Environmentally assisted small crack propagation in an Al-Zn-Mg alloy

• Main Author: Lu, Z.
• Published: Swansea University 1995
• Subjects: 669
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.637956

Small crack growth behaviour in 7017 aluminium alloy under stress corrosion cracking (SCC), fatigue and corrosion fatigue (CE) has been studied and contrasted with long crack growth behaviour. The work explores the effects of crack length, heat treatment and, in the case of fatigue, stress ratio R. The test programme involved corner crack (CC) specimens for small crack studies and compact tension (CT) specimens for long crack characterisation at room temperature. It was evaluated in under-, peak and over-aged microstructural forms. SCC tests were carried out under constant load in 3.5% NaCl solution and free corrosion conditions. Fatigue and CF tests were performed under constant load amplitude at 40Hz with a range of R values in laboratory air and 3.5% NaCl solution respectively. An ACPD crack monitoring system was used to measure the crack length. A calibration procedure has been developed based on analytical-numerical techniques. Results show that a power law can be used to relate normalised PD and normalised crack length for the CC specimens. However, random contributions significantly affect the measured PD levels. A method of allowing for these contributions is introduced. Fatigue crack growth rates are found to be increased significantly by the presence of NaCl solution. The SCC velocity can be substantially enhanced by a cyclic load. The SCC regime II crack growth behaviour is described by a power law rather than the often reported plateau in crack growth rates. Heat treatment has little effect on either small or long crack growth under fatigue and corrosion fatigue loading conditions. However, it does influence the SCC behaviour with the best crack growth resistance occurring in the overaged microstructure. Anomalous small crack growth behaviour is observed in all tests, and the fatigue threshold is significantly decreased by increasing the stress ratio. This is explained by crack closure and differences between small and long cracks in terms of local chemical-electrochemical conditions at the crack tip. The latter also accounts for small crack behaviour under SCC. Fractography reveals that similar mechanisms operate for small and long crack growth. It is suggested that hydrogen embrittlement dominates environmentally assisted crack growth in this aluminium alloy.