SUMMARY

In the present investigation aluminium Alclad 2024-T3 sheet material was tested. The aim of this investigation was to achieve a method by which fracture surfaces of short cracks (crack growth under the rivet heads of lapjoint specimens) can be marked by the use of periodic changes in the environment in the low crack growth rate region, and at very low cycle frequencies. These so-called "beach marks " could then be traced back after the test and enable measurement of crack growth and calculation of crack growth rates. To obtain fracture surface markings through periodic changes in the environment, centre cracked tension specimens with a thickness of 1 mm were tested, using a climate chamber clamped on the specimen. It turned out that testing at frequencies ranging from 0.1 Hz to 10 Hz did not result in beach marks on the fracture surfaces when testing in the crack growth range from 10" ̂ m/cycle to 10" ̂ m/cycle. During investigation in the SEM it turned out that it was very difficult to find any flat facets on which beach marks could be found. Furthermore, when changing from fatigue testmg in wet air to fatigue testing in dry air, the crack tip most probably works like a "pump", so that it takes several cycles before the crack is really dry with a relative humidity < 5%