VISCOPLASTIC AND CREEP CRACK GROWTH ANALYSIS BY THE FINITE-ELEMENT METHOD.

摘要

Creep crack growth in a nickel base alloy at elevated temperatures was analyzed through a hybrid experimental-numerical (HEN) procedure. This HEN procedure consisted of simultaneous use of creep crack growth test displacement data from center cracked plate specimens of IN-100 at 1350(DEGREES)F and a theoretical finite element model of the test specimen.A method for getting creep crack growth behavior solely from high resolution displacement measurements, in conjunction with a cracked specimen model which utilizes realistic constitutive relationships, has been developed. The constitutive law, in the form of the Bodner-Partom material model, was especially tailored to the nickel-base alloy studied which displays time dependent nonlinear inelastic behavior at elevated temperatures. It has been demonstrated that the technique can be applied where crack extension is very small and could not otherwise be resolved by conventional experimental crack measuring techniques. This method provides realistic monotonically increasing crack growth values. The predictions agreed to within 10% of post-test measurements.Crack growth rate and crack growth criteria were studied. Cracktip strain and crack opening displacement were studied in the HEN results for a unique parameter controlling crack growth. Because the parameters were not independent of time due to apparent environmental degradation, it became necessary to establish an empirical criterion for crack growth based on the best fit of HEN results. A damage accumulation criterion based on creep rupture formulations was also developed and applied with promising results.Several crack growth rate criteria were investigated, one of which is the stress intensity factor. The K criterion matched fairly well with an extrapolation of published results. But two other criteria, based on the C* integral and load point displacement rate which are closely related theoretically, were found to be ineffective as crack growth rate criteria.A two-dimensional (constant strain triangular) finite element program was developed which accounts for both nonlinear viscoplastic material behavior and changing boundary conditions due to crack growth. Three viscoplastic material models--(1) Malvern Flow Law, (2) Norton's Creep Law, and (3) Bodner-Partom Flow Law--were incorporated into the program. These time dependent material models were numerically integrated through time by a linear Euler extrapolation technique. A variable time step algorithm was included that maximized time step during the analysis while maintaining good accuracy. This program was used as the plane stress theoretical model for the HEN procedure to analyze sustained load creep crack growth.