Increase In Crack Density Research Articles

High-frequency bulk moduli of fluid-saturated cracked porous rocks

Many experimental data have confirmed velocity dispersion phenomena for body waves in fluid saturated porous rocks. Bulk modulus is a critical parameter for compressional wave velocity, so studing the high-frequency bulk modulus is of importance for ultrasonic compressional wave velocity measurement. Considering the soft pores such as cracks in rocks, Mavko and Jizba proposed a modified Gassmann equation to calculate the liquid saturated bulk modulus for high-frequency condition. However, the equation proposed by Mavko and Jizba is not derived through a rigorous derivation. And there is not an exact expression for high-frequency bulk modulus up to now. Upon above existing problems, we derive an exact expression of high-frequency bulk modulus for fluid saturated cracked porous rocks based on double-porosity model. The derived expression has the same form as Brown-Korringa equation. In the high-frequency limit, there is no time for fluid mass exchange between cracks and pores, and the fluid in cracks become a part of the host, so that the high-frequency effective solid bulk modulus is less than the pure solid bulk modulus. If fluid pressure in cracks has no effect on the fluid content variation in pores, the derived high-frequency drained bulk modulus reduces to Gurevichs result which derived from Sayers-Kachanov method. Numerical results show that the deviation of high-frequency saturated bulk modulus between Mavko-Jizbas equation and this derived expression increases as crack density increases. And, the high-frequency effective pore bulk modulus is always approximate to pure solid bulk modulus. In contrast, the high-frequency effective solid bulk modulus decreases obviously as crack density increases.

Thermal fatigue resistance of H13 die steel repaired by partial laser surface remelting process

In order to study the thermal fatigue resistance of H13 die steel repaired by a partial laser surface remelting process, a strengthening latticed morphology was designed and processed using a pulsed Nd:YAG laser by imitating the structures of certain animals and plants. The ultrafine microstructure within the strengthening unit is composed of martensite, austenite and finer carbides. The average microhardness of the unit is much higher than that of the original surface, even after thermal fatigue cycles. The results show that both percentage increase in crack length and crack density are reduced owing to the blocking effect of strengthening units. Partial laser surface remelting with the smallest spacing is the most effective for repairing cracks and improving the overall thermal fatigue resistance. A manufacturing verification test demonstrates the effectiveness of the partial laser remelting process in repairing thermal fatigue cracks of a casting die and improving its thermal fatigue resistance.

ESPI analysis of crack face displacements in damaged laminates

Abstract Crack opening displacement (COD) and crack sliding displacement (CSD) are the two parameters that define slope of stiffness reduction with increasing crack density. The values of these parameters strongly depend on the crack geometry which is difficult to observe and quantify. In models we usually assume straight crack, no local delaminations (questionable for surface cracks) and use uniform spacing. In this paper COD of cracks in surface layers are analyzed using the displacement field on the edge and on the surface of a [90/0] s and [90 3 /0] s carbon fiber/epoxy laminates subjected to tension. The specimen full-field displacement measurement is carried out using ESPI (Electronic Speckle Pattern Interferometry). It is shown that including in the FEM model crack locations exactly as in the test, the COD of an individual crack is predicted accurately. The model with periodic and symmetric crack distribution significantly underestimates the COD whereas staggered crack distribution model in many cases renders very good results. Crack face sliding displacements, CSD were measured on the edge of [0/70 4 /−70 4 ] s GF/EP laminate, showing that the sliding can not be neglected and that the COD versus CSD ratio for this laminate is similar to model predictions.

Optical properties and laser damage performance of SiO2 irradiated by high-power pulsed electron beam

A 60 kW electron beam is used to study the microstructure and optical property evolutions as well as laser induced damage threshold of fused silica after irradiation at room temperature. Optical microscopic results indicate that cracks appear at the surface of SiO2 after electron beam irradiation, owing to the thermal effect, and that the crack density and size increase with increasing radiation dose. The morphology of the surface cracks is analyzed by using atomic force microscope and the width of crack is about 1 μm. In addition, there are a large number of debris particles with sizes of 0.1-1 μm on the surface. From the optical absorption spectrum of each of all samples, a weak absorption peak at 394 nm is observed and the absorbance increases at the beginning then decreases with increasing electron-radiation dose. Before and after irradiation, three absorption bands at 460 nm, 496 nm and 520 nm are clearly observed and their intensities first increase and then decrease, which is consistent with the results of absorption spectra. The effect of electron dose on the laser induced damage threshold (LIDT) at 355 nm is investigated and the results indicate that the LIDT decreases with increasing dose. At the lower electron doses, the color centers are responsible for the decrease of LIDT. However, at the higher electron doses, the decrease of LIDT is due to the light modulation and absorption induced by microscale cracks and debris particles at the surface of irradiated fused silica.

