Subsurface Cracks Research Articles

Evaluation of Internal Crack Growth of PPS Thrust Bearings under Rolling Contact Fatigue in Water

In this study, in order to evaluate the progress of internal cracks in PPS thrust bearings under rolling contact fatigue in water, cracks were observed by a full-cross-section observation method using a lathe machining. “Main subsurface crack” initiated at the surface toward the inside, then grew in a direction parallel to the surface. They connected with many “Semi-circular cracks” initiating at the surface from the opposite side to the inside, to from a semi-ellipsoidal flaking damage. It was found that the “Semi-circular cracks” and the “Main subsurface crack” dominated the flaking destruction.

Formation Mechanism of a Wide-Face Longitudinal Off-Corner Depression During Thick Slab Continuous Casting

In practical continuous casting, consecutive longitudinal depressions often occur in the off-corner areas of slab wide faces, which may cause surface cracks in the depression areas and subsurface cracks beneath the depressions. To investigate the formation mechanism of the longitudinal depression, a 3D transient thermomechanical-coupled model was developed in the present work with consideration of the heat transfer in steel strand, thermal shrinkage of solidifying shell, ferrostatic pressure, contact with mold and guiding rolls, etc. Based on the model, evolution of the longitudinal off-corner depression and the associated thermal, distortion, and strain behaviors of steel slab were studied partly or fully throughout the continuous casting process. The results showed that the shell shrinkage in mold leads to an obvious shell-mold interfacial gap in the corner and off-corner areas. At the mold exit, the continuous expansion of corner gap at the wide-face side results in a thin spot forming at approximately 50 mm off the corner. When the shell moves out of the lateral strand guide rolls, narrow face of the shell bulges outward and rotates the corner, during which the thin spot in the wide-face off-corner area bends inward and shapes a longitudinal depression. As the shell moves down, the longitudinal depression deepens and widens in zones 2 to 4, and becomes stable in the subsequent cooling zones. When the shell moves through the soft reduction segments, depth and width of the longitudinal depression decrease as the marginal areas of the depressions are flattened. The model in this work is supposed to reveal an insight into the formation of consecutive longitudinal off-corner depressions at the wide face of slab during continuous casting.

Multi-tip indenter tool scratch behavior of glass-ceramics

To reduce the surface flaws and subsurface damages of glass-ceramics as clinical substitute and repair materials, it is necessary to clarify the material removal behaviors and crack propagation modes in the grinding process. In this paper, a multi-tip indenter tool was fabricated and assembled on a Nanomechanical testing system. The experiments of ramp load scratching with the single-tip/multi-tip indenter tools were conducted on glass-ceramics to compare the material removal behavior under different scratch methods. The surface morphology, subsurface cracks propagation, and the brittle-ductile transition region were observed and investigated. Based on the scratch experiments, the finite element simulation model was established to analyze the stress characteristics in the glass-ceramics. The result shows that in multi-tip scratches, as the depth of scratch increases, the growth of surface cracks mainly takes three forms. Compared with non-instant multi-scratch, instant multi-scratch makes the propagation path of lateral cracks closer to specimen surface, suppresses the propagation severity of lateral and median cracks, and increases the depth of brittle-ductile transition region. This paper's results provide a fundamental understanding of multiple abrasive grains instant interaction on the material removal mechanism, and help to improve the clinical performance of glass-ceramics as biomedical materials.

Models of grinding-induced surface and subsurface damages in fused silica considering strain rate and micro shape/geometry of abrasive

