Issue 35

J. Albinmousa, Frattura ed Integrità Strutturale, 35 (2016) 182-186; DOI: 10.3221/IGF-ESIS.35.21 183 Conversely, satisfactory predictions were obtained by evaluating the Jahed–Varvani [7] energy model on the observed planes. Albinmousa and Jahed suggested that the assumption of critical plane based on maximum driving parameter such as normal or shear strains might be idealistic. It was rather suggested that redefining the physical size of the analysis by adopting an average measure could resolve the aforementioned conflict. They proposed that if a crack grows in zigzag path, in average and for a limited size it could still be considered as a single crack with a single orientation as illustrated in Fig. 1. In this figure, the projected crack length, l p , is defined as the straight distance between the tips of the crack and the orientation of this line is defined by the angle β . The objective of this study is to investigate the applicability of the proposed idea by Albinmousa and Jahed [6]. β 1 β 2 β 3 β 4 β 5 N 1 N 2 N 3 N 4 N 5 Figure 1 : Definition of the projected crack length, l p , and its orientation, β . N i+1> N i. E XPERIMENT AND RESULTS ecause the focus of this paper is fatigue, which includes both initiation and early growth stages, tests should be performed on smooth specimens. Unlike fracture mechanics experiments in which a determined flaw size is required or introduced before the application of loading, detection and monitoring fatigue crack growth in fatigue tests is a demeaning task. Experimental analysis considered in this study was conducted by Hoffmeyer et al . [8]. Combined axial-torsional cyclic tests were performed on smooth specimens machined from and aluminum Al5083 (AlMg4.5Mn). The monotonic and cyclic properties of this alloy are as listed in Tab. 1 [8]. Summary of investigated cases in this paper is listed in Tab. 2. Monotonic Cyclic E (MPa) 68,000 K ’ (MPa) 544 ν 0.33 n ’ 0.075 S y0.2% (MPa) 169 σ ’ f (MPa) 780.3 S ut (MPa) 340 ε ’ f 1.153 % AR 20 b -0.114 Table 1 : Monotonic and cyclic properties Al5083 aluminum [8]. Incremental measurements of actual crack length, projected crack length l p and orientation β were performed for the tests listed in Tab. 2. Variations of projected angle β and crack actual crack length with cycling are plotted in Fig. 2. In addition, planes of maximum normal and maximum shear strains were also determined for each case. Three general observations can be made from Fig. 2. Fatigue crack grows exponentially with cycling. The variation of the projected angle β with cycling is relatively steady and closer to the plane of maximum normal strain than the plane of maximum shear. B