Characterisation of crack tip fields utilizing digital image correlation

Digital image correlation (DIC) is more and more utilized in experimental mechanics. It is a contactless measuring system which is able to obtain full field measurements of displacement and strain fields on the surface of specimens. Especially in the case of fracture mechanics it offers the opportunity to observe the crack tip field very locally. For ductile materials like aluminum alloys this includes the plastic zone (PZ) and the elastically deformed region outside. Here, especially the plastic deformations affect the stress redistribution in the vicinity of the crack tip. For quantification of the actually present crack tip loadings the J or interaction integral, as well as the Williams series expansion can be used. Therefore, a special post processor was developed to determine J, KI and KII for flat specimens which are commonly used in fracture mechanical experiments. In the case of light weight structures like aircrafts the local crack behavior is crucial for the evaluation of service live and damage tolerance. For the aircraft fuselage aluminum alloys like AA2024 still play a decisive role. Determining a complete da/dN-ΔK curve with the DIC postprocessor provides a unique insight into the effective cyclic stress intensity factors and the effective R ratio which could deviate from the theoretical values. The evaluated ΔKeff also includes intrinsic and extrinsic effects affecting the crack tip strain field. The presentation shows the evaluation of a fatigue crack propagation experiment of a center cracked specimen with a width of 950 mm. With a nominal stress of 120 MPa cyclic stress intensity factors of more than 100 MPa√m together with crack propagation rates of more than 1 mm/cycle were obtained. It is shown that even for large plastic zones with a diameter of more than 70 mm the postprocessor can be utilized in order to determine the actually present stress intensity factors and the J integral. Finally, it is shown that the K-concept is still applicable despite the huge plastic deformations which is crucial for lifetime estimations for commercial aircraft fuselage structures