ISO 14125
Flexural Properties of Fibre-Reinforced Composites

Downloads: ISO 14125 Composite Flexure Fixture – Universal Grip Data Sheet (PDF)

Written by Chris Thomas, Universal Grip Company · Reviewed by Frank Bertini · Updated October 24, 2026

ISO 14125 is the international standard for the flexural properties of fibre-reinforced plastic composites. A flat bar is supported near its ends and bent at constant speed, either by a single central loading nose (Method A, three-point) or by two noses at the third points of the span (Method B, four-point), while force and deflection are recorded. The results are flexural stress, flexural strength, flexural strain at failure and the flexural modulus between 0.05 and 0.25 % strain. It is the composites counterpart of ISO 178, which covers unreinforced and short-fibre plastics, and the ISO twin of ASTM D7264. Carbon and glass laminates, SMC, BMC, GMT, mat and fabric composites for aerospace, automotive, wind and marine work are all screened and quality-checked against it.

What makes ISO 14125 different from every other flexure standard is that the specimen and the span depend on the material class: four classes, from short-fibre thermoplastics to high-modulus carbon, with span-to-thickness ratios from 16 to 40. Get the class wrong and the specimen fails in interlaminar shear instead of bending, which the standard explicitly rejects. This guide gives the full specimen and span tables for both methods, the roller radii, the speed arithmetic, every formula with a worked example, the valid and invalid failure modes, a numeric comparison with ASTM D7264 and D790, and the fixture, deflection measurement and machines Universal Grip supplies for the test. The bend testing page gives the lab overview and the ISO standards hub lists every ISO method we equip.

In this guide: What ISO 14125 Covers and What It Measures · Editions: 1998, Corrigendum 2001, Amendment 2011 · The Four Material Classes · Specimen and Span Tables for Method A and Method B · Fixture Geometry: Roller Radii, Span Setting and Shims · Test Speed: the 1 %/min Rule and the Table · Conditioning and Step-by-Step Procedure · Formulas and a Worked Example · Valid and Invalid Failures, Large Deflections and Common Errors · ISO 14125 vs ASTM D7264 vs ASTM D790 vs ISO 178 · Equipment for ISO 14125: Fixture, Deflection Measurement, Machines · FAQ

What ISO 14125 Covers and What It Measures

The standard applies to fibre-reinforced thermoplastic and thermoset composites in sheet or moulded form: discontinuous-fibre thermoplastics, mat, fabric and mixed reinforcements including DMC, BMC, SMC and GMT, and unidirectional and multidirectional laminates of glass, carbon or aramid. Unreinforced, particle-filled and short-fibre (under 1 mm) plastics are tested to ISO 178 instead. The standard says plainly that the results are for screening and quality control and are not suitable for design parameters; design allowables for laminates come from tension (ISO 527-4, ISO 527-5), compression and shear tests, with flexure used as a cheap, fast comparison between batches, resins and layups.

• Flexural stress σf: the nominal stress in the outer fibre at mid-span, calculated from simple beam theory.

• Flexural strength σfM: the maximum flexural stress sustained during the test.

• Flexural strain εf: the nominal strain in the outer fibre, calculated from the mid-span deflection.

• Flexural modulus Ef: the chord modulus between flexural strains of 0.0005 and 0.0025 (0.05 % and 0.25 %), from the deflections that correspond to those strains.

• Flexural strain at break or at maximum force: the strain at which the specimen fails.

Two loading methods are allowed. Method A is three-point bending with one loading nose at mid-span; it is simpler, needs less material and is the usual choice for glass composites and quality control. Method B is four-point bending with the two loading noses at the third points of the span, so the central third of the beam sees a constant bending moment with no shear force and no contact stress at the point of maximum stress. It is preferred for high-modulus carbon laminates whose outer plies would otherwise crush under a single nose, and it gives a more representative modulus. Both methods use the same specimens; only the spans and the equations change.

