Real-time measurement of shear fatigue crack propagation at high-temperature using the potential drop technique

in this paper, a direct method for real-time measurement of the advancement closed cracks propagating in Mode II (inplane shear) is presented. The method is based on a modified potential drop technique applied on a patented shear specimen, the geometry of which ensures that under certain loading conditions a uniform shear field is developed in its central region (gauge area). Therefore, by continuously measuring the change in the electrical resistance (potential drop) between the two electrodes attached on the specimen it is possible to correlate this change with the progression of the crack within the specimen and in turn correlate the crack length with the stress intensity factor at the crack tip. This information can be used to create d alpha/dN - Delta K-II plots as the experiment is progressing on-line from the acquired electrical data. The electrical field and the stress intensity factor in Mode II (K-II) have been calculated for the shear specimen using finite element analysis (FEA) for various crack lengths and graphs indicating the change in crack length versus the change of the resistance of the specimen have been plotted. The method has been calibrated with optical measurements using a long distance observation microscope and it can be used in high-temperature testing of electrically conductive materials (i.e. nickel-based superalloys). (C) 2007 Elsevier Ltd. All rights reserved.