Thickness Crack Research Articles

Micro-crack detection of nonlinear Lamb wave propagation in three-dimensional plates with mixed-frequency excitationyg**Project supported by the National Natural Science Foundation of China (Grant Nos. 61571222, 61602235, and 11474160) and the Six Talent Peaks Project of Jiangsu Province, China.

We propose a nonlinear ultrasonic technique by using the mixed-frequency signals excited Lamb waves to conduct micro-crack detection in thin plate structures. Simulation models of three-dimensional (3D) aluminum plates and composite laminates are established by ABAQUS software, where the aluminum plate contains buried crack and composite laminates comprises cohesive element whose thickness is zero to simulate delamination damage. The interactions between the S0 mode Lamb wave and the buried micro-cracks of various dimensions are simulated by using the finite element method. Fourier frequency spectrum analysis is applied to the received time domain signal and fundamental frequency amplitudes, and sum and difference frequencies are extracted and simulated. Simulation results indicate that nonlinear Lamb waves have different sensitivities to various crack sizes. There is a positive correlation among crack length, height, and sum and difference frequency amplitudes for an aluminum plate, with both amplitudes decreasing as crack thickness increased, i.e., nonlinear effect weakens as the micro-crack becomes thicker. The amplitudes of sum and difference frequency are positively correlated with the length and width of the zero-thickness cohesive element in the composite laminates. Furthermore, amplitude ratio change is investigated and it can be used as an effective tool to detect inner defects in thin 3D plates.

Corner cracking model for non-uniform corrosion-caused deterioration of concrete covers

The failure of concrete covers due to corrosion of the corner rebars is different from that of the interior rebars. To date, there is no specific analytical model for simulating corner cracking in the literature. In response, this paper proposes a novel analytical model, which considers both the non-uniform distribution of rust and the boundary conditions of the cover surface, to study the process and propagation of corner cracking. This nondestructive evaluation method is capable of obtaining the volume of corroded steel by measuring the bulging of the cover surface and/or the crack opening. This model has been verified by comparing the analytical results with the numerical results. Using the verified analytical model, the effects of the tensile strength of concrete, cover thickness, rebar diameter and diagonal crack angle on the bulging of the cover surface and crack opening are studied, which provide useful information for the design and prediction of the service life and crack width in reinforced concrete structures.

Vibration and buckling analyses of FGM plates with multiple internal defects using XIGA-PHT and FCM under thermal and mechanical loads

In this paper, the vibration and buckling analyses of the FGM (functionally graded material) plates with multiple internal cracks and cutouts under thermal and mechanical loads are numerically investigated using the combined XIGA-PHT (extended isogeometric analysis based on PHT-splines) and FCM (finite cell method). Material properties are graded only in the thickness direction. The effective material properties are estimated by using either the rule of mixture or the Mori-Tanaka homogenization technique. The plate displacement field is based on the HSDT (higher-order shear deformation plate theory) without any requirement of the SCF (shear correction factor). The HSDT model can exactly represent the shear stress distribution and improve the accuracy of solutions. The PHT-splines can naturally fulfill the C1-continuous requirement of the HSDT model. The representation of internal defects is mesh-independent. The discontinuous and singular phenomena induced by the cracks are captured using the enrichment pattern in the XIGA, and the influence of cutouts is implemented by the FCM. The geometries of cutouts are captured by means of adaptive quadrature procedure based on a simple unfitted structural mesh, which avoids the need for multiple patches to describe the complex geometry and eliminates the enforcement of C1-continuity patch-coupling across the patch boundaries. The initial mesh density around the cracks and cutouts can be controlled flexibly utilizing the local refinement property of the PHT-splines. After validating the results of the developed approach with those available in the literature, the effects of material gradient index, side to thickness ratio, boundary conditions, cutout size and crack length on the normalized frequency and the critical buckling parameter are investigated. Numerical results illustrate the effectiveness and accuracy of the present approach.

