PSI - Issue 13

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

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the correlation between displacement and strain is linear. This way we obtain a constant strain amplitude on specimen. The specimen geometry is shown on Fig. 2. Additionally, during the test, torsional moment is monitored. At the time, when this moment drops significantly (by 15%), the initiation of fatigue crack occurs but further tests allow us to obtain total fatigue life of the tested specimen.

(a)

(b)

Fig. 1. Strain controlled stand for bending loading (a) and torsion (b)

Fig. 2. ‘Diabolo’ type specimen geometry and cross-section after test

The 6082-T6 aluminium alloy subjected to torsional loads experimental data form this new machine (Fig. 1) was then used to calculate fatigue curves according to three different models: Kandil (4), Langer (3), and Kurek- Łagoda (5). Those models were used because the most common MCB (1) model for strain-life curve could not be implemented here. On figures 3-5 the data is described by those three models where fitting was done according to least squares method according to ASTM (E606-92) and Bronstein (2004). The method relies on fitting the curve to the experimental points by searching the minimum of the sum of distances between the curve described by (4-5) and the experimental points in direction of the independent variable. All material constraints obtained for those three models are gathered in table 1.

Table 1. Material constrains and fitting parameters obtained using least squares fitting method 6082-T6 – least squares a b c d R 2 Kandil -0.575 0.7101 0.05978 - 0.892 Langer -1.609 0.2088 -1.81 - 0.9529 Kurek - Łagoda -0.7078 0.6386 0.05608 -0.000797 0.8895