Editions: 1998, Corrigendum 2001, Amendment 2011

The only edition is ISO 14125:1998, published in March 1998, with Technical Corrigendum 1 of 2001 and Amendment 1 of January 2011. The standard was reviewed and confirmed again in December 2024, so the 1998 text plus the two add-ons is the current document. The European consolidated version is EN ISO 14125:1998 + AC:2002 + A1:2011, which is why German and British catalogues show DIN EN ISO 14125:2011 and BS EN ISO 14125:1998+A1:2011. There is no 'ISO 14125:2011' edition: a page that names one is referring to the amendment.

Amendment 1 corrected the flexural-modulus symbol and the stress and strain expressions, undated the normative references, replaced the text before the Method A and B equations, specified the edge strains for high-modulus carbon-fibre composites and expanded the report item on the method. A lab procedure written from the bare 1998 PDF that circulates online is missing those changes.

The Four Material Classes

Everything in ISO 14125 hangs on the material class, which is chosen from the reinforcement format and, for continuous-fibre laminates, from the ratio of the flexural modulus along the fibres to the interlaminar shear modulus G13 (typically 3,000 to 6,000 MPa). The class sets the specimen size, the span-to-thickness ratio and the support radius:

• Class I: discontinuous-fibre-reinforced thermoplastics, for example glass-filled nylon, PP or PBT moulding compounds. Short span (16:1), the stiffest of the ratios because the shear modulus is high relative to the flexural modulus.

• Class II: mat, continuous mat, fabric and mixed-format composites, including DMC, BMC, SMC and GMT. Also 16:1.

• Class III: transverse (90°) unidirectional laminates, and 0° unidirectional or multidirectional laminates with 5 < Ef1/G13 ≤ 15, which in practice means glass-fibre systems. Span 20:1.

• Class IV: 0° unidirectional or multidirectional laminates with 15 < Ef1/G13 ≤ 50, which means carbon-fibre systems. Span 40:1, because the high fibre modulus against a low interlaminar shear modulus would otherwise put the beam into shear failure before the outer fibres reach their strength.

A simple decision tree: short-fibre thermoplastic moulding → Class I; SMC, BMC, GMT, chopped mat or woven glass laminate → Class II; glass unidirectional or quasi-isotropic glass laminate → Class III; carbon or other high-modulus laminate → Class IV. Picking a class that is too low for a carbon laminate is the single most common ISO 14125 error: the specimen delaminates under the nose, the force-deflection curve rolls over early, the 'strength' is low and the result is invalid. The standard's remedy is to move to a longer span, and clause 9.2 lists 16, 20, 40 and 60 as the permitted span-to-thickness ratios when shear failures occur.

Specimen and Span Tables for Method A and Method B

The preferred specimens are small flat bars, 2 or 4 mm thick, cut from moulded sheet or laminate. Dimensions below are length × width × thickness in millimetres with the preferred span L, and for Method B the outer span L and inner (loading) span L′ = L/3. Tolerances: length +10/−0 mm, width ±0.5 mm, thickness ±0.2 mm, spans ±1 mm.

Method A (three-point), Table 3

• Class I: 80 × 10 × 4 mm, span 64 mm, L/h = 16.

• Class II: 80 × 15 × 4 mm, span 64 mm, L/h = 16.

• Class III: 60 × 15 × 2 mm, span 40 mm, L/h = 20.

• Class IV: 100 × 15 × 2 mm, span 80 mm, L/h = 40.

Method B (four-point, third-point loading), Table 4

• Class I: 80 × 10 × 4 mm, outer span 66 mm, inner span 22 mm, L/h = 16.5.

• Class II: 80 × 15 × 4 mm, outer span 66 mm, inner span 22 mm, L/h = 16.5.

• Class III: 60 × 15 × 2 mm, outer span 45 mm, inner span 15 mm, L/h = 22.5.

• Class IV: 100 × 15 × 2 mm, outer span 81 mm, inner span 27 mm, L/h = 40.5.

