Issue 35

S. Barter et alii, Frattura ed Integrità Strutturale, 35 (2016) 132-141; DOI: 10.3221/IGF-ESIS.35.16 133 Understanding this behaviour provides a researcher with the valuable ability to use loading changes to steer the crack front, and has been used here to delineate the crack growth under a desired loading spectrum [1-3]. These path changes are investigated further here for small cracks, and are used in the measurement of crack growth rates at low- to-very low stress intensity ranges (∆ K ), and to investigate the influence of load history on local retardation (closure) in such cracks. B ACKGROUND he purpose of the work being reported here is to make improvements to current fatigue crack growth models so that better predictions of fatigue life can be made for critical airframe components. Predicting the life of aircraft structural metallic components loaded with VA spectra remains difficult since current methods do not achieve accurate results for crack growth in the small 1 to intermediate size range, which often governs the total fatigue life of a component [4-6]. The influence of small crack growth rates on the total fatigue life can be significant for a typical combat aircraft with a highly stressed structure, where the critical crack sizes are often less than 10mm [7] in depth, at least two thirds of the total fatigue life is consumed when the fatigue cracks are small. Further, the accurate prediction of such small cracks is of interest to aircraft maintainers and operators since it may be used to set inspection intervals, or schedules for repairs and modifications in managing the airframe throughout the service life.  Constant amplitude (CA) crack growth data is the common input for VA fatigue crack growth predictions. For small cracks the data is usually drawn from methods that rely on measuring the incremental changes in the surface length of a crack, rather than its depth [1, 7, 8]. This is thought to introduce considerable scatter [9] into the results as the surface extension of small cracks occurs in a plane stress state - compared to the internal crack tip, which is influenced by plane strain. An additional complication to the prediction of VA crack growth is the influence that spectrum effects can have, such as those produced by large loads. These loads may produce retardation (and/or acceleration) of crack growth, and this is often described in literature as a product of crack tip closure ; although this and other effects are generally grouped together under the same term [10]. To address these effects, adjustments are typically made to the predictive algorithms to match measured experimental observations i.e. those discussed in [11]. Other influences, such as the environment, may also be present but this is usually addressed, in part, by using data collected in an environment with similar conditions to that of service. Regardless of these adjustments, predictions of natural cracks that have grown from small initial discontinuities remains poor for many of the metallic materials that may be found in aircraft structures. Further, actual growth when cracks are in the small to intermediate range (from tens of microns to several millimetres [12]) is often under predicted, whereas it may be over predicted for intermediate sizes up to failure. Here, some appropriate data and observed crack growth features for small cracks in a selection of specimens, along with suitable adjustments to allow the predictive models to address closure effects amongst other important issues, are briefly discussed. VA loading sequences are known to produce progression marks on fatigue crack surfaces [1, 13, 14] , at growth rates well below those where striations 2 can be seen. These marks, for small cracks, are usually associated with either a local change in the crack path that leads to a visible feature on the fracture surface or a change in the fracture surface texture . Such a local crack path change may take the form of a relatively prominent ridge, and a corresponding depression on the matching face, where a large change in load amplitude or peak load is/are followed by cycles that are significantly different in range or peak, i.e. with low or negative R [15, 16]. Such features, often have considerable variety, form the markings that are visible on the fracture surface of cracks grown with complex VA loading spectra. However, when such a VA loading sequence is repeated during a fatigue test, these markings may be particularly evident as repeating patterns even at very low crack growth rates. An example of a small crack in aluminium alloy (AA)7050-T7451, the material of interest here, growing away from multiple initiation sites on a surface coated with ion vapour deposited aluminium is shown in Fig. 1, where the repeat of the complex VA spectrum (wing root bending moment in this case) is evident. In this figure, the crack growth bands produced by the repeat of the VA spectrum blocks (containing ≈6000cycles) are visible as progression bands down to approximately 1µm in width (at higher magnifications [3] smaller progression bands can be measured). This suggests an average crack growth rate of approximately 10 -9 m/cycle if all the loads cause growth, which for the case of the example shown has been found to be likely due to the growth 1 The definition of ‘small’ in this context may be found in [13]. 2 Striations, the result of a single load cycle, can be found down to a width of ~2 x10 -8 m/cycle for aluminium alloys [3]. Progression bands are the result of a series of load cycles (that cannot be individually identified); these multi load cycle sequences can also be seen down to about the same growth increment [3]. T