PSI - Issue 18

Rainer Wagener et al. / Procedia Structural Integrity 18 (2019) 490–500 Rainer Wagener, Andreas Maciolek, Heinz Kaufmann/ Structural Integrity Procedia 00 (2019) 000–000

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3.3. Evaluation of the strain-life relation In order to extend the scope of application for the Fatigue Life Curve to other alloys, the high strength aluminium alloys of the 7xxx family come into the focus of interest. Therefore, EN-AW7075 T6 has been chosen. The target strain amplitudes are appointed according to the pearl-string method in order to estimate the course of the strain-life curve. Regarding the elastic strain portion, a knee-point is visible at 2 ∙ 10 3 cycles to crack initiation, which is in the expected range. The analysis of the stress-strain behaviour delivers different results. Due to the high monotonic strength, the stress-strain behaviour is up to  a,t = 0.65% macroscopic elastic. Furthermore, no work hardening or softening takes place within specimens with number of cycles to crack initiation above 5 ∙ 10 2 cycles. The established means of classifying the test results to the different regimes, based on the stress-strain behaviour, is not possible in this case. Presuming that the elastic strain – life relation could be described by a line in a double logarithmic coordinate system, the derived properties should be independent of the used test results, but it should not be forgotten that fatigue test results will always scatter, even in the case of sound material specimens. Therefore, an additional, (maybe optical) method is needed in order to decide if the determined properties describe the material behaviour or not. As mentioned above, using the compatibility conditions to derive the properties of the cyclic stress-strain curve will offer such an option. In doing so, two options are enabled to assess the quality of the determined cyclic properties. The first is to vary the test results, i.e. do not use all test results, for the determination of the cyclic properties, draw the lines and assess if all test results are matched by the line. The second step consists of deriving the properties of the stress-strain curve, drawing the charts and examining if all test results are represented by the line. For this purpose and performing the variation of the test result methodically, the test results are sorted by ascending number of cycles to crack initiation and from iteration loop to iteration loop, the last test result being skipped. The resulting stress-strain curves and strain life curves are depicted in Figure 8.

Figure 8: Elastic strain-life curves and cyclic stress-strain curves considering different sets of used test results for the derivation of the properties

With a decreasing number of considered test results, the derived stress-strain curve matched all test results in a better way than by regarding all test results for the evaluation. At the same time, the slope of the elastic strain-life curve flattens, but the resulting strain-life curve does not match the test results with the very high fatigue lives. Due to this fact, it is not possible to describe the elastic strain-life curve with only one line. On the other hand, the knee point of the elastic strain-life curve could be identified optically. The iteration can be stopped immediately, if the derived stress-strain curve represents the test results. Mathematically, the classification of the results to the first regime can be done by the empirical standard deviation of the plastic strain amplitude in the logarithmic plastic-strain – logarithmic stress amplitude regime, Figure 1. For