A 25 mm width is allowed for coarse reinforcements such as heavy woven roving or long-fibre SMC where a 15 mm bar would cut through too few tows. Annex A scales the geometry for other thicknesses while keeping the same span-to-thickness ratios (16/16/20/40 for Method A, 16.5/16.5/22.5/40.5 for Method B) and overall-length-to-thickness ratios of 20/20/30/50, with a width table that grows with thickness: for Classes II to IV, 15 mm up to 10 mm thick, 30 mm for 10 to 20 mm, 50 mm for 20 to 35 mm and 80 mm for 35 to 50 mm. A 10 mm thick Class II pultrusion, for example, is tested as a 200 × 15 × 10 mm bar on a 160 mm span. The overhang beyond each support is whatever the length tolerance gives; there is no separate overhang rule as there is in ASTM D790.

Specimens are machined per ISO 2818 from sheet moulded per ISO 1268, edges parallel and free of delamination. Width and thickness are measured at the centre to 1 %. Thickness must not vary by more than ±2 % over the central third (Method A) or the full span (Method B), width by no more than 3 %, and specimens more than ±2 % from the set mean thickness are discarded. Record the mould face, because the report states which face contacted the nose.

Fixture Geometry: Roller Radii, Span Setting and Shims

Table 2 of the standard sets the contact geometry. The loading member has a radius of 5 ± 0.2 mm (a 10 mm diameter roller). The supports have a radius of 2 ± 0.2 mm (4 mm diameter) when the specimen is 3 mm thick or less, and 5 ± 0.2 mm when it is thicker than 3 mm. So the 2 mm Class III and IV laminates run on an R5 nose over R2 supports, and the 4 mm Class I and II bars on R5 over R5. The rollers must be longer than the specimen is wide and parallel to each other; the span is set within 1 % of the nominal value and measured to 0.2 %. A fixture with pinned, interchangeable rollers in both radii and a scaled rail for the supports covers all four classes and both methods without re-shimming.

Contact stress under the nose is why the radii matter: a carbon laminate at 1,500 MPa carries only a few hundred newtons on a 15 × 2 mm section, but concentrated on a line, and a sharper nose crushes the top plies and starts a compression failure under the nose instead of in the free span. The standard allows a 0.2 mm polypropylene cushion under the noses, and Method B exists largely to move the loading points away from the point of maximum stress. A scored or flattened roller changes the effective radius and the result.

Deflection is measured at mid-span. The simplest route is crosshead travel, which works for a stiff fixture and a short, thick specimen but includes machine and fixture compliance; for the modulus of a 2 mm Class IV bar, where the 0.05 to 0.25 % strain window is only a millimetre of deflection, a deflectometer or a video extensometer tracking the mid-span point gives a cleaner chord. The standard requires deflection to be measured to ±1 % of full scale and force to ±1 % (an ISO 5893 class 1 machine).

Test Speed: the 1 %/min Rule and the Table

ISO 14125 lists the permitted crosshead speeds as 0.5, 1, 2, 5 and 10 mm/min with a ±20 % tolerance, and 20, 50, 100, 200 and 500 mm/min with ±10 %. The speed is chosen to give an outer-fibre strain rate of about 1 %/min, using v = ε′L²/(6h) for Method A and v = ε′L²/(4.7h) for Method B with ε′ = 0.01 per minute, then rounded to the nearest table value. The arithmetic for the preferred specimens:

• Method A, Class I and II (L 64, h 4): 0.01 × 64² / (6 × 4) = 1.7 → 2 mm/min.

• Method A, Class III (L 40, h 2): 0.01 × 40² / 12 = 1.3 → 1 mm/min.

• Method A, Class IV (L 80, h 2): 0.01 × 80² / 12 = 5.3 → 5 mm/min.

• Method B, Class I and II (L 66, h 4): 0.01 × 66² / (4.7 × 4) = 2.3 → 2 mm/min.

• Method B, Class III (L 45, h 2): 0.01 × 45² / 9.4 = 2.2 → 2 mm/min.

• Method B, Class IV (L 81, h 2): 0.01 × 81² / 9.4 = 7.0 → 5 mm/min (the nearest table value; 10 mm/min is also within the spirit of the rule and some material specifications call it).

