Proof stress indicator

Claim 1

A proof stress indicator for use with a stress/strain testing apparatus in which there is produced during the testing of a specimen two electrical signals which are respective analogues of the increases in the stress and strain of the specimen above the particular values thereof obtained at a selected earlier stage of the testing, said proof stress indicator comprising: A. a voltage-matching device operable in dependence upon an input voltage for supplying an output voltage equal in magnitude to said input voltage and including: 1. an analogue memory device having: a. a supply line; b. first and second control inputs; c. a capacitor for storing an analogue voltage; d. charging means connected between said capacitor and said supply line for charging said capacitor from said supply line and including a first switching means connected with said first control input and responsive for charging said capacitor only when a first control signal is being applied to said first control input; e. discharging means connected across said capacitor for discharging said capacitor and including a second switching means connected to said second control input and responsive for discharging said capacitor only when a second control signal is being applied to said second control input; and f. a readout circuit of high input impedance having input connected to said capacitor and an output for providing a readout of analogue voltage stored in said capacitor; 2. command means connected to said memory device for applying said second control signal to said second control input; 3. a comparator circuit having an input connected to said memory device and an output connected to said command means, and being connected and operable for comparing with one another said input voltage and a voltage taken as a percentage of the output of said analogue device and stopping application of said second control signal to said second control input when said compared voltages become substantially equal; and 4. reset means connected with said first control input for applying said first control signal thereto and operable for causing said first control signal to be applied before said comparison is commenced to insure that said voltage taken as a percentage is greater than said input l voltage when said comparison is commenced; and B. proof stress insertion means for causing a third electrical signal to be fed to said comparator circuit of said voltage device and including an electronic timer connected to cause said third signal to be fed to said comparator circuit after said two electrical signals have become substantially equal; one of said two electrical signals being the input voltage of said voltage matching device, the other of said two electrical signals being said voltage taken as a percentage, and said third electrical signal being an analogue of a preselected proof stress percentage; aNd said proof stress indicator being operable for providing a response when said strain electrical signal has increased above said stress electrical signal by an amount equal to said third electrical signal.