Gustavo M. Castelluccio

Assessment of small fatigue crack growth driving forces in single crystals with and without slip bands

Strain localization under low amplitude cyclic loading is a manifestation of plastic irreversible deformation associated with early crack growth. However, traditional constitutive models cannot usually reproduce strain localization in smooth single crystals, which can affect crack growth predictions for crystallographic fatigue cracks. This work analyzes the influence of bands of localized plastic shear strain on the cyclic crack tip displacement and on a fatigue indicator parameter by making special provision of a crack along the interface of a deformation band. Furthermore, the quality of local and volume-averaged fatigue indicator parameters are assessed using finite element models of a Cu single crystal cycled to induce plastic deformation under multiple loading conditions.

A mesoscale approach for growth of 3D microstructurally small fatigue cracks in polycrystals

In the high cycle fatigue regime, microstructure attributes such as grain size, shape, and crystallographic orientation usually affect fatigue crack formation and early growth. However, most computational strategies and theoretical models for assessing the influence of the microstructure on early stages of fatigue crack formation and growth rely on simple constitutive models and 2D microstructures, which limit their applicability in design of microstructure of engineering materials. This work employs finite element simulations that explicitly render the 3D microstructure of an Face-centered cubic (FCC) alloy to evaluate the change of the driving force for fatigue crack formation and early stages of transgranular growth, including consideration of growth within individual grains and stress redistribution as the crack extends. The methodology is implemented using a crystal plasticity algorithm in ABAQUS and used to study the effect of microstructure on early fatigue life of a powder processed Ni-base RR1000 superalloy at 650℃ subjected to constant amplitude loading. The effects of the microstructure in extending a fatigue crack over the first few grains are analyzed.

Fatigue Life Prediction of Microstructures

The formation and early growth of fatigue cracks in the high cycle fatigue regime is influenced by microstructuctural features such as grain size and morphological and crystallographic texture. However, most fatigue models do not predict the influence of the microstructure on early stages of crack formation, or they employ parameters that should be calibrated with experimental data from specimens with microstructures of interest. These post facto strategies are adequate to characterize materials, but they are not fully appropriate to aid in the design of fatigue-resistant engineering alloys. This paper presents a modeling framework that facilitates relative assessment of fatigue resistance among different microstructures. The scheme employs finite element simulations that explicitly render the microstructure and a methodology that estimates transgranular fatigue growth for microstructurally small cracks on a grain-by-grain basis, including consideration of growth within grains (embedded analytically) and stress redistribution as the cracks extend. The methodology is implemented using a crystal plasticity algorithm in ABAQUS and calibrated to study fatigue crack initiation of a bimodal grain size distribution found in RR1000 powder processed Ni-base superalloys for turbine disk applications.Copyright

Quantitative in Situ SEM High Cycle Fatigue: The Critical Role of Oxygen on Nanoscale-Void-Controlled Nucleation and Propagation of Small Cracks in Ni Microbeams

This Letter presents a quantitative in situ scanning electron microscope (SEM) nanoscale high and very high cycle fatigue (HCF/VHCF) investigation of Ni microbeams under bending, using a MEMS microresonator as an integrated testing machine. The novel technique highlights ultraslow fatigue crack growth (average values down to ∼10-14 m/cycle) that has heretofore not been reported and that indicates a discontinuous process; it also reveals strong environmental effects on fatigue lives that are 3 orders of magnitude longer in a vacuum than in air. This ultraslow fatigue regime does not follow the well documented fatigue mechanisms that rely on the common crack tip stress intensification, mediated by dislocation emission and associated with much larger crack growth rates. Instead, our study reveals fatigue nucleation and propagation mechanisms that mainly result from room temperature void formation based on vacancy condensation processes that are strongly affected by oxygen. This study therefore shows significant size effects governing the bending high/very high cycle fatigue behavior of metals at the micro- and nanoscales, whereby the stress concentration effect at the tip of a growing small fatigue crack is assumed to be greatly reduced by the effect of the bending-induced extreme stress gradients, which prevents any significant cyclic crack tip opening displacement. In this scenario, ultraslow processes relying on vacancy formation at the subsurface or in the vicinity of a crack tip and subsequent condensation into voids become the dominant fatigue mechanisms.

Influence of Weld Mismatch on the Structural Integrity of Pipes for Reeling

Structural integrity analysis of tough materials based on Elastic-Plastic Fracture Mechanics (EPFM) has been successfully employed in the assessment of components. EPFM has originally been developed for homogeneous materials and its applicability to inhomogeneous materials has some peculiarities. In particular, Fitness for Service design of welded pipes requires to know the weld fracture toughness and to estimate accurately the J-integral applied on the actual structural member. In this work, finite element analyses of simulated welds have been carried out in order to qualify and quantify the lack of accuracy of experimental methodologies for measuring fracture toughness of welds and the influence of welds on the applied J-integral in a pipe under bending. Different weld widths and cracks positions are characterized for single edge notch specimens in tension (SE(T)) and pipes. It has been found that inhomogeneity affects elastic-plastic fracture parameters for cracks centered in welds of certain widths. Moreover, the applied J-integral on pipes with circumferential cracks depends significantly on the weld width and crack position.Copyright

Stress Waves Propagating Through Bolted Joints

This paper examines the mechanical response of a simple bolted joint, the Brake–Reu\(\ss\) beam, under shock loading. This is done by creating a high-fidelity finite element model of the beam and subjecting it to a quasi-static bolt load followed by a dynamic shock load. The influence of several parameters on the beam’s response is studied, which include impact force, impact duration, impact location, and residual stress. The results indicate that when the energy input into the beam is held constant, the most influential parameter is the shock’s frequency and that increasing its frequency significantly increases dissipation. The next most influential parameter is the impact location, though its effect is frequency dependent and becomes stronger for higher frequencies. Finally, the results show that while residual stresses can significantly modify the contact-pressure distribution, they have minimal influence on the energy dissipated due to friction resulting from shock loading.