High Velocity Three-Point Bending Test – Fiber Cement Materials

The mechanical properties of plastic materials are sensitive to loading rate and the fracture toughness under dynamic loading has received considerable attention because it often concerns the failure of structural materials during their service life.

Static three-point bending tests investigate the deformation and failure behavior however for a complete mechanical characterization of the materials it is necessary to analyze the response under dynamic conditions. This is because the force-deformation curve depends on the speed with which the load is applied (the strain rate). As the strain rate increases, the modulus of the material increases and so does its yield point. To simulate the behavior of a material under dynamic conditions it is therefore necessary to have and validate data at high strain rates. For the high velocity three-point bending test, an instrumented impact drop tower equipped with an insert of a given shape attached to the tup can be used. The specimen is suspended on two cylindrical supports and the insert strikes it in its center. During the impact, the load is recorded by the sensor (piezoelectric or strain-gauge) embedded in the tup. The load-time curve is then processed to obtain a force-deflection curve.

We were recently asked by a manufacturer of fiber cement building products to recommend an impact system that would allow them to test their products for impact resistance – specifically as related to the installation process. Fiber cement products offer an alternative to owners looking to refurbish the exterior of their homes. A composite material made of sand, cement and cellulose fibers it is fire, rot and insect (termite) resistant. Considered green and sustainable in appearance it can imitate clapboards or shingles; in sheet form it can be used as cladding & as a soffit/eave lining. Besides exterior siding fiber cement can be used to substitute timber fascias.

Either the Model 9440 or the Model 9450 Impact Test Machines is capable of performing the tests. A 3 point bend fixture to support the specimens was chosen by the customer. By using tup inserts of different shapes and sizes impacts of various types can be performed. Adding in an instrumented tup, Bluehill Impact software, Data Acquisition and Analysis System, allows the customer to gather information such as energy absorbed, and incipient damage point from each of the tests performed.

The 9440 or the 9450 are well suited for helping customers determine the performance characteristics of their materials when subjected to an impact event. The information gathered by doing instrumented impact testing can be used to provide the customer with a data baseline whereby they can determine how changes made in the mix of materials will perform in real life situations. A broader range of testing could be accomplished with the use of an environmental chamber to study impact characteristics at different temperatures as well as using different fixtures and inserts to simulate other impact events.