Modifications to existing simple shear device

Major modifications have been made to the existing CU simple shear device in order to measure very small shear strains with minimal noise disturbance and to perform automated quasi cyclic shearing.

First major modification is the replacement of existing small horizontal D/C LVDT that is used to measure small shear strains with a more sensitive A/C LVDT in very small strain tests. The noise level in the existing LVDT readings was too large to measure very small strains accurately. In fact, it was never intended to measure shear strains up to 0.01% in the past. Hence, it is not possible to carry out very small strain controlled shearing to calculate Modulus and Damping with this LVDT.

Therefore it was decided to include a very small-range ‘Honeywell’ A/C LVDT. This LVDT has a range of ± 0.2 mm and has a very high resolution (0.02 m). It is less sensitive to electrical noise and other disturbances. The inclusion of this small LVDT facilitates measurements up to 10-3% shear strain amplitudes. The fluctuation of this LVDT due to noise and other disturbances during a period of 8 hours is shown in Fig 3.5 and compared with that of existing horizontal D/C LVDT. Furthermore, this new small LVDT was fixed in a specially designed LVDT holder as shown in Fig 3.6. Such arrangement of this LVDT (measuring towards left) using the special assembly minimizes any accidental damage due to an unexpected impact or large strain event. This is important as the LVDT is vulnerable to damage due to its very small range and spring mechanism.

This LVDT is connected to the signal conditioner through a special device which transforms A/C signal from this LVDT to D/C signal. Furthermore, the data acquisition

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programme has been modified to switch automatically between the two horizontal LVDTs when the shear strain exceeds the limits of small LVDT.

Fig 3.5 Noise level readings in horizontal small A/C and large D/C LVDTs

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Another important aspect of undrained simple shear tests is maintaining ‘zero’ volume change. Even though this is achieved by locking a vertical clamp in the device, it is important to closely monitor any kind of vertical displacement due to wear and tear and/or defects in the vertical clamp. The current large D/C vertical LVDT employed in the device to measure consolidation is vulnerable to noise disturbances, hence, less sensitive in measuring very small vertical displacements accurately during shearing phase. In addition, it shows false large displacement readings due to noise levels.

Therefore a second very sensitive small A/C LVDT (similar to the small horizontal LVDT) was included to monitor the vertical displacements during undrained shearing. By employing this LVDT, it was possible to confirm that vertical strains are smaller than 0.01% during shearing. The fluctuations of this small vertical LVDT during a period of 8 hours are compared with that of the A/C LVDT in Fig 3.7; it clearly illustrates that the small LVDT is less affected due to noise level and other fluctuations and provide reliable measurements.

Another major modification is made to the data acquisition programme to felicitate very small strain controlled quasi cyclic loading. Existing system enables the user to program only one loading step at a time in the monotonic mode. Therefore, user must interact at the end of each loading or unloading stage, in order to perform a strain controlled quasi cyclic loading. This can cause interruptions and delays which result in stress and / or strain variations at the end of each stage.

In order to overcome this issue, data acquisition system was modified to program all the loading steps at the beginning of the shearing through a user friendly interface as shown in Fig 3.8. Here, the user can define the maximum strain limit, direction of loading and

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strain rate for each loading step. Hence a cyclic strain controlled loop can be performed without an interruption.

In addition, existing data acquisition system controls stepper motor based on pulse count during monotonic loading. Required strain is converted to the number of necessary pulses needed to be produced by stepper motor. Hence, any possible delays or inaccuracies would result in lack of target strain. The motor would stop even before achieving target strain, if the full pulses would have been applied. In order to overcome this issue, data acquisition system was modified to check the strain at the end of pulse count. It was programmed to calculate and apply required pulses to achieve target strain in case of a deficit. This new feature ensures the achievement of intended target strain without any deficit.

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Fig 3.8 User interface for programming automated repeated monotonic loading

Similarly, a modification was made to the cyclic loading system as well. Existing system allows user to program one specific cyclic stress value ( ) at a time. This was modified to program multiple stages with different cyclic stress amplitudes at the beginning of shearing similar to multi stage strain controlled loading. Parameters such as the required number of cycles, frequency and stop criterion can be specified for each step. This feature is helpful in performing a multi stage cyclic stress controlled test with different cyclic stress amplitudes without any interruption.

These modifications enable CU simple shear device to run tests smoothly and provide confident measurements at small strain and stress amplitudes which cannot usually achieved in a typical simple shear device.

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