Fracture mechanics of mining dragline booms

Abstract

Mining draglines are large mining machines which have a booms of about 100 m in length. The booms are tubular structures made up of three large chords connected by smaller lacing tubes. There is a regular cracking phenomenon at the welds which means that the cracking must be remotely detected, exactly located and repaired. This paper describes the cracks which grow in the weld joints (called clusters) and the system by which the booms are protected from catastrophic collapse and the maintenance and repair techniques used to keep them in service.

Weld-induced residual stresses in a prototype dragline cluster and comparison with design codes

“…The booms are often more than 100 m long and are constructed using tubular structural elements with welded nodal connections termed as clusters. The main chords are subjected to predominantly compressive stress cycles but the lacing members, which stabilize the structure against buckling and twisting [1], undergo both compression and tension during each digging cycle. This cycle is one of the most extreme fatigue loading situations in industrial use and very often the booms crack at welded nodal connections due to fatigue related issues.…”

“…Fig. 2 shows the cracks exposed by cutting a window and weldrepair made in a typical mining dragline cluster [1,2].…”

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

“…These connections consist of a main chord member with 3-to 5-bracing members, all welded to the chord at one point forming a complex overlapped joint. Multiplanar double overlapping CHS NN-joints (consisting of 4-bracing members) are the most common type of joints in the dragline boom [3,23]. NN-joints are a special case of KK-joints when two of the braces are vertical to the chord member.…”

Influence of variations in geometric parameters and an alternative design for improved fatigue life of a mining dragline joint

“…These draglines are expected to bear very high levels of applied stresses (usually compressive) when lifting and transporting material in the field. On the average, a dragline in the coal mining industry has to undergo cycles of the order of half a million a year [2][3][4], and is expected to last about three decades in the field in operation.…”

“…There is also no Post Weld Heat Treatment (PWHT) conducted on the joint welds for reducing any weld induced residual stresses, which if performed, has been suggested to improve the fatigue life substantially [7]. With such a poor quality control of welds and usually no effective mechanisms in place to reduce the effect of weld induced residual stresses in dragline joints, cracks start appearing on the main chords within the first five years of their life [3,4,8]. Hundreds of large fatigue cracks have been observed in the field due to undetected and/ or unrepaired initial cracks, which are first noticed by loss of air pressure inside the tubes unless they are on the surfaces of the tubes and can be seen by visual inspection.…”