Full-size testing to determine stress concentration factors of dragline tubular joints

This paper describes the determination of stress concentration factors (SCFs) of dragline tubular joints through laboratory testing of four full-size dragline tubular joints. Strip gauges are installed both outside and inside the footprint of the joints to measure stress distribution at the weld toe and weld root. Measurements for SCF are carried out for two load cases, i.e. tension or compression forces in the main chord alone and tension or compression forces in the vertical bracings alone with reaction forces in the inclined braces. The relationship of the SCF between the corresponding weld toes and weld roots are presented in the paper. The weld profile and weld root gaps that have been measured using the silicon imprint technique and feeler gauges, provide valuable information for future FE simulation and weld root failure study using the effective notch stress approach

Structural health monitoring of a dragline cluster using the hot spot stress method

'Hot spot stress' is an approach often used to consider fatigue loadings in heavily welded tubular joints. This article reports the determination of hot spot stresses in mining dragline booms, which are often ≥100 m in length, using strain gage measurements and finite element analysis (FEA) modeling as part of a structural health monitoring concept. Strain gages were installed on a typical A11 cluster for estimating hot spot stresses, as recommended in the existing fatigue design guidelines by the International Institute of Welding (IIW) and the International Committee for the Development and Study of Tubular Construction (CIDECT). The results from the experimental measurements and the FEA were found to be comparable to a large measure. It was concluded that while hot spot stresses were high enough at the weld toes to cause cracking, they could not explain the cracking that occurs at the welds in the main chords on their own. Issues in comparing theoretical and experimental measurements are discussed

Cumulative damage analysis for fatigue life in dragline clusters

Stresses close to the weld toes in a dragline cluster were measured for a dragline under service conditions. Hot spot stresses were determined at the weld toes in the cluster using the linear extrapolation method as recommended by existing fatigue design guidelines. A rainflow counting analysis was carried out on the resultant hot spot stress-time histories for strain gauges located in the main chord and the lacing members. Existing fatigue design curves are used to estimate the fatigue life in the main chord and the lacing members using cumulative damage analysis. A comparison of fatigue life estimates from the cumulative damage analysis and average lives observed in the field is used as a basis for rec:ommending a modified method for cumulative damage analysis of the chord members. This investigation focuses on a cluster that is close to the boom foot and in which compressive stresses are significant in service. The findings in this investigation are compared to previous research

Notch stress factor of welded thick-walled tubular dragline joints by effective notch stress method

In Australia, draglines are used extensively in the coal mining industry. The tubular booms of the dragline are susceptible to fatigue cracking due to the large number of cycles they are subjected to during operation. Fatigue cracks may occur at either weld toe or weld root of the dragline joints. This paper acts as the first attempt to use the effective notch stress (ENS) method to determine the notch stress factors (NSFs) of the dragline joints. Only simplified 2D finite element analysis (FEA) was carried out at this stage. It was found that the NSF at the weld toe is higher than the NSF at the weld root for LC(I), the NSF at the weld root is in general higher than the NSF at the weld toe. The NSF results obtained are also used to compare with those obtained by using ENS method in the literature

Service loads in dragline tubular structures : a case study of cluster A5

Draglines are used extensively for removal of overburden in the coal mining industry. Draglines with tubular booms are among the structures most susceptible to fatigue cracking due to the large number of high load cycles to which they are subjected during operation. Circular hollow section tubes are used as both lacing and chord members. In this paper, a study was carried out to better understand the stresses in a 4-lacing cluster during operation. Strain gauges were installed on a typical dragline cluster A5 to measure strains generated while in operation. Static and dynamic (swing and digging) tests were carried out, and strains obtained during the different tests were used to calculate both nominal stresses and hot spot stresses. For cluster A5, the hot spot stresses at weld toes in the lacing members were found to be significantly larger than those at weld toes in the chord members. Bending stresses were found to form a relatively larger portion of the nominal stresses at the weld toes in the lacing members compared to chord members. The results of this work highlight a conclusion found in the authors' previous work that the high tensile residual stresses resulting from welding are an important issue not measured in hot spot stress testing, but these stresses are relevant to the levels and location of cracking observed in practice

Service stresses and implications for fatigue strength in dragline tubular structures

Draglines are the powerhouses of open cut mining in the coal industry in Australia. They have the capability of moving more than 100 tonnes of overburden in a cycle. Following site measurements of stress in one of the clusters where cracking is prevalent under service, this paper looks at the applicability of predicting fatigue life in draglines clusters using cumulative damage analysis. The design curves used in the analysis are in the hot stress method. Variable stress as well as equivalent stress range is used in estimating the fatigue life of the cluster. In the main chord of the cluster, where the cycle is largely compressive it is found that predicting fatigue life using a compressive stress reduction based on the British Standard BS7608 results in a prediction that is relatively closer to observed fatigue lives for new clusters. Using the full stress range on the other hand predicts fatigue lives in the main chord that are comparable to the fatigue life in repaired connections