Crack Nucleation Research Articles

Scratch-induced surface formation mechanism in C/SiC composites

Due to the extremely complex composition of C/SiC composites, their material removal mechanisms are significantly different from those of conventional single-phase ceramic materials. Therefore, the grinding removal mechanism of C/SiC composites must be investigated so that the components based on these composites can be machined efficiently and with low damage. In this paper, nano-scratch experiments with various cutting depths are carried out along different fiber orientations to simulate the material removal process in grinding and theoretically clarify the damage failure mechanism based on different cutting thicknesses. According to the assessment of surface and subsurface morphology of the various constituents in the scratch grooves regarding different fiber orientations, the multiple brittle breakage modes and surface integrity of carbon fiber are revealed. Green’s function is applied to establish a half-space anisotropic analytic elastic stress field considering the microstructural characteristics of material, and the crack nucleation and propagation principles during micro brittle fracture regime for nano-scratch experiment are elucidated based on the fracture mechanics and the maximum principal stress criterion. The formation mechanism of machined surface morphology in macro brittle removal process is investigated under the crack deflection effect at the weak interface and based on the elastic foundation beam theory. The related findings can provide a reasonable reference framework for optimizing the grinding technology, material and tool design regarding C/SiC composites.

Failure assessment of hip stem due to pelvic positioning and femoral stem loosening

AbstractThe hip joint is subjected to different loads, leading to hip stem failure from the crack nucleation to the final implant failure. The main objective of the present work is to assess hip stem damage due to pelvic positioning and femur movement. The three‐dimensional finite element analysis (3D FEA) was created to simulate the artificial hip joint subjected to static or cyclic loading. The fatigue crack growth rate in the hip stem was investigated based on the extended finite element method (XFEM) with the cycle fatigue approach. The femoral stem loosening during adduction and abduction movements, which may have occurred, was considered. The numerical results showed that the von Mises stress value increases as the loosening distance increases, which appears with the decrease of the cup positioning angle. Furthermore, the pelvic positioning does not affect the crack initiation site, while the loosening and femur movement affect the crack initiation site. On the other hand, the fatigue crack growth rate was affected by pelvic positioning, loosening, and femur movement.

Pit-to-crack mechanisms of 316LN stainless steel reinforcement in alkaline solution influenced by strain induced martensite

The pit-to-crack transition of AISI 316LN stainless steel reinforcement exposed to stress corrosion cracking (SCC) in chlorides contaminated alkaline environment, was studied by a combination of slow strain rate testing (SSRT) and electrochemical impedance spectroscopy (EIS). The phase angle shift (Δφ) obtained by EIS at low frequencies was utilized to determine the pit-to-crack transition, differentiating from crack nucleation and propagation as identified by shifts in the frequency range of phase angle (θ) peaks. The pit-to-crack transition was developed once the maximum θ value shifted from the low to high frequencies. EIS analysis was corroborated by assessment of repassivation rates and pit growth, in addition to calculating {Delta G}^{{rm{gamma }}to {rm{alpha }}{rm{mbox{'}}}}. Crack nucleation at lath martensite developed transgranular SCC. Strain-induced martensitic transformation was associated with the brittle failure of AISI 316LN stainless steel, where α’–martensite phase preferentially incubated the pit, and favored crack nucleation, thus promoting pit-to-crack transition.

In-situ investigation of tensile anisotropy mechanism in an advanced Ti2AlNb-based alloy associated with CRSS ratio and damage model

