Fatigue Crack Growth Thresholds of Aerospace Alloys

两种合金，细晶(FG)RR1000和一种新的铸态TNM钛铝化物(TiAl)正在进行测试，以确定在一系列条件下的阈值。对FG RR1000进行了卸载门槛值和增载门槛值试验，以了解R比、加载频率和裂纹扩展机制对门槛值的影响以及它们之间的相互作用。新的铸态TiAl合金正在进行初步测试，以与当前的TiAl制造工艺和成分进行比较。这可能会导致进一步的测试，以了解R比和温度对合金阈值的影响。新的表征技术，数字图像相关(DIC)和同步加速器2D X射线衍射(2D-X射线衍射)正被用于更多地了解裂纹尖端以及塑性变形之前的应变场。这两项在研究小组中都是新的，因此正在开发中。DIC是使用开放源代码的DIC软件进行的，该软件已经通过与文献中使用的方程和算法进行比较来进行基准测试，以确保其健壮性并突出任何弱点。DIC的问题是空间分辨率和误差之间的权衡，因此已经实施了文献中的算法来帮助选择良好的子集大小，以节省时间和测试之间的适应性，以确保分析针对每个单独的情况进行优化。这些算法中的一些已经被修改，以帮助显示用于DIC的斑点图案的质量不佳的区域，从而降低DIC分析的准确性。DIC是一种可以在室内执行的技术，可以提供有关存在的应变和导致裂纹尖端张开位移的信息的更多信息。如果通过DIC获得足够的分辨率，则可以使用裂纹尖端张开位移来更多地了解裂纹尖端钝化。同步加速器2D-X射线衍射分析正在开发一种独特的方法，通过最初使用y‘的超晶格反射来分离y和y’，然后将峰位置推断到共享的y/y‘峰上，以解卷共享的峰。峰值位置和半最大值的全宽用来测量弹性应变，也表示在裂纹尖端附近发生的塑性变形程度。这项分析的目的是了解导致阈值随骑自行车时间增加的潜在过程亚阈值，这在长时间居住中明显观察到(3600 S)。

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).