A single speed is used for the whole test, modulus and strength alike, unlike ISO 178 Method B, where the speed changes after the modulus segment. Faster speeds raise the apparent strength of most resins by a few percent per decade, so a speed outside the table is not comparable to published data.

Conditioning and Step-by-Step Procedure

Specimens are conditioned as the material specification requires, otherwise per ISO 291 in the standard atmosphere, and tested in that atmosphere. Elevated or reduced temperatures are allowed by agreement with the fixture inside an environmental chamber. At least five valid specimens are tested; more if a 95 % confidence interval is wanted (ISO 2602). The procedure:

• Assign the material class and cut the specimens to the Table 3 or 4 size, recording orientation and the moulded face.

• Measure width and thickness at the centre to 0.01 mm; check the ±2 % thickness and 3 % width uniformity rules and discard outliers.

• Fit the correct rollers (R5 nose; R2 supports for h ≤ 3 mm, R5 above) and set the span to the nominal value within 1 %. For Method B set the inner span to one third of the outer span.

• Place the specimen symmetrically with the recorded face against the nose; add the 0.2 mm polypropylene cushion if the material specification calls for it.

• Set the speed from the 1 %/min rule, apply a small preload to seat the specimen, zero deflection, and run at constant speed to failure or to the agreed strain.

• Record force and mid-span deflection continuously. Compute the stresses at the two modulus deflections, the strength and the strain at failure.

• Inspect the failure. Tension or compression failure in the outer plies away from the loading points is valid; failure that starts as interlaminar shear is not, and the set is repeated at a longer span. Specimens that break outside the central third (Method A) or outside the loading noses (Method B) are discarded and replaced.

• Report the 18 items of Clause 12: standard, method, class, specimen dimensions and orientation, span, radii, cushion, speed, which face was loaded, conditioning, the equations used, individual and mean results with standard deviation, failure type and date.

Formulas and a Worked Example

With F the force in newtons (for Method B, F is the total force; each nose carries F/2), L the outer span, b the width and h the thickness in millimetres, and s the mid-span deflection:

Method A (three-point)

• Flexural stress: σf = 3FL / (2bh²).

• Flexural strain: εf = 6sh / L².

• Deflections for the modulus window: s′ = 0.0005 L² / (6h) and s″ = 0.0025 L² / (6h).

• Flexural modulus: Ef = (L³ / 4bh³) × (ΔF/Δs) between s′ and s″, or equivalently Ef = 500 (σ″f − σ′f).

Method B (four-point, third-point loading)

• Flexural stress: σf = FL / (bh²).

• Flexural strain: εf = 4.7sh / L².

• Deflections for the modulus window: s′ = 0.0005 L² / (4.7h) and s″ = 0.0025 L² / (4.7h).

• Flexural modulus: Ef = 0.21 (L³ / bh³) × (ΔF/Δs), or Ef = 500 (σ″f − σ′f).

The 4.7 and 0.21 come from third-point beam theory (the mid-span deflection of a beam loaded at L/3 and 2L/3 is 23FL³/1296EI), which is why they differ from the 4.36 and 0.17 in ASTM D7264's quarter-point equations. Using the ASTM constants on an ISO 14125 Method B test, or vice versa, shifts the modulus by about 8 % and the strain by 7 %.

Worked example: Class IV carbon laminate, Method A

Specimen 100 × 15.0 × 2.00 mm, span 80 mm, speed 5 mm/min. The modulus deflections are s′ = 0.0005 × 6400 / 12 = 0.267 mm and s″ = 0.0025 × 6400 / 12 = 1.333 mm. The forces read there are 24.0 N and 120.0 N. σ′f = 3 × 24 × 80 / (2 × 15 × 4) = 48.0 MPa and σ″f = 3 × 120 × 80 / 120 = 240.0 MPa, so Ef = 500 × (240 − 48) = 96,000 MPa. Cross-check with the slope form: L³/(4bh³) = 512,000 / 480 = 1,067 mm⁻¹ and ΔF/Δs = 96 / 1.067 = 90.0 N/mm, giving 1,067 × 90.0 = 96,000 MPa. The specimen fails at 400 N: σfM = 3 × 400 × 80 / 120 = 800 MPa. If the deflection at failure is 12.0 mm, the nominal strain is 6 × 12 × 2 / 6400 = 2.25 %; because 12 mm exceeds 0.1 × L = 8 mm, the large-deflection corrections of Annex B apply and should be reported with the result.

