Magnesium alloys are attractive lightweight structural materials that have the potential to be used in aerospace, automotive and medical industries. However, fatigue is a major cause of failure in structural components. It is also known that notches, which are unavoidable, have a detrimental effect on fatigue resistance. Fatigue damage is associated with the formation of crack(s). Therefore, it is necessary to understand the fatigue crack growth behavior for better design analysis. Conventional crack growth measurement techniques such as the replica method only allow for measuring a one-dimensional aspect of the crack such as the crack length. X-ray tomography is an advanced non-destructive imaging technique that is gaining a significant attraction in engineering analysis. The method depends on the reconstruction of x-ray micrographs to represent a 3D visualization of the material. Advanced segmentation techniques can be used to isolate different three-dimensional features of the internal structure of the examined sample such as cracks or pores.  

 

In this research, a V-notched specimen machined from ZK60 magnesium extrusion is tested under cyclic axial loading. The V-notched specimen was machined as a 16 mm diameter rod with a V-notch that has an opening angle of 60 deg, a root radius of 0.8 mm and a depth of 1.5 mm. Fatigue experiment was performed under completely reserved force-controlled loading with an amplitude of 12 kN and under standard laboratory conditions. The fatigue test was interrupted 14 times in order to perform ex-situ x-ray tomographic analysis using the Nikon XTH 225 ST system. The sample failed after 184,005 cycles. The volumes were reconstructed using Avizo 8.1 software. Then, they were cut into slices with a thickness of 10.72 microns. After that, segmentation analysis was performed on all slices that have crack-like features. It is important to note that crack features were only observed in the last 4,000 cycles. It is expected that earlier detection of fatigue crack could have been possible if a micro-CT system was used. Examination of the slices showed that fatigue crack grew within 30 slices. A gray-level thresholding process was performed using MATLAB on the selected slices in order to convert each pixel to either white or black color. Then, these slices were stacked using imaging software to make 2D representations of the crack surface at different cycles. As a result, the areas of the crack surfaces were measured for all scans. The result suggests an exponential relationship between the crack surface area and the number of cycles.


Fatigue Crack Analysis of Magnesium ZK60 V-Notched Specimen Using X-Ray Tomography

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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