This paper focuses on the crack propagation mechanisms under pure torsional loads of a cast Al-Si alloy. This article is the second part of recent work concerning the characterisation of torsional fatigue crack formation mechanisms using 3D synchrotron observation techniques. While the aim of the first part was to investigate the crack initiation mechanisms, this second article is focused on the crack growth mechanisms. For this, synchrotron observations including X-ray tomography and Diffraction Contrast Tomography were used i) to obtain the full 3D microstructure and ii) to perform 3D in-situ monitoring of the crack growth. It is shown that there are two coexisting crack propagation modes: a shear-mode and an opening-mode. Shear-mode cracks, which are crystallographically controlled, propagate predominantly on the planes of maximum shear stress corresponding to the slip system with the highest Schmid factor. The growth rates of those cracks can be relatively high (da/dN approximate to 10(-8) m/cycle) but frequent and long-lasting arrests at grain boundaries are observed. These crack arrests are governed mainly by low Schmid factors in adjacent grains and large misorientation angles (defined by tilt and twist angles). Unlike shear-mode cracks, cracks propagating in an opening-mode propagate steadily and are not significantly hindered by the surrounding microstructure. They initiate only from pores and do not initiate in defect free zone of the aluminium matrix. They propagate on plane of maximum normal stress with lower growth rates (da/dN approximate to 10(-9) m/cycle) and have a near-threshold stress root intensity range observed to be approximately Delta K-1 approximate to 1 MPa root m.

This study details an experimental procedure developed to investigate the growth of mechanically small fatigue cracks in cast AlSi7Mg alloy and their response to single tensile overloads occurring during constant amplitude loading.The AlSi7Mg alloy underwent grain refinement (~125 µm median grain size) and a heat treatment transformed the network of eutectic Si into a near-random pattern of speckled particles. Small fatigue specimens (1.2-1.6 mm cross-section side length) machined from this alloy were subjected to cyclic tensile loading (featuring occasional tensile overloads) using a custom in situ fatigue rig. This setup was placed on synchrotron X-ray computed tomography beamlines, leveraging phase contrast imaging to reconstruct volumes of the samples featuring the Si particles.Detection of the crack front in the reconstructed volumes allowed the computation of local fatigue crack growth rates. Results revealed a typical reaction to an overload with a period of crack growth arrest followed by a differentiated recovery process: crack growth resumes first in the bulk of the material then near the free surfaces of the specimen.The natural speckle pattern is exploited for running digital volume correlation (DVC) between two mechanical states of the sample as captured via tomography. Crack opening displacement (COD) maps are derived from the DVC displacements field to measure the immediate effects of an overload, notably how residual opening and blunting develop in the near-crack tip region.

The performance of steel parts produced by additive manufacturing (AM) is still inadequate for some industrial applications, particularly in the fields of production and tooling, which require increased durability specifications. They demand high hardness (HRC>60), while maintaining other mechanical performances such as fatigue, toughness and thermal conductivity. The key challenges of printing hard steels occur during the fusion process, due to high carbon contents cracks are likely to occur during the manufacturing process. In this work, laser powder bed fusion (LPBF) and fused pellet fabrication (FPF) are used to print M2 steel parts.Both technologies generate unique microstructures. Here, the microstructure is thoroughly investigated (Optical, XRD, SEM, Leco…) to understand the influence of these technologies on microstructural features such as grains, carbides, porosity and consequences on hardness.

This research investigates the effect of the formation of an oxygen-stabilised titanium alpha layer - called alpha-case at the surface - on the fatigue properties of Ti-6Al-4V (Ti64) alloy components produced by Laser Powder Bed Fusion (L-PBF). Three post processing heat treatments with different controlled atmospheres were carried out on samples with as-built surfaces to evaluate how differences in alpha-case layer thickness and hardness affect the material's susceptibility to surface embrittlement and its overall fatigue performance. The investigation includes bulk and subsurface microstructural analysis, surface characterisation by X-ray computed tomography (XCT), and fatigue testing. Key findings show that alpha-case layers can reduce the fatigue resistance of L-PBF fabricated Ti64. The presence of a 70 +/- 3 mu m thick alpha-case layer was found to promote crack initiation. This is emphasised by a higher density of initiated cracks, thus leading to a reduction in fatigue life. Conversely, thinner alpha-case layers were found to have a reduced impact on the fatigue performance, highlighting the critical role of post processing heat treatments in modulating the fatigue resistance of the material. The use of XCT to characterise the surfaces of the specimens in 3D confirms that fatigue cracks primarily initiate at surface notches, highlighting the predominance of as-built surfaces over microstructure in determining the fatigue resistance of L-PBF Ti64 components.