Combined with in-situ tensile test and electron backscatter diffraction technology, the critical resolved shear stress (CRSS) ratio of O phase and the damage model of grain boundaries were proposed to reveal the tensile anisotropy mechanism of Ti–22Al–25Nb alloy isothermally forged in B2 phase region. In-situ observation suggests that the single slip mode of lamellar O phase is the major deformation behavior, and its preferential slip dominates the yield of the material. Based on the types, number, Schmid factor (SF) and orientation of slip activation, the CRSS ratio of basal <a>, prism <a>, first-order and second-order pyramidal <c+a> slips for O phase is estimated to be 1.19: 1: 1.37: 1.39. Meanwhile, basal and prism <a> slips mainly appear the radial and axial directions (RD and AD) samples with [012] and [100] texture, while the [001], [010] and [212] texture of OD (45° to RD) sample shows an increase in pyramidal <c+a> slips. Thus, the slip activation of O phase depends on the grain orientation and CRSS, resulting in significant strength anisotropy. Furthermore, the microcracks and secondary cracks at grain boundaries are significantly affected by the slip accumulation of lamellar O phase at the O/O and O/B2 interfaces. Considering the stress state of the grain boundary, a damage model about crack nucleation and propagation is proposed to explain the ductility anisotropy. From this, the RD sample has a higher crack nucleation and propagation resistance than the AD and OD samples due to the flattened morphology of B2 grains.

Towards understanding the crack suppression mechanism in brittle materials with high grinding speed at different temperatures

Ductile-regime grinding has been used to eliminate the formation of subsurface cracks by setting an extremely small depth of cut (DOC). The critical DOC is affected by multiple factors, including the grinding speed and material temperature. The underlying mechanism of DOC affected by the grinding speed is still unclear. To reveal the role of grinding speed and material temperature during the formation of cracks, we conducted a series of single-point grinding experiments with the different grinding speeds (26.7–192.3 m/s) and the initial material temperatures (25–200 °C). The experimental results showed that cracks were suppressed with an increase in the grinding speed and initial material temperature even when the DOC was much deeper than the critical DOC determined by the ductile-regime grinding. To understand the mechanisms underlying crack nucleation and suppression, we conducted systematic molecular dynamics simulations. Both simulation and experimental results showed that a crack can be formed by a single slip band. The crack nucleates from a microvoid within the slip band. With the aid of the local tensile stress on one side of the slip band tip, the crack nucleation forms an opening crack. The crack suppression is primarily caused by the high‐pressure field during high‐speed grinding, where the high‐pressure field superposes the local tensile stress to forming a compressive stress state that prevents crack nucleation. In addition, the brittle‐ductile transition is induced by the high temperature on the surface during high‐speed grinding. This study provides insights into building the DOC criterion for different grinding speeds and temperatures based on a ‘bottom-up’ approach.

Quantitative analysis of local plasticity accompanying hydrogen-related fracture in low-carbon martensitic steel

In this study, the local plasticity accompanying hydrogen-related fracture in a low-carbon martensitic steel was quantitatively investigated by correlative microscopy analysis combining digital image correlation technique and electron backscatter diffraction. The results revealed that strain localization occurred mainly in the bulk of martensite blocks and at prior austenite grain boundaries, both with and without the presence of hydrogen. Moreover, hydrogen enhanced strain localization during deformation and facilitated crack nucleation and propagation. The quantitative analysis showed that the hydrogen-related quasi-cleavage cracking was closely related to the local plastic deformation, and increasing hydrogen content decreased the local strain level required for quasi-cleavage cracking. It was found that strain localization also occurred around the intergranular cracks, suggesting that hydrogen-induced intergranular cracking was not a classical simple decohesion process and strain localization was involved in the sequence of hydrogen-induced intergranular cracking. These findings shed light on the understanding of the intrinsic nature of hydrogen embrittlement.

