Damage Occurrence in Welded Structures of the Bucket-Wheel Boom

Causes of damage occurrence in vital components of welded structures of the bucket-wheel excavator boom (DU1) at the coal landfill of the thermal power plant 'Nikola Tesla A' in Obrenovac (Serbia) are investigated. Bucket-wheel excavator was produced by French company 'Ameco' and it moves along the circular track. Taking into account lack of technical documentation, all tests and were carried out under the assumption that welded structures were made of structural steels S355 and S235. Investigation of causes of damage occurrence are based on results of non-destructive tests (NDT) and tensometric measurements.


Introduction
There are 2 bucket-wheel excavators at the coal landfill of the thermal power plant 'Nikola Tesla A' in Obrenovac (Serbia), designated by DU1 and DU2, and produced in France. These bucket-wheel excavators move along the circular track (widely known as polar track). Taking into account the long period of operation under severe working conditions (dynamic loading with varying amplitudes), as well as the fact that during their design there were practically no possibilities to carry out the detailed stress -strain analysis, the most loaded elements and their connections have to be checked continuously [1][2][3][4][5][6][7][8][9][10][11][12]. This especially refers to welded joints and welded structures [3,4,7,8].
Bucket-wheel excavator with designation DU1 is presented in figure 1. Considering the fact that there was no technical documentation, all tests and researches presented in this paper were carried out under the assumption that welded structures were made of structural steels S355 and S235 [13].

Damages at Vital Sections of the Welded Lattice Structure of the Bucket-Wheel Boom
Damage was detected mainly through visual testing (VT) of parent material and welded joints. No defects were detected with other non-destructive testing methods, as well as no deviations from expected results during hardness testing. In figure 2 the reinforcements in the damaged support structure of the cylinder, which role is to enable the moving of the bucket-wheel boom, are shown, while in figure  3 the reinforcements that were embedded in the damaged section of one of the vital girders of the structure are shown. Sections of the vital structure in the upper zone of the bucket-wheel boom with damage in the area of welded joints are shown in figure 4. a) View from the right side to the bucket-wheel b) View from the left side to the bucket-wheel

Stress state of Vital Structures of Bucket-Wheel Boom Based on Measured Local Strains
Stresses were determined on the basis of tensometric measurements of local strains in the areas of parent material and welded joints at the lattice structure of the bucket-wheel boom of the excavator with designation DU1. Measurements were executed through the use of electro-resistant extensometers -measurement gauges TML PL-10 and TML PL-20. Measurement equipment for detection and processing of signals from measurement gauges to readable strain values is shown in figure 5.    Table 3. Stresses at the vital structure of bucket-wheel boom (cross-section 11, Fig. 10

Analysis of Causes of Damage in Vital Components of the Bucket-Wheel Boom
Based on the analysis of results of non-destructive tests executed at vital structures of the bucketwheel boom, it can be concluded that initial cracks within welded joints can propagate until they reach the critical length, which confirms the assumption that damages at vital structures occurred due to inadequate welding technologies during the manufacture of the bucket-wheel excavator and/or during previous repairs performed on parent material and welded joints, figures 2-4. Significant presence of defects in the area of welded joints, figure 4, is caused by complex geometry, figure 11. In accordance with recommendations [14], it was adopted that critical value of fatigue safety is σDwj = 45 MPa. Tensile strength of the weld metal is determined by the following expressions [15]:  [13].
It is obvious that in both representative combinations of static stresses (figure 12), measured stresses in critical areas of vital components of the bucket-wheel (tables 1-3) lie beneath the limit line which connects fatigue strength σDwj and tensile strength σUTSmin of weld metal, which proves that damages at vital components of welded lattice structure of the bucket-wheel boom occurred due to the application of inadequate welding technologies during the manufacture of the bucket-wheel excavator and/or during the previous repairs.
One should notice that stresses, calculated here on the basis measured strains, are in good agreement with the numerical results, obtaing by the FEM, as shown in [16].

Conclusion
Results of tensometric measurements showed that welded structure of the bucket-wheel boom was designed in complete agreement with its function and operational loads with low service stresses which could not produce damage shown in this investigation. Therefore, it is clear that damage at vital components of welded lattice structure of the bucket-wheel boom occurred due to the application of inadequate welding processes and/or procedures during the manufacture and/or during the previous repairs.