Multiscale fatigue crack initiation and propagation in engineering materials: Structural integrity and microstructural worthiness. Fatigue crack growth behaviour of small and large bodies (Q931084)

Earlier fatigue crack growth models assumed functional dependence on the maximum stress and on the size of the pre-existing crack or defect. Various possibilities were examined in the hope that the data could be grouped such that linear interpolation would apply. The idea of associating fatigue test data with design was made when the data led to the straight line relationship with the range of mode I stress intensity factor on a log-log plot. The \(y\)-intercept and slope of this line were found by curve fitting and considered to be material specific even though they also depend on the specimen size. Wide acceptance of this approach was soon adopted due to the simplicity of integrating the two-parameter relation for determining the critical crack length for different specimen configuration. Moreover, the stage of slow and stable crack growth was able to be delineated from the onset of the unstable and rapid crack propagation. Even at this date efforts are being made to better understood the physical meaning of the crack growth rate data outside the range of the two-parameter straight line zone referred to as region II. Region I and region III are associated, respectively, with fatigue crack initiation and fast crack propagation, not to mention the two transitional zones where region II connects with region I and III. Despite the exhaustive undertakings to examine microphotographs, the phenomenon of multiscale fatigue crack growth remains elusive. One of the main objectives of this book is to examine size-scale dependent microstructure evolution of fatigue damage affiliated with small specimens ranging from micrometer to nanometer. These details enhance the understanding of small scale fatigue damage mechanisms. The scale of investigation is also expanded by several orders of magnitude to include slip mismatch due to differences in grain size, orientation, micro-texture, etc. which arise because of the irregular propagation of short cracks. The fatigue damage map method was used to quantify the results which are compared with multiscale test data. In addition the effect of strain rate, temperature and hold time on the high-temperature low cycle fatigue of austenitic and ferritic steels and Nimonic PE16 superalloy are also examined to study their microstructural changes and crack initiation and propagation behavior. The book attempts to model the crack growth rate under simple and complex loads differing from the conventional approaches invoking crack closure or constrained effects. Micro- to macro-ranges of crack sizes in a variety of metals are used to correlate data on a straight line referred to a linear scale covering regions I, II and III with two empirical parameters. A large number of problems are considered. They include the 1969 Lockheed F-111 wing fatigue test; the F/A-18 final test program; the round robin helicopter test program; cracking in composite patched panels; USAF C-141 repair; and NASA centre notch panel tests. An analytical dual scale model is presented to describe the transitory behavior of micro-/macro-cracking for the fatigue of 7075-T6 and 2024-T3 aluminium pre-cracked panels. Accounted for are small cracks that can close in compression and open in tension as in the case of alternating fatigue loading. Interactive effects of load, geometry and material are considered by using three independent micro/macro parameters to render the fatigue growth rate data from the log-log plot. The straight line correlation enables the connection of data referred to micro- and macro-cracking in fatigue. The same approach was applied to analyze the fatigue life of cables and steel wires of the Rungyang cable-stayed bridge based on the designed data with and without traffic. The wide variations of predicted life for the 52 cables can be problematic for the future maintenance of the bridge as some cables may need early replacement while others can still be used. Improved fatigue damage tolerance behavior of aerospace high purity 2024 aluminium alloy is discussed in connection with load sequence and spectrum effects. Of equal concern for aerospace application is thermo-mechanical fatigue in the presence of oxidation and creep strain accumulation. Virtual testing methodology is explored for fatigue crack growth associated with the safe-line assessment of aerospace structural components. Fatigue data of butt welded joints of aluminium alloys for 6061-T6 and 6082-T6 coupon specimens are found and needed to quantify the influence of three different welding techniques. They are the metal inert gas welding, laser beam welding and friction stir welding. It should be said that the consideration of length alone is not enough for distinguishing micro- from macro-defects since the change of defect morphology may take place during scale transition. Creature of macro-fracture free surface is caused predominantly by mechanical load giving rise to symmetric or skew-symmetric geometric pattern whereas asymmetry typifies the creation of micro-fracture that are obstructed by the highly irregular microstructure. Microcracks tend to wonder by curving and branching because of the inherent different constraint of the adjoining crack surface. Micro-photos of fatigue striations suggest that the tip of a microcrack always remains open on account of geometric asymmetry. This additional tip distance suggests a micro-stress singularity that can differ from the macro-stress singularity. It is shown that a micro-/macro-crack can possess a double singularity: one weak and one strong. The dual singularity gave allowance to scale transition where a microcrack can turn to a macrocrack or vice versa. This is precisely what occurs in tension/compression alternating fatigue loading. Micro- and macro-cracking correspond, respectively, to crack opening (tension) and closing (compressing). Reference can be made to macromechanical and microstructural to accentuate the influence of scale with cause. Nanochemical effect would refer to imperfection size resulting from chemical reaction. The cause of fatigue can be traced to the loss of electrons from metal atoms in chemical reaction. Such a process in the presence of repeated mechanical loading even for small amplitude can greatly reduce the life of metals. The destruction can involve oxidation that increases the susceptibility of the local region to nanocrack initiation. The grain boundary is a likely location where the atoms are more loosely packed. A simple approach would be to use a mean characteristic length to describe a nano-defekt with the corresponding energy density weighted by a scale factor related to inhomogeneity of the atoms or atomic lattices. The same can be done at the micro and macro scale level. When needed, scale divisions can be further refined by introducing meso-regions to fill the gaps from one scale range to another. Cause-and-effect studies of material damage in fatigue require a sufficiently long stretch of scaling such that nano-data can be bought up to the macroscopic scale. Use can be made of the overwhelming amount of micrograph data presented in this book. They demonstrate the different fatigue crack growth details due to strain rate, temperature, loading type, material, processing technique and other factors. In this way the practitioner can benefit.