EDXRD analysis of stress corrosion cracking in 4140 steel

The analysis of the strains around the immediate vicinity of a crack tip to encompass the plastic zone in 4140 steel specimens was undertaken. Each specimen was subjected to a cyclic loading in air with a KI of 24.2 MPa✓m to generate a fatigue crack around 5.7mm. After a mechanical overload corresponding to a KImax of 38.5 MPa✓m in varying environments (Air, 3.5% NaCl solution simulating sea water and zinc couple to the 3.5% NaCl solution for generating hydrogen) ranging from inert up to a stress corrosion cracking (SCC) environment were carried out.;During the overloading process only the specimens in an SCC environment were subjected to crack activity as expected. For this reason method that hydrogen attacks steel (hydrogen embrittlement) was a point of curiosity as well. Three SCC specimens were prepared to stay within different phases: i) no crack growth, ii) onset of crack growth and iii) crack growth of ∼1mm. Two of the specimens (Air and SCC with the onset of crack growth) were measured by mapping up to all three 3D principle elastic strains (PES) using a polychromatic high energy synchrotron x-ray probe with photon energies up to 200 keV in the Laue mode. The PES epsilon11 and epsilon22 were directly measured, while the PES epsilon33 was obtained from the combined measurement of epsilon22 and epsilon23 of the elastic strain matrix. By using the generalized Hooke's law and the von Mises yield criterion for 3D state of stresses in conjunction with the measured PES, maps of the von Mises equivalent stress (sigmaeq) were obtained. In addition, of the air specimen, the principle plastic strains and total strains were calculated and used to show the principle stress fields and equivalent stress field. The results indicated that the PES in the direction orthogonal to the crack path i.e. the epsilon22 is sufficient for measuring the plastic zone in 4140 steel.;The use of three models (Linear Elastic Fracture Mechanics (LEFM), Hutchinson-Rice-Rosengren (HRR) and a blunted crack tip) to define the border of the plastic zone was employed. These three models were overlaid on the epsilon22 strain maps. The juxtaposition of the models with the data showed some varying agreement with the projected plastic zone size. The models helped estimate that SCC causes the plastic zone to grow when an SCC crack occurs, due to a lowering of the yield strength of the material and decreases it ductility.