Flexural/Modules Testing as per ASTM D790

Importance of Flexural Testing:

The primary purpose of flexural testing is to measure how materials perform under flexural stress conditions. This is critical because many engineering applications involve materials being exposed to bending forces rather than pure compression or tension. Flexural strength and modulus are key properties that can indicate how a material will react to bending and deformation, providing insight into its stiffness, strength, and resistance to permanent deformation.

Key Parameters Measured

1. Flexural Strength: The maximum stress the material can withstand before yielding or breaking during bending.
2. Flexural Modulus: The ratio of stress to strain in the elastic deformation region, indicating the stiffness of the material.
3. Strain at Break: The degree of deformation a material undergoes at the point of fracture.

The ASTM D790 Standard ASTM D790 outlines the method for testing the flexural properties of plastics and similar materials. Here is an overview of the procedure:

Test Specimen Requirements

According to ASTM D790, the test specimens are prepared from molded, extruded, or fabricated sheets. Specimen sizes are critical, with the standard dimensions typically being 125 mm in length, and either 12.7 mm in width and 3.2 mm in thickness, or other dimensions as specified in the material’s classification.

Equipment and Setup

The test is conducted using a universal testing machine equipped with a flexural fixture. The specimen is positioned horizontally over two supports, and a load is applied at the center via a loading nose until the specimen fractures or the maximum strain level is achieved.

The typical test setup involves a three-point bending fixture, where the specimen is supported at two points and loaded at the midpoint. The span length between the supports is calculated based on the thickness of the specimen to ensure a valid test.

Testing Procedure

1. Conditioning: Specimens are conditioned at a specific temperature and humidity level as defined by ASTM D618 before testing to ensure consistent results.
2. Testing Speed: The speed of testing is based on the specimen thickness and ranges from 1.3 mm/min to 13 mm/min, ensuring that the rate of loading is suitable for the material.
3. Flexural Strength Calculation: The flexural strength is calculated using the formula provided in the standard, based on the load at failure, span length, and dimensions of the specimen.
4. Flexural Modulus Calculation: The flexural modulus is derived from the slope of the initial straight-line portion of the load-deflection curve. This is done using the stress and strain values recorded during the test.

Test Significance

The data obtained from ASTM D790 flexural testing provides essential insights into the material’s behavior under bending stresses. These include:

• Application Suitability: Engineers use flexural modulus and strength to determine if a material is suitable for structural applications that require a specific level of stiffness and resistance to deformation.
• Material Comparison: Flexural properties allow for comparison between different materials, especially in selecting plastics for industries such as automotive, construction, and consumer products.
• Quality Control: Flexural testing is also an integral part of quality control in manufacturing processes, ensuring that materials meet required standards for strength and performance.