Proof Stress Indicators for Stress/Strain Testing Machines

摘要

1,156,243. Determining stress/strain characteristics. INSTRON Ltd. Feb. 14, 1968 [Feb.15, 1967], No.7357/67. Heading G1N. The stress/strain characteristics of a test material obtained electrically is continuously supervised as the load increases to determine when the relationship diverges from linear by a predetermined amount, corresponding to the proof stress value of the load. The linear relationship is represented by the portion of the characteristic 2 (Fig. 1) between two predetermined points A, B. The load on the test material is measured by e.g. strain gauges, connected by a servolink to the slider of potentiometer 7 (Fig.2), whereby the signal voltage at the slider represents the instantaneous stress. The load values corresponding to points A, B and the displacement of line 3 from line 2 are set up on potentiometers 10, 11 and 17 respectively. When the stress reaches the first value A, amplifier 8 emits a signal which energizes clutches 12 causing the slider of potentiometer 13 to follow the load and the slider of potentiometer 14 to follow the extension of the test material. The stress and strain signals being fed to a recorder. When the load reaches the second valve B, amplifier 9 emits a signal which causes a memory circuit 19 to reduce the voltage across potentiometer 13 until the stress and strain signal inputs to amplifier 16 are equal and opposite. A timer circuit including capacitor 28 and resistor 30 subsequently causes switching device 37 to close switch a, whereby the input to amplifier 16 is augmented by the signal from potentiometer 17. When the amplifier input is again zero, indicating the attainment of point 4 on the stress/strain characteristic 2, the signal output from amplifier 16 is differentiated at 33, 34 and indicated as a short pulse by the recorder. The proof stress value of the load is obtained from the record.