Presence Of Microcracks Research Articles

New Experimental Evidence for Drying Shrinkage of Alkali-Activated Slag with Sodium Hydroxide.

Alkali-activated slag (AAS) is emerging as a possible and more sustainable alternative to Ordinary Portland Cement (OPC) in the construction industry, thanks to its good mechanical and chemical properties. Conversely, the effects of its high drying shrinkage are still a concern for its long-term durability. This study aims to investigate the drying shrinkage behaviour of six AAS/sodium hydroxide mortar compositions and the main phenomena affecting their drying shrinkage behaviour. Specifically, the molarity, solution-to-binder ratio (s/b), autogenous shrinkage, creep compliance, microcracking, and carbonation are considered as possible causes of the differences between AAS and OPC. The results show that it is not possible to correlate the shrinkage magnitude with the molarity of the activating solution, while an increase in the s/b increases the drying shrinkage. Concerning the other factors, autogenous deformation remains significant even after a period of 112 days, while the creep compliance is definitely affected by the drying process but does not seem to affect the shrinkage magnitude. Furthermore, the presence of microcracks caused by the drying process definitely influences the drying shrinkage. Finally, carbonation depends on the molarity of the activating solution, even though its effects on the material are still unclear.

Effects of Wire Electrical Discharge Finishing Cuts on the Surface Integrity of Additively Manufactured Ti6Al4V Alloy.

The Selective laser melting (SLM) technology of recent years allows for building complex-shaped parts with difficult-to-cut materials such as Ti6Al4V alloy. Nevertheless, the surface integrity after SLM is characterized by surface roughness and defects in the microstructure. The use of additional finishing technology, such as machining, laser polishing, or mechanical polishing, is used to achieve desired surface properties. In this study, improving SLM Ti6Al4V alloy surface integrity using wire electrical discharge machining (WEDM) is proposed. The influence of finishing WEDM cuts and the discharge energy on the surface roughness parameters Sa, Svk, Spk, and Sk and the composition of the recast layer were investigated. The proposed finishing technology allows for significant improvement of the surface roughness by up to 88% (from Sa = 6.74 µm to Sa = 0.8 µm). Furthermore, the SEM analyses of surface morphology indicate improving surface integrity properties by removing the balling effect, unmelted particles, and the presence of microcracks. EDS analysis of the recast layer indicated a significant influence of discharge energy and the polarization of the electrode on its composition and thickness. Depending on the used discharge energy and the number of finishing cuts, changes in the composition of the material in the range of 2 to 10 µm were observed.

Analysis of Shear Bond Strength and the Quality of the Interface Between Composite Resin and Novel Hybrid Calcium Silicate Cements

Hybrid calcium silicate cements (HCSC) are new bioactive materials of the fourth generation with the main application as pulp capping agents and/or as liners in indirect pulp capping. The aim is to analyze the shear bond strength and the quality of the interface between a composite restorative material and a new bioactive material and other hybrid cements at the same conditions. The used pulp-capping hybrid materials: TheraCal and the new hybrid biomaterial BioCal-Cap. In this study only one type of adhesive system (Scotchbond 3M, St. Paul, MN, US) and resin-based composites (Filtek Ultimate 3M, USA) were used. For this purpose, 60 molds were prepared and divided into 4 groups. Half of them were filled with TheraCal and the other 30 with BioCal-Cap. The shear bond strength between calcium-silicate cements and the composite material was investigated. Mann-Whitney test and Wilcoxon Signed Ranks Test were used for statistical analysis of the results. In both of the materials there was no statistically significant difference in the results after the immediate and postponed placement of the composite material. Nevertheless, in both of them there was a tendency for a slight increase in shear strength in the samples which delayed the application of the restorative material. The novel hybrid material BioCal-Cap was reported to have higher shear strength values (18.06 – 20.25 MPa) at both time periods in comparison to the values for TheraCal (13.71 – 15.58 MPa). The data clearly showed that the time of the composite material placement did not affect the shear bond strength in each of the observed groups. The median value was shifted higher after the immediate composite placement, which supported one-step treatment approach to the novel hybrid material BioCal-Cap, while the TheraCal LC exhibited the opposite tendency. The adhesive layers in both hybrid cements were homogeneous, properly structured, without the presence of microcracks and gaps, and had different thicknesses at different recovery times.

