ASTM E756-05(2017)

5.3.1 All damping measurements are made in the linear range, that is, the damping materials behave in accordance with linear viscoelastic theory. If the applied force excites the beam beyond the linear region, the data analysis will not be applicable. For linear beam behavior, the peak displacement from rest for a composite beam should be less than the thickness of the base beam (See Appendix X2.3).

5.3.2 The amplitude of the force signal applied to the excitation transducer is maintained constant with frequency. If the force amplitude cannot be kept constant, then the response of the beam must be divided by the force amplitude. The ratio of response to force (referred to as the compliance or receptance) presented as a function of frequency must then be used for evaluating the damping.

5.3.3 Data reduction for both test specimens 2b and 2c (Fig. 2) uses the classical analysis for beams but does not include the effects of the terms involving rotary inertia or shear deformation. The analysis does assume that plane sections remain plane; therefore, care must be taken not to use specimens with a damping material thickness that is much greater (about four times) than that of the metal beam.

5.3.4 The equations presented for computing the properties of damping materials in shear (sandwich specimen 2d - see Fig. 2) do not include the extensional terms for the damping layer. This is an acceptable assumption when the modulus of the damping layer is considerably (about ten times) lower than that of the metal.

5.3.5 The equations for computing the damping properties from sandwich beam tests (specimen 2d–see Fig. 2) were developed and solved using sinusoidal expansion for the mode shapes of vibration. For sandwich composite beams, this approximation is acceptable only at the higher modes, and it has been the practice to ignore the first mode results. For the other specimen configurations (specimens 2a, 2b, and 2c) the first mode results may be used.

5.3.6 Assume the loss factor (η) of the metal beam to be zero.

5.4.1 With the exception of the uniform test specimen, the beam test technique is based on the measured differences between the damped (composite) and undamped (base) beams. When small differences of large numbers are involved, the equations for calculating the material properties are ill-conditioned and have a high error magnification factor, i.e. small measurement errors result in large errors in the calculated properties. To prevent such conditions from occurring, it is recommended that:

5.4.2 Test specimens Fig. 2b and Fig. 2c are usually used for stiff materials with Young's modulus greater than 100 MPa, where the properties are measured in the glassy and transition regions of such materials. These materials usually are of the free-layer type of treatment, such as enamels and loaded vinyls. The sandwich beam technique usually is used for soft viscoelastic materials with shear moduli less than 100 MPa. The value of 100 MPa is given as a guide for base beam thicknesses within the range listed in 8.4. The value will be higher for thicker beams and lower for thinner beams. When the 100 MPa guideline has been exceeded for a specific test specimen, the test data may appear to be good, the reduced data may have little scatter and may appear to be self-consistent. Although the composite beam test data are accurate in this modulus range, the calculated material properties are generally wrong. Accurate material property results can only be obtained by using the test specimen configuration that is appropriate for the range of the modulus results.

5.4.3 Applying an effective damping material on a metal beam usually results in a well-damped response and a signal-to-noise ratio that is not very high. Therefore, it is important to select an appropriate thickness of damping material to obtain measurable amounts of damping. Start with a 1:1 thickness ratio of the damping material to the metal beam for test specimens Fig. 2b and Fig. 2c and a 1:10 thickness ratio of the damping material to one of the sandwich beams (Fig. 2d). Conversely, extremely low damping in the system should be avoided because the differences between the damped and undamped system will be small. If the thickness of the damping material cannot easily be changed to obtain the thickness ratios mentioned above, consider changing the thickness of the base beam (see 8.4).

5.4.4 Read and follow all material application directions. When applicable, allow sufficient time for curing of both the damping material and any adhesive used to bond the material to the base beam.

5.4.5 Learn about the characteristics of any adhesive used to bond the damping material to the base beam. The adhesive's stiffness and its application thickness can affect the damping of the composite beam and be a source of error (see 8.3).

5.4.6 Consider known aging limits on both the damping and adhesive materials before preserving samples for aging tests.