Subsurface Cracks Research Articles

Quantitative detection of lateral subsurface cracks based on laser-generated Rayleigh waves in the frequency domain

Quantitative detection of lateral subsurface cracks based on laser-generated Rayleigh waves in the frequency domain

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.

The influence from PTFE on surface and sub-surface damages of glass fiber reinforced PPS

Polymer composites are common materials for tribological components, even at relatively high operating temperatures. One advantage is the possibility to add low friction additives, such as PTFE, to the base material. In this paper the influence of such additives was studied, with focus on surface and sub-surface damages. The polymer composites were PPS with glass fibers (PPS-F), PPS with glass fibers and PTFE (PPS-L) and PEEK with carbon fibers, graphite and PTFE (PEEK-L). A reciprocating ball-on-disc test set-up, with ball bearing steel balls as counter material, was used with 5 N and 15 N normal load. The tests were run for 2,000 and 10,000 cycles in room temperature, 80 °C and 120 °C. PPS-F showed high friction (µ ≈ 0.4–0.5) and severe surface damage of both the polymer and the counter surface. Sub-surface cracks, detachment of fibers from the polymer and a deformed surface layer was revealed, when studied in cross section. The load and temperature had negligible effect on the friction, however a higher temperature resulted in more surface damage of the PPS-F. PPS-L and PEEK-L showed relatively low friction (µ ≈ 0.1) and superficial surface damage. XPS analysis revealed a thin tribofilm of PTFE in the wear track for PPS-L.

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.

Enhancement of fatigue properties of selective laser melting fabricated TC4 alloy by multiple shot peening treatments

This study investigated the impact of multiple shot peening (SP) treatments on the surface morphology, microstructure, residual stress and fatigue performance of TC4 alloy manufactured by selective laser melting (SLM). The multiple SP treatments were conducted sequentially using cast steel beads, ceramic beads and glass beads at different peening intensities. In comparison to the conventional single SP process, multiple SP treatments reduced surface roughness and stress concentration coefficient while further increasing the residual compressive stress on the alloy surface. SP significantly enhanced the fatigue life of the alloy by 6.4 to 9.6 times, with a more pronounced improvement observed with multiple SP. Fatigue fracture analysis revealed that the untreated alloy showed multiple fatigue sources which all located on the surface, with surface printing defects acting as potential sites for crack initiation. After SP treatments, all fatigue cracks were initiated beneath the surface. The improved fatigue performance of SLM-formed TC4 alloy could be mainly attributed to the SP-induced surface microstructure refinement and residual compressive stresses. Furthermore, results indicated that, for subsurface crack initiation, the initial cracks primarily stemmed from the fracture of α/α' martensite in the primary β columnar grains.

An experimental investigation of micro-channels on transparent material using Nd: YAG laser transmission technique

Among the fastest growing basic materials, polymers have good strength-to-weight ratios, easy to shape, relatively economical, and recyclable which enabled it to become substitutes for metals and alloys in the fields of medical, automobile, aerospace, electronics, thermal and chemical engineering. But a thick transparent PMMA plate creates deep sub-surface cracks due to the burning and charring of polymeric chains, making the operation with Nd: YAG laser radiation extremely difficult. In the current research, a 75 W nanosecond pulsed Nd: YAG laser system is applied to make micro-channel with adequate depth on a thick transparent PMMA plate by using transmission cutting technique. The input process factors considered are pulse width, laser power, cutting speed, and pulse frequency in order to examine the quality characteristics, the depth of cut and heat affected zone (HAZ) in and around the formed micro-channel. To design the experiment of laser transmission channelling of thick PMMA plate of 11 mm, a central composite design (CCD) methodology is used. Due to open-air laser processing, undesirable laser processing effects have been minimised, resulting in a finer and cleaner micro-channel structure with adequate depth of cut, kerf width and minimal Heat Affected Zone (HAZ) width with laser power of around 9 W, pulse frequency of around 33 kHz and cutting speed of 2 mm/sec.

Evolution of point defects in mechanical cracks of fused silica after CO2 laser melting

The traditional polishing method inevitably results in subsurface cracks in the fused silica, which seriously degrades their ultraviolet laser damage resistance. CO2 laser irradiation can melt these cracks and improve their laser induced damage threshold (LIDT). Photoluminescence spectrum and SEM-FIB were employed to investigate the changes in the material microstructure at the crack location with CO2 laser melting. The density of the oxygen-deficient centers of type II (ODC II) defects decreases, while the density of the non-bridging oxygen hole center (NBOHC) defects increases after high-temperature melting. The reason for this change is related to the dihydroxylation reaction and the participation of environmental oxygen in the defect type conversion. The reduction of ODC II defects is most likely the reason for the improvement of LIDT.

