The American Society for Testing and Materials (ASTM), formerly called ASTM International, has established and documented more than 12,000 standards used voluntarily around the world to improve product quality, enhance safety, facilitate trade and facilitate market access. The 80-volume Annual Book of ASTM Standards contains them.
ASTM Standards have fewer professional experts involved in the decision-making process for the establishment of various standards than ISO Standards. ISO Standards, in contrast, are based on the opinions of 165 expert professionals from around the world.
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Heat cure acrylic resin was evaluated according to ISO 20795.1.2013 and ASTM D 6272 standards. These test methods are typically used for rigid and semi-rigid materials. They measure the flexural strength of materials.
Most of these methods are used for quality control and research. A study was conducted to determine which of the three and four-point testing protocols would be most appropriate.
The ASTM is a national organization that is a member of the ISO. ISO has representatives from all countries, including ASTM. With the global consensus of the experts of the associated national organizations, ISO establishes documents and updates standards of testing materials. Such products are safe, reliable, and of high quality. Since ISO standards are developed and updated to meet the needs of international experts, they are more valid.
ISO’s initial protocols had varying testing procedures. They have been modified and changed over time. There have been constant efforts to align the testing protocols between the organizations to reduce duplication of testing and better serve the community.
Currently, ISO is the standard for determining flexural strength. Many publications still use outdated ANSI or ISO protocols, despite directions for universal adaptation of the latest ISO standards. The differences between four-point and three-point bending tests include specimen size, shape, thickness, load nose radius, bending moment, maximum allowable strain, and axial stress.
For three-point testing, the specimen measured 65mm x 40mm x 5mm; for four-point testing, it measured 127mm x 12.7mm x 3.2mm. Compared to the larger specimens of a four-point bending test in dentistry, the samples for ISO testing were easier to fabricate in regular dental flasks. To fabricate and polymerize the ASTM samples, larger flasks were required. The variations in the sample size and protocol did not significantly affect the results.
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Generally, these test methods are applicable to rigid and semi-rigid materials. The flexural strength of heat-cure acrylic resin was evaluated based on ISO 20795.1.2013 and ASTM D 6272 (3). In the study, the flexural properties of 3 and 4-point test protocols were evaluated to determine which protocol was more appropriate for quality control and research (8,9).
ASTM is a national organization that is a member of the ISO organization. ISO is an international organization with representation from all countries, including ASTM. With the agreement of the experts of the associated national organizations, ISO documents, and updates the standards of testing materials. Thus, products are created that are safe, reliable, and of high quality. It is better for ISO standards to be valid since they are developed and updated with the opinions of internationally established experts. The initial protocols of ISO had variations in testing procedures.
In order to reduce duplication of tests and better serve the community, constant efforts have been made to match testing protocols between the organizations. Currently, ISO is the standard for determining flexural strength. In spite of directions aimed at universally adopting the latest ISO standards (8,9), many kinds of literature use outdated ANSI protocols.
A four-point bending test differs from a three-point bending test in terms of specimen size, shape, thickness, load nose radius, bending momentum, maximum strain, and axial stress. For three-point testing, the specimen measured 65mm x 40mm x 5mm, and for four-point testing, it measured 127mm x 12.7mm x 3.2mm. It was easier to manufacture ISO test samples in dental flasks than larger specimens for four-point bending tests. Despite the differences in sample sizes and protocols, the ASTM samples required larger flasks to fabricate and polymerize PMMA specimens.
Comparison of ISO and ASTM Standards
Groups ISO and ASTM have a mean flexural strength of 60.49 MPa and 61.44 MPa. There were no statistical differences between the two methods, but ASTM has slightly higher flexural strength quantitatively than ISO. The results matched the manufacturers and ideal values of flexural strength for denture base materials. Both protocols may show different flexural characteristics depending on specimen depth, temperature, atmospheric conditions, and rate of strain. Quantitative variability can be attributed to stress distribution in this study. As opposed to 3-point bending tests, where the maximum axial stress is located immediately under the loading points, 4-point bending tests have a uniform distribution of axial stress between the loading points.
It is possible for the mechanism of stress evaluation to vary the strength value in a minor way. Both protocols are reliable testing methods. The 4-point bending test has a lower test sensitivity than the 3-point bending test. ASTM and older ISO protocols have been found to differ by 10% (9) between composite and brittle materials. For dental materials, ISO has adapted and modified them to meet their needs, including sample fabrication and mechanical testing (8).
Without any modifications to the compositions of the specimens, the study evaluated the conventional heat cure specimens. In order to determine the effect of testing protocols on the composition, reinforcement, and composite materials of PMMA, as well as their influence on testing protocols, further studies are required.