Experimental study of crack growth in thin sheet 2024-T3 aluminum under tension-torsion loading

ABSTRACT: To assess the viability of using a critical COD criterion for flaws in 2024-T3 aluminum experiencing tension stresses (S p) and torsion stresses (Sr), the enclosed work presents (a) a complete set of measurements for critical COD during crack growth under nominal tension-torsion loading, (b) the evolution of crack path with crack growth and (c) crack surface shape as a function of loading. Data from this work will provide an important experimental database for use in assessing the predictive capability of advanced, three-dimensional, crack growth simulation tools. Results for COD during crack growth under tension-torsion loading indicates that the measured critical COD for tension-torsion loading is constant during crack growth. In addition, the value of COD measured using image correlation methods is approximately 8% larger than observed for in-plane tension-shear, with much of the increase apparently due to specimen deformations in the crack tip vicinity. In addition, crack path evolution data for the range of Sr / S p considered in this work show that the crack experiences both tunneling and slant fracture during loading, with tunneling rapidly decreasing (a) as crack growth progresses for all Sr / S p values or (b) as Sr / S p increases. Furthermore, results indicate that tearing during tension-torsion loading always occurs in a manner so that the crack surfaces tend to interfere during growth. Finally, crack surface shape data indicates that, with the exception of a small secondary transition, the direction of crack growth remains stable along a straight line oriented along the initial fatigue crack direction for the range of Sr / S p being considered.

Tension-torsion specimen

Non-dimensional measure of tunneling during initial stages of crack growth
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