Font Size: 

The resistance of metals against fatigue crack growth is experimentallydetermined with standardized specimens and procedures (for example ASTM E-647).We have studied the environmental effect on the crack growth resistance of 7075-T651aluminum alloy. Three environmental conditions have been chosen: humid air,technically purified nitrogen and a fine vacuum. Load increasing and load decreasingtests with three R-ratio’s have been performed. The loading frequency was 83 or 54 Hz.In all the tests we observed, as expected, a crack path which followed macroscopicallythe symmetry plane of the CT-specimen, with one exception: In nitrogen, with R = 0.1and 83 Hz macroscopical crack branching was observed when ∆K reachedapproximately 5.0 MPa√m. The two crack branches propagated simultaneously in the+/- 38°-plane. This crack branching was observed both in the load increasing and loaddecreasing test mode. Such a change of the crack path can be understood as a result ofa change of the dislocation motion at the crack tip. Possible explanations of thisphenomenon are discussed