Crack Nucleation Sites Research Articles

A Data-Driven Bayesian Model for Predicting Fatigue Crack Nucleation in Polycrystalline Ni-Based Superalloys

This paper develops a Bayesian, probabilistic crack nucleation model in the Ni-based superalloy Ren\'e 88DT for fatigue loading. A data-driven, machine learning approach is developed to identify the underlying mechanics driving crack nucleation. An experimental set of fatigue-loaded microstructures is characterized near crack nucleation sites using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) images to correlate grain morphology and crystallography to the spatial location of crack nucleation sites. A concurrent multiscale model that embeds polycrystalline microstructures, created from the EBSD images, in a self-consistent homogenized material is developed for low cycle fatigue simulations needed to create a database of state variables. The polycrystalline domain is modeled by a crystal plasticity finite element model (CPFEM), while a homogenized anisotropic plasticity model is used for the exterior domain. A Bayesian classification method is introduced to optimally select the most informative state variable predictors of crack nucleation and constructs a near-Pareto frontier of models with varying complexity. From this principal set of state variables, a simplified scalar crack nucleation indicator is formulated which encompasses all of the relevant components derived from the main discriminators. This Bayesian approach allows the micromechanical state variables responsible for causing crack nucleation events to come out naturally from existing data. The final result is a model that predicts the probability of nucleating a crack at a microstructural location, given the mechanical state of the material.

Crystallography and elastic anisotropy in fatigue crack nucleation at nickel alloy twin boundaries

Fatigue crack nucleation at annealing twin boundaries (TBs) within polycrystal nickel-based superalloy René88DT is investigated with a microstructure-sensitive crystal plasticity (CP) model, digital image correlation strain measurements and experimental SEM crack nucleation observations. Strong slip localizations at TBs were experimentally observed and predicted by the CP model, which also showed high predicted geometrically necessary dislocation and corresponding stored energy densities, capturing experimental observations of crack nucleation. In a systematic study, elastic anisotropy was found to drive local elastic constraint and hence resolved shear stress, slip activation, GND density and stored energy density, demonstrating for this reason that TBs are preferential sites for crack nucleation in this alloy. The parent grain / twin pair crystallographic orientation with respect to remote loading was also demonstrated to be key to slip activation parallel to TBs and hence to stored energy density and fatigue crack nucleation, and the range of most damaging parent grain orientations has been identified.

Microstructure and mechanical properties of AA7075/Al3Ni composites produced by compocasting

The present study investigates the effects of the parameters of the compocasting method on the microstructure and mechanical properties of AA7075/Al3Ni composites. By applying compocasting in the semisolid-liquid condition, the Ni reinforcement was varied from 0 to 5 wt%. The results indicate that an increase in the weight percentage of nickel does not have a major impact on the α-Al grain size. However, the most significant changes were in the distribution coefficient and the porosity percentage of the composite. Moreover, the iron-rich contaminant with strip-like morphology increased in the composites due to the increase in the stirring time by increasing the weight percentage of nickel. Compared to the reinforcement-free matrix, the hardness, strength, and toughness of the composite fabricated under optimal conditions increased by 37%, 49%, and 100%, respectively. The highest hardness, tensile strength, and toughness values were achieved using 3 wt% of Ni reinforcement, a stirring temperature of 530 °C, and a stirring speed of 450 rpm. The eutectic structure in the microstructure of the composite acted as a crack nucleation site, and the expansion of this structure resulted in lower strength and toughness in the composites.

Slip localization in Inconel 718: A three-dimensional and statistical perspective

The slip localization behavior of the polycrystalline nickel base superalloy Inconel 718 during monotonic tensile loading at room temperature, is investigated for the first time in relation to the 3D microstructure. Multi-modal data merging tools are used to recombine high resolution digital image correlation (HR-DIC) data with 3D electron back-scatter diffraction tomography (3D EBSD), over a wide region of interest. This procedure enables reconstruction of the slip band planes in the 3D microstructure. Statistical analyses conducted over 500 individual slip bands reveal strong correlations between their location and specific microstructure configurations. In particular, over half of the slip bands emanate from triple junction lines (3D lines defined by the junction of three crystals). Moreover, the most intense and longest slip bands, which would become critical fatigue crack nucleation sites during cyclic loading, are located close and parallel to particular annealing twin boundaries and are simultaneously connected to triple junction lines. Crystal plasticity finite elements calculations are performed on the experimental microstructure to identify the slip activity that results in the formation of high intensity slip bands (localized plasticity) or zones of high lattice rotation (non-localized plasticity) in these particular microstructure regions.

