FUNDAMENTAL STUDIES OF FATIGUE CRACK PROPAGATION IN METALS

Previously, the plastic work required to create a unit area of fatigue work, U, has been shown to be one of the parameters controlling the rate of propagation. The present research extended the correlation between U and the rate of fatigue crack propagation to low strength metals, 99.99('+) Al, 1100 Al alloy and annealed 0.05 w/o C steel, and to the low temperature, 77(DEGREES)K. The additional data allow a better correlation with microstructure.; When U is measured using foil strain-gages, an extrapolation is required to estimate the contribution closer than 100 (mu)m to the crack tip, U(,c), since the gages fail when the crack enters them. An electron channeling technique may be used to determine U(,c) directly in alloys which form subgrains on cyclic straining. Using foil strain-gages, U was measured at (DELTA)K = 8 MN/m('3/2) on the identical annealed 0.05 w/o C steel used by Davidson and Lankford in an electron channeling study. Extrapolation of strain-gage data gave 2.0 x 10('6) J/m('2) for U(,c) compared to 1.7 x 10('6) J/m('2) by electron channeling.; The plastic work, U, is made up of two parts, hysteretic and non-hysteretic. The non-hysteretic plastic work was found to be about three to four orders of magnitude smaller than the hysteretic plastic work in the three low strength materials studied and thus may be neglected.; A map of the plastic zone results from the strain-gage measurements. Rice's theory predicts the measured plastic zone size if the stress corresponding to the plastic zone boundary is substituted into the formula.; A fatigue crack propagation rate equation,; (DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI); with a universal dimensionless constant, A, of {lcub}2.8 (mean) (+OR-) 0.9 (standard deviation){rcub} x 10('-3) fits the crack propagation rate data for 7 aluminum alloys and 6 steels ranging in (sigma)(,y)', the 0.2% offset cyclic yield stress, from 42 to 868 MN/m('2). In the equation, (, )(' ) is the fatigue crack propagation rate, (DELTA)K is the stress intensity range and (mu) is the shear modulus. Mura and Vilmann subsequently obtained a theoretical vaue of A of approximately 2 x 10('-3).; For improving fatigue crack propagation resistance in alloys, U and (sigma)(,y)' need to be considered together. These factors usually vary in the opposite direction because increased strength gives a smaller plastic zone size. In order to maximize the resistance to fatigue crack propagation, the product (sigma)(,y)'('2)U needs to be maximized. Examination of the available data shows that strain localization reduces this product.; A closed-loop servo-controlled electrohydraulic fatigue machine was constructed for fatigue crack propagation and strain-controlled fatigue studies at 77(DEGREES)K. A large decrease in the fatigue crack propagation rate in 99.99('+) Al and 1100 Al alloy on cooling from room temperature to 77(DEGREES)K is due to large increases in both (sigma)(,y)' and U. These increases correlate with the dislocation structures after cyclic deformation. Kiritani and Ono showed that after cycling at 77(DEGREES)K, the dislocations are quite homogeneously distributed and the density is high, while at 298(DEGREES)K dislocation cells form. Cyclic hardening occurs for annealed 99.99('+) Al at 298 and 77(DEGREES)K. However, cold-rolled 99.99('+) Al cyclically softens at room temperature and cyclically hardens at liquid nitrogen temperature.