l, Fig. 6. In a first approximation, the largest quotient K R of Δ h to Δ l has to be determined in order to obtain a representative value for comparing the local stress due to the surface profile. However, the surface profile does not consist of one notch but of a sequence of many peaks and valleys. Accordingly, it is not sufficient to determine only one K R value, but as many as possible. The counting of the different KR values can be visualized by the application of one-parameter counting method. This way enables a good impression about the surface finish. Even though only the largest K R value is of interest for the fatigue, other information such as any singularities due to manufacturing defects can be obtained from the shape of the cumulative frequency. Fig. 5: Influence of the as-built surface on the fatigue life under variable amplitude loading
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