In-situ observation of environmentally assisted crack initiation and short crack growth behaviour of new-generation 7xxx series alloys in humid air

Large area, time resolved, in-situ optical scanning of constant displacement (four-point bend) tests has been used to monitor the environmentally assisted cracking (EAC) initiation and crack growth behaviour of AA7085 and AA7449 T7651 thick plate materials, exposed to warm-humid air (50 % RH & 70 degrees C). Subsequent fractography has revealed the dominant initiation sites originating at surface connected gas pore clusters, in combination with reactive Mg-containing constituent intermetallic compounds co-located on grain boundaries. Cracks were also shown to grow at stress intensities substantially below the KIEAC threshold measured via DCB testing, which should not therefore be taken as a limit below which EAC cannot occur.