PSI - Issue 39

Jesús Toribio et al. / Procedia Structural Integrity 39 (2022) 488–493 Author name / Procedia Structural Integrity 00 (2021) 000–000

490

3

The test environment was an aqueous solution of 1 g/l calcium hydroxide plus 0.1g/l sodium chloride. The pH value of the solution was 12.5 and tests were performed at constant electro-chemical potential ranging between the two values of –1100 mV versus SCE and –1400 mV versus SCE (where SCE means saturated calomel electrode ) to maintain environmental conditions associated with a hydrogen embrittlement mechanism. The region of analysis is represented by a red line in the Pourbaix diagram shown in Fig. 2. Two testing rates ( applied displacement rates ) were used in the environmentally assisted fracture tests: one moderately fast (0.1 mm/min) and another moderately slow (0.01 mm/min). In all performed tests the hydrogen charging and the mechanical loading were simultaneous, i.e. , no hydrogen pre-charging (prior to mechanical loading) was used in the tests.

Potential (mV SCE)

1.2

Passivation 3

0.6

Corrosion 1

0

Study region

-0.6

Immunity 2

-1.2

Corrosion

2 4 6 8 10 12 14

0

pH

Fig. 2. Pourbaix diagram and region of analysis.

3. Macro-crack paths This section analyzes the fracture behaviour and the crack paths in the initially-smooth samples of cold drawn pearlitic steel wires (commercial prestressing steel wires) tested under tension loading in an aqueous solution with electrochemical potential ranging between –1100 to –1400 mV SCE ( saturated calomel electrode ). Figs. 3 and 4 show the transverse fracture surfaces and the fracture profiles ( crack paths ) in two wires tested respectively at faster (0.1 mm/min) and slower (0.01 mm/min) applied displacement rate (crosshead speed) under electrochemical potentials of –1400 mV SCE and –1200 mv SCE. The main phenomenological fact regarding crack paths is the anisotropic hydrogen embrittlement behaviour of high-strength cold-drawn pearlitic steel wires (commercial prestressing steel wires). The anisotropy of hydrogen embrittlement is reflected in the crack path (fracture profile) in the form of local crack deflections (Figs. 3 and 4). The afore said crack path is produced by the oriented pearlitic microstructure after manufacturing by cold drawing, as described by Toribio and Ovejero (1997; 1998a; 1998b; 1998c) and they do not transform into real macro-crack deflection due to the lack of stress triaxiality in the smooth specimens. It has been proved by Toribio and Ayaso (2020) that there is a necessity of both triaxiality and microstructural orientation to produce fracture path deflection in cold drawn pearlitic steel wires. This axial micro-cracking also arises in standard tension tests with no environmental assistance (Toribio et al., 2016).