Short and long crack data for fatigue of 2024‐T3 Al sheets: binariness of scale segmentation in space and time

Pedagogically speaking, crack initiation–growth–termination (IGT) belongs to the process of fracture, the modelling of which entails multiscaling in space and time. This applies to loadings that are increased monotonically or repeated cyclically. Short and long crack data are required to describe IGT for scale ranges from nano to macro, segmented by the SI system of measurement. Unless the data at the nano scale can be connected with the macro, IGT remains disintegrated. The diversity of non-homogeneity of the physical properties at the different scale ranges results in non-equilibrium. These effects dubbed as non-equilibrium and non-homogeneous are hidden in the test specimens and must be realized. They can be locked into the reference state of measurement at the mi-ma scale range by application of the transitional functions and transferred to the nano-micro and macro-large scale ranges. The aim of this work is to convert the ordinary crack length data to those referred to as short cracks that are not directly measurable. All test data are material, loading and geometry (MLG) specific. The results obtained for the 2024-T3 aluminium sheets hold only for the MLG tested. The differences are more pronounced for the short cracks. These effects can be revealed by comparing the incremental crack driving force (CDF) for the ma-mi range the ma-large range and the na-mi range The CDF is equivalent to the incremental volume energy density factor (VEDF). The incremental mi-ma CDF is found to be 10–105 kg mm−1 for cracks 3–55 mm long travelling at an average velocity of 10−5 mm s−1. The crack velocity rises to 10−3 mm s−1 when the incremental CDF is increased to 105–106 kg mm−1, while the crack lengths are 49–260 mm. The crack velocity for the na-mi range of 0.040–0.043 mm slowed down to 10−8 mm s−1, and the incremental CDF reduces further to 10−8–10−2 kg mm−1. Note that changed several orders of magnitude while the crack advanced from 0.040 to 0.044 mm. Such behaviour is indicative of the highly unstable nature of nanocracks. All results are based on using the transitionalized crack length (TCL). The TCL fatigue crack growth increment Δa is postulated to depend on the incremental CDF ΔS or ΔVEDF. The form invariance of , and is invoked by scale segmentation to reveal the multiscale nature of IGT that is inherent to fatigue crack growth. While the choice of directionality from micro to macro is not the same as that from macro to micro, this difference will not be addressed in this work.