Relation between stress intensity factor of circumferential crack and adhesive thickness in bonded round bar

The fracture of the bonded dissimilar materials generally occurs near the interface or at the interface edge. In this study, the stress intensity factors of the circumferential crack near the interface and the circumferential interface crack are analysed by the crack tip stress method. The relation between the dimensionless stress intensity factor of the circumferential crack and the adhesive thickness is investigated. The dimensionless stress intensity factors are defined by using the singular stress value at the crack tip point in bonded round bar without crack. The dimensionless stress intensity factors based on the singular stress value become constant irrespective of the adhesive thickness. In addition, the dimensionless stress intensity factor of the circumferential crack in bonded round bar is compared with that of the edge crack in bonded plate. Both factors coincide with each other when the geometrical condition and material combinations are the same.

Experimental investigation of surface integrity after wire electro-discharge machining of Ti–6Al–4V

This article presents the relationship of machining parameters containing pulse-on time (Ton), pulse-off time (Toff), peak current (IP) and servo voltage (SV) on surface integrity characteristics, including white layer thickness (WLT), heat-affected zone (HAZ) and surface crack density (SCD) and also on material removal rate (MRR), after wire electric discharge machining of Ti–6Al–4V. Taguchi’s method was utilized to design the experiments, and response surface methodology (RSM) was employed for developing the empirical models. Results indicated that Ton and IP played a significant role on surface integrity characteristics. In addition, the microstructure of selected machined samples was analysed using a field emission scanning electron microscope (FESEM) and energy-dispersive X-ray (EDX) analysis. Accuracy of models was examined using residual analysis and confirmation runs. Finally, multi-response optimization of process parameters was obtained using desirability approach. Results can be used to improve the quality of the machined workpiece significantly to fulfil the requirements of the various industries. The novelty of this research is mainly investigation and multi-response optimization of all the surface integrity characteristics at the same time.

Multi-objective optimization in wire-EDM process using grey relational analysis method (GRA) and backpropagation neural network–genetic algorithm (BPNN–GA) methods

PurposeThe purpose of this paper is to investigate prediction and optimization of multiple performance characteristics in the wire electrical discharge machining (wire-EDM) process of SKD 61 (AISI H13) tool steel.Design/methodology/approachThe experimental studies were conducted under varying wire-EDM process parameters, which were arc on time, on time, open voltage, off time and servo voltage. The optimized responses were recast layer thickness (RLT), surface roughness (SR) and surface crack density (SCD). Arc on time was set at two different levels, whereas the other four parameters were set at three different levels. Based on Taguchi method, an L18 mixed-orthogonal array was selected for the experiments. Further, three methods, namely grey relational analysis (GRA), backpropagation neural network (BPNN) and genetic algorithm (GA), were applied separately. GRA was performed to obtain a rough estimation of optimum drilling parameters. The influences of drilling parameters on multiple performance characteristics were determined by using percentage contributions. BPNN architecture was determined to predict the multiple performance characteristics. GA method was then applied to determine the optimum wire-EDM parameters.FindingsThe minimum RLT, SR and SCD could be obtained by setting arc on time, on time, open voltage, off time and servo voltage at 2 ms, 3 ms, 90 volt, 10 ms and 38 volt, respectively. The experimental confirmation results showed that BPNN-based GA optimization method could accurately predict and significantly improve all of the responses.Originality/valueThere were no publications regarding multi-response optimization using a combination of GRA and BPNN-based GA methods during wire-EDM process available.

Analysis of the coal seam spalling-failure mechanism based on the seepage instability theory.

