In-situ investigation of shear-mode fatigue crack growth in various alloys

VÉRONIQUE DOQUET; GRACIELA BERTOLINO

Francia

Conferencia; EMMC9 CONFERENCE - Locale approache to fracture; 2006

Ecole des Mines

<!-- @page { size: 8.5in 11in; margin: 0.79in } P { margin-bottom: 0.08in } --> The mechanisms and kinetics of mode II fatigue crack growth in maraging steel, ferritic-pearlitic steel and TA6V were investigated, using precracked CTS or tubular specimens submitted to repeated (R = 0) or fully reversed loading (R =-1) inside a SEM. For high enough KII, decelerating mode II propagation took place - along a distance that increases with KII - before bifurcation occured. Friction stresses along the crack flanks -evidenced by the large quantity of fretting debris continuously coming out from the crack- shield more and more the applied load and explain this deceleration. Although pronounced ratchetting under repeated loading is evidenced ahead of the crack tip by an increasing distortion of the microgrids and behind the tip by an increasing permanent shift of the lips, the R ratio has a very limited influence on the crack growth rate. This is coherent with the limited influence of a mean stress or strain on the fatigue life in shear. The influence of the R ratio on the extent of mode II crack growth is discussed. For a given nominal KII, coplanar crack growth in mode II was much more extensive in maraging steel than in ferritic-pearlitic steel and TA6V. This result is discussed in relation with the microstructure and cyclic behaviour of the materials as well as the importance of crack flanks friction. An inverse procedure is used to derive the effective stress intensity factor - allowance made for friction effects - from the displacement profiles measured in the SEM, using microgrids as reference marks. Crack tip shielding by friction appears much stronger in TA6V than in both steels, this partly explaining the limited coplanar crack growth in this material. The measured crack growth rates correlate much better with the effective stress intensity factor than with the nominal KII and for high effective KII, mode II crack growth appears to be faster than mode I. Coplanar crack growth in mode II is not predicted by classical bifurcation criteria, but the crack paths observed in the three materials investigated can be explained by a maximum growth rate criterion.