ASTM E190 Guided Bend Testing of Welds for Ductility

What the test does

A coupon is cut transverse to a welded joint, its reinforcement removed flush with the base metal and its edges rounded. It is then placed in a guided bend jig — a plunger of specified radius forced into a die of specified opening, or a wrap-around former — with the surface of interest facing the tension side, and bent in one movement to the full angle. The convex surface is afterwards examined under magnification for cracks and open defects, their length judged against the acceptance limit in the code that called up the test.

What it measures, and why it matters

Strictly nothing is measured; the outcome is a judgement on a surface. What the method contributes is comparability. In a free bend the specimen finds its own radius according to how it work-hardens, so two laboratories bending nominally identical coupons impose different strains and reach different verdicts. Guiding the bend fixes the geometry, which is why the plunger radius and die opening belong in the report rather than any force. Weld ductility is then a defensible finding rather than an impression. It is used wherever a welding procedure or a welder has to be qualified before production starts — pressure vessels under ASME rules, structural steelwork under the AWS codes, pipelines, heavy plant — and in production testing, where coupons welded alongside the real joints are bent to confirm that what was qualified in the laboratory is still being achieved on the shop floor. In failure investigation it runs the other way round, to establish whether a joint that cracked in service was ductile enough to begin with.

Coupon and jig

The word guided is the whole point. The jig imposes the curvature, so two laboratories bending the same material get the same strain.

Face bend

Weld face on the convex surface

Root bend

Weld root on the convex surfaceFinds incomplete penetration, which is the most common serious joint defect.

Side bend

Through-thickness slice bent on edgeUsed on thicker sections; exposes every weld pass simultaneously.

Plunger radius

Set by the governing code, from the materialA tighter radius is a more severe test. It is not a free choice.

Reinforcement

Removed flush with the base metal

Record the jig dimensions used

Radius and die openingDakWithout them the report cannot be reproduced, because the severity of the test lives entirely in that geometry.

Nothing here is graded. The coupon meets the code's crack limit or it does not, and the number that matters in the report is the plunger radius rather than any force.

Test speed

Rate of plunger travel

No rate is setThe severity of the test is fixed by the jig, not by how fast the coupon enters it.

What governs instead

Plunger radius 2t, width 4t; die opening 6t + 3.2 mmt is the coupon thickness, so a 9.5 mm coupon is bent by a 38 mm plunger of 19 mm radius into a 60 mm opening — about 20 % outer-fibre elongation from ε = t / (2r + t).

Movement

Steady, without significant lateral motion, to the full bendThe bend ends by geometry, not by force: a U-shape closed until a 3.2 mm diameter wire will not pass between specimen and die. Stopping and restarting can leave a mark that reads as a defect.

Examination

Convex surface, against the code's crack length limit

Corner tears

Usually judged separately from face cracksMost codes treat an edge tear differently from a crack in the body of the weld.

Examine under magnification

Not by eye aloneDak

What the machine must be capable of

Enough force to complete the bend — commonly five to a hundred kilonewtons depending on section and former size — with the stroke to reach the full angle without repositioning, and a jig holding the plunger radius and die opening the code requires. Force accuracy matters less here than in most methods, since nothing is calculated from the load; what the frame needs is the stability to drive the plunger straight and the travel to finish the bend in one movement.

E190 sets no rate of plunger travel, and does not need one: any convenient means may be used so long as the motion is steady and free of significant lateral movement. The severity is fixed by the jig instead. In the standard's own jig figure the plunger is 4t wide with a radius of 2t, the die opening is 6t + 3.2 mm and the shoulders are radiused 3t + 1.6 mm, t being the coupon thickness — so a 9.5 mm coupon is bent by a 38 mm plunger of 19 mm radius into a 60 mm opening. Work the outer-fibre strain out from those numbers, ε = t / (2r + t), and that coupon is being asked for about 20 % elongation. The end of the bend is geometric too: force is applied until the specimen conforms to a U-shape and a 3.2 mm diameter wire will no longer pass between it and the die. Those figures and the code's crack-length limit are what a report has to carry. Crosshead speed is not one of them.

Grips and fixtures for this method

Adjustable spanTJ-124

Three Point Bend Fixture

A three-point bend fixture with adjustable span and the roller diameters the method specifies — span-to-depth ratio is set on the fixture, not assumed.

Specifications

Uniform momentTJ-165

Four Point Bend Fixture

Four-point loading where the specification calls for it, putting a length of the specimen under constant moment rather than concentrating it under one nose.

Specifications

How the test runs

1. 01Cut coupons transverse to the weld in the specified orientation.
2. 02Remove the weld reinforcement flush with the base metal.
3. 03Round the coupon edges to remove machining corners.
4. 04Take the plunger radius and die opening from the governing code.
5. 05Set the guided bend jig to that geometry.
6. 06Place the coupon with the correct surface facing the tension side.
7. 07Force the plunger through in one continuous movement to the full bend.
8. 08Remove the coupon and examine the convex surface under magnification.
9. 09Measure the length of any crack or open defect.
10. 10Judge face cracks and corner tears against their separate limits.
11. 11Report the jig geometry alongside the result.

Calculations

What the report has to contain

• Reference to ASTM E190 and the edition
• Base metal and welding procedure identification
• Bend orientation — face, root or side
• Coupon thickness and width
• Whether reinforcement was removed
• Plunger radius and die opening used
• The code the jig geometry was taken from
• Condition of the convex surface
• Length and location of any crack or open defect
• The acceptance limit applied, and pass or fail

What goes wrong in practice

Using a larger plunger radius than the code specifies has the greatest consequence, because it quietly makes the test easier while producing a report that looks identical; recording the jig geometry is the defence. Sharp coupon edges generate corner tears indistinguishable from real defects. Stopping mid-bend leaves a witness mark on the very surface being examined. Inspecting by eye rather than under magnification misses fine root cracks a few millimetres long, the defect the test exists to find. And reporting a pass without naming the code the jig geometry came from leaves a reader unable to tell whether the coupon met a demanding requirement or a lenient one.