Analysis of stiffened panels with multiple site damage
Abstract
An analytical model based on displacement compatibility was used to analyze the link-up and residual strength behavior of stiffened panels with lead and multiple site damage (MSD) cracks. Fifteen inch wide stiffened aluminum specimens with various lead and MSD crack lengths and two different stiffener configuration were tensile tested to determine crack link-up and panel failure loads. A tip stress intensity reduction factor was verified experimentally through fatigue tests of stiffened panels, and used in two different link-up and panel failure criteria to predict the lead crack link-up and panel failure loads. The ligament yield criterion with full area compounding, and modified for the stiffened panel, was shown to accurately predict lead crack link-up load. An alternate version of the ligament yield criterion with moderate area compounding also gave reasonable results. The apparent fracture toughness criterion consistently overpredicts link-up loads for specimens with MSD. The stiffener failure criterion was shown to be able to predict panel failure to within less than 4 percent for specimens with a true lead crack. The net section failure criterion gives good results for specimens where the lead crack was arrested at a hole. The stiffened panel model was also incorporated into a fatigue crack growth prediction code and used to predict the crack growth behavior of 15 inch wide stiffened panel specimens with lead and MSD cracks. The stiffened panel fatigue crack growth model was able to predict the crack link-ups and total panel lives of the stiffened panel specimens with good accuracy. Parametric studies of sheet link-up stress and fatigue life to lead crack formation for 75 inch wide specimens was performed using the stiffened panel models that had been verified experimentally. A simple model was developed for the effect of cracks in the stiffeners on the link-up and fatigue behavior of the sheet. That model does not predict stiffener failure loads and stiffener crack growth. The parametric sheet link-up stress study showed that even small MSD cracks (0.05 inches) reduce the sheet link-up stress between 15 and 28 percent depending on stiffener size and material. Also, panels with heavy stiffeners are affected more severely than panels with light stiffeners. The presence of cracks in the stiffeners has very little effect on the sheet link-up stress until the stiffener is almost completely cracked. The fatigue study indicates that the time to lead crack formation is the critical parameter. Once two MSD cracks have linked to form a lead crack, further link-up occurs within a few cycles, until the lead crack reaches the stiffener. Increasing the stiffener size does not increase the lead crack formation life very much, but increasing the stiffness of the stiffener (i.e. going from aluminum to titanium) can increase lead crack formation life by a factor of 2 to 4. Cracks in the stiffeners have very little effect on the lead crack formation and propagation in the sheet. (Abstract shortened by UMI.)
Degree
Ph.D.
Advisors
Grandt, Purdue University.
Subject Area
Aerospace materials|Mechanical engineering
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