Sandwich Core Shear Fixture
ASTM C273 is used to determine the shear properties of construction materials that have a sandwich core. An example of a sandwich panel, (two sandwich facings and one core), would be rigid foam insulation that is sandwiched in between two pieces of plywood.
The test is a compression based load deflection test but can also be used in the tensile mode if the sandwich facings are glued to the support plates of the fixture. The two ends of the shear fixture push or pull parallel to the sandwich facings to create shear stress. The goal of the test is to cause a 100% shear failure of the sandwich core. Any adhesive failures that are seen between the sandwich panels and core, or the sandwich panels and loading plates, should be discarded and not included in the final data analysis.
The thickness of the sandwich specimen being tested can vary and is based on the thickness of the sandwich that is to be used in the field. The width of the sample should be over two inches and the length should be over 24 inches in a 12:1 ratio unless otherwise agreed upon.
The direction of the loading force should be such that the loading plane intersects two diagonally opposite corners of the specimen.
The ASTM C273 shear fixture is designed specifically for testing shear in both tension and compression. The fixture has a swivel joint which allows the apparatus to self-align during the test. The sandwich panels can be bonded directly to the fixture, or there is a small step that juts out and can support the sample when performing the test in compression.
A contact strain gauge is sometimes used for this test, however that is not very common.
Shear Stress is calculated using a formula supplied in the official ASTM publication. The shear modulus can also be calculated in a similar fashion and is a function of the load, slope of the deflection curve, displacement of the loading plates, and the thickness of the core. The max load is also a commonly recorded and simple test result.