Lateral Cracks Research Articles

Intricacies involving the evaluation of fracture toughness obtained by the surface‐crack‐in‐flexure method

AbstractAmong the several methods available for the measurement of the fracture toughness (KIc), the surface‐crack‐in‐flexure (SCF) has the advantages of being based on an easy‐to‐produce short (application relevant) crack size of defined geometry with a naturally sharp crack tip. The production of ready‐to‐test cracked specimens is fast and uncomplicated, rendering accurate results with low scatter. However efficient and reliable, the procedural steps for the obtainment of a valid crack need revisions, especially on aspects concerning the polishing depth, the removal of lateral cracks, the obtainment of an appropriate crack geometry, and the use of adequate solutions for the geometry factor. In this study we attend to these issues by systematically polishing a large number of specimens of two ceramic materials having very distinct subsurface Knoop crack systems to assess the effect of remnants of lateral cracks. By evaluating the geometry factor of the median precrack along the entire crack front, we provide valid ranges for the crack geometry regarding the crack‐surface intersection angle as resulting from the polishing depth, extending the applicability of the Strobl et al. solution to shallow cracks in detriment of the Newman and Raju formula that is poised to overestimations of KIc of up to 10%. Our results debunk the belief that lateral cracks affect the obtained KIc, but demonstrate the necessity of a minimum polishing depth for the attainment of valid crack geometries.

An Investigation into the Densification-Affected Deformation and Fracture in Fused Silica under Contact Sliding.

Subsurface damage of fused silica optics is one of the major factors restricting the performance of optical systems. The densification-affected deformation and fracture in fused silica under a sliding contact are investigated in this study, via three-dimensional finite element analysis (FEA). The finite element models of scratching with 70.3° conical and Berkovich indenters are established. A refined elliptical constitutive model is used to consider the influence of densification. The finite element models are experimentally verified by elastic recovery, and theoretically verified by hardness ratio. Results of densification and plastic deformation distributions indicate that the accuracy of existent sliding stress field models may be improved if the spherical/cylindrical yield region is replaced by an ellipsoid/cylindroid, and the embedding of the yield region is considered. The initiation sequence, and the locations and stages of radial, median, and lateral cracks are discussed by analyzing the predicted sliding stress fields. Median and radial cracks along the sliding direction tend to be the first cracks that emerge in the sliding and unloading stages, respectively. They coalesce to form a big median–radial crack that penetrates through the entire yield region. The fracture behavior of fused silica revealed in this study is essential in the low-damage machining of fused silica optics.

Crack Propagation Behavior of Fused Silica during Cyclic Indentation under Incremental Loads

Fused silica is an important optical material with important applications, where the surface must be precisely machined without subsurface damage. In this study, multi-cyclic indentations under incremental loads were performed on fused silica using two kinds of indenters to clarify the mechanisms of crack generation and propagation induced by precision grinding. It was found that incremental loading cyclic nanoindentation induced various patterns of subsurface cracking and surface spalling. Four kinds of surface spalling were identified at different locations around an indent, the temporal formation mechanisms of which were clarified by microscopic observation and topographical measurement. Load–displacement curve analysis demonstrated that incremental propagation of lateral cracks during early indentation cycles caused large-scale brittle fractures during later cycles. Compared with a Berkovich indenter, a cube-corner indenter caused more significant brittle fractures and surface spalling. The findings in this study will deepen the understanding of subsurface damaging mechanism of fused silica and other brittle solids caused by cyclic tool-workpiece interactions in grinding and other mechanical machining processes.

