Shallow Scratch Research Articles

Quasi-brittle fracture criterion of CFRP with shallow surface scratch based on boundary effect model

This study investigates the quasi-brittle fracture parameters of carbon fiber composites with shallow surface scratches by considering single-layer prepreg thickness as the characteristic composite microstructure. Three-point bending fracture tests were conducted on single-edge notched specimens of various-sized carbon fiber composites. The boundary effect model was employed to establish the relationship between the microstructure and macroscopic mechanical characteristics of carbon fiber composites. The maximum fracture load was used to determine the quasi-brittle fracture characteristics. By employing normal distribution analysis, the tensile strength and fracture toughness of each specimens were determined asft = 453.28 MPa andKIC = 22.2 MPa √ m, respectively. The analysis results exhibited an error of only 0.39 % compared to the least-squares fit, and encompassed almost all discrete points of the specimens within a 95 % reliability range. Using standard laboratory dimensions, fracture intervals for different notched depths at the same thickness can be predicted. Furthermore, the fracture parameters demonstrated an increasing trend within a certain range as the crack-thickness ratio increases, which aligns with the theoretical findings.

Scratch-induced micro-fracture behaviour and translational failure of glass: An experimental and numerical study

The structural safety of aged glass members shows significant sensitivity to surface scratches. The distribution and geometrical features of scratch impacts the glass fracturing. In this study, the fracture behaviour of scratched thin glass pane was investigated by the ordinary state-based peridynamic (OSB-PD) theory. Coaxial double ring test was first conducted to identify the mechanical and fracture behaviour of scratched glass plates that serve as the benchmark. The applicability of simplifying and reconstructing the actual scratches in numerical models was then examined. The refined three-dimensional fracture process from micro-crack coalescence to macro-propagation of featured cracks can be well described by the developed PD model. The propagation of in-plane radial cracks was found prior to median cracks along the thickness for scratched glass. Crack branch was observed due to shallow scratch depth and the primary crack branching point lies near the edge of the scratch. The transition from scratch-induced failure to strength-induced failure due to particular scratch spatial location was also tackled, which benefits the accurate performance assessment of scratched glass.

One-shot colored reflectance direction field imaging system for optical inspection.

Detecting microscale defects on the surface of an object is often difficult with conventional cameras. Microscale defects are known to greatly affect the bidirectional reflectance distribution function (BRDF) of light rays reflected from the surface. Therefore, an imaging system for capturing the reflectance direction field by color mapping using a multicolor filter placed in front of an imaging lens is proposed, which can have a simple structure. From the color variations of light rays passing through several different color regions of the multicolor filter, this imaging system can detect the extent of broadening of the BRDF. The effectiveness of the imaging system for optical inspection is experimentally validated by testing it on a plastic surface that has a shallow scratch with a depth of a few micrometers.

Quasi-brittle fracture performance of glass fiber composites based on boundary effect model

Light surface scratch around 50 μm or less in-depth was a formidable engineering challenge for the application of quasi-brittle materials. In the present study, a boundary effect model (BEM) was used to analyze the fracture performance of glass fiber reinforced polymer (GFRP) composites with shallow surface scratch. The ply thickness of single-layer prepreg was introduced as the controlling GFRP microstructure parameter or characteristic composite unit Cch of the model, and the parameter of ply thickness was set to 1.2 mm. Quasi-brittle transverse fracture parameters of the tensile strength ft and the fracture toughness KIC of GFRP were discussed. Finally, the average values of the fracture parameters uf and uk of the GFRP were obtained by normal distribution analysis. The means, and upper and lower bounds of the fracture parameters were within the range of 95% reliability covering almost all test discrete points. The laboratory samples of conventional sizes were used to predict the fracture parameters of GFRP. The result reflected that the error between the test results of all samples is only 5.7%. In addition, as the ratio α of the gap height of the GFRP sample increases, the fracture parameters showed a trend of increase firstly and then declining. The average fracture parameter estimated value has decreased by 14.5%, which was in good agreement with the test results.

A simple closed-form model for fracture of CFRP panel with shallow surface scratch confirmed by comprehensive flexural/tensile tests

Comprehensive tests using more than 200 CFRP laminate specimens with and without shallow surface scratches (20 μm - 3 mm) have been performed under three-point-bending (3-p-b) and direct tensile conditions. Critical flexural loads of scratched 3-p-b specimens have been linked to the tensile strength ft of CFRPs by a simple closed-form solution of a non-Linear Elastic Fracture Mechanics (non-LEFM) model for composite failures, which models the crack tip damage using the characteristic composite microstructure Cch (ply thickness of CFRP). The average ft measured from 30 direct tensile tests is virtually identical to that from 170 different 3-p-b tests with only 2% relative error, indicating potential applications of the simple closed-form solution in structural integrity analysis for scratched CFRP components. The statistical reliability has been combined into the predictive model, which can be important when not only the mean strength but also the prediction interval is required.

