ASTM D6272
4 Pt. Bend Fixture 
Plastics

ASTM D6272 - Standard Test Method for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials by Four-Point Bending - PDF

ASTM D6272 outlines the procedure for measuring the flexural properties of unreinforced plastics, reinforced plastics (like composites), and electrical insulating materials using a four-point bending configuration.
Four-point bending differs from three-point bending because it provides a constant moment region between the two loading noses, minimizing shear effects and allowing a more accurate measurement of material behavior under pure bending.

This article explains the scope of ASTM D6272, the necessary equipment and setup, how the test is performed, and related ASTM standards commonly referenced with it.

Purpose of ASTM D6272

The primary goal of ASTM D6272 is to determine two key material properties:

• Flexural Strength (Modulus of Rupture) – The maximum stress a material can withstand before failing in bending.

• Flexural Modulus (Modulus of Elasticity in Bending) – The stiffness of the material under bending loads.

Four-point bending is particularly important for materials where the presence of shear forces (as in three-point bending) could distort flexural property measurements.

ASTM D6272 is widely used for quality control, product development, research, and engineering design purposes.

Equipment Required

Testing according to ASTM D6272 typically requires:

• Universal Testing Machine (UTM):

  • Must have a load cell appropriate for expected failure loads.

  • Must control crosshead movement precisely at a specified test rate.

• Four-Point Bending Fixture:

  • Two support rollers (lower supports) and two loading rollers (upper noses).

  • Adjustable span length based on specimen size and material type.

  • Rollers typically have a diameter between 1/8 inch and 1 inch, depending on specimen dimensions.

• Deflection Measurement System:

  • Can use the crosshead displacement, or more accurate mid-span devices such as LVDTs or dial indicators.

• Specimen Cutting and Conditioning Equipment:

  • To prepare standardized test specimens.

  • Specimens should be conditioned at standard laboratory conditions (typically 73°F and 50% RH) unless otherwise specified.

Test Specimens

Specimens are typically rectangular in cross-section. Standard dimensions are:

• Length: At least 16 times the nominal depth (thickness) of the specimen.

• Width: Recommended to be 1/2 inch to 1 inch (12 to 25 mm) wide.

• Thickness: Typically between 1/8 inch and 1/2 inch (3 to 13 mm), depending on the material and application.

The surfaces of the specimen must be smooth and free from obvious flaws unless the purpose is to study imperfections.

Test Procedure for ASTM D6272

Here’s how a typical ASTM D6272 test is performed:

1. Setup:

   • Mount the specimen on the support rollers.

   • Position the two loading noses symmetrically between the supports to create equal spans.

   • Set the outer span and inner span according to the specifications (outer span = 2 × inner span).

2. Loading:

   • Apply force steadily at a specified crosshead speed so that failure occurs between 0.5 and 5 minutes after loading begins.

   • Typical strain rate: about 0.01/min based on outer fiber strain.

3. Measurement:

   • Measure load and mid-span deflection continuously.

   • Record the maximum load for calculating flexural strength.

   • Use the slope of the load-deflection curve to calculate the flexural modulus.

4. Calculations:

   • Flexural Strength (σ):
     Based on the maximum load carried before rupture.

   • Flexural Modulus (E):
     Calculated from the slope of the stress-strain curve in the elastic region.

5. Observations:

   • Note the type and location of failure (crack initiation, fiber breakage, crushing, etc.)

Results and Reporting

The final test report should include:

• Flexural strength (modulus of rupture) in psi or MPa.

• Flexural modulus in psi or MPa.

• Specimen dimensions and span lengths.

• Loading configuration details.

• Rate of loading and conditioning environment.

• Description of failure mode.

Multiple specimens are usually tested to obtain average values and standard deviations.

Related ASTM Standards

Several ASTM standards are similar to or often referenced with ASTM D6272:

• ASTM D790 – Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials:
  The most widely used flexural test for plastics, but uses three-point bending instead of four-point bending. ASTM D790 is the most direct complement to D6272.

• ASTM D2344 – Standard Test Method for Short-Beam Strength of Polymer Matrix Composite Materials and Their Laminates:
  Used for testing shear strength under flexural loads in composites using a very short span-to-depth ratio.

• ASTM D7264 – Standard Test Method for Flexural Properties of Polymer Matrix Composite Materials:
  Focuses on the flexural properties of fiber-reinforced composites using three-point or four-point bending, depending on the test setup.

• ASTM D4065 – Standard Practice for Plastics: Dynamic Mechanical Properties: Determination and Report of Procedures:
  While D4065 covers dynamic flexural testing, it is related when a comparison between static and dynamic flexural properties is needed.

• ASTM E855 – Standard Test Methods for Bend Testing of Metallic Flat Materials for Spring Applications Involving Static Loading:
  While intended for metals, E855 shares conceptual similarities for bending evaluations of materials.

Applications of ASTM D6272 Testing

Materials tested under ASTM D6272 are commonly used in:

• Electrical insulating components (boards, panels, casings)

• Structural composite panels (automotive, aerospace, and construction sectors)

• Unfilled and filled plastics (in consumer goods, piping, medical devices)

• Fiber-reinforced plastics (marine applications, sporting goods, infrastructure)

Four-point bending is preferred in many industries when minimizing shear effects is important for product validation and design.

Conclusion

ASTM D6272 provides a critical testing method for understanding the flexural behavior of plastics, composites, and insulating materials under a pure bending moment.
It is especially important for applications requiring precise material characterization with minimal shear influence.
While ASTM D790 remains more commonly used overall (for three-point bending), ASTM D6272 is indispensable when a more uniform stress distribution is needed during flexural testing.