Hydromat Testing of Composite Laminates
Because of the anisotropic characteristics (different proprieties in different directions) of composite fibers, the traditional methods of flexural testing such as ASTM D790 do not always reflect the true strength of the tested laminates. This is also true with the standard tensile and compression strength and modulus tests such as ASTM D 3039, 695 or 3410. This is especially true when the laminates contain fibers that are off-axis, from the primary test coupon direction, be it in plane or in the z-direction. On any triaxial fabric it is impossible to test physical properties with a normal above-mentioned tests.
A second aspect to consider is the complex transfer of shear stresses in sandwich laminates. If a panel is designed for the shear strength of the core to be adequate in a large panel, the shear strength can be disproportional weaker in a smaller panel suitable for a test fixture. This is why so little testing of complete sandwich panels is performed.
An area of particular interest is the boundary layer between fibers and core materials. With the arrival of new resins systems and better impregnation techniques, resin infusion in particular, it has become questionable whether it is necessary to have a bedding layer between fibers and core.
In the 90's, Bill Bertelsen (Gougeon Brothers, Inc. Bay City, MI) developed a testing method, which solves the problems involved in obtaining true values for multi-axis, multi-layer composite panels. This testing method became patented in 1995 but now universally shared as ASTM D6416, commonly referred to as "Hydromat".
In an effort to obtain more precise data for typical boat laminates, particularly for bottom panels which are submitted to heavy bending and impact loads, GRPguru.com will conduct a series of tests in cooperation with Gougeon Brothers, Inc. and Riley Composites, Inc.
The tests will focus on the use of high quality epoxy and vinyl ester resins, multi-axial non-crimp fabrics and high-density core materials.
The values of some master panels will be verified by FEA, and used as a benchmark to obtain values for the submitted panels using the Hydromat tests. The results of the tests will then be used to generate updated values for laminate strength prediction programs such as Panalam and PPJ.
Companies that are interested in participating in this program, which will start June 15, 2001, or want to submit their materials for testing can contact GRPguru.com via the website or contact Andre Cocquyt by tel.: 561 286 6125 or e-mail at GRPguru@yahoo.com