Surface Crack Propagation Research Articles

Silver-loaded mesoporous silica nanoparticles enhanced the mechanical and antimicrobial properties of 3D printed denture base resin

The aim of this study is to develop a novel 3D printed denture base resin material modified with mesoporous silica nanocarrier loaded with silver (Ag/MSN) to enhance mechanical and antimicrobial properties. Acrylate resin-based was incorporated with various proportion of Ag/MSN (0.0–2.0 wt%). Specimens with different geometry were printed and characterized accordingly for the effect of modification on properties such as: mechanical and physical properties, chemical composition and degree of conversion, as well as biological response in term of biocompatibility and antimicrobial against oral fibroblast and candida biofilm (C. albicans), respectively. The consecutive addition of Ag/MSN improved significantly surface hardness and crack propagation resistance, while flexural strength remained similar to control; however, a negligible decrease was observed with higher concentrations ≥1 wt%. No significant difference was noticed with water sorption, while water solubility had a remarkable trend of reduction associated with filler content. The surface roughness significantly increased when concentration of Ag/MSN was ≥1.0 wt%. A significant reduction in C. albicans biofilm mass, as the inhibition proficiency was correlated with the proportion of the filler. With respect to the amount of Ag/MSN, the modification was compatible toward fibroblast cells. The sequential addition of Ag/MSN enhanced significantly the mechanical and antimicrobial properties of the 3D printed resin-based material without affecting adversely compatibility. The acrylic resin denture base material has susceptibility of microbial adhesion which limits its application. Silver loaded MSN showed a significant performance to enhance antimicrobial activity against C. albicans which is the main cause of denture stomatitis. The proposed invention is a promise technique for clinical application to provide an advanced prosthesis fabrication and serve as long-term drug delivery.

Finite element simulation of thermal stress in RF magnetron sputtered SiC coating as tritium penetration barrier

The magnitude of thermal stress in the tritium penetration barrier coating is crucial during RF (radio frequency) magnetron sputtering preparation. The thermal stress would cause the delamination or crack on the coating, which seriously affects the structural stability of the coating. In this work, a finite element model represented the SiC tritium penetration barrier coating system was established. The relevant parameters which affected the magnitude of thermal stress of the system were discussed in detail. The shear and radial stresses in the system were also evaluated in order to identify the locations where the materials were prone to failure. Then, the propagation of multiple surface cracks in the system was investigated. Finally, the thermal stress was optimized by an Al2O3 interlayer. The outcomes indicated that the addition of the Al2O3 interlayer alleviated the stress at the interface and enhanced the mechanical stability and adhesive strength of the coating.

Mechanical behavior in liquid lead of Al2O3 coated 15-15Ti steel and an Alumina-Forming Austenitic steel designed to mitigate their corrosion

The mechanical behavior in liquid lead of the 15-15Ti steel coated by an alumina layer and an Alumina Forming Austenitic (AFA) steel, with a chemical composition allowing for the formation of alumina at the surface of the steel, has been studied. To investigate the liquid metal embrittlement (LME) sensitivity by liquid lead, tensile tests in air and in liquid lead have been carried out at 400 °C and 500 °C. Then analyses of the cracking and of the fracture surfaces have been performed. No LME sensitivity at the tested conditions was observed for the Al2O3 coated 15-15Ti steel. The AFA steel is not sensitive to LME at 400 °C but suffers from LME at 500 °C. For the AFA steel, the liquid lead promotes at 500 °C an intergranular propagation of surface cracks which suggests a grain boundary wetting-dominated LME mechanism of this steel at 500 °C.

