Micro-crack Defects Research Articles

Electromagnetic Acoustic Detection of Steel Plate Defects Based on High-Energy Pulse Excitation

The conventional electromagnetic ultrasonic transducers (EMATs) rely on the static magnetic field created by magnets. The magnet increases the size of the EMATs, and the strong magnetic force of the magnet attracts the detected steel and even ferromagnetic particles. It can cause mechanical damage to the transducer and the detected objects. A new high-energy acoustic excitation system, without a static bias magnetic is designed, which does not include any magnets. As the core of the system, the high-energy pulse excitation power supplies a transient high voltage to the excitation coil by the LC oscillation circuit. The maximum amplitude of current can reach 1700 A, which is much larger than the current in the conventional EMATs. Compared with the conventional EMATs, the intensity of the ultrasonic signal is greatly strengthened and the size of the EMAT is effectively reduced. Therefore, it can detect high-temperature steel plates at a higher lift-off distance. In this paper, the transduction mechanism of high-energy pulse electromagnetic acoustic on ferromagnetic materials was studied, and the high-energy pulse excitation coil used for the A0 mode Lamb wave was designed. The interaction rule of the magnetic field, the force field, and the acoustic field, was obtained. Then, the EMAT lift-off characteristic experiment of the high-energy pulse excitation was carried out, and the defect detection experiment was conducted on a cracked steel plate. The results show that the A0 mode Lamb waves have caused a high signal-to-noise ratio and can accurately locate the crack, which has a great advantage in detecting the microcrack defects of ferromagnetic materials.

Effect of pre-deformation and artificial aging on fatigue life of 2198 Al-Li alloy

The effects of pre-deformation and artificial aging on fatigue life of 2198 Al-Li alloy were explored to further reveal its fatigue behavior and improve the damage tolerance. Fatigue life was investigated on 2198-T3 alloy after pre-deformation and aging treatments. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to study the fracture morphology, precipitation behavior, fatigue crack initiation and propagation. Results indicated that T1 (Al2CuLi) phase dominated the strengthening of 2198-T3 alloy after the further aging. Pre-deformation increased the number of dislocations in the matrix, which provided favorable position for nucleation of T1. The precipitation of θ′ (Al2Cu) and δ′ (Al3Li) were inhibited. Also, the formation and coarsening of precipitate-free zone (PFZ) were effectively avoided in this condition. Dislocation density in the 2198 Al-Li alloy matrix increased with the amount of pre-deformation, leading to an increase in micro-crack defects on the surface and inside alloy. As a result, decreased fatigue life and an increased crack growth rate of the alloy were observed. Overall, 2198 Al-Li alloy had the best fatigue life and damage tolerance enhancement after it underwent 3% pre-deformation and was aged at 155 °C for 15 h.

Theoretical study on infrared thermal wave imaging detection of semiconductor silicon wafers with micro-crack defects

The semiconductor silicon wafer with micro-crack defects was detected using infrared thermal wave imaging technique. The 3-D thermal conduction model in semiconductor silicon wafer excited by linear frequency modulated continuous laser was established, and it was solved by finite element method. The results show the effectiveness of the proposed method for detecting micro-crack defects in semi?conductor silicon wafers.

Comparative Evaluation of Microcrack Formation in Different Kinematics Using Rotary and Reciprocating File Systems: An In Vitro Study

To compare r oot microcrack formation after r oot canal preparation using ProTaper Next in r otation or forward r eciprocation and Waveone gold in r everse r eciprocating motion. Buccal r oots of 60 maxillary premolars with mature apices were selected, for different instrumentation techniques and divided into three groups. Coronal access was achieved and the canals were confirmed for apical patency. The canals were then instrumented using the following instrumentation techniques: ProTaper Next in r otation or forward r eciprocation or Waveone gold in r everse reciprocation. The tooth was then subjected to sectioning using a diamond saw under water cooling and then was visualized under the stereomicroscope for dentinal microcrack. The results showed that the maximum dentinal microcrack formed at apical 3 and 6 mm was in Waveone gold in reverse reciprocation followed by ProTaper Next in forward reciprocation and rotation. However, the p value was found to be not significant at 3 and 6 mm (p value-0.082 and 0.23). Nickle titanium rotary instruments tend to induce varied degrees of root dentinal damage during canal instrumentation. ProTaper Next files in rotation as well as forward reciprocation presented with minimal microcrack defects when compared with Waveone gold. Root canal preparation, when performed by manual or engine-driven techniques, has shown to produce structural defects in the root dentin. One of the causes of failures in root canal treatment is because of fracture in the dentin that occurs due to these procedures. Though all the motion kinematics caused microcracks in this study, it was seen that rotational motion produced the least structural damage to the dentin.

