ASTM D790 Flexural Testing Machine: Plastic Bending Test Method, Specimen Preparation, and Flexural Properties

1. Introduction to ASTM D790

Flexural testing is an important method for evaluating the bending performance of plastic materials. In many industrial applications, plastics are not exposed only to direct tension or compression. They also experience bending loads when used in structural components, housings, panels, supports, and mechanical parts. ASTM D790 is a widely used standard for determining the flexural properties of unreinforced and reinforced plastics, high-modulus composites, and electrical insulating materials. It uses a three-point loading arrangement in which a rectangular specimen is supported at two points and loaded at its midpoint. During the test, a Universal Testing Machine applies a controlled downward force through a loading nose. The specimen bends under the applied force, and the machine records the load and corresponding deflection. The resulting data can be used to evaluate flexural stress, flexural strength, flexural strain, and flexural modulus. The results are useful for material comparison, quality control, product development, and specification verification. They help engineers understand how a material behaves when subjected to bending rather than direct tensile or compressive loading. ASTM D790 is particularly relevant to rigid and semi-rigid plastics. It is also used for reinforced plastic materials and electrical insulating materials when the specimen and test configuration are appropriate. A properly configured Universal Testing Machine is essential for obtaining consistent results. The machine must provide stable loading, an appropriate force measurement range, suitable bending fixtures, and reliable deflection measurement.

2. Scope and Purpose of the Standard

ASTM D790 covers the determination of flexural properties using a three-point loading system applied to a simply supported rectangular specimen. The specimen rests on two supports, while a loading nose applies force midway between them. The standard is generally applicable to rigid and semi-rigid materials. However, flexural strength cannot be determined in the usual way for materials that do not break or yield at the outer surface within the specified strain limit of the method. The main objectives of ASTM D790 testing include:

• Determining the flexural strength of plastic materials.
• Measuring flexural modulus and material stiffness.
• Evaluating the relationship between applied load and specimen deflection.
• Comparing different plastic grades and material formulations.
• Monitoring production quality and material consistency.
• Supporting product development and engineering material selection.
• Verifying material performance against technical specifications.

ASTM D790 includes two procedures. Procedure A is generally intended for materials that break at comparatively small deflections and is commonly used for flexural modulus measurement. Procedure B is designed for materials that undergo larger deflections and may be used for flexural strength determination where appropriate. The applicable material specification takes priority when it provides specific requirements for specimen preparation, conditioning, dimensions, or testing parameters. ASTM D790 should not be treated as interchangeable with every other bending standard. For example, four-point bending uses a different loading arrangement. The correct method must be selected according to the material and testing objective.

3. Principle of Three-Point Bending

The three-point bending test evaluates the response of a specimen when a load is applied at its midpoint while the specimen is supported at both ends. The specimen is positioned horizontally on two support rollers. A loading nose moves downward and applies force at the center of the specimen. As the load increases, the specimen bends and develops tensile stress on one outer surface and compressive stress on the opposite outer surface. The maximum bending moment in a simply supported specimen under a central load occurs at the midpoint. This is also the location of maximum bending stress in the conventional beam model. A Universal Testing Machine measures the applied force. Deflection is measured through crosshead position or a suitable deflection-measuring device, depending on the test configuration. For the conventional three-point bending arrangement, the nominal flexural stress can be expressed as:

σf = 3PL / 2bd²

Where:

• σf: Nominal flexural stress.
• P: Applied load.
• L: Support span.
• b: Specimen width.
• d: Specimen thickness or depth.

For the conventional small-deflection beam calculation, nominal flexural strain can be expressed as:

εf = 6Dd / L²

Where D represents the measured midspan deflection. These equations are based on the assumptions and limitations of the applicable beam model. The standard provides the detailed calculation requirements, including considerations for large deflections and particular specimen conditions. Laboratories should use the equations and procedures in the current official ASTM D790 edition rather than relying on simplified formulas alone.

4. Materials Covered by ASTM D790

ASTM D790 can be used to evaluate the flexural properties of many unreinforced and reinforced plastic materials.

4.1 Polypropylene (PP)

Polypropylene is commonly used in automotive components, packaging, household products, and industrial parts. Flexural testing helps evaluate its stiffness and resistance to bending deformation. The flexural response can vary according to polymer grade, filler content, processing conditions, and specimen orientation.

4.2 Polyethylene (PE)

Polyethylene is used in pipes, containers, sheets, and molded products. Its bending behavior depends on density, molecular structure, and formulation. For materials that undergo substantial deformation without breaking, the test result must be interpreted according to the limitations of the selected procedure.