Tensile properties and damage evolution in vascular 3D woven glass/epoxy composites

Vascularization enables multifunctional composites capable of self-healing, thermal regulation, electrical and magnetic modulation, and damage sensing. In this study, the effect of vascular channels on the in-plane tensile properties and damage progression of three-dimensional orthogonally woven textile composites is examined. Vascular channels are manufactured by Vaporization of Sacrificial Components (VaSC). Sacrificial fibers composed of poly(lactic acid) treated with tin(II) oxalate catalyst are integrated into 3D woven glass fiber preforms. Composites with straight channel and undulating wave-shaped channel architectures are created and tested in both longitudinal and transverse orientations. Damage evolution is monitored by acoustic emission and optical microscopy. Vascular channels have minimal effect on tensile behavior when fiber alignment is unaltered, while reductions in strength and modulus and increased crack density occur when channels distort the reinforcement fiber architecture.

Crack initiation behavior of neutron irradiated model and commercial stainless steels in high temperature water

The objective of this study was to isolate key factors affecting the irradiation-assisted stress corrosion cracking (IASCC) susceptibility of eleven neutron-irradiated austenitic stainless steel alloys. Four commercial purity and seven high purity stainless steels were fabricated with specific changes in composition and microstructure, and irradiated in a fast reactor spectrum at 320°C to doses between 4.4 and 47.5dpa. Constant extension rate tensile (CERT) tests were performed in normal water chemistry (NWC), hydrogen water chemistry (HWC), or primary water (PW) environments to isolate the effects of environment, elemental solute addition, alloy purity, alloy heat, alloy type, cold work, and irradiation dose. The irradiated alloys showed a wide variation in IASCC susceptibility, as measured by the relative changes in mechanical properties and crack morphology. Cracking susceptibility measured by %IG was enhanced in oxidizing environments, although testing in the lowest potential environment caused an increase in surface crack density. Alloys containing solute addition of Ni or Ni+Cr exhibited no IASCC. Susceptibility was reduced in materials cold worked prior to irradiation, and increased with increasing irradiation dose. Irradiation-induced hardening was accounted for by the dislocation loop microstructure, however no relation between crack initiation and radiation hardening was found.

Experimental and FEM study of thermal cycling induced microcracking in carbon/epoxy triaxial braided composites

The microcrack distribution and mass change in T700s/PR520 and T700s/3502 carbon/epoxy braided composites exposed to thermal cycling was evaluated experimentally. Acoustic emission was utilized to record the crack initiation and propagation under cyclic thermal loading between −55°C and 120°C. Transverse microcrack morphology was investigated using X-ray computed tomography. The differing performance of two kinds of composites was discovered and analyzed. Based on the observations of microcrack formation, a meso-mechanical finite element model was developed to obtain the resultant mechanical properties. The simulation results exhibited a decrease in strength and stiffness with increasing crack density. Strength and stiffness reduction versus crack densities in different orientations were compared. The changes of global mechanical behavior in both axial and transverse loading conditions were studied. By accounting for the obtained reduction of mechanical properties, a macro-mechanical finite element model was utilized to investigate the influence of microcracking on the high-speed impact behavior.

