ASTM D790 – Flexural Properties of Plastics
A complete lab guide to the 3-point bend test for unreinforced and reinforced plastics: specimen geometry, 16:1 span, loading-nose radius, Procedures A and B, flexural strength and modulus calculation, and compliant reporting – engineered around Testometric UK universal testing machines and supported by FITCO India.
Overview & scope
ASTM D790 defines the test methods for determining the flexural properties of unreinforced and reinforced plastics and electrical insulating materials using a three-point bending configuration. It is maintained by ASTM Committee D20 on Plastics and is a core QC and material-qualification reference for moulders, compounders, and NABL/ISO 17025-accredited laboratories across India.
The test reports flexural strength, flexural modulus, and stress at a defined strain, characterising how a rigid plastic resists bending — a property closely tied to part stiffness in service.
Tip: ASTM D790 covers three-point bending. For four-point loading (where peak stress is distributed over a central region), use ASTM D6272 instead.
What does it measure?
Flexural strength (MPa), flexural modulus of elasticity (MPa or GPa), and flexural stress at 5% strain for ductile materials that do not break. It does not measure tensile or impact behaviour — see ASTM D638 and ASTM D256.
Sample materials
Polypropylene (PP), ABS, polycarbonate (PC), nylon (PA6/PA66), POM, PBT, glass-fibre reinforced compounds, thermosets, and electrical insulating laminates — including recycled and compounded grades for incoming QC.
Industries & applications
Automotive & mobility
Structural and trim mouldings where stiffness and load-bearing performance are specified by the OEM.
Electrical & electronics
Insulating laminates and housings — flexural modulus governs rigidity under mounting loads.
Polymer compounding
Glass-fibre and filler loading is verified quickly via flexural modulus shifts in QC.
Consumer & appliances
Rigid components and panels where bending stiffness drives perceived quality and durability.
Specimen & fixture geometry
The standard bar specimen is moulded or machined with smooth, parallel faces. The support span is set to 16 times the specimen depth (16:1) unless the material requires an adjusted ratio.
| Parameter | Typical value | Note |
|---|---|---|
| Specimen length | ~127 mm | 125–200 mm depending on thickness/modulus |
| Specimen width (b) | 12.7 mm | Measure at the midspan |
| Specimen depth (d) | 3.2 mm | 3.2–6.4 mm range |
| Support span (L) | ~51 mm | 16 × depth for a 3.2 mm bar |
| Loading-nose radius | 5 mm ± 0.1 mm | Central loading nose |
| Support radius | 5 mm typical | Both supports equal |
Tip: Always recompute and reset the span when the specimen thickness changes — using the wrong span is the most common cause of non-comparable flexural data between batches.
Specimen preparation
- Mould or machine bars with smooth, defect-free surfaces and parallel faces; deburr cut edges.
- Inspect for sink marks, voids, and weld lines in the central span — discard defective specimens.
- Condition at 23°C ± 2°C and 50% ± 5% RH per ASTM D618 unless the specification states otherwise.
- Measure width and depth at the midspan with a calibrated micrometer (≤ 0.01 mm).
- Test at least five specimens; use ten for anisotropic or highly variable materials.
Fixture & instrumentation
- Use a rigid three-point bend fixture with a central loading nose (5 mm radius) and two adjustable supports of equal radius.
- Select a load cell so peak force falls between 10% and 90% of rated capacity for best resolution.
- Measure midspan deflection from crosshead travel for modulus, or use a deflectometer for higher accuracy on stiff materials.
- Sample at a rate sufficient to capture the peak cleanly, especially for brittle or filled compounds.
Test procedure (Procedures A & B)
ASTM D790 defines two procedures distinguished by strain rate. Procedure A (0.01 mm/mm/min) is preferred and used for both modulus and strength. Procedure B (0.10 mm/mm/min) is used for flexural strength when a material does not break by 5% strain under Procedure A.
- Record temperature, humidity, and specimen identification.
- Verify load-cell calibration; set and confirm the 16:1 support span for the measured depth.
- Centre the specimen on the supports with the loading nose at midspan; zero load and position.
- Apply load at the crosshead rate computed for Procedure A (or B), recording the full load–deflection curve.
- Continue to break, or to 5% strain for ductile materials, per the selected procedure.
- Export flexural strength, modulus, and stress at 5% strain with the curve for traceability.
| Aspect | Procedure A | Procedure B |
|---|---|---|
| Strain rate (Z) | 0.01 mm/mm/min | 0.10 mm/mm/min |
| Primary use | Modulus & strength (preferred) | Flexural strength only |
| When to use | Default for most materials | Materials that don't break by 5% strain under A |
Calculations & outputs
- Flexural stress: σ = 3PL / (2bd²) — P = load, L = support span, b = width, d = depth. Result in MPa.