4.3 ABS

ABS is used in electronic housings, automotive interiors, appliance components, and consumer products. Flexural testing can help compare material grades and evaluate the effect of additives or manufacturing changes.

4.4 Polycarbonate (PC)

Polycarbonate is known for its toughness and impact resistance. Flexural testing provides information about its bending stiffness, strength, and deformation behavior.

4.5 Polyamide (PA)

Polyamide is used in gears, mechanical components, fasteners, and automotive parts. Moisture absorption can influence its mechanical behavior, so conditioning is important when comparing results.

4.6 Fiber-Reinforced Plastics

Glass-fiber-reinforced and other composite plastics are frequently tested to evaluate their flexural performance. Their results are influenced by fiber orientation, fiber content, matrix properties, and laminate structure. For highly anisotropic laminates, conventional homogeneous beam calculations may not represent the actual stress distribution completely. The material's reinforcement architecture must be considered when interpreting results.

4.7 Electrical Insulating Materials

Rigid electrical insulating plastics and related materials can be evaluated using flexural testing to assess their mechanical performance under bending loads.

5. Specimen Preparation and Geometry

Specimen preparation has a direct influence on ASTM D790 test results. The test specimen is generally a solid rectangular bar with a consistent cross-section. Specimens may be injection molded or cut from molded sheets, extruded sheets, plates, or other suitable molded shapes. The preparation method should be consistent with the applicable material specification. Important specimen considerations include:

• Specimen width: Width is used in flexural stress calculations and should be measured accurately.
• Specimen thickness: Thickness has a significant influence on calculated stress and strain.
• Specimen length: The overall length must be sufficient for the selected support span and fixture arrangement.
• Support span: The distance between the two support points must be configured according to the standard and specimen dimensions.
• Surface condition: Scratches, edge defects, and machining damage may cause premature failure.
• Specimen orientation: Mold-flow direction, extrusion direction, and fiber orientation can influence bending performance.

Specimens should be prepared with uniform dimensions and without unnecessary surface damage. For reinforced plastics, cutting direction and fiber orientation should be documented because they may produce significant differences in flexural properties. The support span-to-depth ratio is another important consideration. An incorrect span can change the stress distribution and the contribution of shear deformation. The standard and relevant material specification should be consulted for exact specimen dimensions and test conditions. A single specimen configuration should not be assumed to be appropriate for every plastic material.

6. Procedure A and Procedure B

ASTM D790 provides two test procedures to accommodate different material deformation behaviors.

6.1 Procedure A

Procedure A is designed principally for materials that break at comparatively small deflections. It is generally used for flexural property measurements, particularly flexural modulus, unless the material specification states otherwise. The procedure is suitable when the specimen response and deflection remain within the assumptions of the method. For modulus measurement, accurate deflection data is important. The test configuration should minimize errors caused by machine compliance, fixture movement, and inaccurate specimen positioning.

6.2 Procedure B

Procedure B is designed particularly for materials that undergo larger deflections during testing. It may be used for flexural strength measurement when appropriate. Large deflections can influence the relationship between applied force, bending moment, and specimen geometry. The calculation method must therefore account for the applicable procedure and its limitations.

6.3 Choosing the Correct Procedure

The choice between Procedure A and Procedure B depends on the material behavior and the required test result. Before testing, laboratories should review:

• The material specification and required test procedure.
• Whether the specimen is expected to break at small or large deflections.
• Whether flexural modulus or flexural strength is the primary measurement.
• The specimen dimensions and support span.
• The required deflection measurement method.

Procedure A is commonly selected for modulus measurement. Procedure B may be appropriate for flexural strength testing of materials that undergo larger deflections. The selected procedure must be reported with the results.

7. Universal Testing Machine Requirements

A Universal Testing Machine used for ASTM D790 should provide controlled loading and accurate measurement of the force and specimen deflection.

7.1 Appropriate Force Capacity

The required force capacity depends on the material strength, specimen dimensions, and support span. Plastic flexural tests are often performed using lower force capacities than heavy metal tensile tests. Common machine configurations may include 1 kN, 5 kN, 10 kN, and 20 kN systems. These are examples of available capacities rather than mandatory ASTM D790 requirements. The expected maximum force should be estimated before selecting the machine.

7.2 Accurate Load Measurement

The load cell must be suitable for the expected force range. Accurate force measurement is necessary for calculating flexural stress and strength.

7.3 Stable Crosshead Movement

The machine should provide smooth and controlled movement throughout the test. Sudden changes in movement can affect the recorded load-deflection relationship.