The Role of Nickel in Low Alloy Steels Exposed to H2S Containing Environments. Part II: Effect of the Electrochemical Potential and Stress Level on Trench Formation

In the first part of this paper, trenches were reported for Ni-containing steels tested using the slow strain rate test (SSRT) method in H2S-containing environments at the open-circuit potential (OCP). Trenches are deep elongated pits, and their appearance is more similar to small blunt cracks. These features can develop a sharp sulfide stress crack at their bottom under certain conditions, which are still not fully understood. In this second and final part, the effect of the electrochemical potential and the stress level in trench nucleation and growth was investigated for the same set of Ni-containing steels with up to 5 wt% Ni. The anodic nature of the trench formation mechanism was verified, and under an anodic polarization a critical stress value for trench formation, σtrench, was estimated from SSRTs and finite element modeling. Applying a cathodic potential suppressed trench formation, but not cracking, because cracks nucleated and propagated by hydrogen stress cracking. The resulting environmental and stress-level dependencies for Ni steels confirmed that trenches could be considered a form of environmental-assisted cracking. It is concluded that the main role of trenches is to provide favorable spots for hydrogen stress crack nucleation at OCP, but their presence is neither necessary nor sufficient for cracking occurrence.

Effects of crystallographic orientation and lamellar configuration on fatigue crack propagation in single-colony structures of Ti–6Al–4V alloy: Alternating shear crack growth vs. damage accumulation crack propagation

In this study, the fatigue crack propagation mechanisms in lamellar colonies of Ti–6Al–4V alloy were examined using miniature compact-tension specimens with a single-colony structure at the crack tip. Fatigue tests were performed on colonies with different α-phase crystallographic orientations and lamellar configurations with respect to the notch plane and direction, followed by post-fatigue metallographic analysis. Crack propagation occurs by three main mechanisms: 1) alternating shear due to in-plane prismatic slip, 2) damage accumulation via dislocation–dislocation interaction due to out-of-plane slip, and 3) interlamellar decohesion due to damage accumulation via lamellar interphase boundary–dislocation interaction. The slope of da/dN vs. ΔK is higher for alternating shear crack growth than for damage accumulation crack propagation via dislocation–dislocation interaction. This is due to an incubation period before substantive crack extension in the latter case, which is dominated by degree of strain accommodation associated with the activated slip systems. When the lamellar interphase boundaries are nearly perpendicular to the loading axis, the crack growth rates drastically increase by interlamellar decohesion. This is attributed to the reduced incubation period due to slip incompatibility at the interphase boundary where numerous crack nuclei were formed during damage accumulation process.

Effect of pre-corrosion on crack initiation behavior of friction stir welded aluminum alloy 2024-T3 in high and low cycle fatigue regimes

The effect of microstructural inhomogeneity and pre-corrosion in 3.5% NaCl solution is investigated on the fatigue crack nucleation behavior of welded AA2024-T3. The localized corrosion affects the heat-affected zone more than the weld nugget zone. The formation of corrosion pits often occurs in the region between the adjacent Al-matrix and intermetallic particles due to the reverse polarity behavior of S-phase precipitates (Al2CuMg) in the saltwater environment. Further, the long-term stability of the welded joint is examined by immersing the specimens in 3.5% NaCl solution for different time intervals ranging from 10 hrs to 200 hrs. After 100 hrs of immersion in the salt water, high and low cycle fatigue behavior of the corroded samples has been examined. Long-term exposure to the saltwater environment results in intergranular corrosion due to localized pitting. The micro-mechanism of the pitting process under salt water and its effect on fatigue lives is studied.

TEM analysis of quasi in-situ formed tensile and fatigue cracks in a dual-phase Ti alloy

Designing dual-phase titanium (Ti) alloys with exceptional damage tolerance holds profound scientific and engineering significance. A comprehensive understanding of microstructure evolution around cracks under diverse loading conditions is pivotal in advancing this pursuit. Here, tensile and fatigue cracks were formed quasi in-situ in a dual-phase Ti alloy, and the microstructures around the cracks were systematically analyzed and compared using TEM. Our findings reveal that the deflection of tensile cracks is primarily influenced by the hindrance posed by the straight α/β heterointerfaces and is further facilitated by the β-to-α" martensite transformation within the β matrix. In contrast, fatigue cracks exhibit deflection within the β matrix, located far from the α lamellae. Furthermore, the presence of spindle-shaped short α lamellae holds promise for enhancing resistance to fatigue crack nucleation. These results contribute to fundamental comprehension of the crack nucleation and propagation in dual-phase Ti alloys and provide valuable guidance for materials design.

