Experimental techniques to evaluate fatigue crack growth in preflawed bolts under tensile loads

Fatigue crack growth testing of Inconel and multiphase 159 bolts is presented. The direct-current potential difference technique is shown to be successful in obtaining crack length data from the test bolts with short grip lengths. Three stress intensity solutions for round sections are presented with a preferred solution suggested. This preference is based on the ability to apply a geometric correction that is variable with crack depth and crack aspect ratio. The crack aspect ratio functionality was determined experimentally by "marker banding" the fracture surface and adjusting the stress intensity solution, following the test, by adjusting the crack aspect ratio. The fatigue crack growth of the Inconel was completely successful. The MP159 cracking was initially Mode I but quickly turned mixed mode, thus invalidating the Mode I crack growth data.