Coal and gas outburst is a common coal-rock dynamic disaster. Such accidents frequently occur, and the mechanism underlying the occurrence of these outbursts is complex. As a typical failure mode of a gas-filled and pressure-relieved coal body, the spallation mechanism should be investigated to reveal the mechanism of coal and gas outburst and guide outburst-prevention strategies. In this paper, a fluid-solid coupling model for coal seam gas flow is established. This model considers the adsorption characteristics of coal. Numerical calculations are used to simulate the stress field distribution and evolution of gas-filled coal bodies under different boundary conditions. The mechanical mechanism of the spallation occurrence after the pressure relief of coal is explained from the perspective of seepage breaking coal. The control of the flow and stress state of the gas to the spallation failure is analyzed. The mechanical-quantitative conditions for the initial failure of the coal body under seepage and the mechanical-qualitative conditions for the continuous advancement and termination of spallation are studied based on numerical solution results. The numerical calculation results show that the formation of a flow field after pressure relief will apply a drag force (tensile stress) on the porous media of coal. The presence of this force plays a crucial role in promoting the spallation and cracking of coal and, thus, the promotion of spallation. The tensile strength, initial adsorption pressure, and pressure relief rate of the coal body jointly control whether the initial failure can occur and the thickness of the fracture layer cracks. Spallation propulsion is mainly determined by the pressure relief conditions of the undestroyed coal body and pressure changes in the spallation space; the former can be quantitatively obtained by numerical calculations, whereas the latter is related to the thickness of the spalled layer and the degree of the layer-crack structure.

Effects of Molybdenum on HIC Susceptibility in Normalized Pressure Vessel Steels for Sour Service Applications

This paper discusses a molybdenum-added alloy design for normalized pressure vessel steels, to reduce hydrogen induced cracking (HIC) and inhibit crack propagation. The change in microstructure produced by the modified alloy composition was analyzed to determine its effect on HIC characteristics. The microstructural change was observed by optical microscopy, hardness tests, scanning electron microscopy, and electron backscattered diffraction. The crack length ratio and crack thickness ratio were evaluated using the NACE TM 0284 standard, and ultrasonic testing was used for HIC analysis. The formation of polygonal ferrite and pearlite during the processing of the alloy creates localized areas of high stress concentration at the polygonal ferrite/pearlite interface, due to the expansion/contraction of various structures during the transformation. This results in the generation of potential hydrogen-trapping sites, subsequent HIC, and crack propagation. The addition of molybdenum leads to a decrease in the volume fraction of the pearlite structure in the steel in favor of a more beneficial bainitic ferrite microstructure, which is generated during the normalizing process. This bainitic transformation creates a more favorable expansion/contraction compatibility and reduces/breaks up the ferrite/pearlite banding. The combination of these two characteristics can result in an overall lower stress-intensity state, which can minimize hydrogen-trapping and crack propagation. This study demonstrates that the resistance of normalized pressure vessel steels to HIC can be significantly improved by incorporating molybdenum in the alloy design. Key words: sour service, molybdenum, hydrogen induced crack (HIC), normalizing steel, pressure vessel steel

Coupling effects of interfacial friction and layer thickness on soil desiccation cracking behavior

A series of desiccation tests were performed to investigate the effects of both interfacial friction and layer thickness on desiccation cracking behavior of soil. Nine groups of slurry specimens with different interfacial roughness and layer thicknesses were prepared and dried under a constant temperature of 30 °C. During drying, the change in water content was recorded and the dynamic process of soil cracking was monitored with a digital camera at different time intervals. Using digital image processing technique, the propagation characteristics of desiccation cracks on soil sample surfaces were quantitatively described with a series of geometric parameters. The results show that the initiation and propagation of cracks occur at relatively high water content ranging from 10% to 40% during drying. For the same soil layer thickness, the larger the interfacial roughness, the faster the propagation velocity of soil cracks. However, the influence of interfacial friction on soil cracking decreases with increase in soil layer thickness. Besides, there is a negative linear relationship between soil layer thickness and peak propagation velocity of soil cracks. During drying, the critical water content (wf, refers to the water content when the propagation of soil cracks tends to stabilize) decreases with increase in interfacial roughness under the same soil layer thickness. The final morphology of the desiccation crack network on each soil sample is significantly influenced by both the interfacial friction and soil layer thickness. The intensity of soil cracking increases with increase in interfacial roughness or decrease in soil layer thickness. These results also confirm that the effects of interfacial friction and layer thickness on soil desiccation cracking are coupled. Generally, the interface friction effect is more pronounced for thinner soil layers.

