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Fractures nucleated from defects and subjected to cyclic loading can experience propagation for a range of stress intensity factor ΔK well below the so‐called long crack threshold. This phenomenon is attributed to the development of crack closure mechanisms which may differ from those observed in laboratory tests conducted in accordance with current standards. Cracks originating from material defects require a specific degree of extension to develop the plastic wake, thus achieving a stabilized condition called the long crack threshold. However, in certain materials, this stabilization length can extend up to several millimeters, effectively encompassing a significant portion of the component's fatigue life. Therefore, understanding and quantifying the development of ΔKth with crack extension is important for implementing a reliable assessment procedure based on the fracture mechanics theory. Herein, R‐curve measurements are presented for five distinct structural metallic alloys widely used in various industrial applications. Additionally, the impact of load ratio is investigated, providing a comprehensive analysis of fatigue crack resistance in terms of the R‐curve concept. This study underscores the necessity of ruling new experimental techniques to measure and implement the long crack threshold, thereby ensuring the development of a reliable and robust framework for fatigue assessment. [ABSTRACT FROM AUTHOR]

Nowadays, several components for aerospace and energy applications (i.e., turbine disks, offshore pipelines, and combustor chambers) are designed following a safe-life approach in the low cycle fatigue (LCF) regime. As also in the LCF regime it is worth adopting a damage tolerance design, fatigue life is assessed considering a crack propagating from the first load cycle, starting from an initial defect present in the component’s most-stressed region or from a microstructural feature. In this frame, fatigue crack growth is described considering LCF conditions, where the driving force at the crack tip can be described with the ΔJeff parameter. The paper aims to give an overview of the potential applications and perspectives for the adoption of digital image correlation (DIC) measurements to study LCF nucleation and propagation. A visual technique based on the DIC displacement fields was developed to measure crack opening and closing levels in the presence of large strains. The technique is applied to different materials (steels, AlSi10Mg, nickel-based super-alloys). In addition, further applications of the DIC strain measurements are revised for the study of the nucleation stage and the crack tip process zone.

Combining thermography and Digital Image Correlation (DIC) measurements, this paper aims to experimentally capture the fatigue damage phenomenon in steels, with a methodology allowing to rapidly estimate the fatigue limit. Surface temperatures and displacement fields are collected during fatigue tests performed with blocks of progressively increased stress amplitude, at different stress ratios. Results show that both thermography and DIC-based parameters have bilinear trends as a function of the stress amplitude. These changes in the thermal and mechanical behaviours, highly sensitive to the selected stress ratio, are related to damage initiation due to microplasticity and give a precise and rapid estimation of the fatigue limit. [ABSTRACT FROM AUTHOR]