Effects of Cracks on the Electrical Conductivity of a Fissured Laterite: A Combined Experimental and Statistical Study

Abstract This paper presents an experimental study of the effects of cracks on the electrical conductivity (EC) of a fissured laterite in Hunan Province, People’s Republic of China, followed by multi-variant statistical analyses of the sensitivity of the EC dependence on different crack parameters (e.g., depth, length, width, orientation, and crack density). The results show that the soil EC is highly dependent on the crack properties. In general, the EC decreases linearly as the crack depth, length, width, orientation angle, or density increases. However, the EC decreases non-linearly, and the rate of reduction increases more dramatically, as the depth of cracks exceeds 1/4 of the thickness of the soil sample. When the width of the crack increases to 2.8 cm, the EC decreases less obviously and tends to remain at a constant value. Moreover, the EC decreases more dramatically with the orientation angle when it is less than 45° (otherwise the EC decreases more slowly with the orientation angle). With an increase in crack density (or the number of cracks), the EC exhibits much higher dependence on crack depth, length, and width. Finally, of all these crack parameters, the depth and orientation are the two primary factors influencing the soil EC, and the crack length, width, and density are secondary ones. As such, the degree of effect of different crack parameters on the soil EC follows the order of depth > orientation angle > density > length > width.

Non-steady state chloride diffusion in concrete with different crack densities

With an increasing concern about long-term durability, chloride transport and service-life prediction of cracked concrete are now a hot area in concrete research. However, only few researches up to now investigated the effect of crack density on chloride transport in cracked concrete. Nevertheless, in real concrete structures, multiple cracks instead of a single crack are most commonly observed. The present study investigates non-steady state chloride diffusion in concrete samples with different crack densities prepared by a non-destructive notch method. The relationship between diffusion coefficient and crack density is discussed. Water and acid soluble chloride contents increase significantly with increasing crack density up to a certain value. A linear function is found for the relationship between diffusion coefficient determined by acid soluble chloride and crack density. A piecewise function including linear and exponential function is proposed to describe the relationship between diffusion coefficient determined by water soluble chloride and the crack density.

Monte Carlo Simulation on the Matrix Failure Considering the Interfacial Debonding Energy

Considering the interfacial debonding energy at the fiber/matrix interface, an improved Monte Carlo simulation has been done on the matrix failure of the unidirectional ceramic matrix composites. Firstly, the debond length formula was given, then the stress/strain formula was also reviewed, and the matrix failure was simulated by the improved Monte Carlo model, the effect of the interfacial debonding stress on the matrix cracks evolution and the stress/strain curve was considered especially. Lastly, the result was compared to the experiment data. The result shows that the larger interfacial debonding energy at the fiber/matrix interface, the larger debond stress, the crack density increase more slowly, the mean crack density smaller, the final failure strength lower. The improved Monte Carlo simulation result is agreed well with the experiment data.

Crack patterns corresponding to the residual strength plateau of ceramics subjected to thermal shock

The formation mechanism of the residual strength plateau of ceramics subjected to thermal shock is addressed. A set of thermal shock experiments of 99Al2O3 are conducted, where the thin specimens of 1 mm×10 mm×50 mm exhibit parallel through edge cracks, and thus permit quantitative measurements of the crack patterns. The cracks evolve with the severity of thermal shock. It is found that there is a correlation between the length and density of the thermal shock cracks. The increase of crack length weakens the residual strength, whereas the increase of crack density improves it. In a considerably wide temperature range, the two contrary effects just counteract each other; consequently a plateau appears in the variation curve of the residual strength. A comparison between the numerical and experimental results of the residual strength is made, and they are found in good agreement. This work is helpful to a deep understanding of the thermal shock failure of ceramics.

Modeling of electromechanical behavior of woven SiC/SiC composites

A coupled electro-mechanical model was developed to predict the mechanical behavior of woven SiC/SiC ceramic matrix composites and electrical resistance response to mechanical damages in the composites. The matrix is explicitly included in the model such that the matrix cracking and fiber break can be linked to the electrical resistance change during loading. The results show that the electrical resistance increases linearly with an increase of matrix crack density and the number of fiber breaks. The predictions are compared to the experimental results on 2D woven SiC/SiC ceramic composites. With proper materials parameters input, the models can accurately predict the stress–strain curve and electrical resistance change during the loading. The model is further compared to an analytical solution of electromechanical coupling to get an insight into the electrical–mechanical interaction mechanisms in the composites.