Microstructural inhomogeneity and associated hetero mechanical behavior in advanced bainitic weathering steel weldments

A new filler metal was developed to join the new generation weathering steel Q690qNH. Microstructural evolutions and the associated mechanical behavior of weldments were comparatively studied in hot rolled and tempered thick Q690qNH plates with initial microstructure of lath bainite and granular bainite respectively. Phase transformations in the HAZ promote the work hardening capacity of the weld jointed tempered Q690qNH (Q690T) plate and enable the joints to exhibit a different mechanical response upon the tensile loading in comparison to the weld jointed hot rolled Q690qNH (Q690R) plate. Prior austenite reconstruction was carried out to understand the crack nucleation behavior in HAZ. The plastic incompatibility between martensitic-austenitic (M-A) and bainitic ferrite leads to the nucleation of plastic micro-voids from the prior austenite grain boundary or M-A boundary. The fracture of Q690R joint is caused by coalescing micro-voids in HAZ with a strength reduction of 10%–14% that caused by the welding process. The continuous yielding behavior regarding the retained austinite in HAZ results in a competition of the plastic deformation in each subzone of the Q690T joint, of which the catastrophic fracture is dominated by the initial microstructure comprising the ferrite and the granular bainite.

A multi-scale formulation for polycrystalline materials accounting for cohesive micro-cracks: Homogenisation of the traction-separation law

This contribution investigates the modelling of failure in polycrystalline materials due to the nucleation and propagation of cohesive cracks at the micro-scale. First, a two-scale formulation that enables the analysis of failure, even when the evolution and propagation of cohesive micro-cracks induce material instability, is proposed. The development of this macro-continuous/micro-discontinuous formulation is based on the Method of Multi-Scale Virtual Power, from which dynamic equilibrium and homogenisation equations are derived, ensuring a variationally consistent scale transition. The respective minimal kinematical constraint is retrieved and enforced with the Lagrange multiplier method. The formulation is also extended for periodic boundary conditions. The discretisation of the resulting equilibrium equations, with the finite element method and the generalised-α method, is described in detail.Second, a fracture-based computational homogenisation procedure is developed to obtain homogenised traction-separation laws and fracture properties from microstructural volume elements. It is derived from a crack-based Hill-Mandel principle and employs a novel energetic-based damage variable, within the Park-Paulino-Roesler cohesive model, proposed to define the crack domain and compute the crack homogenised quantities. A strategy for accurately computing the homogenised unit normal vector of the equivalent macroscopic crack is also suggested.Numerical examples demonstrate the formulation’s ability to analyse fracture problems, including a study on the impact of different microscopic boundary conditions. This work also reveals the convergence of the traction-separation law with increasing microstructure model size when using the fracture-based computational homogenisation. Furthermore, it investigates how advanced deformation mechanisms, like slip plasticity and martensitic transformation, affect intergranular crack propagation mechanisms.

The strength of the Brazilian fracture test

Since its introduction in the 1940s until present times, the so-called Brazilian test has been embraced by practitioners worldwide as a method of choice to indirectly measure the tensile strength of concrete, rocks, and other materials with a large compressive strength relative to their tensile strength. This is because of the ease that the test affords in both the preparation of the specimen (a circular disk) and the application of the loads (two platens compressing the specimen between them). Yet, this practical advantage has to be tempered by the fact that the observations from a Brazilian test — being an indirect experiment in the sense that it involves not uniform uniaxial tension but non-uniform triaxial stress states throughout the specimen — have to be appropriately interpreted to be useful. The main objective of this paper is to carry out a complete quantitative analysis of where and when fracture nucleates and propagates in a Brazilian test and thereby establish how to appropriately interpret its results. We do so by deploying the phase-field fracture theory of Kumar et al. (2020), which has been recently established as a complete theory of fracture capable of accurately describing the nucleation and propagation of cracks in linear elastic brittle materials under arbitrary quasistatic loading conditions. The last section of this paper puts forth a new protocol to deduce the tensile strength of a material from a Brazilian test that improves on the current ISRM and ASTM standards.