Picosecond laser seal welding of glasses with a large gap.

Ultra-fast lasers can realize selective welding of glasses through nonlinear effect and have the advantages of having high welding accuracy, having small heat-affected zone, having high joint strength and not requiring intermediate absorption layer, which provides a new idea for chip packaging. However, this method is limited to the condition of optical contact, which is difficult to use in engineering applications. To solve this problem, an innovative seal welding method by picosecond laser is presented in this paper. In this method, a picosecond laser performs firstly rapid oscillating scan local welding of the two natural overlap glasses with a large contact gap to form a closed area consisting of spot welds. The glass contact gap in the closed area can be reduced to about 1.5 μm through the solidification shrinkage effect of spot welds. Then a good seal weld without plasma ablation, micro-hole, and micro-crack defects can be achieved by picosecond laser in this closed area to realize seal welding of glasses with a large contact gap. The sealing test results show that the seal welding samples can keep good sealing performance for more than one week.

Cracking failure analysis of convection tube of heat recovery steam generator

The failure causes of cracks on a convection tube of heat recovery steam generator were analyzed by the optical microscope and scanning electron microscope analysis. The results show that the circumferential crack of the convection tube belongs to fatigue fracture. The micro-crack defect on the surface of fracture promotes the fatigue crack propagation.

The Rapid Detection Technology of Lamb Wave for Microcracks in Thin-Walled Tubes

Thin-walled tubes are a kind of pressure vessel formed by a stamping and drawing process, which must withstand a great deal of sudden pressure during use. When microcrack defects of a certain depth are present on its inner and outer surfaces, severe safety accidents may occur, such as cracking and crushing. Therefore, it is necessary to carry out nondestructive testing of thin-walled tubes in the production process to eliminate the potential safety hazards. To realize the rapid detection of microcracks in thin-walled tubes, this study could be summarized as follows: (i) Because the diameters of the thin-walled tubes were much larger than their thicknesses, Lamb wave characteristics of plates with equal thicknesses were used to approximate the dispersion characteristics of thin-walled tubes. (ii) To study the dispersion characteristics of Lamb waves in thin plates, the detection method of the mode was determined using the particle displacement–amplitude curve. (iii) Using a multi-channel parallel detection method, rapid detection equipment for Lamb wave microcracks in thin-walled tubes was developed. (iv) The filtering peak values for defect signal detection with different depths showed that the defect detection peak values could reflect the defect depth information. (v) According to the minimum defect standard of a 0.045-mm depth, 100,000 thin-walled tubes were tested. The results showed that the missed detection rate was 0%, the reject rate was 0.3%, and the detection speed was 5.8 s/piece, which fully meets the actual detection requirements of production lines. Therefore, this study not only solved the practical issues for the rapid detection of microcracks in thin-walled tubes but also provided a reference for the application of ultrasonic technology for the detection of other components.

Experimental investigation on True Triaxial Deformation and Progressive Damage Behaviour of Sandstone

Studying the true triaxial deformation characteristics and progressive damage behavior of sandstone is of great significance for the stability control of roadways. Both the conventional triaxial test (CTT) and the true triaxial compression test (TTT) were conducted for sandstone to investigate its deformation characteristics and the variation laws of volume strain during the progressive damage process under different confining pressures. The conducted experiments showed that both the axial and lateral strains of the rock prior to failure under CTT conditions increased with increasing confining pressure. However, with increasing intermediate principal stress (σ2) under TTT conditions, both the axial strain, and the lateral strain (ε2) gradually decreased, and the lateral strain (ε3, expansion) first slow down and then accelerated. Moreover, the anisotropic characteristics first gradually weakened and then enhanced. The variation of the volume strain increment and the volume strain rate of rock combined with the acoustic emission activity and a three-dimensional rock theoretical model with microcrack defects were analyzed in detail. During the stable crack growth stage III, the volume strain increment and volume strain rate increased with increasing confining pressure under CTT conditions, while they decrease after the initial increase with increasing σ2 under TTT conditions. During the unstable crack growth stage IV, the volume strain increment increased sharply, while the volume strain rate gradually slowed down with increasing confining pressure under CTT conditions. The internal cracks of the rock were gradually suppressed and the lateral expansion was gradually constrained. The volume strain increment first increased followed by a decrease, and the volume strain rate gradually slowed down after a noticeable acceleration with increasing σ2 under TTT conditions. The internal micro-cracks gradually evolved from inhibition (in the planes parallel to plane 1–2 and plane 2–3) to accelerated expansion (the planes along the σ2 direction), and the lateral deformation first weakened and then strengthened.

