PSI - Issue 45

James Martin Hughes et al. / Procedia Structural Integrity 45 (2023) 44–51 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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piezoelectric strain sensor. These measurements are subsequently incorporated into the crack growth rate curve (see Eq. 2) to remove the dependency between the fatigue crack growth rate and stress ratio, effectively collapsing the data into a single master curve. These results exhibit good correlation with existing data sets, which generally contain only constant amplitude loading extracted over a small portion of the overall fatigue life. This paper is structured as follows: Section 2 presents the experimental methodology and describes the load sequence, instrumentation, specimen geometry, and the data processing techniques used. The main results of the study are presented in Section 3, where the fatigue crack growth rate curve and crack opening loads are discussed. The overall outcome, collapsing the growth rate curve, is then demonstrated, and the results are compared against existing literature. Finally, Section 4 concludes the paper, and future work arising from this research is suggested. 2. Methodology 2.1. Loading Sequence and Instrumentation A 100 kN MTS testing machine was used to load a compact-tension (CT) specimen manufactured from 7075 T7351 aluminium. The length of the specimen (W) was 50.8 mm, the width (B) 12.7 mm, and the initial notch length (a i ) 10.16 mm. A compression-compression pre-cracking load regime was used to nucleate a crack from the notch until the pre-crack length reached 0.75 mm (total length a 0 = 10.91 mm). Following this, continuous sequences of constant amplitude loading blocks were applied to grow the fatigue crack until failure. Each load sequence consisted of an initial 200 cycles of stress ratio R = 0.5 loading, followed by 100 cycle load blocks of descending stress ratios from 0.4 to 0 in increments of 0.1, interspersed with R = 0.5 load blocks (see Fig. 1 below). This specific sequence was originally used in White et al. (2018). The piezoelectric strain gauge was bonded along the centreline of the back side of the CT specimen. The gauge was indented from the top surface to ensure that the maximum strain range of the instrument was not exceeded, as damage can occur for strains greater than 100 με (Piezotronics Model 740B02 Manual, 2021). The specimen was also fitted with a traditional back-face strain gauge and a clip gauge attached to the crack mouth. More information regarding the specimen, loading sequence, and piezoelectric strain sensor can be found in Wallbrink et al. (2023).

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Cycle Number (hundreds) 1 234567891011

Fig. 1. Load sequence applied to the compact tension specimen, indicating the stress ratio.

2.2. Measurement of Crack Growth Rate The growth rate of the fatigue crack was measured using an optical microscope. Images were taken at a number of locations along the fracture surface which correspond to different crack lengths. Specific load blocks can be identified in the fractographic images as different shaded bands. In particular, the R = 0.5 load blocks act as marker bands, which appear darker in colour (see Fig. 3 ahead) and separate the other load blocks. The fatigue crack growth rate was calculated by measuring the progression of the crack front during each of the loading blocks. Several measurements were taken within the same stress ratio band to improve the accuracy of the measurements; the average value was taken as the growth rate and the standard deviation as the error. Macroscopic crack length measurements were estimated with the standard compliance method as documented in the ASTM standards (2009).