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.
SpecificationsTesting standard
ISO 5173
Destructive tests on welds in metallic materials — Bend tests
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ISO 5173 bends a coupon cut from a welded joint over a former of specified diameter, then examines the stretched surface for cracks. It is a pass-or-fail examination rather than a property measurement, and it finds defects — lack of fusion, root cracks, inclusions — that a transverse tensile test can pass straight over.
At a glance
- Test type
- Flexure & bend — the specimen is bent
- Published by
- ISO
- Edition
- ISO 5173:2023
- Material
- Metals, alloys & welds
- Runs on
- Series 7200 and Series 9000
What the test does
A coupon is cut from a welded joint and its reinforcement machined flush. It is then bent over a former whose diameter is chosen from the parent material's specified minimum elongation — more ductile materials get tighter formers — either pressed between rollers in a guided bend fixture or wrapped around a mandrel, to a specified angle that is commonly 180°. The orientation depends on what is being examined: a root bend puts the weld root on the stretched surface, a face bend the cap, and a side bend takes a through-thickness slice and bends it on its edge. Afterwards the tension surface is examined for cracks.
What it measures, and why it matters
Strictly it measures nothing — it is a pass-or-fail examination. What it does is find defects that a tensile test can walk straight past. A transverse tensile coupon can pass a joint containing a significant lack of fusion, because enough sound metal remains to carry the load and the flaw is simply averaged over. Bending puts one surface into large tensile strain and forces any defect there to open into a visible crack. That is why welding procedure qualification requires both: tensile establishes that the joint is strong enough, bending establishes that it is sound and ductile.
Specimen and former
The former diameter is the whole severity of the test. A larger former is a gentler bend; the diameter is set by the material's ductility, not by convenience.
- Root bend
- The weld root on the tension surfaceFinds lack of penetration and root cracking, which is where most joint defects live.
- Face bend
- The weld face on the tension surface
- Side bend
- A through-thickness slice bent on its sideUsed on thicker plate, and the most searching of the three because it exposes every pass at once.
- Former diameter
- Specified from the material's elongationA more ductile material is bent over a tighter former. Using a larger former than specified makes the test easier and the result meaningless.
- Reinforcement
- Machined flush
- Round the specimen edges
- Before bendingDakA sharp machined corner cracks from the edge and that crack is not a weld defect — it is a preparation defect.
This test is not graded. The examination is of the tension surface after bending, against an acceptance limit on crack length — a coupon either meets it or does not, and there is no number that partially passes.
Test speed
- Rate
- Slow and steadyThe force is incidental. What is assessed is the surface afterwards.
- Bend angle
- As specified, commonly 180°
- Examination
- Visual, on the tension surface, against a crack-length limit
- Examine under good light and magnification
- Not at arm's lengthDakA fine root crack in a bent coupon is easy to miss and is exactly what the test exists to find.
Calculations
Former diameterd
Set from the material's minimum elongation, per the standard's table
A more ductile material gets a tighter former. This is the parameter that fixes the test's severity, and substituting a larger one silently makes the coupon easier to pass.
Acceptance—
Maximum permitted crack or defect length on the tension surface
A pass-or-fail limit from the application standard, not a value computed here.
How the test runs
- 01Cut coupons transverse to the weld, in root, face or side orientation as specified.
- 02Machine the reinforcement flush.
- 03Round the specimen edges.
- 04Select the former diameter from the material's specified elongation.
- 05Set up the guided bend fixture with that former.
- 06Bend slowly and steadily to the specified angle.
- 07Remove the coupon and examine the tension surface under good light and magnification.
- 08Measure any cracks or open defects.
- 09Compare against the acceptance limit.
- 10Record which orientation was tested and the former diameter used.
- 11Photograph any defect found.
Grips and fixtures for this method
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.
SpecificationsWhat the report has to contain
- Reference to ISO 5173 and the edition
- Parent material and welding procedure identification
- Bend orientation — root, face or side
- Coupon dimensions and whether reinforcement was removed
- Former diameter and the elongation it was chosen from
- Bend angle achieved
- Condition of the tension surface after bending
- Length and location of any crack or open defect
- The acceptance limit applied, and pass or fail
- Photographs of any defect
What the machine must be capable of
A guided bend fixture with formers of the required diameters, and force enough to bend the section — commonly between five and a hundred kilonewtons depending on thickness and former size. Force accuracy matters less here than in most tests, because the force is incidental: nothing is calculated from it, and the assessment is entirely of the surface afterwards. What the frame does need is the stroke to complete the bend angle in one continuous movement and the stability to do it without the coupon skewing off the former.
