Increase In Crack Density Research Articles

A Highly Sensitive Bending Sensor Based on Controlled Crack Formation Integrated with an Energy Harvesting Pyramid Layer

AbstractA highly sensitive bending sensor composed of patterned Pt lines, integrated with energy harvesting capability, is reported. The sensitivity of the bending sensor increases as the width of the Pt lines decreases, owing to the increase in crack density with decreasing line width. Furthermore, sensitivity increases with increasing bending cycles, but saturates at around 1000 cycles. Such a behavior corresponds to the increase and eventual saturation of crack density with increasing bending cycles. A microstructured polydimethylsiloxane layer is placed on top of the Pt lines to serve as a triboelectric energy harvesting layer, where human skin and the Pt lines are utilized as electrodes. Voltage and current of 18.6 V and 209 nA are generated, respectively, from gentle finger tapping. These demonstrations make the device highly useful for a wide variety of portable and wearable flexible electronic applications.

Microstructural Control of Physical Properties During Deformation of Porous Limestone

AbstractWe performed triaxial deformation experiments in Purbeck limestone (13.8% average porosity) across the brittle‐ductile transition and monitored the evolution of permeability and wave velocities as a function of strain. In the brittle regime, the rock yields in dilation. In the ductile regime, the rock first yields in compaction and then undergoes net dilation at some critical level of strain. The permeability increases after failure in the brittle regime and decreases with increasing compaction in the ductile regime. The wave velocities decrease with increasing strain, and the material becomes transversely isotropic. At axial strains of the order of 5%, the anisotropy parameters (from Thomsen, 1986) are around ε ≈ −0.2 and δ ≈ decrease with increasing axial strain beyond the yield point−0.3. Under hydrostatic conditions, the rock also yields in compaction. The hydrostatic yield point is not marked by any significant drop or increase in wave velocity during loading, but wave velocities decrease (and therefore crack density increases) significantly upon unloading. In all our tests, the permeability change is proportional to the initial porosity change until the point of net dilation is reached. The strain at that point also acts as a scaling factor for the relative drop in P and S wave velocities during deformation. All our experimental data point to a disconnection between the evolution of permeability, porosity, and wave velocities during deformation in the ductile regime: permeability is controlled by a fraction of the micropore network, while wave velocities are mostly influenced by microcracks that do not contribute significantly to either the total rock porosity or fluid flow properties.

Fracture characterization of inhomogeneous wrinkled metallic films deposited on soft substrates

This study investigated the fracture properties of wrinkled metallic films on a polydimethylsiloxane (PDMS) soft substrate. In particular, the crack density of the wrinkled film during tensile deformation was examined. In order to achieve better deformability of metallic thin films, a method to fabricate a wrinkled thin film on a PDMS soft substrate was first established. The copper (Cu) nano-film fabricated in this study possessed a wrinkled geometry, which plays a critical role in determining the extent of large elastic deformation. To create the wrinkled structure, wet-etching with a polymeric sacrificial layer was used. A sacrificial layer was first deposited onto a silicone rubber sheet. During the curing process of the layer, a compressive strain was applied such that the hardened surface layer buckled, and a wrinkled form was obtained. Subsequently, a PDMS solution was used to cover the layer in order to form a wrinkled PDMS substrate. Finally, the Cu film was deposited onto the wrinkled PDMS, such that the wrinkled Cu film on a soft PDMS substrate was fabricated. The use of uni-axial tensile tests resulted in film crack generation at the stress concentration zone in the wrinkled structure of the films. When the tensile loading was increased, the number of cracks increased. It was found that the increase in crack density was strongly related to the inhomogeneous nature of the wrinkled structure. Such a trend in crack density was investigated using FEM (finite element method) computations, such that this study established a simple mechanical model that may be used to predict the increase in crack density during tensile deformation. This model was verified through several experiments using various wrinkle patterns. The proposed mechanical model may be useful to predict the crack density of a wrinkled metallic film subject to tensile loading.