Tensile fracture behaviors of a laser powder deposited Fe–30Mn–10Cr–10Co–3Ni high-entropy alloy: In situ x-ray computed microtomography study

The defect evolution and fracture mechanism of a laser powder deposited Fe–30Mn–10Cr–10Co–3Ni alloy in the course of uniaxial tensile tests was investigated by in-situ high-resolution computed microtomography (CT) and electron back-scattered diffraction (EBSD). The influences of surface wrinkling and the long and straight grain boundary on the defect evolution and fracture mechanism were discussed emphatically. The results showed that pores were not the source of crack initiation. During plastic deformation, the pore volume did not change significantly, but the pore shape varied. The development of voids was observed only in the zones with strain localization. However, the plastic damage voids were not the source of fracture cracks, neither growing nor aggregating into macroscopic cracks. The cracks in the specimens were initiated at the long and straight grain boundary with an angle of about 45° relative to the applied load and the Taylor factors on both sides of the grain boundary differed greatly. Severe plastic damage occurred at the surface and near-surface of the tensile specimens as a result of surface wrinkling. The void volume fraction of a superficial zone was several times higher than that in the bulk of the specimens, and there were many cracks with an angle of about 45°relative to the applied load. The tensile fracture behaviors of the laser powder deposited Fe–30Mn–10Cr–10Co–3Ni alloy were different from the microvoid coalescence fractures of traditional polycrystalline ductile alloys, and the fracture in the former ones was caused by the propagation of surface and near-surface cracks. Fractures had neither shear lips nor radial zones. The fundamental understanding will provide a theoretical basis for future fabrication and post treatment to enhance the mechanical properties of similar alloys.

Design of high strength and wear-resistance β-Ti alloy via oxygen-charging

Titanium (Ti) is a promising biomedical material due to its superior corrosion resistance, low elastic modulus and favorable biocompatibility. Nevertheless, Ti faces a dilemma because of its inferior abrasion performance and strength-ductility trade-off, which poses a limitation in application as biomedical implants. Here, we developed an oxygen-charging method to fabricate a β-Ti alloy with combination of ultrahigh surface hardness, strength, toughness and remarkable wear resistance. The superior mechanical performance of β-Ti alloy originates from a 200 μm-thick α+β phase hard shell, a 600 μm oxygen gradient region and an oxygen-free β-Ti core. The gradient phase and composition structures display different deformation mechanisms, transforming from simple but unusual basal slip in α phase to multiple-slip activities in β phase. The unique oxygen gradient distribution makes β-Ti alloy much stronger and tougher that can resist surface crack propagation and sample catastrophic failure. Oxygen charging is a novel technique to design high-performance Ti implants for biomedical applications.

Influence of ultrasonic surface rolling process on surface characteristics and micro-mechanical properties of uranium

Ultrasonic surface rolling process (USRP) is a novel surface modification technique based on surface severe plastic deformation (SSPD). The present study aims to investigate the surface characteristics and micro-mechanical properties of uranium by USRP treatment. By means of theoretical calculations and experimental tests, it was concluded that the USRP treated specimen showed a smoother and more densified surface. The surface roughness Rv, Rp and Ra were reduced to 0.590 μm, 0.669 μm and 0.249 μm, respectively. Meanwhile, the U(002) basal texture and stronger residual compressive stresses were presented on the surface. The gradient microstructure including severe deformation layer and deformation twinning layer was generated after USRP treatment. Furthermore, there was a typical gradient strength distribution with significantly higher values on the top surface, followed by a gradual decrease until the matrix was reached. As a result, the improvement of surface strength was achieved and the ductility of matrix in core was maintained. These improvements in the micro-mechanical properties were attributed to the results of surface integrality, orientation of grains, residual compressive stress, severe deformation layer and deformation twinning layer. However, excessive passes of USRP treatment led to the nucleation and propagation of surface cracks and the release of residual compressive stress, which were not conducive to the improvement of micro-mechanical properties.

Investigation of microplastics release behavior from ozone-exposed plastic pipe materials

