Subsurface Crack Propagation Research Articles

Discrete element method to study the subsurface crack propagation trend during vertical ultrasonic vibratory cutting of sapphire

Discrete element method to study the subsurface crack propagation trend during vertical ultrasonic vibratory cutting of sapphire

Scratching properties of 4H–SiC single crystal after oxidation under different conditions

Single-crystal silicon carbide (SiC) has poor machinability because of its low fracture toughness, and surface modification has become the first choice for SiC polishing to obtain a high removal rate and a smooth surface. However, the removal mechanism of SiC after modification remains unclear. In this study, diamond scratching experiments were performed on 4H–SiC after oxidation under two conditions. To reveal the material removal mechanism, the scratch morphology and subsurface defects were analysed using Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy. The results showed that oxidation improved the scratch depth and critical depth for ductile removal, resulting in a smooth scratch surface with low damage. However, weak oxidation increased the median crack length and exacerbated the subsurface damage. The SiOxCy, C = O, C–O–C, and Si–O–Si functional groups produced during oxidation reduced the hardness and improved the machinability of the 4H–SiC substrate. On the other hand, strong oxidation reduced the generation of surface cracks, tearing, and spalling, and weakened the propagation of subsurface median cracks. These results prove that ultrasonic-assisted photocatalytic oxidation provides high removal efficiency and defect control, providing a new approach for the synergistic polishing of SiC substrates.

Molecular dynamics simulation and experimental study of the material machinability characteristics and crack propagation mechanisms for fused silica double nanoscratches

Fused silica is a high-purity, hard and brittle, difficult-to-machine amorphous material that is widely used in the aerospace, optical instruments, semiconductors, microelectronics and other important fields. However, the influence of adjacent scratches on material deformation and subsurface damage during ultraprecision machining by abrasive particles is not well understood. This study aims to investigate the influence of micron-spaced double scratches on the deformation and damage for fused silica through molecular dynamics simulations and double nanoscratch experiments. An amorphous model of fused silica is created using a two-step simulated annealing method with three-directional periodic boundary conditions. Variable loaded double scratches with different nanometer distances are simulated on the surface of the model, and variable loaded double scratch experiments with varying micron-sized distances between scratches are conducted for fused silica wafers. The simulation and experimental results reveal that the deformation of fused silica during nanoscale double scratching involves both elastic and plastic deformation, and the surface and subsurface damage primarily consists of radial, lateral, Hertzian, and median cracks. The distance between the double scratches affects the load variation, surface and subsurface crack propagation, extent of material damage and removal rate, and surface roughness. Examination of the material subsurfaces using the FIB-TEM sample preparation and testing technique shows that the distance between the double scratches also influences the subsurface damage to the material.

Anisotropic mechanism of material removal and ductile-brittle transition in sapphire scratching based on acoustic emission signal

Sapphire exhibits typical anisotropy due to its unique crystal structure, and there are significant property differences in different processing areas during the machining of curved sapphire component, making it essential to investigate the anisotropy in the removal process of sapphire materials for efficient curved surface machining. In this study, the force and acoustic emission (AE) signals were monitored at different scratch stages and orientations, and the anisotropic characteristics of ductile-brittle transition (DBT) depths, material removal morphology, and subsurface crack propagation direction were analyzed. The results indicate that the anisotropy of force signals and DBT depths is mainly reflected in the differences between crystal planes. In addition, the removal morphology of materials with different crystal orientations showed smooth scratch grooves with unclear anisotropic characteristics during the ductile removal (DR) stage, while the crack propagation direction with different scratch orientations during the DBT stage showed significant anisotropic characteristics. The brittle removal (BR) stage is characterized by brittle spalling in the form of shell-like, blocky, and fishbone spalling, and there is a significant anisotropic difference in the spalling shapes on both sides of the scratch. The original AE signals, Fast Fourier Transform (FFT), and Short Time Fourier Transform (STFT) results show that the AE frequencies of different crystal planes are different, and the differences in different scratch orientations on the same crystal plane are reflected in the peak amplitude. Finally, the propagation directions of subsurface cracks with different directions are anisotropic due to different scratching crystal planes, and propagate mainly along directions of 15°, 29°, 32°, 58°, and 105°.