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.

On fatigue crack origin with a fine granular area in matrix without defect

Fatigue crack initiation in metallic materials during very high cycle fatigue has been characterized by a subsurface crack origin with defect such as an inclusion. In this paper, fatigue damage behavior in an austenitic stainless steel has been studied using a novel progressive stepwise load increasing test method with each cycle step higher than 108 cycles. Subsurface crack origin with a fine granular area has formed in the matrix without defect. This is a new phenomenon. The mechanism has been investigated using Focused Ion Beam cross-sectioning and electron channeling contrast imaging techniques. Strain localization, grain fragmentation and local plasticity exhaustion are the main factors that cause fatigue damage and crack initiation in the matrix. This study provides a fundamental understanding how material damage and crack initiation occur in material matrix during very high cycle fatigue.

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.

Damage quantification of a metallic beam under thermo-mechanical loads using novel empirical correlations and neural network

This research examines the interactions between fracture depth and crack location on a cantilever beam’s dynamic response under thermomechanical loads. The structure’s stiffness is significantly influenced by temperature, and changes in stiffness can alter the response’s damping, frequency, and amplitude. The basis for measuring damage to an Aluminium 2024 specimen under thermomechanical loads is provided by these variations. Cantilever beams are used in experiments that are carried out at elevated temperatures, such as 50°C, 100°C, 150°C, and 200°C, as well as at room temperature (non-heating). This analysis takes into account a cantilever beam with different initial fracture depths and positions. The outcomes of the experimental, analytical, and numerical work are found to be in good accord. This research mainly fill the gaps of recent techniques for structure health monitoring of metallic structures where the coupled loads exist. Dynamic response formulation is presented experimentally on beam for the first time under thermo-mechanical loads. The dynamic parameters vary with material stiffness, thus; this change is also investigated by introducing the temperature as a key variable using a specially designed temperature controller. A novel technique is presented for damage estimation in metallic structures using the experimental data that is currently accessible which is later compared with the neural network approach. This tool helps in finding out and quantifying damagesusing dynamic response, temperature along withfinding of subsurface cracks. The results establisha clearway of diagnosing the crack growth at any particular instant under thermo-mechanical loads within the operational condition.

Modelling and experimental study of laser-assisted milling of fibre reinforced SiC/Ti-6Al-4V metal matrix composite

Metal matrix composites (MMCs) offer unique advantageous mechanical properties by strengthening a ductile metal matrix with a ceramic reinforcement (e.g., Ti6Al-4 V/SiCf). However, their heterogeneous composition poses machining challenges including fibre pullout, matrix cracking, and increased tool wear. Whilst pre-heating via laser-assisted machining (LAM) shows promise for improving machinability, traditional LAM implementations with a fixed laser spot size and straight laser path prevent uniform heating. By introducing spatially and temporally controlled LAM that generates homogeneous heating by varying the laser scanning velocity, a technique called fully inverse LAM can be applied for MMCs. This involves calculating separate temperature fields for the matrix and reinforcement to minimise thermal mismatch stresses. The fully inverse LAM decreases subsurface cracks and delamination resulting from conventional milling, whilst localised matrix softening reduces cutting forces by over 62 %. Flank tool wear is also diminished, increasing tool life by 120 %. Material analysis reveals reduced machined surface damage, lower surface roughness, and less formation of intermetallic compounds (Ti2C) compared to traditional LAM.