Microplastics (MPs) are detected in drinking water and plastic used during water treatment and distribution is one of the possible sources of MPs. This work aimed to investigate the MPs release behavior from ozone-exposed plastic pipe materials. The changes on physicochemical properties of the plastic materials were analyzed. The carbonyl groups introduction, the oxidation induction time variation, and the surface topography altering were detected after ozone exposure. The MPs release behavior varied between different plastic materials. As the ozone exposure duration of plastic materials prolonged, the released MPs abundance from the materials sharply increased, especially for LDPE, HDPE and PP. PVC was an exception where the released MPs abundance had little changes (p > 0.05). The total released MPs concentration from 20 h-aged samples could be ranked in order as follows: LDPE (656 ± 20 MP L−1) > PP (349 ± 20 MP L−1) > HDPE (337 ± 22 MP L−1) > PVC (63 ± 13 MP L−1). MPs release behavior was more likely to occur for LDPE, which was possibly related to the low oxidation resistance and weak stability of LDPE under ozone exposure. There was a more dominant contribution to MPs abundance increase caused by MPs release from aged plastic pipe materials than secondary MPs generation from original plastic particles. The generated MPs from 20 h-aged LDPE, HDPE, and PP accounted for 88.4%, 82.2%, and 88.3% of the total released MPs, respectively. For ozone-exposed plastic materials, the surface crack propagation and fragmentation posed an entry point for MPs generation. The proportion of generated MPs with polymer composition consistent with pipe materials (PE/PP) increased as the ozone exposure proceeded. Small-sized particles, especially 1–10 μm, were released more predominantly. This study provides an implication that possible MPs release from long-term aged plastic pipe materials under proper conditions could not be ignored.

Vickers crack extension and residual fracture strength of annealed and thermally tempered glass in water

The effect of water contact on the propagation of microscale surface cracks is investigated in two types of glass: annealed and tempered glass. Initial flaw is artificially created on the glass surface using a Vickers indenter, and is covered with a water droplet for 20 min. Micrographs of the flaw taken before and after water contact confirms the increase in crack length from around 61–103 μm for thermally tempered glass. After water dipping, the maximum length to which the crack growth is approximately 57 % smaller in the thermally tempered glass than in the annealed glass. Despite the severe effect of water contact on crack propagation, it is found that the fracture strength is not substantially altered by water dipping; even though the crack length is enlarged, the fracture strength of glass is similar, and in the case of tempered glass, its fracture strength is slightly changed within 7% due to the blunting of the crack tip by water or others.

The effect of epoxy coating on the fatigue strength of grade-A mild steel

In this study, tension fatigue tests of grade-A marine mild steel with epoxy type of coating were carried out and the corresponding S–N curves with a survival probability of 97.5% are derived. Compared to the uncoated specimens studied previously, Epoxy coating has increased fatigue life of Mild steels by about 50% in the case of high cycle fatigue and fatigue limit by 6%. This improvement on fatigue strength is mainly due to crack-tip bridging phenomenon of coating which fills micro surface cracks and delayed surface crack propagation. The effect of surface finishing method on adhesion strength of Epoxy coating is investigated by pull-off tests. The DIC method is used to determine the in-plane strain distribution in the necking region of tensile test of uncoated specimens. The final image before fracture shows that the vertical and horizontal local strain in the necking position is 155 and 32%, respectively.

Research on the anisotropic fracture behavior and the corresponding fracture surface roughness of shale

Understanding the anisotropic fracture mechanism of shale is of significance to assess the development of a stimulated reservoir volume. In this study, three-point bending tests were carried out using notched deep beam specimens of shale with 7 different bedding dip angles to investigate the anisotropic fracture behavior. The anisotropic characteristics of the fracture toughness, energy release rate, inelastic zone and crack propagation path were comprehensively analyzed and highlighted. The results showed that the fracture toughness decreases with increasing bedding dip angle. When the bedding dip angle is 0° and 90°, the fracture toughness reaches the maximum and minimum values, respectively, with a ratio of 1.60. The variation tendency of the energy release rate with the bedding inclination angle is consistent with that of the fracture toughness, but the ratio of the maximum value to the minimum value of the energy release rate is about 3.24. The size of the inelastic zone first decreases and then increases at different bedding inclination angles from 0° to 90° and reaches the maximum size of 1.51 mm when the bedding plane inclination angle is 45°. The crack propagates along the bedding plane rather than straight along the initial crack direction when the bedding dip angle is higher than 45°. As the bedding plane dip angle increases from 0° to 45°, the crack surface roughness of the tested shale first increases, then decreases and finally reaches the maximum. The discussion indicated that the presence of the Mode II fracture has a considerable influence on the crack surface roughness and crack propagation path, although it has little effect on the failure load. Compared with the maximum energy release rate (MERR) criterion, the maximum tangential stress (MTS) criterion seems more accurate in predicting the anisotropic Mode I fracture toughness. The applicability of these two classical crack initiation criteria of fracture mechanics has been analyzed based on our experimental results, which demonstrates the validity of the MTS criterion to assess the anisotropic fracture toughness of shales.