The fatigue crack closure and propagation behavior of 1Cr18Ni9Ti stainless steel under cyclic loading with single tensile overloads at different crack growth stages were investigated using in-situ digital image correlation. The evolution of the crack tip opening displacement and stress intensity factors was analyzed by extracting displacement fields from high-resolution optical microscopy images. The results indicate that under constant amplitude loading, crack closure intensifies with crack growth, with the crack opening stress root root intensity factor (Kop) increasing from 9.36 MPa m at a crack propagation length of 0.3 mm to 28.83 MPa m at 1.3 mm. The application of a single overload caused significant crack tip blunting, temporarily eliminating crack closure and leading to a sharp reduction in the effective stress intensity factor. Overload retardation is more pronounced in the short crack stage, leading to a 70% reduction in crack propagation rate and requiring approximately 25,000 cycles for recovery, whereas in the long crack stage, recovery occurs within 8,000 cycles. SEM fracture surface analysis confirmed that crack tip blunting played a key role in post-overload retardation, with crack re-initiation occurring at the blunted tip, governing delayed propagation. These findings provide new insights into the fatigue behavior of 1Cr18Ni9Ti steel under variable amplitude loading, contributing to improved fatigue life predictions for aerospace structures.

Ultrafast laser machining has been researched extensively over the last few decades to create features such as holes in a variety of materials. The effects of laser parameters including power and polarization on the dynamics of hole formation and resulting hole geometry have been studied. Grooves formation, especially deep ones, on the other hand, has not attracted as much attention, even though grooves are essential to most laser cutting operations. One aspect limiting the study of deep machined features such as grooves is the difficulty in imaging not only the geometry but also the associated collateral damage produced in the material during machining. Here, we employed x-ray tomography for three-dimensional imaging of deep ultrafast laser-machined grooves in various metals. The 3D images of the deep grooves were quantitatively analyzed, revealing the significant effect of laser polarization on groove morphology. Under rotating polarization (also called “scrambled polarization” or “polarization trepanning”), the deep grooves are smooth and uniform, while under linear polarization, extensive branching is observed along the groove, and becomes more pronounced with increasing laser energy and groove entrance length. A mechanistic picture based on laser light reflection off the groove walls is proposed to qualitatively explain the polarization-dependent groove branching observed experimentally. These findings provide new insights into high-precision deep groove laser machining, highlighting the effectiveness of x-ray tomography as a powerful tool for in-depth three-dimensional studies of laser machining processes.

Ultrasonic fully reversed tension fatigue tests have been performed in the Very High Cycle Fatigue (VHCF) regime (N-R > 10(7)- 108 cycles) on Ti-6Al4V specimens containing a controlled internal notch. Two sets of samples have been used. The first one contains a central chimney along the specimen longitudinal axis which brings air to the internal notch; in the second series the notches are not connected to the surface. The microstructure present below the fracture surface of the broken specimens has been studied by electron microscopy (EBSD, TKD and TEM). The formation of nanograins and nanovoids was observed below the surface of the cracks growing in a vacuum environment but not below the surface of cracks connected with ambient air. In the latter case extensive striations were observed. Below each striation the formation of tensile {1012} twins was observed.