The effect of cracks on the stability of the functionally graded plates with variable-thickness using HSDT and phase-field theory

In this paper, the stability in a rectangular functionally grade material (FGM) plate with central crack is studied. The plate thickness is changed exponentially following the length of the plate. The properties of the FGM plate are assumed to vary along the thickness direction according to a simple power law distribution. Based on the phase-field theory, the new third order shear deformation plate theory (TSDT) and the finite element method (FEM), the stability of cracked FGM plate is determined. The obtained numerical results are compared with the published articles to ensure credibility. The work also considered effects of changing of the plate thickness ratio, length, crack angle and volume fraction exponent of the functionally graded material on the stability of the plate. Lastly, some visual images of the mechanical instability forms of cracked FGM plates will be introduced.

Optimization of parameters affecting horizontal cracking in continuously reinforced concrete pavement (CRCP)

Field evaluation of distresses in continuously reinforced concrete pavement (CRCP) indicated punch-out distress associated with horizontal cracking at the depth of the longitudinal steel is the most severe performance problem in CRCP. The developed 3-D model was used to perform a parametric analysis to determine the effects of critical loading location, concrete properties, and longitudinal steel design on horizontal cracking potential. The maximum vertical tensile stresses in the concrete were slightly affected by the coefficient of thermal expansion of the concrete. The critical tensile stresses in the concrete were observed to decrease as the base modulus, slab–base friction, slab thickness, and transverse crack spacing increase. The vertical tensile stresses significantly decreased when the longitudinal steel spacing decreased. The use of varying longitudinal steel spacing and reducing the depth of steel may be one of the ways to reduce the horizontal cracking potential without changing the steel ratio of the slab.

Hygrothermal effect on the moisture absorption in composite laminates with transverse cracks and delamination

The stiffness degradation of the cross-ply composite laminates containing a transverse cracking and delamination in 90o layer is predicted by using a modified shear-lag model by introducing the stress perturbation function. The prediction shows better agreement with the experimental results published by Ogihara and Takeda 1995, especially for laminates with thicker 90o plies in which extensive delamination occurs. A homogenised analytic model for average transient moisture uptake in composite laminates containing periodically distributed matrix cracks and delamination is presented. It is shown that the model well describes the moisture absorption in a cross-ply composite laminate containing periodically distributed transverse matrix cracks in the 90o plies. The obtained results represent well the dependence of the stiffness degradation on the crack density, thickness ratio and moisture absorption. The present study has proved to be important to the understanding of the degradation of the material properties in the failure process when the laminates in which the delamination grows extensively.

Elastic wave scattering by a fluid-saturated circular crack and effective properties of a solid with a sparse distribution of aligned cracks.

Hydraulic fractures and preexisting cracks in natural aquifers and hydrocarbon reservoirs are often saturated with fluids. Understanding the elastic wave properties in such a cracked fluid-saturated medium is of importance for many physical and engineering applications such as hydrology, petroleum engineering, oil exploration, induced seismicity, and nuclear waste disposal. In this paper, the scattering of a normally incident longitudinal (P-) wave by a fluid-saturated circular crack in an infinite elastic non-porous matrix is studied. In particular, the mechanism of hydraulic conduction (including the effects of the crack permeability and fluid inertia) inside the crack is incorporated. A semi-analytic solution for this scattering problem is derived. Based on the solution and multiple scattering theorem, an effective medium model is developed to determine the velocity dispersion and attenuation due to wave scattering in an elastic matrix with sparse distribution of aligned cracks. It is shown that the effective P-wave velocity is consistent with Gassmann's theory in the low-frequency limit. The effect of crack permeability on scattering is negligible, but the effect of fluid inertia is important. Specifically, it is found that resonance phenomena can take place inside the cracks at frequencies much lower than the scattering characteristic frequency so that rapid velocity variation can occur at relatively low frequencies. The fluid viscosity plays a damping role in weakening the resonance. The effects of crack thickness and fluid compressibility on scattering dispersion are similar to those in the case of plane-strain (two-dimensional) slit crack.

