Bilinear CTOD/CTOA scheme for characterisation of large range mode I and mixed mode stable crack growth through AISI 4340 steel

An investigation has been reported concerning characterisation of large span stable crack growth (SCG) through AISI 4340 steel in terms of CTOD/CTOA under both mode I and mixed (I and II) mode loadings. The characterisation has been possible through finite element analysis of published experimental results with compact tension type of specimens. As against the earlier observations by many investigators of a bilinear CTOD/CTOA variation, decreasing initially and constant later, characterising the crack growth through other materials, an increasing initially and constant later type of variation is found to be suitable for AISI 4340 steel. The same variation is found to characterise both mode I and mixed mode SCG. The starting value of CTOD/CTOA is 0.035 mm/0.0875 rad; the value at the later stages is 0.08 mm/0.20 rad. The same variation is found to predict accurate enough for engineering applications the initiation (P_i) and maximum (P_max) loads and the variation of load-displacement diagrams over a span of crack growth up to 10 mm.