This paper focuses on the characterisation of fatigue crack initiation mechanisms under fully reversed torsional loads for the porosity-containing cast AlSi7Mg0.3 aluminium alloy by synchrotron X-ray tomographic imaging and Diffraction Contrast Tomography (DCT). The aim is to analyse the relation between crack initiation and the microstructure, in the fatigue regime close to the fatigue limit of the material (fatigue lives of approximately 2 x 106 cycles). In-situ torsion fatigue tests have been conducted at the European Synchrotron Radiation Facility. A large number of cracks (approximately 80 cracks) were analysed, at the surface and in the bulk, highlighting the role of the surrounding microstructure on the crack initiation mechanisms. Contrary to uniaxial loads in which fatigue crack initiation from casting pores in an opening mode (mode I) is generally only observed for this material, it is shown in this work that multiple crack initiations mechanisms can appear simultaneously under torsional loads. This is very interesting because it is possible to analyse different mechanisms (and the effect of the surrounding microstructure) using the same specimen, that is under identical loading conditions. More precisely, two different crack initiation mechanisms were observed. The first mechanism involves intra-granular fatigue crack initiation from Persistent Slip Bands (PSBs) formed on the slip system with the highest Schmid factor. For this mechanism, the grain orientation is the key parameter. A shear stress threshold of approximately 80 MPa is observed to trigger this mechanism. The presence of a pore, in the crack initiation zone reduces this threshold. The second mechanism concerns crack initiation from pores, in mode I, on planes of maximum principal stress. No link to the local grain orientation was found in that case. The pore size is the key factor governing this crack initiation mechanism. Regarding the competition between these mechanisms in the formation of the final crack, it was observed that at high applied stress, the cracks initiated from PSBs are dominant due to their high density and high growth rates resulting from coalescence. At lower stress, cracks initiated from pores are dominant, mainly because of the little effect of grain boundaries and eutectic zones, while most of cracks initiated from PSBs are arrested within the grain where they have initiated.

Many studies highlight the significance of three-dimensional surface topography characterization in assessing its effect on the mechanical or functional properties of materials. This is especially obvious for parts made by additive manufacturing (AM), known for their complex shape and surface topographies. However, a vast majority of 3D characterizations have constraints regarding the macroscopic geometry of the parts they can probe. At the microscale, they are also unable to account for hidden surface features, e.g. notches hidden by unmelted powder particles. Even with the use of X-ray Computed Tomography (XCT) - a tool with the potential to circumvent these issues -data is often reduced to 2D or 2.5D formats for easier analysis, but this leads to a loss of information. This underscores the need for XCT data post-treatment tools to perform thorough 3D surface characterizations. Herein, we introduce a methodology for local roughness and curvature characterization of surfaces of complex shapes using XCT. This method has been designed to be user-friendly, especially for those without extensive data analysis expertise. It provides a comprehensive 3D characterization and efficiently tackles the issues caused by hidden features. After a detailed description of our methodology, we give a first illustrative example based on architected structures fabricated by Electron Powder Bed Fusion (E-PBF). By integrating roughness and curvature metrics, we also derive a parameter indicative of the stress concentrations caused by surface irregularities.

Digital volume correlation (DVC) of laboratory X-ray tomography images has been used to characterize the closure of artificial internal fatigue cracks in nodular graphite cast iron. DVC allows to produce crack opening displacement (COD) map from 3D tomography images at the sub-voxel scale thanks to the graphite nodules which can be used as internal natural markers. Fatigue samples containing controlled internal defects have been produced in a nodular cast iron by diffusion bonding plates containing laser-machined sharp notches. Specimens with internal cracks and surface cracks were respectively characterized by laboratory tomography at different loads during fatigue crack propagation tests. The local stress intensity factors (SIFs) have been extracted from the measured displacement fields by using Williams series, based on those results the closure levels of internal cracks and surface cracks were comparatively analyzed and discussed. Crack closure levels were found to be linearly related to the location of the surface crack front, and the level of internal crack closure was comparable to that of the surface crack in bulk.

3D tomographic images of a cast AlSi7Cu3Mg alloy were obtained using synchrotron X-ray tomography during in-situ Low Cycle Fatigue tests at 250 degrees C. While image analysis highlights the role of eutectic Si particles close to pores in damage mechanisms, high resolution digital volume correlation reveals the relationship between strain heterogeneity at the microstructural scale and hard particles failure or cracks. Monitoring strains evolution with cycles within hard particles, i.e. eutectic Si and Fe or Cu intermetallics, allows measuring their local failure strains and drawing a hierarchy of the deformation to failure. Then, a local Manson-Coffin curve per hard phase is proposed.

Fatigue specimens of a Ti-6Al-4V alloy containing internal artificial defects with controlled and reproducible size and shape have been produced. These defects systematically led to the initiation of a fatigue crack which propagation has been monitored in situ by synchrotron X-ray tomography during R=0.1 uniaxial fatigue tests at 20 Hz. The crack growth curves of the internal cracks have been obtained for 6 samples. Ex situ fatigue tests have been performed on samples submitted to a supplementary heat treatment or containing a defect put into contact with air. The results obtained tend to support the fact that internal fatigue cracks grow from the notch in a vacuum environment. On the fracture surfaces of samples containing an artificial defect not connected to air, two regions have been observed. They correspond to the Rough Area and the Fish Eye regions observed for internal cracks initiated from natural defects. The transition between those two regions takes place when the plastic radius size is equivalent to the grain size.

