🔩 Flexural properties of rigid and semi-rigid plastics — three-point bending of a freely supported beam, method A (1 %/min) and B, flexural strength and flexural modulus; PN-EN ISO 178:2019-06

Materials and NDT PN-EN ISO 178:

Overview

WHAT THE STANDARD COVERS. Scope from the PKN card of PN-EN ISO 178:2019-06 (English version), in full (with the bibliographic references in square brackets, as on the card; our translation): "This document specifies a method for determining, under defined conditions, the flexural properties of rigid and semi-rigid plastics. Requirements for the recommended test specimen are specified, but parameters of an alternative test specimen, which is used where justified, are also given. A range of test speeds is given. The method is used to investigate the flexural properties of test specimens and to determine the flexural strength, the flexural modulus and other aspects of the flexural stress/strain relationship under the conditions defined. It applies to a freely supported beam, loaded at midspan (three-point bending). The method is used for testing the following materials: - thermoplastic materials intended for moulding, injection and extrusion, including filled and reinforced compounds; rigid thermosetting sheets; - thermosetting materials, including filled and reinforced compounds; thermosetting sheets; In accordance with ISO 10350-1 [5] and ISO 10350-2 [6], this International Standard applies to materials reinforced with fibres with a length, prior to use, not greater than 7.5 mm. For materials reinforced with long fibres (laminates), when the fibre length exceeds 7.5 mm, ISO 14125 [7] applies. The method is usually not used for rigid cellular materials or multilayer (sandwich) structures containing cellular materials. In such cases ISO 1209-1 [3] and/or ISO 1209-2 [4] applies. NOTE 1 For certain types of fibre-reinforced plastics a four-point bending test is used. This is described in ISO 14125. The method uses test specimens moulded to specified dimensions, machined from the central part of a multipurpose test specimen (see ISO 20753), or prepared by machining from finished products or semi-finished products such as mouldings, laminates and extruded and cast sheets. The method gives recommended dimensions of the test specimens. Tests carried out on specimens differing in dimensions, or prepared under different conditions, may give non-comparable results. Other factors, such as the test speed and the conditioning of the specimens, can also affect the results. NOTE 2 Especially for injection-moulded semi-crystalline polymers, the thickness of the oriented skin layer, which depends on the processing conditions, also affects the flexural properties. The method is not suitable for determining design parameters, but can be used in materials testing and in quality control." For the first group of materials the card says "rigid thermosetting sheets", whereas the text of ISO 178:2019 at this point has "rigid thermoplastics sheets". We read the content of the method in the sample extract of ISO 178:2019 (sixth edition, 2019-04) from the cover to clause 5.4.3 (measurement of deflection) with the table of contents; we do not have clauses 6–11 (specimens, conditioning, procedure, calculations, precision, test report) or Annexes A–D.

ACCORDING TO THE TEXT OF ISO 178:2019 (foreword, clauses 1–5.4.3). The standard was prepared by ISO/TC 61 "Plastics", SC 2 "Mechanical properties"; the sixth edition replaced ISO 178:2010 and incorporated the amendment ISO 178:2010/Amd.1:2013; changes: calibration requirements differentiated by test type, introduction of deflectometers, reintroduction of the machine compliance correction and a new Annex D (tensile versus flexural modulus). Scope (1): rigid and semi-rigid plastics — thermoplastic moulding, extrusion and casting materials (also filled and reinforced), rigid thermoplastic sheets, thermosetting moulding materials and sheets; composites with fibres up to 7.5 mm long prior to processing; for laminates with longer fibres — ISO 14125; usually not for rigid cellular plastics and sandwich structures with a cellular core (ISO 1209-1, ISO 1209-2); specimens moulded to size, cut from the centre of a multipurpose specimen (ISO 20753) or from products; results from specimens of other dimensions or prepared differently may be non-comparable; the method is not used for determining design parameters. Definitions (3): conventional deflection sC = 1.5 times the specimen thickness (with a span L = 16h it corresponds to a strain of 3.5 %); flexural modulus Ef = (σf2 − σf1)/(εf2 − εf1) at εf1 = 0.000 5 and εf2 = 0.002 5 — only an approximation of Young's modulus; rigid plastic — modulus above 700 MPa, semi-rigid — 70–700 MPa. Principle (4): the specimen is deflected at a constant speed until the outer surface breaks or until a strain of 5 %, whichever occurs first; method A — 1 %/min throughout the test; method B — 1 %/min when determining the modulus, then 5 %/min or 50 %/min depending on ductility; with non-linear behaviour the results are only nominal. Machine (5): conforming to ISO 7500-1 and ISO 9513; speeds from Table 1 — 1, 2, 5 and 10 mm/min (± 20 %) and 20–500 mm/min (± 10 %), 1 mm/min for specimens 1–3.5 mm thick; radius of the loading edge R1 = 5.0 ± 0.2 mm, radius of the supports R2 = 2.0 ± 0.2 mm for thicknesses up to 3 mm and 5.0 ± 0.2 mm above; parallelism ± 0.2 mm. Test types (Table 2): I — only stress and strength; II — also strains above 1 %; III — repeatable modulus (crosshead displacement with compliance correction, ISO 9513 class 2); IV — true and precise modulus (deflectometer, ISO 9513 class 1), needed for interlaboratory comparisons; force — ISO 7500-1 class 1. A modulus from uncorrected crosshead displacement is "not suitable"; for type IV the deflection is measured to an accuracy of 1 % (± 3.4 µm at L = 64 mm and h = 4.0 mm), for type III — 2 % (± 6.8 µm).