Unveiling the strengthening mechanisms of as-cast micro-alloyed CrMnFeCoNi high-entropy alloys

The strengthening effects introduced by the addition of 2 at% titanium, vanadium, and niobium, as the well-known micro-alloying elements, to the model CrMnFeCoNi high-entropy alloy (HEA) were studied in the present work. Accordingly, the microstructure, mechanical properties, and strengthening mechanisms of the as-cast CrMnFeCoNi, (CrMnFeCoNi)98Ti2, (CrMnFeCoNi)98V2, and (CrMnFeCoNi)98Nb2 HEAs were investigated by electron-backscattered diffraction (EBSD), tensile testing, differential scanning calorimetry (DSC) thermal analysis, and theoretical calculations and measurements. Depending on the nature of the added elements and their segregation tendency during solidification, different degrees of microstructural refinement were observed in the as-cast ingots. The segregation tendency of Ti was found to be more pronounced compared to that of V (as predicted by the Scheil-Gulliver model), leading to a more refined secondary dendrite arm spacing (SDAS) and grains (resulting from the growth restriction factor and constitutional undercooling). Moreover, Nb addition led to the formation of the (Cr,Fe,Ni)2(Nb) Laves phase at the last stages via the eutectic solidification. The effect of the Laves intermetallic compound (type C14) and twinning-induced plasticity (TWIP) effect on the strength-ductility synergy was discussed. Moreover, a detailed modeling of the strengthening mechanisms revealed that the grain boundary strengthening (represented by the Hall-Petch relationship) and solid solution hardening (due to the lattice distortion) were the primary contributors to the increase in yield strength of V- and Ti-containing HEAs. On the other hand, the formation of the Laves phase, besides solid solution and grain boundary strengthening mechanisms, could lead to a considerable increase in the yield strength of the Nb-containing sample; although it would deteriorate the ductility of the alloy, as also discussed based on its brittle fracture surface appearance and the presence of micro-cracks. Accordingly, the present study is applicable to the design of modified Cantor-based HEAs.

Influence of milled and acid-treated graphene oxide on the self-healing properties of graphene oxide reinforced polyurethane

The presence of microcracks in polymer coatings degrades the structural stability and lifespan of the protective layers. Enhancing the polymer-coated surfaces with self-healing properties has attracted considerable attention as it increases the reliability and lifespan of polymer systems. Herein, the strength and self-healing properties of polyurethane/graphene oxide (PU/GO) composite materials were improved using two GO treatments, referring to mechanical milling and chemical treatment with HF. When GO was surface treated for 5 h with an HF concentration of 40%, and PU/GO was heated 80 °C for 6–8 h, the over 80% healing efficiency can be obtained. The HF-treated GO-reinforced composites demonstrated high self-healing efficiency and remarkable mechanical performance, making them a promising self-healing material for protecting structural composites.

Durability of GGBS based alkali-activated binder treated peat through wetting-drying cycle

This research is aimed at investigating the effect of wetting-drying (NWD) cycles on pH, electrical conductivity, unconfined compressive strength (UCS), and mass loss of ground granulated blast furnace slag (GGBS)-NaOH stabilised sapric, fibric, and haemic peat. The ordinary Portland cement stabilised peats were selected as control samples for comparison. The microstructure of GGBS-NaOH stabilised peats before and after NWD cycles were analysed through X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), and Fourier transform infrared spectroscopy (FTIR). The pH, electrical conductivity decrease with increasing NWD cycles. The GGBS-NaOH-treated peats exhibits higher UCS values of 3.1, 4.2, and 1.2 times more than cement-treated sapric, fibric, and haemic peats respectively. The FESEM micrographs reveal the presence of smooth and dense cementitious materials before NWD cycles, and the presence of microcracks after NWD cycles. Further, the FTIR confirms the presence of physical water in cavities of treated peats after NWD cycles.