Multiscale frictional behaviors of sp2 nanocrystallited carbon films with different ion irradiation densities

sp2 nanocrystallited carbon films with large nanocrystallite sizes, smooth surfaces, and relative high hardness were prepared with different ion irradiation densities regulated with the substrate magnetic coil current in an electron cyclotron resonance plasma sputtering system. Their multiscale frictional behaviors were investigated with macro pin-on-disk tribo-tests and micro nanoscratch tests. The results revealed that, at an ion irradiation density of 16 mA/cm2, sp2 nanocrystallited carbon film exhibits the lowest friction coefficient and good wear resistant properties at both the macroscale and microscale. The film sliding against a Si3N4 ball under a contact pressure of 0.57 GPa exhibited a low friction coefficient of 0.09 and a long wear life at the macroscale. Furthermore, the film sliding against a diamond tip under a contact pressure of 4.9 GPa exhibited a stable low friction coefficient of 0.08 with a shallow scratch depth at the microscale. It is suggested that sp2 nanocrystallites affect the frictional behaviors in the cases described differently. At the macroscale, the contact interface via the small real contact area and the sp2 nanocrystallited transfer layer dominated the frictional behavior, while the sp2 nanocrystallited structure in the film with low shear strength and high plastic resistivity, as well as the smooth surface morphology, decided the steady low nanoscratch properties at the microscale. These findings expand multiscale tribological applications of sp2 nanocrystallited carbon films.

Fracture evolution in coalbed methane reservoirs subjected to liquid nitrogen thermal shocking

Thermal shocking effect occurs when the coalbed methane (CBM) reservoirs meet liquid nitrogen (LN2) of extremely low temperature. In this study, 3D via X-ray microcomputer tomography (µCT) and scanning electron microscope (SEM) are employed to visualize and quantify morphological evolution characteristics of fractures in coal after LN2 thermal shocking treatments. LN2 thermal shocking leads to a denser fracture network than its original state with coal porosity growth rate increasing up to 183.3%. The surface porosity of the μCT scanned layers inside the coal specimen is influenced by LN2 thermal shocking which rises from 18.76% to 215.11%, illustrating the deformation heterogeneity of coal after LN2 thermal shocking. The cracking effect of LN2 thermal shocking on the surface of low porosity is generally more effective than that of high surface porosity, indicating the applicability of LN2 thermal shocking on low-permeability CBM reservoir stimulation. The characteristics of SEM scanned coal matrix in the coal powder and the coal block after the LN2 thermal shocking presented a large amount of deep and shallow progressive scratch layers, fracture variation diversity (i.e. extension, propagation, connectivity, irregularity) on the surface of the coal block and these were the main reasons leading to the decrease of the uniaxial compressive strength of the coal specimen.

Friction and wear properties of phenolic composites with dual inorganic oxide‐modified titanate whiskers

AbstractInorganic oxides, alumina and silica, were successively modified on titanate whiskers surface. Enrolled into phenolic resin, the phenolic/whisker composites were prepared by melting molding, with intentions for wearing property. The alumina‐coated whisker surface fulfilled wear resistance, while the outer silica layer was less stiff for compatible with polymer matrix. The dual surface modifications improved the whiskers dispersion in the matrix, enhanced interfacial adhesion, and significantly triggered wear resistance of the composites. With 7.5 wt% multiply modified whiskers involved, the phenolic composite showed specific wear rate of 0.49 × 10−5 mm3/Nm, remarkably lower than that of phenolic resin (4.79 × 10−5 mm3/Nm). The worn surface of the composites detected by a scanning electron microscope showed abrasive wear with slight fatigue wear. Optical microscope images indicated that the whisker‐filled composite had a shallow scratch on the worn surface, which positively favored wear resistance.

Mechanical property of YCF101 coating under different overlap modes by laser cladding

The specimens with three different overlap modes were successfully manufactured. The mechanical properties of the clad layer were measured along the scanning direction and z-axis direction respectively. The results show that the increased uniformity of remelting temperature distribution made the cracks and pores decreased and the microstructure refined. The coatings had a ower friction coefficient, and shallow and narrow scratch along the scanning direction. YCF101 alloy is a plastic material and the compressive property of the clad layer was improved by the uniformity of grain. Improving the consistency of the clad track and the loading direction can improve the coating compressive performance.

