Damage-Based Assessment of the Fatigue Crack Initiation Site in High-Strength Steel Welded Joints Treated by HFMI

This study aimed to identify the fatigue crack initiation site of high-frequency mechanical impact (HFMI)-treated high-strength steel welded joints subjected to high peak stresses; the impact of HFMI treatment residual stress relaxation being of particular interest. First, the compressive residual stresses induced by HFMI treatment and their changes due to applied high peak stresses were quantified using advanced measurement techniques. Then, several features of crack initiation sites according to levels of applied peak stresses were identified through fracture surface observation of failed specimens. The relaxation behavior was simulated with finite element (FE) analyses incorporating the experimentally characterized residual stress field, load cycles including high peak load, improved weld geometry and non-linear material behavior. With local strain and local mean stress after relaxation, fatigue damage assessments along the surface of the HFMI groove were performed using the Smith–Watson–Topper (SWT) parameter to identify the critical location and compared with actual crack initiation sites. The obtained results demonstrate the shift of the crack initiation most prone position along the surface of the HFMI groove, resulting from a combination of stress concentration and residual stress relaxation effect.

Initiation and Propagation Processes of Internal Fatigue Cracks in β Titanium Alloy Based on Fractographic Analysis

Uniaxial fatigue tests were conducted for a β titanium alloy Ti-22V-4Al up to a very high cycle fatigue (VHCF) regime. The initiation and propagation processes of the internal fatigue cracks were investigated using 3D fractographic analysis. Multiple facets were observed at the crack initiation site. Three facet initiation models were proposed based on the surface appearances and the 3D facet bonding patterns of the multiple facets, and the major facet was determined to be the true crack initiation site. Using the size of the major facet, a Tanaka–Akiniwa model, which can determine the material constants for the Paris law using only conventional fatigue tests, was applied to reveal the propagation process of the internal cracks. A reverse fatigue life prediction was also conducted to evaluate the accuracy of the material constants obtained using the Tanaka–Akiniwa model. When the facet initiation models were applied, the predictions showed less deviation and better agreement than when the facet initiation process was not considered. The findings of this study indicate that the formation of multiple facets in β titanium alloys is sequential rather than simultaneous.

Analysis of Cold Dwell Fatigue Crack Initiation Site in a β-Forged Ti-6242 Disk in Relation with Local Texture

The α and prior β textures and microtextures of a lamellar Ti6242 forged disk were characterized by advanced electron back scattered diffraction (EBSD) and related to crystallographic features of faceted crack initiation sites of dwell fatigue specimens tested along the radial direction (RD). Large feather-like structures of α colonies with close orientations were observed at boundaries of elongated prior β grains. Their orientation belongs to the <11-20>α//z fiber, aligning the c-axis in RDs (z: axial direction of the disk). They are inherited from prior β grains belonging to the major <100>β and minor <111>β//z fibers. These feather-like structures are strong regions of the forging that act as a preferential crack initiation site. Adjacent to them, one can observe large colony with evidence of prismatic slip. Thus, the facet formation seems triggered by stress redistribution from “weak” to “strong” regions due to the elastic and plastic anisotropy. Finally, the occurrence of neighboring β grains able to share close oriented feather-like colonies is discussed considering the reconstructed β microtexture and texture. This study may be helpful for further texture control during the forging process.

Forecast of the fatigue crack initiation site of commercially pure Titanium miniature specimens with local surface topography data

Surfaces of technical components rarely appear in perfectly smooth condition. During fatigue loading, stress concentrations at surface asperities cause localized plastic deformation that can lead to crack initiation. Therefore, we have established a computer-aided method based on material ratio curves to investigate the possibility to predict the crack initiation site in fatigue tests by using detailed information on the local surface topography. The present study shows the results of investigations on the mutual influence of the average grain size and the surface condition on the fatigue behavior of commercially pure Titanium (cp-Ti) miniature specimens. Three cp-Ti states were investigated: two types of coarse-grained cp-Ti Grade 2 with 35 µm and with 100 µm average grain size and one ultrafine-grained cp-Ti Grade 4 state with less than 2.5 µm average grain size. Confocal microscopy provided the surface topography data of all specimens and data post-processing was applied to the topography in order to locate critical areas where crack initiation may preferentially occur. These areas were compared with the actual crack initiation areas in fatigue test. Finally, scanning electron microscopy (SEM) images of the fracture surfaces were studied to analyze fatigue crack initiation site and crack path of the three microstructural states.