STATUS (status of PKN cards as of 08.10.2026). The card of PN-EN ISO 178:2019-06 (English version) has no withdrawal label: title "Plastics -- Determination of flexural properties" (our translation), publication 13-06-2019, 36 pages, price group R, Chemistry Sector, KT 141 Plastics, "Introduces: EN ISO 178:2019 [IDT], ISO 178:2019 [IDT]", "Replaces: PN-EN ISO 178:2011 – English version, PN-EN ISO 178:2011/A1:2013-06 – English version, PN-EN ISO 178:2011 – German version", ICS 83.080.01. Card of PN-EN ISO 178:2011 (English version): "Withdrawn and replaced by PN-EN ISO 178:2019-06", publication 27-01-2011, withdrawal 13-06-2019, it introduced EN ISO 178:2010 and ISO 178:2010 and replaced PN-EN ISO 178:2006 (Polish version). Card of the amendment PN-EN ISO 178:2011/A1:2013-06 (English version): "Withdrawn and replaced by PN-EN ISO 178:2019-06", publication 26-06-2013, withdrawal 13-06-2019, it introduced ISO 178:2010/Amd 1:2013; scope of the amendment, in full (our translation): "The content of clauses 5.5.1.1. and 5.5.1.2, describing, respectively, the way of measuring the thickness and the width of the test specimen, was changed. Figure 5 illustrating the way of determining the dimensions of the test specimen was changed". The current edition has only an English version; the 2006 edition had a Polish version.

HOW MANY LABORATORIES AND IN WHAT FORM (copy of the accreditation scope database and current PCA documents, read on 08.10.2026). Number 178 appears in 12 records at 11 laboratories; in the current PCA documents (issues from 16.07.2025 to 06.10.2026) — in 12 entries at the same laboratories (AB 163 has two: strength and modulus); no entry is suspended. Notations: "PN-EN ISO 178:2019-06" — AB 001, AB 005 (alongside PN-EN ISO 14125), AB 067, AB 163 (alongside PN-EN ISO 14125); in one entry the 2019-06 edition and the 2011 edition with amendment A1:2013-06 — AB 044 (with EN ISO 178 and ISO 178 of both editions), AB 1340 and AB 1640 (at AB 1640 "flexural strength" with 2011/A1, "modulus of elasticity" with 2019-06); only the 2011 edition with amendment A1 — AB 1376 and AB 1808 (at AB 1808 "Method A", alongside PN-EN ISO 11296-4:2011 Annex B); without a year — AB 023 ("Method: A") and AB 128. The "Laboratories" tab searches for records beginning with the term entered; the term of this entry "PN-EN ISO 178:" (with a colon) shows 9 laboratories — without AB 023 and AB 128, which write the number without a year. Without the colon the term would show 58 laboratories, mostly with standards whose numbers begin with 178 (PN-EN ISO 17852, PN-EN ISO 17892-1…-12 and others).

