The effect of dents on fatigue life and fatigue crack growth of aluminum 2024-T3 bare sheet

The purpose of this study is to determine the effect of dents on the fatigue properties of 0.04 inch thick 2024-T3 bare aluminum sheet, which is the most widely used material for aircraft skin. The study is divided into two parts. The first part (Part I) is a study on the effect of dents on the fatigue life of the material, while the second part (Part II) is a study on the effect of dents on crack growth in the material. The test specimens were either pristine (no dents), dented or reworked. For Part I, the dented and reworked specimens were divided into two groups, depending on the dent depth ranges. The dent depths for each of these two groups ranged from 0.030" to 0.0335" and 0.0605" to 0.065". Dents were produced with a drop tower having a 0.5 inch spherical hardened steel indenter head. The fatigue life of the material in these three conditions was determined experimentally. Constant amplitude fatigue tests were conducted according to ASTM-466 with a single stress level and a single frequency for the fatigue cycle. The results were then compared to determine the effect of dents and the reworking of dents on the fatigue life of the material. Results of the study indicate that the fatigue life of a dented specimen is significantly less than that of the pristine and the reworked specimens. The study also showed that the fatigue life decreases as the dent depth increases in the dented specimens. Statistical analysis of the results showed that there is no significant difference in the fatigue life between the reworked population and the pristine population of specimens, while reworking considerably improves the fatigue life of the dented material.; For Part II, the specimens are again either pristine dented or reworked. The edge-cracked pin-loaded specimens of 8" in width were tested at constant amplitude loading with a stress ratio of 0.2 producing stable crack growth of close to 4 inches completely through two dents on the crack line. Dents were produced the same as described for Part I. Dent depths ranged from 0.03" to 0.0325" measured on the convex side of the specimen. A starter notch of 0.3" was produced at the edge of the specimen with a jeweler's saw blade. The specimen was fatigue loaded under constant amplitude loading to produce an initial crack length of 0.37" t which time readings of crack length vs. cycles began. The same constant amplitude cyclic loading used to produce the initial crack length was used during the testing. The crack lengths were measured with an optical microscope at 160X magnification. Nine specimens were tested including three replications for each of the three conditions. Crack growth data is given in both tabular and graphical form for all specimens. Crack growth rate data is also presented in graphical form. The overall crack growth in the dented specimens was significantly greater than in the pristine specimens. It was also, on the average, faster in the reworked specimens however, reworking, in general, did not recapture the life displayed by the pristine specimens.