Formation Of Subsurface Cracks Research Articles

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.

Dual composite architectonics: Behavior of ZrB2–SiC–TaB2 composites under abrasive conditions

The influence of abrasive impact on ZrB2–SiC–TaB2 ceramic composites with homogeneous distribution of components in ZrB2 matrix and with «dual composite » structure (ZrB2–SiC)-(TaB2–SiC) has been studied. It was shown that at the same content of components the abrasive wear of the ceramic with the «dual composite » structure is significantly less than that of the composite with homogeneous distribution of single-phase components in a single-phase matrix. On the surface of the studied composites there were no furrows and grooves typical for the abrasive action on the material. At abrasive impact on ceramic composite with homogeneous distribution of components there was a formation of subsurface cracks with the subsequent tearing off of material segments with the size exceeding the size of inclusions in the matrix. Abrasive wear of ceramics with the «dual composite » structure at a high content of composite inclusions occurs by "erosion" of the matrix at the interface with composite inclusions with subsequent removal of inclusions. Composites with «dual composite » structure had higher residual strength compared to ceramic with homogeneous distribution of components. It was found that the difference in the character of abrasive wear of composites is largely in relation to the distribution of residual stresses formed due to the difference in the thermal linear expansion coefficients of the components.

Wear Mechanisms of Cold-Sprayed Stellite-6 During Reciprocated Dry Sliding Under Different Sliding Speeds

Cobalt–chromium alloys are often employed in those environments that require reliable wear and friction properties. Cold Gas Dynamic Spray offers the opportunity to obtain good quality deposits of Stellite-6, that can be successfully used in harsh environments, where good surface performance, in terms of wear resistance, is required. It is also well-known that Stellite-6 is subjected to several physical changes at the interface during dry sliding, that are often related to the loading conditions. As a consequence, wear behavior of this alloy can undergo some variations that linear models are not able to capture, since micro-structural modifications occur during operation. To better understand the wear mechanisms of cold-sprayed Stellite-6 coatings together with the occurring physical phenomena, a systematic experimental study was performed, in fact, to date, no such in-depth tribological studies have been performed. Tests were conducted under combinations of two sliding speeds (0.1 and 0.5 m/s) and four contact pressure in the range of 2-5 MPa. In low-speed tests, abrasive wear is evident, where detachment and pull-out phenomena mainly affect the worn surface of coatings. On the other hand, subsurface cracking was observed in high-speed tests, as well as evidence of plastic deformation on the wear surface. These results suggest that observed wear mechanisms are more likely a consequence of adhesive wear. Unique to this study, the cross-sectional nano-indentation tests showed how the stiffness of the coating, near to wear interface, increases significantly in the case of the lowest value of sliding speed (i.e., v = 0.1 m/s), whereas tends to decrease at high speeds, i.e., v = 0.5 m/s, as a consequence of the formation of subsurface cracks into the coating.

Erratum: “Numerical simulation and validation of subsurface modification and crack formation induced by nanosecond-pulsed laser processing in monocrystalline silicon” [J. Appl. Phys. 127, 085106 (2020)

Erratum: “Numerical simulation and validation of subsurface modification and crack formation induced by nanosecond-pulsed laser processing in monocrystalline silicon” [J. Appl. Phys. 127, 085106 (2020)

Early stages of subsurface crack and WEC formation in 100Cr6 bearing steel under RCF and hydrogen influence

Hydrogen precharged specimens were tested in a disc-on-disc tribometer to study the early stages of crack formation that, with cycling, eventually generate WEC or WSF. Specimens whose contact surface was intact were analysed and at subsurface numerous small transgranular cracks were found, as well as sequences of unlinked voids. Premature WEC with fine cracks and little microstructural alteration volume were also found. This paper aims to clarify certain concepts of debate concerning WEC such as the formation of WEAs, the initiation and propagation of cracks and the influence of defects in steel, the mechanisms by which hydrogen favours the development of cracks and WEAs and the transgranular character of cracks. Finally, a crack formation mechanism is proposed which may lead to a novel hypothesis on the mechanism of WEA formation.

