Crack Linkup at Multiple Site Damage

Abstract

Abstract A commonly used design criterion for predicting the onset of crack link up at a multiple site damage (MSD) is when the sum of the plastic zones of two adjacent cracks is equal to the remaining ligament (Swift 1985, Broek et al 1994). The collapse load predicted by this criterion, however, will differ with the analytical plastic zone and in general, will yield a nonconservative load. Recently, Pyo et al (1997) have shown that the T* integral, which was designed for elastic-plastic fracture mechanics, can predict the extent of stable crack growth and the subsequent collapse load of wide-panel MSD experiments (deWit et al, 1994). The purpose of this paper is to present experimental/numerical results on stable crack growth and linkup of idealized MSD based on the T* integral. Moiré interferometry was used to determine the orthogonal displacement fields surrounding stably growing cracks in 0.8 mm thick 2024-T3 aluminum specimens with two or three cracks. Fatigue precracked cracks extended toward each other from two circular holes, which were 25.4 mm apart to simulate fastener holes in airplane fuselage, close to the outside edges of the specimens. A longer center crack from a center hole in the wider, three-crack specimens with three holes spaced 25.4 mm apart, provided information on the lead crack effect. T*ε integral, where ε is the distance of an elongating integration contour from the stably growing crack, was evaluated from the Moiré data using the procedure established by Okada et al (1996). ε in all studies was equated to the specimen thickness. An elastic-plastic finite element (FE) model of the fracture specimens were executed in its generation mode (Kobayashi, 1979) by inputting the measured Moiré displacement data along the width, 20 mm away from the crackline, of the specimen. T*ε integral was evaluated using an equivalent domain integral (Nikishkov et al, 1987). The experimentally and the FE determined T*ε’s were in excellent agreement with each other. Moreover, the T*ε variation with stable crack growth closely followed the stable crack growth results generated previously using standard fracture specimens (Omori et al, 1998). Thus stable crack growth data generated by standard fracture specimens can be used to predict crack growth of MSD cracks. Collapsed load was reached when the remaining ligament exceeded the tensile strength of the 2024-T3 sheet and the two cracks rapidly propagated toward each other in the two-crack specimens. For the three-crack specimen, the lead crack propagated toward the smaller two MSD cracks.