The same laminate under Method B on the 81/27 mm spans at 600 N total force gives σf = 600 × 81 / (15 × 4) = 810 MPa; a slightly higher strength from Method B is typical of carbon laminates because the constant-moment zone removes the contact-stress penalty. Stress and modulus are reported to three significant figures, deflection to two.

Valid and Invalid Failures, Large Deflections and Common Errors

Figure 6 of the standard shows the acceptable failures: fracture of the tension face, crushing or buckling of the compression face, or both, starting remote from the loading points. The unacceptable failure is one that initiates as interlaminar shear, seen as a horizontal crack at mid-thickness, delamination spreading from under the nose, or a force-deflection curve that drops in steps while the specimen stays in one piece. A shear-initiated result is not a flexural strength; the fix is a longer span (clause 9.2 allows 16, 20, 40 or 60) or Method B. Deflections larger than 0.1 L put the beam into large-deflection territory, where the support reactions develop horizontal components; Annex B gives corrected stress and strain equations, with versions that include roller friction. For a 2 mm Class IV bar that is any deflection beyond 8 mm, which many carbon laminates exceed before they break.

• Wrong class: a carbon laminate run as Class II at 16:1 delaminates early; the reported 'strength' can be 30 to 50 % low and is invalid.

• Span measured to the roller edge, not the axis: a 5 mm error on an 80 mm span shifts stress by 6 % and modulus by 19 % (the L³ term).

• Thickness error: h appears squared in stress and cubed in modulus, so a 0.05 mm error on a 2 mm laminate is 5 % in stress and 7.5 % in modulus. Measure at the centre with a 0.01 mm micrometer.

• Wrong rollers: R3 supports from an older D7264 kit, or R5 supports on a 2 mm laminate, change the contact stress and are not the ISO geometry; report the radii used.

• Modulus from crosshead on a thin bar: fixture compliance can be a third of the 1 mm deflection window; use a deflectometer or correct for compliance.

• Mixed equations: ISO third-point and ASTM quarter-point constants are not interchangeable (8 % on modulus).

• Face orientation: the mould face and the machined face bend differently; load the same face every time and report it.

• No cushion on soft matrices: nose indentation on a thermoplastic Class I bar reads as extra deflection and lowers the modulus.

ISO 14125 vs ASTM D7264 vs ASTM D790 vs ISO 178

The four flexure standards share the beam equations and differ in specimen, span and loading geometry, so results are similar but not interchangeable:

• ISO 14125: composites only; four classes; 3-point 16/16/20/40:1 or 4-point third-point 16.5/16.5/22.5/40.5:1; preferred bars 80 × 10 × 4, 80 × 15 × 4, 60 × 15 × 2, 100 × 15 × 2 mm; R5 nose, R2 or R5 supports; 0.5–500 mm/min table at 1 %/min; chord modulus 0.05–0.25 % strain; ≥5 specimens.

• ASTM D7264: composites; standard span-to-thickness 32:1 (16, 20, 40, 60 optional); specimen 4 mm thick × 13 mm wide, length about 20 % longer than the span; Procedure A 3-point, Procedure B 4-point with the loading span at half the support span (quarter-point); nose and support radius 5.0 ± 0.1 mm in the current editions (3.0 mm in the 2007 edition, which is why older fixtures carry R3 rollers); 1.0 mm/min; chord modulus 0.1–0.3 % strain; stress for Procedure B is 3PL/(4bh²).

• ASTM D790: plastics and composites; 16:1 (32, 40, 60 for anisotropic materials); 127 × 12.7 × 3.2 mm bar on a 51.2 mm span; R5 nose and supports; crosshead rate ZL²/(6d) with Z = 0.01, about 1.3 mm/min; test ends at 5 % strain for materials that do not break; tangent, secant or chord modulus.

