Integrated system for monitoring the initiation and propagation of fatigue cracks

Damage tolerant design requires accurate prediction of fatigue crack growth under service conditions and typically this is accomplished with the aid of numerical codes and experimental tests on simple specimens. In this work an in house made integrated system for automated fatigue tests is presented: such system is aimed to provide crack initiation detection, propagation monitoring and remote test machine control. The initiation is automatically devised by the variation of a strain gauge signal. The strain gauge is placed on the specimen close to the most probable initiation site and bonded in such a way to measure the strain in that specific direction which enhances its sensitivity to crack length variations. The crack length is correlated to the measured strain by means of a previous calibration based on a numerical simulation with the Boundary Element method (BEM). The automatic crack length assessment is provided in a twofold manner, with different level of accuracy: the strain gauge measurement is directly converted in the crack length, via an in house made Labview software and the aforementioned calibration; such measurement is used to control the movement of a micrometric motion system that keeps the camera aligned to the crack tip in order to grab focused images of the advancing crack, that, post-processed by an image analysis software, will provide a more accurate crack length assessment. The remote machine control, by a personal computer, is realized by means of a Labview software and a GPIB (General Purpose Interface Bus) hardware. The whole system is applied to assess the crack retardation for a specimen undergoing a fatigue load spectrum.