7.4 Rigid Load Frame

A rigid frame helps reduce unwanted deformation in the machine structure. This is particularly important when measuring small specimen deflections and calculating flexural modulus.

7.5 Digital Data Acquisition

The control software should record load and deflection data throughout the test. It should support the required calculations and allow the laboratory to generate complete test reports.

7.6 Compatibility with Bending Fixtures

The machine must provide a suitable mounting arrangement for the three-point bending fixture. The loading nose and support rollers must be compatible with the specimen geometry and applicable requirements.

8. Bending Fixtures and Deflection Measurement

The bending fixture is an essential component of an ASTM D790 testing system. It supports the specimen and transfers the applied load through the loading nose.

8.1 Three-Point Bending Fixture

The fixture consists of two lower support points and one upper loading nose. The specimen rests on the lower supports, and the loading nose applies force at the midpoint. The support span must be adjustable or otherwise configured to suit the specimen dimensions and test requirements.

8.2 Support Rollers

The support rollers allow the specimen to rotate or move appropriately as it bends. Their alignment and condition can influence the test results.

8.3 Loading Nose

The loading nose applies the central force to the specimen. Its geometry and alignment must comply with the applicable fixture requirements.

8.4 Crosshead Position Measurement

Crosshead position is a convenient method for recording deflection and is often used in production quality control. However, it can include deformation from the machine frame, fixture, and other components.

8.5 Deflectometer Measurement

A deflectometer measures specimen deflection more directly. It can provide more accurate deflection data in suitable configurations, especially when modulus measurement requires higher precision. The method used to measure deflection must be recorded in the test report. Crosshead-position data and deflectometer data should not automatically be treated as identical.

8.6 Fixture Alignment

The loading nose should be positioned at the center of the support span. The support points should be aligned and stable to prevent uneven loading.

9. ASTM D790 Test Procedure

The following workflow describes a typical three-point bending test. Exact specimen dimensions, span, speed, conditioning, and calculation requirements must follow the current ASTM D790 edition and the applicable material specification.

Step 1: Identify the Material

Record the plastic material type, grade, manufacturer, batch information, and relevant product specification.

Step 2: Select the Test Procedure

Determine whether Procedure A or Procedure B is appropriate based on the material behavior and required properties.

Step 3: Prepare the Specimens

Prepare rectangular specimens according to the specified dimensions. Check that the specimens have consistent geometry and no obvious surface defects.

Step 4: Condition the Specimens

Condition the specimens when required by the applicable material specification or testing procedure. Record relevant environmental conditions.

Step 5: Measure Specimen Dimensions

Measure specimen width and thickness accurately. Record the values required for calculating flexural stress and strain.

Step 6: Configure the Testing Machine

Select a suitable load cell and install the three-point bending fixture. Confirm that the fixture is properly secured to the machine.

Step 7: Set the Support Span

Adjust the support span according to the specimen dimensions and the selected test procedure.

Step 8: Position the Specimen

Place the specimen centrally on the two supports. Confirm that the loading nose is aligned with the midpoint of the specimen.

Step 9: Configure Deflection Measurement

Select crosshead position or an appropriate deflection-measuring device. Confirm that the measurement system is suitable for the required accuracy.

Step 10: Apply the Bending Load

Start the test and apply the specified loading conditions. The machine records force and deflection as the specimen bends.

Step 11: Continue to the Required Endpoint

Continue loading until the required endpoint is reached, such as fracture, yield, or the specified deformation limit.

Step 12: Calculate the Flexural Properties

Use the recorded data and specimen dimensions to calculate flexural stress, strain, strength, and modulus as applicable.

Step 13: Prepare the Test Report

Record the specimen information, dimensions, procedure, support span, deflection measurement method, test conditions, and calculated results.

10. Flexural Properties and Calculations

ASTM D790 testing can provide several important mechanical properties.

10.1 Flexural Stress

Flexural stress represents the nominal stress calculated from the applied bending load and specimen geometry. For the conventional three-point bending calculation, the applied load, support span, specimen width, and specimen depth are used to determine the nominal flexural stress.

10.2 Flexural Strength

Flexural strength is the maximum flexural stress sustained by the specimen during the test, according to the applicable calculation procedure. For materials that show a yield point before breaking, the standard defines how the relevant load is selected for calculating flexural strength.

10.3 Flexural Stress at Break

Flexural stress at break is calculated using the load associated with specimen fracture. For some materials, the maximum stress and stress at break may be the same. For others, a yield point may occur before fracture.

10.4 Flexural Strain

Flexural strain describes the nominal deformation of the outer surface of the specimen at the midpoint. It is calculated from deflection and specimen geometry using the applicable standard equation.