Identification of critical material thickness for eliminating back reflected shockwaves in laser shock peening – A numerical study

Laser Shock Peening (LSP) is one of the emerging surface treatment processes being considered for inducing beneficial compressive surface residual stresses in fatigue critical components. Owing to the ease in handling multiple parameters during optimization of process parameters, simulation based parameterization studies using finite element (FEM) based numerical models are widely gaining importance. Most of the LSP modeling performed so far considered infinite elements in both thickness and lateral directions. However, infinite elements in thickness direction would neglect the deleterious effect of shock wave back reflections for certain sample thicknesses. These back reflections have been reported to result in formation of subsurface cracks in the specimen. Therefore, in this study, using an alternative modeling strategy, effect of thickness on the back reflection of shock waves and its subsequent effect on residual stresses induced are discussed. A 2-D axi-symmetric model with infinite elements in lateral direction and finite elements in thickness direction is developed to simulate a single spot LSP process using ABAQUS/CAE FEM package. It is found that there exists a critical material thickness depending on spot diameter below which the effects of back reflection are predominant.

Вивчення процесів формування підповерхневих тріщин у ромбічній сортовій заготовці при виробництві в умовах металургійного мікро-заводу

Вивчення процесів формування підповерхневих тріщин у ромбічній сортовій заготовці при виробництві в умовах металургійного мікро-заводу

Numerical simulation and validation of subsurface modification and crack formation induced by nanosecond-pulsed laser processing in monocrystalline silicon

We describe a numerical simulation of subsurface modification and crack formation in monocrystalline silicon induced by nanosecond-pulsed laser irradiation. In this model, we assume the residual stress generation due to material transfer caused by volume reduction during melting and resolidification to be the dominant factor in creating subsurface mechanical stress and cracks. In order to quantitatively determine the geometry of the modified region, we numerically model the nonlinear propagation and absorption of the laser beam and the thermal transport. We find that during a single pulse, the lattice temperature distribution results in melting, material transfer, and structural changes on resolidification. The residual stress generated within the monocrystal adjacent to the modified region is subsequently assessed for crack formation in the substrate. The validity of the proposed model is confirmed through agreement with a number of experimental results, including the transmitted power, the timing of the onset of the phase transition during laser irradiation, the processing threshold, the geometry of the modified region, and the formed crack length.

Влияние интерметаллидных включений на образование подповерхностных трещин в сплаве АМг6 при гигацикловой усталости

Влияние интерметаллидных включений на образование подповерхностных трещин в сплаве АМг6 при гигацикловой усталости

Subsurface crack formation and propagation of fretting fatigue in Ni‐based single‐crystal superalloys

AbstractThe fretting fatigue crack formation and propagation behaviors of Ni‐based single‐crystal (NBSX) superalloys are investigated in this paper. Subsurface crack formation process is revealed by in situ fretting fatigue experiment. The crack is observed to form on subsurface area, then propagates to the contact surface. Inclusions in materials are found to have obvious effects on crack propagation, and slip lines are closely related to the crack propagation direction. Crystal plastic finite element method (CPFEM) simulation is used to simulate crack formation position. The accumulative plastic strain peaks at the edge of contact zone and the subsurface area. The results show that the CPFEM simulation and in situ observation achieve good agreements.

Formation of subsurface cracks in silicon wafers by grinding

Single-crystal silicon is an important material in the semiconductor and optical industries. However, being hard and brittle, a silicon wafer is vulnerable to subsurface cracks (SSCs) during grinding, which is detrimental to the performance and lifetime of a wafer product. Therefore, studying the formation of SSCs is important for optimizing SSC-removal processes and thus improving surface integrity. In this study, a statistical method is used to study the formation of SSCs induced during grinding of silicon wafers. The statistical results show that grinding-induced SSCs are not stochastic but anisotropic in their distributions. Generally, when grinding with coarse abrasive grains, SSCs form along the cleavage planes, primarily the {111} planes. However, when grinding with finer abrasive grains, SSCs tend to form along planes with a fracture-surface energy higher than that of the cleavage planes. These findings provide a guidance for the accurate detection of SSCs in ground silicon wafers.

Investigation of crack initiation mechanisms responsible for the fish eye formation in the Very High Cycle Fatigue regime

The present paper aims to explain the subsurface crack formation occurring in the Very High Cycle Fatigue domain for bcc steels. For this purpose, two high strength steels (a low alloyed steel AISI 5120 and a martensitic stainless steel X40CrMo13) were studied under ultrasonic push-pull fatigue tests. For all tests, a fish eye developed before fracture and, in most cases, the subsurface crack initiation originated from inclusions. Also, during fatigue tests for both materials, the thermal field on specimen surface was recorded by infrared camera and fracture surface observations were performed by Scanning Electron Microscopy (SEM). The SEM pictures reveal the presence of a Fine Granular Area (FGA) surrounding the inclusion in the crack initiation stage. Moreover, earlier in the crack process, slips markings were found in the matrix inside the inclusion dimple indicating that some similarities exist between surface and subsurface crack initiation mechanisms.

