A four-channel crack propagation timing recorder has been developed to determine the rate of growth of surface cracks in small concrete beams subjected to flexural fatigue loading. To determine the velocity of a crack the recorder counts and displays the number of pulses from an oscillator that occur during the time taken for the crack to extend through a series of parallel conducting wires bonded to the surface of the specimen. The maximum frequency of counting is 10 mhz with a switch to allow this rate to be reduced, in decade steps, to 10hz. Digital integrated circuits are used for logic control and numerical indicators display a count of four decades. Although developed specifically for the testing of concrete beams, the recorder has potential application to a wide range of material and structural tests.

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