Defect Enhancement Generative Adversarial Network for Enlarging Data Set of Microcrack Defect

This paper presents a micro defect data set expansion method focuses on the microcrack defect of magnetic ring. Deep neural networks require a mass of training samples to be fully optimized. However, it is difficult to obtain a mass of defective samples in industrial field. In the case of insufficient samples, using GANs (Generative Adversarial Networks) for data expansion can effectively solve the problems of model over-fitting and low detection accuracy caused by insufficient training samples. However, it is difficult for conventional GANs to generate microcrack defective samples of high quality. This paper presents Defect Enhancement Generative Adversarial Network (DEGAN). This model can generate microcrack defects with obvious defect characteristics and high diversity. The experimental results show that the defective samples generated by DEGAN are very close to the real ones. The data set amplified by this model can significantly optimize deep neural network and achieve higher defect detection accuracy.

Structure and half-metallic ferromagnetism of quaternary Heusler compounds CoMnZn<i>Z</i>

Using the first principle full-potential linearized augmented wave method we study the electronic structure and elastic and magnetic properties of CoMnZn<i>Z</i> (<i>Z</i> = Si, Ge, Sn, Pb) LiMgPdSn-type Heusler alloys. These compounds have the composition CoMnZn<i>Z</i> with 1︰1︰1︰1 stoichiometry, where <i>Z</i> denotes the main group element Si, or Ge, or Sn, or Pb. The exchange-correlations are treated within the generalized gradient approximation of Perdewe-Burke-Ernzerhof. For each of all studied Heusler alloys, the ferromagnetic state is considered to be more stable than the paramagnetic state, judged by the energy. The total energy of the magnetic calculation is lower than that of the nonmagnetic state for each of all three serise compounds at the equilibrium lattice constant, indicating that the magnetic state is more stable than the nonmagnetic state. We determine the elastic constants <i>C</i><sub>11</sub>, <i>C</i><sub>12</sub> and <i>C</i><sub>44</sub>, which have not been established previously in experiment nor in theory. The elastic constant indicates the weakened resistance to sheardeformation compared with the resistance to unidirectional compression. We derive other mechanical parameters, i.e., the shear modulus <i>G</i>, Young’s modulus <i>E</i>, Poisson’s ratio <i>ν</i>, and shear anisotropic factor <i>A</i>, which are the important elastic moduli for applications. These compounds each have a lower anisotropy and possess a low probability to develop micro-crack or structural defect in its growing process. The sound velocity and Debye temperature for each of the CoMnZn<i>Z</i> (<i>Z</i> = Si, Ge, Sn, Pb) compounds in their stable structure are calculated. The CoMnZnPb exhibits the lowest Debye temperature, and the highest value is observed for CoMnZnGe. The electronic structure calculations show that CoMnZn<i>Z</i> (<i>Z</i> = Si, Ge, Sn) each exhibit a gap in the band of minority states, and they are clearly half-metallic ferromagnets, except for the CoMnZnPb. The CoMnZn<i>Z</i> (<i>Z</i> = Si, Ge, Sn) compounds and their magnetic moments are in reasonable agreement with the Slater-Pauling rule, and they comply with a Slater-Pauling rule of <i>M</i><sub>t</sub> = <i>Z</i><sub>t</sub> – 28, which indicates the half metallicity and high spin polarization for these compounds. The CoMnZnSi compound has the largest half-metallic gap value and the gap is about 0.66 eV. The magnetic properties are primarily determined by the Mn atoms, which contribute the highest magnetic moments. The localmoment of the <i>Z</i> element atom is negligibly small. The hybridization of the d orbitals between Co and Mn can explain the origin of the Slater-Pauling rule in half-metallic quaternary Heusler alloys. The half-metallic gap comes mainly from the interaction between the Co and Mn atoms.

The infrared thermal wave imaging detection of micro-crack defects in Ti-Al alloy plate

The Ti-Al alloy has good physical, chemical and mechanical properties, and it is the preferred material in aerospace and other special fields. The laser spots array is used for thermal excitation of the Ti-Al alloy specimen. Based on the fractional differential equation and Fourier heat conduction equation, the fractional heat transfer model of Ti-Al alloy plate specimen ex-cited by short pulse laser spots array is established, and the infrared thermal imaging simulation analysis is carried out by finite element method. The effects of crack width, crack depth and the distance between the crack and its nearest laser spot center on temperature abrupt jump is analyzed. With the increase of crack width and depth, the temperature abrupt jump increases, but the trend gradually slows down. The distance between the crack and its nearest laser spot center has a significant effect on the temperature abrupt jump. With the increase of the distance, the temperature abrupt jump first increases and then decreases. When the distance equals the laser spot radius, that is, the crack is tangent to the spot, the temperature abrupt jump reaches its maximum.