Effect of cristobalite formation on the delamination resistance of an oxide/Si/(SiC/SiC) environmental barrier coating system after cyclic high temperature thermal exposure

Delamination toughness of a mullite/Si/(SiC/SiC) environmental barrier coating (EBC) system has been measured using a shear-loading test. The EBC system is initially heated to a temperature of 1470°C (above the melting point of Si (1414°C)), and then repeated thermal exposure is carried out at a temperature 1320°C (below the melting point of Si) for 2h/cycle. Change of the delamination toughness with heat exposure is successfully obtained using the shear-loading test. Formation of cristobalite and transformation from α↔β during thermal cycling cause an increase of crack density in the Si bond coat layer and a resulting reduction in delamination toughness.

Measurements of Seismic Anisotropy in Synthetic Rocks with Controlled Crack Geometry and Different Crack Densities

Seismic anisotropy can help to extract azimuthal information for predicting crack alignment, but the accurate evaluation of cracked reservoir requires knowledge of degree of crack development, which is achieved through determining the crack density from seismic or VSP data. In this research we study the dependence of seismic anisotropy on crack density, using synthetic rocks with controlled crack geometries. A set of four synthetic rocks containing different crack densities is used in laboratory measurements. The crack thickness is 0.06 mm and the crack diameter is 3 mm in all the cracked rocks, while the crack densities are 0.00, 0.0243, 0.0486, and 0.0729. P and S wave velocities are measured by an ultrasonic investigation system at 0.5 MHz while the rocks are saturated with water. The measurements show the impact of crack density on the P and S wave velocities. Our results are compared to the theoretical prediction of Chapman (J App Geophys 54:191–202, 2003) and Hudson (Geophys J R Astron Soc 64:133–150, 1981). The comparison shows that measured velocities and theoretical results are in good quantitative agreement in all three cracked rocks, although Chapman’s model fits the experimental results better. The measured anisotropy of the P and S wave in the four synthetic rocks shows that seismic anisotropy is directly proportional to increasing crack density, as predicted by several theoretical models. The laboratory measurements indicate that it would be effective to use seismic anisotropy to determine the crack density and estimate the intensity of crack density in seismology and seismic exploration.

Simulation of acoustic wave propagation in a borehole surrounded by cracked media using a finite difference method based on Hudson’s approach

Cracked media are a common geophysical phenomena. It is important to study the propagation characteristics in boreholes for sonic logging theory, as this can provide the basis for the sonic log interpretation. This paper derives velocity–stress staggered finite difference equations of elastic wave propagation in cylindrical coordinates for cracked media. The sound field in the borehole is numerically simulated using the finite-difference technique with second order in time and tenth order in space. It gives the relationship curves between the P-wave, S-wave velocity, anisotropy factor and crack density, and aspect ratio. Furthermore, it gives snapshots of the borehole acoustic wave field in cracked media with different crack densities and aspect ratios. The calculated results show that in dry conditions the P-wave velocity in both the axial and radial directions decreases, and more rapidly in the axial direction while the crack density increases. The S-wave velocity decreases slowly with increasing crack density. The attenuation of the wave energy increases with the increase in crack density. In fluid-saturated cracked media, both the P-wave and S-wave velocity increases with the aspect ratio of the cracks. The anisotropy of the P-wave decreases with the aspect ratio of the cracks. The aspect ratio of the crack does not obviously affect the energy attenuation.