The effect of stress state and strain partition mode on the damage behavior of a Mg-Ca alloy

The ductility of magnesium alloys is greatly affected by damage formation and accumulation. Due to its low-symmetry close-packed hexagonal structure, magnesium alloy's damage behaviors are different from most cubic crystal metals. A comprehensive study was conducted to uncover the underlying damage mechanism of a Mg-0.5%wtCa alloy by using mechanical tests with different stress states, microstructure characterization and the crystal plasticity finite element method (CPFEM). It is found that the central hole tension has a greater resistance to damage than uniaxial tension, as confirmed by the results of digital image correlation (DIC) measurement and X-ray micro-computed tomography (XCT) characterization. This unusual phenomenon is related to the variation of fracture mechanisms in the two scenarios. For the uniaxial tension, plentiful grain boundary cracks are formed and the fractography shows an intergranular feature; while for the central hole tension, the propensity of intergranular fracture is less evident. Through combining the SEM observations of the boundary cracks and the strain distribution determined by the CPFEM simulation, it is revealed that the boundary cracking is caused by the intense deformation heterogeneity and the large strain gradient. Uniaxial tension has a diffuse deformation mode with the randomly distributed ″soft″ grains and ″hard″ grains. This strain partition mode results in a number of spots with high local strain gradient; these spots serve as the nucleation sites of the grain boundary cracks. The results of the notched tests with different notch radii also show the fact that the localized deformation mode is beneficial than the diffuse one. Therefore, in addition to the stress state, the strain partition mode also plays a vital role in the fracture of the dilute Mg-Ca alloy. Further corroboration was done by a three-bending test with gradient deformation mode. A large fracture strain was obtained in the test, indicating that the magnesium alloy has a potentially good formability when the forming process is reasonably designed.

Coexistence of two distinct fatigue failure mechanisms in Super304H welded joint at elevated temperatures

Super304H steel is a promising candidate for boiler tube materials in thermal power plants operating under ultra-supercritical conditions, and the fatigue properties of its welded joint play a key role in the structural reliability of the thermal power plants. Herein, we report that there is a transition in the fatigue failure location (i.e., fatigue failure mechanism) from the base metal at a high strain amplitude (≥~0.4%) to the weld metal at a low strain amplitude (≤~0.3%) at a constant temperature in the operating range of 500–700 °C, and this leads to a significant reduction in the fatigue resistance of the welded joint as compared to that of the base metal. We found that a longer fatigue test time at a low strain amplitude induces a thermal aging effect that promotes different microstructural evolutions in the base metal and weld metal, deepening material inhomogeneity in the welded joint, and thereby triggering strain localization in the weld metal. With increasing fatigue test time (i.e., thermal aging time), Cr23C6 carbides precipitated in the interdendritic region of the weld metal and at the austenitic grain boundary of the base metal, and Nb (C, N) phases precipitated in the interior of austenitic grain in the base metal, which increased local hardness, whereas there was no significant microstructural change in the dendrite core of the weld metal, retaining its initial hardness. The intensified local hardness inhomogeneity caused the softest zone in the welded joint, wherein strain localization occurred, serving as a crack nucleation site and propagation path, to shift from the base metal to the weld metal (dendrite core). This induced a shift in the fatigue failure location from the base metal to the weld metal with decreasing strain amplitude.

Influence of Sheet Conditions on In-Plane Strain Evolution via Ex-Situ Tensile Deformation of Ti-3Al-2.5V at Room Temperature

Localised plastic deformation evolution was examined in a near alpha Ti-3Al-2.5V alloy with indent defect and defect free surfaces using digital image correlation, an interrupted uniaxial tensile test and scanning electron microscopy. The main aim was to understand the role of the localised strain evolution at micro scale and the underlying deformation mechanisms that influence the global mechanical behaviour of the material. The microstructures captured at different stages of deformation were processed using a digital image correlation system, whose outputs were analysed through Matlab, to ascertain the localised strain evolution observed in each surface condition. This work found that the strains observed at the deformation bands along the indent defect edge, were significantly higher than those observed in the deformed β phase field. The deformation bands concentrating at the tip of the indent defect acted as a fertile site for early crack nucleation and propagation with a reduced localised fracture strain. For a defect free surface, the absence of defect zones acting as a high stress concentration site meant that strain aggregation was minimised and the α phase field was able to sufficiently accommodate the β phase deformation resulting in higher fracture strains.