Interior cracking behavior of laser powder bed fused TiC/Ti6Al4V composites under vacuum environment

High and very high cycle fatigue tests were carried out to elucidate the interior crack nucleation behavior in a vacuum environment of a TiC-reinforced Ti6Al4V composite fabricated by laser powder bed fusion. To investigate the failure mechanisms, advanced microscopy techniques such as scanning electron microscopy and electron backscatter diffraction are employed. The first failure mode is the cracking of the inhomogeneous hardening zone (IHZ). The cleavage planes observed inside IHZ suggest that the essence of cracking is lamellar α′-grain fracture. The second failure mode is the lack of fusion (LOF) assisted IHZ cracking. The presence of LOF increases stress concentration and reduces the effective bearing capacity of the material, leading to a significant decrease in fatigue life. The knowledge gained can be utilized to optimize the fabrication process and design of composites, ultimately improving their fatigue resistance and extending their service life in applications.

Strength and toughness of tissue adhesives depend on thickness

Adhesives are commonly assessed by two properties: strength and toughness. Here we study how strength and toughness are affected by adhesive thickness. We sandwich gelatin adhesives of various thicknesses between glass substrates. The transparency of the adhesives and substrates enables us to observe crack nucleation and growth. We measure strength by lap shear of samples without precrack, and measure toughness by lap shear of samples with precrack. Our data show a characteristic adhesive thickness, about 0.5 mm. For adhesives below the characteristic thickness, strength is independent of thickness, but toughness increases with thickness. For adhesives above the characteristic thickness, strength decreases as thickness increases, but toughness is a constant. Strength scatters narrowly for samples of a thin adhesive, but broadly for samples of a thick adhesive. By contrast, toughness scatters narrowly for samples of all thicknesses. This work shows the importance of assessing adhesives of various thicknesses.

Investigations of the mechanical properties of dissimilar joint of GTD-111 superalloy and 316 stainless steel obtained by laser welding

The objective of this study is to improve the mechanical properties of dissimilar joint between 316 stainless steel and GTD-111 superalloy obtained by laser welding. Hence, the input variables of pulse frequency, welding speed and laser power were employed to optimize the output variables of weld penetration, hardness and tensile strength through the desirability function technique and response surface methodology. In order to study the changes of microstructure in the welded samples, the optical and scanning electron microscopies were applied. It was observed that an increase in the laser power from 1500 to 2500 W enhanced the penetration depth of weld, while it worsened the hardness and tensile strength owing to nucleation of solidification cracks and formation of Laves phase caused by segregation of Ta, Ti, Nb, W, and Mo elements. Moreover, the faster welding speed up to 1.5 mm/s resulted in a decrease in the penetration depth and an increase in the hardness and tensile strength, which is due to the nucleation of a small dendritic structure and the nucleation of fewer Laves phase owing to lesser heat input. When the variables of pulse frequency and laser power increased simultaneously, the liquation cracks were formed within HAZ of GTD-111 superalloy due to nucleation of γ′-γ eutectic phase, while the cracks and shrinkage voids were observed within HAZ of 316 stainless steel. The results of desirability function technique indicated that the optimum mechanical properties have been obtained at a pulse frequency of 15 Hz, welding speed of 0.8 mm/s and a laser power of 1550 W.

Fatigue behavior of transverse attachments under constant and variable amplitude loading from a Swiss motorway bridge

This paper investigates the fatigue behavior of non-load-carrying transverse welded steel attachments by reporting on an experimental characterization under constant and variable amplitude loading. Fatigue cracks have been monitored using DIC and ACPD techniques and fracture surfaces are analyzed to obtain information about nucleation sites and crack depth. The applied variable amplitude load history has been obtained from traffic measurements from a Swiss road bridge. A procedure has been proposed resulting in a Markov transition matrix representative for the measured strain signal and the random resampling of the stress history to be applied for the variable amplitude fatigue tests.

