Cracked weld on a trolley bracket
A student is making a mild steel L-bracket for a school trolley. The bracket is 150 mm × 100 mm, made from 25 mm × 3 mm flat bar. They MIG weld the corner at 120 A with 0.8 mm wire and CO2 shielding gas. The weld looks tall and narrow. During a load test with 20 kg, a crack appears at the weld toe and the bracket bends. The student cleaned mill scale from the flat bar on one side only. The weld bead sits proud of the surface and shows little penetration at the root. The student used a wire brush, not a grinder, before welding. They have two days and enough material for one more bracket, but the trolley must carry 25 kg in the assessment.
1. Using the evidence, explain why the bracket joint failed.
Read the worked answer
The joint failed because the weld did not achieve fusion at the root. Mill scale was cleaned from only one face, so the other face remained contaminated; mill scale melts at a higher temperature than steel and prevents the weld pool from wetting the parent metal. The tall, narrow bead suggests the student welded too fast or with too little heat input for the joint, so penetration was limited. The crack started at the weld toe, which is a sharp transition and a stress concentration. Under the 20 kg test load, the effective throat was small, so the bend occurred at the joint. The bracket may also have been poorly fitted, leaving a gap that the weld bridged rather than fused. The combination of contamination, low penetration and a sharp toe explains the failure.
2. Recommend a redesign or process change that would make the bracket carry 25 kg, and justify it.
Read the worked answer
Process change: clean both faces and the edges to bare metal with a grinder, set a root gap of about 1–1.5 mm, tack in place, and weld with a current around 100–130 A for 3 mm steel, using a slightly slower travel speed to get penetration. That should produce a sound joint. Redesign: add a triangular gusset of 3 mm plate across the inside corner, or increase the flat bar to 5 mm. A gusset lowers the stress at the weld toe by spreading the load over a larger area and shortening the unsupported leg. Trade-off: the gusset adds weight, cutting and welding time, and may interfere with the trolley frame; a thicker bar adds weight and cost but avoids extra fabrication. For a school assessment with two days, the gusset plus proper cleaning is the more reliable change because it addresses both joint quality and bracket stiffness.
3. Why is 'just run a bigger weld' not a reliable fix?
Read the worked answer
A bigger weld adds metal at the surface but does not guarantee root penetration or fusion. If the uncleaned face remains, the extra weld still sits on contamination and can peel away. A larger bead also increases heat input, which can distort the thin 3 mm bar and enlarge the heat-affected zone; it creates a thicker section at the toe, which can raise stress concentration if the transition is poor. The extra weld adds weight without necessarily increasing the effective throat. The failure was a lack of fusion, so the fix must clean the joint and achieve penetration, or reduce the stress with a gusset. Adding weld size alone treats the symptom, not the cause.