Testing standard
Standard Test Methods for Bend Testing of Material for Ductility
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM E290 is the bend test for ductility of metallic material. A coupon is bent to a specified angle or around a specified inside radius, and the stretched outer face is examined for cracking. The result is a condition rather than a number — pass or fail against a criterion the PRODUCT specification sets, not this standard.
A coupon — flat strip, round bar or hexagonal section — is laid across two hardened supports or rollers and a mandrel or plunger is pressed into its mid-length, bending it to a specified angle or around a specified inside radius. Nothing is pulled apart and no curve is plotted: the bend is made, the load released, and the convex outer face examined for cracking. Four degrees of constraint are recognised — a guided bend, with the plunger forcing the piece down between the supports; a semi-guided bend, bent in contact with a mandrel to an angle judged while still under load; a free bend, with the ends brought together and nothing touching the concave face; and a bend-and-flatten, in which the legs are driven into contact along their length.
The result is a condition rather than a number: whether the outer fibre survives an imposed strain intact. The acceptance criterion — how many cracks, of what size, visible to the unaided eye — belongs to the product specification, not to E290, which supplies only the mechanics of getting there.
That makes it a direct proxy for the press brake: material that cracks at the specified radius on the bench will crack at that radius in production. Because the outer fibre is worked hardest, the test is unusually sensitive to defects a tensile coupon averages away — inclusion stringers, banded structure, embrittled or decarburised surface layers, coarse grain in a heat-affected zone — and it is a fast lot check on heat treatment.
Four bend arrangements are recognised, and they are not interchangeable. Which one applies comes from the product specification.
This test has no curve and no number of its own. A report that gives an angle without naming the mandrel radius, or a pass without naming the criterion it passed, has recorded nothing that can be checked.
The same adjustable-span beam a flexural test uses, with the span and plunger set to what the product specification calls for. Nothing is measured on a curve here — the fixture only has to deliver the geometry accurately.
SpecificationsThis is a fixture-led method. Bend force follows the section and the span, not the standard: a 3 mm strip needs only a few kN, while a 25 mm plate coupon on a short span can call for 100 kN to 600 kN, which is why bend jigs are commonly bolted into frames of 300 kN to 600 kN. Stroke matters as much as capacity — enough plunger travel to take the legs into contact for a bend-and-flatten.
E290 fixes no rate of bending, and the omission is deliberate: ASTM A370, which calls E290 up for its bend clause on steel products, states that the speed of bending is ordinarily not an important factor. Geometry is what the method fixes. The plunger or mandrel radius r comes from the product specification and the support span follows from it, C = (2r + 3t) ± t/2 — so a 10 mm plate bent around a 20 mm radius sits on a 70 mm span. Bend continuously to the required angle; stopping and restarting marks the very face being examined. No extensometer is required. Whether Practices E4 is invoked for force accuracy could not be confirmed, so no accuracy class is asserted here; the load reading is not the reported result in any case.
Supports must be hard, parallel and unable to shift under load, and a convenient mandrel radius is no substitute for the specified one — it changes the outer-fibre strain the coupon actually sees. No temperature or humidity limits are stated.
Edge cracking is the most common false failure. A crack starting at a sheared or burred edge and running inwards is a preparation defect, and logging it as convex-face cracking condemns sound material.
Friction at the mandrel is the next. An unlubricated or galled mandrel drags on the outer fibre, adding tensile strain the geometry never intended, and leaves score marks that are then argued over as cracks.
Springback catches out semi-guided and free bends. The angle or inside radius has to be read while load is still applied; measured after release the bend looks shallower than it was, and a piece that should have gone further is passed.
Finally, supports that creep apart under load, or a span set too short to begin with, clamp the coupon and turn the bend into stretch forming — a different strain history, and a quietly invalid test.
| ASTM E290 | ISO 7438 | |
|---|---|---|
| Purpose | Ductility by bending | Ductility by bending |
| Arrangements | Guided, semi-guided, free, bend-and-flatten | Defined bend arrangements |
| Acceptance | From the product specification | From the product specification |
| Output | A condition, not a number | A condition, not a number |
Both are mechanics rather than acceptance standards, and both defer the criterion to the product specification. That is what makes a bend result meaningless without the specification it was judged against.
It is the ASTM standard for bend testing of metallic material for ductility. A coupon is bent to a specified angle or around a specified radius and the stretched outer face is examined for cracking. It reports a condition rather than a number, and it supplies the mechanics while the product specification supplies the acceptance criterion.
A guided bend, where a plunger forces the piece down between supports; a semi-guided bend, made in contact with a mandrel with the angle judged while still under load; a free bend, where the ends are brought together and nothing touches the concave face; and a bend-and-flatten, where the legs are driven into contact along their length. They differ in constraint and are not interchangeable.
Whatever the product specification says — how many cracks, of what size, visible to the unaided eye. E290 does not set one. That is why a bend result reported without naming the specification it was judged against cannot be checked by anybody else.
Because the outer fibre is worked harder than anything in a tensile coupon, and it is worked at the surface. Inclusion stringers, banded structure, an embrittled or decarburised skin and coarse grain in a heat-affected zone all live near the surface, and a tensile test averages them away over the whole section. The bend concentrates strain exactly where they are.
E290 sets no rate of bending, and ASTM A370 — which calls E290 up for its bend clause on steel products — states that the speed of bending is ordinarily not an important factor. Geometry is the controlled variable: the plunger or mandrel radius r from the product specification, the support span C = (2r + 3t) ± t/2, and the angle achieved. A 10 mm plate bent around a 20 mm radius therefore sits on a 70 mm span. Bend in one continuous movement, because a stop and restart leaves a mark on the surface under examination.
No. A flexural test such as ASTM D790 measures stress and modulus from a force-deflection curve. A bend test to E290 plots nothing — it imposes a strain of a known severity and asks whether the surface survived. The fixture can be the same; the question is not.
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 | Bend force follows the section, not the standard: a 3 mm strip needs only a few kN, while a 25 mm plate coupon on a short span can call for 100 kN to 600 kN, so laboratories usually bolt the bend jig into a 300 kN to 600 kN frame. | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | unknown | ISO 7500-1 Class 0.5 — the method sets no class of its own |
| Gripping | Guided-bend jig with a plunger or mandrel over two hardened supports or rollers, plus U-bend, V-block, wrap and flattening variants | Our bend fixtures, built to the specimen |
| Environment | Ambient laboratory air; the method states no temperature or humidity limits | 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.