Study of the inhibition mechanism of sapphire subsurface damage by tangential ultrasonic vibration

Ultrasonic vibration has a certain effect on the crack propagation tendency during sapphire grinding, but the mechanism of the effect on the crack propagation tendency has not yet been elucidated. In this study, tangential ultrasonic vibratory scratching and normal scratching sapphire processes were simulated using the discrete element method, and the expansion tendency of sub-surface cracks in the scratching sapphire process was analyzed in terms of cutting forces and stresses. The results of the study show that the crack depth decreases with increasing scratch speed and increases with increasing scratch depth; The greater the variation in cutting force, the greater the number of cracks formed and the crack propagation is mainly distributed near the maximum residual tensile stress; The introduction of tangential ultrasonic vibration induces a decrease in residual tensile stresses and an increase in residual compressive stresses, thus inhibiting subsurface crack propagation; The suppression of subsurface cracks becomes more pronounced as the amplitude increases. Finally, tangential ultrasonic vibratory scratching and normal scratching sapphire experiments were comparatively studied and the experimental results were in agreement with the simulation results. This study provides some theoretical guidance for the suppression of subsurface cracks in sapphire materials during the grinding process.

Interaction of erosion and corrosion on high-strength steels used for marine dredging engineering

In this work, the erosion–corrosion performances of three high-strength steel samples: high manganese austenitic steel (HMnS), dual-phase steel (D-PS), and martensitic steel (MS), which are potentially used to construct marine dredged pipelines were investigated in comparison to the Q235 carbon steel. The corrosion-enhanced erosion (C-E) and erosion-enhanced corrosion (E-C) were electrochemically studied in conjunction with surface and subsurface characterizations. Results showed that the fracture of the steel extrusions induced by micro-cutting was the main erosion-corrosion feature under normal sand impacts. The C-E played a significant role in the erosion–corrosion process, which resulted in the dissolution of the work-hardening layer and facilitated the subsurface crack propagation, particularly for D-PS and MS. Although the general E-C rate was low, the concentration of the major anodes at the highly eroded area significantly enhanced the erosion process. Evaluating erosion resistance solely based on hardness is significantly limited. The twinning deformation of the austenite grain in HMnS effectively hindered the crack propagation during the erosion–corrosion. The addition of Al and Cr elements improved the erosion–corrosion resistance of high-strength steels.

Reciprocating dry sliding friction and wear behavior of B319 aluminum alloy-graphene composites

In the current study, the B319 aluminum (Al) alloy-graphene composites were fabricated using a stir-casting process, and the composites' tribological behavior was examined under dry sliding conditions. The B319–0.3 wt% graphene exhibited ∼42% and ∼52% higher friction and wear resistance than the baseline B319 alloy. Both friction and wear resistance of the B319 alloy deteriorated at higher graphene contents (0.5 and 0.7 wt%), indicating the best tribological performance was achieved with 0.3 wt% graphene addition. The wear in the base alloy primarily occurred by mechanically mixed layer (MML) delamination via subsurface crack propagation, while the wear in the B319 - 0.3 wt% graphene composite was driven by MML fragmentation, accompanied by adhesive, oxidative, and abrasive wear mechanisms.

Microstructure and tribology of cold spray additively manufactured multimodal Ni-WC metal matrix composites

Cold spray produces thick and dense deposits that can be applied in additive manufacturing or as a repair to recover damaged components. One potential materials system is WC-reinforced Ni composites, which are known for a combination of high hardness and toughness. WC particle size and distribution are some of the most important variables that impact the mechanical properties and wear resistance. In this study, thick multimodal Ni-24 vol%WC composite coatings were additively manufactured using the cold spray technique. Spherical Ni powder and two types of WC particles, i.e., cast and agglomerated, were used as feedstock materials. The microstructure, porosity, WC distribution, and microhardness of coatings were investigated in comparison with cold-sprayed Ni and Ni-agglomerated WC coatings with similar WC fill ratios. Dry sliding wear of multimodal Ni-WC coatings was studied with a sliding speed of 3 mm/s under normal loads of 5 and 12 N and was then compared to Ni and composite coatings made with only agglomerated WC. Multimodal Ni-WC coatings with an altered distribution of WC particles offered an improvement in friction and wear as compared to Ni-agglomerated WC coatings. Characterization of worn surfaces revealed the formation of tribofilms following compaction of wear debris. A more stable tribofilm that developed to a higher coverage was identified for coatings with multimodal WC particles at both tested loads. However, the protective role of tribofilms was reduced at a higher load, caused by subsurface crack propagation. Subsurface microstructure of worn surfaces that are induced by sliding wear was studied, and a correlation between the mechanical properties and microstructure and wear mechanism of coatings was made. Overall, our findings highlight the potential of cold spray additive manufacturing in producing multimodal Ni-WC composite coatings with enhanced wear resistance.