Effect of zeolite on saturated hydraulic conductivity and crack behavior of silty clay paddled soil

The addition of zeolites to soil modifies soil physical and chemical properties. The objectives of this research were to study the effect of zeolite on saturated hydraulic conductivity, K s, and crack behavior in a silty clay paddled soil. Soil samples were mixed with 0, 4, 8 and 12 g kg−1 of zeolite for K s determination, and 0, 2, 4, 8 and 12 g kg−1 for soil crack measurements. Saturated hydraulic conductivity was measured using the constant head method. The results indicated that K s was significantly increased at a zeolite application rate of 8 g kg−1. Furthermore, an increase in zeolite content up to 8 g kg−1 decreased soil crack area after paddling and first rewetting. Higher amounts of zeolite (e.g. 12 g kg−1) increased crack density after the second rewetting. However, a 50% reduction in crack depth occurred with zeolite application rates of 8 and 12 g kg−1 in comparison with controls. Thus, a zeolite application rate of 8 g kg−1 is recommended for soil crack reduction in intermittent-flood irrigation. Furthermore, a relationship was obtained between crack area density (Ln), gravimetric soil water content and zeolite application rate. After the second irrigation, a relationship was also obtained between crack depth, gravimetric soil water content and zeolite application rate. Crack depth showed a positive and highly significant linear correlation with crack width.

On Effective Properties of Layers in Damaged Composite Laminates

Methodology is suggested for calculation of effective elastic constants of layer containing intralaminar cracks. Trends are presented for layers with low and medium high crack densities, where the interaction between cracks can be neglected. The effective properties of the damaged layer are determined by back- calculation using previously developed GLOB-LOC model for thermo-mechanical properties of damaged laminate [1]. It is shown that the effective transverse modulus and shear modulus of a layer decrease linearly with increasing crack density, whereas longitudinal modulus and major Poisson’s ratio do not change at all. Experimental data for multiple cracking are analyzed showing that linear approximation of crack density versus applied strain may have sufficient accuracy. These two linear trends are used to calculate the effective elastic properties of a layer as a function of strain.

Residual Stiffness of Cracked Cross-Ply Composite Laminates Under Multi-Axial In-plane Loading

In this paper, the Equivalent Constraint Model (ECM) together with a 2-D shear lag stress analysis approach is applied to predict residual stiffness properties of polymer and ceramic matrix [0/90n/0] cross-ply laminates subjected to in-plane biaxial loading and damaged by transverse and longitudinal matrix cracks. It is found that the longitudinal Young’s modulus, shear modulus and major Poisson’s ratio undergo large degradation as the matrix crack density increases, with Poisson’s ratio appearing to be the most affected by transverse cracking. In cross-ply laminates with thick 90° layer strip-shaped delaminations begin to initiate and grow from the tips of matrix cracks at the 0°/90° interface. These delaminations contribute to further stiffness degradation of such laminates, and hence have to be taken into account in failure analysis models. The thickness of the 90° layer plays an important role; the thicker the 90° layer, the bigger stiffness reduction suggesting a size (volume) effect at ply level. In SiC/CAS cross-ply laminates reduction in the longitudinal modulus occurs mainly due to transverse cracks, while the shear modulus appears to be the most affected by the presence of longitudinal cracks. The shear modulus reduction ratio predicted previously by a semi-empirical formula is, in the most of cases, within 10% of the current ECM/2-D shear lag approach value. In some cases, though, the error of the semi-empirical finite element expression can be as big as 20% since it fails to capture damage mode interaction.

The electrical signature of rock samples exposed to hydrostatic and triaxial pressures

The electrical signature of sedimentary (carbonate) and crystalline rock samples was studied in hydrostatic and triaxial pressure experiments up to 300 MPa. The aim was to establish a relation between an electrical signal stimulated by an external pressure acting on the sample and the mechanical stability of the rock. Natural open fractures tend to be closed under hydrostatic pressure conditions, whereas in triaxial pressure experiments new fractures are generated. These contrary processes of either decrease or increase in crack density and geometry, cause a decrease or increase in the inner surface of the sample. Such pressure induced variations in pore geometry were investigated by an interpretation and modelling of the frequency dependence of the complex electrical conductivity. In a series of hydrostatic pressure experiments crack-closure was found in the electrical signature by a decrease of the model capacitor C being related to crack geometry. This capacitor increases in the triaxial experiments where new fractures were formed.

