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Fatigue damage at mesoscopic level. Fatigue life prediction in conjunction with acoustic emission signals

Periodic Reporting for period 1 - FADAMES (Fatigue damage at mesoscopic level. Fatigue life prediction in conjunction with acoustic emission signals)

Reporting period: 2018-07-01 to 2020-06-30

Fatigue failure is a major damage mechanism of structural components and is a global issue with economic and societal impact. 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, [1]. The early cracking of blades in Trent 1000 aircraft engine produced by Rolls Royce caused a loss of £450m in one single year, [2]. 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 microstructural features of the material. Reference studies describe 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. 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 mechanism-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. In this context, the objectives of the action are summarized as follows: 1) Development of a novel concept for the evaluation and prediction of fatigue damage. Fundamentally the concept consists of correlating the real stress/strain state with the physical mechanisms of material degradation at mesoscopic level; 2) Link results of the concept with detected AE signatures during experimental validation testing; 3) Develop the researcher’s knowledge and ability so he is recognized as an established independent researcher whilst benefiting the Cardiff University knowledge of the AE techniques to industry and academia.

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The project is based on a methodology that includes numerical analysis and experimental studies. Within the numerical analysis, submodeling technique was used to evaluate the stress and strain state at mesoscopic level. Submodeling techniques are introduced in the stress analysis due to the limitation of the full model to capture the correct stress concentrations at critical locations. The submodeling technique is based on the St. Venant’s principle. Numerical and experimental studies were performed during the project on high strength steel, Weldox 960, and stainless steel, respectively. The results showed a great capability of the AE technique to detect microstructural changes from the early stages of the material damage process. During tensile tests on high strength steel, the analysis of the acoustic signal revealed in the macroelastic area (before the yield limit) damage mechanisms such as dislocation multiplication at the grain boundaries. Also, the tests of fatigue crack growth in stainless steel were able to identify the dominant fracture mode by distributing AE events in different clusters depending on the similarity of their features. The results of this study will be presented at the Fatigue 2021 conference in Downing College, Cambridge, UK, 29-31 March 2021. A microstructural analysis followed by EBSD (Electron Back Scatter Diffraction) was performed on Weldox 960 steel determining the microstructural characteristics. Thus, Weldox 960 is fully martensitic steel with a crystallographic BCC structure (Body-Centred-Cubic) and an average grain size of 10 μm. In a numerical analysis on non-proportional multiaxial loadings, the dependence of fatigue damage on the evolution of the strain state inside a loading cycle was highlighted. This study was presented at the 12th International Conference on Multiaxial Fatigue and Fracture, Bordeaux, France, June 24-26, 2019.
The project presents a real progress of the research in the field from two perspectives. First, the multidisciplinary approach to fatigue damage of materials. This implies knowledge of the acoustic emission technique and especially the processing and analysis of acoustic signals and respectively the theories and techniques for analyzing the fatigue of materials phenomenon. This approach allows a qualitative assessment of the fatigue damage mechanisms. It has been shown that the acoustic emission technique can identify fatigue damage mechanisms in all phases of this process. Thus, from the formation of slip bands at the level of the grains to different fracture modes during the fatigue crack propagation, all these can be highlighted by analyzing the corresponding acoustic signals. Second, the introduction of the submodeling technique in numerical analyses of fatigue damage prediction. This favors the transmission of the macrostructural loadings of the components on mesoscopic models that include microstructural characteristics of the material. In this way, the fatigue damage is analyzed in the earlier phase, and qualitative predictive analyses are obtained due to the proximity to the real situation.
Overall, the project contributes both to the understanding and characterization of fatigue damage mechanisms, and to the failure’s prevention through predictive analysis. All these have a great economic impact by designing reliable safety components and also socially through the significant contribution in ensuring and maintaining the safety and comfort of the population.