Thermo-elasto-visco-plastic finite element analysis on formation and propagation of of-corner subsurface cracks in bloom continuous casting

The formation and propagation of the popular off-corner subsurface cracks in bloom continuous casting were investigated through thermo-mechanical analysis using three coupled thermo-mechanical models. A two-dimensional thermo-elasto-visco-plastic finite element model was developed to predict the mould gap evolution, temperature profiles and deformation behavior of the solidified shell in the mould region. Then, a three-dimensional model was adopted to calculate the shell growth, temperature history and the development of stresses and strains of the shell in the following secondary cooling zones. Finally, another three-dimensional model was used to analyze the stress distributions in the straightening region. The results showed that the off-corner cracks in the shell originated from the mould owing to the tensile strain developed in the crack sensitive regions of the solidification front, and they could be driven deeper by the possible severe surface temperature rebound and the extensive tensile stress in the secondary cooling zone, especially upon the straightening operation of the bloom casting. It is revealed that more homogenous shell temperature and thickness can be obtained through optimization of mould corner radius, casting speed and secondary cooling scheme, which help to decrease stress and strain concentration and therefore prevent the initiation of the cracks.

Synchrotron X-ray measurement of residual strain within the nose of a worn manganese steel railway crossing

Switches and crossings are an integral part of any railway network. Plastic deformation associated with wear and rolling contact fatigue due to repeated passage of trains cause severe damage leading to the formation of surface and sub-surface cracks which ultimately may result in rail failure. Knowledge of the internal stress distribution adds to the understanding of crack propagation and may thus help to prevent catastrophic rail failures. In this work, the residual strains inside the bulk of a damaged nose of a manganese railway crossing that was in service for five years has been investigated by using differential aperture synchrotron X-ray diffraction. The main purpose of this paper is to describe how this method allows non-destructive measurement of residual strains in selected local volumes in the bulk of the rail. Measurements were conducted on the transverse surface at a position about 6.5 mm from the rail running surface of a crossing nose. The results revealed the presence of significant compressive residual strains along the running direction of the rail.

Sub-surface crack formation in ultrasonic vibration-assisted grinding of BK7 optical glass

Due to the inherent properties of high brittleness and low fracture toughness of optical glass materials, sub-surface cracks would be inevitably induced into the ultrasonic vibration assisted grinding process. Knowledge of the formation mechanisms of sub-surface cracks plays a key role in implementing high efficiency and precision machining of this kind of materials. In this work, the ultrasonic vibration assisted grinding experiments of BK7 optical glass were carried out. Processed by cross-sectional polishing and assisted by HF acid etching, quite a few sub-surface cracks with different shapes were observed beneath the machined surfaces. Categorized by the shapes and formation mechanisms, four kinds of sub-surface cracks (i.e., straight median sub-surface crack, arc median sub-surface crack, lateral sub-surface crack, and bifurcated sub-surface crack) and their corresponding formation mechanisms were both clarified. Experimental and analytical results suggest that the arc sub-surface cracks were formed by the relative shear stresses parallel to the plane of median crack frontiers, which are generated by nonsymmetric contacting between abrasive grains and glass specimen in ultrasonic vibration grinding. The bifurcation cracks were formed by the distortion in frontier fields caused by the impact effect provided by ultrasonic vibration as the abrasive grains vibrate down to the glass material. Effects of grinding and ultrasonic vibration parameters on the maximum depth of sub-surface cracks were also investigated in this work. The value of the maximum depth of sub-surface cracks showed a great dependence on the processing parameters.