Experimental analysis and modeling of subsurface cracks with random propagation for ceramic material on rolling contact fatigue

During the degradation of critical rotating components using ceramic materials, due to the uncertainty of the failure generation mechanism, the high nonlinearity of failure evolution and the diversity of failure modes, the unpredictability of sudden failure triggered by random propagation of subsurface cracks poses a great challenge to the operation and maintenance. Based on the finite element model reconstructed from fatigue tests, an early failure evolution model for ceramic bearings considering random propagation of subsurface cracks is constructed in this paper. Throughout the work, the position coordinates of critical nodes characterizing the degree of crack propagation (α, β, γ) are introduced to visualize the influence of propagation on the finite element of crack front during the whole rolling contact process, whose errors (ɛ) are stabilized at 10%. Then Monte Carlo Simulation is used to generate the randomness events of crack propagation, which satisfies the uncertainty characteristics of early failure initiation and propagation under actual working conditions. The results show that the first deflection shows strong randomness. Afterwards, the main cracks move away from the surface in mixed fracture mode, and finally form abrupt spall after several deflections. The proposed model not only reveals the sudden failure mechanism of ceramic bearing under uncertain crack propagation, but also effectively evaluates performance degradation of critical rotating components using ceramic materials.

Experimental and theoretical analysis of rock failure characteristics by a single PDC cutter under downhole pressurized conditions

The development of high-efficiency polycrystalline diamond compact (PDC) bits plays a major role in developing unconventional oil and gas. Thereby, a series of single cutter tests have been conducted in the past few years. However, most of them were performed under atmosphere conditions due to the limitation of the experimental setup. In the present study, a series of single cutter experiments were performed under pressurized conditions, in which both principal stress and hydrostatic pressures were loaded with a self-developed facility. The cutting force, topography of cutting grooves, and mechanical specific energy (MSE) were analyzed to evaluate rock failure efficiency. Furthermore, a theoretical model was developed to study stress evolution. Combined with experimental results, the rock failure mechanism and the effects of bottom hole pressures on rock breaking characteristics were revealed. The results indicate that the increase in principal stress and hydrostatic pressures cause larger cutting forces and reduced rock cutting efficiency. Additionally, a larger hydrostatic pressure will promote the propagation of subsurface cracks, leading to larger roughness of cutting grooves and facilitating the subsequent rock cutting process. In this study, the hydrostatic pressure has the greatest impact on rock failure process, followed by the principal stress when parallel to the cutting direction. Comparing the experimental and simulation results, we can find that only a fraction of cutting force is directly utilized in breaking rocks, while the remaining force is applied to overcome the friction induced by the flow of cuttings along the cutter surface. Consequently, measures should be taken to prevent bit balling and decrease the friction force, thus weakening the effects of hydrostatic pressures and improving the efficiency of PDC bits.

Subsurface deformation and crack propagation between 3C-SiC/6H-SiC interface by applying in-situ laser-assisted diamond cutting RB-SiC

Reaction-bonded silicon carbide (RB-SiC) is considered as an excellent optical material in aerospace engineering. However, the multiple complex interfaces and high hardness make it difficult to be precisely machined. In this research, the in-situ laser-assisted diamond cutting was applied to precisely machining RB-SiC. Firstly, Raman spectroscopy was adopted to identify 3C-SiC and 6H-SiC components in RB-SiC. As following, the in-situ laser-assisted diamond cutting was experimentally carried out to analyze the subsurface deformation and fracture in detail. It was clarified that the cross slips and high-density stacking faults significantly enhance the plastic deformation of RB-SiC. Moreover, the dislocations rapidly plugged at the intersection of 3C-SiC/6H-SiC interfaces resulting in the lateral crack which propagates along the [111]//[0001] direction.

