Finite Element Calculation of Stress-Intensity Factors for Thumb-Nail Cracks inDEN Specimens

摘要

The accepted techniques for stress intensity factor (SIF) extraction from three-dimensional finite element method (FEM) calculations are reviewed. Results are then summarized for thumbnail cracks emanating from a semicircular notch of double edge notch (DEN) specimens. Two different specimen geometries corresponding to AGARD and Institute of Aerospace Research dimensions were analyzed. In the AGARD specimen, the ratio of the notch radius to the specimen width (R/W) is 1/16 while in the other the R/W is 1/8. Three-dimensional models were run on NISA 2/DISPLAY 3 and ABAQUS FEM software packages to calculate SIFs. Results obtained for standard AGARD specimens are compared with the predictions of Newman's 1990 model and found to be very close. Graphs showing the absolute values of SIF for small thumbnail cracks emanating from semicircular notches of Institute DEN specimens and the dependence of SIFs on crack dimensions are also presented. The numerical results indicate that in the crack depth range of 0.3-0.4 mm, a semicircular crack will tend to grow into a semi-elliptical crack. An increase in R/W increases the SIF for a given crack depth and a/c ratio in the DEN specimens, where a is half of the surface crack length and c is the crack depth.