160 K. Perry et al. Fig. 20.3 Schematic load history for the multiple and incremental step test methods Applied Stress Load Steps Multiple Step Method Incremental Step Method testing control software to trigger taking phase shifted moiré interferometry data while the fatigue test machine is paused. Using the block configuration mode within the fatigue test machine software, the machine could fatigue the specimen for a given time, pause the machine, start phase shifting and acquiring image data, and automatically restart cycling after the data was recorded. To permit fully automatic data collection, a quantitative signal to noise measurement algorithm was implemented to verify the quality of data taken during automatic tests, and to recollect any data smeared by vibrations or other environmental factors. The surface images produced by the wrapping algorithm are digitally correlated to automate the measurement of strain in the presence of moving bimaterial interfaces. 20.2.4 Loading Sequences and Cyclic Test Parameters Various test methods are available to obtain the cyclic stress-strain behavior for a given material [8]. In all methods, a specimen is cycled between fixed load or displacement conditions until some criteria of stability is met. The cyclic stressstrain curve is determined from the stabilized load or stress plotted as a function of strain. Three test methods are considered in this study: the multiple step, incremental step and single step tests. The multiple step method uses a single specimen tested at progressively higher load levels. Specimens tested according to this protocol received a minimum of five cycles at load levels from 50 to 300 N in increments of 50 N. Additional cycles were applied as necessary at each level until a stabilized response was obtained. The load history of a multiple step test is shown in the top of Fig. 20.3. The incremental step method applies a specific sequence or ‘block’ of loading to a single specimen. The block loading sequence is applied again until the stress-strain curve of the specimen has stabilized. The block sequence is an increasing then decreasing waveform of peak loads and unloads of the specimen, generally with five load levels within the block. A typical load history for the incremental method is shown in the lower part of Fig. 20.3. Five blocks of loading are shown in the diagram. Several combinations of test parameters for the incremental step method were explored and results for two cases are included here. The first specimen was tested with peak loads from 150 to 250 N in 25 N increments and the other from 150 to 275 N in 30 N increments. The single step method uses individual specimens tested at constant load levels. Results for a single load level at 275 N are included here.
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