Environment-sensitive fracture of aluminium alloy 6013

The aim of this paper is to contribute to a better understanding of the rea sons of the fatigue behaviour in corrosive environment of aluminium alloys used in aircraft industry. Fatigue crack propagation experiments were carri ed out at various test frequencies on CCT specimen of Al6013T6 in air and i n 3.5% sodium chloride solution. The rate of crack propagation is accelerat ed in sodium chloride solution, and the da/dN-Delta K curve for Al6013T6 sh ows characteristics which differ from the curve in air. In order to clarify the influence of the corrosion medium the composition and the pH value of the electrolyte were varied. In this way the crack propagation was inhibite d or accelerated during fatigue test. During the fatigue test in corrosive environment the dissolution of aluminium occurs and at the same time hydrog en evolves. Hydrogen embrittlement means a loss of ductility caused by the incorporation of hydrogen into the lattice. For the fatigue behaviour in co rrosive medium it is decisive how atomic hydrogen is generated and which co nditions allow the absorption of the hydrogen into the material. The result s show that chloride ions and an acid pH value are necessary for the absorp tion of hydrogen. Besides the hydrogen embrittlement a second process of me chanical degradation occurs. Secondary cracks were determined after finishi ng the fatigue crack propagation test. These cracks could be observed as we ll in air as in corrosion medium. But in corrosion medium the occurrence of secondary cracks was more pronounced. The degradation mechanism in the pre sence of corrosion medium and cyclic stresses is discussed. It is shown tha t superposition of hydrogen embrittlement due to hydrogen absorption and fo rmation of secondary cracks plays an important role in the fatigue behaviou r of Al6013T6 in corrosive medium.