🔩 Determination of flexural properties of fibre-reinforced plastic composites — three-point (method A) and four-point (method B) flexure of a freely supported beam: flexural strength, flexural stress and deflection at break, flexural modulus, PN-EN ISO 14125

Materials and NDT PN-EN ISO 14125

Overview

WHAT THE STANDARD COVERS. Scope from the PKN catalogue card of PN-EN ISO 14125:2001 (Polish version), in full (our translation): “A method is specified for determining flexural properties under three-point and four-point loading of thermoplastic and thermosetting plastics reinforced with fibres. Standard test specimens are specified and factors used for alternative specimen dimensions are given. Ranges of test speeds are given. The method may be used for classifying materials or for quality-control testing”. Scope from the card of amendment PN-EN ISO 14125:2001/A1:2011 (English version), in full (our translation): “The introduction and the normative references in Clause 2 have been changed, an additional paragraph has been introduced into 4.9; Subclauses 10.1.2 and 10.2.2 and Clause 12 on page 13 have been changed”. We read the text in the sample of ISO 14125:1998 from the cover to 6.2.2 (preparation of specimens from plates) with the table of contents and in the sample of amendment ISO 14125:1998/Amd.1:2011 in full; we do not have Clauses 6.2.3–12 or Annexes A (other specimens) and B (large-deflection corrections).

ACCORDING TO THE TEXT OF ISO 14125:1998 (foreword, introduction, Clauses 1–6.2.2, table of contents). The standard was prepared by ISO/TC 61 “Plastics”, SC 13 “Composites and reinforcement fibres”; Annexes A and B are normative. Introduction: the standard is based on ISO 178 but deals with fibre-reinforced composites — it retains the conditions for glass-fibre systems and extends them to four-point loading (method B) and to composites with carbon and aramid fibres; sources: ASTM D 790 (four-point), prEN 2562 (test conditions), CRAG 200 and JIS K 7074 (shims in four-point loading); the overall specimen length for four-point loading is the same as for three-point loading; the scope of ISO 178 was to be limited to unreinforced and filled plastics, and EN 63:1977 (three-point test of glass-reinforced plastics) — withdrawn. Scope (1): method A — three-point flexure, method B — four-point; standard specimens and parameters for alternative sizes; a range of test speeds; the method is not suitable for determining design parameters, but for screening materials or quality control (the flexural modulus is lower than the tensile modulus because of the additional deflection from shear — the span/thickness ratio is chosen to minimize this effect); for fibre-reinforced thermoplastic and thermosetting composites; unreinforced, particle-filled plastics and those reinforced with short fibres (below 1 mm) — according to ISO 178; specimens moulded, machined from the multipurpose specimen (ISO 3167) or from products; results from specimens of other dimensions or prepared differently may not be comparable, and for materials not homogeneous through the section the result applies only to the thickness and structure tested. References (2, as worded in 1998): ISO 178:1993, 291:1997, 293:1986, 294-1:1996, 295:1991, 1268:1974, 2602:1980, 2818:1994, 3167:1993, 5893:1993. Principle (3): the specimen, supported as a beam, is deflected at a constant rate until fracture or a predetermined deformation, measuring force and deflection; the geometry is to limit shear deformation and avoid interlaminar shear failure; in four-point loading the bending moment between the central loading members is constant, and contact stresses are lower than under the single loading member of the three-point test; the inner span is one third of the outer span, and the outer span is the same as in three-point loading, so the same specimen can be used. Definitions (4): speed of testing v (mm/min), flexural stress σf (MPa, from equation (3) or (8) in Clause 10), stress at break σfB, flexural strength σfM (at maximum load, for acceptable failure modes — 9.9 and Figure 6), deflection s, deflection at break sB, deflection at flexural strength sM, flexural strain εf, flexural (chord) modulus Ef — the ratio of the stress difference to the strain difference εf″ = 0,002 5 and εf′ = 0,000 5 (with computer-aided equipment a linear regression between these points is permitted), interlaminar shear modulus G13 (for materials with mainly in-plane reinforcement of the order of 3 000–6 000 MPa), specimen axes 1 (along the fibres, 0°) and 2 (transverse, 90°). Apparatus (5): test machine according to ISO 5893; recommended speeds 0,5, 1, 2, 5, 10 mm/min (tolerance ± 20 %) and 20, 50, 100, 200, 500 mm/min (± 10 %); radius of the loading member R1 = 5 ± 0,2 mm, radius of the supports R2 = 2 ± 0,2 mm for h ≤ 3 mm and 5 ± 0,2 mm for h > 3 mm; axes parallel, span L adjustable; error of the indicated force and deflection not more than ± 1 % of full scale; deflection from crosshead movement is corrected for loading-train deflection and indentation at the loading points; micrometer reading to 0,01 mm (width b and thickness h), callipers accurate to 0,1 % of the span. Specimens (6): dimensions according to the material standard or preferred; test direction in one of the principal axes, with strong anisotropy testing in both is recommended, and with loading in another direction — in that direction. Preferred specimens, method A (length l / span L / width b / thickness h, mm): class I (discontinuous-fibre-reinforced thermoplastics) 80 / 64 / 10 / 4; class II (mats, fabrics, DMC, BMC, SMC, GMT) 80 / 64 / 15 / 4; class III (transverse unidirectional, unidirectional and multidirectional 0° with 5 < Ef1/G13 ≤ 15, e.g. glass) 60 / 40 / 15 / 2; class IV (unidirectional and multidirectional with 15 < Ef1/G13 ≤ 50, e.g. carbon) 100 / 80 / 15 / 2. Method B (l / L / L′ / b / h): class I 80 / 66 / 22 / 10 / 4; class II 80 / 66 / 22 / 15 / 4; class III 60 / 45 / 15 / 15 / 2; class IV 100 / 81 / 27 / 15 / 2. For coarse reinforcement a width of 25 mm is permitted. The thickness in the central third of the length (method A) or over the whole length (method B) shall not deviate from the mean by more than 2 %, the width by more than 3 %; rectangular cross-section, without rounded edges. Other specimens — dimensions according to Tables A.1 and A.2. Preparation: moulding compounds — according to the material standard or compression or injection moulding according to ISO 293, 294-1, 295; plates — machining according to ISO 2818. Amendment Amd.1:2011 (read in full): new Clause 2 with undated references (among others ISO 1268, all parts — methods of producing test plates); a paragraph added to 4.9: for materials with a failure strain of less than 0,002 5 (e.g. high-modulus carbon-fibre-reinforced plastics), the strain difference for the modulus is reduced to 0,001 0 − 0,000 5; in 10.1.2 and 10.2.2 the deflections s′ and s″ are calculated accordingly for εf′ = 0,000 5 and εf″ = 0,002 5 or 0,001 0; the report (Clause 12, item n) gives the equation and the strain range used.