Developing methods and strategies to reduce the impact of rumble strips (RS) on pavement performance

ABSTRACT Despite numerous research studies inspecting the safety benefits of centreline and shoulder rumble strips (RS) on pavements, there has been limited exploration into the impact on pavement durability due to the installation of RS. The RS milling process creates micro-cracks in the pavement structure that could propagate to macro cracks over time with freeze and thaw, moisture damage, and traffic loading, thereby decreasing the lifespan of the pavements. The primary objective of this study was to develop testing procedures for RS samples as well as analysis methods to determine the impact of potential controlling factors such as permeability and resulting moisture damage; freeze–thaw; the presence of microcracks; surface layer sealants (chip seal); RS size, depth, type, and RS location in relation to the longitudinal construction joint. The flexural beam fatigue test suggests that sinusoidal rumble strips had 38% increased fatigue cracking resistance. Moreover, the sinusoidal rumble strip resulted in 41% improved cracking and structural performance in the Hamburg Wheel Tracking Test (HWTT) tests. Also, the rutting performance of sinusoidal rumble strips was similar to the control section without rumble strips. In addition, 0% moisture infiltration was observed in sinusoidal rumble strip specimens.

Both network architecture and micro cracks effects on lacuno-canalicular liquid flow efficiency within the context of multiphysics approach for bone remodeling

When physical forces are applied to bone, its mechanical adaptive behaviors change according to the microarchitecture configuration. This leads to changes in biological and physical thresholds in the remodeling cell population, involving sensor cells (osteocytes) interacting with each other and changes in osteocyte shape due to variation in lacunar shape. The resulting alterations in fluid flow leads to changes in the membrane electrical potential and shear stress. Eventual creation of microcracks, may lead in turn to modify cell activity. In contrast, the redundancy in the lacuno canalicular network (LCN) interconnectivity maintains partial flow. Our goal was to investigate the role of fluid flow in LCN by proposing a model of electro-mechanical energy spread through inhomogeneous microarchitectures. We focused on mechano-sensitivity to changes in load-induced flow impacted by neighboring micro cracks and quantifying its critical role in changing, velocity, shear stress and orientation of liquid mass transportation from one cell to another. To enhance the concept of intricacy LCN micro-structure to fluid flow, we provide a new combined effects factor considered as osteocytes sensor efficiency. We customized an influence function for each osteocyte, coupling: in one hand, the spatial distribution within remodeling influence areas, conducting a significant fluid spread, leading hydro-dynamic behavior and impacted further by presence of micro cracks and; in other hand, the fluid electro kinetic behavior.As an attempt to fill the limitations stated by many of the recent studies, we reveal in numerical simulation, some results which cannot be measured in vitro/in vivo studies. Numerical calculations were performed in order to evaluate, among many others, how liquid flow conditions changes between lacunas, how the orientation and the magnitude of the governing flow in LCN can regulate osteocytes efficiency. In addition to be regulated by osteocytes, a direct effects of fluid flow are also acting on osteoblast activity. In summary, this new approach considers mechano-sensitivity in relation to liquid flow dynamic and suggests additional pathway for Osseo integration via osteoblast regulation. However, this novel modeling approach may help improve the mapping and design bone scaffolds and/or selection of scaffold implantation regions.

