Issue 41

M.V.C Sá et alii, Frattura ed Integrità Strutturale, 41 (2017) 90-97; DOI: 10.3221/IGF-ESIS.41.13 91 I NTRODUCTION luminium alloys have been used mainly in the aeronautical industry for more than eighty years due to its low density and good mechanical resistance. The Al 7050-T7451 alloy was developed in the 1970´s and has been used not only in the fuselage but also to build structural parts of the wings and landing gears of aircrafts. Despite its technical and economical relevance for such industry, there are few works available in the literature on fatigue of this alloy. Further, most of the research carried out has been concentrated on the effects of shot peening or surface treatments on the fatigue resistance of the material under simple push-pull loadings. For instance, Carvalho and Voorwald [1] investigated the effect of shotpeening and hard Chromium coating on the fatigue resistance of the Al 7050-T7451. They found out that the deleterious effect of such a coating on the fatigue behavior of the alloy was caused by the presence of high tractive residual stresses and micro-cracks, both generated by the electrodeposition process. They also found that this deleterious effect could be mitigated by the use of shot peening. In 2011, Gao [2] tested Al 7050-T7451 specimens subjected to laser peening and conventional shot peening. The obtained results showed that laser peening was more effective to enhance the fatigue resistance than shot peening provoking an increase of 42% in strength when compared to specimens, which were not surface peened. This improvement was of 35% due to shot peening. Under multiaxial fatigue conditions, as far as the authors are aware, only one article by Chen et al. [3] reported experimental data on Al 7050-T7451. These authors carried out seven different types of tests under axial-torsional variable strain amplitude. They sought to evaluate the accuracy in estimating life of four multiaxial fatigue parameters that did not include any weight factors in their formulation. They concluded that the use of such models provided good fatigue life estimates for this alloy. However, tests were in low cycle fatigue regime and no stress gradient was present. For components operating in the medium or high cycle fatigue regime (MHCF or HCF) the presence of geometric discontinuities is usual. There is a number of multiaxial fatigue criteria available in the literature to estimate fatigue life under more complex stress states [4-7]. The introduction of stress gradient effects caused by notches (or contact mechanics, etc) into such models is still a matter of strong debate among fatigue scientists. An interesting approach to do so is to consider the existence of a critical distance [8-10]. More recently, it has been suggested [11] that a combination of a critical plane based multiaxial model with a simple linear relation between critical distance and life extracted from push- pull data could provide good estimates of life for notched components under multiaxial loading in the MHCF or HCF regime. The purpose of this work is to further investigate this problem. In order to do so, the critical distance against life relation will be raised not only for fully reversed push-pull tests but also for alternated torsion. Both curves will be combined with a critical plane multiaxial fatigue criterion to estimate life. The aim here is to assess the role of combined shear and normal stress gradients in the life methodology. To validate the analysis multiaxial fatigue tests were conducted on notched Al 7050-T7451 specimens. M ULTIAXIAL MODEL AND LIFE DEPENDENT CRITICAL DISTANCE his work will require que use of a multiaxial fatigue criterion and a method to incorporate the stress gradient effects in the life estimation approach. The multiaxial model considered here is the Modified Wöhler Curve Method (MWCM) [6]. It is based on a diagram where  a is plotted against the number of cycles to failure, N f . This diagram contains different fatigue curves characterized by different values of  ratio.  is the ratio between the shear stress amplitude,  a , and the maximum normal stress,  n,max , acting on the material plane experiencing the maximum shear stress amplitude (i.e., the so-called critical plane). Each modified Wöhler curve is defined by its negative inverse slope,  , and by a reference shear stress amplitude,  A,Ref , extrapolated at an appropriate number of cycles to failure, N A . For a given material, the corresponding modified Wöhler diagram can directly be built provided that the  vs.  and  A,Ref vs.  relationships are calibrated by running at least two sets of basic experiments such as fully reversed push-pull (  =1) and torsion (  =0) as a function of life. After determining the appropriate material constants (see [6,11] for more details) any intermediate modified Wöhler curve can be obtained. From the specific modified Wöhler curve for the  ratio that characterizes the local stress history being assessed, the number of cycles to failure can then be estimated as [11]:                   , A Ref f A a N N (1) A T