Analysis of Microdamage in Thermally Aged CF/Polyimide Laminates

Microdamage in the layers of CF Thornel® T650 8-harness satin-weave composites with a thermosetting NEXIMID® MHT-R polyimide resin, designed for high service temperatures, is analyzed. After cooling down to room temperature (RT), a multiple intrabundle cracking due to tensile transverse thermal stresses was observed in the [(+45/–45)/(90/0)]2s laminates studied. Then, the composite was subjected to two ramps of thermal cycling quantifying the increase in crack density in its layers. A comparison of two ramps with the same lowest temperature showed that the highest temperature in the cycle where thermal stresses were low had a significant detrimental effect on the thermal fatigue resistance of the composite. The effect of holding it at 288°C for 40 days was also studied: many new cracks formed in it after cooling down to RT. During the time at the high temperature, the mechanical properties degraded with time, and the crack density versus aging time was measured at RT. Then, both aged and nonaged specimens were tested in uniaxial quasi-static tension quantifying the damage development in layers of different orientation. Cracking in the layers was analyzed using fracture mechanics arguments and probabilistic approaches: a) a simple one, not considering crack interaction; b) Monte Carlo simulations. It is shown that cracking in the off-axis layers which are not in contact with the damaged 90°-layer can be predicted based on the Weibull analysis of the 90°-layer, whereas in the off-axis layer contacting the 90°-layer, the crack density is much higher due to the local stress concentrations caused by cracks in the 90°-layer. The thermal treatment degraded the cracking resistance in the surface and adjacent layer, whereas the composite close to the midplane was not changed.

Dynamic electromechanical behavior of single-crystal α-quartz

Electrical response from dynamic compression tests on x-cut single crystal α-quartz specimens at strain rates of 103 s−1 reveal nonsimilar stress-charge behavior during damage evolution. Specifically, when quartz is undergoing extensive and irreversible dynamic brittle fracture under a compressive stress impulse of up to 1.8 GPa, the effective piezoelectric stress coefficient (d) exhibits approximately a 24% increase from loading to unloading. The experimental results are examined in the framework of the theory of linear piezoelectricity in order to understand the role of increasing crack density and crystal orientation on electroelastic properties.

Accelerated aging effects on carbon fiber/epoxy composites

Abstract The influence of operational environments on the long-term durability of structural components fabricated with carbon fiber reinforced composites is an ongoing concern. Exposures to ultraviolet radiation, temperature cycles and moisture are known to degrade the polymeric matrix. In this work, carbon-epoxy composites were subjected to accelerated aging in an aging chamber with controlled conditions of temperature, humidity and UV-radiation. Changes within the material are evaluated by Fourier-Transform Infrared (FTIR) Spectroscopy, Dynamic Mechanical Analysis (DMA), interlaminar shear strength and compressive strength, Scanning Electron Microscopy (SEM), and also in terms of mass variation. Although significant changes in mechanical properties were not observed, the effects of accelerated aging on the composite material were evidenced by mass loss, fiber exposure, chemical alterations, increased crack density in interlaminar shear tests and fiber buckling in fractured specimens after compression testing.

Microcracking in thermally cycled and aged Carbon fibre/polyimide laminates

Carbon fibre T650 8-harness satin weave fabric composites with thermosetting polyimide resin designed for high service temperatures are solidified at 340°C. High thermal stresses develop after cooling down to room temperature, which lead to multiple cracking in bundles of the studied quasi-isotropic composite. The composites are subjected to two thermal cycling ramps and the increase of crack density in each bundle is quantified. Comparison of two ramps with the same lowest temperature shows that the highest temperature in the cycle has a significant effect on thermal fatigue resistance. During thermal aging tests at 288°C the mechanical properties are degrading with time and the crack density after certain aging time is measured. Aging and fatigue effects are separately analysed showing that part of the cracking in thermal cycling tests is related to material aging during the high temperature part of the cycle. Numerical edge stress analysis and fracture mechanics are used to explain observations. The 3-D finite element edge stress analysis reveals that there is large edge effect that induces a large difference in the damage state between the different layers on the edge. The linear elastic fracture mechanics explains the higher initiated and propagated crack density in the surface layers comparing to the inner layers.

