2.6 MATERIAL PROPERTIES
2.7.1 UNIAXIAL TEST PROGRAM (STRESS-STRAIN)
The basic features of this program include the continuous acquisition of stress-strain data and subsequent plotting of these data. An existing program by Tipton [Ref. 16] was modified for these purposes. Calibration of the load cell and extensometer is the first
objective of the program. This requires, under the control and directives of the program, that calibration weights be added to the load cell calibration fixture. The program tells the user to add the correct weight at the proper time. Then, the program asks for calibration data for the extensometer. The central idea in supplying these strain data is the same as for the load data.
Load cell or extensometer data can be entered manually, recalled from memory or obtained directly from a calibration while the program is in operation. After these
calibration data have been entered into the program, a least squares fit will be performed on both sets of data. The slope of the linear calibration curve, intercepts and standard
deviations are calculated and printed for interpretation of accuracy. These variables now form the basis of the uniaxial test and if these calibration data are inaccurate, the test data will be inaccurate.
At this point the program asks for the total length of the test and for the required data resolution The program will then compute the number of data points to be taken by the RIMS system.
When the actual test is started, the stress-strain behavior of the material can be directly observed on the plotting system. The program will obtain data on a continuous basis from the load-cell and extensometer for a preset time. After the test is finished, these data are transferred from local memory to a more permanent medium.
An extension of this program helps in the determination of the average stress-strain curves by generation of generic Ramberg-Osgood stress-strain curves. The user can vary hardening and yield parameters to obtain the best fit with the actual experimental data.
After the user selects the most suitable stress-strain curve, the relevant parameters such as yield and hardening are printed for input into the numerical analysis.
2,7.2 BUCKLING TEST PROGRAM (TUBE)
The main features of this program (TUBE) are control of circumferential scanning and
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recording of load and displacement data. This program has been written to record all pertinent data obtained during tests with the DSD and load-control systems.
After the DSD has been properly positioned in the test chamber, a scan is initiated with no loads imposed on the structure. The Analog-to-Digital converter will not provide the program with scan data until the Beta channel on the converter has been activated (or opened). Data acquisition starts at the same location on the shell because of a second sensing system, which is located over the position sensing gear in the DSD. Once the Beta channel is opened, the program waits for the external clock pulse before data are taken from the displacement sensing circuitry. The external clock will trigger each time a polished gear tooth passes the detection system until the complete shell has been scanned.
Resolution of the data points is picked high enough for buckling detection. Immediately after the scan the program obtains load-data from the load cell and pressure transducer and stores these data along with the DSD scan data. Tube axial displacements are read directly from the LVDT in the axial loading machine and are recorded for a number of experiments.
Besides plotted output of the scan or displacement profile, the program also prints load-path data on the system printer. This output is kept with the plotted output and provides a means of identifying where and how the data are stored. The program will then return to its starting point and wait for a new signal to proceed with another scan. After the complete test has been finished and the specimen has buckled, the data set for the whole experiment is transferred from local memory to permanent memory in the form of a floppy disk. Figure 2.24 depicts some of the experimental scans together with results obtained after manipulation of experimental data. Load-path and a buckled specimen are also shown in this figure.
Extensions of this program include several plotting options of the scan and load data.
Data can be recalled from memory and replotted after the initial imperfection has been subtracted. This type of manipulation shows more clearly the formation of the buckling waveform and the number of buckling waves. Another option allows the user to plot load
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.