ISO 18437-5:2011 pdf free download – Mechanical vibration and shock – Characterization of the dynamic mechanical properties of visco-elastic materials – Part 5: Poisson ratio based on comparison between measurements and finite element analysis.
8 Sample preparation and mounting
Samples may be either moulded to the desired shape or cut from a slab of the material. Ideally the sample is moulded directly to rigid metallic end plates but may also be bonded securely with a rigid adhesive. It is recommended that a minimum of three representative samples (six for the two-sample method) be tested.
9 Sample conditioning
Test samples shall be adequately aged after moulding or vulcanization. The test samples shall be thermally conditioned at each test temperature. In addition, materials are often sensitive to humidity. ISO 23529 shall be used as a guide for determining the temperatures, humidity and times for conditioning and testing.
10 Main sources of uncertainty
When measurements are performed by the compressional stiffness method the possible reasons for uncertainty may be:
— improper acoustical contact;
— inaccuracy when manufacturing the test samples;
incorrect choice of sample sizes;
large phase shift between the device channels for measuring the values of forces and strains;
— improper or inadequate calibration of the sensors.
If acoustical contact between the test device and the test sample is insufficient (loose), the measured values of stiffness can be less than the actual value.
Uncertainty connected with manufacturing of the test sample can lead to the appearance of displacements, which are different from the measured displacements. Therefore the measurement results cannot be attributed to the specified sort of displacement. To avoid these uncertainties it is necessary to achieve the required parallelism of contact surfaces with the test device, and also to assure the test sample’s symmetry for eliminating sample misalignments.
Failure to conform to the sample size requirements given in 5.3 can result in errors, for example:
if the sample stiffness is comparable with stiffness of test device, the stiffness properties of the test device influences the measurement results;
— if the sample stiffness is small, the force transducer produces a signal which is comparable to background noise of test device;
if the condition for the stiffness characteristics of the test sample is not fulfilled, the sample’s resonance has an influence on the measurement.
NOTE The uncertainty of this method is not universally known. However Reference [5] reports a Poisson ratio of 0,450 with a standard deviation of 0,003.
11 Time-temperature superposition
In each of the single-sample measurement and two-sample measurement methods, the limited frequency range of measurement may be extended through the application of the time-temperature superposition method. See ISO 18437-2 and ISO 18437-3.
In principle, the dynamic properties of a vibro-acoustic isolator are dependent on static preload, vibration amplitude, frequency, and temperature.
The assumption of linearity implies that the principle of superposition holds and that the dynamic stiffness at a given frequency is independent of amplitude. For many isolators, this assumption is approximately satisfied when under the appropriate static preload the dynamic deformation amplitudes are small compared with the static deformation. However, it should be noted that this depends on the materials of which the isolators are composed and a simple check should be carried out by comparing the dynamic stiffness characteristics for a range of input levels. If these are nominally invariant, then linearity may be assumed to hold.
For butyl rubber (IIR), Reference [2] presents data for the in-phase component and the phase angle of the dynamic shear modulus as a function of strain amplitude and of the percentage of carbon black. For strain amplitudes smaller than about iO, the in-phase component and phase angle are hardly dependent on the vibration amplitude. However, a significant decrease of dynamic stiffness is seen when strain amplitudes exceed about 2 x 1 , especially for rubber with a high percentage mass fraction of carbon black.