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

What ASTM C158 Measures

ASTM C158 measures the flexural strength of glass and glass-ceramics, reported as the modulus of rupture (MOR). A flat glass bar or a rod is broken in four-point bending, and the MOR is the tensile stress on the bottom surface at the moment it breaks. Because glass fails from surface flaws, the result depends heavily on surface condition, edge finish and specimen size, so these must be reported with the result.

Specimens and Loading

• Flat specimens cut from sheet or plate, or round rods and tubes

• Four-point bending: two loading points set inside two supports, so the region between the loading points has a constant bending moment

• Support and loading points must be free to roll or articulate so the specimen is not twisted and friction does not add to the load

• Load at the constant stress rate specified in the standard; glass strength rises with faster loading, so the rate must be held and reported

• Test enough specimens for a statistical result (glass strength is usually analyzed with Weibull statistics)

Confirm the specimen sizes, spans and stress rate for each procedure against your copy of C158.

Calculations

Rectangular bar: MOR = 3P(L − a) ÷ (2bd²)

Round rod: MOR = 8P(L − a) ÷ (πD³)

P is the breaking load, L the support span, a the loading span, b the width, d the thickness and D the rod diameter. MOR is a stress (MPa or psi), not a load.

Example: a 25 mm wide, 5 mm thick glass bar on a 200 mm support span with a 100 mm loading span breaks at 300 N. MOR = 3 × 300 × (200 − 100) ÷ (2 × 25 × 5²) = 90,000 ÷ 1,250 = 72 MPa.

Fixtures and Equipment

Universal Grip offers a fully articulating 4-point bend fixture with wide contact spans for large glass panels, and a dual-column universal testing machine that can run at the very low speeds glass testing needs. Wide, rubber-padded loading bars help spread contact so the glass does not break at the loading points.

Related Standards

• ASTM C1161 – flexural strength of advanced ceramics

• ASTM C1341 – flexural properties of fiber-reinforced ceramic composites

• ASTM C1499 – equibiaxial (ring-on-ring) strength of advanced ceramics

• ASTM C1048 – heat-strengthened and tempered flat glass

• ASTM D7264 – flexural properties of polymer matrix composites

ASTM C158 FAQ

What is the modulus of rupture of glass?

The tensile stress on the bottom surface of a glass specimen at the moment it breaks in bending. It is calculated from the breaking load and the specimen and span dimensions, and it is reported in MPa or psi.

Why does C158 use four-point bending?

Four-point bending puts a constant bending moment over the whole region between the loading points, so the test samples a larger area of surface flaws than three-point bending.

How is MOR calculated for a flat glass bar?

MOR = 3P(L − a) ÷ (2bd²), where P is the breaking load, L the support span, a the loading span, b the width and d the thickness.

Does loading rate affect glass strength?

Yes. Glass is subject to slow crack growth, so faster loading gives higher measured strength. The stress rate must be held constant and reported.