Nano-indentation investigation on the local softening of equiaxed zone in 2060-T8/2099-T83 aluminum-lithium alloys T-joints welded by double-sided laser beam welding

Double-sided laser beam welding of 2-mm-thick 2060-T8 and 2099-T83 aluminum-lithium alloys T-joint was performed with AlSi12 wire. Microstructure and nano-indentation hardness characteristics were studied especially in nondendritic equiaxed zone (EQZ). In comparison with the precipitates of LiAlSi, Mg2Si and Al2Cu in weld zone except for the EQZ, an AlCuFeMn icosahedral quasicrystal was detected in the EQZ and Al3Zr particle could be the heteromorphic core of this quasicrystal phase. Local softening problem was confirmed in the EQZ's localized enrichment regions by nano-indentation tests. The cross sections of different indents in and out of the EQZ's local softening area prepared by focused ion beam (FIB) were examined by using transmission electron microscopy (TEM). For the first time, it was directly observed that an intergranular tiny hot crack was formed right underneath the indent tip in the EQZ's local softening area, and mainly thin Al2Cu precipitates were generated and sparsely distributed in equiaxed grains and their boundaries. However, thick and continuous Al2Cu intergranular precipitates with no microcrack defect were observed below the indent in the other area in the EQZ without softening. Obviously, local softening in the EQZ was most likely to be induced by a combined action of the formation of microcrack and the absence of abundant Al2Cu precipitates in equiaxed grain boundaries. Since the quantity and size of Al3Zr particle were both much smaller than those of Al2Cu, it had more limited impact on the EQZ's local softening than Al2Cu particle in equiaxed grain.

The effect of external magnetic flux field in the QTS weldment on the change of fatigue crack propagation behaviors

This investigation discusses fatigue crack propagation behaviors on the welded joint of Hot Rolled Quench Tempered Steel (QTS) in which during welding process the fusion zone of the joint was subjected to magnetic flux field. The QTS weldability is not really excellent due to the change of microstructure into tempered martensite, and the possibility of microcrack defect on the welding area is still high. The purpose of the investigation is to know the effect of External Magnetic Flux (EMF) field during welding process on fatigue crack propagation behaviors. The external magnetic flux is applied transversely from two sides of the workpiece using a DC powered solenoid of 0, 3, 6, 9 and 15 Amperes. The effect of EMF is more sensitive to decrease the tensile strength and the fatigue crack propagation rate of the weld area. The result shows that the electromagnetic force on the weld pool increases. It causes the liquid metal circulation rate to increase and welding defects to decrease. This indicates that the liquid metal and filler metal are easily mixed, the release of gas from liquid metal to surface before solidification easily happens. The finding shows that the effect of EMF is more efficient.

Detection of micro-cracks using nonlinear lamb waves based on the Duffing-Holmes system

To solve the problem that weak nonlinear Lamb waves caused by micro-cracks are often buried in background noises, a Duffing-Holmes system was used to enhance the weak nonlinear Lamb waves and the crack size was quantitatively characterized using Lyapunov exponent(LE). The frequency and amplitude critical threshold of the external driving force were determined according to the center frequency of excitation signal and the Poincaré map of the Duffing-Holmes oscillator, respectively. After periodic extending and filtering, the Lamb waves were input to the Duffing-Holmes system, and then the nonlinear second harmonic was identified according to the phase trajectory and Poincaré map. Based on the phase space reconstruction of Duffing system output, the maximal LE was calculated to characterize the magnitude of the second harmonic. Based on S0 Lamb wave mode, the simulations on models with different micro-crack sizes showed that the Duffing-Holmes system could accurately identify the structural micro-cracks under strong noise interference. Compared with the traditional acoustic nonlinearity parameter β′, the damage index defined here had a better linear relationship with the crack size, and the micro-cracks could be accurately quantified. The proposed method has obvious advantages for the detection of micro-crack defects under noise interference, and can greatly improve the sensitivity of nonlinear Lamb waves detection.

