A quarry contractor reported that a John Deere 210G LC crawler excavator had developed a visible crack in the boom's lower plate near the bucket-lug root - the welded connection between the boom's bottom plate and the lugs that support the bucket pivot pin. The operator first noticed the crack during routine daily inspection, and over the following three days of continued operation, the crack lengthened visibly by approximately 40 millimeters, running along the weld's toe in a direction roughly parallel to the boom's longitudinal axis. The operator also reported a faint "popping" sound during aggressive bucket-curl operations, indicating the crack was propagating further.
The technician immediately took the excavator out of service - a cracked boom is a catastrophic structural-failure risk, and continued operation under load could cause the boom to fracture and fall onto the operator cab or surrounding structures. He then performed a "Visual Inspection" and marked the crack's visible extent with paint. Following visual inspection, he performed "Dye Penetrant Testing" (DPT) - spraying a penetrant liquid onto the suspect area, wiping away the excess, and applying a developer that draws out any penetrant that has seeped into surface cracks - to confirm the crack's full surface extent. The DPT results showed the crack extended approximately 80 millimeters beyond the visible end, indicating the crack had propagated further beneath the surface than the visible crack suggested.
He then performed "Magnetic Particle Inspection" (MPI) - a more sensitive non-destructive testing method that reveals subsurface cracks by inducing a magnetic field in the steel and applying magnetic particles that concentrate at any discontinuity. The MPI results confirmed the crack extended through approximately 60 percent of the bottom plate's thickness, running parallel to the weld's toe - the classic signature of a "fatigue crack nucleated at a weld toe."
The root cause was traced to a combination of cyclic fatigue loading and weld-toe stress concentration. Boom welds at the bucket-lug root are subjected to high cyclic stress every time the bucket is loaded and curled, and the weld toe - where the weld's external surface transitions into the parent steel - is a natural stress concentration point. Over extended service in a demanding quarry environment, the repeated cyclic stress had nucleated a small fatigue crack at the weld toe, which had then propagated through the bottom plate over time. The machine's service history showed the boom had been visually inspected during routine maintenance but had never received non-destructive testing of the boom's welded joints - meaning the crack had nucleated and propagated for some time before becoming visible.
The repair involved gouging out the entire cracked weld and its heat-affected zone using carbon-arc gouging, followed by grinding the gouged surface to clean bare metal and performing "Magnetic Particle Inspection" to confirm that all crack material had been removed. The joint was then re-welded using a matched-strength welding procedure - specifically, a low-hydrogen electrode (E7018) with pre-heat and post-weld heat treatment to minimize residual stress and prevent hydrogen-induced cracking in the weld and its heat-affected zone. The completed weld was dressed with a grinder to remove any sharp weld-toe geometry that would create a new stress concentration, and the repaired area was inspected again with MPI to confirm the repair's integrity. After reassembly and a 30-minute loaded operational test, the excavator returned to service. Quarterly non-destructive testing of the boom's welded joints was added to the machine's preventive maintenance schedule, and operators were briefed to report any visible crack or unusual structural noise immediately.
