Numerical simulation of cracks behaviour and their propagation need true stress - true strain diagrams of materials. The new Gurson's approach needs dependence of a loading force F and change in diameter (DELTA)d of the specimen under tensile test, to determine some of necessary constants. Therefore two types of special extensometers were designed. Electrical resistance strain gauges and full-bridge circuits were used in both cases. In the first case the distance-adjusting device with two connected built-in beams was designated, Fig. 1. To avoid a time consuming modification for the specimen diameter adjusting a distance-adjusting mechanism was used. Disadvantage of this gauge is that it measures only a change in specimen diameter in one position. It is obvious that a change in a circular shape of the specimen can occur during a tensile testing, especially during necking. Therefore a new design of extensometer was designed using a loop around the cross-sectional circular area. That approach gives possibility to take into account appropriate shape change of the cross-sectional area giving a mean value of the change in diameter.
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