ASTM C1869 Test for Open-Hole Tensile Strength of Advanced Ceramics

Scope:

In ASTM C1869, the open-hole tensile (OHT) test determines the effect of a single hole (notch) on the tensile strength of CMCs. This test is used for the material and design development of composite materials. It informs how a composite performs with a notch and how to design one for notches. 

The tensile strength of the notched specimen is compared to that of an unnotched specimen to determine the effect of the notch on the tensile strength. If the material is sensitive to a notch, then its open-hole tensile (OHT) strength varies with the notch size. Usually, larger holes reduce the OHT strength because a notch reduces the area available to carry an applied force, causes stress concentrations, and introduces new edges where delamination can occur. 

The mechanical properties of a notched specimen, calculated from this test method, are the open-hole (notched) tensile strength, the net section tensile strength, the proportional limit stress, and the stress response of the specimen. 

ASTM C1869 test method is used for ceramic matrix composites with continuous fiber reinforcement in multiple directions. Usually, The CMC material tested is fiber reinforced, 2D, and laminated. The laminate is balanced and symmetrical. Composites with different reinforcements (1D, 3D, braided, unbalanced) are also tested with this method. Composites with different architecture are affected differently with a notch. Therefore, the composite architecture is kept into consideration. 

Although the OHT test is not used for discontinuous fiber-reinforced, whisker-reinforced, or particulate-reinforced ceramics, it can be equally applied to these composites. The CMCs are essentially matrices reinforced by fibers. The matrices and fibers of an OHT test specimen can be made of a wide range of materials (oxide-oxide composites, silicon carbide fibers in SiC matrices, carbon fibers in SiC matrices, and carbon-carbon composites).

Test Procedure:

ASTM C1869 defines two specimen geometries and a test procedure for reproducible and comparable results. The test specimens are flat, straight-sided ceramic composites with a defined laminate fiber architecture. The test specimens contain a single notch of either 6 mm or 3 mm diameter at the center of the specimen. 

The tensile test is uniaxial and monotonic, performed along the defined test reinforcement axis at ambient temperature, measuring the applied force versus time/displacement. Measurement of the gauge length extension/strain is optional, using extensometer/displacement transducers. Bonded strain gauges are optional for measuring localized strains and assessing bending strains in the gauge section.

 Specimen size:

The test specimen in ASTM C1869 testing has a defined single notch with a diameter of either 6 mm or 3 mm, centered by length and width in the defined gauge section. Minimum ten test specimens are required. 

Data:

Tensile strength calculation for ASTM C1869 testing:

T_OHT = P_max /A

T_OHT = the tensile strength
P_max = the maximum load
A = Average shear stressed area, which is calculated as,

A = Wh

W = average specimen width, and
h = average distance between the notches

Conclusion:

ASTM C1869 is used to determine the open-hole (notched) tensile strength of continuous fiber-reinforced ceramic matrix composite (CMC) at ambient temperature.