• ISO 178: plastics; 80 × 10 × 4 mm bar on a 64 mm span (16:1); R5 nose, R5 or R2 supports; 2 mm/min for modulus; chord modulus 0.05–0.25 %; stress at 3.5 % strain for non-breaking materials. Class I and II ISO 14125 specimens are the same bar on the same span, which is why short-fibre data are often quoted to either standard.

A laboratory that tests to both ISO 14125 and D7264 needs rollers in R5 and R2 for ISO, R5 (and legacy R3) for ASTM, and a fixture whose loading noses can be set at one third and one half of the support span. EN 2562 (carbon) and EN 2746 (glass) are the aerospace analogues.

Equipment for ISO 14125: Fixture, Deflection Measurement, Machines

The Universal Grip ISO 14125 bend fixture is a combined three- and four-point flexure fixture for composites. Support and loading anvils slide along scaled rails for fast span setting, three-point and four-point loading members are supplied in one system, and interchangeable pinned rollers of different radii give the Table 2 geometry (R5 nose; R2 and R5 supports) with hardened, ground contact surfaces. It is rated 10 kN standard with higher capacities on request, built in stainless steel with hardened tool-steel rollers, and ships with clevis-pin or threaded adapters to suit your frame. When ordering, specify the loading method, span range, specimen width and thickness, roller radii, capacity and machine connection. The same fixture base covers ASTM D7264, ASTM D790 and, with the short-span inserts, ISO 14130 and ASTM D2344 short-beam shear.

Capacity. Flexural forces are small: a Class II glass SMC bar at 600 MPa on the 64 mm span needs F = 2bh²σ/(3L) = 2 × 15 × 16 × 600 / 192 = 1,500 N, and a Class IV carbon bar at 1,500 MPa on the 80 mm span only 750 N. A 10 mm thick Class II pultrusion on a 160 mm span at 600 MPa needs about 3.8 kN. A 5 or 10 kN load cell gives the resolution the modulus segment needs; the fixture's 10 kN rating is sized for thick laminates and for the Annex A scaled specimens.

Deflection. For strength, crosshead travel is acceptable on a stiff frame; for modulus, especially on 2 mm Class III and IV bars whose modulus window is under 1.1 mm of deflection, use a deflectometer at mid-span or a video extensometer tracking a mark on the specimen edge, or run a compliance calibration with a steel bar and subtract it. Our extensometer range includes deflection transducers for bend fixtures.

Machines. A 5 kN or 10 kN benchtop UTM handles every preferred specimen; a 25 kN frame adds headroom for thick laminates and shares the fixture with tensile work on ISO 527-4 coupons. Hot and cold testing uses the fixture inside an environmental chamber with the deflectometer brought out through the port. Specimen preparation, cutting and edge finishing are covered on the bend testing page.

ISO 14125 FAQ

What is ISO 14125?

ISO 14125 is the international standard for determining the flexural properties of fibre-reinforced plastic composites by three-point (Method A) or four-point third-point (Method B) bending. It reports flexural stress, flexural strength, flexural strain and the 0.05–0.25 % chord flexural modulus. The edition is ISO 14125:1998 with Corrigendum 1:2001 and Amendment 1:2011, confirmed in 2024.

What are the ISO 14125 material classes?

Class I: discontinuous-fibre thermoplastics. Class II: mat, fabric and mixed reinforcements including DMC, BMC, SMC and GMT. Class III: transverse unidirectional, and 0° or multidirectional laminates with 5 < Ef1/G13 ≤ 15 (glass systems). Class IV: 0° or multidirectional laminates with 15 < Ef1/G13 ≤ 50 (carbon systems). The class sets specimen size and span-to-thickness ratio.

What are the ISO 14125 specimen dimensions and spans?