10.5 Flexural Modulus

Flexural modulus indicates the stiffness of the material under bending. It is determined from the relationship between load and deflection within the specified region. A higher flexural modulus generally indicates greater resistance to bending deformation under comparable test conditions.

10.6 Tangent Modulus

Tangent modulus is determined from the slope of the initial straight-line portion of the load-deflection relationship, using the applicable calculation method.

10.7 Secant and Chord Modulus

Secant modulus is determined from the relationship between stress and strain at a selected point. Chord modulus is determined using two selected points on the load-deflection curve. When reporting these values, the selected stress or strain points should also be identified.

11. Understanding the Load-Deflection Curve

The load-deflection curve is a key result of ASTM D790 testing. It shows how the specimen responds as the bending force increases.

11.1 Initial Linear Region

At the beginning of the test, many rigid plastics show an approximately linear relationship between load and deflection. This region is used to evaluate flexural stiffness and modulus.

11.2 Nonlinear Deformation

As the load increases, the material may begin to show nonlinear behavior. This can result from plastic deformation, internal damage, or changes in the material's stress distribution.

11.3 Yield Behavior

Some materials reach a point where the load no longer increases proportionally with deflection. This may indicate the onset of yielding or other nonlinear deformation behavior.

11.4 Maximum Load

The maximum recorded load is used in the applicable flexural strength calculation. The corresponding stress depends on specimen dimensions and support span.

11.5 Fracture or Large Deflection

Brittle materials may fracture at relatively small deflections. Ductile or semi-rigid materials may continue bending without a clear fracture point. The interpretation of the curve must consider the selected ASTM D790 procedure. A material that does not break or yield at the outer surface within the specified strain limit may not provide a meaningful conventional flexural strength result under this method.

12. Universal Testing Machine for ASTM D790 Testing

A Universal Testing Machine equipped with a suitable three-point bending fixture can perform ASTM D790 testing on a wide range of plastic specimens. An electronic Universal Testing Machine is commonly suitable for plastic flexural testing because it can provide controlled crosshead movement and digital data acquisition. The required configuration may include:

• A suitable load cell for the expected bending force.
• A three-point bending fixture with compatible supports and loading nose.
• An appropriate deflection measurement system.
• Software for recording load and deflection data.
• Calculation and reporting functions for flexural properties.

For laboratories that perform tensile, compression, and bending tests, a multi-purpose Universal Testing Machine can offer greater equipment flexibility. By changing the fixtures and accessories, the same machine frame may be configured for different material testing applications. The final equipment configuration should be selected according to specimen dimensions, expected force, required measurement accuracy, and the applicable ASTM D790 procedure.

13. Factors Affecting Test Accuracy

ASTM D790 results can be affected by specimen preparation, fixture configuration, deflection measurement, and environmental conditions.

13.1 Specimen Thickness

Thickness is especially important because it has a strong influence on the calculated flexural stress and strain. Small measurement errors can produce noticeable differences in calculated results.

13.2 Support Span

The support span must be set correctly. An incorrect span changes the bending moment and the relationship between applied load and specimen deflection.

13.3 Specimen Alignment

The specimen should be centered on the supports. Incorrect positioning can create uneven loading and inconsistent results.

13.4 Fixture Condition

Damaged, worn, or misaligned supports and loading noses can affect force transfer and specimen deformation.

13.5 Deflection Measurement Method

Crosshead position and direct deflection measurement can produce different results. The selected method should be appropriate for the required measurement accuracy and clearly reported.

13.6 Loading Rate

The loading rate can influence the measured response of plastic materials. It must follow the applicable procedure and material specification.

13.7 Temperature and Conditioning

Plastic properties can change with temperature and moisture. Consistent conditioning helps improve repeatability.

13.8 Material Orientation

Molded, extruded, and reinforced plastics may have directional properties. Specimen orientation should be controlled and recorded.

13.9 Machine Calibration

The force measurement system and relevant measuring devices should be maintained and verified according to applicable requirements.

14. Industrial Applications

ASTM D790 testing is used in many industries where plastic materials must resist bending loads.

• Automotive manufacturing: Evaluating plastic brackets, interior components, covers, panels, and structural parts.
• Electrical and electronics: Testing plastic housings, insulating materials, support components, and enclosures.
• Construction: Evaluating plastic profiles, sheets, panels, and building components.
• Consumer products: Testing furniture components, appliance parts, and molded plastic products.
• Packaging: Evaluating rigid packaging components and plastic structural elements.
• Aerospace: Studying suitable polymer and reinforced plastic components for bending performance.
• Composite manufacturing: Comparing the flexural performance of fiber-reinforced plastic materials.
• Research and development: Evaluating new polymer formulations, fillers, and processing methods.

