ASTM D790: Flexural Properties Testing of Unreinforced Plastics – Full Guide

What Is ASTM D790?

ASTM D790 is the standard test method for determining the flexural properties of unreinforced and reinforced plastics, high-modulus composites, and electrical insulating materials. It uses a three-point loading system to apply a bending load to a simply supported rectangular beam specimen, measuring the material’s response to bending forces across its outer fiber.

Flexural testing per ASTM D790 is one of the most widely performed mechanical characterization tests in plastics quality control and product development. It provides engineers with quantitative data on how a plastic material will perform when subjected to bending loads in real-world applications — from smartphone housings and automotive dashboards to PCB substrates, window lineals, and structural insulating panels.

Why Flexural Properties Matter

Many engineering applications subject plastic components to bending forces during service — not just tension or compression. A structural beam deflecting under distributed load, a PCB substrate flexing under mechanical or thermal stress, an automotive trim panel resisting an impact load — all rely on the material’s ability to resist bending without fracture or excessive deflection.

Flexural properties as determined by ASTM D790 are particularly useful for:

• Material selection: Identifying the most appropriate polymer grade or composite for a given bending load condition
• Quality control and specification compliance: Verifying that incoming materials or molded parts meet defined flexural property thresholds
• Comparative ranking: Objectively comparing competing materials during product development
• Design validation: Providing inputs for finite element analysis (FEA) of parts under bending loads
• Regulatory documentation: Supporting material qualification in regulated industries

Key Flexural Properties Measured by ASTM D790

Industries That Depend on ASTM D790 Testing

Automotive

ASTM D790 flexural data is routinely required for dashboard components, door panels, bumper fascia materials, under-hood polymer parts, and load-bearing structural plastic components across OEM and Tier 1 supplier qualification programs.

Aerospace

Lightweight composite laminates and polymer-matrix materials used in aircraft secondary structures are characterized for flexural properties to confirm structural adequacy under aerodynamic and mechanical loading.

Electronics and Electrical

PCB substrates, enclosure materials, and electrical insulating plastics must maintain structural rigidity under mechanical and thermal cycling. ASTM D790 is a standard qualification requirement in many electronic component and material specifications.

Medical Devices

Instrument panel housings, structural polymer chassis, and device enclosures in medical equipment are tested per ASTM D790 to confirm required stiffness and strength before regulatory submission and product launch.

Infrastructure and Building Products

Plastic pipes, window profiles, roofing panels, and decking materials are tested for flexural properties to verify compliance with building code requirements and product performance specifications.

ASTM D790 Test Procedure: Step by Step

Specimen Preparation and Conditioning

ASTM D790 specimens are rectangular bars that may be injection molded, extruded, or cut from sheets, plates, or shapes. Specimen dimensions depend on material thickness:

• Molded materials (standard): 3.2 mm thick × 12.7 mm wide × 127 mm long; support span = 51.2 mm (16:1 span-to-depth ratio)
• Sheet materials (1.6–3.2 mm thick): Width = 12.7 mm; support span = thickness × 16
• Thin sheet materials (<1.6 mm thick): 50.8 mm × 12.7 mm; support span = 25.4 mm
• Sheet materials (>3.2 mm thick): Width and span proportioned to thickness

Specimens must be conditioned at standard laboratory conditions (23°C ± 2°C, 50% ± 5% RH) for a minimum of 40 hours prior to testing to eliminate residual processing stresses and ensure reproducible moisture content.

Test Equipment Setup

A universal testing machine (UTM) equipped with a three-point bending fixture is used. The fixture consists of two cylindrical lower support spans and a central loading nose. ASTM D790 specifies minimum radii for both the loading nose and the supports to prevent stress concentration artifacts. The specimen is placed symmetrically across the two lower supports and the loading nose is positioned precisely at the midspan.

Procedure A vs. Procedure B

Procedure A uses a crosshead rate calculated as R = ZL²/6d, where R = crosshead rate (mm/min), Z = strain rate (0.01 mm/mm/min), L = support span (mm), d = specimen depth (mm). Procedure A is recommended for all flexural property measurements, particularly flexural modulus.

Procedure B uses a higher crosshead speed — ten times the Procedure A rate — and is specified only for measurement of flexural strength, not modulus. The faster rate may introduce inertia artifacts that reduce modulus accuracy.

Test Execution and Data Collection

The crosshead advances at the calculated rate while load and deflection are continuously recorded. The test continues until the specimen fractures, yields, or reaches 5.0% strain — whichever comes first. Materials that do not fracture or yield within the 5% strain limit are reported with a stress-at-5%-strain value, as the true failure load was not reached within the test boundaries.

Calculation of Results

From the load-deflection data:

• Flexural Stress: σf = 3PL / (2bd²)
• Flexural Modulus: Ef = L³m / (4bd³)

Where P = applied load (N), L = support span (mm), b = specimen width (mm), d = specimen depth (mm), m = slope of initial linear region (N/mm).

ASTM D790 vs. ISO 178: Key Differences

Both ASTM D790 and ISO 178 are three-point flexural test methods for plastics, and results are often cited interchangeably in material data sheets — but important technical differences exist:

The ISO 178 requirement for deflectometer-based modulus measurement generally produces more accurate modulus values than ASTM D790 crosshead-only measurement, because it eliminates machine compliance from the deflection reading.

Conclusion

ASTM D790 flexural testing is not merely a quality control checkbox — it is a fundamental engineering characterization that directly informs material selection, structural design, and regulatory qualification decisions for plastic and composite components across virtually every manufacturing industry. The data it produces — flexural modulus, flexural strength, and stress-at-strain values — defines the boundary between a component that performs reliably in service and one that deflects excessively or fractures unexpectedly under load. Testing with a qualified, accredited laboratory ensures those results carry the traceability and procedural rigor that regulatory bodies, customers, and internal quality systems demand.

Why Choose Infinita Lab for ASTM D790 Flexural Testing?

Infinita Lab is a trusted USA-based testing laboratory offering comprehensive ASTM D790 flexural testing and full-scope polymer mechanical testing across a nationwide network of 2,000+ accredited partner labs. Our specialists support automotive, aerospace, electronics, medical device, and industrial plastics testing programs with fast turnaround, ASTM-compliant procedures, and clear, actionable reports.

Infinita Lab’s SPOC model ensures seamless coordination of your testing program — from sample receipt and conditioning through testing, calculation, and report delivery — with no administrative burden on your engineering team.

Looking for a trusted partner to achieve your research goals? Schedule a meeting with us, send us a request, or call us at (888) 878-3090 to learn more about our services and how we can support you. Request a Quote

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