Surface integrity and wafer-thickness variation analysis of ultra-thin silicon wafers sliced using wire-EDM

ABSTRACTThe defect-free ultra-thin Si wafers with high surface quality have a huge demand in the solar cell-based industry. However, the currently employed wafer slicing methods result in various defects like wafer warpage, variation in thickness, surface and sub-surface cracks, and twin boundaries along with high surface roughness of around 3–5 µm. Therefore, in order to minimise the effect of mechanical machining on the wafer produced, wire-EDM as an alternative method has received attention in the recent past. This work presents the experimentation to understand the effect of wire-EDM on the surface quality and the wafer-thickness variation along the wafer height. The results show that the variation in wafer thickness can be minimised by increasing the parametric values of wire tension, wire feed, and dielectric flushing pressure. These parametric conditions help in better removal of clogged debris in the inter-electrode gap, which in turn improves the slicing process efficiency. The surface roughness values were found to be present in the range of 1 to 2.3 µm. In general, craters due to plasma sparks were visible on the wafer surface while analyzing through SEM micrographs, which also contributes to the surface roughness.

Ultrasonic spot welding of 5182 aluminum alloy: Evolution of microstructure and mechanical properties

The present study is focused on the evaluation of microstructures, tensile lap shear strength and fatigue resistance of ultrasonic spot welded joints of lightweight AA 5182 aluminum alloy. The temperature quickly increased during ultrasonic spot welding (USW), with the peak temperature increasing with increasing welding energy. A “necklace”-like ultrafine structure occurred at the weld interface due to the presence of dynamic recrystallization. The tensile lap shear strength increased with decreasing number of impedance setting. When the impedance was set below 5, the tensile lap shear failure load of nearly 6 kN was attained, surpassing the requirement for the average peak load of AWS D17.2 standards. The tensile lap shear strength, failure energy, and critical stress intensity factor first increased, reached their maximum and then decreased with increasing welding energy. The maximum tensile lap shear strength of ∼150 MPa was achieved at a welding energy of 4000 J and an impedance setting of 2. The load-controlled fatigue behavior displayed a bi-linear characteristic due to the change in the failure mode from interfacial failure at the higher cyclic loads to transverse-through thickness (TTT) crack growth mechanism at the lower cyclic loads.

Detection of sub-500-μm cracks in multicrystalline silicon wafer using edge-illuminated dark-field imaging to enable thin solar cell manufacturing

High capital expenditures associated with manufacturing thin silicon wafers make it difficult for the industry to scale up and prevent novel technologies from entering the market. Thin wafers fail largely due to breakage during solar cell processing and handling. One of the root causes for breakage is sub-mm edge cracks in the silicon wafer, and these cracks cannot be reliably detected by most commercially-available crack detection tools. In this work, we first investigate the correlation between wafer thickness and critical crack length, and explain the importance of detecting sub-500-μm edge cracks as the wafer thickness is reduced. Secondly, we extend our previous work of micro-crack detection to demonstrate an edge illumination technique using a near-infrared laser to image edge cracks less than 500 μm in length in multicrystalline silicon. Thirdly, we investigate two fundamental edge illumination mechanisms based on dark-field imaging; namely, direct and vicinal illumination. We will then compare these methods to a state-of-the-art rear illumination method. The advantages and disadvantages of both illumination methods are presented and provide an in-depth analysis of light-crack interaction. In particular, we find that the robustness of vicinal illumination is due to diffuse reflectance. The diffuse reflectance has less dependence on crack configurations, while direct illumination has more dependence on the crack configurations because it utilizes specular reflectance. Our results show that our proposed prototype can detect sub-mm edge cracks in multicrystalline silicon wafers, which is an important step in enabling thin silicon wafer manufacturing.

