CTOA measurements in aluminium alloys using direct techniques

Fracture mechanics has come a long way since the introduction of Hooke's law. The ability to predict the structural integrity of components or structures has improved significantly but there is still a need for improvement. A promising recent development in fracture analysis is the Crack Tip Opening Angle (CTOA) fracture criterion. It was developed to characterize the material fracture resistance of low constraint structures because the other more established fracture criteria such as K and J-integral were found to be unable to do so. The CTOA criterion hypotheses that for a given alloy, a constant CTOA value is obtained in the stable crack growth phase and this value can be used to characterise the crack resistance behaviour. However, CTOA has yet to be used extensively because there are doubts about CTOA being a true material property. This is because the exact definition of CTOA has yet to be agreed upon and the values are affected by various parameters which are discussed further in the report. There are also disagreements on the measurement techniques. This dissertation focuses predominantly on developing and evaluating experimental techniques used for CTOA characterisation. The materials used were aerospace AA2024T3 and an aluminium alloy supplied by Airbus. A range of tests were conducted to evaluate the mechanical and fracture properties of the alloys in a bid to find a correlation with CTOA. The measurement of CTOA was done using the optical method for· measuring crack edges, digital image correlation (DIC) for displacement measurements which CTOA is then derived and a grid technique which derives CTOA from gridline rotation. Results further suggest CTOA to be a material property but it also highlights the importance of measurement techniques due to its influence on CTOA. The techniques used were able to produce good quantities of consistent data which corresponded well to literature data. However, the conclusion of this study was that CTOA evaluation is not a straight forward process due to effects of constraints, fracture behaviour, crack initiation conditions and more importantly the different measurement techniques which can sometimes produce contradicting results due to its intrinsic nature or operator error.