Fatigue Crack Growth Thresholds of Aerospace Alloys

Two alloys, Fine Grain (FG) RR1000 and a new as-cast TNM Titanium Aluminide (TiAl) are being tested to find the thresholds across a range of conditions. For FG RR1000 load shedding thresholds and load increasing thresholds are being conducted, to understand the impact of R ratio, loading frequency and crack growth mechanism on threshold, along with their interactions. The new as-cast TiAl alloy is undergoing preliminary testing to compare to current TiAl manufacturing processes and compositions. This will likely lead onto further testing to understand how R ratio and temperature impact on the threshold of the alloy.
New characterisation techniques, Digital Image Correlation (DIC) and Synchrotron 2D X-ray Diffraction (2D-XRD) are being used to understand more about the strain fields ahead of the crack tip along with plastic deformation. Both of these are new in the research group and so are being developed.
DIC is performed using an open source DIC software that has been benchmarked by comparing the equations and algorithms used against literature to ensure it is robust and highlight any weaknesses if any. The issue with DIC is the trade off between spatial resolution and errors so algorithms from literature have been implemented to aid in selecting a good subset size to enable time saving and adaptability between tests to ensure that the analysis is optimised to each individual case. Some of these algorithms have been modified to aid in showing areas of speckle patterns used for DIC that are lacking in quality, subsequently reducing the accuracy of the DIC analysis. DIC is a technique that can be performed in house giving more information about the strains present and displacements leading to information about crack tip opening displacements. The crack tip opening displacements can then be used to understand more about crack tip blunting should sufficient resolution be achieved through DIC.
Synchrotron 2D-XRD analysis is being developed with a unique method to separate the y and y' by initially using the super-lattice reflections of y' and then inferring the peak position onto the shared y/y' peaks to de-convolute the shared peaks. The peak positions and Full Width at Half Maximum's are used to measure elastic strains and also represent the degree of plastic deformation that has occurred in a vicinity around a crack tip. The aim with this analysis is to understand the underlying processes that cause threshold to increase with time spent cycling sub-threshold, which is clearly observed during prolonged dwells (3600 s).