Effect of crack opening and orientation on dispersion and attenuation of antiplane coherent wave

SUMMARY Propagation of antiplane coherent wave in solids containing randomly and uniformly distributed cracks is studied by calculating the associated effective wave number. Attention is focused on the influence of crack opening and orientation. Strip-like cracks or empty elliptical cavities are either mutually parallel or randomly oriented, and their surfaces are stress-free. Different multiple-scattering formulations for the effective wave number, in terms of the farfield displacement scattered by a single crack in the matrix, are compared for various crack systems. They coincide at low crack density and show similar trends when the crack density increases. Simple analytical estimates enable a direct evaluation of the crack density from the speed and attenuation of the coherent wave. In the low- and high-frequency ranges, the effective wave speed and attenuation appear to be mainly governed by the projection of cracks on the incident plane wave front.

면저항 45 ohms/sq.의 ITO/PET Sheets의 변형률 속도에 따른 균열 형성 거동

The stress-strain behavior and its effects on the crack initiation and growth of ITO film on PET substrate with a sheet resistance of 45 ohms/sq were investigated. Electrical resistance increased gradually at the strain of 0.7% in the elastic to plastic transition region of the stress strain curves. Numerous cracks were observed after 1% strain and the increase of the resistance can be linked to the cracking of ITO thin films. The onset strain for the increase of resistance increased with increasing strain rate, suggesting the crack initiation is dependent on the strain rate. Upon loading, the initial cracks perpendicular to the tensile axis were observed and propagated the whole sample width with increasing strain. The spacing between horizontal cracks is thought to be determined by the fracture strength and the interfacial strength between ITO and PET. The crack density increased with increasing strain. The spacing between horizontal cracks (perpendicular to the stress axis) increased with decreasing strain rate, The increase of crack density with decreasing strain rate can be attributed to the higher fraction of the plastic strain to the total strain at a given total strain. As the strain increased over 5% strain, cracks parallel to the stress axis were developed and increased in number with strain, accompanied by drastic increases of resistance.

Insights into the effects of tensile and compressive loadings on microstructure dependent fracture of trabecular bone

The micro-scale finite element models used in the past to understand yielding failure of trabecular bone have not addressed the microcrack formation and its effect on microstructure dependent fracture. An understanding of microcrack based failure mechanisms can be important to develop insights into response of trabecular bone to external loading before final failure. With this goal, we analyze tensile and compressive fracture failure at two different ages in two trabecular bone micrographs obtained from an ovine femur using a recently developed cohesive finite element method (CFEM) framework. The results and analyses indicate that examined trabecular microstructures are optimally designed for resisting compressive loading. Under tensile loading, initial damage in a microstructure is localized in a single random trabecula. Final microstructure failure occurs immediately after the failure of the trabecula. However, under compressive loading, failure of the first trabecula does not precede immediate complete failure of microstructure. Under compression the propagation fracture toughness (characterized by change in energy release rate as a function of crack density) increases with increase in crack density. However, under tension the propagation fracture toughness decreases with increasing crack density. The fracture mechanism remains unaffected by age variation. Effect of tissue property random variation on the variation in fracture strength diminishes under tension and increases under compression with increase in the age. Overall, results indicate that structural arrangement of the trabecular bone (besides the hierarchical chemical composition) can be an important contributor to its unique fracture resistance properties.

The impact of porosity and crack density on the elasticity, strength and friction of cohesive granular materials: Insights from DEM modelling

This study uses the Discrete Element Method (DEM), in which rock is represented by bonded, spherical particles, to investigate the dependence of elasticity, strength and friction angle on porosity and crack density. A series of confined triaxial extension and compression tests were performed on samples that were generated with different particle packing methods, characterised by differing particle size distributions and porosities, and with different proportions of pre-existing cracks, or uncemented particle contacts, modelled as non-bonded contacts. The 3D DEM model results demonstrate that the friction angle decreases (almost) linearly with increasing porosity, and is independent of particle size distribution. Young's modulus, strength and the ratio of unconfined compressive strength to tensile strength (UCS/T) also decrease with increasing porosity, whereas Poisson's ratio is (almost) porosity independent. The pre-eminent control on UCS/T is, however, the proportion of bonded contacts, suggesting that UCS/T increases with increasing crack density. Young's modulus and strength decrease, while Poisson's ratio increases with increasing crack density. The modelling results replicate a wide range of empirical relationships observed in rocks and underpin improved methods for the calibration of DEM model materials.