Based on the indentation fracture mechanics of brittle material, the correlations between the subsurface crack depth and the scratch depth induced by pyramidal, conical, and spherical indenters are established, respectively. Combined the kinematics of grinding process, the theoretical models of surface damage (SD) and subsurface damage (SSD) depths are developed considering the strain rate effect and the micro shape/geometry of abrasive grit. The mechanical properties of fused silica under different strain rates are measured by nanoindentation test. Many fused silica samples are processed under different grinding parameters, and their SD and SSD depths are measured. In combination with the experimental results, the theoretical models from differently shaped grits are assessed, and the effects of grinding/abrasive grit parameters are analyzed theoretically. The results show that compared with experimental SD and SSD depths, those calculated from spherical or pyramidal grit have average errors of less than 11.0% and 6.0%, respectively, while those from conical grit have average errors of more than 50.0% and 33.0%, respectively. The models from hybrid grit can be used to predict the SD and SSD depths efficiently, with average errors of 5.3% and 4.6%, respectively. The results also show that both SD and SSD depths increase with the grit apex angle, diameter, tip radius, or extraction depth. Moreover, the strain rate decreases with increasing grinding depth, feed speed, or grit diameter. The research is useful to optimize the grinding/abrasive grit parameters to reduce the damages in ground brittle materials.

Nondestructive diagnostics and detection of subsurface cracks in non-rigid pavements with GPR

Nondestructive diagnostics and detection of subsurface cracks in non-rigid pavements with GPR

Interlaboratory comparison: Round robin test for the damage reproduction of white etching crack in cylindrical roller thrust bearings

White Etching Cracks (WEC) is a damage pattern in roller bearings characterized by sub-surface crack networks within local microstructural changes. Nowadays, neither the causes of occurrence are clarified, nor are there commercial calculation methods for damage prediction. The comparability of current research results is hampered by the use of different testing methods and their transferability is, therefore, limited. Until today, there is no generally accepted test method for the risk assessment of predefined tribological systems regarding WEC occurrence. To establish such a test method, the statistically valid transferability of experimental results with a commercially available test rig is necessary. The present work, therefore, concentrates on the transferability of the WEC occurrence using cylindrical roller thrust bearings, a standardized test rig, and a detailed test guideline. The statistical validation of the WEC occurrence and therefore of the proposed reference test method was carried out using an interlaboratory comparison of 20 tests in 10 laboratories. Subsequent metallographic examinations confirmed that all failures were caused by surface anomalies related to WEC. Furthermore, the WEC's position and orientation were systematically and extensively analyzed. The high reproducibility (Weibull coefficient of correlation of 0.975) and low variance (Weibull exponent of 6.4) show the suitability of the test method for risk assessment of predefined tribological systems.

Ultrasonic fatigue testing in as-built and polished Ti6Al4V alloy manufactured by SLM

Additive Manufacturing (AM) has carved a significant leap in mechanical design history, offering an almost infinite geometric freedom. It enables complex geometries with lower weight and enhanced behaviour, like topology optimisation. AM material mechanical properties is under continuous research due to the ever-increasing available materials, methods and an overall higher number of influential variables, such as printing parameters and powder shape and size (associated with metal AM). A rough and low quality-built surface is obtained from most AM methods. It has been proven to have a far from neglectable influence on the final mechanical properties and fatigue strength. Different post-treatment methods have shown a positive impact on correcting the rough surface impact on fatigue strength. In this work, the Ti6Al4V titanium alloy additively manufactured by Selective Laser Melting (SLM) is placed under Ultrasonic Fatigue Testing (UFT) within the Very High Cycle Fatigue (VHCF) regime. Ti6Al4V is widely applied in the aerospace industry due to its high strength, low-density relation. This work aims to evaluate the influence of surface roughness, defect size and location on fatigue strength. Several non-heat treated Ti6Al4V specimens were manufactured in one printing session. The present defect type, size and density, were quantified, and their tension, ductility and fatigue life properties evaluated. Tensile UFT were conducted with R =-1 to as-built and polished mirror-like specimens. The resulting fatigue fracture showed surface crack initiation for as-built specimens and sub-surface in polished specimens. A significant fatigue strength increase accompanied the crack initiation change. Sub-surface crack initiation occurred on the most common AM present defects, gas pores and smooth facets. Their location in respect to the surface appeared to influence fatigue resulting life directly.

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.