Surface Morphology Evaluation and Material Removal Mechanism Analysis by Single Abrasive Scratching of RB-SiC Ceramics

Reaction-Bonded Silicon Carbide (RB-SiC) ceramics possessing excellent mechanical and chemical properties, whose surface integrities have an essential effect on their performance and service life, have been widely used as substrates in the core parts of aerospace, optics and semiconductors industries. The single abrasive scratching test is considered as the effective way to provide the fundamental material removal mechanisms in the abrasive lapping and polishing of RB-SiC ceramics for the best surface finish. In this study, a novel single abrasive scratching test with an increasing scratching depth has been properly designed to represent the real abrasive lapping and polishing process and employed to experimentally investigate the surface integrity regarding different scratching speeds. Three typical and different material removal stages, including the ductile mode, ductile–brittle transition mode and brittle mode, can be clearly distinguished and it is found that in the ductile material removal stage by increasing the scratching speed would inhibit the plastic deformation and improve its surface integrity. It is also found that in the ductile–brittle transition and brittle material removal stages, to increase the scratching speed would inhibit the plastic deformation due to the fast scratching speed that limits the time of plastic deformation on the target, but it also results in the increased length of lateral cracks with the increased scratching speed which can reflect that the size of brittle chips, like brittle fractures and large grain fragmentations, increases as the scratching speed increases. It can provide the references for the optimization of the abrasive lapping and polishing of RB-SiC ceramics with high efficiency and surface quality.

A full-section asphalt concrete waterproof sealing structure for the high-speed railway subgrade

PurposeThis study aims to investigate the service performances of a new full-section asphalt concrete waterproof sealing structure (FSACWSS) for the high-speed railway subgrade through on-site tracking, monitoring and post-construction investigation.Design/methodology/approachBased on the working state of the waterproof sealing structure, the main functional characteristics were analyzed, and a kind of roller-compacted high elastic modulus asphalt concrete (HEMAC) was designed and evaluated by several groups of laboratory tests. It is applied to an engineering test section, and the long-term performance monitoring and subgrade dynamic performance testing system were installed to track and monitor working performances of the test section and the adjacent contrast section with fiber-reinforced concrete.FindingsResults show that both the dynamic performance of the track structure and the subgrade in the test section meet the requirements of the specification limits. The water content in the subgrade of the test section is maintained at 8–18%, which is less affected by the weather. However, the water content in the subgrade bed of the contrast section is 10–35%, which fluctuates significantly with the weather. The heat absorption effect of asphalt concrete in the test section makes the temperature of the subgrade at the shoulder larger than that in the contrastive section. The monitoring value of the subgrade vertical deformation in the test section is slightly larger than that in the contrastive section, but all of them meet the limit requirements. The asphalt concrete in the test section is in good contact with the base, and there are no diseases such as looseness or spalling. Only a number of cracks are found at the joints of the base plates. However, there are more longitudinal and lateral cracks in the contrastive section, which seriously affects the waterproof and sealing effects. Besides, the asphalt concrete is easier to repair, featuring good maintainability.Originality/valueThis research can provide a basis for popularization and application of the asphalt concrete waterproof sealing structure in high-speed railways.

Surface integrity and material removal mechanisms in high-speed grinding of Al/SiCp metal matrix composites

SiC particle reinforced Al metal matrix composites (Al/SiCp MMCs) are typical difficult-to-machine materials due to the heterogeneous constituent. Poor surface integrity is commonly caused in conventional machining methods. To explore material removal mechanisms in high-speed grinding, this study carries out high-speed grinding (HSG) on an Al/SiCp MMC at a grinding speed from 30.4 m/s to 307.0 m/s, and assesses surface integrity including surface damage and subsurface damage (SSD) to explore how different grinding speeds take effect therein. The results reveal that improved surface quality is attained in HSG in which continuous and discontinuous dynamic recrystallization mechanisms govern Al grain refinement, and the latter is inclined to occur in the upper part of the ground surface. The distribution of the O-rich zone is closely associated with subsurface cracks. The workpiece ground at a higher grinding speed is with less damage than at a lower grinding speed due to the reduced O-rich zone. Three different layers in the subsurface below ground workpiece are identified based on various features, which are relatively narrower compared to that in low-speed grinding. The range of plastic deformation of the Al alloy matrix is suppressed in HSG because of larger Al grains and a reduced depth of lateral cracks in Al alloy matrix. Distinctly denser dislocation kinks formed at the boundary of SiC particles in HSG indicate the increased ductility of SiC particles. In HSG of Al/SiCp MMCs, strain-rate effect prevails for Al alloy matrix as a result of reduced ductility, and size effect plays the dominant role for SiC particles due to increased ductility, which facilitate reducing the property discrepancies between these two very different components. Therefore, an improved surface integrity of the Al/SiCp MMCs is realized through HSG. This study enhances the understanding of the surface and subsurface formation and material removal mechanisms in HSG of Al/SiCp MMCs, which can provide a theoretical basis and practical reference for achieving better surface quality for Al/SiCp MMCs and other composites machined by HSG.