Analysis of the Cutting Features of the Damaged Rock by Shock Disturbance

This article aims to study the effects of shock-disturbed cracks on the cutting features of cone bits. By utilizing the Walsh’s model, a crack model was established for the damaged rock surrounding the induced hole, the intrusive coefficient of the cone bit was derived, and the drilling rate equation around the induced hole was formulated. The effects of crack density and disturbance frequency on elastic modulus, intrusive coefficient, and cutting force were studied. With an increase in crack density, both the effective elastic modulus and the intrusive coefficient decreased. Under a certain range of internal friction angle, with an increase in crack density, the drilling rate increased. Under a certain axial load, as the disturbance frequency increased, the cutting force of one-cone bits decreased.

Microdamage analysis in thermally aged CF/polyimide laminates

Microdamage in layers of CF Thornel® T650 8-harness satin woven composite with thermosetting polyimide NEXIMID® MHT-R resin was analysed. After cooling to room temperature multiple intra-bundle cracking due to tensile transverse thermal stresses was observed in the studied [(+45/-45)/(90/0)]2s composite. The composite was subjected to thermal cycling quantifying the increase of crack density in layers. Comparison of two ramps with the same lowest temperature shows that the highest temperature in the cycle has a significant detrimental effect. Exposure for 40 days to 288°C caused many new cracks after cooling down to room temperature. Both aged and not aged specimens were tested in uniaxial quasi-static tension. Cracking was analysed using fracture mechanics and probabilistic approaches. Cracking in off-axis layers was predicted based on Weibull analysis of the 90- layer. The thermal treatment degraded the cracking resistance of the surface layer and of the next layer.

Mechanical damage of hard chromium coatings on 416 stainless steel

Abstract Hard chromium coatings are widely used for high performance components due to their superior wear and corrosion resistance. In this study, 416 stainless steel bars were chromium electroplated commercially with different thickness. Mechanical properties and residual stress were measured using a nano-indentation system and X-ray diffraction, respectively. Image analysis technique was used to measure inherent crack density. Residual stress in the coating increases with increasing thickness leading to an increase in inherent crack density. An indentation study was carried out to investigate the mechanical damage of hard chromium coating as a function of thickness. Different failure modes were observed when hard chromium coatings were indented with a spherical indenter including Hertzian cone cracks, radial cracks, bend cracks, lateral cracks and coating delamination. Three thickness regions of distinctive cracking modes were identified. Thin coating region exhibited through-thickness Hertzian cracks. Hertzian and coating-bend rings, cone, surface radial cracks and coating bottom surface radial cracks were observed in intermediate coating region. This region exhibited highest cracking damage density. In thick coatings Hertzian cracks were observed. It was found that two factors affect the cracking damage: residual stress that increases with increasing thickness and bending stresses that decrease with increasing thickness. The resultant effect was maximized at intermediate coating thickness.

Monitoring fatigue-induced transverse matrix cracks in laminated composites using built-in acousto-ultrasonic techniques

This work investigated the use of ultrasonic wave propagation in a composite laminate to monitor transverse matrix cracking. It was found that the change in the signal’s power spectral density correlates to crack density increase, and its sensitivity was dependent on the actuator to sensor orientation. This orientation dependence was found to be the same as that of the laminate’s stiffness degradation. A crack density damage index model was developed that relates signal changes to laminate stiffness degradation due to transverse matrix cracks. With this damage index model, a matrix cracking monitoring system was proposed, implemented, and validated. The developed system requires a calibration test for the composite layup of interest and a minimum of one actuator and two sensors at different angle orientations. The calibration test and general approach steps are described and validated. The system’s crack density estimates match experimentally measured crack density values within established error bounds.

Influence of Fatigue Loading and Bone Turnover on Bone Strength and Pattern of Experimental Fractures of the Tibia in Mice.