Effect of Welding Polarity on Mechanical Properties of Submerged Arc Welded Railway Vehicle Wheels

When a railway vehicle moves on a curved rail, sliding contact between the rail head side and wheel flange causes wear on the wheel flange. Traditionally, a wheel with thinned flange is machined to get a minimum flange thickness specified for structural safety. This operation reduces the rim thickness and shortens the life of the wheel. In the present study, the thinned flanges were hard-faced by submerged arc welding. A welding wire, which has good weldability to the base material of the wheel and does not generate thermal cracking, was developed. The effects of welding polarity on the microstructure, hardness, friction coefficient, and wear characteristics of the welded wheel were studied. The hardness of the wheel welded with reverse polarity was similar to that of welded with straight polarity. The wear rates of the wheel disc welded with reverse polarity and its counterpart rail disc were 11% and 27% lower than those welded with straight polarity. Delamination wear due to subsurface crack propagation and oxidation wear were mixed. The hardness of the rail before the wear test was in the range of 250–300 HV. After the wear test, it soared to 500 HV.

Influence of welding residual stress on subsurface fatigue crack propagation of rail

To accurately evaluate the influence of welding residual stress on the fatigue crack propagation behavior of rail, the distribution of residual stress of welded rail was predicted, the subsurface fatigue crack propagation type, path, rate and lifetime of the rail and the welded rail with residual stress were calculated and compared. The results showed that the subsurface crack propagated with II + III mixed-mode and the slip propagation of mode II was dominant for both the cases of residual stress existing and not, while the crack propagation path, rate and lifetime were quite different for the two cases. For horizontal circular cracks, when there was no welding residual stress, the crack propagation path was mainly transverse under wheel-rail contact load, the propagation rate was relatively small, and the propagation lifetime was longer. When there was welding residual stress, the crack propagated longitudinally, the crack propagation rate increased significantly, and the fatigue lifetime was shorter.

Relationship between subsurface stress and wear particle size in sliding contacts during running-in

The results of an experiment set using a pin-on-disk rig on samplers made of ST37 disk and bearing steel pin are reported to characterize the wear rate of the specimen, weight loss, friction coefficient behavior, and wear particle size during running-in. Using the equations of subsurface stress for spherical contact, the location and value of maximum subsurface stress in the running-in period in three dimensions is predicted. The results indicate that there exists a relation between the size of the detached particles and the location at which the maximum subsurface shear stress occurs. Based on wear particles’ SEM images, there is a direct relationship between the size of the wear particles with the applied load, applied speed, and the initial surface roughness. It is shown that increasing the load and speed results in a rise in the friction coefficient, which affects the magnitude and location of the maximum subsurface shear stress. The latter is responsible for the nucleation and propagation of subsurface cracks.

Antagonistic effect of electrochemical corrosion on the mechanical wear of Monel 400 alloy in seawater

The tribocorrosion behavior as well as the corrosion-wear interaction of Monel 400 alloy in seawater were investigated under varied loads. An abnormal antagonistic effect of corrosion on wear was found, which was highly related to the peculiar corrosion products in the shape of nanospheres only generated under the tribocorrosion condition due to the selective corrosion of Ni-rich components. A boundary lubrication layer, consisting of the nanospheres squeezed and spread by friction stress, leaded to the reduction of friction coefficient. Moreover, the material loss rate was greatly alleviated because the friction-induced surface delamination and sub-surface crack propagation were prevented.

Quality and flexural strength of laser-cut glass: classical top-down ablation versus water-assisted and bottom-up machining.

The growing applicability of glass materials drives the development of novel processing methods, which usually lack comprehensive comparison to conventional or state-of-art ones. That is especially delicate for assessing the flexural strength of glass, which is highly dependent on many factors. This paper compares the traditional top-down laser ablation methods in the air to those assisted with a flowing water film using picosecond pulses. Furthermore, the bottom-up cutting method using picosecond and nanosecond pulses is investigated as well. The cutting quality, sidewall roughness, subsurface damage and the four-point bending strength of 1 mm-thick soda-lime glass are evaluated. The flexural strength of top-down cut samples is highly reduced due to heat accumulation-induced cracks, strictly orientated along the sidewall. The subsurface crack propagation can be reduced using water-assisted processing, leading to the highest flexural strength among investigated techniques. Although bottom-up cut samples have lower flexural strength than water-assisted, bottom-up technology allows us to achieve higher cutting speed, taper-less sidewalls, and better quality on the rear side surface and is preferable for thick glass processing.