Effect of reduced graphene oxide on the mechanical properties of rGO/Al2O3 composites

We fabricated reduced graphene oxide (rGO)-reinforced Al2O3 composites by spark plasma sintering and studied the effect of graphene oxide (GO) dimensions on the microstructure and mechanical properties of composites. The thin GO led to both grain size refinement and grain boundary pinning effects. The Al2O3 composite reinforced by thin and large GO had higher hardness than that reinforced by thick and small GO. The addition of thick GO leads to a reduction of hardness and generation of lateral cracks at the Al2O3 grain boundaries under indentation due to low rGO interlayer strength. The rGO distribution in the Al2O3 matrix was studied. A model calculation showed that the spacing between two parallel rGO platelets was one Al2O3 grain at rGO addition of 0.77 wt%. We concluded that the increased hardness of rGO/Al2O3 composites is due to the fine grain size and thin rGO at Al2O3 grain boundaries.

Inclusion orientation dependent flaking process in rolling contact fatigue observed by laminography using ultrabright synchrotron radiation X‐ray

AbstractThe formation and propagation of cracks in rolling contact fatigue were observed by synchrotron radiation computed laminography, and the effect of stringer‐type inclusion orientation was examined. For longitudinal inclusions, cracks started forming at their tips. After cracks propagated toward the rolling direction, a longitudinal crack was kinked simultaneously at both its tips, and propagated toward the direction perpendicular to the rolling direction to form lateral cracks. After kinking, horizontal cracks were formed from the deepest point of a lateral crack, leading to flaking. On the other hand, for specimens with lateral inclusions, cracks propagated to the lateral direction without the formation of longitudinal cracks. Since the propagation life of lateral cracks and that of horizontal cracks were unrelated to the inclusion orientation, the rolling contact fatigue life of specimens with longitudinal inclusions was considerably longer than that of specimens with lateral inclusions.

Lateral cracks damaging the stability of tensile sheet during tandem rolling conveying

In tandem cold-rolling production, sheet fractures are interrelated to both conveyance stress and transverse cracks during conveyance process. The residual tension or sheet contour cannot completely explain the early fracture problem. It is found that small lateral crack reduces fracture threshold of sheets destructively during cold rolling and coiling processes without rolling stress. Several lateral notches were machined on cold-rolled intermediates of silicon steel to simulate edge cracks in rolling processes. Based on tensile test and laser confocal microscope, the damages of lateral notches on tension resistance of sheets were investigated. Then, the shape effect and size effect of lateral notches were analyzed. The notch depth could negatively affect energy absorption capacity of sheets before tensile fracture. Meanwhile, the sharp notch could deteriorate elongation ability of sheets before necking. Both factors reduced fracture threshold of sheets and tensile redundancy and conveyance stably capacity of tandem rolling mills, promoting sheet fracture.

Morphology of subsurface cracks in glass-ceramics induced by Vickers indentation observed by synchrotron X-ray multiscale tomography

The characterization of subsurface cracks induced by indentation is a challenge for understanding contact damage, impact, wear, erosion, and abrasion of brittle materials, because the crack pattern observable on the surface is only a part of the total crack system. Here we applied synchrotron X-ray multiscale tomography to observe the morphology of subsurface cracks produced by Vickers indentation in a novel CaO–Al2O3–SiO2 glass-ceramic with plate-like crystals forming a house-of-cards microstructure. It revealed a diverse type of crack systems around the semispherical microcrack zone beneath the indent, including a new mode II inclined lateral crack driven by the maximum shear stress. Tomography images provided knowledge on how the heterogeneous microstructure affected the toughening processes such as crack deflection, crack bridging, and microcracking.

