Fracture toughness of additively manufactured 316L stainless steel measured with miniature C(T) specimens - compliance versus normalization technique

The paper concentrates on experimental challenges associated with fracture toughness measurement with miniature C(T) specimens. The ductile fracture toughness of additively manufactured (AM) 316L steel was measured following the ASTM E1820 standard. Three material conditions were investigated: stress relieved, stress relieved + solution annealed, and stress relieved + hot isostatically pressed conditions. J-R curves were obtained by the compliance and normalization techniques, and a comparative assessment of their reliability was performed. Two different crack opening displacement sensors were used: an internal crack-mouth-opening-displacement (CMOD) and an external load-line displacement (LLD) sensor. Many tests produced a slanted S-shaped J-R curve for the hot isostatically pressed material when the compliance technique was used in combination with the CMOD data. The evidence points towards issues with the reliability of the CMOD measurement. The normalization technique was found to give more realistic J-R curves without an S-shape. However, it should be noted that the small error induced in the CMOD measurement also affected the normalization technique, since the same peak values of CMOD are used as in the compliance technique analysis, but the effect to the J-R curve is smaller. This is most likely due to smaller signal-to-error ratio of the peak values of CMOD when compared to the error induced in the slopes of the unloadings of the compliance technique.