Corrosion behavior in seawater of arc thermal sprayed Inconel 625 coatings with sealing treatment

In this study, zinc powder (Zinc), inorganic ceramic (Ceramic), and organic-inorganic composite ceramic (Hybrid) sealing processes were applied to improve the corrosion resistance of arc thermal sprayed Inconel 625 coatings. The Inconel 625 coating in seawater solution showed poor corrosion resistance due to its porosity and Cr depletion resulting from Cr oxide formation during the arc thermal spraying process. Zinc sealing formed a stable cathodic protection, but the local corrosion damage occurred largely in the anodic polarization experiment caused by an excessive dissolution reaction and a weak bonding force between the zinc particles. Hybrid sealing gave a better sealing effect, but surface corrosion damage appeared due to interconnected micro-crack defects on the surface. Ceramic sealing gave the best corrosion protection due to a high sealing efficiency and a lower surface damage tendency resulting from effectively blocked defects of the Inconel 625 coating.

Micro-crack Detection of Polycrystalline Silicon Solar Wafer

ABSTRACTThis paper proposes a machine vision scheme for detecting micro-crack defects in polycrystalline solar wafer manufacturing. Micro-crack inspection is very challenging because this type of defect is very small and completely invisible to naked eyes. The presence of other heterogeneous structures in solar wafer like grainy materials and dark regions further complicates the problem. In this paper, an efficient micro-crack inspector comprising of near infrared illumination and improved Niblack segmentation algorithm is proposed. Empirical and visual results demonstrate that the proposed solution is competitive when compared to the existing Niblack thresholding formulas and achieve better precision and performance in terms of Pratt's figure of merit.

Micro-crack detection of multicrystalline solar cells featuring an improved anisotropic diffusion filter and image segmentation technique

This paper presents an algorithm for the detection of micro-crack defects in the multicrystalline solar cells. This detection goal is very challenging due to the presence of various types of image anomalies like dislocation clusters, grain boundaries, and other artifacts due to the spurious discontinuities in the gray levels. In this work, an algorithm featuring an improved anisotropic diffusion filter and advanced image segmentation technique is proposed. The methods and procedures are assessed using 600 electroluminescence images, comprising 313 intact and 287 defected samples. Results indicate that the methods and procedures can accurately detect micro-crack in solar cells with sensitivity, specificity, and accuracy averaging at 97%, 80%, and 88%, respectively.

Experimental Study on the Influences of Deffctive Concrete on Chloridion Penetration Resistance

Micro-crack defects in concrete can affect the Chloride ion permeability of concrete in a certain extent. In the paper, the splitting experiments of concrete specimens from the four different dimensions are carried out in order to build the even damage fields. The displacements of the concrete specimens’ surface are analyzed through DSCM (Digital Speckle Correlation Method). Therefore damage parameters are determined that are damage area based on the MCOD (Micro-Crack Opening Displacement) and damage variable DE based on the weighting-damping elastic modulus. At the same time, Clˉ permeation of concrete specimens before and after the splitting test are studied by NEL method, and Clˉ diffusion coefficients D are recorded in detail. Further relationships between damage parameters and diffusion coefficients of Clˉ are discussed.

Silicon PV Wafers: Mechanical Strength and Correlations with Defects and Stress

Nanoindentation was used to measure the mechanical properties of 200mm diameter (100) CZ Si wafers subjected to the initiation and propagation of micro-crack defects. Silicon amorphization and phase changes were observed and accompanied by a monotonic decrease in hardness and elastic modulus, as the nanoindent tip approached the micro-crack shank or point. Identification and profiling of these localized phase transitions was obtained in the vicinity of the micro-cracks using electron back-scattered diffraction (EBSD) and Raman spectroscopy. It was found that the amorphous Si regions extend for about 10 µm at the edges and ahead of a moving crack tip. Wafers from ingots grown at faster growth rates with enhanced thermal gradients and associated point defect/impurity produce large localized stresses in the wafer core, which are capable of changing the path of propagating cracks. FTIR and Raman spectroscopy analysis were used to quantify local stresses due to radial oxygen precipitate variations. The resulting stress modified crack deviates considerably from energetically favorable [110]/(111) directions, following a radial path suggesting a ductile fracture failure mode.

Effect of Pre-Shot Peening on the Fretting Fatigue Behavior of Plasma Nitrided AISI 420 Stainless Steel at 350 °C

AISI 420 martensite stainless steel was plasma nitrided with/without shot peening (SP) previously at 350 °C. The FF resistance of researched material was evaluated using a rotating bending fatigue machine and a home-made apparatus. The results indicated that low-temperature nitriding alone and the combined treatment both improved the FF resistance of AISI 420 stainless steel significantly. However, the later did not lead to higher FF resistance than the former. FF cracks tended to initiate at microcrack defects induced by SP.