STATUS (PKN catalogue cards as of 02.10.2026). The card of PN-EN ISO 14125:2001 (Polish version, published 15-10-2001, 23 pages, price group L) has no “Withdrawn” header: Chemistry Sector, KT 168 Plastics Products, ICS 83.120, “Introduces: EN ISO 14125:1998 [IDT], ISO 14125:1998 [IDT]”, additional elements PN-EN ISO 14125:2001/AC:2003 (Polish version; we did not read the card of the corrigendum — we do not give its content) and PN-EN ISO 14125:2001/A1:2011 (English version). The card of amendment A1:2011 (published 18-04-2011, 5 pages, price group C) has no “Withdrawn” header, “Introduces: EN ISO 14125:1998/A1:2011 [IDT], ISO 14125:1998/Amd 1:2011 [IDT]”. The PKN search engine (query “PN-EN ISO 14125”, 02.10.2026) also shows the standards PN-EN 14125 (editions 2005, 2013-10 and 2025-08) — on a different subject, unrelated to this method. What A1 changes for the result: for materials with a failure strain below 0,002 5 the modulus is calculated in the range 0,000 5–0,001 0 — under the wording before A1, a specimen of such a material breaks before reaching a strain of 0,002 5, so the modulus in that range cannot be determined, and the report must state the strain range used.

HOW MANY LABORATORIES AND IN WHAT FORM (copy of the accreditation scope database, data up to 17.09.2026, read 02.10.2026). The number 14125 appears in 7 records of 6 laboratories: AB 001, AB 005, AB 023, AB 163, AB 1490, AB 1640. In the current scopes on the PCA website (read 02.10.2026, issues from 16.07.2025 to 23.06.2026) AB 001, AB 163 and AB 1640 cite “PN-EN ISO 14125:2001” (AB 1640 with the note “method A”), AB 005 — “PN-EN ISO 14125:2001+AC:2003+A1:2011”, and AB 023 and AB 1490 — the designation alone without a date. The “Laboratories” tab (“PN-EN ISO 14125”) shows 5 laboratories — without AB 005, whose field begins with PN-EN ISO 178 — checked with a query of the tab on the labcoda.pl production server on 02.10.2026.