A material point method based investigation on crack classification and transformation induced by grit geometry during scratching silicon carbide

The microcracks and residual stress are commonly regarded as non-negligible obstacles while realizing non-damage surfaces in the grinding of brittle materials. In this paper, with the combination of stress field analysis, material point method simulation and high-speed scratch experiment, a variety of cracks in the classical indentation-induced crack system were decoupled to determine the inducing conditions and coupling modes of different cracks. The tests on pressureless sintered silicon carbide (S–SiC) exhibited that the grit geometry was crucial in inducing different cracks. The Hertzian and lateral crack systems were mainly determined by the rake angle of scratching grit and can transform from one to another near the critical value of −60°. Further, a two-point stress field model was proposed to prove that large contact width is the key factor to induce radial cracks. By contrast, large contact depth is related to the appearance of bottom debris. Furthermore, Raman spectroscopy was used to investigate the velocity effect of grit on cumulative damage. Besides the stacking disorder and residual stress, even chemical disorders such as the fracture of the Si–C bond and the formation of the homonuclear bond were found to be variable at the bottom of the scratch under different contact speeds and depths. Based on the above findings, a multi-layer superposition model was proposed to re-couple the effect of different cracks and predict the workpiece's damage/force state scratched by spherical grits. It proved that blunt grit could also realize low damage machining under specific conditions.

Designed Ti/TiN sub-layers suppressing the crack and erosion of TiAlN coatings

Solid particle erosion damage seriously affects the service life and operational safety of engine blades, while the erosion resistant coating is one of the effective measures to effectively improve the erosion resistance of the blade material and extend its service life. Here, we deposited the Ti/TiN/TiAlN multilayer coatings with different thickness ratio of the Ti to TiN layer on Ti-6Al-4V substrates by a home-made hybrid multisource cathodic arc system. The relationship between coating structure and coating properties including adhesion strength, residual stress, load-bearing capacity, crack resistance and erosion resistance was focused. Results showed that the Ti layer was beneficial in improving adhesion strength and reducing residual stress of the multilayer coatings, which was benefited from coordinating the deformation and absorbing strain energy induced by plastic deformation. The TiN layer was important in maintaining load-bearing capacity. Radial cracks initiation and propagation from the Ti/TiN interface occurred at most cases, while wide lateral cracks were formed only in the TiAlN layer for its high brittleness and internal stress during scratching. When the thickness ratio of the Ti to TiN layer was 1:2, the coating exhibited the best combination of hardness, adhesion strength, crack resistance, and load-bearing capacity. As a consequence, the coating behaved the lowest erosion rate of 0.060 ± 0.0002 mg/g and 0.024 ± 0.0068 mg/g at the erosion angle of 90° and 30°, respectively.

Increasing and Decreasing Depth Taper Scratching: Force Response of Silicon

Mechanical loading and unloading of silicon is a characteristic feature of grinding and diamond turning processes. Such rapid loading and unloading induces damage and phase transformations. While, indentation tests are often used to study such normal loading and unloading via characteristic events in the force-depth plot, such tests involve only normal loading and lack tangential loading. A better alternative is scratch test, both constant and varying depth ones, involving normal and tangential loading on the scratching tool; this better simulates conditions of machining, or grinding. In this research, the mechanical load/unload behavior response of silicon is studied under scratching conditions by comparing increasing and decreasing depth scratch behaviour. In-situ force responses show that after ductile-brittle transition occurs, higher forces, at a given scratch depth, are required to deform the material during increasing depth scratching for a given depth than in decreasing depth scratch. Large surface and sub-surface damages with the presence of radial, median, and lateral cracks are seen to make the material weaker, ahead of the advancing tool, in decreasing depth scratch. Raman intensity ratio of amorphous silicon (a-Si) to nanocrystalline silicon (nc-Si) shows that high amorphization of silicon occurs during increasing depth scratching than decreasing depth. Using such force-depth plots an attempt is made to compare the normal loads while indenting and scratching. This study can help optimize the processing of silicon by grinding and diamond turning.

