versus displacement of any particular location on the shell wall. Good results are obtained if the user specifies these points to be those locations on the shell wall where maxima and minima occur in the buckling waveform. Included in this section of the program is the capability of generating Southwell plots from the load versus displacement data. These Southwell plots turn out to be very helpful in the determination of critical loads, as will be explained later.
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insertion. A long screwdriver is inserted into the lower end plug and with the help of a flashlight, the positioning bolt is turned until the probe to wall distance is within the linear range of the DSD calibration. A digital Voltmeter connected to the proximitor provides the necessary information for proper positioning. Next, a scan is completed and on the basis of this scan the probe is repositioned. This procedure is repeated until a satisfactory position has been found. This location will be such that all circumferential points are within the linear range, but skewed towards the higher part of the calibration curve. As the load increases, the deformations will tend to be more radially inward than outward and the probe reading will therefore remain within the linear region during the experiment.
Position data are picked up by the DSD and sent to the data acquisition system. Green blinking lights on the control circuitry indicate data acquisition of each individual point.
An external platform supports the pressure sleeve since the end plugs are free to move within the pressure chamber and do not provide support. Hydraulic fluid enters near the bottom of the pressure chamber through an external valve. The bleed valve on top of the dial indicator helps air escape during filling operations. Figure 2.26 a and b shows the support equipment and the test set-up. The initial scan is made with no load applied to the system.
To initiate the scan, the DSD electronic circuitry is reset and one of the two green indicators on the system lights up, indicating that the system is waiting for the correct starting position. The data acquisition program must now be advanced to the point where it waits for the external clock to trigger before data can be taken and stored. When the green light goes out and the neighboring light starts to blink at a constant rate, the beta channel has been activated and data are being acquired until a full circumferential scan has been completed. Pressure and tension data are also recorded by the program and printed together with displacement data by the system printer. The option of rejecting the scan allows the user to redo the scan before plotting and storing the scan at that load level.
After the pressure chamber has been filled with hydraulic fluid and properly bled of all
remaining air, the actual loading of the specimen can start when the Signal Interruption System (SIS) has been activated. Function generator control is now running the experiment and the operator only specifies when the scanning system should perform a scan. Regular intervals during low loading conditions with increased scanning near the buckling point are advised for accurate buckling load determination. This way more points are obtained near the buckling load producing better Southwell plots as shown in Chapter 3.
Buckling often occurs quite suddenly accompanied by a noticable "pop" and is immediately followed by a load decrease due to rapid decrease of control signals by the SIS. Often the probe gets stuck between large buckling waves and unwinds itself from the DSD drive shaft. This "unwinding" prevents probe damage since the scanning motor continues turning the probe-shaft after buckling The thread on the shaft is such that continued turning of the shaft causes the probe to unwind from its support and drop down into the lower endplug, a highly desired feature that saved the probe many times. Data acquisition continues but because of a constant probe signal, the last plot, after Fourier analysis, is often quite different and out of bounds when compared to previous scans during the same experiment.
Sometimes during high tension operations the shell is able to carry load after buckling and shell buckling deformations are of the order of prebuckling deformations. The Signal Interruption System does not shut off the load control signals during these loading
conditions. The next scan after buckling will then produce the postbuckling shape, since the probe is still able to scan the shell wall. Besides the changed shape of the shell wall, a temporary vibration in the loading system also indicates buckling. The vibration is stopped by reducing the amplifier gain of the load feedback loop. The load-path is traced on a plotter that is connected directly to the load signals during the experiment, and this plot provides another means of determining approximate buckling load and any other operational problems of the system.
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Removal of the buckled specimen is possible by reversing the assembly process.
Figures 2.27 a and b depict some of the specimens after they have been removed from the end plugs through the application of localized heat. It is important to note that often the specimen shows little indication of buckling when the SIS is in operation. Large postbuckling deformation is prevented when the load is removed from the system.
However, the specimens shown in Figures 2.27 a and b have been exposed to continued loading after bifurcation, to bring out the buckling wave form for visual inspection.
Normally, one would not be able to see the buckling waveform with the naked eye because of the load interruption by the SIS. Finally, all pertinent data are transferred to permanent storage and the system is ready for the next test.
3.0 EXPERIMENTAL RESULTS