EHL modeling of nonhomogeneous materials: The effects of polycrystalline anisotropy on RCF

Bearing steel is comprised of a polycrystalline aggregate of anisotropic crystals which affects the stress distribution and subsurface fatigue crack formation. In the present investigation an approach is proposed to determine the contact pressure, film thickness and subsurface stresses in a polycrystalline anisotropic aggregate material operating under elastohydrodynamic lubrication (EHL). The approach is based on a fully coupled finite element EHL model which uses 1D finite elements to solve the Reynolds equation and 2D finite elements to resolve the deformations for a line contact EHL problem. The polycrystalline material is simulated using a Voronoi polygon discretization with each Voronoi polygon receiving a unique crystallographic orientation. The anisotropic material definition used for this model has a cubic crystal structure. Results from the FE Voronoi EHL model show that the contact pressures vary due to crystallographic orientation and stress concentrations occur at the polycrystalline grain boundaries. EHL film thickness profiles were not significantly affected by the addition of crystal anisotropy effects to the microstructural model. However, relative life prediction obtained from the model with the anisotropic stress profiles showed that significant life scatter is generated by the addition of crystal anisotropy into the microstructural model. The results obtained from the FE Voronoi EHL model show much closer agreement to experimental results than models that assume isotropic material models.

Improved wear resistance of nanotube- and other carbon-filled PTFE composites

Microfillers have long been shown to reduce the wear rate of PTFE by up to two orders of magnitude. One hypothesized mechanism resulting in this wear reduction is the interruption of subsurface crack formation, which prevents the formation of delamination debris. Recent studies into nanofillers have shown some promising results, but only for very few types of fillers. Notably, alpha-phase alumina and activated nanocarbon have been shown to impart exceptional wear resistance to PTFE composites, outpacing that provided by the same materials on the microscale. As described here, carboxyl-functionalized carbon nanotubes and mesoporous nanocarbon have both been additionally identified as effective nanoscale fillers, with composites approaching wear rates as low as 10−7mm3/Nm. It is believed that the functionalized carbon nanotube fillers presented here may combine microscale mechanisms, such as subsurface arrest of crack propagation, with a uniquely nanoscale mechanism as held by the mesoporous nanocarbon, and that the effectiveness of such mechanisms increases with increased filler content.

Non-destructive Analysis of Material Detachments from Polychromatically Glazed Terracotta Artwork by THz Time-of-Flight Spectroscopy

The damage caused by the environment to exposed glazed terracotta objects is usually not externally visible. For instance, the detachment of the glaze owing to subsurface crack formation or whole cavities in the area of the terracotta cannot be located visually. In this article, we demonstrate that terahertz time-of-flight spectroscopy is suitable to locate and measure the air gaps under the glaze detachments which could only be done by X-ray axial tomography before. This tool will be very useful to guide the restoration process, particularly for pieces that are still attached to buildings or other structures that cannot be transported to a tomography facility.

The role of surface chemistry and fatigue on tribocorrosion of austenitic stainless steel

Tribocorrosion is combined with fatigue (multi-degradation) in many engineering applications. In the present study, the effect of surface chemistry and electrochemical conditions on multi-degradation of austenitic stainless steel has been investigated. Mechanical and chemical wear loss has been calculated by modeling and measuring anodic current at open circuit potential and passive potential respectively. Surface and subsurface structure and chemistry has been investigated by SEM/FIB and XPS. Observations showed that cyclic and static 4-point bending affect the composition of the passive film and concentration of Mo and Cr cations in the oxide layer as well as formation of subsurface voids and cracks at different electrochemical conditions. Formation Cr(OH)3 is found to have significant effect on multi-degradation.

Fatigue strength of self‐piercing riveted joints in lap‐shear specimens of aluminium and steel sheets

AbstractThe self‐piercing riveting (SPR) process is gaining popularity because of its many advantages. This study investigated the fatigue strength of SPR joints in tensile‐shear specimens with dissimilar Al‐5052 and steel sheets. A structural analysis of the specimen was conducted. For this specimen, the upper steel sheet withstood applied load in a monotonic test and played a major role in the low‐cycle region. In the high‐cycle region, however, the harder surface of the upper steel sheet reduced the fatigue strength by enhancing fretting crack initiation on the opposite softer aluminium surface. Therefore, the fatigue endurance of the specimen was reduced. The fatigue endurance of a SPR joint with the combination of steel and aluminium sheets was found to be governed by the strength of the lower sheet, which is more vulnerable to the applied loading. Thus, it is desirable to use a stronger metal sheet as the lower sheet with regard to the fatigue performance. Scanning acoustic microscopy was effectively used to reveal and prove the formation and growth of subsurface cracks in SPR joints. The structural stress can predict the fatigue lifetimes of the SPR joint specimens within a factor of three.