Comparison of fracture properties of different amorphous carbon coatings using the scratch test and indentation failure method

In this work, fracture properties of different types of hydrogen-free amorphous carbon coatings were studied in detail, using ta-C, ta-C:B and a-C:Mo coatings deposited at nominal coating thickness of 1 μm and 6 μm. Two common test methods with additional advanced evaluation techniques were used. Scratch testing was conducted with load and indenter size scaled to the coating thickness, evaluating critical loads and relative area of delamination. Instrumented indentation failure testing at different final loads was carried out, analyzing the load-depth curve for pop-ins, and acoustic emission for fracture events. Afterwards, indentation sites were studied using SEM and FIB cross sections, following a new approach where crack pattern imaging was directly correlated with fracture events obtained from indentation. Observed crack modes were assigned to radial, circumferential, parallel and lateral cracks. The crack types were classified with respect to location, timing, causative stress situation, amount of plastic substrate deformation, and conclusion about the coating behavior. Furthermore, the causes for the different crack types were discussed in detail.In the case of the studied coatings, a specific fracture behavior was found for each type of carbon coating, which cannot be simply attributed to the coating hardness. Instead, it is shown that during indentation not only the first fracture events but also the entire behavior up to the maximum indentation load should be considered, allowing a more distinct differentiation of fracture behavior of different types carbon coatings.

Scratch test in boride layers: Influence of indenter tip radius on failure mechanisms

In this work, the influence of indenter tip radius (ITR) on scratch tests in boride layers was studied. The thermochemical treatment of boriding was performed on an AISI H13 steel at 1000 °C for 1 h. Depth sensing indentation test was developed to estimate both, hardness, and elastic modulus of the boride-layer/substrate system. Indenter radius of 200, 100, 50 and 20 μm were used for the scratch tests on borided steel. Failure mechanisms were analyzed by scanning electron microscopy (SEM). Predominant failure mechanisms were lateral cracking, chipping, and gross chipping. ITR of 20 μm developed more severe damage on the borided AISI H13 surface and the more severe substrate plastic deformation was obtained by ITR of 200 μm.

Revealing the surface structural cause of scratch formation on soda-lime-silica glass

Scratch formation on glass surfaces is a ubiquitous phenomenon induced by plastic deformation, often accompanied by radial, lateral or median cracks with consequent chipping and brittle fracture caused during and after the event of dynamic abrasion instigated by shear stress by a harder material. This paper addresses the fundamental aspect of scratch formation on soda-lime-silica (SLS) glass surfaces. A constructive combination of surface-sensitive characterization tools, including field emission scanning electron microscopy (FESEM), laser scanning microscopy (LSM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and instrumented indentation technique (IIT), helped to investigate the structural cause of generation of visible scratches on SLS glass surfaces. The experimental results indicate that a silicate network possessing a mechanically weakening structural characteristic in terms of network connectivity confined to the region between 5 and 100 nm below the glass surface is likely to cause a destructive surface scratch eminently visible to the naked eye.

Material removal behavior and crack-inhibiting effect in ultrasonic vibration-assisted scratching of silicon nitride ceramics

To investigate the material removal mechanism and crack-inhibiting effect of ultrasonic vibration-assisted grinding of ceramic materials, a series of double-scratch tests with variable scratching depths and interval distances was conducted. Ultrasonic vibration-assisted scratching (UVS) and conventional scratching (CS) were compared in terms of cutting force, scratch morphology, crack system, and material removal mode, and the effects of ultrasonic vibration and scratch interval distance on the material removal mode and crack-inhibiting effect were analyzed. The results indicate that the crack-inhibiting effect between scratches gradually decreases with an increase in the scratch interval distance. In comparison with CS, UVS can increase the critical load of the brittle-ductile transition and promote the propagation of shallow lateral cracks to the surface. Meanwhile, ultrasonic vibration increases the critical scratch interval distance of the crack-inhibiting effect. The research results also reveal that the material is mostly removed by the interlacing of shallow lateral cracks in UVS; however, this is due to the combined action of deep lateral cracks and radial cracks in CS.