In situ X-ray imaging of pore formation mechanisms and dynamics in laser powder-blown directed energy deposition additive manufacturing

Directed energy deposition (DED) additive manufacturing (AM) is receiving growing attention in many applications, such as repair, remanufacturing, and fabrication of functionally graded structures. However, the laser-matter interactions and melt pool dynamics in laser DED with powder flow are still unclear, particularly in how pores form and flow inside the melt pool during the process. Understanding the porosity formation mechanisms is critical in the qualification, certification, and overall properties of a DED AM part. Porosity is a common phenomenon and can significantly hinder the quality of DED fabricated parts, as the pores can act as sites of crack nucleation and propagation. Here, we reveal four types of pore formation mechanisms through in-situ and operando high-speed high-resolution X-ray imaging in the DED AM process. Our results confirm that porosity within the feedstock powder induces pores in the process. We also observed pore formation mechanisms unique to the laser-based, powder-blown DED process as a result of powder delivery, keyhole dynamics, melt pool dynamics, and shield gas. High-speed X-ray images provide direct evidence for pore formation mechanisms and show that the pores related to the interaction between the delivered powder and melt pool are the largest in size in laser-based powder-blown DED AM. These results will guide porosity mitigation, elimination, and control in DED AM.

The effect of pearlite morphology and crystallographic texture on environmentally assisted cracking failure

Mechanism of failure by environmentally assisted cracking was extensively investigated in the present work to find the effect of pearlite morphology and crystallographic texture on crack nucleation and propagation sites. The macrotexture analysis revealed that (001) crystallographic texture components parallel to the normal direction was predominant in the investigated sample. Crack nucleated at surface pits and propagated in low angle grain boundaries. The refined microstructure prevents crack growth and can deflect crack propagation. High angle grain boundaries were found to suppress crack propagation. In the crack-free region (223) [11¯0] and (1¯11) [11¯2] texture components were found. Therefore, the control of crystallographic orientation and engineering the grain boundary characteristics can effectively improve the environmentally assisted cracking resistance.

Effect of stress and surface finish on Pb-caustic SCC of alloy 690TT

The effect of surface preparation on lead-induced stress corrosion cracking (PbSCC) of Alloy 690 TT in Pb-caustic solution was investigated using C-rings manufactured from tubing that was either as-manufactured or polished. The surface deformed layer on the as-manufactured samples promoted the formation of an oxide, which reduced the number of sites for crack nucleation. Conversely, the polished surface developed crystallographic corrosion slots from which some of the cracks seemed to initiate. Advanced microstructural characterization complemented with stress analysis of the samples, suggested that the early stages of PbSCC are consistent with a film-rupture/slip-dissolution mechanism and influenced by de-alloying.

Crystal Plasticity Modeling of Laser Peening Effects on Tensile and High Cycle Fatigue Properties of 2024-T351 Aluminum Alloy

Abstract This study aims to model the effects of multiple laser peening (LP) on the mechanical properties of AA2024-T351 by including the material microstructure and residual stresses using the crystal plasticity finite element method (CPFEM). In this approach, the LP-induced compressive residual stress distribution is modeled through the insertion of the Eigenstrains as a function of depth, which is calibrated by the X-ray measured residual stresses. The simulated enhancement in the tensile properties after LP, caused by the formation of a near-surface work-hardened layer, fits the experimentally obtained tensile curves. The model calculated fatigue indicator parameters (FIPs) under the following cyclic loading application show a decrease in the near-surface driving forces for the crystal slip deformation after the insertion of the Eigenstrains. This leads to a higher high cycle fatigue (HCF) resistance and the possible transformation of sensitive locations for fatigue failure further to the depth after LP. Experimental observations on the enhancement in the HCF life, along with the relocation of fatigue crack nucleation sites further to the depth, reveal the improvement in the HCF properties due to the LP process and validate the numerical approach.

Fatigue strength of austempered ductile iron-to-steel dissimilar arc-welded joints

Nowadays, the use of different classes of materials in the same structure is increased to keep pace with innovation and high structural performances. In this context, structural components made of different materials need to be joined together and a possible solution is given by arc welding. Dissimilar welded joints must often be able to withstand fatigue loads; however, Design Standards provide fatigue strength categories only for homogeneous welded joints. The aim of the present paper is to compare the fatigue behaviour of EN-GJS-1050 austempered ductile iron-to-S355J2 steel dissimilar joints to the categories of the corresponding homogeneous steel welded joints, as suggested in International Standards and Recommendations. For this purpose, experimental fatigue tests were performed on a selection of dissimilar welded details. First, the microstructure was identified by metallographic analysis; micro-hardness measurements were collected and residual stress profiles were obtained by using the X-ray diffraction technique on a selection of joints. Misalignments were quantified for all specimens. Then, experimental fatigue tests have been performed on a number of joint geometries subject to axial or bending fatigue loadings and tested in the as-welded conditions. The fracture surfaces of the joints have been analysed to locate fatigue crack nucleation sites.

