What Are ASTM D790 and ISO 178 Flexural Modulus Testing Methods?
ASTM D790 and ISO 178 are standardized methods for measuring the flexural properties of rigid and semi-rigid plastics. The techniques are essentially the same, involving a three-point bending setup with a specimen supported at each end and the loading nose at the center. From this force and deflection data, the laboratory can calculate the material’s bending resistance. However, the two standards differ. ASTM D790 (Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials) is a North American and international plastics testing standard developed by ASTM International. ISO 178 (Plastics – Determination of flexural properties) is an international standard for measuring flexural properties of plastics. When manufacturers provide plastic materials or components to various international markets, it is especially critical to choose the proper standard. Reported flexural modulus values using the specified method meet the requirements of the material specification or customer.
Principle of Flexural Modulus Testing
The mechanical principle is the same for both ASTM D790 and ISO 178: the engineer applies a controlled bending force to a plastic specimen and measures the amount of bending. The specimen is set between two supports with a loading nose in the middle. The testing machine then applies force at a predetermined rate. The specimen is bent, and when the load is increased, the machine measures the force and deflection of the specimen. Analysts use these measurements to determine the material’s flexural behavior. The flexural modulus is a measure of the stiffness of the material during bending. The higher the flexural modulus, the more resistant the material is to bending deformation. This data can also be used to calculate flexural strength and other specified properties. As such, the test gives engineers quantitative data to use when choosing materials, designing products, checking quality, and verifying specifications.
ASTM D790 vs. ISO 178: Key Differences
ASTM D790 and ISO 178 both use the same basic three-point bending principle, but with a few technical differences. There are a number of differences between the two, but the main points are specimen preparation, test conditions, the conditions for measurement, and calculation procedures. The following table summarizes key differences that engineers and technicians should note when making a selection.
| Feature | ASTM D790 | ISO 178 |
| Developing organization | ASTM International develops and maintains the standard for testing the flexural properties of plastics. | The International Organization for Standardization develops and maintains the international method for determining plastic flexural properties. |
| Primary purpose | Technicians use ASTM D790 to determine flexural strength and flexural modulus of plastics under specified bending conditions. | Technicians use ISO 178 to determine flexural properties, including flexural modulus and flexural strength, under controlled conditions. |
| Loading arrangement | Engineers use a three-point bending configuration with two supports and a central loading nose. | Engineers use a three-point bending configuration with the specimen supported at two points and loaded at the center. |
| Specimen requirements | Technicians prepare standardized rectangular specimens and follow the dimensional requirements specified by the applicable ASTM procedure. | Technicians prepare specimens according to ISO 178 dimensional and preparation requirements before testing. |
| Test conditions | Technicians control test speed and other conditions according to the selected ASTM D790 procedure and material requirements. | Technicians control test speed, conditioning, and other parameters according to ISO 178 requirements. |
| Measurements | The testing machine records force and deflection, while analysts use the data to determine the required flexural properties. | The testing machine records force and deflection, and analysts use the resulting data to calculate the specified flexural properties. |
| Result reporting | Laboratories can report flexural modulus, flexural strength, strain, and other required properties according to the applicable procedure. | Laboratories can report flexural modulus, flexural strength, strain, and other properties required by the standard. |
| Typical use | Manufacturers and laboratories commonly use ASTM D790 for material characterization, quality control, product development, and specification testing. | Manufacturers and laboratories commonly use ISO 178 for international material characterization, quality control, product development, and conformity assessment. |
| Compliance | Engineers report results specifically as ASTM D790 results and follow the exact procedure required by the applicable specification. | Engineers report results specifically as ISO 178 results and follow the conditions required by the applicable specification. |
Which Instruments Do Technicians Use for Flexural Modulus Testing?
The ASTM D790 and ISO 178 flexural testing are primarily done on a Universal Testing Machine (UTM) with a three-point bending fixture. The machine applies a controlled force on the specimen and records the deflection. A suitable load cell measures the applied force, and a deflection measurement system or extensometer records specimen displacement during loading. The width and thickness of the specimen are also measured prior to testing using a micrometer or Vernier caliper. If a standard specifies environmental conditions, engineers use a conditioning chamber to achieve them before testing. Test software records force and deflection data and helps analysts determine flexural modulus, flexural strength, and other properties. When engineers need accurate, repeatable measurements, they can achieve them with proper fixture calibration and alignment.
What Are the Applications of Flexural Modulus Testing?
Manufacturers and engineers use flexural modulus testing to evaluate the stiffness and bending performance of plastics and polymer-based materials. In the automotive industry, engineers test plastics, reinforced polymers, and composites used in panels, covers, interior components, and structural parts. Aerospace manufacturers evaluate lightweight polymers and composites when they need materials with suitable stiffness and mechanical performance. In plastics manufacturing, analysts use ASTM D790 or ISO 178 results to compare material formulations, monitor production consistency, and verify specifications. Electrical and electronics manufacturers evaluate polymers used in housings, supports, insulating components, and other parts that require adequate rigidity. Similarly, consumer product manufacturers test plastics used in appliances, containers, panels, and housings. Furthermore, research and development laboratories use flexural testing to study new polymer blends, reinforced plastics, composites, and material formulations and to determine how composition and processing affect flexural performance.
