A B S T R A C TThe 6000 series aluminium alloys (Al-Mg-Si systems) are commonly used as mediumstrength structural materials; in particular, the 6061 (Al-1Mg-0.6Si) alloy is widely utilized as a general-purpose structural material due to its excellent formability and corrosionresisting capabilities. The objective of this study was to obtain a correlation between the small punch (SP) test estimated equivalent fracture strain (ε qf ) and fracture toughness (J 1C ) property for 6061 aluminium, and determine its viability as a non-destructive fracture toughness test technique for remaining life assessment of in-service components. Samples of 6061-T6 aluminium were cut from bulk plate, in both the longitudinal and transverse directions, for the as-received condition as well as subjected to three different over-ageing heat-treatment schedules. A strong linear correlation between valid J 1C and SP estimated biaxial fracture strain ε qf is presented for aluminium 6061 at room temperature.Keywords aluminium alloy 6061; equivalent fracture strain; in-service component; J integral fracture toughness; remaining life assessment; small punch test. N O M E N C L A T U R EF UPTHRUST = upthrust due to sample in water J 1C = valid J integral fracture toughness J 0 = material/environment dependent empirical constant k = material/environment dependent empirical constant m DRY = dry weight of aluminium m WET = wet weight of aluminium n = empirical parameter in small punch correlation R 2 = correlation coefficient for least squares curve fit t 0 = original thickness of small punch disk t = thickness of small punch disk at fracture T SP = transition temperature in small punch tests w CT = critical strain energy density of compact tension sample w SP = critical strain energy density of small punch sample x = material dependent characteristic averaging distance β = empirical parameter in small punch correlation δ = small punch deflection at fracture ε qf = equivalent biaxial strain at fracture ρ Al = density of aluminium ρ H2O = density of water

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