Application of a Crack Length Measurement with a Laser Micrometer to R-Curve Tests

Abstract

The application of the damage tolerance philosophy to aeronautical thin structures requires that R-curves tests be run. The automation of such tests can be achieved through crack length monitoring using the compliance technique. However such a method faces experimental errors when a clipon gage extensometer is used to record the crack opening displacement on a CCT specimen because of the out-of-plane displacements. In order to write these displacements off, it is proposed to resort to a noncontacting device such as a scanning plane laser micrometer. The sensor used consists of two parts which are symmetrically placed on each side of the central notch of the specimen. The emitter sends a 60 mm high laser plane wave to the receiver. Only the beams passing through the central hole of the specimen hit the receiver. The intensity of the transmitted laser beam is then converted into an electrical signal by the sensor electronics. A numerical processing of the signal enables a calculation of the crack opening displacement of a machined slot with a resolution greater than 1 μm and an excellent digital stability. The measurement is then integrated into a PC microcomputer, which calculates the corresponding effective crack length using the appropriate compliance formula (ASTM E 561). The stress intensity factor is computed from the testing machine load signal. A stability test (no crack growth under constant applied load step) is checked out in order to store the relevant data points for plotting the R-curve and to decide for the next load increment the computer is going to apply. This laser technique was validated in comparison to a classical mechanical extensometer method on an aluminum alloy. This method bears fruitful applications in testing under controlled environment (temperature, moist or corrosive atmosphere).