Atom scale study of cracks in the carbide phase of WC–Co alloys

ABSTRACT In this study, cracks propagating in WC grains are investigated at the atom scale along the [0001] axis. A brittle crack, characterised by cleavage along {1, 0, −1, 0} prismatic facets and the absence of dislocations is observed. The crack follows a zig-zag path allowing a higher deformation energy to be absorbed by the crystal. A ductile crack is also characterised. The plastic zone is limited and contains mainly multipoles consisting in unextended dislocations. The crack path is tortuous and delineated by dislocation arrangements sometimes involving extended dislocations. Close to the crack tip, the fracture is discontinuous and nanocracks occur at the level of dislocation arrangements. These dislocation configurations could provide crack nucleation sites for the crack propagation.

Effect of nanosized NbC precipitates on hydrogen-induced cracking of high-strength low-alloy steel

We investigated the effect of nanosized NbC precipitates on hydrogen-induced cracking (HIC) of high-strength low-alloy steel by conducting slow-strain-rate tensile tests (SSRT) and performing continuous hydrogen charging and fracture analysis. The results reveal that the HIC resistance of Nb-bearing steel is obviously superior to that of Nb-free steel, with the fractured Nb-bearing steel in the SSRT exhibiting a smaller ratio of elongation reduction (Iδ). However, as the hydrogen traps induced by NbC precipitates approach hydrogen saturation, the effect of the precipitates on the HIC resistance attenuate. We speculate that the highly dispersed nanosized NbC precipitates act as irreversible hydrogen traps that hinder the accumulation of hydrogen at potential crack nucleation sites. In addition, much like Nb-free steel, the Nb-bearing steel exhibits both H-solution strengthening and the resistance to HIC.

Coupling a discrete twin model with cohesive elements to understand twin-induced fracture

The interplay between twinning and fracture in metals under deformation is an open question. The plastic strain concentration created by twin bands can induce large stresses on the grain boundaries. We present simulations in which a continuum model describing discrete twins is coupled with a crystal plasticity finite element model and a cohesive zone model for intergranular fracture. The discrete twin model can predict twin nucleation, propagation, growth and the correct twin thickness. Therefore, the plastic strain concentration in the twin band can be modelled. The cohesive zone model is based on a bilinear traction-separation law in which the damage is caused by the normal stress on the grain boundary. An algorithm is developed to generate interface elements at the grain boundaries that satisfy the traction-separation law. The model is calibrated by comparing polycrystal simulations with the experimentally observed strain to failure and maximum stress. The dynamics of twin and crack nucleation have been investigated. First, twins nucleate and propagate in a grain, then, microcracks form near the intersection between twin tips and grain boundaries. Microcracks appear at multiple locations before merging. A propagating crack can nucleate additional twins starting from the grain boundary, a few micrometres away from the original crack nucleation site. This model can be used to understand which type of texture is more resistant against crack nucleation and propagation in cast metals in which twinning is a deformation mechanism. The code is available online at https://github.com/TarletonGroup/CrystalPlasticity.Graphic

Volume, Side-Area, and Force Direction of Berkovich and Cubecorner Indenters, Novel Important Insights

The iteration-free physical description of pyramidal indentations with closed mathematical equations is comprehensively described and extended for creating new insights in this important field of research and applications. All calculations are easily repeatable and should be programmed by instrument builders for even easier general use. Formulas for the volumes and side-areas of Berkovich and cubecorner as a function of depth are deduced and provided, as are the resulting forces and force directions. All of these allow for the detailed comparison of the different indenters on the mathematical reality. The pyramidal values differ remarkably from the ones of so-called “equivalent cones”. The worldwide use of such pseudo-cones is in severe error. The earlier claimed and used 3 times higher displaced volume with cube corner than with Berkovich is disproved. Both displace the same amount at the same applied force. The unprecedented mathematical results are experimentally confirmed for the physical indentation hardness and for the sharp-onset phase-transi- tions with calculated transition energy. The comparison of both indenters provides novel basic insights. Isotropic materials exhibit the same phase transition onset force, but the transition energy is larger with the cube corner, due to higher force and flatter force direction. This qualifies the cube corner for fracture toughness studies. Pile-up is not from the claimed “friction with the indenter”. Anisotropic materials with cleavage planes and channels undergo sliding along these under pressure, both to the surface and internally. Their volumes add to the depression volume. These volumes are essential for the exemplified pile-up management. Phase-transitions produce polymorph interfaces that are nucleation sites for cracks. Technical materials must be developed with onset forces higher than the highest thinkable stresses (at airliners, bridges, etc.). This requires urgent revision of ISO 14577-ASTM standards.