Molecular dynamics analysis on the effect of grain size on the subsurface crack growth of friction nanocrystalline 6H-SiC

The purpose of this paper is to investigate whether the grain size of ceramic materials follows the classical Hall–Petch effect, and the effect of grain boundaries and grains on the subsurface crack propagation of nano-polycrystalline 6H-sic by molecular dynamics.

Crystallographic evaluation of low cycle fatigue crack growth in a polycrystalline Ni based superalloy

We present the microscale fracture mechanics aspects of LCF cracks non-propagating till failure in the cylindrical specimens of Ni based superalloy Haynes 282 and attempt to unravel the underlying crystallographic factors. Two key parameters, Crack Tip Opening Angle (CTOA) and maximum tangential stress (θMTS) have been opted for characterization. CTOA variations along with a propagating crack, exhibit a non-linear decay followed by a stabilized regime and the fraction of the stabilized regime increases in lower strain amplitude where the fatigue life is more. Mixity of local KI and KII fields is directly proportional to θMTS and that has been assessed by measuring local deflections. The modal mixity has been found to be microstructure sensitive and an entirely statistical parameter. Different growth modes of these non-propagating cracks have been identified from the microstructural observations. The role of elasto-plastic incompatibility of neighboring grains has been addressed by conducting crystallographic analyses. There is a critical bound for Elastic Modulus (EM) and Schmid factor (SF) for the grains favoring subsurface crack propagation below which grains (mostly with high elastic compliance and low SF) are not participating in crack propagation. The favourable twin-matrix incompatibility of the microstructure has also been identified about the fatigue crack growth and twins in (211) plane is abundant in the cracked region. From the results of a detailed slip transfer analysis based on the Luster-Morris parameter (LMP), it is observed that there exists microstructurally controlled lower bounds that inhibits slip transfer (LMP value <0.1). Dislocation debris analysis using TEM has also been carried out to understand micromechanisms clearly. Both non coplanar and planar mode of dislocation activity has been observed and twin boundaries has been see to act as barrier to the planar dislocation motion.

Comparison of erosion rate of EB-PVD and plasma sprayed TBC

ABSTRACT A thermal barrier coating (TBC) consisting of platinum-aluminide bond coat and 7wt-% yttria stabilized zirconia (7YSZ) ceramic top coat deposited by electron beam physical vapour deposition (EB-PVD) system cast Ni-base superalloy CM-247LC. The erosion behaviour of the EB-PVD and air plasma sprayed (APS) TBCs are compared. The APS TBC consists of 7YSZ top coat and CoNiCrAlY bond coat on Inconel 738 substrate. Erosion responses of TBCs have been examined as a function of temperature. The erosion rate of EB-PVD TBC was significantly lower than that of APS TBC. Erosion rate of EB-PVD TBC decreased with test temperature. APS TBC exhibits mostly temperature independent erosion rate. The material removal mechanism of EB-PVD TBC was by the formation of parallel cracks in the ceramic layer close to its surface and their subsequent joining. The material is removed by propagation of subsurface cracks along the splat boundaries in case of APS TBC.

A New Dual-Mode Wear Model of Abrasion–Fatigue for Plane (100Cr6)-Cylinder (C45E4) Steel Pairs With Fractal Surfaces

Understanding dual-mode wear behavior, including fatigue and abrasive wear, is essential for determination of surface damage under the sliding-impact condition, which can induce early motion-pair failure. A new dual-mode wear model is presented in this article for plane–cylinder contact pairs with fractal surfaces. First, fractal theory is employed for elastic–plastic contact analysis of the mating surfaces. Then the negative inhibitory effect of the abrasive surface damage on subsurface fatigue crack propagation is considered to explore abrasion–fatigue interactions at the asperity level. Subsequently, dual-mode wear behaviors are derived for the asperity and fractal surface based on four wear-combination scenarios of the abrasive and fatigue mechanisms. Furthermore, the validity of the proposed prediction model is evaluated by comparisons between the expected results and laboratory tests with 100Cr6 plane and C45E4 cylinder steel pairs. The results show that the new model can reasonably explain the dual mode wear behavior and predict the wear rate of friction pairs. These insights can potentially help in guiding the tribological design of these dual-mode wear scenarios.