CTOA AND CRACK-TUNNELING MEASUREMENTS IN THIN SHEET 2024-T3 ALUMINUM-ALLOY

The stable tearing behavior of 2.3-mm thick sheets of 2024-T3 aluminum alloy was experimentally investigated for middle crack tension specim ens having initial flaws that were: (a) flat fatigue cracks (low fatig ue stress) and (b) 45-deg through-thickness slant cracks (high fatigue stress). The critical CTOA values during stable tearing were measured by two independent methods, optical microscopy and digital-image corr elation. Results from the two methods agree well. The CTOA measurement s and observations of the fracture surfaces have shown that the initia l stable tearing behavior of low and high fatigue stress tests is sign ificantly different, The cracks in the low fatigue stress tests underw ent a transition from flat-to-slant crack growth, during which the CTO A values were high and significant crack tunneling occurred. After cra ck growth equal to about the thickness (Delta a > B), CTOA reached a c onstant value of 6 deg and after crack growth equal to about twice the thickness (Delta a > 2B), crack tunneling stabilized. The cracks in t he high fatigue stress tests reach the same constant CTOA value after crack growth equal to about the thickness, but produced only slightly higher CTOA values during initial crack growth. The amount of tunnelin g in the high fatigue stress tests was about the same as that in the l ow fatigue stress tests after the flat-to-slant transition. This study indicates that stress history has an influence on the initial portion of the stable tearing behavior. The initial high CTOA values, in the low fatigue crack tests, coincided with large three-dimensional crack front shape changes due to a variation in the through-thickness crack- tip constraint. The measured CTOA reached a constant value of 6 deg fo r crack growth of about the specimen thickness, This coincided with th e onset of 45-deg slant crack growth and a stabilized, slightly tunnel ed (about 20 percent of the thickness) crack-front shape. For crack gr owth on the 45-deg slant, the crack front and local field variables ar e still highly three dimensional. However, the constant CTOA values an d stable crack-front shape may allow the process to be approximated wi th two-dimensional models.