- Flexural strain: ε = 6Dd / L² — D = midspan deflection.
- Flexural modulus (tangent): E_B = L³m / (4bd³) — m = slope of the initial linear region of the load–deflection curve.
- Crosshead rate: R = Z L² / (6d) — Z = 0.01 (Procedure A) or 0.10 (Procedure B) mm/mm/min.
Example (L = 51 mm, b = 12.7 mm, d = 3.2 mm): at P = 100 N, flexural stress = (3 × 100 × 51) / (2 × 12.7 × 3.2²) = 15,300 / 260.1 = 58.8 MPa. Procedure A crosshead rate = (0.01 × 51²) / (6 × 3.2) = 1.35 mm/min.
Crosshead rate guidance
- Always compute the crosshead rate from R = ZL²/(6d) for the actual span and depth — it is not a fixed value.
- Procedure A uses Z = 0.01 mm/mm/min; Procedure B uses Z = 0.10 mm/mm/min.
- Document the procedure, span, and computed rate in every report.
Tip: Reporting a flexural result without the span ratio and procedure makes it impossible to compare across labs — both materially change the measured values.
Reporting requirements
- Material identification, grade, lot/batch number, and preparation method.
- Specimen dimensions, support span and span ratio, number tested, and any rejected specimens with reasons.
- Conditioning, temperature and humidity, procedure (A or B), and computed crosshead rate.
- Results with statistics: flexural strength, flexural modulus, stress at 5% strain (mean, SD, CV%).
ASTM D790 vs ISO 178 — key differences
Both determine flexural properties in three-point bending, but specimen size and rate conventions differ, so results are not directly numerically comparable. Use the standard cited by your customer or market.
| Parameter | ASTM D790 | ISO 178 |
|---|---|---|
| Primary market | Americas, global OEM | Europe, Asia, global OEM |
| Typical specimen | 127 × 12.7 × 3.2 mm | 80 × 10 × 4 mm |
| Span ratio | 16:1 (adjustable) | 16:1 (span 64 mm for 4 mm bar) |
| Rate basis | Strain rate via R = ZL²/6d | Fixed test speed (e.g. 2 mm/min) |
| Conditioning | ASTM D618 | ISO 291 |
Recommended Testometric setup
Frame
Testometric X-Series (X250/X350/X500) twin-column UTM with precise low-speed crosshead control for accurate flexural modulus.
Fixture
Three-point bend fixture with 5 mm loading nose and adjustable equal-radius supports for quick span changes.
Deflection
Crosshead or deflectometer-based midspan deflection integrated with WinTest Analysis.
Software
WinTest Analysis with ASTM D790 templates, automatic R = ZL²/6d rate calculation, and audit-ready exports.
- ±0.5% load accuracy and 0.000001 mm position resolution for tight modulus reporting.
- Fine low-speed control for sub-2 mm/min Procedure A rates.
- Quick-change three-point and four-point fixtures; span set against the measured depth.
- Optional environmental chambers for elevated/low-temperature flexural testing.
- FITCO India support: installation, operator training, spares, after-sales, and 2-year comprehensive warranty.
Model suggestions for flexural testing
| X-Series model | Force capacity | Typical use |
|---|---|---|
| X250-5 | 5 kN | Commodity plastics (PP, ABS) flexural QC |
| X350-10 | 10 kN | Engineering thermoplastics and laminates |
| X500-25 | 25 kN | Glass-fibre reinforced and high-stiffness compounds |
Specifications vary by configuration; contact FITCO India for a tuned method and fixtures aligned to ASTM D790.
FAQs
When do I use Procedure B instead of A?
Use Procedure B (0.10 mm/mm/min) for flexural strength when a ductile material does not break by 5% strain under the preferred Procedure A.
What support span should I set?
16:1 by default — 16 times the measured specimen depth. Recompute and reset the span whenever thickness changes.
Is the crosshead speed fixed?
No. It is computed from R = ZL²/(6d) for the actual span and depth, so it changes with specimen geometry.
Is ASTM D790 the same as ISO 178?
Both are three-point flexural tests but use different specimens and rate conventions, so results are not directly comparable. Use the standard your specification cites.
How many specimens are required?
At least five; use ten for anisotropic or highly variable materials per the governing specification.
Related Standards
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