Fatigue damage at mesoscopic level. Fatigue life prediction in conjunction with acoustic emission signals
Project link to Cordis: https://cordis.europa.eu/project/id/792652
Once the industrial revolution starts, which made the transition from manual to mechanized production, a series of operating failure began to be reported. One such case was the failures of some mining chains in the mines of the Oberharz, Germany, 1829. These failures determined Wilhelm August Julius Albert to initiate an experimental program that reproduced the loading conditions of the mining chains. Thus, he published the first fatigue tests results, concluding that the chains failure was due to repeated loads. The phenomenon continued with a series of in-service failures of the railway wagon axles. Among these, the most resonant at that time was the derailment of the Paris – Versailles train in 1842, which resulted in the death of 60 people. In the years 1842 – 1843, William John Macquorn Rankine a Scottish engineer with formidable achievements in mechanical engineering was tasked with examining many broken railway axles. He publishes a paper in the Institution of Civil Engineers, UK, where he describes the failure of the axles as being due to the progressive growth of a brittle crack from a shoulder or stress concentration on the shaft. Gradually, fatigue failure became a serious concern and between the years 1850 – 1860 August Wohler, a German engineer, carried out a series of experimental investigations on the axles of railway wagons. He developed a machine for repeated loading of railway axles and showed that fatigue failure occurs by crack growth from surface defects until the load can no longer be supported. Wohler represented his studies in the form of tables. His successor, Spangenberg, director of the Mechanisch-Technische-Versuchsanstalt in Berlin, plotted the results of Wohler as curves obtaining the well-known S-N curves. These were called later as Wohler curves, . This moment represents a turning point in the analysis of fatigue of materials phenomenon and from this moment the researches begin to evolve in parallel with the technological evolution and with a long series of failures, some of them catastrophic.
Fatigue life prediction in the design stage of the structural components, as well as the identification respectively the follow-up of this process from the early stages represent preventive actions against catastrophic failures in operation. With all the advanced level of scientific research and technological development, accidents continue to occur due to different causes (aging of structures, manufacturing processes, increasing loads and optimisation, new materials, etc.). Even some do not cause human loss, they produce very large material damage. In 2018, a woman died after being partially sucked out of a plane through a window broken by a bladder detached from the engine after a fatigue failure, . The early cracking of blades in Trent 1000 aircraft engine produced by Rolls Royce caused a loss of £450m in one single year, . The examples can continue and even some incidents are not publicized. All of these supports the idea that efforts to predict and analyse fatigue damage must continue in close cooperation between the research environment and industry.
Fatigue damage occurs in three stages: crack initiation, growth from small to short and long crack and final fracture of the component. The first two stages, which cover most of the fatigue life, are dominated by the interaction with the microstructural features of the material, [4, 5]. Schijve, , divided the fatigue phenomenon into four periods, micro crack nucleation, micro crack growth, macro crack growth and final failure. Hochhalter et al., , developed a comprehensive study of micro crack nucleation within grain boundaries and found that grain orientation has a significant effect on the nucleation metrics. Hoshide and Socie, , studied the nucleation and propagation of fatigue cracks in biaxial stress loading based on a model that considers two cracking modes, shear crack growth and single crack propagation. The shear crack growth mode was evaluated based on the resolved shear stress on the slip plane, while the single crack propagation mode was treated by fracture mechanics parameters. Sangid, , described the physical mechanisms of fatigue crack initiation based on the persistent slip bands which are formed at grain level and cause an accumulation of plastic deformation. The slip always occurs on a particular set of crystallographic planes and directions when the shear stress acting in the slip direction reaches a critical value. A major contribution was made by David McDowell and now Fionn Dunne et al. in microstructure-sensitive computational modelling of cyclic plasticity and fatigue crack formation, [10-12].
In most cases, the prediction models for fatigue crack initiation based on physical damage mechanisms have been developed and applied on simplified computational models incorporating several grains with well-defined crystallographic characteristics (called Representative Volume Elements – RVE) which are then subjected to simple loads. A current problem for accurate prediction of fatigue damage of structural components is the implementation of physical damage mechanisms-based models for a real loading case characterized by a multiaxial stress/strain state. This is the primary area of investigation for this project. On the other hand, the accumulation of plastic deformation occurs with the release of strain energy, that can be captured using the acoustic emission technique.
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