Detecting meso-damage and subsurface cracks in a hard rock using frequency-modulated thermal wave imaging (FMTWI)

Mines, tunnels, and hillside roadways that are subjected to high levels of stress are prone to massive and violent occurrences of rock failures. It results in a multitude of irreversible consequences, including the loss of human lives. Nevertheless, preceding rock failures, the development of micro and macrocracks, which are sometimes not discernible on the surface, takes place. Subsurface cracks indicate the degradation of rock and can be employed as a means to anticipate occurrences of rock failures and bursts. Therefore, the utilization of subsurface imaging techniques for rocks facilitates the estimation of the true strength of the rock mass. Nevertheless, in many instances, rock masses are not easily reachable, posing difficulties for standard techniques such as ground-penetrating radar or computed tomography (CT) scan imaging, to identify the cracks. Hence, this research endeavours to explore the feasibility of employing frequency-modulated thermal wave imaging (FMTWI) for identifying subsurface cracks and their coalescence in hard rocks through the utilization of numerical simulation and experimental methods. A model was constructed using the finite element method wherein artificial cracks were intentionally introduced into a cylindrical granite specimen based on the CT scan data acquired during the meso-damage analysis. The thermograms obtained were subjected to pre-processing and post-processing techniques, and afterwards compared with the CT scan images. The FMTWI tests were conducted in the laboratory to calibrate and validate the simulation results. The findings derived from the analyses of temperature profiles and thermograms indicate that this particular technology is a promising one and offers several advantages in comparison to alternative methods for detecting micro- or macrocracks in deep mines and tunnels.

Integration of digital shearography and FEM for non-destructive evaluation of internal cracks in GRE pipes

Pipes made of glass fiber-reinforced epoxy (GRE) are often exposed to complex environmental conditions, which can make them prone to internal cracking. Quantitative, non-destructive evaluation of cracks can reduce unintended failures and improve system reliability. The use of digital shearography, a modern non-destructive method based on laser interferometry, in the diagnosis of composite materials has drawn attention in recent years. In this paper, a model for the quantitative evaluation of subsurface cracks in various locations on GRE pipes was developed by integrating the finite element method and shearography with thermal stimulation. The response surface method was used to design the simulations, and the effect of crack dimensions and location on the phase map and out-of-plane surface strain component was investigated. The results demonstrated that the crack placement angle relative to the shearography device is effective on crack detectability due to the change in the actual shearing size. A model was then presented to estimate the length and depth of the crack based on the displacement derivative graph by using analysis of variance and regression techniques. By determining the out-of-plane displacement derivative, the proposed model enables precise estimation of the dimensions and location of subsurface cracks.

Surface wear resistance and microstructure characteristic in longitudinal–torsional ultrasonic vibration side milling of GH4169 superalloy

The intrinsic link between the surface wear resistance and microstructural evolution of a material was revealed. The surface wear resistance of the GH4169 superalloy was systematically investigated in relation to the surface machining topography and mechanical properties at different cutting widths in conventional side milling (CSM) and longitudinal–torsional ultrasonic vibration side milling (LTUVSM). The wear resistance was evaluated by analyzing the wear topography, friction coefficient, duration of the initial wear stage (IWS), and wear volume of dry sliding wear. The specimens in LTUVSM exhibited higher wear resistance than those in CSM owing to the inhibition of friction and growth of inter-surface friction nodes by the uniform ultrasonic vibration texture (UVT) and the inhibition of subsurface crack nucleation and extension by the superior mechanical properties. The mechanism of the ultrasonic vibration intervention in grain refinement was revealed. The involvement of ultrasonic vibration enhanced the dislocation accumulation, the grain boundary fractures, and increased the recrystallization fraction, which resulted in a thorough grain refinement with a 7.3 % decrease in average grain diameter (AGD) relative to that obtained with CSM. The increased grain refinement in LTUVSM improved the mechanical properties of the surface-layer material, thereby improving the wear resistance of the specimen.

Ultrasonic array tomography-oriented subsurface crack recognition and cross-section image reconstruction of reinforced concrete structure using deep neural networks

Manual interpretation from technicians with prior expertise knowledge is still needed for ultrasonic array tomography when detecting subsurface damages in reinforced concrete (RC) structures. Ultrasonic scans were usually utilized in qualitative manners that have not been leveraged for quantification purposes in prior studies and practical engineering. This study presented an ultrasonic array tomography-oriented subsurface crack recognition and pixel-wise cross-section image reconstruction method for RC structures based on deep neural networks (DNN). B-scans were acquired by testing entity RC components with preset artificial cracks using shear-wave low-frequency ultrasonic array. A DNN with the basic encoder-decoder architecture was developed, which was improved by introducing skip connections and residual modules to adapt to semantic structure of ultrasonic B-scans. A supervised generative adversarial DNN was proposed to generate fake but convincing B-scans for data augmentation. Results reported that F-scores were higher than 78 %, and satisfactory agreements were observed between ground truth annotations and reconstructed cross-section images, substantiating the effectiveness of the proposed method. Furthermore, with the physical information-assisted registration, individual local reconstructed cross-section images were combined to global representations indicating crack distribution in the entire component to facilitate convincing structural assessments.

