CERAMIC EQUIBIAXIAL FLEXURAL STRENGTH TEST

                                             FIXTURE  (ASTM C 1499 & ASTM F 394)

                                             Model No. CU-CB  (low carbon steel)
                                             Model No. WTF-CB  (17-4PH stainless)

ASTM C 1499 (Reference 1) is a relatively new standard, introduced in 2001.  A thin circular disk  (a square specimen is also permitted) of a monolithic ceramic material, or a whisker- or particle-reinforced ceramic matrix composite, is supported on a ring and loaded by a smaller diameter concentric ring, a so-called ring-on-ring loading.  The required diameters of the support and loading ring are dependent on the thickness and material properties of the specimen being tested.

In order to attain a uniform load distribution, either the specimen surfaces must be very flat (typically by surface grinding), or a relatively thick (one-half the specimen thickness) sheet of a compliant material (e.g., rubber) is placed between each ring and the specimen.  As another aid to achieve uniform loading, the loading head is articulated, being supported on a spherical seat.  Because of these uniform load introduction considerations, this test method has not become very widely used yet, although it is gaining in popularity.

This fixture, assembled and disassembled, is shown in the following two photographs.

Another, much older ASTM test method, ASTM F 394 (Reference 2), first introduced in 1974, is still used frequently, in spite of being discontinued by ASTM in 2001.  A 1.25" diameter thin disk of the ceramic material to  be tested is supported on three 0.125" diameter balls equally spaced around a 1" diameter circle. A compressive loading is applied at the center of the disk specimen by a 0.0625" diameter, flat-ended hardened steel rod. A thin (0.002" thick) polymer film (e.g., polyethylene) is placed under the 0.0625" diameter flat-ended loading rod, to more uniformly distribute the applied loading. The minimum thickness of the specimen must be such that its center deflection at failure does not exceed one-half the specimen thickness.

One important advantage of this biaxial test configuration is that specimen warpage is not an influencing factor, as opposed to the ring-on-ring test (ASTM C 1499).  Specimen warpage is also a problem when flexural testing a straight beam (Reference 3), requiring an articulating test fixture.

Both biaxial test methods have an advantage relative to a flexural test of a straight beam in that the maximum (biaxial) tensile stress occurs at the center of the specimen on the lower (convex when deflected) side.  Thus, the surface finish of the edge of the specimen is not critical.  This is not the case when flexural testing a straight beam.

Shown below is an ASTM F394 test fixture.

Assembled ASTM F 394 Test Fixture, No Specimen Installed

Sources of Additional Information:

1)  ASTM Standard C 1499-03 (2003), “Monotonic Equibiaxial Flexural  Strength of Advanced Ceramics at Ambient Temperature,” American Society   for Testing and Materials, West Conshohocken, PA (first issued in 2001).

2)  ASTM Standard F 394-78 (Reapproved in 1996 and then Discontinued in 2001), "Biaxial Flexure Strength (Modulus of Rupture) of Ceramic Substrates," American Society for Testing and Materials, West Conshohocken, PA (first issued in 1974).

3)  ASTM Standard C 1161-02c (2002), "Flexural Strength of Advanced Ceramics at Ambient Temperature," American Society for Testing and Materials, West Conshohocken, Pennsylvania (first issued in 1990).