Carbonate Pore Shape Evaluation Using Digital Image Analysis, Tomography, and Effective Medium Theory

Carbonate rocks have a wide variety of pore shapes and different types of grains, which greatly affect the elastic properties and characteristics of the reservoir. This causes certain difficulties in petroelastic modeling. One of the problems is the scale of the input data, which is then used to build the rock physics model. The paper presents the results of studying three core samples of carbonate rocks of the Upper Devonian and Lower Carboniferous age, which are located in the South Tatar arch (Volga-Ural oil and gas basin (Russia)). To evaluate the structural characteristics of the pore space, the effective medium theory is used. The input data are the results of laboratory studies that include measurements of the velocities of longitudinal and transverse waves, porosity, and thin section and computed tomography analysis. When using the computed tomography, the core samples are analyzed at different resolution (12–37 µm/voxel). The tomography studies of pore space at different scales provide rather different values of porosity and pore aspect ratio. The tomography-based porosity estimations also differ from the experimentally measured porosity (up to 10%). The pore space characteristics provided by different datasets are used to build a rock physics model for the studied rocks that helps to estimate the elastic wave velocities with three different methods of effective medium theory (self-consistent approximation, differential effective medium (DEM), and the Kuster–Toksöz method). A comparison of the velocity estimations with their experimental analogs for dry rocks may indicate the presence of microcracks whose size is beyond the tomography resolution. Improved rock physics models incorporating both pores and microcracks are then used to predict the elastic wave velocities of fluid-saturated rock in a wide porosity range. It is demonstrated that the predicted values significantly differ (up to 30%) from those provided by the rock physics (RP) models constructed without the support of the tomography results. Moreover, other types of models are considered in which the difference in experimental and theoretical velocities is attributed to changes in the host matrix properties as compared to the calcite polycrystal, which are caused by various reasons.

Study of superhydrophobic concrete with integral superhydrophobicity and anti-corrosion property

Due to the natural hydrophilicity of concrete, steel reinforced concrete structure is vulnerable to be corroded by chloride ions in marine environment. Fabricating superhydrophobic concrete is a good measure to improve the water repellency of concrete and to prevent steel reinforced corrosion damage by chloride ions. Herein, STA solution was put into fresh mortar to prepare superhydrophobic concrete with good overall hydrophobicity and corrosion resistance. The results indicate that superhydrophobic concrete has stable hydrophobicity under different pH values. Compared with ordinary concrete, the water absorption rate of superhydrophobic concrete is reduced by 30%. Moreover, the ground powder of superhydrophobic concrete also shows hydrophobicity. Furthermore, superhydrophobic concrete has good anti-corrosion property even in the presence of micro cracks. Expectedly, the proposed environmentally-friendly method may have application value in increasing durability and anti-corrosion property of coastal concrete structures.

Study on Strength and Durability Properties of Polypropylene and Carbon Fiber Reinforced Concrete Composite

Abstract: Concrete is the most widely utilized “manmade” material globally for construction in many developing countries in all types of civil engineering works. Also, concrete is an environmental - friendly material and in areas of growing environment – related awareness that is of prime importance. The weakness of concrete is due to the presence of micro cracks at mortar aggregate interface. To overcome the existing problems addition of fibers in the concrete has been come in to practice. In fiber reinforced concrete the fibers are added to the concrete mix so that those are discontinuous fibers will be uniformly distributed in the mix and improve the concrete properties in all directions. In this research paper, an attempt is made to use fibers only .The experimental investigation consisting of casting and testing of compression tests were conducted on 150x150x150mm cube and 150mmx300mm, cylindrical specimens using test method that gave the complete compressive strength, split tensile test using with and without fiber.The strength and durability properties are carried out for various mix designations and compared with normal conventional concreteIn present project work M25 grade considered and the polypropylene (PP) fiber at 0.5,1 ,1.5 ,2 and 2.5% volume fractions and carbon fibers at 1, 1.5, 2,2.5 and 3% volume fractions are introduced into fiber‐reinforced. Based on the results will find the superior crack resistance, improved tensile strength. The proposed sample will be tested at 7 days, 21 days and 218days age of curing. These results will show whether the FRC is suitable for proper confinement or not for structures subject to extreme load conditions