Effects of processing parameters and grinding direction on the material removal mechanism of (100) surface single crystal diamond in self-rotating mechanical grinding

The current commonly used method for diamond processing is mechanical polishing using the cast iron grinding wheel (Scaife). However, this method is challenging to meet the processing requirements of semiconductor applications. This study proposes a more efficient grinding method that employs a high-speed self-rotating grinding device, coupled with a bonded abrasive diamond grinding wheel with high self-sharpening properties, to achieve high precision grinding of single crystal diamonds. Different cutting depths, grit sizes, and linear velocities are employed in the processing, and the material removal mechanism is studied through morphology characterization, element content, and other detection methods. The experimental results indicate that the material removal method of diamond (100) crystal surface is influenced by the cutting depth. It is a collaborative removal process in the 〈110〉 and 〈100〉 directions, and the brittle-ductile transition in the <110> direction significantly affects the machining quality. Surface fractures and subsurface cracks primarily occur through cleavage along the (111) plane, so a method to distinguish the orientation of the (100) crystal plane is proposed. Additionally, this study demonstrates the influence of different processing directions on the surface of the diamond within the same plane. By using this method, the surface roughness of the 7 × 7 mm as-grown crystal can be reduced to Ra 0.523 nm within 10 min, with a material removal rate of 32.4 μm/h.

Polarized laser scattering detection of low-density and micron-scale subsurface cracks in silicon wafer

Subsurface cracks (SSCs) of silicon wafers is inevitably generated during grinding due to the hard and brittle characteristics of the material. This paper studied a polarized laser scattering (PLS) detection method for low-density and micro-scale SSCs in silicon wafer. Samples with specific depths and densities of SSCs were prepared by using micro-indentation method. A PLS system was established to detect SSCs on the prepared samples. A numerical model of laser depolarization at SSCs was developed by using finite difference time-domain (FDTD) method. The depolarization mechanism at different inclination angles of SSCs is revealed. The correlation between the depth and density of SSCs and PLS detection signal is investigated experimentally. The results show that the PLS system could detect the SSCs in a low-density level and micron-scale depth. The developed PLS method could provide guidance to determination of the material removal amount in the subsequent ultra-grinding or polishing processes.

Characterizing and controlling infrared phonon anomaly of bilayer graphene in optical-electrical force nanoscopy

We, for the first time, report the nanoscopic imaging study of anomalous infrared (IR) phonon enhancement of bilayer graphene, originated from the charge imbalance between the top and bottom layers, resulting in the enhancement of E1u mode of bilayer graphene near 0.2 eV. We modified the multifrequency atomic force microscope platform to combine photo-induced force microscope with electrostatic/Kelvin probe force microscope constituting a novel hybrid nanoscale optical-electrical force imaging system. This enables to observe a correlation between the IR response, doping level, and topographic information of the graphene layers. Through the nanoscale spectroscopic image measurements, we demonstrate that the charge imbalance at the graphene interface can be controlled by chemical (doping effect via Redox mechanism) and mechanical (triboelectric effect by the doped cantilever) approaches. Moreover, we can also diagnosis the subsurface cracks on the stacked few-layer graphene at nanoscale, by monitoring the strain-induced IR phonon shift. Our approach provides new insights into the development of graphene-based electronic and photonic devices and their potential applications.

Insight into crack propagation induced by fiber orientations during single grain scratching of SiCf/SiC composites using FEM

Due to the multiphase structure and anisotropy of ceramic matrix composites (CMCs), crack propagation is extremely complex during grinding. However, there is no full comprehension of the crack propagation law of CMCs in varying fiber directions, which limits high-quality CMCs machining. Based on experimental and numerical results, the effect of fiber directions on crack propagation of SiCf/SiC composites was analyzed. Results demonstrate that the change in crack propagation path is the primary reason why the removal behavior of CMCs is different from traditional ceramics. For CMCs, the interface is the priority path of crack propagation no matter along any fiber direction. Besides, the fiber orientation determines the propagation paths of radial and subsurface cracks, which affects material removal behavior. Comparing the scratched groove, the groove edge damage is the most severe when scratching along the fiber transverse direction, whereas the scratched groove surface quality is best in the perpendicular direction.