3-Point vs. 4-Point Bend Testing: ASTM D790, D6272 and C1161 Explained
Three-point and four-point bend tests both measure flexural strength and modulus, but they load the specimen very differently, and picking the wrong one (or the wrong span) is one of the most common reasons flexural data doesn't match a data sheet. This guide explains the difference, then walks through the three most-searched flexural standards, ASTM D790, ASTM D6272 and ASTM C1161, with spans, speeds, formulas, the right bend test fixtures and videos from our lab.
Quick answer: 3-point or 4-point?
3-point bend: one loading nose in the center. Simple, fast and the basis for most plastics data sheets (ASTM D790, ISO 178). Peak stress sits in a narrow line under the nose.
4-point bend: two loading noses. The section between them sees constant bending moment and no shear, so a larger volume of material is tested. Preferred for brittle materials (ceramics, glass), composites and any material where flaws control strength (ASTM D6272, C1161, C1609).
Rule of thumb: use the standard your customer or material spec names. If you have a choice, 3-point for routine plastics QC, 4-point for brittle materials and design data.
How 3-point and 4-point loading differ
Stress distribution: in 3-point bending the maximum moment occurs only directly under the center nose. In 4-point bending the moment is constant over the whole inner span.
Shear: 3-point bending combines bending with shear everywhere; the inner span of a 4-point test is in pure bending. That matters for laminates and composites that can fail in shear instead of flexure.
Strength values: 4-point tests usually give lower, more representative strengths for brittle materials, because more of the specimen (and more flaws) is highly stressed. Don't mix 3-point and 4-point results in the same comparison.
Contact damage: splitting the load over two noses reduces local crushing and indentation, useful for soft or brittle surfaces.
Setup: 3-point is quicker to align. 4-point fixtures need both noses to contact evenly, which is why articulating fixtures exist.
ASTM D790: 3-point flexural properties of plastics
ASTM D790 is the most widely used flexural test for plastics and electrical insulating materials. Key setup values:
Specimen: molded bars are typically 127 × 12.7 × 3.2 mm (5 × 0.5 × 0.125 in).
Span: support span = 16 × thickness (16:1 span-to-depth), so about 51 mm for a 3.2 mm bar. Higher ratios (32:1 or 40:1) are used for some laminates.
Loading nose and supports: 5 mm radius unless otherwise specified.
Speed: crosshead rate R = Z·L² / (6·d). Procedure A uses Z = 0.01 mm/mm/min (about 1.4 mm/min for a 3.2 mm bar on a 51 mm span) and is preferred; Procedure B uses Z = 0.10 mm/mm/min (about 13.6 mm/min) when the specimen does not break at the slower rate.
End of test: break, or 5 % outer-fiber strain. Materials that reach 5 % strain without breaking are outside D790; consider ASTM D6272 or tensile testing to ASTM D638.
Formulas: flexural stress σ = 3PL / (2bd²); flexural strain ε = 6Dd / L²; modulus from the slope of the initial linear region.
Our 3-point bend fixture has bottom spans adjustable from 4 to 150 mm and comes in a 2.5 kN aluminum or 10 kN steel version:
ASTM D6272: 4-point flexural properties of plastics
ASTM D6272 is the 4-point counterpart to D790 for plastics and composites. It is used when you want a region of constant moment, for stiff or reinforced materials, or when your customer specifies it.
Spans: the load span is either one-third of the support span (third-point loading) or one-half of the support span. Span-to-depth is 16:1, with 32:1 and 40:1 options.
Formulas: for third-point loading σ = PL / (bd²); for a load span of one-half the support span σ = 3PL / (4bd²).
Speed: calculated from the outer-fiber strain rate, the same way as D790.
Fixture: our 4-point bend fixture has adjustable roller spans in 2.5 kN aluminum or 10 kN steel.