Additively manufactured novel Al-Cu-Sc-Zr alloy: Microstructure and mechanical properties

An in-depth understanding of microstructure and resultant properties is paramount in the design of a novel alloy system, especially for additive manufacturing (AM). The present investigation aims to characterize a prototypical AM Al alloy with great potential for structural applications. An Al-1.5Cu-0.8Sc-0.4Zr alloy designed using integrated computational material engineering was printed using the laser powder bed fusion AM process. This novel alloy shows promising combination of strength and ductility in as-built and peak-aged conditions. This improvement in the tensile properties is attributed to the presence of both coherent L12 Al3Sc/Al3(Sc,Zr) precipitates and Cu-rich regions. The microstructures were studied via extensive microscopy at different length scales using X-ray microscopy, scanning electron microscopy, and transmission electron microscopy. Fractography revealed that the columnar grain boundaries in as-built condition allow easy slip transfer as compared to the equiaxed grains, with the apex of the melt pool acting as the crack nucleation site. However, the peak aged condition resulted in improved strength while marginally sacrificing ductility due to precipitates decorating dislocations, grain boundaries and melt pool boundaries thus acting as obstacles to slip transfer.

Failure analysis of cemented carbide roller for cold rolling: Material characterisation, numerical analysis, and material modelling

Cemented carbides, also known as hard metals (HM), are composite materials that are widely used in many technological applications requiring favourable mechanical properties, specifically a high wear resistance. In the steelmaking industry, these materials have been effectively used to produce rollers for both hot and cold rolling operations.In this study, failure analysis of a roller used for cold rolling was performed, starting from a detailed material investigation to a fracture surface characterisation using scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM + EDXS). A finite element (FE) analysis of the roller was performed to determine the Hertzian-type stress distribution in the contact area. To identify the most critical zone, the Crossland multiaxial criterion was applied, and the results were then compared with measurements obtained from a component failure analysis. An additional investigation of the possible causes of failure was performed by adopting a material model (the Murakami–Endo equation) to distinguish the actual causes of failure more clearly, mainly at the crack nucleation site.The failure analysis results showed that the roller failed primarily because of rolling fatigue, which started from an internal pore acting as a source of local stress concentration. The component reached the fatigue limit of the material in proximity to the pore, which behaved as a crack nucleation site.

How does surface integrity of nanostructured surfaces induced by severe plastic deformation influence fatigue behaviors of Al alloys with enhanced precipitation?

Effects on fatigue behaviors and microstructures of Surface Mechanical Attrition Treatment (SMAT) before or after precipitation aging have been investigated on two hardenable Al alloys.The aluminum alloy with high notch sensitivity (7075) should not be processed by SMAT as the generated low surface integrity is always detrimental to fatigue performances. On less notch sensitive alloys (2024), SMAT before aging formed smaller and denser precipitates, resulting in a high hardened depth, and microstructures more resistant to residual stress relaxation than after conventional shot-peening. SMAT after aging resulted in a significant improvement of fatigue performance with only subsurface crack nucleation sites.

Notch fatigue behaviour of low-temperature gaseous carburised 316L austenitic stainless steel

ABSTRACT The influence of low-temperature gaseous carburisation on notch fatigue behaviour of 316L steel under cyclic axial loading was investigated. After carburisation, the carburised case was well distributed at the surface region and was not influenced by the notch geometry. Low-temperature carburisation considerably enhanced the notch fatigue performance, which led to 32% and 44% increase in the endurance limits for the specimens with stress concentration factors K t = 1.91 and 3.91, respectively. The notch sensitivity of 316L steel reduced after carburisation. Irrespective of the applied stress amplitude, the fatigue crack nucleation sites were always at the notch root surface for the untreated specimens. For the carburised specimens, fatigue cracks nucleation changed from surface at high-level stress to subsurface at low-level stress.