Performing standard tests

When running a test through GDSLab, make sure to reference all of the GDSLab handbook sections involving relevant test modules.

Running the first test

Install a dummy test specimen. This could be made up using a metal cylinder (of aluminium alloy, say, 70 dia. by 140mm high) and one large (or, say, three smaller) laboratory rubber bungs. Place the rubber bungs between the top of the metal cylinder and the triaxial test specimen top-cap. Perform a constant rate of strain test (perhaps using advanced loading or standard triaxial modules). Choose parameters appropriate to the dummy test specimen. Use a strain rate of 50mm per hour say and terminate after 2mm. Plot the results and check for noise levels. On the screen, plot deviator stress against axial displacement. From the plot, calculate the stiffness of the dummy test specimen in kN/mm. This stiffness will be useful in the dynamic test.

Running the first dynamic test

Select the Dynamic Triaxial Testing module and choose axial displacement control. Select 1Hz at +/- 1mm say (if the dummy test specimen will stand it). Choose 64 data points per cycle, 2 cycles on, 8 cycles off, terminate after 10 cycles.

Running other tests

Going back to the GDSLAB object display, you can check that the load control is working by setting a small change in target load. Observe the motion of the pulleys and the belts and the deformation of the dummy test specimen. From your ad hoc calibration of the stiffness of the dummy test specimen, check that the observed deformations are about right. On the screen, note the resolution (i.e. variation) of control of axial force (load). Normally, you should expect better than 1% of full range at loads greater than about 1/20th of full range.

If all is OK you can then start testing with real soil.

General Maintenance

General maintenance consists of keeping the machine and its components clean and free of grit. The system comprises of many o-rings and seals, which can be compromised if loose sand/particles get into contact with them, so it is important that if you do take anything apart on the machine, to contact GDS for maintenance instructions prior.

There are no user-serviceable parts in any of the GDS controllers or data acquisition instruments.

User Adjustments

The main user adjustments are concerned with the calibration of the data acquisition system. The simplest way to achieve this is to calibrate the cell pressure controller as a master device and then use this to pressurise the cell to 1000 kPa and check the calibration of the pore, back and alternate pressure transducers. If any adjustments need to be made they can be made within the transducer setup in the Object Display, in the GDSLAB software.

The load cell calibration can be achieved by ensuring that the 10v excitation is exactly 10v and then setting the sensitivity as stated on the manufacturer calibration certificate into the transducer setup in GDSLAB. There is a helpsheet (helpsheet 125) available on the GDS website to download for further instructions of how to calibrate specific transducers.

The main user software adjustment (if necessary on your system) is the selection of the gain parameters for the axial controller to give optimum cyclic performance for LOAD control tests. The axial gain parameter is requested as the initial (or maximum) stiffness of the test specimen. This is expressed as kN per mm deformation. The higher this value is the less responsive the axial control system is. If you suspect uncontrolled oscillation is occurring in the axial controller the value of axial stiffness used should be increased.

Adaptive Control

What is adaptive control

Adaptive Control allows the user to run a dynamic test from start to finish under load control without having to worry about the changing stiffness of the specimen. Under normal control the system relied on the user inputting a stiffness estimate value. The stiffness of the specimen would change during a test and the user would have to input further stages with a different stiffness estimate that would then aid the system in reaching its target values.

With Adaptive control turned on the system uses the following two components to change how its target values are archived.

• Feed-forward control – this is the main driving component of the control loop. The feed-forward control component drives the system with what the control system has calculated should be the correct driving voltage for the desired output. The PID control loop then does the fine control adjustment on top of this feed-forward control. The feed-forward component also has a correction applied to the feed-forward term, which adjusts the feed-forward term by a filtered difference between the calculated feed-forward value and total driving force sent to the system on the previous control loop.

• Adaptive control – the control system continually calculates a filtered value of the compliance (stiffness) of the specimen. When performing load or stress control, all of the control terms (feed-forward & PID) are factored by the compliance of the specimen to adapt the control system to the changing specimen behaviour.