In-situ study of damage mechanisms in Mg–6Li dual-phase alloy

Interfaces play a crucial role in influencing the mechanical properties of Mg alloys. For Mg–Li dual-phase alloy, the type of interfaces is complex, which includes both grain boundary and phase boundary, and the influence of such interfaces on the damage nucleation is yet to be explored. In this paper, in-situ scanning electron microscopy (SEM) based measurements were carried out to investigate the meso‑scale damage nucleation mechanisms of the Mg–6Li dual-phase alloy. Results show that 94.8% of cracks are nucleated at the α-Mg grain boundary in the post-uniform elongation stage, while 5.2% are at phase boundary and almost no crack at the β-Li grain boundary. The initiation of α-Mg grain boundary cracks is attributed to strain incompatibility, which induces micro-strain localization, and then causes grain boundary sliding (GBS) and crack nucleation. Deformation compatibility analysis reveals that the geometric compatibility factor (Mk) can be used to predict the nucleation of α-Mg grain boundary crack. When Mk is lower than 0.075, α-Mg grain boundary cracks tend to form. Few cracks are generated at the phase boundary is due to the mild strain partitioning between α-Mg phase and β-Li phase and may also be partly attributed to multiple slip systems in body-centered cubic (BCC)-structured β-Li phase, which can accommodate well with the deformation of adjacent α-Mg phase.

A crystal plasticity based methodology for crack initiation lifetime prediction of fretting fatigue in AA2024-T351 alloy

AA2024-T351 alloy finds extensive use as a component material in the fields of civil engineering, machinery and aerospace, which are prone to long-term contact loading during service that may cause catastrophic failure such as fretting fatigue. In this work, a crystal plasticity finite element (CPFE) model has been established to predict the fretting fatigue crack initiation (FFCI) lifetime of AA2024-T351 alloy. Submodel methods for frictional contact parts are adopted, and crystal plasticity theory is utilized to obtain more accurate simulation results. The hotspots for fretting fatigue crack nucleation are determined by the difference of maximum value of maximum principal strain. The accumulated plastic slip and accumulated energy dissipation are utilized as fatigue indicator parameters (FIPs) to accurately predict FFCI lifetime. The results show that both FIPs predicted crack initiation lifetime are within twice the scatter band. Furthermore, the submodel is employed to generate diverse grain orientation sets while maintaining a constant total of 800 grains. The results indicate that different sets have little effect on the prediction of FFCI lifetime, but has a significant impact on the cumulative shear strain distribution between each individual grain at the mesoscale.

An experimental and statistical study on the characteristics of non-metallic inclusions that serve as hydrogen-induced crack nucleation sites in pipeline steel

This study consists of a statistical study to identify spatial distribution parameters of non-metallic inclusions (NMIs) at hydrogen-induced cracking (HIC) nucleation sites in both low-strength and high-strength steel pipes. The electrochemical cathodic charging method was used to induce HIC in pipeline steel plates, and the nucleation of the HIC was monitored using straight beam ultrasonic testing. Optical microscopy (OM) and scanning electron microscopy (SEM) were used to evaluate the shape, size, area fraction, and separation distance of NMIs. The hydrogen microprint technique (HMT), electron backscattered diffraction (EBSD) analysis, and finite element (FE) stress analysis were performed to characterize HIC nucleation sites. The findings showed that cubical and spinal NMIs, characterized by strong hydrogen trapping capacity due to high misfit strain and von Mises stress, are favored sites for HIC nucleation. The main finding of this study is that the shape and sharpness of NMIs are the factors that determine when NMIs will be a HIC nucleation site, rather than size, as generally accepted.