Stress release waves in plastic solids

Better understanding of damage under high strain rate deformation is critical; whether assessing the impact of new weapon systems, calculating safety issues, or even modeling modern high speed machining. Mott waves, one of the keystones needed to predict bomb debris size, are reexamined. Mott assumed instantaneous through thickness crack propagation and stress relaxation waves unloading the material surrounding the crack. But, following experimental evidence, crack extension velocity actually has limits. A new energy balance based framework, considering damage development during stress release waves propagation is presented. Exact boundaries for the time and wave position at failure are established. Particular wave paths close to these boundaries are analyzed. This work will be the basis to predict more accurately fragment size distributions having industrial as well as military applications.

Controlling cracking formation in fuel cell catalyst layers

Since cracks in catalyst layers of a fuel cell affect performance and durability, it is important to obtain the factors that control crack formation. Cracking characteristics are likely to differ due to the formation of agglomerates in catalyst inks. Herein, a critical crack thickness (CCT) is examined as a parameter for cracking characteristics by changing the formation of Pt/carbon by the presence or absence of an absorbed ionomer on Pt/carbon particles. The CCT was found to be four times greater for catalyst layers obtained from catalyst inks that are well dispersed by ionomer adsorption than for catalyst layers obtained from catalyst inks with a network of structured agglomerates. This shows that crack behavior can be controlled by ionomer adsorption into the Pt/carbon in catalyst ink. A well-dispersed catalyst ink produce a homogeneous Pt/carbon and ionomer distribution in catalyst layers with high fracture toughness, while catalyst inks with a network of structured agglomerates produce dense aggregates with small primary pores that generate high drying stress, and a self-organizing free ionomer that causes stress concentration in catalyst layers with a high risk of cracking. This research provided useful knowledge for controlling cracks and examining the effects of cracks on performance and durability.

Sensitive Damage Detection of Reinforced Concrete Bridge Slab by “Time-Variant Deconvolution” of SHF-Band Radar Signal

In this paper, we focus on ground-penetrating radar (GPR) for infrastructural health monitoring, especially for the monitoring of reinforced concrete (RC) bridge slab. Due to the demand of noncontact and high-speed monitoring technique which can handle vast amounts of aging infrastructures, GPR is a promising tool. However, because radar images consist of many reflected waves, they are usually difficult to interpret. Furthermore, the spatial resolution of system is not enough considering the thickness of target damages, cracks, and segregation are millimeter-to-centimeter order while the wavelength of ordinary GPR ultrahigh-frequency band is over 10 cm. To address these problems, for the purpose of sensitive damage detection, we propose a new algorithm based on deconvolution utilizing a super high-frequency (SHF) band system. First, a distribution of reflection coefficient is inversely estimated by 1-D bridge slab model. Because concrete is found to be a lossy medium at SHF band, we consider the attenuation of signal in deconvolution. The algorithm is called “time-variant deconvolution” in this paper. After the validation by simulation, the effects of the algorithm and frequency band on damage detection accuracy are evaluated by a field experiment. Though the results show a 1-mm horizontal crack is not detected by measured waves, when it is filled with water, it is detected by time-variant deconvolution. Moreover, the 1-mm dried crack is detected only by time-variant deconvolution at SHF band, which greatly emphasizes the peaks of the reflection coefficient of the crack.

Numerical analysis of the BWR-5 beltline region containing a crack postulated for an extended period of operation

In the case of a BWR-5, the wall of the reactor vessel located adjacent to the core is named beltline and it is subjected to neutron embrittlement, lowering the fracture toughness and increasing the yield strength of the material. Cracks can be developed in the internal surface of this area. In this paper, a semi elliptical part through thickness crack has been postulated along the axial direction. Its depth is 1.25 inch which is 0.25 of the thickness of the wall vessel and its length is 6 times the crack depth (7.5 inches). The material of the vessel is a low alloy carbon steel.In order to consider an extended period of operation, the analysis must cover sixty years of operation. The normal conditions of operation and start-up and shutdown transients have to be taken into account. For this purpose, a numerical evaluation was done with the Finite Element Method in conjunction with linear elastic fracture mechanics. This analysis was compared with the results of the procedure under the scope of the Appendix G of the ASME Code Section XI and the NUREG-0800. The results were used to establish the pressure and temperature limits.