阿尔托大学的Abinab Niraula等人在国际杂志Welding in the World上发表文章Local weld geometry-based characterization of fatigue strength in laser-MAG hybrid welded joints.
Laser & Electron Beam Processing
01
Dissertation Introduction
Since the 90s of the last century, numerical simulation has played an important role in exploring the influence of weld downcut parameters on fatigue properties. Although these simulations provide a great deal of valuable information, their experimental validation has been a challenge due to the limitations of weld geometry measurement methods. In the 21st century, the development of high-resolution, high-precision optical measurement systems has made it possible to accurately measure the geometry of welds, including random changes along the length of the weld. In recent years, these techniques have been widely used to study the influence of weld geometric parameters on fatigue properties, providing new perspectives and tools for welding engineering design and quality control.
Laser & Electron Beam Processing
02
Full text overview
In this study, high-resolution 3D scanning and fatigue fracture testing technology were used to explore the influence of local weld notch parameters on fatigue crack initiation location in laser composite butt welds. The gap parameters of different local geometries are analyzed in detail to understand their applicability in characterizing fatigue strength, and the current challenges in measuring these parameters are highlighted. It is found that there are significant fluctuations in the shape of the weld notch, which leads to a large change in the notch parameters over a short distance. It is important to note that in some cases, fatigue cracks have developed from gaps with large radius, which contradicts the expected results based on previous studies. The results of this study further show that the depth of weld depression can be used as an effective indicator of the fatigue strength of high-quality laser-MAG composite welds.
Laser & Electron Beam Processing
03
Graphic analysis
Figure 1 shows the macroscopic cross-sections of the two series (A and B) laser-MAG composite welds in this study.
Figure 1. Two fatigue specimen geometries, as well as a cross-section of two welds.
For in-depth analysis, a 2D profile of the weld with a distinct notch was selected for study. Subsequently, with the help of the MatLab semi-automated program, the undercut depth (d), the gap radius (r) and the lateral angle (θ) of the weld were accurately measured for these 2D cross-sections, as shown in Figure 2.
Figure 2. (a) 3D model of the weld scanned by LCI (b) 3D model of the notch defect (c) Notch profile of the weld by the destructive method (d) Notch profile of the weld extracted from the LCI scan.
The notch radius and side angle distribution of the two welds are similar, and there is no significant difference in the average value, as shown in Figure 3. However, there is a significant difference between the two welds in terms of undercut depth. Specifically, the average notch depth of A-series welds is almost three times that of B-series welds, a significant difference that reflects the difference in fatigue performance between the two.
Figure 3. The sink depth (d), notch radius (r), flank angle (θ) and d/r distribution of the two welds.
Fracture analysis of individual specimens shows that the primary fatigue crack starts at or near the deepest notch, although sharper notch radii are present elsewhere. In addition, the notch depth contour plot shown in Figure 4 also shows that the initiation of the secondary crack also occurs in the adjacent area of the deep local weld notch.
Figure 4. The fracture section of the specimen shows the fatigue crack initiation site.
The results show that the deepest weld notch in the specimen is always the most critical initiation point of fatigue cracks, rather than the sharpest radius or the largest side angle. Of particular note is the fact that in the two anomalous fatigue crack initiation cases (A-02 and B-02) shown in Figure 5, the cracks are generated at relatively shallow notch depths.
Figure 5. Fracture diagram of abnormal crack initiation and notch depth profile.
In order to further explore the relationship between the weld gap parameters and their occurrence in the fatigue critical notch, the researchers performed a statistical correlation analysis, as shown in Figure 6. In this analysis, the researchers defined the profile of the weld notch at the selected location, which was based on a combination of local weld notch parameters measured from each cross-section. Through the study of the measurement results, the distribution of the three parameters of notch depth, notch radius and side angle in the combination was further explored to reveal the potential correlation between them.
Figure 6. The distribution of weld notch parameters in their respective combinations.
Figure 7 shows the depth of depression, radius of depression, and side angles measured for the transition area between all welds and substrates in a test sample. It was observed that although different transition regions had similar distribution of local weld parameters, when we combined data from multiple transition regions, changes in the mean and range of individual transition regions significantly affected the distribution of measured parameters. This effect is particularly pronounced in the depth of the weld depression and the angle of the sides.
Figure 7. The specimen measures the depth of the depression, the radius of the notch, and the angle of the sides.
04
summary
Initial fatigue cracks typically occur at the deepest notch, rather than at the sharpest radius. At the same time, the average value can effectively reflect the fatigue performance, so the researchers found that this characterization method is suitable for the characterization of welds based on fatigue strength. In addition, since deeper notches tend to have sharper radii, blunt notches that have less of an impact on fatigue strength can be excluded from characterization using the notch depth criterion. This method not only significantly reduces the amount of data that needs to be analyzed, but also greatly reduces the analysis time for fatigue strength characterization based on local geometric features, opening up new possibilities for quality control of high-resolution weld notches in industrial applications.
Laser & Electron Beam Processing
Paper Links:
https://doi.org/10.1007/s40194-023-01488-5
Source: High Energy Beam Processing Technology
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