Bone fragility depends on bone mass, structure, and material properties, including damage. The relationship between bone turnover, fatigue damage, and the pattern and location of fractures, however, remains poorly understood. We examined these factors and their integrated effects on fracture strength and patterns in tibia. Adult male mice received RANKL (2mg/kg/day), OPG-Fc (5mg/kg 2×/week), or vehicle (Veh) 2days prior to fatigue loading of one tibia by in vivo axial compression, with treatments continuing up to 28 more days. One day post fatigue, crack density was similarly increased in fatigued tibiae from all treatment groups. After 28days, the RANKL group exhibited reduced bone mass and increased crack density, resulting in reduced bone strength, while the OPG-Fc group had greater bone mass and bone strength. Injury repair altered the pattern and location of fractures created by ex vivo destructive testing, with fractures occurring more proximally and obliquely relative to non-fatigued tibia. A similar pattern was observed in both non-fatigued and fatigued tibia of RANKL. In contrast, OPG-Fc prevented this fatigue-related shift in fracture pattern by maintaining fractures more distal and transverse. Correlation analysis showed that bone strength was predominantly determined by aBMD with minor contributions from structure and intrinsic strength as measured by nanoindentation and cracks density. In contrast, fracture location was predicted equally by aBMD, crack density and intrinsic modulus. The data suggest that not only bone strength but also the fracture pattern depends on previous damage and the effects of bone turnover on bone mass and structure. These observations may be relevant to further understand the mechanisms contributing to fracture pattern in long bone with different levels of bone remodeling, including atypical femur fracture.

Multi-level modeling of viscoelastic microcracked masonry

• We propose a multi-level model to predict the orthotropic behavior of cracked masonry • This model relies on the coupling between Griffith’s theory and homogenization methods • Overall estimates of Cecchi & Taliercio model are softer than the Cecchi & Tralli’s ones • The Modified Maxwell model is relevant for the mortar’s creep at short and long terms • Both short and long terms, constant and time-dependent crack densities were investigated This paper provides multi-level modeling of microcracked viscoelastic masonries generally present in historical masonries or refractory linings. It is an extension of the Cecchi and Tralli (2012) or Cecchi & Barbieri (2008) and Cecchi & Taliercio (2013) works for the typical Burgers and Modified Maxwell models followed by both undamaged and microcracked masonries. For the sake of simplicity and in order to provide rigorous analytical global estimates, only the mortar is assumed to be viscoelastic and microcracked. Bricks are assumed to be undamaged, elastic or quasi-rigid. The distribution of microcracks is assumed to be isotropic. The effective behavior of the viscoelastic microcracked masonry is provided by two steps. The first one relies on the coupling between the Griffith’s brittle fracture theory and linear mean-field homogenization scheme in order to account for the effect of microcracks on the macroscopic deformation of the mortar and establishes a linear relation between apparent macroscopic stress and strain. This step allows to easily and fast determine the effective creep function of the microcracked mortar without recourse to ’complex’ or heavy numerical inversion of the Laplace–Carson transform. The second step is based on the coupling between asymptotic analysis and homogenization theory applied for a periodic masonry. The proposed models provide analytical solutions - explicit functions of the crack density parameter - for the effective orthotropic behavior of a microcracked viscoelastic periodic masonry cell. This study proves that the Cecchi & Tralli’s and Cecchi & Taliercio’s extension estimates are close and that the later are softer. Such overall properties are used to perform finite element computations on a compressed masonry panel as a first application. These models allow then the prediction of mostly stressed and deformed areas in microcracked masonry structures. This study demonstrates that modeling a mortar (at its undamaged or microcracked state) with this Burgers formulation is only suitable for a masonry with too high values of the Maxwell’s relaxation time otherwise it yields to vanishing effective properties with the increase of time and crack density leading thus to a premature collapse of the masonry. On the other hand, the Modified Maxwell model permits the masonry to preserve a certain resistance for every range value of Maxwell’s relaxation time. These conclusions are valid for masonry either with elastic or quasi-rigid undamaged bricks.