Effect of simultaneous Cu and Nb doping Bi4V2O11 on structural and electrical properties of Bi4V2−xCux/2Nbx/2O11−3x/4

BiMeVOx compounds Bi4V2-xCux/2Nbx/2O11-3x/4 doubly substituted solid solution, with identical compositions of Cu2+ and Nb5+ ions, show an area of existence from x ​= ​0 to 0.5. X-ray diffraction measurements and thermal analysis have shown that, depending on the composition, the three main Bi4V2O11 polymorphs α, β and (γ/γ′) are observed at room temperature. The evolution of the electrical conductivity with the rate of substitution has been investigated by electrochemical impedance spectroscopy (EIS) in the temperature range 120–720 ​°C. The highest values of conductivity are observed for samples with x ​= ​0.2. Scanning electron microscopy (SEM) shows an important grain growth and the presence of micro-cracks in the ceramics with x ​= ​0.5 composition.

Abfractive lesions of teeth in dogs

Introduction . The study of patterns and features of the structural state of the dentoalveolar apparatus in representatives of the Canidae family is one of the urgent problems of veterinary dentistry. Understanding the etiological factors of pathologies of the dentoalveolar system allows the veterinarian and pet owners to preserve the function of the tooth as an organ, as well as its functionality by preserving the dentition. It has long been known and confirmed that the pathology of the dentition leads to the development of chronic diseases of the gastrointestinal tract, musculoskeletal system, cardiovascular system and respiratory organs. To systematize the clinical signs of defects and lesions of hard tissues of the teeth, a brief description of these lesions, their classification and clinical signs are given, which are taken from specialized sources and the practical experience of the authors. The purpose of the study was to systematize the clinical signs of defects and lesions of hard dental tissues in dogs of dwarf, small and medium breeds. Objects and methods . A sufficient analysis of significant specialized literature data and a clinical examination of the oral cavity organs were carried out using additional research methods, adopted in dentistry, on dogs (n = 70, dwarf, small and medium breeds). Results . To eliminate the gap in information about abfractional lesions of the teeth, a brief description of these lesions, their classification and clinical signs are given. Based on the study, the following specifications of clinical signs of dental lesions should be distinguished: the presence of microcracks, slight damage to tooth enamel, the formation of a mesh pattern of enamel and the predominance of horizontal lines over vertical ones, and the absence of gum recession. Our studies allow us to identify some features of the systematization of dental lesions in dogs, which is only the initial stage in the study of this complex process. Further research will improve the treatment and prevention measures and methods for the differential diagnosis of abfraction lesions of the teeth in dogs of small and dwarf breeds.

A Micromechanical Model for Damage Evolution in Thin Piezoelectric Films

Thin-film piezoelectric materials are advantageous in microelectromechanical systems (MEMS), due to large motion generation, high available energy and low power requirements. In this kind of application, thin piezoelectric films are subject to mechanical and electric cyclic loading, during which damage can accumulate and eventually lead to fracture. In the present study, continuum damage mechanics and asymptotic theory are adopted to model damage evolution in piezoelectric thin films. Our purpose is to develop a new interface model for thin piezoelectric films accounting for micro-cracking damage of the material. The methods used are matched asymptotic expansions, to develop an interface law, and the classic thermodynamic framework of continuum damage mechanics combined with Kachanov and Sevostianov’s theory of homogenization of micro-cracked media, to characterize the damaging behavior of the interface. The main finding of the paper is a soft imperfect interface model able to simulate the elastic and piezoelectric behavior of thin piezoelectric film in the presence of micro-cracking and damage evolution. The obtained interface model is expected to be a useful tool for damage evaluation in MEMS applications. As an example, an electromechanically active stack incorporating a damaging piezoelectric layer is studied. The numerical results indicate a non-linear evolution of the macroscopic response and a damage accumulation qualitatively consistent with experimental observations.

