Tests on Plastic sheets
Plastics include thermo hardening and thermoplastic polymers. For practical engineering applications, plastics are usually considered as globally isotropic materials. The elastic behavior can hence be described by 3 engineering constants, a Young’s modulus E, Poisson’s ratio v and the shear modulus G. Because isotropic materials have only two independent elastic properties, there is a relation between the engineering constants:
The Engineering constants E, G and v can be identified with 3 resonance frequencies measured on a square test sheet. Hence, no test beams are necessary.
Because many plastics objects are manufactured with injection molding machines, the material properties are oriented randomly and so they are not “perfectly” isotropic. However, the Resonalyser procedure on a square test sheet, considered as globally isotropic material, yield a global average over the area of the sheet. This is the value engineers can use in software packages for designing polymer parts. The Resonalyser automatically provides a result together with an uncertainty value. The more the plastic test sheet deviates from perfect isotropic behavior, the larger the computed uncertainty bounds will be.
Tests on Plastic bars
Tests on plastic bars with the Resonalyser will provide (only) the longitudinal Young’s modulus. Because of the longitudinal orientation of the bars produced with injection molding, the longitudinal modulus will be higher than the average modulus found on test sheets.
Example: Measurement on a test beam