PSI - Issue 28

Jan Seyda et al. / Procedia Structural Integrity 28 (2020) 1458–1466 Author name / Structural Integrity Procedia 00 (2019) 000–000

1459

2

Keywords: multiaxial fatigue, small cracks, replication, elastic-plastic strain;

1. Introduction The initiation and growth of small cracks is usually an important portion of fatigue life as described by Shamsaei and Fatemi (2014). Often, the overall direction of macrocrack can be different than the direction of small crack, from which it originated. McClaflin and Fatemi (2004) showed that in SAE 9254 AL FG quenched and tempered steel subjected to torsion some of the main cracks formed on the maximum principal stress plane. However, initially cracks grew in the maximum shear stress direction. Consequently, the fatigue life prediction based on principal stress or strain values gave poor results. Foletti et al. (2018) showed that the application of the fatigue damage parameter, which reflect the cracking behavior, resulted in an accurate fatigue life prediction for high strength quenched and tempered 30NiCrMoV12 steel. Hence, the observation of small cracks behavior is very useful to understand the fatigue damage mechanism and to associate the damage mechanism with multiaxial fatigue models, as presented by Kurath and Socie (1988). The present research is a continuation of previous work aimed to study the fatigue behavior of materials subjected to multiaxial loadings, especially asynchronous cases conducted by Pejkowski and Skibicki (2019) and observation of small cracks on surfaces of fatigued specimens conducted by Pejkowski et al. (2019). The first stage of research concerns identification of fatigue damage mechanism in PA38-T6 (AW 6060-T6) aluminum alloy. The development of small cracks was registered by replication using cellulose acetate think foils, which is a convenient and known technique used by Foletti et al. (2018), Main et al. (2019) and Molari et al. (2020). Three cases of basic loadings were applied in elastic-plastic strain range – fully reversed axial, torsional and 90° out-of-phase loading. 2. Materials and methods Thin-walled tubular specimens, manufactured from PA38-T6 aluminum alloy, were used. The chemical composition of this material is given in Table 1. Basic mechanical properties, Young modulus E , 0.2% offset yield stress ��� , ultimate tensile strength � and corresponding strain � � , elastic Poisson ratio � , cyclic strength coefficient K’ and cyclic strain hardening exponent n’ are given in Table 2. The dimensions of the specimens are presented in Fig. 1. The specimens’ surfaces were carefully mirror-polished using different grades of diamond paste in order to remove all machining marks and scratches. Specimens were subjected to fully reversed axial, torsional and 90° out-of-phase loading with sine waveforms. All tests were performed on Instron 8874 servohydraulic axial/torsional testing system. Axial and shear strains were measured and controlled using an Epsilon 3550 biaxial extensometer. The loading parameters, programs and signals recording were conducted using Instron WaveMatrix software.

Table 1. Chemical composition of PA38-T6 aluminum alloy according to EN 573-3 standard

Element

Si

Fe

Cu 0.1

Mn 0.1

Mg

Cr

Zn

Ti

Other

Al

Share in %

0.3-0.6

0.1-0.3

0.35-0.6

0.05

0.15

0.1

0.15

Balance

Table 2. Basic mechanical properties of PA38-T6 aluminum alloy determined experimentally , GPa ��� , MPa � , MPa � � , mm/mm � , – , MPa , – 68.3 191.5 229.1 0.094 0.35 288.1 0.051