QF marker research

icaf2023 Tracking Number 90

icaf2023 poster

Experimental crack growth rates using actual load histories increase confidence level of a structural lifetime in two ways: damage rates can be used as a direct estimation method in defining the remaining useful lives, and in selection of the proper calculation parameters. Post-test quantitative fractography (QF) together with embedded marking sequences in the fatigue loading provides a way to the experimental determination of crack growth curves. The periodically applied markers leave a trace along the fracture surface of the progressing crack. The identification of the traced markers depends on the load and condition history, and can be done with e.g. an optical microscope or a scanning electron microscope (SEM). The main research questions are: a) how to define and add marker blocks to a given real usage spectrum of a fighter aircraft, b) the former in terms of effects on both small aluminium specimen and full-scale fatigue tests, c) the former two without affecting to the structural lifetime of the test article, and finally d) the applicability of the QF in analysing small crack growth. The future research question is how to use machine learning to automate the markers’ identification process. Functionality of marker loads especially in relatively short and slow fatigue crack growth (FCG) area is of primary interest. If every loading spectrum contains natural or inserted markers, distinguishability might become a problem during the early stages of the FCG. The marker loading can also appear differently on the surface as the crack growth progresses. The work started with a literature review, followed by a pre-planning of the marker loads. The pre-chosen marker blocks were calculated with crack growth analysis to see the effects resulted on the structural life. Specified fatigue tests were arranged to obtain experimental results. The research is ongoing, thus only preliminary, and intermediate results can be presented. Taken the intermediate objectives into account, the differences between the QF results were examined: the hypothesis to use an optical microscope alone, cost-effectively, and appropriately in tracing the crack growth in small crack sizes was tested by comparing findings to SEM usage of this and previous projects.