Analysis of the crack propagation mechanism of multiple scratched glass-ceramics by an interference stress field prediction model and experiment

The crack propagation mechanism of glass-ceramics is the theoretical basis for the improvement of surface quality after grinding. The unclear mechanism of the multi-scratch interference effect on the crack propagation of glass-ceramics makes it difficult to propose methods to suppress crack propagation and control strategies to improve the surface quality. This research is aimed at understanding the influence of interference between scratches on lateral crack propagation, material removal mode, and surface quality during the grinding of glass-ceramics. In this study, the stress field model of the interference effect is established to reveal the influence mechanism of the interference effect on crack propagation in glass-ceramics. The amplitude and orientation distribution of the characteristic stress for the lateral crack are analysed. The results show that an increase in amplitude and the influence range of tensile stress and expansion merging under interference promote the further propagation of lateral cracks and brittle removal of glass-ceramics. Multiple scratches experiments produced by single grit were conducted, emulating the interference caused by multiple grits, and they were analysed by an interference stress field prediction model. For glass-ceramics, lateral crack/brittle fracture phenomena are predominant in the case of multi-scratch/double-scratch, whereas major plastic deformations are dominated in the case of a single scratch. A significant reduction in the critical cut depth is noted with an increase in the interference intensity which corresponds to the number of scratches. This preliminary work enables the understanding of the implications of the interference effect on lateral crack propagation in grinding glass-ceramics and provides a scientific basis for the establishment of a control method to suppress crack propagation and improve grinding quality.

Determination of intrinsic defects of functional KDP crystals with flawed surfaces and their effect on the optical properties

A practically nonlinear optical material, the functional KH2PO4 (KDP) crystal, has an extremely low laser-induced damage threshold (LIDT) due to manufacturing-induced surface defects. The low LIDT induced by these surface defects of KDP crystals hinders their application in laser fusion facilities. Herein, the effect of intrinsic point defects introduced by manufacturing-induced lateral cracks on laser damage was particularly investigated by a combination of spectral detection (i.e., photoluminescence, Raman and Fourier transformed infrared spectra) and first-principles calculation. It was determined that the manufacturing-induced lateral cracks would introduce more hydrogen vacancy and oxygen vacancy intrinsic defects than the ideal surface. These intrinsic defects would form cluster-type defects, lowering the LIDT to 6.600 J cm-2 by introducing two defect levels of 2.83 eV and 4.89 eV within the band gap of the KDP crystal. To further investigate the effect of intrinsic defects introduced by lateral cracks on laser-induced damage growth, the transformation of charge states of intrinsic defects under laser irradiation was calculated. Combined with the spectral information of the lateral crack-induced laser damage site and the laser damage growth experiments, it was found that the intrinsic defects of lateral cracks have a large amount of evolution, making the crystal prone to severe damage growth. Overall, the avoidance of such lateral cracks with a large number of intrinsic defects is very beneficial for improving the laser damage resistance of KDP crystals.

The pre-stress scratching test investigation on silicon carbide ceramics

Based on the theory of pre-stress machining, In this paper the pre-stress scratching test was carried out on SiC ceramics, and the single-point diamond indenter moved with uniform speed under constant normal load. Combining with acoustic emission testing, the acoustic emission signal, the depth of scratch and the damage of surface / subsurface were analyzed under different pre-stress. The results showed that the existence of pre-stress can shorten the depth of median cracks and reduce the surface / sub-surface damage when adopting pre-stress machining method to study the machining of ceramic materials, moreover, with the increasing of pre-stress to a certain extent, the depth of median cracks were decreased and the propagation length of lateral cracks were shortened.