ASTM C365 Compression Testing of Sandwich Core Materials

ASTM C365 is a test method that determines the compressive strength and modulus of sandwich core specimens. These sandwich constructions can include core materials with continuous bonding surfaces such as balsa wood and foam, as well as those with discontinuous bonding surfaces such as honeycomb. The properties are determined in the same direction the sandwich core will face during use, but this test method can also be applied to determine compressive forces in different directions if those results are desired. The goal of the test is to calculate results for the ultimate flatwise compressive strength, the stress at 2% deflection, and the compressive chord modulus. Determination of the compressive strength and modulus of sandwich cores are absolutely critical to the design of sandwich panels, which are mainly used in the aircraft industry, but are also seeing adoption in the transportation, automotive, and construction industries.

ASTM C365 testing should be performed on an electromechanical or static hydraulic universal testing machine such as Instron's 5980 Series.  The sandwich core will be compressed using compression platens. One platen has to be spherically seated and self-aligning, while the other platen is rigid. Instron recommends installing the spherically seated platen on the crosshead and the rigid platen on the base of the machine. The platens' surface area must be larger than the test specimen’s edges. If specimen conditioning is required, the environmental chamber has to be capable of maintaining the required temperature within + - 3° C.

• Any damage caused by poor material fabrication processes or specimen machining can cause high data scatter in sandwich structures. Joints, voids, or any other gaps in the core will contribute to data scatter as well. The testing environment can also have significant influence on the strength and failure mode of sandwich cores. Extra care must be taken to ensure proper specimen alignment and temperature conditioning in order to generate accurate results.

• LVDTs, compressometers, or similar devices can be used as displacement indicators if machine compliance is significant. These devices are required to be accurate within +- 1 %. 
• To read the full standard, purchase ASTM C365.