Theory:

ASTM D7249 cover a procedure for flexure testing of sandwich constructions. Composite sandwich materials are commonly used in the construction and building industry. Bend testing can be used to determine the flexural strength, core shear strength, shear modulus, as well as the tensile and compressive properties of the sandwich facings. 

​

Geometry:

A 75x600mm composite sandwich is subjected to either three point or four point loading. For 4 point loading, the top loading beam span is separated by 100mm and then loaded directly in the middle of the sample. 

​

This test can be used on on both continuous and non-continuous core materials. Continuous core material would be a uniform piece of wood or particle board.  A non-continuous core material would be a honeycomb construction. 

Solution:

Universal Grip offers either a basic or specialized 3 point or 4 point bend fixture specifically designed for testing ASTM D7249. The fixture attaches to any universal testing machine via clevis pin style adapters. The basic fixture includes some simple rollers for contact points.

​

The specialized ASTM D7249 fixture includes the articulating loading pads with the required width and thickness. 

​

Along with bend fixtures, UGC also offers a full line of universal testing machines from the Italian manufacturer Galdabini. All testing machines include software and support and can be pre-programmed with the desired testing standards and analysis calculations. 

Analysis:

There are several failure modes for this test and each type of failure should be noted in the test report. An LVDT sensor can be used to achieve more accurate and precise results. Force displacement behavior is used to identify any transition zones where the behavior of the force-displacement curve changes.

​

Facing Ultimate Stress is calculated using the given formula. This formula is used when the facings are considerably thinner than the core material. The effective facing chord modulus as well as the flexural stiffness can also be calculated using the formulas given in the test standard. 

​

Similar Specifications:

ASTM C393 - Sandwich Composite Flexure Strength