7.2 Guided Bend, Nick Break, and Fillet Weld Break Tests

Guided Bend Test

The guided bend test is the principal test for weld soundness — freedom from internal and surface discontinuities. A rectangular specimen is bent through a specified angle (normally 180 degrees) by forcing it around a mandrel (plunger) in a guided bend jig, so the surface of interest is stretched in tension. Because bending magnifies any lack of fusion, slag, porosity, or crack into an open tear, the test exposes flaws a tensile test would not. AWS D1.1 uses three specimen orientations.

The mandrel diameter is typically 4T (four times specimen thickness) for the common 36-ksi/50-ksi structural steels, giving a defined strain on the convex surface; lower-ductility materials use a larger radius (6T or 8T) to avoid false rejection. For a complete-joint-penetration groove PQR, AWS D1.1 commonly requires four bend specimens (two face + two root for thin plate, or four side bends for thicker plate).

Acceptance criteria (AWS D1.1)

The bent specimen is examined on its convex (stretched) surface:

• No single open discontinuity may exceed 1/8 in (3 mm) measured in any direction.
• The sum of the greatest dimensions of open discontinuities between 1/16 in and 1/8 in must not exceed 3/8 in (10 mm).
• Corner cracks at the specimen edges are disregarded unless there is clear evidence they originate from slag inclusion or another weld discontinuity. A corner crack longer than 1/4 in with no visible discontinuity may require the inspector to re-test or examine for cause.

Exam trap: the 1/8 in single-discontinuity limit and the 3/8 in cumulative limit are favorite questions. "Corner cracks are ignored unless caused by a discontinuity" is another.

Nick-Break Test

The nick-break test is a simple, rugged field test that reveals internal discontinuities in a groove weld by fracturing it open for visual examination. Saw cuts (nicks) are made into the weld at both edges so the break is forced through the weld metal, then the coupon is fractured by a press, hammer blow, or bending. The exposed fracture face is studied for incomplete fusion, slag inclusions, porosity, and internal cracks. It is the workhorse test of API 1104 (pipeline welding) and is widely used for field welder qualification because no precision specimen machining is needed.

Typical API 1104 nick-break acceptance: the exposed surface must show complete penetration and fusion; gas pockets (porosity) must not exceed 1/16 in (1.6 mm), the combined area of all gas pockets must not exceed about 2% of the exposed surface, and slag inclusions are limited in length and depth per the code. Any single defect cluster beyond these limits fails the coupon.

Fillet Weld Break Test

The fillet weld break test checks the soundness of a fillet weld that cannot be radiographed or easily bent. A T-joint with a fillet welded on one side only is loaded so the unwelded side is the root in tension, and force (press or hammer) folds the top member over until the fillet fractures or bends flat without breaking.

The fractured surface is examined for:

• Complete fusion to the root of the joint (the single most important requirement).
• Internal porosity and slag not exceeding code limits.
• Adequate leg size and absence of overlap.

AWS D1.1 acceptance for the fillet break specimen: the specimen passes if it bends flat without breaking, or, if it breaks, the fracture face shows complete fusion to the root with no inclusion or porosity larger than 3/32 in (2.4 mm) and the sum of porosity and inclusions not exceeding 3/8 in within any 1 in of weld.

Exam trap: the nick-break and fillet-break tests both reveal internal discontinuities on a fracture surface — they do not measure strength, toughness, or hardness. Fusion to the root is the headline requirement for the fillet break.

How the Specimens Are Removed and Why Orientation Matters

Bend, nick-break, and fillet-break coupons are not taken at random — the code specifies where each comes from on the qualification test plate so the most critical region of the weld is sampled. On a groove test plate the specimens are cut transverse to the weld and the weld reinforcement and backing are removed and machined flush before bending; leaving a crown on a bend specimen creates a stress riser that can cause a false rejection. For a side bend, the plate is sliced through its thickness into roughly 3/8 in (10 mm) thick straps so the full cross-section, including every pass, is presented in tension.

The direction a surface faces in the jig decides what is tested:

• A face bend stretches the cap, exposing surface and near-cap lack of fusion and crown cracks.
• A root bend stretches the root, exposing incomplete root penetration, root cracks, and lack of root fusion — the most failure-prone zone in many open-root welds.
• A side bend stretches the entire wall, catching mid-thickness slag lines and lack of interpass fusion that face and root bends can miss in thick plate.

Reading the Broken Coupon

The value of nick-break and fillet-break tests is that the inspector reads an actual fracture face, not an indirect signal. Typical findings and their meaning:

For the fillet break, the inspector also confirms the leg dimensions with a fillet gauge and looks for overlap or insufficient throat before judging fusion to the root. A specimen that bends flat without fracturing is itself a pass under AWS D1.1 because it demonstrates the fillet is too sound and ductile to break open.

Exam trap: removing the reinforcement before bending is mandatory; a bend failure caused by a leftover crown is an invalid test, not a weld rejection.