Speed effect on the material behavior in high-speed scratching of BK7 glass

Scratch tests are often performed at a speed that is significantly lower than the real application like machining and grinding. However, brittle materials like BK7 behave very differently under high-speed conditions due to the more promising temperature and stain rate effects Therefore, it is important to study its material behavior under high-speed condition. In this study, single scratch tests and consecutive scratch tests were performed on BK7 under scratch speeds of 1, 5 and 20 m/s, which were much higher than the traditional scratch tests. The surface morphology as well as the subsurface cracks of the scratch grooves was inspected under AFM and FIB-SEM. The thermal effect that caused the changes in ductile-brittle transition (DBT) and scratch morphology was simulated and explained by a thermal-stress coupled finite element analysis. Finally, the changes in material removal behavior as well as the crack initiation mechanism due to speed effect was revealed.

High-Order Wave-Damage Interaction Coefficients (WDIC) Extracted through Modal Decomposition.

This paper presents a new technique for the extraction of high-order wave-damage interaction coefficients (WDIC) through modal decomposition. The frequency and direction dependent complex-valued WDIC are used to model the scattering and mode conversion phenomena of guided wave interaction with damage. These coefficients are extracted from the harmonic analysis of local finite element model (FEM) mesh with non-reflective boundaries (NRB) and they are capable of describing the amplitude and phase of the scattered waves as a function of frequency and direction. To extract the WDIC of each wave mode, all the possible propagating wave modes are considered to be scattered simultaneously from the damage and propagate independently. Formulated in frequency domain, the proposed method is highly efficient, providing an overdetermined equation system for the calculation of mode participation factors, i.e., WDIC of each mode. Case studies in a 6-mm aluminum plate were carried out to validate the WDIC of: (1) a through-thickness hole and (2) a sub-surface crack. At higher frequency, scattered waves of high-order modes will appear and their WDIC can be successfully extracted through the modal decomposition.

Influence of spatial winding distribution of flexible eddy current sensor on quantitative monitoring of subsurface cracks

This paper proposes a flexible eddy current sensor based on TMR sensor. Firstly, the influence of three kinds of sensors with different spatial winding distributions on the penetration depth of magnetic field is analyzed through theoretical derivation. Then, the finite element model is established to analyze the eddy current distribution on the cross-section of the specimen under different excitation frequency and spatial winding distribution. The distribution of eddy current can be improved by increasing the spatial winding distribution. Finally, the corresponding sensors are prepared for subsurface crack monitoring. The experimental results show that the sensor with a spatial winding distribution of 10 mm can quantitatively monitor the cracks below 3 mm of material surface, and increasing the space winding distribution of the sensor to 20 mm, it can quantitatively monitor cracks below 5 mm of material surface, and the crack monitoring accuracy is consistent with the spacing between excitation lines.

Impacts of Mode Mixity on Controlled Spalling of (100)-Oriented Germanium

Controlled spalling is a technology to prepare single-crystal thin films of semiconductors by fracture with a subsurface crack propagating nearly parallel to the substrate surface. Practical applications require uniform thickness and a smooth surface across the whole film. Both wafer-scale and patterned-stressor-defined small-area spalling of germanium substrates are conducted experimentally and numerically. River line features are observed on spalled surfaces close to lateral edges of the spall, regardless of the spall direction and the size of the spalled area. Three-dimensional finite element method modeling shows the river lines are caused by mixed mode I + III loading near the lateral edges of spall and predicts a spall depth variation near the lateral edges of spall due to mixed mode I + II loading. The absolute range of river lines increases with lateral size of spall, while the relative range of river lines decreases, consistent with variations in mode mixity.

The Influence of Crystal Orientation on Subsurface Damage of Mono-Crystalline Silicon by Bound-Abrasive Grinding.