MATHEMATICAL AND MECHANICAL RELATIONSHIP OF CRACKS GRIFFITH AND A NEW TYPE IN SOME NANOTECHNOLOGIY

In this paper, for the first time, mathematically strictly, models of Griffiths cracks and cracks of a new type are constructed simultaneously. Accurate models are constructed as a result of the convergence of the ends of two semi-infinite deformable stamps located on a deformable multilayer base. The mechanism of destruction of the medium by cracks of a new type is radically different from the mechanism of destruction of the medium by Griffiths cracks, and has so far been poorly studied. Griffiths formed his cracks with a smooth border as a result of compression from the sides of an elliptical cavity in the plate. Cracks of a new type have a piecewise smooth border, resulting from the replacement of an ellipse with a rectangle compressed from the sides. The construction of models in the work is based precisely on these descriptions of the origin of cracks formed by the ends of approaching deformable stamps. When creating models, the results of previously performed studies on the construction of exact solutions to a number of integral equations of contact problems, published in this journal, are used. The paper found that the destruction of the medium, in the case of cracks of a new type, occurs earlier than in the case of Griffith cracks, which, according to the authors, explains the reason for the destruction of Griffith cracks in the experiment earlier than the theory predicts. Griffiths explained this by the presence of microcracks on the border of the main crack. The authors tend to believe that the cause is the transformation of one type of crack into another, under certain conditions. The results obtained make it possible to compare and quantify the behavior of the medium in the presence of both one and another type of cracks. It is applicable both for the study of mutual transitions of Griffith cracks and cracks of a new type, and for the construction of models of adhesion of wetted nanoparticles.

Ways to solve welding problems of bridge structures for arctic service

The problems of material selection for steel bridge structures for arctic service and welding of such structures in pre-fabricated conditions and at sites are considered. The sources for the research included data on the requirements for steels for the manufacture of metal bridge structures for arctic service, the specifics of their welding, control of structure formation and mechanical properties of welded joints. The main reason for the insufficient impact strength at low temperatures is the presence of microcracks on the grain boundaries of the metal crystal structure, the cause of which is the presence of phosphorus and sulfur impurities in structural steels. To improve cold resistance properties, welding modes should be chosen in such way, that liquid metal stays for the minimum amount of time at the melting point of iron sulfide on the grain boundaries of the metal crystal structure. The necessity of engineering comparative analysis with the application of modern research methods of available methods for welding of bridge structures for arctic service is shown.

Wear resistance of deposited layer with Kh25N10V8 alloy on 08Kh14N5M2DL steel at elevated temperatures

The results of the study of the effect of defects in the deposited layer on the durability of deposited samples made of steel 08Kh14N5M2DL operating under friction conditions at high heating temperatures up to 800...900 °С and significant loads in the zone of tribological contact are presented. Defects in the deposited layer were pores, non-fusion and cracks. Defects in the friction tested specimens were determined by X-ray computed tomography. It has been established, that the wear resistance of the surfacing layer without defects decreases both with increasing test temperature and the load on the tribological contact. A particularly serious decrease in wear resistance was caused by the presence of microcracks in the deposited layer of the mesh. To a lesser extent, the amount of wear was affected by the presence of porosity in the deposited layer.

Low carbon bainitic steel processed by ausforming: Heterogeneous microstructure and mechanical properties