Critical bending load of CFRP panel with shallow surface scratch determined by a tensile strength model

Carbon fibre reinforced polymer (CFRP) composite structures may experience light surface damage such as shallow surface scratch around 50 μm or less in applications. In this study, critical bending loads of 2D woven CFRP panels with and without shallow surface scratches were assessed under three-point-bending (3-p-b) conditions and then modelled by a simple strength theory for composite fracture. This closed-form tensile strength solution is a non- Linear Elastic Fracture Mechanics (LEFM) model derived specifically for heterogeneous composites. Direct tensile tests of the CFRP panels (without shallow surface scratch) were also performed, and measured tensile strength was compared with that from 3-p-b tests. The relative error between the two different tests was only around 6.7%. The ply thickness around 140 μm was selected as the controlling composite microstructure parameter or characteristic composite unit Cch in this non-LEFM model. This simple model shows that a suitable composite characteristic microstructure measurement such as Cch = ply thickness can significantly simplify composite fracture analysis. A simple statistical analysis using a 2D normal distribution methodology for strength measurements was also introduced so that the experimental scatters can be analysed. The 95% reliability band can be conveniently used in safe design of composite structures.

Experimental neoichnology of crawling stalked crinoids

Stalked crinoids have long been considered sessile. In the 1980s, however, observations both in the field and of laboratory experiments proved that some of them (isocrinids) can actively relocate by crawling with their arms on the substrate, and dragging the stalk behind them. Although it has been argued that this activity may leave traces on the sediment surface, no photographs or images of the traces produced by crawling crinoids have been available. Herein, we present results of neoichnological experiments using the shallowest species of living stalked crinoid, Metacrinus rotundus, dredged from Suruga Bay (near the town of Numazu, Shizuoka Prefecture, ~ 140 m depth). Our results demonstrate that isocrinids produce characteristic locomotion traces, which have some preservation potential. They are composed of rather deep and wide, sometimes weakly sinuous, central drag marks left by the stalk and cirri, and short, shallow scratch marks made by the arms. Based on the functional morphology and taphonomy, it has been argued that the ability to autotomize the stalk and relocate had already evolved in the oldest stem-group isocrinids (holocrinids), likely in response to increased benthic predation pressure during the so-called Mesozoic marine revolution. Our data show that this hypothesis may be corroborated in the future by ichnological findings, which may provide more direct proof of active locomotion in Triassic holocrinids.

Observation of a Latent Scratch on Chemo-Mechanical Polished 4H-SiC Wafer by Mirror Projection Electron Microscopy

A latent scratch which is an extremely shallow scratch induced on a SiC wafer during chemo-mechanical polishing (CMP) has been investigate by mirror projection electron microscopy (MPJ), low-energy scanning electron microscopy (LESEM), atomic force microscopy (AFM) and scanning transmission electron microscopy (STEM). The latent scratch, which is difficult to detect by using optical microscopes, was easily visualized by MPJ as a high contrast dark line. The morphology of detected latent scratch is less than 1nm in depth and about 30nm in full width at half depth by AFM evaluation. The STEM observation revealed the latent scratch was accompanied two dislocation arrays. One contains loop-like dislocations and the other contains spiky dislocations, both lying in the wafer at a few ten nm in depth.

Dark-field detection method of shallow scratches on the super-smooth optical surface based on the technology of adaptive smoothing and morphological differencing

There exist some shallow scratch defects on the super-smooth optical surface. Their detection has a low efficiency with the existing technologies. So a new detection method, dark-field detection of adaptive smoothing and morphological differencing (DFD-ASMD), is proposed. On one hand, the information of shallow scratches can be kept in dark-field images. On the other hand, their weak characteristics can be separated and protected from being overly reduced during the elimination of noise and background in the image. Experiments show the detection rate of shallow scratches is around 82%, and DFD-ASMD can lay a foundation for quality control of defects on the high-quality optical surface.

Rebuttal: Shallow and deep scratch tests as powerful alternatives to assess the fracture properties of quasi-brittle materials

We recall the principles of Dimensional Analysis. We do not violate the principle of geometrical similarity: the Size Effect Law can be applied to dissimilar specimens when the energy release function is known. By considering a two-dimensional and a three-dimensional similitude case, we can predict the fracture toughness using scratch tests. Finally, we recall our Scratch Fracture Mechanics model that was validated via an exhaustive campaign of numerical simulations. In sum, whether deep or shallow, the ductile-to-brittle transition in scratch tests is governed by the out-of-plane width and the width-to-depth ratio.

