Mechanical Test Procedure
The tension, compression, and shear tests at the low movement rate (v = 50 mm/min) were performed using a conventional, screw-driven 10 kN mechanical test machine (Zwick GmbH & Co., Ulm, Germany). For higher movement rates (v2 = 0.5 m/s to v5 = 5 m/s) a 20 kN servo-hydraulic mechanical test machine (Zwick REL 1856 with a clamp speed range of 1 to 10 m/s) was used. All machines were fitted with precision strain gage load cells for continuous operation at maximum dynamic loading. Different adapters were fabricated to mount the specimens in the test equipment (see Fig. 8).
In tests with higher movement rates (v2 to v5) the striking cylinder of the testing machine was accelerated up to the required speed before making contact with the receptor on the specimen holder. The impact of the striker against the receptor caused an elastic compression wave (incident wave) to propagate along the specimen holder. Therefore, in the tension tests a conical attachment
FIG. 8—Adaptors used for mounting the various specimens in the test equipment (shown from left to right: adaptors for specimens H0, M, and L).
was used as a receptor to minimize vibrations induced by the impact in the test specimen, as the vibrations, which increased with higher movement rates, interfered with the measurement of the force signal. The amplitude, frequency and damping behavior of these vibrations depended on the test setup including the adapter used and the specimen type, especially its mass, stiffness, and damping characteristics. The tensile responses of the silicone sealants were measured to the point of fracture of the specimens. The experimental setup allowed for a constant movement rate in all tension tests until rupture of the specimen occurred.
In the compression tests the striking cylinder decelerated towards the end of the test, however, the experimental setup was such that it always achieved a displacement of about 9 mm after making contact with the test specimen holder.
The ambient conditions during the test were 20±2°C and 50±5 % relative humidity. The load-displacement histories were recorded by digitally measuring the signals of force F(t) and displacement x(t). Fast piezoelectric quartz load-cell devices were used to measure the forces at higher movement rates (v2 to v5). A fast noncontact optical displacement sensor was employed in the compression tests with higher movement rates (v2 and v3). The maximum sampling rate was 15 kHz (used at v5 = 5 m/s) and the sampling rate was adjusted such that the load-displacement curves within time intervals down to below 10 ms were represented by a minimum of 100 measurement values.
The measured data were evaluated to determine the maximum stresses, crmax and rmax, and the corresponding strains emax and ymax, in tension or shear, respectively, at maximum loads Fmax. The stresses and strains were related to the actual values of the cross-sectional area (width (W) by height (H) by length (L)) determined for each test specimen with an accuracy of ±0.1 mm.
Omax = FmJ(W ■ L) Tmax = Fmax/(W ■ L) (6)
Strain at maximum load:
Gmax = xmax/H X Ю0 % ^ax = xmax/H X Ю0 % (7)
The maximum loads in the compression tests were given by the limits of loading devices Fmax=10 kN and 20 kN, respectively. In this case stresses o50 % at strains of є = 50 % were evaluated:
Compressive stress at є = 50 %:
°50 % = F50 %(W ‘ L)