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.

Bringing to Life a Newly Discovered Signature of Fatigue Crack Initiation via Multimodal Characterizations

Fatigue is the most prevalent failure mode in structural materials, yet remains challenging to study due to the seemingly unpredictable nature of crack initiation. To elucidate the driving forces of crack initiation in ductile polycrystalline metals, we employ a multimodal approach to identify and track grains with a suspected potential to initiate fatigue cracks via a newly founded signature. We discover this crack initiation potential (CIP) signature under the hypothesis that slip localization, a well-known precursor to crack initiation, is linked to intragrain misorientation, which can be quantified through single grain orientation distributions. We verify the CIP signature in an Inconel-718 material via static two-dimensional and three-dimensional electron microscopy and “bring to life” the dynamics of the CIP signature via in-situ synchrotron X-ray diffraction. With this CIP signature, we move to better focus studies of fatigue crack initiation on the individual grains and processes that drive fatigue failure.

The effect of corrosion location relative to local stresses on the fatigue life of geometrically-complex, galvanically corroded AA7075-T6

Aluminum components used in aerospace structures are commonly coupled with stainless-steel fasteners. These through-hole geometries on the aluminum substrate cause a concentrated stress field. The high-stresses at the fastener sites can preferentially initiate coating damage allowing for moisture ingress which can lead to the formation of a galvanic couple between the aluminum alloy and the stainless-steel fastener. Corrosion damage is known to favorably initiate fatigue cracks thus severely reducing the total life of the component. This work aims to understand the relative impact and interaction of fastener hole geometry induced stress concentrations and corrosion damage on the fatigue crack initiation behavior. Specifically, by imparting various levels of corrosion severities at different locations within the macro-scale stress field, the relative impact of each on the initiation process can be determined. This work demonstrated a dominant role of the macro-scale stress field on the crack formation location. Specifically, crack formation was found to preferentially occur at high stress regions in lieu of forming at lower stress regions, regardless of corrosion severity. Critically, the findings of this work will inform the means by which coatings are evaluated and will serve as a controlled validation of experiments for fracture mechanics modeling.

Fracture Surface Behavior of 34CrNiMo6 High-Strength Steel Bars with Blind Holes under Bending-Torsion Fatigue

The present study evaluates the fracture surface response of fatigued 34CrNiMo6 steel bars with transverse blind holes subjected to bending with torsion loading. The analysis of the geometric product specification was performed by means of height parameters Sx, functional volume parameters Vx, and fractal dimension Df. Surface topography measurements were carried out using an optical profilometer with focus variation technology. The experimental results show that the doubling the bending to torsion moment ratio B/T from B/T = 1 to B/T = 2, maintaining the same normal stress amplitude, greatly reduces both Sa, Vv as well as the fractal dimension Df of the analyzed specimen fractures by 32.1%, 29.8%, and 16.0%, respectively. However, as expected, a two-fold increase in the B/T ratio, maintaining the same normal stress amplitude, resulted in a larger number of cycles to fatigue crack initiation, Ni, which can be explained by the lower shear stress level. These experiments prove that parameters Sx, Vx, Df are smaller for larger Ni values, which is an important finding. In addition, it was found a high consistency of surface topography measurements for the two sides of the broken specimens. The proposed methodology is both reliable and applicable for other engineering applications involving different geometries and loading conditions.