Thermodynamic stability of austenite is a primary factor affecting mechanical properties of low-carbon bainitic steels. In this work, ausforming was applied to a low-carbon steel, in which additional dislocations and defects were induced in austenite prior to isothermal transformation of bainite at a relatively low temperature. Hence, the stability of the retained austenite was improved, and the formation of fresh martensite during the secondary stage of cooling was suppressed. Results of microstructure characterizations by electron backscatter diffraction (EBSD), X-ray diffraction (XRD), and dilatometry were examined. The correlations between the process parameters and developed heterogeneous microstructures were established. It was found that the volume fraction of retained austenite was greatly promoted along with the reductions of fresh martensite and bainitic ferrite. Tensile properties of the bainitic steel were enhanced due to the occurred heterogeneous microstructure and fair balance of its phase constituents. The significantly increased strengths were mainly attributed to microstructure refinement as well as the enrichment of geometrically necessary dislocations (GNDs) within the bainitic ferrite and retained austenite. Furthermore, a large fraction of retained austenite showed a significant impact on the enhancement of both uniform elongation and strength, benefitting from the transformation-induced plasticity (TRIP) effect. The presence of microcracks and tiny dimples with thick tearing edges in the vicinity of the inclusions in the fractographs were directly related to the refined microstructure, which certainly provided a remarkable ability to reduce the formation of macro/micro-cracks and prolong the post-necking. • Ausforming improves the thermodynamic stability of the retained austenite. • Ausformed microstructure with high GND densities hinder martensite transformation. • Strength and ductility are enhanced by the developed heterogeneous microstructure. • Microstructural refinement retards macro/microcracks formation up to fracture.

Effect of types of curing environments on the self-healing capacity of mortars incorporating crystalline admixture

The presence of microcracks can lead to a reduction in the durability of concrete structures. In recent years, crystalline admixture has been used to repair microcracks in concrete. In this study, the effect of different amounts of crystalline admixture on the performance of mortar was firstly determined. In order to reveal the purpose of this study, a mortar with 4% crystalline admixture has been healed under three types curing environments (standard curing, water immersion, and saturated calcium hydroxide solution). The self-healing effect of the specimens was characterized macroscopically by water absorption tests and visualized crack images after repair. The type of crack filler was determined by microscopic analysis experiments XRD and SEM-EDX. The experimental results show that, firstly, the self-healing effect of mortar with crystalline admixture is limited by standard curing environments. Secondly, water immersion environment plays a weak role in promoting crack healing of mortars without crystalline admixture, and the addition of crystalline admixture leads to self-healing of the mortar due to a large amount of production of hydrated calcium silicate. Finally, the saturated calcium hydroxide solution has a beneficial effect on the self-healing of cracks in mortars without crystalline admixture due to the precipitation of calcium hydroxide from the healing solution. After the addition of crystalline admixture to the mortar, calcium ions and water promote the generation of calcium silicate hydrate, and the precipitation of calcium hydroxide acts together to fill the crack, resulting in the crack being completely healed.

Non-contact microcrack detection via nonlinear Lamb wave mixing and laser line arrays

In this study, a non-contact nonlinear Lamb wave mixing technique based on laser line-array excitation is developed for microcrack detection in plate-like structures. Specifically, a pulsed laser with a line-array pattern (LAP) source is created to generate two narrowband Lamb waves with distinctive frequencies; subsequently, a vibrometry is used to measure the corresponding ultrasonic responses. The presence of microcrack caused nonlinear wave mixing between the two generated Lamb waves; consequently, nonlinear mixed components are generated at the sum and difference of the two input frequencies. The novel contributions of this study are as follows: (i) a fully non-contact laser ultrasonic system with a Sagnac interferometer is developed for narrowband Lamb wave generation; additionally, a vibrometry is employed to identify the mixed components; (ii) the proposed system allows two Lamb waves with distinctive frequencies to be simultaneously generated by a single laser source; (iii) the proposed system allows the user to selectively implement the Lamb wave mode at the desired input frequency by simply adjusting the optical lens; and (iv) unlike the slit mask method, the Sagnac interferometer system yields laser LAP sources that can produce sufficient energy (exceeding 80%) to generate input waves in structures. Moreover, the performance of the developed laser ultrasonic system is experimentally validated by applying it to aluminum specimens containing microcracks. The results indicate that the proposed system can locate and detect microcracks in a plate by scanning the wave-mixing zone.