Strain Pacing

Metals test type only. Strain pacing is an outer loop software algorithm that monitors the current data and alters the position rate to achieve a user defined tensile strain rate as determined from the tensile strain sensor. The pacing facility within the software allows the straining rate of the specimen to be controlled to a pre-defined level. It is intended for use with materials that show smooth, continuous deformation characteristics in the elastic region, such as metals and rigid or semi-rigid plastics. Many such materials are sensitive to the straining rate applied during the materials test and it is therefore desirable (and sometimes mandatory) to control, or at least limit, the strain rate. The actual maximum strain rate that can be controlled depends upon the characteristics of the specimen to be tested and the strain rate accuracy requirements. The lower the specimen stiffness and the more gradual the yield, the higher the strain rate that can be used. Instron electromechanical testing machines normally run at a constant rate of crosshead displacement (i.e. crosshead speed) by using a position controlled servo-loop. The characteristics of the testing system and specimen are such that a constant rate of crosshead displacement may not result in a constant strain at the specimen, particularly in the yielding region. Hence the need for controlling the straining rate. In pacing mode, the software constantly alters the crosshead speed to achieve the demanded rate. The extensometer monitors the actual rate on the specimen, and the software computes the crosshead speed required to maintain the demanded rate as the test proceeds. The crosshead speed slows down as the material yields in order to maintain a constant straining rate due to the fact that most of the crosshead displacement is translated into permanent specimen deformation. The pacing mode is intended for monotonic (unidirectional) tests only and is not suitable for controlling the straining rate during transient effects such as upper and lower yield or discontinuous yield phenomena.
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