Fatigue upgrade of a crane runway bracket
An existing crane runway girder has a bracket connected by two 8 mm fillet welds, each 200 mm long, giving a throat of about 5.6 mm. The bracket carries a fluctuating load of 20–120 kN for 2×10^6 cycles. After 12 years, magnetic particle inspection found toe cracks at both ends of the bracket. A static check gives only 45 MPa nominal stress, so the structure is not overloaded statically. The client proposes to gouge out the cracks, increase the fillet leg to 12 mm and reweld, arguing that the larger weld will reduce stress. The detail is a sharp, unwrapped bracket end; the calculated stress range at the toe is 70 MPa. The applicable fatigue curve for this detail gives a fatigue limit of about 56 MPa at 2×10^6 cycles. The area is accessible for grinding but not for major modification.
1. Is increasing the fillet leg to 12 mm an adequate fatigue remedy? Show your reasoning.
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The static check only shows that general yielding is not the issue. The crack started at the toe, so the governing detail is the toe geometry, not the nominal throat.
Increasing the leg from 8 to 12 mm raises the throat from about 5.6 to 8.4 mm and lowers nominal shear stress by roughly one third. That may bring the nominal stress range from 70 MPa to about 47 MPa, below the 56 MPa detail limit. However, the fatigue class depends on the sharp bracket end, the weld toe angle and residual stress. A larger weld also increases restraint and can make the toe worse if it is not blended. Simply adding weld metal does not change the stress concentration at the toe.
The reliable remedy is to remove the crack, improve the toe by grinding or TIG dressing, or change the detail, then verify by calculation and inspection. The leg increase may be part of the repair, but it is not sufficient on its own.
2. Recommend a repair and verification plan, including the inspection method and timing.
Read the worked answer
Gouge or grind out the cracks until MT or PT is clear, with a smooth excavation. Repair with a qualified WPS and a low-hydrogen consumable. Dress the weld toe by grinding to about 0.5 mm depth and blend the bracket end into the weld; TIG dressing is an alternative if the procedure is qualified. HFMI can improve the fatigue class but must be applied correctly and verified.
Verify the repair by MT after cooling and again after 48 h if the steel is crack-sensitive. Reassess the fatigue life using the improved detail category and the actual stress range. Record the repair on the drawing. If the calculated stress range remains above the detail limit, reduce the load or modify the bracket.