What goes wrong in practice
Using a larger former than specified is the error that matters most, because it makes the test easier to pass while leaving no trace in the report unless the diameter is recorded — which is why it is recorded. Sharp specimen edges produce cracks that are preparation artefacts and lose the coupon. And examining the bent surface casually rather than under good light and magnification defeats the purpose entirely: a fine root crack a few millimetres long against a deformed surface is easy to overlook, and it is exactly the defect the test exists to reveal.
ISO 5173 or ASTM E190
| ISO 5173 | ASTM E190 | |
|---|---|---|
| Scope | Root, face and side bends on welds | Guided bend for ductility of welds |
| Former | Diameter from material elongation | Specified by the governing code |
| Result | Pass or fail on surface condition | Pass or fail on surface condition |
| Framework | ISO 15614 procedure qualification | ASME BPVC Section IX and AWS codes |
Both are examinations rather than measurements and both are run beside a transverse tensile test. Which applies is contractual, and a coupon bent to one does not automatically satisfy the other.
Questions we are asked about this test
What is ISO 5173?
It is the ISO bend test for welds in metallic materials. A coupon cut from the joint is bent over a former of specified diameter — root, face or side orientation — and the stretched surface is then examined for cracks against an acceptance limit. It is a pass-or-fail examination rather than a property measurement.
Why bend a weld when you have already pulled one?
Because they find different things. A transverse tensile coupon can pass a joint that contains a lack of fusion, if enough sound metal remains to carry the load. Bending puts one surface into large tension and forces any defect there to open, so a flaw that a tensile test averaged over ignores becomes a visible crack. The two together are why procedure qualification requires both.
What does the former diameter control?
The severity of the test. A tighter former imposes more strain on the tension surface, so it is a harder test to pass. The diameter is chosen from the material's specified minimum elongation — more ductile materials get tighter formers — which means substituting a larger former than specified silently makes the coupon easier to pass and the result meaningless.
Which orientation should I use?
Whichever the application standard names. Root bends put the weld root in tension and find lack of penetration and root cracking, which is where most joint defects live. Face bends stretch the cap. Side bends, used on thicker plate, expose the whole thickness and every pass at once, which makes them the most searching of the three.
Why round the specimen edges?
Because a sharp machined corner is a stress raiser at the very edge of the tension surface, and a crack starting there is a preparation defect rather than a weld defect. It is indistinguishable from a genuine failure once it has run, so the coupon is lost. Rounding the edges beforehand removes a whole class of false failures.
Is there a number to report?
Not a strength. What gets reported is the orientation, the former diameter, the bend angle achieved, and the length and location of any crack found on the tension surface, judged against the acceptance limit in the governing standard. The force required to make the bend is incidental — the examination afterwards is the test.
How carefully does the surface need examining?
Under good light and magnification, not at arm's length. A fine root crack in a bent coupon can be a few millimetres long and easy to overlook against the deformed surface, and it is precisely the defect the test exists to reveal. Photographing anything found, with a scale, makes the judgement reviewable afterwards.
Running ISO 5173 on the Series 7200 and Series 9000
Dak verifies against whichever standard the method names, and where a class applies our frames sit a class tighter than it asks.
| The method asks for | Dak supplies | |
|---|---|---|
| Capacity | Moderate to high — commonly 5 to 100 kN depending on thickness and former diameter | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | ISO 7500-1 Class 1 over the working range | ISO 7500-1 Class 0.5 — a class tighter than the method asks |
| Gripping | Guided bend fixture: a former of specified diameter pressed between two rollers, or a wrap-around former | Our bend fixtures, built to the specimen |
| Environment | 23 ± 2 °C standard laboratory atmosphere | 3009 series chambers, −150 °C to +400 °C — temperature only |
This page describes the method as practised. The governing text is the current edition from the issuing body. Tell us what you are testing and we will answer with the machine, the fixture and a quotation.