Method A: Class I 80 × 10 × 4 mm on 64 mm; Class II 80 × 15 × 4 mm on 64 mm; Class III 60 × 15 × 2 mm on 40 mm; Class IV 100 × 15 × 2 mm on 80 mm. Method B uses the same bars with outer/inner spans of 66/22, 66/22, 45/15 and 81/27 mm. Tolerances: length +10/−0, width ±0.5, thickness ±0.2, spans ±1 mm.

What roller radii does ISO 14125 require?

The loading nose is R5 ± 0.2 mm (10 mm diameter). Supports are R2 ± 0.2 mm for specimens 3 mm thick or less and R5 ± 0.2 mm for thicker specimens. So 2 mm Class III/IV laminates run on R5/R2 and 4 mm Class I/II bars on R5/R5.

What test speed does ISO 14125 use?

A speed from the table 0.5, 1, 2, 5, 10 (±20 %) or 20, 50, 100, 200, 500 mm/min (±10 %) chosen to give about 1 %/min outer-fibre strain: v = 0.01 L²/(6h) for Method A and 0.01 L²/(4.7h) for Method B. That gives 2 mm/min for Classes I and II, 1 mm/min for Class III and 5 mm/min for Class IV under Method A.

What is the difference between Method A and Method B?

Method A is three-point bending with one nose at mid-span; stress is 3FL/(2bh²). Method B is four-point bending with two noses at the third points (inner span = L/3), giving a constant-moment zone with no contact stress at the point of maximum stress; stress is FL/(bh²), strain 4.7sh/L² and modulus 0.21(L³/bh³)(ΔF/Δs). Method B is preferred for high-modulus carbon laminates.

How is the flexural modulus calculated in ISO 14125?

As the chord between flexural strains of 0.0005 and 0.0025: Ef = 500 (σ″f − σ′f), where the two stresses are read at the deflections s′ = 0.0005 L²/(6h) and s″ = 0.0025 L²/(6h) for Method A (divide by 4.7h instead of 6h for Method B). For an 80 mm span and 2 mm thickness the deflections are 0.267 and 1.333 mm.

Which failures are valid in ISO 14125?

Tension-face fracture, compression-face crushing or buckling, or both, starting away from the loading points, are valid. Failure that initiates as interlaminar shear (a mid-thickness horizontal crack or delamination from under the nose) is not acceptable; the span-to-thickness ratio is increased (16, 20, 40 or 60 are allowed) or Method B is used. Breaks outside the central third are discarded.

How many specimens does ISO 14125 require?

At least five valid specimens per material and direction; more if a 95 % confidence interval is required per ISO 2602. Results are reported individually with the mean and standard deviation, stress and modulus to three significant figures.

What is the difference between ISO 14125 and ASTM D7264?

D7264 uses a single 32:1 span-to-thickness ratio (16, 20, 40 or 60 optional) with a 4 × 13 mm specimen, quarter-point four-point loading (load span = half the support span), R5 nose and supports, 1 mm/min and a 0.1–0.3 % modulus window. ISO 14125 uses class-dependent 16:1 to 40:1 spans, third-point loading (inner span = L/3), R5/R2 or R5/R5 rollers, speeds from the 1 %/min rule and a 0.05–0.25 % window. The Method B/Procedure B equations differ (4.7 vs 4.36, 0.21 vs 0.17).

Can ISO 14125 results be used as design values?

No. The standard states the method is for screening and quality control and is not suitable for determining design parameters, because flexural strength combines tension, compression and shear and depends on span, thickness and loading geometry. Design allowables come from ISO 527-4/527-5 tension, compression and shear tests.

What fixture capacity is needed for ISO 14125?

Very little: a 4 mm glass SMC bar at 600 MPa on a 64 mm span needs about 1.5 kN and a 2 mm carbon bar at 1,500 MPa on 80 mm about 750 N. A 10 kN fixture with a 5 or 10 kN load cell covers all preferred specimens and thick Annex A laminates with margin; Universal Grip's ISO 14125 fixture is rated 10 kN standard.

Need a three- and four-point flexure fixture, deflectometer or a complete system for ISO 14125? Contact Universal Grip with your material class, laminate thickness, expected strength and machine connection and we will quote the right setup.