Issue 30

M. Da Fonte et alii, Frattura ed Integrità Strutturale, 30 (2014) 360-368; DOI: 10.3221/IGF-ESIS.30.43 361 need of weight reduction in the transportation industry, the use of lightweight materials is being considered and aluminium alloys are one of the most used metals for the weight reduction on mechanical components, therefore their use on axles and shafts may be considered. Nevertheless, aluminium alloys are much more sensitive to fatigue crack growth than steels whereby damage tolerance studies must be performed before any industrial application. Fatigue failure comprises three distinct phases: (i) crack initiation (including early crack nucleation and small crack growth), (ii) long crack growth and (iii) fast final failure. For long cracks, Linear Elastic Fracture Mechanics (LEFM) is a powerful tool to predict fatigue crack growth that has been proved with excellent results in aeronautic industry and has been the main tool to perform damage tolerance analysis [2]. To predict fatigue crack growth through LEFM, a Stress Intensity Factor (SIF) solution for the specific geometry is needed together with materials fatigue crack growth data. Besides, shafts are also widely used in railway, automotive, aeronautic and energy industries and in this mechanical component both a torsion and bending load are combined. Generally, fatigue failures in power shafts have origin on surface cracks that grow with semi-elliptical shape under cyclic bending, mode I (ΔK I ), combined with steady torsion, mode III (K III ) [3]. Large number of power plant systems run with a general steady torsion combined with cyclic bending stress either due to the self-weight bending during the rotation or possible misalignment between journal bearings [4]. In the case of power shafts such as those used in electric power plants, propeller shafts of screw ships, or any other rotary load-transmission devices, the lifetime spent between crack initiation and final fracture is of capital importance to improve the inspection intervals and maintenance procedures. Many researchers have studied the influence of a static mode III loading on cyclic mode I for circumferential notched shafts mainly since 80’s, such as Akhurst and Lindley [3], Hourlier and Pineau [5], Yates and Miller [6]. Tschegg [7, 8] has done an important contribution to clarify the influence of steady or cyclic mode III on fatigue crack propagation behaviour when combined with mode I loading. Fonte and Freitas [9] have studied the influence of steady torsion on fatigue crack growth of semi-elliptical cracks in shafts under rotating bending, and rotating bending combined with steady torsion. For the purpose a new testing machine was design and constructed. Despite the relevant and practical importance of mixed-mode fatigue, the applications of fracture mechanics have been mainly focused on crack growth problems in mode I. Several SIF solutions have been proposed for semi-elliptical surface cracks under pure bending in round bars [10, 11]. Fonte and Freitas [12] have also presented SIF for semi-elliptical surface cracks in round bars for both pure bending and pure torsion. Freitas et al [1] carried out an experimental work in rotary bending specimens to simulate railway axles, and it was shown that the proposed SIF are not directly applicable to the fatigue crack growth analysis because the position of fatigue crack front changes continuously during a load cycle. The maximum value of SIF is not reached at the same rotation angle for all points along the crack tip profile and it is well-known that crack initiation process in rotor shafts has generally origin on the surface and the crack grows with a semi-elliptical shape. Also in [13], both small and long fatigue crack growth data were obtained from rotating or alternate cyclic bending tests on a medium carbon steel (Ck45) are presented and results are discussed. For small crack growth, an effective strain-based intensity factor range was proposed as the parameter which correlates small fatigue crack growth data under proportional or non-proportional multiaxial loading conditions. In this paper, the effect of a steady torsion load on fatigue crack growth due to cyclic bending in bars (axles or shafts) is studied for an aluminium alloy. Results are commented and compared with previous studies on steels under the same loading conditions, allowing designers to have tools for damage tolerance analysis. E XPERIMENTAL PROCEDURE Material and specimens he material used in this study is the Al 7075 T6 in the aged condition. The chemical composition of this aluminium alloy is presented in Tab. 1 and the mechanical properties are presented in Tab. 2. In rotating bending fatigue tests, classic cylindrical hourglass shaped specimens are used for the determination of S-N curves where the hourglass shape is adopted in order to localize the stress at the crack initiation and final failure. In this study, fatigue crack growth data for long cracks from a precrack will be studied, therefore the specimen geometry adopted was straight cylindrical specimens, Fig. 1, with two different diameters, respectively 10 mm and 12 mm. All specimens were machined from round bars of 25 mm diameter. After machining, all specimens were polished and notched on the surface (chordal notch), approximately 3 mm length and 0.1 mm depth. All specimens were pre-cracked under constant amplitude cyclic bending stress until the arc crack length reaches a minimum length of 2 to 3 mm. T