Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials - PDF

 

ASTM D790 – Flexural Testing of Plastics

Introduction to ASTM D790

ASTM D790 is the standard test method for determining the flexural properties of unreinforced and reinforced plastics and electrical insulating materials. This test is crucial for manufacturers, quality control engineers, and material scientists who need to evaluate a material’s bending strength and stiffness.

Industries & Applications

Commonly used in industries such as:

• Automotive and aerospace

• Consumer electronics

• Medical devices

• Electrical insulation

• Packaging and construction materials

These industries rely on ASTM D790 to ensure that plastics and composite materials can withstand bending forces during service.

Sample Preparation for ASTM D790

Specimen Dimensions

ASTM D790 defines two procedures:

• Procedure A: For high modulus materials

• Procedure B: For low modulus materials that may deflect more than 5%

Typical sample dimensions:

• Length: 127 mm (5.0 in)

• Width: 12.7 mm (0.5 in)

• Thickness: 3.2 mm (0.125 in)

The span length is determined based on the thickness using the formula:

Support span = thickness × 16 (for Procedure A)

Number of Samples

• Minimum of 5 specimens per material configuration

Conditioning

• Condition specimens at 23 ± 2°C and 50 ± 5% RH for at least 40 hours before testing, unless otherwise specified.

Equipment Required for ASTM D790 Testing

1. Universal Testing Machine (UTM)

• Must operate at a constant crosshead speed

• Must be capable of capturing load vs. deflection data

2. Flexural Test Fixture

• Consists of two lower supports and a central loading nose

• Support span must follow the required span-to-depth ratio

• Smooth, rounded loading noses help reduce stress concentrations

3. Extensometers (Optional)

• Clip-on or video extensometers can increase accuracy by measuring center deflection

• Usually a special extensomter called a deflectometer is used. It has an arm that sticks out like a lever and sits underneath the sample. 

4. Sample Preparation Dies

• Dies or CNC routers help cut specimens to exact dimensions

• Clean edges and flat surfaces are essential

5. Data Acquisition System

• Records load, deflection, stress, strain, and modulus

• Graphing software enables real-time data monitoring

At Universal Grip Company, we provide ASTM-compliant fixtures, extensometers, cutting dies, and testing machines tailored for D790 flexural testing.

Test Setup and Procedure

Machine Settings

• Crosshead speed depends on span and material stiffness

  • Example: 1.3 mm/min for stiff materials (Procedure A)

  • Example: 50 mm/min for flexible materials (Procedure B)

Procedure

1. Place the specimen on the two lower supports

2. Center the loading nose directly above the midpoint

3. Begin the test at the correct crosshead speed

4. Record load and deflection continuously

5. End the test at break or 5% strain

ASTM D790 Equations 

Flexural Stress

Flexural stress (σ) = (3 × Load × Support Span) / (2 × Width × Thickness²)

Flexural Strain

Flexural strain (ε) = (6 × Deflection × Thickness) / (Support Span²)

Flexural Modulus

Flexural modulus (E) = (Support Span³ × Slope) / (4 × Width × Thickness³)

Where:

• Load = Force at any given point (in Newtons)

• Support Span = Distance between lower supports (in mm or inches)

• Width = Width of the specimen (in mm or inches)

• Thickness = Thickness of the specimen (in mm or inches)

• Deflection = Midpoint deflection (in mm or inches)

• Slope = Slope of the initial straight-line portion of the load-deflection curve

Real-World Applications

• Automotive bumpers require flexural strength to absorb impact

• Consumer electronics enclosures need stiffness for structural integrity

• Construction panels and pipes are tested for flexural rigidity

• Electrical insulation boards undergo this test to ensure durability

Common Pitfalls and Tips for Success

• Be sure to use the correct span-to-thickness ratio

• Keep the test fixture aligned to avoid skewed results

• Ensure specimens are flat and accurately dimensioned

• Calibrate machines and data systems regularly

• Document environmental conditions for repeatability

Comparison to Related Standards

Standard - Description - Difference

ASTM D6272 - Four-point flexural test - Distributes stress more evenly

ISO 178 - Flexural test for plastics (metric) - Metric-based, some minor setup differences

ASTM D695 - Compression strength of plastics - Measures compressive, not flexural, properties

Conclusion: Key Takeaways

The ASTM D790 standard is essential for evaluating the flexural strength, strain, and modulus of plastic and composite materials. When properly executed using the correct test setup, equipment, and sample preparation, it provides reliable mechanical data critical for product development and quality assurance.

Universal Grip Company proudly supplies turnkey testing solutions for ASTM D790 and other related standards. From universal testing machines and flexural fixtures to sample prep dies and extensometers, we’re your expert partner in material testing.