This paper develops and validates a technique for real-time measurement of crack tip opening load using ultrasonic sensors and its application to life-extending control of mechanical structures. To experimentally validate the proposed measurement technique, fatigue tests have been conducted in the laboratory environment on center-notched 7075-T6 aluminium alloy specimens. The energy of reflected ultrasonic waves is used to detect crack closure and opening phenomena that lead to real-time measurement of crack opening load. Experimental results are compared with the Newman's crack opening stress model under constant amplitude cyclic loading. A life-extending control scheme is proposed by taking advantage of the real-time information on fatigue damage in mechanical structures.