PSI - Issue 13

Andrzej Kurek et al. / Procedia Structural Integrity 13 (2018) 2210–2215 Kurek Andrzejet al. / Structural Integrity Procedia 00 (2018) 000 – 000

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Fig. 3. Shear strain-life curve for 6082-T6 aluminium alloy according to Langer model fitted using least squares method

Fig. 4. Shear strain-life curve for 6082-T6 aluminium alloy according to Kandil model fitted using least squares method

Fig. 5. Shear strain-life curve for 6082-T6 aluminium alloy according to Kurek- Łagoda model fitted using least squares method

After finding model constraints according to ASTM standards (E606-92) it became clear that one of the models (Langer) describes the data much better than the other two. It was due to unusual, for strain-controlled tests, scatter of experimental data. In this case few points, that couldn’t be considered as false, determined the curve course. Because of this we decided to use different fitting algorithm – bisquare waged method. This method minimizes a weighted sum of squares, where the weight given to each data point depends on how far the point is from the fitted line. Points near the line get full weight. Points farther from the line get reduced weight. Points that are farther from the line than would be expected by random chance get zero weight. For most cases, the bisquare weight method is preferred because it simultaneously seeks to find a curve that fits the bulk of the data using the usual least-squares approach, and it minimizes the effect of outliers. The experimental data and material constraints and fitting parameters are shown on figures 6-8 and in table 2.