Impact of GaN transition layers in the growth of GaN epitaxial layer on silicon**Project supported by the National Natural Science Foundation of China (Nos. 61474110, 61377020, 61376089, 61223005, 61176126), and the National Science Fund for Distinguished Young Scholars (No. 60925017).

A method for growing GaN epitaxial layer on Si (111) substrate is investigated. Due to the large lattice mismatch between GaN and AlN, GaN grown directly above an AlN buffer layer on the Si substrate turns out to be of poor quality. In this study, a GaN transition layer is grown additionally on the AlN buffer before the GaN epitaxial growth. By changing the growth conditions of the GaN transition layer, we can control the growth and merging of islands and control the transfer time from 3D to 2D growth mode. With this method, the crystalline quality of the GaN epitaxial layer can be improved and the crack density is reduced. Here, we have investigated the impact of a transition layer on the crystalline quality and stress evolution of a GaN epitaxial layer with methods of X-ray diffraction, optical microscopy and in situ reflectivity trace. With the increasing thickness of transition layer, the crack decreases and the crystalline quality is improved. But when the transition layer exceeds a critical thickness, the crystalline quality of the epilayer becomes lower and the crack density increases.

Predicting evolution of ply cracks in composite laminates subjected to biaxial loading

An energy-based model is developed to predict the evolution of sub-critical matrix crack density in symmetric multidirectional composite laminates for the case of multiaxial loading. A finite element-based numerical scheme is also developed to evaluate the critical strain energy release rate, GIc, associated with matrix micro-cracking, a parameter that previously required fitting with experimental data. Furthermore, the prediction scheme is improved to account for the statistical variation of GIc within the material volume by using a two-parameter Weibull distribution. The variation of GIc with increasing crack density is also accounted for based on reported experimental evidence. The simulated results for carbon/epoxy and glass/epoxy cross-ply laminates demonstrate the ability of the improved model to predict the evolution of multidirectional ply cracking. By integrating this damage evolution model with the synergistic damage mechanics approach for stiffness degradation, the stress-strain response of the studied laminates is predicted. Finally, biaxial stress envelopes for ply crack initiation and pre-determined stiffness degradation levels are predicted to serve as representative examples of stiffness-based design and failure criterion.

Scaling shift in multicracked fiber bundles.

Bundles of fibers, wires, or filaments are ubiquitous structures in both natural and artificial materials. We investigate the bundle degradation induced by an external damaging action through a theoretical model describing an assembly of parallel fibers, progressively damaged by a random population of cracks. Fibers in our model interact by means of a lateral linear coupling, thus retaining structural integrity even after substantial damage. MonteCarlo simulations of the Young's modulus degradation for increasing crack density demonstrate a remarkable scaling shift between an exponential and a power-law regime. Analytical solutions of the model confirm this behavior, and provide a thorough understanding of the underlying physics.

Microstructure change and deuterium permeation behavior of the yttrium oxide coating prepared by MOCVD

Deuterium permeation measurements and microstructure analysis on yttrium oxide coating deposited by metal organic chemical vapor deposition (MOCVD) were studied. The composition and microstructure of the coating can be adjusted by tuning sublimation temperature and deposition temperature. It is found that the coating sublimated at 150 °C and deposited at 600 °C possesses compact surface morphology with no pinholes or cracking was observed. Moreover, the oxygen/yttrium ratio corresponding well to the stoichiometric Y2O3 was observed by X-ray photoelectron spectroscopy (XPS) with no moving of Y3d binding energy doublet peaks and O1s binding energy peak at different depth throughout the coating. The permeation properties of coatings depended much on the microstructure. The coating sublimated at 150 °C and deposited at 600 °C exhibits the best permeability property with PRF values 412–244 at 873–973 K. However, a permeability drop, especially for the 800 °C deposited coating was observed. It was believed that the obvious increase of pinhole and crack density caused the increase of deuterium permeability. Furthermore, the existence of (Fe,Cr)2O3, forming a grid structure in the natural oxide layer of stainless steel, increases its hydrogen permeability.