Rebuttal: Shallow wide groove scratch tests do not give fracture toughness

Lin and Zhou (2013) recently showed that by using the depth of cut as a size measure, cutting forces from scratch tests on a quasibrittle material, such as rocks, followed Bažant’s simple size effect law (SEL). It was clear that LEFM applied only at deep cutting which might not be attainable. Akono et al. (2014) disagreed and contended that wide shallow groove cuts gave KIC based on an SEL that was scaled by width. We show herein that their conclusions were not supported by data and that the presence of sidewall friction on groove cuts further complicates efforts to obtain KIC.

Influence of mechanical scratch on the recorded magnetization stability of perpendicular recording media

Stability of recorded magnetization of hard disk drives (HDDs) is influenced by external environments, such as temperature, magnetic field, etc. Small scratches are frequently formed on HDD medium surface upon contacts with the magnetic head. Influence of temperature and mechanical scratch on the magnetization structure stability of perpendicular recording media was investigated by using a magnetic force microscope. The magnetic bit shape started to change at around 300 °C for an area with no scratches, whereas for the area near a shallow mechanical scratch it started to change at a lower temperature around 250 °C. An analysis of magnetization structure under an influence of temperature and mechanical scratch is carried out for the magnetization structure variation and recorded magnetization strength.

Analytical Approach to Elimination of Surface Micro-Defects in Forging

Initial surface micro-scratches due to cropping, rolling or handling change that cause crack or lap defects during forging and reduce the quality of forged products. Elimination of surface micro-defects in forging is pressing problem in forging industry. A new method of finite element analysis featuring effective meshing and an adaptive remeshing system is applied to clarify the mechanism of the formation or the disappearance of surface micro-defects in forging. V-shaped and rectangular scratches on the surface of billet are simulated under the axisymmetric assumption and the basic deformation of scratch during upsetting and the influence of friction on the deformation of micro-defects are discussed. Shallow V-shaped scratch on the top surface of billet tends to disappear and deep scratch becomes lap defect during upsetting. Defects are more likely to disappear under low friction. It is possible to predict that an initial surface micro-scratch on the billet remain or not as a defect after forging.

A Scratch Intersection Model of Material Removal During Chemical Mechanical Planarization (CMP)

A scratch intersection based material removal mechanism for CMP processes is proposed in this paper. The experimentally observed deformation pattern by SEM and the trends of the measured force profiles (Che et al., 2003) reveal that, for an isolated shallow scratch, the material is mainly plowed sideway along the track of the abrasive particle with no net material removal. However, it is observed that material is detached close to the intersection zone of two scratches. Motivated by this observation, it is speculated that the deformation mechanism changes from ploughing mode to shear-segmentation mode as the abrasive particle approaches the intersection of two scratches under small indentation depth for ductile metals. The proposed mechanistic material removal rate (MRR) model yields Preston constant similar to those observed experimentally for CMP processes. The proposed model also reveals that the nature of the slurry-pad interaction mechanism, and its associated force partitioning mechanism, is important for determining the variation of MRR with particle size and concentration. It is observed that under relatively soft pads, small particles and low particle concentration, the pad undergoes local deformation, yielding an increased MRR with increasing particle size and concentration. At the other extreme, the intact walls of the surface cells and the connecting cell walls between the surface pores deform globally, resembling a beam or a plate, and a decreasing trend in MRR is observed with increasing particle size and concentration. The predicted MRR trends are compared to existing experimental observations.

Effects of surface defects on the performance of 4H– and 6H–SiC pn junction diodes

Effects of surface defects on performance of kV-class 4H– and 6H–SiC epitaxial mesa pn junction diodes were investigated. Mapping studies of surface morphological defects have revealed that triangular-shaped defects severely degrade high-blocking capability of the diodes whereas shallow round pits and scratch give no direct impact. The perimeter recombination and generation, instead of the bulk process, are responsible for forward recombination current and reverse leakage current of the diodes, respectively. Effective minority carrier lifetimes are mainly limited not by bulk recombination but by perimeter recombination.

Performance limiting surface defects in SiC epitaxial p-n junction diodes

Effects of surface defects on performance of kV-class 4H- and 6H-SiC epitaxial p-n junction diodes were investigated. The perimeter recombination and generation, instead of the bulk process, are responsible for forward recombination current and reverse leakage current of the diodes, respectively. Mapping studies of surface morphological defects have revealed that triangular-shaped defects severely degrade high-blocking capability of the diodes whereas shallow round pits and scratch give no direct impact. Device-killing defects in SiC epilayers are discussed based on breakdown voltage mapping. Effective minority carrier lifetimes are mainly limited not by bulk recombination but by perimeter recombination.