Issue 38

J. Papuga et al., Frattura ed Integrità Strutturale, 38 (2016) 106-113; DOI: 10.3221/IGF-ESIS.38.14 107 into the validation set was a simple stating of the experimenter or of any other researcher, that the data items are usable for such a validation. Already during preparation of that data set, some bad practice or misleading data interpretation was uncovered and discussed in [1]. Anyhow, only a proper analysis [4] of the most often cited and reused data set defined by Papadopoulos et al. [2] and of other often used data items from the paper by Nishihara and Kawamoto [3] made a start point for an extensive research on evaluating the data quality. M ETHOD OF EVALUATION his paper further extends the analyses provided in [4] by updating them and enlarging the scope of the data check. Due to the multitude of papers available for validation, it can be in no way complete, and some other such papers can be found. On the other hand, it is covering now all data sources found, which were used by the cited authors in their effort to validate new or older multiaxial fatigue limit estimation methods, and which were used for such a purpose by at least one researcher not related to the team of experimenters. Some newer sets from fresh experiment campaigns were reported meantime, but the scope of a conference paper cannot give a thorough overview. Data presented here are quickly touched and only their weak points are commented. The experiments are very valuable, their realization took a lot of time, a lot of work and a lot of funds. Anyhow, only short evaluation can get here. Thus the sets are criticized when necessary, so that the other researchers were aware of potential problems if accepting them, while the detailed description or appraisal cannot be provided here. The measure of acceptability of experimental results for validation purposes has been already discussed in [4]. ASTM [5] e.g. recommends between 6 and 12 specimens to be used for an S-N curve applicable for an exploratory research. Papuga in [4] proposed a system of weights, where 8 and more finished experiments per the S-N curve result in a full weight (1.0) of the item, while the weight decreases linearly to 0.2 for the case of 4 specimens per the S-N curve, and experiments with fewer test points are simply discarded. In this paper, we do not evaluate the appropriateness to the validation in another way than by the check if the S-N curve is described with minimum 4 specimens. The question of the data dispersion, applicability of experiments tested on one load level only, of the importance of the basic material curves in fully reversed push-pull and fully reversed torsion, the comparison between the statistical representativeness of the fatigue limit derived from an S-N curve and from the staircase analyses are only a few of the questions that should be further raised, but cannot be discussed in such a short paper. The basic reference for the next evaluation is Tab. 1, where the individual papers describing validation of some multiaxial fatigue limit estimation method(s) are referred to, and the data sets used for this purpose are marked by “x“ symbol. The papers often just reuse data from a second-hand source, so the fact that some paper uses Nishihara‘s and Kawamoto’s data [3] need not mean that there are not error in interpreting it or in the transcription. The table also does not describe, if the authors referring to a particular data set used in a complete form or if they selected only its parts. The individual test sets described in Tab. 1 are evaluated in the next section including also the test set marks extensively used in FatLim and Finliv databases [6-7]. Due to the limited space available for Tab. 1, these marks could not be preserved there as well, but they can be deduced from the context. D ATA SETS Nishihara and Kawamoto (NKc, NKd, NKh, NKm - [3]) he fact that the data items of this data set on fatigue limits obtained for hard steel, mild steel, cast iron and duralumin are the 1st, 5th, 8th and 9th as regards the frequency of their use shows its importance in validating of the multiaxial fatigue limit estimation methods. The careful analysis of the set in [4] anyhow uncovers that only two S-N curves obtained for the hard steel material can be seen as applicable, if accepting the low coefficient of determination R 2 < 0.75 obtained during the regression analysis of data by the Basquin formula. For the other materials, either the material curves (mild steel, cast iron) or the multiaxial experiments (duralumin) dispose of too few specimens, in some cases limited even to only one finished and one unfinished experiment. Though the set is so broadly used for validation practice, the analysis of the original paper shows that applicability of so poorly documented S-N curves is inadequate. T T