ASTM C1161: flexural strength of advanced ceramics
Ceramics can't be gripped for tensile testing without breaking at the grips, so flexural strength is the standard measure. ASTM C1161 uses small bars and precise fixtures:
Specimen sizes: A = 2 × 1.5 × 25 mm, B = 4 × 3 × 45 mm (the most common), C = 8 × 6 × 90 mm (width × depth × length).
Fixtures: outer spans of 20, 40 and 80 mm for sizes A, B and C. Four-point quarter-point loading (inner span half the outer span) is preferred over 3-point.
Articulation: ceramic bars aren't perfectly flat or parallel, so articulating fixtures let the rollers seat evenly and avoid twisting the specimen. See our articulating bend fixture.
Formula (4-point, quarter-point): σ = 3PL / (4bd²).
Speed: slow, typically around 0.5 mm/min for size B, so the bar breaks in a few seconds without dynamic effects.
Ceramic bars store a lot of energy, so expect violent breaks. Read our full ASTM C1161 guide, and watch what happens:
Other flexural and bend standards
ASTM C393: core shear properties of sandwich panels by beam flexure (3- or 4-point).
ASTM D2344: short-beam strength (interlaminar shear) of composites, a 3-point test with a very short span.
ASTM D7264: flexural properties of polymer matrix composites (3- or 4-point, 32:1 span-to-thickness).
ASTM C158: flexural testing of glass.
ASTM E290: bend testing of metals for ductility (guided and semi-guided bends).
VDA 238-100: tight-radius plate bend test for automotive sheet metal.
How to choose a bend test fixture
Capacity: small 3- and 4-point fixtures in 2.5 kN aluminum or 10 kN steel for plastics and ceramics; 50 kN medium bend fixtures with spans from 20 mm to over 1,000 mm for boards, panels and composites; 100 kN heavy bend fixtures (designs up to 600 kN) for metals.
Span range: make sure one fixture covers the spans your standards call for (for example 51 mm for D790 and 40 mm for C1161 size B).
Roller and nose radius: 5 mm is standard for D790; other standards specify different radii. Interchangeable rollers save buying a second fixture.
Deflection measurement: crosshead displacement includes machine and fixture compliance. For accurate modulus, use compliance correction or a deflectometer under the specimen.
Articulation: required for ceramics and helpful for any specimen that isn't perfectly flat.
For higher-capacity work, here is our 10,000 lb 3-point bend fixture:
Common flexural testing mistakes
Wrong span-to-depth ratio: short spans add shear and give low strength and modulus. Re-set the span for every thickness.
Wrong nose radius: a small radius indents soft plastics and causes early failure under the nose.
Running at a fixed crosshead speed: D790 and D6272 set the speed from strain rate, so speed changes with span and thickness.
Mixing 3-point and 4-point results: they are not interchangeable, especially for brittle materials.
Ignoring failure mode: shear or compression failures under the nose are not valid flexural failures; record and report them.
Measuring dimensions once: thickness is squared in every formula, so measure it carefully at several points.
Flexural testing FAQ
What is the difference between a 3-point and a 4-point bend test? A 3-point test loads the specimen with one central nose, so peak stress is at a single line. A 4-point test uses two noses, creating a region of constant bending moment with no shear between them.
Which is more accurate, 3-point or 4-point bending? Both are accurate when run to their standard. 4-point bending gives more representative strength for brittle and flaw-sensitive materials; 3-point is simpler and is the basis of most plastics data sheets.
What span do I use for ASTM D790? Support span = 16 × specimen thickness, about 51 mm for a 3.2 mm bar.
What is the D790 test speed? R = Z·L² / (6·d), with Z = 0.01 mm/mm/min for Procedure A (about 1.4 mm/min for a standard bar).
Is ASTM D790 the same as ISO 178? They are similar 3-point tests but use different specimens and speeds, so results are not directly interchangeable.
Related reading
Get the right bend fixture
Tell us your standard, specimen size and expected load and we'll recommend the right 3-point, 4-point or articulating fixture for your machine. See all bend testing fixtures, our universal testing machines, or contact us for a quote.
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