WHAT THE LABORATORIES TEST (PCA documents). Flexural strength, flexural stress, flexural modulus of elasticity, deflection (AB 001), strain at maximum stress (AB 128); bending test. Subject: plastics and plastic products, polymer composites (AB 005), plastic pipes and fittings (AB 163), solid insulating materials (AB 044), electrotechnical materials and products (AB 067 — in one entry with several standards, bending up to 10 kN). Force ranges: up to 2.5 kN (AB 1376), up to 25 kN (AB 1640), up to 30 kN (AB 128), up to 50 kN (AB 023, AB 1808), up to 100 kN (AB 163), up to 300 kN (AB 005), up to 1000 kN (AB 1340).

WHERE A SEEMINGLY CORRECT RESULT IS EASY TO GET. First — modulus from crosshead displacement: according to 5.4.1 uncorrected crosshead displacement is not suitable for determining the modulus, because it includes the compliance of the machine (play and deformation of the grips, the load train and the load cell); a repeatable modulus requires a compliance correction or — which the standard ranks higher — a deflectometer. Second — test type: for comparisons between laboratories type IV is needed (deflectometer, ISO 9513 class 1); type III is sufficient only for tracking one's own results over time. Third — the edition: the 2011 edition with amendment A1:2013 (AB 1376, AB 1808, some of the entries of AB 044, AB 1340, AB 1640) does not have the calibration requirements by test type introduced in 2019. Fourth — specimen dimensions: results from specimens other than 80 mm × 10 mm × 4 mm or cut from products may be non-comparable (1); for injection-moulded semi-crystalline plastics the result also depends on the thickness of the oriented surface layer. Fifth — material outside the scope: laminates with fibres longer than 7.5 mm are tested according to ISO 14125 (described in a separate entry of this database), and rigid cellular plastics — according to ISO 1209.

WHAT WE DO NOT GIVE. We do not have clauses 6–11 — we do not give the number of specimens, the conditioning, the span for other specimens, the way of determining the speed, the formulae for stress, strain and modulus, the precision or the content of the report; we do not know the compliance correction procedure (Annex C).

Method principle

A specimen of rectangular cross-section (preferred 80 mm × 10 mm × 4 mm, span 16 times the thickness) is deflected at midspan by a loading edge with a radius of 5 mm at a constant speed — in method A 1 %/min, in method B 1 %/min up to the modulus, then 5 or 50 %/min — until it breaks or until a strain of 5 %, measuring force and deflection; the flexural strength, the stress at the conventional deflection 1.5h and the flexural modulus between strains of 0.000 5 and 0.002 5 are calculated from the curve (ISO 178:2019, clauses 3 and 4).

Applications

• Rigid and semi-rigid plastics (thermoplastics, thermosets, sheets, also filled and reinforced with fibres up to 7.5 mm) — flexural strength and modulus; 11 laboratories in PCA scopes
• Quality control and comparison of materials (not design parameters)
• Solid insulating materials, plastic pipes and fittings, polymer composites — according to the scopes of the laboratories

Key parameters

Standard

Standard number

PN-EN ISO 178:2019-06 (wersja angielska; wprowadza EN ISO 178:2019 [IDT] i ISO 178:2019 [IDT])

Title (PL)

Tworzywa sztuczne — Oznaczanie właściwości przy zginaniu

Title (EN)

Plastics — Determination of flexural properties

Step-by-step procedure

1. Specimens

   Preferred specimens 80 × 10 × 4 mm, moulded (ISO 293, ISO 294-1, ISO 295, ISO 10724-1) or machined (ISO 2818, ISO 20753); conditioning according to clause 7 — WE DO NOT GIVE.

2. Setting up

   Span 16 times the thickness, radii R1 and R2 according to the thickness; test type (I–IV) chosen according to the results needed (Table 2).

3. Bending

   Method A: 1 %/min until break or 5 %; method B: 1 %/min up to the modulus, then 5 or 50 %/min; record force and deflection (with a deflectometer or with the crosshead and compliance correction).

4. Result

   Flexural strength, stress at sC, modulus between 0.000 5 and 0.002 5; WE DO NOT GIVE the formulae (clause 9). PN-EN ISO 178:2019-06 current (status of PKN cards as of 08.10.2026).

Required equipment and apparatus

Health and safety (OHS)

• Brittle plastics break with fragments — guard the test zone according to the machine instructions
• TECHNICAL NOTE: the modulus is not determined from uncorrected crosshead displacement; for interlaboratory comparisons a deflectometer is needed (type IV)

Frequently asked questions