Standard Test Methods for Strength of Glass by Flexure (Determination of Modulus of Rupture)

Flexural strength is determined in three-point or four-point bending on bars of rectangular or circular cross-section. Between ten and thirty specimens per type/brand is recommended by ASTM.

$520 for tests of 10 specimen


Significance and Use

4.1 For the purpose of this test, glasses and glass-ceramics are considered brittle (perfectly elastic) and to have the property that fracture normally occurs at the surface of the test specimen from the principal tensile stress. The modulus of rupture is considered a valid measure of the tensile strength subject to the considerations discussed below.

4.2 It is recognized that the modulus of rupture for a group of test specimens is influenced by variables associated with the test procedure. These include the rate of stressing, test environment, and the area of the specimen subjected to stress. Such factors are specified in the test procedure or required to be stated in the report.

4.3 It is also recognized that the variables having the greatest effect on the modulus of rupture value for a group of test specimens are the condition of the surfaces and glass quality near the surfaces in regard to the number and severity of stress-concentrating discontinuities or flaws, and the degree of prestress existing in the specimens. Each of these can represent an inherent part of the strength characteristic being determined or can be a random interfering factor in the measurement.

4.4 Test Method A is designed to include the condition of the surface of the specimen as a factor in the measured strength. It is, therefore, desirable to subject a fixed and significant area of the surface to the maximum tensile stress. Since the number and severity of surface flaws in glass are primarily determined by manufacturing and handling processes, this test method is limited to products from which specimens of suitable size can be obtained with minimal dependence of measured strength upon specimen preparation techniques. This test method is therefore designated as a test for modulus of rupture of flat glass.

4.5 Test Method B describes a general procedure for test, applicable to specimens of rectangular or elliptical cross section. This test method is based on the assumption that a comparative measurement of strength on groups of specimens is of significance for many purposes, such as determining the effect of environment or stress duration, or the effectiveness of varied prestressing techniques or strengths characteristic of glass-ceramics of differing composition or heat treatment. In this test method the surfaces of the specimens are not assumed to be characteristic of a product or material, but are considered to be determined by the procedures used to prepare the specimens. Though the stated procedure permits a wide variation in both specimen size and test geometry, it is necessary to use identical test conditions and equivalent procedures for specimen preparation to obtain comparable strength values. The use of a controlled abrasion of the specimen as a final normalizing procedure is recommended for such comparative tests.

4.6 A comparative abraded strength, determined as suggested in Test Method B, is not to be considered as a minimum value characteristic of the material tested nor as directly related to a maximum attainable strength value through test of specimens with identical flaws. The operationally defined abrasion procedure undoubtedly produces flaws of differing severity when applied to varied materials, and the measured comparative strengths describe the relative ability to withstand externally induced stress as affected by the specific abrasion procedure.


1.1 These test methods cover the determination of the modulus of rupture in bending of glass and glass-ceramics.

1.2 These test methods are applicable to annealed and prestressed glasses and glass-ceramics available in varied forms. Alternative test methods are described; the test method used shall be determined by the purpose of the test and geometric characteristics of specimens representative of the material.

1.2.1 Test Method A is a test for modulus of rupture of flat glass.

1.2.2 Test Method B is a comparative test for modulus of rupture of glass and glass-ceramics.

1.3 The test methods appear in the following order:



Test Method A
Test Method B

6 to 9
10 to 15

1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. 


Copyright Notice

Extracted, with permission, from ASTM C158 Standard Test Methods for Strength of Glass by Flexure (Determination of Modulus of Rupture), copyright ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428. A copy of the standard may be purchased from ASTM International, phone 610-832-9555, e-mail:, website:


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