JOURNAL OF MATERIALS SCIENCE LETTERS 11 (1992) 1085-1086 On the meaning of thresholds in environmentally assisted cracking J. TORIBIO, A. M. LANCHA Department of Materials Science, Polytechnical University of Madrid, ETSl Caminos, Ciudad Universitaria, 28040 Madrid, Spain Experimental results on environmentally assisted cracking are usually given in terms of fracture mechanics parameters, as a plot of crack growth rate versus stress intensity factor. This curve has three stages representing different physical processes of environmental cracking: stage I corresponds to the threshold level or minimum load required to propa- gate the crack in a given environment; stage II shows a more-or-less stress intensity-independent growth process; in stage III there is an approach to the K~ level, and the crack growth rate rises sharply up to the fracture point. Each of the three stages can be modelled according to the nature of the crack growth rate at that level of loading [1]. From the engineering design point of view it is very interesting to determine, as accurately as possible, the threshold stress intensity level for the given aggressive environment (Kth). It may be denominated according to the specific mechanism of crack growth [Kiscc for stress corrosion cracking (SCC) and KIHAC for hydrogen-assisted cracking (HAC)]. Calculations of stress intensity threshold levels for hydrogen-induced crack growth on the basis of hydrogen diffusion models can be found in [2, 3]. This letter analyses the meaning of thresholds in environmentally assisted cracking. Defining the threshold as the stress intensity level below which no crack propagation takes place, it would be expected for this concept to have an intrinsic character, dependent only on the specific material and environ- ment, but not on mechanical variables. However, this letter makes clear that precracking conditions can significantly affect the environmentally assisted cracking threshold. To achieve this, crack growth rate tests were performed on a commercial eutectoid pearlitic steel in an aqueous solution which promoted environ- mentally assisted cracking. Prismatic specimens 3.85 mm × 7.70 mm× 100.00 mm were machined from 12 mm diameter commercial bars, with a starter notch in its central section perpendicular to the bar axis. Precracking was produced by axial fatigue in air environment before the fracture test in aggressive environment. Load was applied by means of a three-point bending device with a distance between supports of 61.60 mm. Strain was increased stepwise to obtain different points of the curve crack growth rate versus stress intensity factor. The aggressive environment was an aqueous solution of 1 g l-lCa(OH)2 plus 0.1 g 1-1NaC1 (pH 12.5). Test- ing was performed at room temperature (between 16 and 22 °C) and at constant potential by using an 0261-8028 © 1992 Chapman & Hall electrochemical device consisting of a potentiostat and a classical three-electrode assembly: metallic sample (work electrode), saturated calomel elec- trode (SCE, reference electrode) and platinum wire (counterelectrode), as described in [4]. Two poten- tials were used: -600 mV versus SCE (anodic) and -1200mV versus SCE (cathodic). The former corresponds to anodic dissolution or pure SCC, whereas the latter refers to HAC [5]. To evaluate the influence of the precracking procedure, two types of samples were prepared using different fatigue precracking loads during the last step Oust before the fracture test). The max- imum stress intensity factors during fatigue pre- cracking were Kmax = 0.25Kit and 0.50Klc , where Kit is the critical stress intensity factor for the prismatic three-point bending specimens used in the experimental programme (a value greater than the fracture toughness of the material Kic, since the thickness is not enough to fulfil the plain strain condition requirements). As a consequence there were two different distributions of compressive residual stresses in the vicinity of the crack tip, and therefore two distinct degrees of environmentally assisted cracking. Results for E = -1200 mV versus SCE (cathodic regime: HAC) are plotted in Fig. 1. For a maximum fatigue precracking load Km,x = 0.25K1~ the threshold stress intensity factor for HAC is KIHAC = 0.35Klc. Below this value no crack propa- gation is observed. For higher values of KI there is a plateau in the curve da/dt versus KI for which the crack growth rate remains constant and equal to 1.5 x 10 -7 ms -1 (slow crack growth rate) for the different values of K I. When KI exceeds 0.55Klc there is a sudden increase in the crack growth rate, up to about 10 -3 m s-1. This part of the curve is not horizontal, but slopes slightly. For higher KI values final fracture takes place. For a maximum fatigue precracking load Kma x -0.50Klc the threshold stress intensity factor for hydrogen-assisted cracking is KInAC = 0.58K10. No plateau is observed for higher KI, but a sudden increase to about 10 -3 m s -I is observed, as in the previous curve. It is important to notice the large difference in the threshold value (0.35K1c compared with 0.58Klc) under cathodic regime, depending on the maximum stress intensity factor, Kmax, during the last step of fatigue precrack- ing. Experimental results for E---600mV versus SCE (anodic regime: SCC) are shown in Fig. 2. For a maximum fatigue precracking load Kmax= 0.25K1o the threshold stress intensity factor for SCC 1085