⚙️ Bend Test of Metals

Construction PN-EN ISO 7438

Overview

STATUS (as of 02.11.2026). PN-EN ISO 7438:2021-04 — Polish version, published 24.07.2023, implements EN ISO 7438:2020 and ISO 7438:2020 [IDT]; it replaced PN-EN ISO 7438:2016-03 (English version). The card has no withdrawal note — the standard is current. The fourth edition of ISO 7438 (12.10.2020) replaced the 2016 edition; the main change is the new normative Annex B — bend test under plane strain conditions.

SCOPE (clause 1). The standard specifies a method for determining the ability of metallic materials to undergo plastic deformation in bending. It applies to test pieces taken from metallic products as specified in the product standard. It does not apply to certain materials and products for which other standards exist — for example tubes in full section and welded joints.

PRINCIPLE (clause 5). A test piece of round, square, rectangular or polygonal cross-section is subjected to plastic deformation by bending, without changing the direction of loading, until a specified angle of bend is reached. The axes of the two legs of the test piece remain in a plane perpendicular to the axis of bending. In a 180° bend — depending on the requirements of the product standard — the lateral surfaces of the legs lie flat against each other or are parallel at a specified distance, maintained by an insert.

DEVICES (clause 6). The test is carried out in a testing machine or press equipped with one of three devices: two supports and a former, a V-block and a former, or a clamp with a former (the clamp may have a lever for applying the force). The length of the supports and the width of the former are greater than the width or diameter of the test piece, the diameter of the former D is determined by the product standard, and the supports and the former are of sufficient hardness. Unless otherwise specified, the distance between the supports l = (D + 3a) ± a/2, where a is the thickness or diameter of the test piece, and it does not change during the test; a distance not greater than D + 2a can result in clamping and stretch forming of the test piece. The surfaces of the V-block form an angle of 180° − α (α — angle of bend given in the relevant standard), and its edges have a radius of 1 to 10 times the thickness of the test piece. In the clamp, the left face of the jaw should not reach up to or beyond the vertical through the centre of the rounded portion of the former, because its position influences the result.

TEST PIECE (clause 7). Areas of material affected by shearing, flame cutting and similar operations are removed; testing a test piece in which they have not been removed is acceptable if the result is satisfactory. The edges of rectangular test pieces are rounded to a radius not exceeding 3 mm for a thickness of 50 mm or more, 1.5 mm for a thickness from 10 mm to less than 50 mm and 1 mm for a thickness below 10 mm, without transverse burrs, scratches or marks (here too, a test piece with unrounded edges is acceptable if the result is satisfactory). Width of the test piece, unless the relevant standard specifies otherwise: equal to the product width when that does not exceed 20 mm; for a wider product — (20 ± 5) mm for thicknesses below 3 mm and from 20 mm to 50 mm for thicknesses of 3 mm or more. The thickness of test pieces from sheets, strips and sections equals the product thickness; above 25 mm it may be reduced by machining one surface, to not less than 25 mm. Round and polygonal test pieces have the cross-section of the product up to a diameter of 30 mm; from over 30 mm up to and including 50 mm the diameter may be reduced, and above 50 mm it shall be reduced — in both cases to not less than 25 mm. During bending, the unmachined surface is on the tension side. The dimensions and sampling of test pieces from forgings, castings and semi-finished products are defined by the general delivery requirements or by agreement; by agreement, wider and thicker test pieces may be tested; the length of the test piece depends on its thickness and on the device.

PROCEDURE (2005 edition, clause 6 — in the current edition this is clause 8, outside the available text). Adequate safety measures and guarding equipment shall be provided during the test. The test is generally carried out at ambient temperature between 10 °C and 35 °C and, where controlled conditions are required, at (23 ± 5) °C. Depending on the relevant standard, the test piece is bent under an appropriate force to a specified angle, until the legs are parallel at a specified distance, or until the legs are in direct contact. The angle of bend can be determined from the displacement of the former (Annex A, informative). For test pieces thicker than 3 mm, the parties may agree on bending under plane strain conditions according to Annex B (normative).

