NASA Technical Reports Server (NTRS) : Finite pdf

NASA Technical Reports Server (NTRS) : Finite

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A two-dimensional, elastic-plastic finite-element analysis was used with a critical crack-tip-opening angle (CTOA) fracture criterion to model stable crack growth in thin-sheet 2024-T3 aluminum alloy under monotonic loading after precracking at different cyclic stress levels. Tests were conducted on three types of specimens: middle-crack, three-hole-crack and blunt-notch tensile specimens. An experiment technique was developed to measure CTOA during crack growth initiation and stable tearing using a high-resolution video camera and recorder. Crack front shapes were also measured during initiation and stable tearing using a fatigue marker-load technique. Three-dimensional elastic-plastic finite-element analyses of these crack shapes for stationary cracks were conducted to study the crack-front opening displacements. Predicted load against crack extension on middle-crack tension specimens agreed well with test results even for large-scale plastic deformations. The analyses were able to predict the effects of specimen size and precracking stress history on stable tearing. Predicted load against load-line displacements agreed well with test results up to maximum load bu the analyses tended to overpredict displacements as crack grew beyond the maximum load under displacement-controlled conditions.

During the initiation phase, the measured CTOA values were high but decreased and remained nearly constant after a small amount of stable tearing. The constant value of CTOA agree well with the calculated value from the finite-element analysis. The larger CTOA values measured at the sheet surface during the initiation phase may be associated with the crack tunneling observed in the tests. Three-dimensional analyses for nonstraight crack fronts predicted much higher displacements near the free surface than in the interior

• Creator/s: NASA Technical Reports Server (NTRS
• Date: 8/1/1992
• Year: 1992
• Book Topics/Themes: NASA Technical Reports Server (NTRS), ALUMINUM ALLOYS, CRACK PROPAGATION, CRACK TIPS, FINITE ELEMENT METHOD, FRACTURE MECHANICS, MATERIALS TESTS, METAL SHEETS, TWO DIMENSIONAL MODELS, DAMAGE ASSESSMENT, ELASTIC PROPERTIES, FATIGUE (MATERIALS), OPTICAL MEASUREMENT, PLASTIC DEFORMATION, PLASTIC PROPERTIES, Newman, J. C., Jr., Dawicke, D. S., Bigelow, C. A

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