The Crack Propagation Trend Analysis in Ceramic Rolling Element Bearing considering Initial Crack Angle and Contact Load Effect

Silicon nitride ceramic bearings are widely used for their excellent performance. However, due to their special manufacturing method, cracks will occur on ceramic ball surface, and this initial surface crack will propagate under the action of cyclic stress, which will lead to material spalling. This will greatly limit its service life in practical applications, especially under heavy load at high speed. Therefore, it is necessary to study the surface crack propagation of silicon nitride ceramic bearings. In this paper, the effect of initial crack angle and contact load on crack growth is analysed by the finite element method (FEM). A three-dimensional finite element model of a silicon nitride bearing ball containing an initial crack is created by the FEM. The cracks are initially classified based on the angle between the crack and the bearing ball surface, and the location of the most dangerous load for each type of crack is known by theoretical analysis. The stress intensity factors (SIFs) are calculated for the crack front to investigate the effect of load position on crack growth. Subsequently, the SIFs are calculated for each type of crack angle subdivided again to investigate the effect of crack angle on crack propagation.

Analysis of deep crack formation and propagation in railway brake discs

While surface crack formation and propagation in Electric Multiple Unit (EMU) brake discs has been an active research topic, relatively few studies have considered similar behavior in deep cracks which become unstable. Moreover, most analyses assume nominal design conditions which does not reflect changes which may have been occurred during service. In the present study, a steel brake disc was analyzed after a deep crack was discovered during a maintenance check. The objective was to identify the impetus for deep crack propagation and characteristic semi-elliptical shape. Finite element (FEM) and extended finite element (XFEM) models which included residual stress effects from service were used to analyze the thermo-mechanical history and resulting crack propagation. The simulated and actual fracture surfaces demonstrated satisfactory agreement and showed that the crack preferentially initiated at the bolt hole corners and that transient temperature differences between adjacent hot spots help to define the crack orientation. It was also found that unstable propagation along the axial direction was due to a steep temperature gradient. Service induced distortion was also found to indicate the presence of internal stresses which can have a strong effect on crack behavior. It is expected that the findings will contribute toward a better understanding of how braking conditions and service effects influence unstable crack behavior in EMU brake discs.

Experimental study on the evolution of chip morphology, chip formation, and surface topography with cutting parameters, and their relationships in dry milling of cast aluminum alloy with PCD inserter

The evolution of chip morphology, chip formation, and surface topography (Ra, Rz) with cutting parameters has become an increasingly important way to achieve higher manufacturing efficiency and better surface topography. In this investigation, the relationships of chip morphology, chip formation, and surface topography with respect to cutting parameters are developed in dry milling of cast aluminum alloys with a PCD inserter. The feed rate affects the surface roughness, although no unanimous trend is observed between cutting parameters and surface topography. Surface crack propagation directly affects the saw-tooth chip morphology and the surface topography. The chip morphology is transformed from saw-tooth to continuous type at a chip deformation ratio of 3; chip deformation ratios of 2.5, 1.875, and 1.25 achieve cutting velocities of 32.71, 35.98, and 39.25 m/s, respectively, in which a continuous chip is formed by surface crack propagation and the predominant effect of thermal softening and surface topography is significantly improved.

Water vapor volatilization and oxidation induced surface cracking of environmental barrier coating systems: A numerical approach

A numerical model is developed for surface crack propagation in brittle ceramic coatings, aiming at the intrinsic failure of rare-earth silicate environmental barrier coating systems (EBCs) under combustion conditions in advanced gas turbines. The main features of progressive degradation of EBCs in such conditions are captured, including selective silica vaporization in the top coat due to exposure to water vapor, diffusion path-dependent bond coat oxidation, as well as crack propagation during cyclic thermal loading. In light of these features, user-defined subroutines are implemented in finite element analysis, where surface crack growth is simulated by node separation. Numerical results are validated by existing experimental data, in terms of monosilicate layer thickening, thermal oxide growth, and fracture behaviors. The experimentally observed quasi-linear oxidation in the early stage is also elucidated. Furthermore, it is suggested that surface crack undergoes rapid propagation in the late stage of extended thermal cycling in water vapor and leads to catastrophic failure, driven by both thermal mismatch and oxide growth stresses. The latter is identified as the dominant mechanism of penetration. Based on detailed analyses of failure mechanisms, the optimization strategy of EBCs composition is proposed, balancing the trade-off between mechanical compliance and erosion resistance.