The propagation rate of a fatigue crack in a nodular cast iron, loaded in cyclic tension, has been studied in situ by X-ray computed tomography and digital volume correlation. The semi-elliptical crack initiated from an asym-metric corner notch and evolved to a semi-circular shape, initially with a higher growth rate towards one edge of the notch before the propagation rate along the crack front became essentially independent of position. The phase congruency of the displacement field was used to measure the crack shape. The three-dimensional stress intensity factors were calculated via a linear elastic finite element model that used the displacement fields around the crack front as the boundary conditions. Closure of the crack tip region was observed. The cyclic change in the local mode I opening of the crack tip determined the local fatigue crack propagation rate along the crack front.

In the present study, physically short fatigue cracks initiated from an artificial internal notch under vacuum conditions are observed in smooth specimens made of titanium alloy. The crack initiated from such a notch is not in contact with the surface during the majority of its propagation. Another batch of artificially notched specimens in the same alloy having their internal notch linked with the surface to bring air to the initiated short crack were tested too. All the specimens were ultrasonically cycled with synchrotron micro-computed tomography acquisitions which were regularly acquired from crack initiation detection to final failure without demounting the specimen. The number of cycles to failure, the internal crack growth rate and the crack growth mechanisms of internal cracks are compared for propagation under either air conditions or in the core of the alloy (i.e. under vacuum environment). It is shown that the environment plays a key role in the explanation of the very low crack growth rate of internal short crack loaded at very low stress amplitude leading to the gigacycle regime.

This work addresses the fatigue behaviour of two strut-based topologies of micro-lattice materials, focusing in particular on the damage evolution throughout the fatigue life. An experimental campaign is performed on micro-lattice specimens tested in fatigue with different stress ratio (R = 0, R = -1 and R = 10). A methodological approach for metal foams is adopted to analyse the experimental tests and to characterize the fatigue damage evolution. To validate this approach the Digital Volume Correlation technique is applied on two in-situ experimental fatigue tests conducted inside a Micro-Computed Tomography scanner. An analysis based on a finite element model of the as-manufactured geometry and the application of a suitable multiaxial fatigue criterion to evaluate the local stress field shows good results in predicting the fatigue failure location.

Very high cycle fatigue fracture is often associated with internal crack propagation and one major problem to study the initiation and the propagation of this internal crack is to detect its initiation and quantify its propagation rate. The objective of the present work is to develop an experimental methodology to follow the initiation and propagation of the internal crack. The experimental technique presented here is based on temperature field measurement on the specimen surface during an ultrasonic fatigue test using infrared thermography. Indeed, plasticity in the reverse cyclic plastic zone near the crack front generates heat sources due to the dissipation of plastic energy rate and thus an increase of the temperature which propagates through the specimen thanks to heat conduction. By considering the heat source located in the reverse cyclic plastic zone and the geometry of the crack obtained by tomographic observations, finite element simulation of the heat transfer problem enables us to establish a relationship between the internal crack growth and the temperature field evolution on the specimen surface. The results obtained for a cast aluminum alloy are presented and analyzed.

Laser Powder Bed Fusion (LPBF) is an additive manufacturing process used to produce conformal cooling injection molds with complex internal channels, mainly using cobalt-rich 18Ni300 maraging steel. Yet, built parts with this steel powder still demand improved toughness and fatigue strength. As an alternative, this work describes the manufacturibility of two alternative cobalt-free martensitic stainless steels by LPBF. After a quantitative characterisation of the microstructure, defects and mechanical properties of as-built parts, different heat treatments were performed to (i) age the precipitation-hardenable low-carbon maraging steel (CX) or (ii) temper the high-carbon martensitic steel (PM420). The hardness, tensile strength, ductility, impact energy and fatigue behavior of as-built and treated parts were compared. The influence of the microstructure and the critical defects on the mechanical behavior is discussed, with an emphasis on the fatigue life. Finally, the manufacturing of complex injection molds using PM420 powder was assessed.