Papers by Keyword: Multiple Site Damage

Abstract: Fatigue test were performed on 2024-T62 aluminum panels with MSD cracks, and the fatigue behavior of panels with MSD was analyzed. By the experiment results, the speed of crack growth increased stably in most time of life span, and crack growth rate curves were plotted. Then a rough method was presented to evaluate fatigue life of aluminum panels based a constant geometric factor from the curves and the evaluation is useful in engeering.

Abstract: As more aircrafts reach or exceed their design life, it is becoming very important to research multiple cracks damage, especially the multiple site damage (MSD) in order to re-evaluate their service life and damage tolerance/durability performance. The existing of MSD may remarkably reduce the residual strength of an aerospace structural component than those with a singe lead crack. This study investigated the residual strength of aluminum alloy sheet with MSD through three types of aluminum specimens test. Aluminum panels with bare collinear constant diameter holes were chosen as specimens. After some constant amplitude tension-tension load cycles, the MSD were found in these specimens since there were multiple fatigue cracks emanating from the saw cuts of holes. The residual strength was recorded as the maximum load when every specimen was subjected to monotonically increasing tensile load until failure occurred. In different failure prediction criteria that were often used in engineering in order to evaluate the accuracy of these criteria, Swift criterion (ligament yield) criterion got more accurate prediction results than other criteria. Although Swift criterion was more accurate than some other criteria, its error was still big for some specimens. Two modified approaches were proposed in order to get more accurate and appropriate failure criterion for MSD structure.

Abstract: The problem of multiple site damage (MSD) has got more attention in ageing structures. Cumulative effects of interacting cracks may significantly degrade the damage tolerance capacity of structures. Cracks caused by MSD are extremely difficult to detect and greatly reduce the residual strength, fatigue life and overall structural integrity of aircraft panels. This paper presents a simple numerical method, which use the principles of fracture mechanics and the computation results, to predict the fatigue crack growth life of MSD structure. Comparing with calculating crack growth life cycle by cycle, this method will save much time. To verify the validity of the proposed method, experiment was conducted and reported with simulation specimen of representative MSD structure with 5 details. The comparison between the calculated a-N curves and the crack growth lives and the test results shows that the prediction result with this fast method is acceptable. A discussion was carried out by numerical analysis; in with typical MSD structures with different initial crack length were adopted. Crack interaction effect was found obviously, but it occurred mainly in the last part of the crack growth lives. The relative size of MSD cracks depends significantly on the distribution of the initial cracks.

Abstract: This study investigated the residual strength of aluminum alloy sheet with multiple site damage (MSD) through three types of aluminum specimens test. Aluminum panels with bare collinear constant diameter holes were chosen as specimens. And there were saw cuts at both edges of all the holes. After some constant amplitude tension-tension load cycles, the MSD were found in these specimens since there were multiple fatigue cracks emanating from the saw cuts of holes. The residual strength was recorded as the maximum load when every specimen was subjected to monotonically increasing tensile load until failure occurred. The test results were compared with predicted residual strength results by five different failure prediction criteria that were often used in engineering in order to evaluate the accuracy of these criteria. The failure criteria included: fracture criterion, net ligament loss criterion, ligament yield criterion, CTOA/CTOD criterion and average stress criterion. Ligament yield criterion (Swift criterion) and average stress criterion got more accurate prediction results than other criteria.

Abstract: Accurate stress intensity solutions for multiple site damage (MSD) cracks in riveted stiffened panels are difficult to determine due to geometric complexity along with variations in rivet load transfer and corrosion, especially the interaction of MSD cracks. A methodology was proposed for efficiently depicting rivet in stiffened panel using finite elements. Rivet material properties were determined based on an empirical force-displacement relationship, for highly refined rivet model as well as for idealized spring element representations of rivets. Parametric studies of panels with a middle crack and a central stiffener indicate that rivets can provide comparable load transfer and relative displacement if the rivets closed to the crack are explicit modeled. Using idealized combination of explicitly and spring element representations of rivets, the stress intensity factors (SIFs) for uncorroded and corroded one-bay stiffened panels were predicted. The results show that the effect of MSD and thinning of the sheet is to increase substantially SIFs values compare to that of a single crack without corrosion. But the SIF is insensitive to corrosion of stiffener. The particular rivet material has relatively little effect on SIF values, while the rivet diameter has a significant effect.