Effects of Tooth Surface Crack Propagation on Meshing Stiffness and Vibration Characteristic of Spur Gear System

Tooth surface cracks are considered as the early stage of the development of tooth surface spalling failure. Understanding the excitation mechanism of surface cracks has a great significance in the early diagnosis of spalling faults. However, there are few studies on the dynamic modelling of surface cracks, and the influence mechanism of surface cracking on the dynamic characteristics of a gear system is also not yet clear during its propagation process. Thus, an analytical calculation model of the meshing stiffness of gear with tooth surface crack is developed. Then, a dynamic model of a spur gear system with six degrees of freedom (DOF) is established based on the proposed surface crack calculation model. The effects of surface crack propagation on the meshing stiffness and dynamic characteristics of gear system are investigated. The results show that the side frequencies of dynamic transmission error (DTE) are more sensitive than those of the acceleration responses during the surface crack propagation, which is more favorable to the surface crack fault diagnosis. Compared to the traditional spalling fault model, the proposed model can accurately characterize the dynamic characteristics of a gear system with the early spalling defect.

Effect of partial substitution of corn straw fly ash for fly ash as supplementary cementitious material on the mechanical properties of cemented coal gangue backfill

Cemented coal gangue backfill (CCGB) is an engineering mixture that can effectively solve the surface subsidence problem, and comprises coal gangue and cementitious materials. However, there are many constraints regarding cost, performance, and other engineering indicators of CCGB. To optimize the performance, reduce the cost, and recycle agricultural solid waste, the replacement of fly ash in the raw material with the equal weight of corn straw fly ash (CSFA) was studied. The effects of adding CSFA on the uniaxial compressive strength (UCS), surface crack propagation, acoustic emission characteristics, shear strength, drying shrinkage, and microstructure of the CCGB were investigated experimentally by setting different substitution amounts (0, 10, 20, 30, and 40 wt%) and curing ages. The results show that CSFA can be used to replace up to 40 wt% of the fly ash in CCGB, and help improve the mechanical properties of CCGB, subject to the adverse effects of drying shrinkage. At higher substitution amounts, the UCS and shear strength were found to increase. Furthermore, the specimen exhibited a double-crack failure on the surface and X-shaped failure inside; otherwise, a single-crack failure on the surface and single-slope failure were observed. Compared to fly ash, CSFA has a coarser particle size, higher pozzolanic reactivity, more reactive silica/alumina, and later reaction stage as a supplementary cementitious material in CCGB. This introduction of CSFA into CCGB provides an excellent example of green and sustainable development.

Fatigue behaviors of 2205 duplex stainless steel with gradient nanostructured surface layer

Fatigue behaviors of 2205 duplex stainless steel (initially composed of austenite and ferrite phase) with a gradient nanostructured (GNS) surface layer induced by the ultrasonic rolling process (USRP), are investigated in both strain-controlled high-cycle fatigue (HCF) and low-cycle fatigue (LCF) tests. Results showed that the fatigue life is improved in HCF (~5 times longer under 0.3% strain) but decreased in LCF (~53% under 0.8% strain). This is different from the simultaneous enhancement of fatigue behaviors in both HCF and LCF in other materials with GNS. The effect of martensite phase transformation (MPT), residual compressive stress as well as the GNS layers have been investigated to clarify the mechanisms of the fatigue behavior of the USRP samples. Besides the suppression of the surface crack initiation and propagation by residual compressive stress and GNS layers, the MPT during the USRP and cyclic loading processes greatly affects the fatigue behaviors. For HCF tests, not only the hard-brittle martensite phase induced by the USRP process improved the material strength but also the MPT process during cyclic loading absorbed the strain energy released from the crack growth, which increased the HCF life. However, for LCF tests, the hard-brittle martensite phase decreased the ductility of the GNS layer and accelerated the crack growth rate under higher strain amplitude, resulting in the decrement of LCF life. This work presents the first investigation on fatigue behaviors of duplex stainless steel with GNS structure.

