Structural fatigue cracks are an accumulative mode of failure which takes place in materials under cyclic stresses, starting from microscopic defects and developing into macroscopic cracks. The excavator boom, an important component in heavy earth-moving equipment, which is a steel box structure used to support large loads, is very prone to it. An excavator boom is the main working member that is used as an arm between the machine and its tools. As time passes, structural parts of these machines are exposed to extensive fatigue damage because of the complex cyclic loads that they face during everyday excavations (Zhao et al., 2021).
The first thing to do to manage fatigue is to detect it using periodic field inspections. The booms of excavators are constantly under harsh and dynamic operating conditions. Therefore, the only way to determine their condition is by performing non-destructive testing (NDT) which will help in examining the structure without damaging it (Arsić, Arsić, et al., 2021). Although visual inspection can easily find any macroscopic cracks, NDT such as ultrasonic testing plays a major role in detecting internal defects. Inspection can help engineers discover damage early, especially in welded joints which have geometric shape changes.
Once possible areas for cracking have been identified, analysis of the residual strength is best done using computer simulations. Finite Element Analysis (FEA) is used to assess fatigue damage and estimate the safe service life left in the boom. Using the actual load spectra and loading conditions, maintenance personnel can use the FEA techniques to precisely determine the stress distribution through the boom’s complete truss system (Andraș et al., 2021). Through the study of time response to the frequencies of the operation, the exact places where the cyclic load exceeds the yield strength of the material can be found.
The repair of the damaged excavator boom requires a high level of precision in terms of metallurgy. Welding is the most widely used industrial technique to repair cracks and install heavy reinforcement plates on the zones that are highly stressed. Though the process of welding changes the mechanical characteristics of the metal and causes some residual thermal stresses that have an influence on the further fatigue life of the material, the engineers should apply appropriate fatigue models including the modified S-N curve (Arsić, Arsić, et al., 2021; Zhao et al., 2021).
Once the physical work is completed, monitoring the conditions constantly becomes a must for the longevity of equipment. In order to avoid unexpected fatigue fractures of critical welded constructions, it is common practice to conduct tensometric measurements. Installing special strain gauges and measuring the strains in the working process allows the monitoring of real operational stresses on the renovated boom (Arsić, Flajs, et al., 2022). This way, in case of dynamic stresses exceeding the safe level, it is possible to make changes right away in order to avoid new fractures.
The effective management of fatigue fractures of old excavator booms is connected with a combination of timely detecting them, numerical calculations of their parameters, metallurgical renovation and monitoring. Combining regular non-destructive evaluation with computer fatigue analysis makes it possible to foresee potential structural problems. Considering changes in materials due to welding and monitoring the strains in operational process ensures safe operation of renovated equipment. Thus, the use of such engineering method will ensure the longevity of construction machinery and security of its operators.
References
Andraș, A., Radu, S. M., Brînaș, I., et al. (2021). Prediction of Material Failure Time for a Bucket Wheel Excavator Boom Using Computer Simulation. Materials, 14(24), 7897. https://doi.org/10.3390/ma14247897
Arsić, M., Arsić, D., Flajs, Ž., Grbović, A., & Todić, A. (2021). Application of Non-Destructive Testing for Condition Analysis, Repair of Damages and Integrity Assessment of Vital Steel Structures. Russian Journal of Nondestructive Testing, 57, 918–931. https://doi.org/10.1134/s1061830921100053
Arsić, M., Flajs, Z., Sedmak, A., Veg, E., & Sedmak, S. (2022). Damage Occurrence in Welded Structures of the Bucket-Wheel Boom. Engineering Innovations, 2, 41–48. https://doi.org/10.4028/p-15iqnw
Zhao, G., Xiao, J., & Zhou, Q. (2021). Fatigue Models Based on Real Load Spectra and Corrected S-N Curve for Estimating the Residual Service Life of the Remanufactured Excavator Beam. Metals, 11(2), 365. https://doi.org/10.3390/met11020365