In quality control, flexural testing can help identify changes in material stiffness and strength between production batches. In product development, it provides information for comparing alternative materials and optimizing component geometry. The test results should be interpreted alongside the actual loading conditions of the intended application. Flexural testing alone does not replace impact, fatigue, creep, or other tests when those properties are important to product performance.

15. How to Select a Flexural Testing Machine

Choosing a suitable Universal Testing Machine for ASTM D790 requires evaluating the expected bending force, specimen dimensions, required deflection accuracy, and laboratory workload.

15.1 Determine the Required Force Capacity

Estimate the expected maximum bending load based on the material properties and specimen configuration. Select a load capacity suitable for the expected force range.

15.2 Check Fixture Compatibility

Confirm that the machine can accommodate the required three-point bending fixture and support span. The loading nose and supports must be appropriate for the specimen geometry.

15.3 Evaluate Deflection Measurement

If the laboratory needs accurate flexural modulus results, consider a suitable deflectometer or other direct deflection measurement system.

15.4 Review Load Cell Performance

The load cell should provide appropriate measurement performance at the expected bending force. A load cell with an unsuitable range may reduce the usefulness of the test data.

15.5 Check Software Functions

The software should record load and deflection, support the required calculations, and generate clear test reports.

15.6 Consider Multi-Function Testing

If the laboratory also performs tensile and compression tests, a multi-purpose Universal Testing Machine may reduce the need for separate equipment. Additional grips and fixtures can support different test methods.

15.7 Consider Future Testing Requirements

Laboratories should consider whether they may need to test additional plastic materials, reinforced composites, or specimens with different dimensions in the future.

15.8 Confirm ASTM D790 Suitability

Before purchasing, confirm that the complete configuration—including machine, load cell, fixture, deflection measurement, and software—is suitable for the selected ASTM D790 procedure.

16. Frequently Asked Questions

Q1: What is ASTM D790 used for?

ASTM D790 is used to determine the flexural properties of unreinforced and reinforced plastics, high-modulus composites, and electrical insulating materials using a three-point bending arrangement.

Q2: What is a three-point bending test?

A three-point bending test supports a specimen at two points and applies force at the midpoint. The specimen bends under the applied load, allowing the machine to measure load and deflection.

Q3: What is the difference between Procedure A and Procedure B?

Procedure A is designed mainly for materials that break at comparatively small deflections and is commonly used for flexural modulus. Procedure B is intended for materials that undergo larger deflections and may be used for flexural strength measurement where appropriate.

Q4: Which Universal Testing Machine capacity is suitable for ASTM D790?

The required capacity depends on the plastic material, specimen dimensions, and expected bending force. Common configurations include 1 kN, 5 kN, 10 kN, and 20 kN systems, but the correct capacity must be determined from the application.

Q5: What properties can ASTM D790 measure?

Depending on the test procedure and material behavior, the method can determine flexural stress, flexural strength, flexural strain, and flexural modulus.

Q6: Is a deflectometer necessary?

Crosshead position may be used to measure deflection, but a deflectometer can provide more direct and accurate deflection measurement in suitable configurations. The selected method must be reported.

Q7: Can ASTM D790 test reinforced plastics?

Yes. The method covers reinforced plastics, including high-modulus composites, when the specimen configuration and material behavior are suitable for the method.

Q8: What is the difference between three-point and four-point bending?

Three-point bending applies the load at one central point, while four-point bending uses two loading points. The locations and distribution of maximum bending moment and stress are different.

Q9: Can one Universal Testing Machine perform both tensile and flexural tests?

Yes. A suitably configured Universal Testing Machine can perform both tests by using the appropriate grips, bending fixtures, load cell, and software settings.

17. Conclusion

ASTM D790 is an important test method for evaluating the flexural performance of unreinforced and reinforced plastics. Its three-point bending arrangement provides useful information about material stiffness, flexural strength, deformation, and bending behavior. A properly configured Universal Testing Machine is essential for obtaining reliable results. The system should provide suitable force capacity, accurate load measurement, compatible bending fixtures, stable loading, and appropriate deflection measurement. Specimen preparation, support span, loading rate, and environmental conditions must also be controlled. By selecting the correct procedure and equipment configuration, laboratories can use ASTM D790 testing for material comparison, quality control, product development, and engineering evaluation. Always consult the current official ASTM D790 edition and the relevant material specification before conducting tests or reporting results.