PSI - Issue 59

Ivan Pidgurskyi et al. / Procedia Structural Integrity 59 (2024) 314–321 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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changes are peak one-time overloads, cyclical overloads or underloads of a certain duration, software load, etc. Numerous studies (Salvati et al. (2018), Yasnii et al. (2013)) testify to the significant transient effects that make it difficult to estimate the service life of structural elements with cracks. This issue is especially relevant when surface cracks are present in structural elements (Yasnii et al. (2014), Yasniy et al. (2017)). The simplest case of a single tensile overload is studied in the most detail for through-thickness cracks and gives an idea of the probable trends and mechanisms inherent in the more complex transient processes during fatigue of materials. It was established that intense tensile overloading of an element with a crack significantly slows down its growth if the level of further cyclic loading is lower than the overload level by 1.3 times or more (for aluminum alloys) and by 1.4 times or more (for steels) (Yasnii et al. (2013)). It is also evident that as the overload level increases, the number of delay cycles N D increases, and that for many aluminum alloys at an overload factor (or ) = 2.3...3.0, crack growth stops completely, while for structural steels, this threshold rises to 3.2...4.0 (Panasiuk (1990)). This phenomenon is of great practical importance for the development of methods for assessing survivability under irregular loading, in particular when justifying pressing modes during testing or assembly in order to increase the reliability of machines elements and structures elements. The number of cycles of crack slow growth N D for through-thickness cracks depends on the parameters of the overload cycle K OL and the main (base) load level K BL ; initial mechanical characteristics of the material; its structural state; material thickness; properties of strengthening and weakening of metal during plastic deformation; degree of compression of deformations; environment, etc. The influence of the above factors on the delay N D of fatigue crack growth after overloading is ambiguous (Panasiuk (1990), Anderson (2017), Sadananda et al. (1999), Panasiuk et al. (1991), Skorupa (1998)). The uncertainty of the influence of various factors on the rate of fatigue crack growth after overloading calls for the need for additional materials research in order to predict the survivability of structural elements in the presence of overload cycles. Single or multiple tensile overloads cause a slowdown in the development of through-thickness cracks as well as surface cracks (Panasiuk et al. (1991), Pokrovskii et al. (1996)), but consideration of this process is complicated by additional factors that are characteristic of surface cracks. One of these factors is the presence of changing stiffness of the stress-strain state along the crack front, when the growth of surface and near-surface points under cyclic loading is taken place under conditions of a plane stress state, and for points located in the deepest part of the contour is taken place under conditions of plane deformation. Under such conditions, the delay in the surface crack growth after the application of a tensile overload is smaller than for the surface and near-surface contour points (Pokrovskii et al. (1996), Fleck et al. (1983)). This also leads to some change in the shape of the crack. It is worth noting that in these experimental studies, the crack contour, which is characterized by the shape parameter a/c (ratio of the semi-axes), was energetically stable (a/c = 0.75 – 0.95) (Fleck et al. (1983)). For an energetically stable contour the stress intensity factor K 1 have practically the same values (Coules (2016), Brennan et al. (2008)). Another factor is the variability of the shape of the surface crack contour, which is characterized by the shape aspect ratio a/c. Such cracks appear during coalescence of smaller cracks and change shape during further cyclic loading (Pang et al. (2017), Pіdgurskyi et al. (2022)) . For such cracks with 0.1 < a/c < 0.6, a characteristic feature is different values of stress intensity factors (SIF) along their contour. It is obvious that the change in the range of the SIF along with the changing stiffness of the SSS along the thickness of the sample, will affect the delay in the growth of the indicated points of the crack front. In this regard, the aim of the work is to study the delay in the surface cracks growth with different aspect ratio at different levels of tensile overloads and different range of SIF ( ΔK BL ) and different stress ratio R . 2. Methodology The growth of surface semi-elliptical cracks (Fig. 1) under cyclic tensile loading was studied according to the methodology recommended by the ASTM Standard (ASTM E740 (2016)). Testing of large- sized (600×80×20 mm) semi -natural samples (Fig. 1) with a surface crack under regular cyclic loading (or under overloading and subsequent cyclic loading) was carried out on a hydropulsator ZDM-100 at a frequency of 4 Hz and stress ratio R = 0.25. Hydropulsator ZDM -100 allows you to create a maximum cyclic load