Subsurface damage (SSD) produced in a grinding process will affect the performance and operational duration of single-crystal silicon. In order to reduce the subsurface damage depth generated during the grinding process by adjusting the process parameters (added), experiments were designed to investigate the influence of machining factors on SSD. This included crystal orientation, diamond grit size in the grinding wheel, peripheral speed of the grinding wheel, and feeding with the intention to optimize the parameters affecting SSD. Compared with isotropic materials such as glass, we considered the impact of grinding along different crystal directions <100> and <110> on subsurface damage depth (added). The Magnetorheological Finishing (MRF) spot technique was used to detect the depth of SSD. The results showed that the depth of SSD in silicon increased with the size of diamond grit. SSD can be reduced by either increasing the peripheral speed of the grinding wheel or decreasing the feeding rate of the grinding wheel in the <100> crystal orientation, if the same size of diamond grit was employed. In addition, we proposed a modified model around surface roughness and subsurface crack depth, which considered plastic and brittle deformation mechanisms and material properties of different crystal orientations. When the surface roughness (RZ) exceeded the brittle-plastic transition’s critical value RZC (RZC<100> > 1.5 μm, RZC<110> > 0.8 μm), cracks appeared on the subsurface. The experimental results were consistent with the predicted model, which could be used to predict the subsurface cracks by measuring the surface roughness. However, the model only gives the approximate range of subsurface defects, such as dislocations. The morphology and precise depth of plastic deformation subsurface defects, such as dislocations generated in the fine grinding stage, needed to be inspected by transmission electron microscopy (TEM), which were further studied.

Review of the Mechanical Properties of High-Strength Alloys at Cryogenic Temperatures

To advance the understanding of metallic materials for structural components at cryogenic temperatures, a review of previous studies on the mechanical properties of austenitic stainless steels, titanium alloys, aluminum alloys, and copper alloys is presented herein. In particular, the evaluation of strength and toughness balance intends to clarify key differences between these alloys to facilitate better alloy design and potentially bolster further alloy development. Several key points on cryogenic mechanical tests are also introduced. Temperature control, testing tools, and test conditions should be properly calibrated to avoid experimental errors. Furthermore, substantial progress for subsurface crack initiation in high-cycle fatigue at cryogenic temperatures is presented, in which the mechanism of subsurface crack generation is highlighted as a key design consideration in microstructure of high-strength alloys.

Tensile and very high cycle fatigue behaviors of a compressor blade titanium alloy at room and high temperatures

Tensile and fatigue behaviors of a compressor blade titanium alloy TC17 at room and high temperatures (RT and HT) have been investigated up to very high cycle fatigue (VHCF) regime. Compared to RT, the HT tensile strength of TC17 titanium alloy decreases, but the elongation remains basically unchanged at HT. The fatigue S–N curves are changed from single linear mode at RT to bilinear mode at HT owing to the temperature effect, and the failure mode is transformed from the surface and subsurface crack initiations at RT to the surface crack initiation at HT. The brittle oxygen-enriched subsurface (BOES) layer on the specimen surface is responsible for the crack initiation at HT. The failure models of oxide shedding and oxide intrusion are proposed. Dislocation distribution indicates that the fatigue failure at HT is insensitive to the intrinsic material microstructure and is only related to the BOES layer on the specimen surface in the VHCF regime. • Very high cycle fatigue behavior of titanium alloy at room temperature and high temperature was investigated. • The oxide layer on the specimen surface is responsible for the fatigue cracks initiation at high temperature. • The dislocation distribution revealed that the material is insensitive to microstructure at high temperature.

Characterization of subsurface cracks in eddy current testing using machine learning methods

AbstractThe exploitation of nondestructive eddy current testing (NDT‐EC) has become a capital necessity. Therefore, the development of a fast tool for the eddy‐current signal inversion is necessary. This paper proposes an inversion of signals coming from the NDT‐EC sensor response using machine learning methods, to reconstruct the length and depth of the defect and obtain its geometric characterization by solving the inverse problem. In this context, a database comprising the impedance of the sensor‐cracked part system (constituting the crack signature) was constructed from a 3D finite element simulation and validated by an experimental companion. The machine learning algorithms were trained using this database. The results show that the defect can be quantified using these developed approaches. The numerical approach can replace the expensive experimental investigation or the optimization algorithm that has a prohibitive computing time. The results show that the approaches developed estimated the desired parameters of the crack with good precision. These different machine learning methods for solving the inverse problem were implemented using the MATLAB software.

