Fatigue crack growth in metals has been studied intensively for more than half a century. This research field has been closely connected to the engineering industry throughout history. It is therefore no surprise that the main focus has mostly been on developing better prediction models rather than on understanding the phenomenon of fatigue crack growth. This dissertation focuses on improving our understanding of the underlying physics, and presents several novel insights primarily relating to crack closure modeling and crack growth rate modeling....@en

An important figure was omitted from this article, as Fig. 11 was repeated in Fig. 12. The image below shows the correct version of Fig. 12 that should have appeared in the article. [Figure presented]

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To improve resolution of in-situ measurement of crack closure and opening in fatigue, potential drop (PD) measurement technique has been further developed to measure thousands of times through each load cycle with high precision. The results are interpreted with physical phenomena like strain, Poisson's effect, piezo-resistivity, plasticity, and crack growth. Application of the technique to fatigue crack growth tests on aluminum 2024-T3 CCT specimens at different maximum stresses and stress ratios, demonstrate that indeed variations in PD can be associated to development of plasticity and crack opening and closure. Hence, the technique allows to measure timing and magnitude of crack opening and closure stresses in-situ in fatigue crack growth experiments.

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The substantiation of fatigue crack closure corrections is disputed, based on the closure stress definition. The ΔK_eff equation lacks a physical explanation. An inconsistency is observed between the opening stress S_opphen as used by this equation and the physical opening stress S_opphys. This S_opphys is related to S_opphen through an energy equivalent area approach. Furthermore, an elastic spring model is used as a physical approach to crack closure effects. An FEA approach generates S_opphys values, which are reworked into S_opphen. This physical model agrees well with existing closure corrections, and is able to provide a physical explanation for their necessity.

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This paper discusses the appropriateness of crack length as a reference dimension for fatigue damage. Current discussion on short crack versus long crack data is still divided between various approaches to model small crack growth. A proper physical explanation of the probable cause of the apparent differences between short crack and long crack data is not yet provided. Long crack data often comprises crack growth in constant thickness specimens, with a through crack of near constant crack front geometry. This is not true for corner cracks or elliptical surface crack geometries in the small crack regime where the crack front geometry is not symmetric or through-thickness. This affects similitude parameters that are based on the crack length. The hypothesis in this paper is that a comparison between long crack data and short crack data should be made using similar increments in crack surface area. The work applied to the specimen is dissipated in generation of fracture surface, whereas fracture length is a result. The crack surface area approach includes the two-dimensional effect of crack growth geometry in the small crack regime. A corner crack and a through crack are shown to follow the same power law relationship when using the crack area as base parameter. The crack front length is not constant, and its power law behaviour for a corner crack is shown.@en