Issue34

S. Henkel et alii, Frattura ed Integrità Strutturale, 34 (2015) 466-475; DOI: 10.3221/IGF-ESIS.34.52 467 I NTRODUCTION t is known from literature that the nonsingular T-stress parallel to a crack surface influences the shape and the size of the plastic zone at the crack tip [1]. The standard specimens for fracture mechanics testing have different in plane constraint conditions depending on specimen type and crack length [2]. Therefore, in static testing the shape of the static crack resistance curve depends on specimen type [3] and crack length to ligament ratio is limited by standards as ASTM E1820. There are also measurements in literature, which found an influence of T-stress on fatigue crack growth by using different specimen types [4] or cruciform samples in planar biaxial loading [5, 6]. Dalle Donne et al. [7] showed that the shape of the cruciform sample is important for a proper measurement and there is a lot of misunderstanding due to specimen effects, which are treated as material behavior because an uncoupled loading in the two loading axes is possible only for a specimen with long slits. Today, there is also the possibility to consider the coupling effect by finite element analysis. Sunder and Ilchenko tested random flight spectrum load on cruciform specimens from aluminum alloy 2024-T3 with and without phase shift in time between the loading axes and crack direction in the direction of one axis. They showed that considering the coupling between the load axes  K is a proper parameter to describe the fatigue crack growth [8]. The aim of the present study is to clarify if there is an effect of crack parallel tensile forces, which affect the T-stress on fatigue crack growth in a common aluminum alloy. Regarding the size and the shape of the plastic zone, overload effects are taken into account because the retardation after overloads depends on the plastic zone, which is formed by overload cycles [9]. M ATERIAL AND METHODS he investigated material was the aluminum alloy 6061 (EN AW-AlMgSi1Cu) in the condition T651 (solution annealed, quenched, controlled strained and artificially aged). The material had a chemical composition of 0.76% Mg, 0.56% Si, 0.23% Cu, less than 0.7% Fe. The samples were milled out of a 13 mm thick plate. The mechanical properties of the static tensile test as well as the fracture mechanics parameter for stable crack initiation J i/BL calculated from the intersection of the blunting line and the static crack resistance curve from SENB specimen with 10x20x100mm³ are given in Tab. 1. For comparison with the K based loading in the cyclic tests the elastic recalculated K Ji/BL is also given. The microstructure (given in Fig. 1a) contains precipitates which are arranged in lines parallel to the rolling direction. The samples where cut under an angle of 45° to this direction. Fig. 1b shows the cyclic crack growth curves for the material at R =  min /  max = 0.1; 0.3; 0.5 and 0.8, respectively. The measurements were done on SENB specimens (10 x 20 x 100 mm³) according to ASTM E647 [10] using an electrodynamic resonance testing system RUMUL Mikrotron with 20 kN maximum capability. The threshold area was measured with  K decreasing and the Paris region with  K increasing. Notch orientation was 45° to the rolling direction, that means between L-T and T-L orientation. Crack length measurement was done by elastic compliance method. The test frequency was between 65 Hz and 77 Hz. The scatter was reduced using 7 point polynomial method according to ASTM E647 [10]. The material shows the well-known point of inflection in the transition to the Paris region [11], which is caused mainly by switching to grain surface oriented crack growth [12]. There is a difference in crack growth rate especially at low R-values, between  K increasing and decreasing due to load history effects and crack closure. Direction R p0.2 [MPa] R m [MPa] A g [%] A 5 [%] J i/BL [kJ/m²] K Ji/BL [MPa  m] L-T 282 319 10.5 18.1 69 98 T-L 275 321 9.3 14.5 48 82 Table 1 : Mechanical properties of the static tensile test as well as the fracture mechanics parameters for stable crack inition J i/BL calculated from the intersection of the blunting line and the static crack resistance curve. The planar biaxial tests were done using a slotted cruciform specimen with thinned inner measurement area which is shown in Fig. 2a). This type of specimen which was developed in [13] shows an excellent uncoupling between stresses in both principal directions in the center due to the slits [14]. Avoiding notch effects on the end of the slits, the slits were I T