Sliding wear behavior of cold-sprayed Ni-WC composite coatings: Influence OF WC content

Metal Matrix Composites reinforced with WC particles and Co or Ni as matrices are well-known for their unique combination of hardness, toughness, and thus superior wear performance. Previous studies showed that the structure of the coating including carbide size and content, and morphology as well as matrix/binder microstructure must be carefully tailored to achieve desirable wear properties. In this study, Ni–WCNi composite coatings were deposited using the cold spray. Agglomerated WCNi powders and spherical Ni powders were used as feedstock materials. It was found that Ni–WCNi coatings can be tailored to contain 13-54 vol% WC and display a varied hardness ranging between 225 and 550 HV1. A higher concentration of WC in initial feedstock led to more compaction of WCNi particles in coatings, the lower mean free path between WC particles, and harder coatings. The sliding wear behavior of coatings was studied with a sliding speed of 3 mm/s under normal loads of 5 and 12 N. Coatings with higher WC fill ratios were found to be more wear-resistant than coatings with less or no WC. This is related to the uniform distribution of WC particles with the lower mean free path that minimized adhesive wear at the initial stages of wear and helped to develop a higher coverage of protective mechanically mixed layers. However, the protective role of mechanically mixed layers was reduced at higher load, caused by subsurface crack propagation at WC particle interfaces, and was related to strain accumulation in highly compacted WC coatings. A qualitative comparison of coating cohesion strengths was performed using scratch tests on the polished cross-section of coatings and correlated to wear properties.

Predicting white etching matter formation in bearing steels using a fretting damage parameter

White etching cracks (WECs) are deleterious artifacts found in fatigue damaged bearings. The formation of WECs has been attributed to the frictional energy dissipation at subsurface cracks during cyclic rolling contact. This study introduces a novel Ruiz fretting damage parameter (FDP) implementation to predict the frictional energy dissipation at subsurface cracks. A parametric study using a finite element model explores the role of crack length, orientation, depth, and coefficient of friction (COF) on FDP. The FDP is maximized at a normalized length of 0.50a and depth of 0.78a, between the orientation of ±30°, at a COF of μ = 0.3, in agreement with experimental observations. These results provide strong evidence of the FDP as a predictor of WEC formation.

Experimental investigation on thermal healing of subsurface damage in borosilicate glass

Subsurface damage (SSD) is the fracture and deformation near the surface of brittle optical materials, caused by surface lapping or grinding. The existence of SSD dramatically influences the performance of optical glass and reduces the laser-induced damage threshold. Subsurface cracks of borosilicate glass can be spontaneously healed when heated under appropriate conditions. In this paper, thermal healing experiments of borosilicate glass (BK7) subsurface cracks are conducted on typical cracks induced by an indentation process, and the effects of the Beilby layer, temperature, crack depth, and water vapor pressure are studied. A semi-empirical relation is obtained through the regression of experimental results to describe the variation of subsurface crack length. Finally, a healing experiment is performed on the subsurface damage formed by grinding. The detection results show both the damage density and maximum damage depth have been reduced after heat treatment, demonstrating the effectiveness of the thermal healing method on eliminating glass subsurface damage.

Fracture analyses of surface asperities during sliding contact

Fracture of interlocking asperities during sliding contact plays a central role in determining friction and wear of brittle materials. The local force to break surface asperities and subsurface crack propagation directly affect the frictional force, wear rate and evolution of surface roughness. Here, we study the fracture failure of an isolated asperity under a lateral contacting force using a systematic set of experiments and numerical simulations. Combining the generalized elastic fracture mechanics and the Euler-Bernoulli beam theory, we also develop an analytical model to predict the critical force at the onset of asperity fracture as a function of the asperity's shape and material properties. The developed model can be used to develop physics-based friction and wear models for brittle solids.