ASSESSMENT OF THE RESULT. The standard refers the interpretation of the result to the requirements of the product standard (clause 9, outside the available text). According to a technical note by a manufacturer of testing machines describing this standard, when the product standard specifies no criteria, it is checked that the test piece shows no visible macro-cracks.

WHAT WE DO NOT GIVE HERE. Details of the procedure, assessment and test report from clauses 8–10 of the current edition (bending rate, how the legs are pressed together in a 180° bend, the exact crack criterion) and the dimensions from Annexes A and B — the available text of the 2020 edition ends at 7.7, and the procedure is given from the 2005 edition up to 6.2. We do not give former diameters or bend angles for specific products (e.g. reinforcing bars or sheets) — they are specified in product standards.

Method principle

The test piece, placed on two supports (or in a V-block or a clamp), is bent with a former of diameter D specified in the product standard, without changing the direction of loading, to a specified angle, until the legs are parallel at a given distance, or until they touch. Unless otherwise specified, the distance between the supports is (D + 3a) ± a/2 and does not change during the test. The result is assessed against the requirements of the product standard.

Applications

• Assessment of the ability to undergo plastic deformation in bending of test pieces taken from metallic products according to the product standard
• Sheets, strips and sections — test pieces of product thickness (above 25 mm a reduction to at least 25 mm is permitted)
• Bars, wire and products of round or polygonal cross-section — cross-section of the product up to a diameter of 30 mm
• Forgings, castings and semi-finished products — test piece dimensions according to the delivery requirements or by agreement
• Bending under plane strain conditions (Annex B) — by agreement between the parties, for thicknesses over 3 mm
• The standard does not apply to tubes in full section or welded joints — other standards exist for them

Key parameters

Standard

Standard number

PN-EN ISO 7438:2021-04

Title (PL)

Metale — Próba zginania

Title (EN)

Metallic materials — Bend test

Step-by-step procedure

1. Sampling and preparation of the test piece

   Take the test piece according to the product standard. Remove areas affected by shearing or flame cutting; round the edges of a rectangular test piece to a radius not exceeding 3 mm, 1.5 mm or 1 mm (depending on thickness), without transverse burrs or scratches (7.1–7.2).

2. Test piece dimensions

   Width equal to the product width up to 20 mm, for a wider product (20 ± 5) mm (thickness below 3 mm) or from 20 mm to 50 mm (thickness of 3 mm or more); thickness equal to the product thickness, above 25 mm a reduction from one side to at least 25 mm is permitted; round and polygonal cross-sections according to 7.4.2 (7.3–7.7).

3. Setting up the device

   Select the device (supports with a former, V-block or clamp) and a former of diameter according to the product standard. With supports, set the distance l = (D + 3a) ± a/2 unless otherwise specified (clause 6).

4. Positioning the test piece

   Place the test piece so that the unmachined surface is on the tension side (7.4).

5. Bending

   Bend without changing the direction of loading, at ambient temperature between 10 °C and 35 °C (under controlled conditions (23 ± 5) °C), depending on the relevant standard: to a specified angle, until the legs are parallel at a specified distance (with an insert) or until the legs touch. Provide safety measures and guarding (2005 edition, clause 6; principle — clause 5).

   🌡 Temperature: 10–35 °C

6. Angle of bend

   The angle can be determined from the displacement of the former according to informative Annex A.

7. Assessment of the result

   Assess the test piece against the requirements of the product standard (clause 9 — outside the available text; when the product standard gives no criteria, the absence of visible macro-cracks is checked — according to the technical note of a testing machine manufacturer).

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• The standard (2005 edition, clause 6) requires adequate safety measures and guarding equipment during the test
• Detailed rules for working at the machine or press are set by the laboratory on the basis of a risk assessment

Frequently asked questions