Horizontal (effect of axial load)

The effect of different axial compression loads on the horizontal hysteretic behaviour of the LRB are investigated by comparing horizontal hysteresis loops from horizontal displacement controlled characterization tests with compression levels equal to 50%, 100%, and 200% of the design compression force. Results are shown in Figure 95. Within each plot, all other loading parameters are the same except for the compression force acting on the bearing. The comparison is conducted at different loading rates:

(a) standard loading rate and (b) slow loading rate. The horizontal hysteresis loops are almost identical considering the different axial compression loads in each plot. It can be concluded that for the specific LRB specimens tested here, the compression force acting on the isolator has a negligible effect on the horizontal shear behaviour of the bearing.

(a) (b)

FIG. 95. Comparison of horizontal force-deformation behaviour of LRB for different levels of axial

compression. (a) Results from characterization tests conducted at normal test rate (0.33 Hz). (b) Results from characterization tests conducted at low test rate (0.005 Hz).

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5.3.2.1 Horizontal (effect of loading rate)

The effect of loading rates was investigated by comparing horizontal force-deformation hysteresis loops for a range of different loading speeds. Given the same sinusoidal shape and design displacement amplitude of the loading histories, horizontal displacement-controlled characterization tests were conducted with three different loading rates of 0.005 Hz (quasi-static), 0.33 Hz and 1.15 Hz.

From the comparison of the horizontal hysteresis loops shown in Figure 96, it can be observed that the loading cases with 0.33 Hz and 1.15 Hz are almost identical. For the quasi-static case (0.005 Hz), the hysteresis loop is thinner and the isolator force from cycle to cycle does not change much. While for the other two faster loading cases (0.33 Hz and 1.15 Hz), the isolator forces become smaller with each subsequent cycle. This behaviour is mainly caused by the heating of the lead core when loading speeds are large. For the very slow test the heat developed in the lead cores is negligible. The larger characteristic strength at the faster loading rates and the increased post-yield stiffness in the first half-cycle is caused by the strain-rate dependent behaviour of the lead core. Scragging effects in the rubber also contribute to the decrease of the post-yield stiffness after the first half-cycle, but it is believed that the stiffness contribution from the lead core is the main cause of this phenomenon. Almost identical observations were reported by Kasalanati [33].

FIG. 96. Comparison of horizontal force-deformation behaviour of LRB for different loading rates.

5.3.2.2 Horizontal (effect of temperature)

The effect of temperature was investigated by conducting characterization tests at ambient temperature and hot temperature (after a previous characterization run). However, due to the time required between each run to recharge the accumulators back to the required oil pressure, the difference between the starting temperatures of the two cases was quite small (28.4°C versus 30.7°C); therefore, as shown in Figure 97, the hysteresis loops of the two runs are nearly identical, with the hysteresis loop for the hot condition being slightly thinner as expected.

105 FIG. 97. Comparison of horizontal force-deformation behaviour of LRB for different bearing temperatures (warm: 28.4°C, hot: 30.7°C).

However, within each characterization test, the temperature increase was significant (as much as 10°C).

Therefore, if the isolator shear force history during the test is investigated as shown in Figure 98, the degradation of the isolator shear strength with each loading cycle can clearly be observed. This indicates that the horizontal shear strength, meaning the yield strength of the four lead cores, decreases with the increase of the temperature in the bearing. A numerical model which can capture the strength reduction due to heating of the lead cores in these types of bearings will be useful for future evaluations of this phenomenon.

FIG. 98. Comparison of horizontal shear force histories of LRB for different loading temperatures (warm:

28.4C, hot: 30.7C).

5.3.2.3 Horizontal (effect of displacement amplitude)

The isolator behaviour under different horizontal displacement amplitudes was investigated by comparing results of experimental runs for a range of loading amplitudes. As indicated in Table 23, the design displacement for the LRB is around 210 mm, which corresponds to 100% shear strain of the rubber. In a series of displacement-controlled characterization tests, loading displacement amplitudes were increased to 200%, 300% and 350% of the design level shear strain. The corresponding horizontal hysteresis loops of the isolator are shown in Figure 99. As indicated in the figure, when bearing deformations are larger, a significant hardening behaviour is observed starting at about 400 mm horizontal deformation (200% of the design displacement). The shear behaviour of the bearing before the hardening commences is almost identical for all cases. It is also interesting to notice that for the first half cycle of the hysteresis loops the shear forces in the LRB are generally larger than for the successive cycles. This phenomenon was already explained in Section 5.3.2.1.

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FIG. 99. Comparison of horizontal shear force-deformation behaviour of LRB for different displ. amplitudes.

The nonlinear hardening behaviour at large deformations indicates the need for a numerical isolator model which incorporates this unique behaviour. This becomes especially important when simulating the response of nuclear power plants for beyond design basis events. In the hybrid simulations conducted in this phase of the test program, the ground motion records were developed to match the design level target spectrum. Therefore, the isolator displacement demand was not expected to be significantly larger than the original design displacement of 210 mm. A model which can accurately capture the hardening behaviour will not be necessary for the purpose of these hybrid simulation tests. However, it will be important when isolator safety under extreme events is studied and evaluated.

5.3.2.4 Horizontal (failure test)

Finally, a test was conducted to characterize the horizontal failure behaviour of the LRB at extremely large shear strains. The characterization test was executed using an elliptical input motion targeting 580% shear strain. Failure of the bearing was observed at approximately 899 mm longitudinal displacement amplitude as indicated in Figure 100, this corresponds to 428% shear strain, a factor of 4.3 times the design shear strain. The failure mechanism of the bearing specimen is shown in Figure 101, which indicates that the failure was due to delamination between rubber and steel shim.

FIG. 100. Responses for horizontal failure test of LRB specimen (UET2). Upper left: Longitudinal shear deformation hysteresis loop. Upper right: Horizontal displacement orbit. Lower left: Lateral shear force-deformation hysteresis loop. Lower right: Shear force orbit.

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FIG. 101. Failure mechanism of LRB specimen (UET2). (a) Large horizontal shear strain before failure. (b) Failure observed between rubber and steel shim and extreme deformation of lead cores.

5.3.2.5 Summary

The LRB characterization test results were discussed in this section while focusing on the effect of different loading conditions. The results indicated that the horizontal shear behaviour of the tested LRB was quite stable and was not much affected by axial compression force and horizontal loading rate.

However, at faster loading rates an increase of the post-yield stiffness in the first half-cycle was observed. It was concluded that this stiffness increase was caused by the strain-rate dependent behaviour of the lead core. Temperature changes in the bearing had a moderate effect on the strength of the isolator.

Increasing core temperatures reduced the yield strength of the four lead cores which led to an overall bearing strength reduction from cycle to cycle. At larger shear deformations, the LRB isolator exhibited a gradual hardening behaviour commencing at about 200% horizontal shear strain. Horizontal shear failure occurred at approximately 430% shear strain, which was more than 4 times the design deformation of the LRB.

5.3.3 EQSB isolator