Issue 38

A. Eberlein et alii, Frattura ed Integrità Strutturale, 38 (2016) 351-358; DOI: 10.3221/IGF-ESIS.38.45 353 of α = 90° and varying the loading angle β by rotating the turrets in the range of 0° till 90° by 15°-steps the loading situation at the crack front of the specimen can be adjusted from pure mode II-loading to pure mode III-loading, as Fig. 2 b) shows. Are both loading angles in a range between 15° and 75° this concept enables investigating the crack growth under 3D-mixed-mode-loading conditions. Figure 2 : CTSR-specimen and corresponding loading device. (a) Specimen’s geometry and dimensions: length l , width w , initial crack length a 0 , thickness t and ligament in sectional view; (b) Adjustment of loading angles α and β on loading device. E XPERIMENTS ithin this contribution different series of crack growth experiments were performed. Therefor the CTSR- specimens were made from aluminium alloy 7075-T651. The different test series are described below in detail. Series of experiments In the first test series various in-plane mixed-mode-ratios with Δ K I ≠ 0 and Δ K II ≠ 0 respectively spatial mixed-mode- ratios with Δ K I ≠ 0 and Δ K III ≠ 0 by shifting the loading device are adjusted after a mode I-crack growth of Δ a = 3.5 mm under constant cyclic stress intensity factor Δ K I = const. After changing the loading direction the tests again start under constant cyclic loading force conditions Δ F = const., from which a cyclic comparative stress intensity factor Δ K V results, which is equivalent to the cyclic stress intensity factor Δ K I just before changing the loading direction. Thereby similar loading conditions before and after the mixed-mode-adjustment are given at the crack front. The second test series investigate possible impacts of changing loading levels on crack growth by interspersed mixed- mode-block loads in mode I-, mode II- as well as in mode III-base loads. In mode I- and in mode II-base load the cyclic stress intensity factor is Δ K I,Bl = Δ K II,Bl = 90 MPa mm . Whereas the level of mode III-base load is Δ K III,Bl = 160 MPa mm . The R -ratio of the base load is 0.1. After a crack growth of Δ a = 2.0 mm in the base loading the mixed-mode-block loads are interspersed for N block = 10,000 cycles with a block loading ratio of R V,block = 2.0 (cf. Eq. 1). Thereafter the loading direction is changed again to the base loading by shifting the loading device. Varying loading directions with constant cyclic comparative stress intensity factor ΔK V Starting from a pure mode I-loading Fig. 3 shows the impacts on fatigue crack growth after changing loading directions from mode I-loading to mode I and mode II loading combinations. The region, where Δ K V is nearly constant W