Observation of Surface and Subsurface Crack Propagation in PPS Thrust Bearings under Rolling Contact Fatigue in Water

In this study, crack propagation in PPS thrust bearings under rolling contact fatigue (RCF) in water was observed in order to investigate the relation between cracks and flakings. RCF tests in water under loads of 700 N and 900 N were performed. The semi-circular cracks propagated in a direction perpendicular to the rolling direction were observed under a load of 700 N. The line cracks propagated in a direction parallel to the rolling direction at periphery of contact area and the semi-circular cracks were observed under a load of 900 N. To study the subsurface cracks, full section of rolling contact area was observed. It is concluded that the flaking mechanism in PPS thrust bearing has three features as follows: Initiation and propagation of surface cracks depend on the load; When load is 700N, the semi-circular cracks growing from the surface and the cracks branching from the main subsurface cracks join to form the flaking; When load is 900N the line cracks and the semi-circular cracks growing from the surface join to form the flaking.

Penetration behaviour of TBM disc cutter assisted by vertical precutting free surfaces at various depths and confining pressures

The ability to improve rock-breaking efficiency of tunnel boring machines in hard rock stratum is significant to improve driving speed and reduce construction cost and time. The undercutting method has been used to improve the rock-breaking efficiency of disc cutters, including the design of curved and multistage cutterheads. The limited radian of curved cutterheads and the number of multistage cutterheads, however, are restricted by the current manufacturing level. We explored mechanical response and rock-breaking efficiency assisted by water jet kerfs as the vertical free surface. We conducted a quasi-static penetration test of the disc cutter considering four kerf depths and three groups of confining pressures. We analysed the surface crack propagation and fractures inside the sample using acoustic emission and computed tomography scanning. We studied the influence of confining pressure and cutting depth of the water jet on penetration force, rock chips, and specific energy. We analysed the failure evolution and stress field of kerf specimens using numerical simulation. The results showed that increasing cutting depth relieved restraint stress and the resulting influence of the confining pressure on removed rock volumes and specific energy was not remarkable. We recommended a kerf depth of greater than 18 mm to reduce penetration force and significantly improve rock-breaking efficiency.

Experimental Study on Anchorage Mechanical Behavior and Surface Cracking Characteristics of a Non-persistent Jointed Rock Mass

It is significant to conduct anchorage on the non-persistent jointed rock mass by bolts to inhabit the unstable fracture evolution of rock engineering. In this research, a series of experiments are done on 70 specimens to investigate the effect of anchorage method on strength and deformation behavior of a non-persistent jointed rock mass. First, based on the stress–strain curves of anchorage jointed specimens, the effect of anchorage method on the peak strength and elastic modulus of non-persistent jointed rock mass is investigated. The experimental results show that the peak strength and elastic modulus of jointed specimens changes with anchorage method for the same joint angle, while first decreases and then increases from 0° to 90° for the same anchorage method. Second, the effect of pretightening force on the strength and deformation behavior of anchorage jointed specimens is analyzed. The axial stress-axial strain curves of anchorage jointed specimens with various pretightening forces can be characterized into five types: (I) strain softening after the specimen drops to yield platform from peak strength; (II) strain hardening after the specimen drops to yield platform from peak strength; (III) strain hardening after the specimen yields; (IV) stepwise strain softening after the peak strength; (V) single stress drop after the peak strength. The peak strength and elastic modulus of jointed specimens for the same dip angle all increases nonlinearly with the pretightening force. And then, based on a series of surface failure mode of non-persistent jointed rock specimen, it can be seen that the initiation, propagation and coalescence of surface cracks depend on not only the joint angle, but also the magnitude of pretightening force. Twelve crack coalescence types are identified to analyze the surface crack mode of anchorage non-persistent jointed rock specimens. Finally, the effect of pretightening force on brittleness index of non-persistent jointed rock mass is made a discussion.