WHAT THE LABORATORIES TEST (PCA entries). “Flexural strength” (AB 005: maximum load 300 kN; AB 163: 100 kN), “Flexural stress at break”, “Modulus of elasticity in flexure” (AB 023: up to 50 kN, method A), “Stiffness modulus” and a bending test in a force range of 0–200 kN (AB 1490), “Flexural properties” of glass-fibre-reinforced composites (AB 1640, method A); objects: plastics, polymer composites, plastic pipes and fittings.

WHERE A SEEMINGLY CORRECT RESULT IS EASY TO GET. With dimensions: results from specimens of other dimensions, another class or prepared differently may not be comparable (1.5) — the specimen class depends on the material, and in classes III and IV on the Ef1/G13 ratio. With deflection: deflection from crosshead movement must be corrected for the deflection of the loading train and indentation under the loading members (5.1.4) — without correction the modulus comes out too low. With the support radius: for h ≤ 3 mm R2 = 2 mm, for h > 3 mm R2 = 5 mm (Table 2). With the modulus: for materials breaking before a strain of 0,002 5 the range from A1:2011 has to be used and stated in the report. With failure: the strength is calculated only for acceptable failure modes (4.4, 9.9) — failure by interlaminar shear does not give a flexural strength. With interpretation: the result is not a design parameter (1.2).

WHAT WE DO NOT GIVE. We do not have 6.2.3–12 or the annexes: the number of specimens, conditioning, the test procedure (including 9.9 on failure modes), equations (3) and (8) for stress and the equations for the modulus, large-deflection corrections (Annex B), precision and the full content of the report — so we give no equation or these values.

Method principle

A specimen of fibre-reinforced composite, freely supported as a beam, is deflected at a constant rate by one loading member at mid-span (method A, three-point) or two loading members one third of the span apart (method B, four-point), recording force and deflection to fracture or to a specified strain; from the curve the flexural strength, the stress and deflection at break and the chord flexural modulus are calculated.

Applications

• Plastic composites reinforced with glass, carbon or aramid fibres (laminates, SMC, BMC, GMT, reinforced thermoplastics) — flexural strength and modulus; 6 laboratories in PCA scopes
• Classification of materials and quality control of batches (scope of the standard)
• Pipes, fittings and products of reinforced plastics — together with PN-EN ISO 178 for unreinforced plastics

Key parameters

Standard

Standard number

PN-EN ISO 14125:2001 (wersja polska) z PN-EN ISO 14125:2001/AC:2003 i PN-EN ISO 14125:2001/A1:2011 (wersja angielska); EN ISO 14125:1998 IDT, ISO 14125:1998 IDT

Title (PL)

Kompozyty tworzywowe wzmocnione włóknem — Oznaczanie właściwości przy zginaniu

Title (EN)

Fibre-reinforced plastic composites — Determination of flexural properties

Step-by-step procedure

1. Specimens

   Machine or mould the preferred specimens for the material class (Table 3 or 4) in a principal axis; check the thickness (2 %) and width (3 %) tolerances. PN-EN ISO 14125:2001 with AC:2003 and A1:2011 current (PKN cards as of 02.10.2026).

2. Conditioning and measurement of dimensions

   Conditioning according to Clause 8 (we do not have its content; ISO 291 is referenced); measure b and h with a micrometer to 0,01 mm, span L to 0,1 %.

3. Setting up the fixture

   Method A — span L, one loading member; method B — spans L and L′ = L/3; choose R2 according to the specimen thickness; correct deflection for the loading train.

4. Bending

   Deflect at a constant speed from Table 1 to fracture or a specified strain, recording force and deflection; assess the failure mode (9.9 — we do not have its content).

5. Calculation and report

   Stress, strength and modulus according to Clause 10 (we do not have the equations); modulus between 0,000 5 and 0,002 5 or — for materials breaking earlier — 0,000 5 and 0,001 0; the report states the equation and strain range (A1:2011).

Required equipment and apparatus

Health and safety (OHS)

• The standard contains no safety warnings in the part we read; carbon and glass composites fracture in a brittle way with fragments — shielding of the test area according to the machine instructions
• Machining composite plates produces fibre dust — extraction and respiratory protection according to the laboratory’s risk assessment

Frequently asked questions