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 Bend Testing of Welds in Metallic Materials
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
From the test method to your testing system
Explore the DAK machines already listed for ISO 5173, then review the grips, measurement and setup requirements below.
Universal Testing MachineSeries 7200Explore the machine →
Universal Testing MachineSeries 9000Explore the machine →Jump to a specific section
01Understand the method
What the test does
A coupon is cut from a welded joint, its reinforcement machined flush, 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 or wrapped around a mandrel, to a specified angle, commonly 180°. Orientation depends on what is being examined: a root bend puts the weld root on the stretched surface, a face bend the cap, a side bend takes a through-thickness slice and bends it on edge. The tension surface is then 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 a tensile test walks straight past. A transverse tensile coupon can pass a joint containing significant lack of fusion, because enough sound metal remains to carry the load and the flaw is 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.
02Prepare the specimen and test settings
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
- The method sets no rate of traverse; clause 5 asks only that the speed be constant and recorded where knownThe force is incidental. Nothing is calculated from it, and the verdict is the state of the tension surface afterwards. Clause 9 j) then requires the testing speed in the report — so a figure is needed even though none is prescribed.
- Rate Dak sets where nothing else governs
- (1 ± 0,2) mm/s — 60 mm/minDakThe figure ISO 7438 fixes for the same operation on unwelded metal, and the one to adopt so clause 9 j) can be answered with a number. Slow enough to permit the free plastic flow the test exists to observe. An application standard or contract naming a different speed overrides it.
- What governs instead
- Former diameter d = 4ts, or (100ts / A) − ts below 20 % elongationRollers at d + 2ts + 1 ≤ l ≤ d + 3,5ts, roller radius 20 mm or more. A 10 mm plate of 22 % elongation takes a 40 mm former on a 61 to 75 mm span.
- 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.
03Build the test setup on a DAK machine
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 five to a hundred kilonewtons. Force accuracy matters less than in most tests: nothing is calculated from it. The frame needs stroke to complete the bend in one continuous movement, and stability enough that the coupon does not skew off the former.
ISO 5173 fixes no rate of traverse. Clause 5 asks only that the speed be constant and recorded where known — declared, not prescribed, because the verdict is the state of the convex surface, not a force. What it does fix, tightly, is geometry. The former or inner roller diameter d is 4ts for parent metal of 20 % elongation after fracture or more, and d = (100ts / A) − ts below that; roller spacing lies between d + 2ts + 1 and d + 3,5ts, roller radius at least 20 mm; the bend runs to the angle the application standard names, or to about 180°. On a 10 mm plate of 22 % elongation, that is a 40 mm former on a span of 61 to 75 mm. With the rate left open, Dak sets and reports the figure ISO 7438 fixes for the same operation on unwelded metal: (1 ± 0,2) mm/s — 60 mm/min. It is slow enough to permit the free plastic flow this test exists to observe, and it answers the report requirement with a number rather than an adjective.
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.
SpecificationsRunning 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 |
04Run the test
How the test runs
- Cut coupons transverse to the weld, in root, face or side orientation as specified.
- Machine the reinforcement flush.
- Round the specimen edges.
- Select the former diameter from the material's specified elongation.
- Set up the guided bend fixture with that former.
- Bend slowly and steadily to the specified angle.
- Remove the coupon and examine the tension surface under good light and magnification.
- Measure any cracks or open defects.
- Compare against the acceptance limit.
- Record which orientation was tested and the former diameter used.
- Photograph any defect found.
05Calculate, report and interpret
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.
What 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 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.
06Compare methods and find answers
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.
Related
Materials tested to it
The test it standardises
Industries that test to it
Other standards explained
Planning ISO 5173 testing?
Discuss your specimen, test requirements and reporting needs with DAK engineering.
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.