Transverse Cracks Research Articles

Laser cleaning process of high-pressure turbine blade: Characterization and removal of surface contaminants

Laser cleaning has been introduced as an effective method for removing contaminants from the surface of a turbine blade. In this investigation, a continuous wave fiber laser with a power density of 7.96 × 106 W/cm2, a wavelength of 1064 nm, and a focusing distance of 305 mm under an argon gas was applied. The used laser removed CaO-MgO-Al2O3-SiO2 (CMAS) as contaminants from the Y2O3-stabilized ZrO2 layer of a thermal barrier coating (TBC) on a high-pressure turbine blade made of a nickel-based superalloy substrate. At first, the characterization of the CMAS and TBC properties, such as thickness, roughness, and morphology, as well as phase identifications were performed. CMAS on the high-pressure turbine blade (HPTB) surface had a mixture of granular and plate structures. The major imperfections of the surface of the out-of-service HPTB were infiltration of CMAS into the TBC, chemical interaction between CMAS and the TBC, transverse cracks, delamination of the TBC, structural changes of the TBC, separation of YSZ from TGO, and deformation of the TBC on the pressure side and the leading edge of HPTB. The main aim of this work was to remove non-infiltrated CMAS from the TBC coating through the laser cleaning process. Then, various parameters of the laser cleaning process such as power and scanning speed were investigated. After that, various analyses of laser-cleaned samples such as scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), optical microscopy (OM), X-ray diffraction (XRD), thickness test, and roughness measurement were accomplished for the optimum parameters of the laser cleaning process. Finally, the mechanism involved in the laser cleaning process to remove the contamination was presented in detail. The results demonstrated that the optimum parameters of the laser cleaning process to completely eliminate targeted contaminants were a power density of 7.16 × 106 W/cm2, a scanning speed of 0.01 mm/s, and a focusing distance of 305 mm under an argon gas atmosphere.

The formation mechanism of metal-ceramic interlayer interface during laser powder bed fusion

ABSTRACT Experiments on Laser powder bed fusion (LPBF) of powdered Ti on Al2O3 substrate were conducted and the interface formation was studied using a multi-material fluid dynamics model. Results show that the melt pool is relatively shallow, with relatively flat interlayer interface under LPBF’s conduction mode. In this condition, a thin sheath of molten Al2O3 forms and acts as a lubricating film for the molten Ti, leading to Rayleigh instability due to high flow inertia. Keyhole formation penetrates the Al2O3 substrate, resulting in a wavy interlayer interface. The recoil pressure from the keyhole and overall melt inertia are suppressed by the highly viscous molten Al2O3, thereby improving single-track melt pool stability. However, the thermal expansion coefficient difference between Ti and Al2O3 led to the formation of transverse cracks. Achieving a defect-free metal-on-ceramic single track remains a challenge, despite this study serving as a guide for melt track and interface control.

A study on cracks and IMCs in laser welding of Al and Cu

Aluminum-copper laser welding has become a crucial joining technology for battery manufacturing. However, dissimilar material mixing in welding often causes the formation of intermetallic compounds (IMCs) and cracks, which reduce weld quality. In this study, the effects of laser power and welding speed on material mixing, IMCs formations, and crack formation were investigated by 3D micro-X-ray computed tomography imaging, computational fluid dynamics, and chemical/elemental analyses. Of crack formation during Al-Cu laser welding, transverse cracking was found to be the major type. The crack formation is associated with the IMC distributions generated throughout the weld fusion zone when high laser energy input caused more extensive Al/Cu material mixing.

DAMAGE DETECTION IN A STEEL BEAM USING VIBRATION RESPONSE

Changes in vibration signals, natural frequency and modal parameters were used for damage detection in a steel beam. Several tests were performed on simply supported steel beams having open transverse cracks with varying depths and locations. Frequencies for the first three vibration modes were generated from the analytical, numerical, and experimental models. Analytical frequencies were calculated assuming Euler-Bernoulli beam and solving partial differential equations. Vibration signals were collected using a portable digital vibrometer and analyzed through Fast Fourier Transform. The finite element beam models were calibrated using experimental results. The graphical plots of the normalized frequency from the experimental study and FE models showed similar pattern for the first mode. The nodal points were identified as the locations where the damaged frequency equals the frequency of an intact beam. The existence of damage in the beam specimen was confirmed by comparing the natural frequency of a beam at its intact and damaged states. Equations relating normalized frequency with varying crack depth and location were proposed.

Shape distortion in composites: sources, characterization, and remedies

This paper reviews the shape distortions in a polymer composite, its sources, characterization techniques, and remedies. The shape distortion may include warpage, spring-in, or change in enclosed diameter for flat, angled, and circular parts. Residual stresses are considered to be the major source of shape distortion (dimensional instability) in laminated composites. These stresses are the result of the difference in thermal expansion behavior of the different plies or between matrix and reinforcement. Additionally, fiber buckling, transversal cracking, and delamination are also produced in the composite, affecting its mechanical properties like tensile, flexural, and compression. The produced residual stresses are determined through different techniques like layer removal of symmetrical laminates, first ply failure, and x-ray diffraction. The common remedies for shape distortion reported in the literature include nanoparticle addition and variable thickness at the base or flanges.

Comparison study on bending properties and failure of 3D5d and 3D6d carbon/phenolic braided composites at room and elevated temperatures

Three-dimensional five-directional and six-directional braided composites (3D5dBCs and 3D6dBCs) are widely used in aerospace industry for their excellent axial and transverse properties. This work successfully fabricated 3D5dBCs and 3D6dBCs with carbon/phenolic, and compared their bending properties and failure mechanisms at room and high temperatures (RT and HT). The influences of braiding structure and temperature on bending properties and failure mechanisms of 3D5dBCs and 3D6dBCs were analyzed. The load/deflection curves are linear elastic at RT, showing prominent plastic characteristics at HT. From RT to HT, the bending properties decrease, but 3D5dBCs always have higher bending properties than 3D6dBCs. The 3D5dBCs and 3D6dBCs are primarily damaged near the surface at RT; fiber shearing fracture feature becomes apparent and the interior damage worsens at HT. 3D5dBCs exhibit obvious shear failure mode, where the fracture is bent and rough, and the damage patterns are braiding and axial fibers shearing fracture, matrix softening, and interface debonding. 3D6dBCs show transverse sawtooth fracture cracks, and the damage mechanisms are braiding fiber shearing fracture, fifth fiber tearing, sixth fiber shedding and pulling-off, matrix plasticization, and interface debonding.

Self-healing of transverse crack damage in carbon fiber composites

Laminated fiber-reinforced composites are susceptible to transverse cracking at relatively low stresses. These cracks cause a reduction in stiffness and can lead to premature and dangerous failure modes. Here, we investigate the self-healing efficiency of carbon fiber composites in isolated transverse crack events. The composites contain solvent-filled microcapsules in an epoxy matrix toughened with 20 wt% of thermoplastic poly(bisphenol A-co-epichlorohydrin) (PBAE). Self-healing is triggered upon release of the encapsulated solvent and subsequent transport of dissolved thermoplastic into the damaged volumes. Following solvent evaporation, redistributed thermoplastic re-bonds the crack faces to heal the damage. To evaluate self-healing efficiency, we develop a new protocol which leverages digital image correlation (DIC) to compare the stress required to open and re-open a transverse crack before and after healing. Self-healing is evaluated after 2, 4, and 6 days and compared to control specimens that are healed thermally. We report a self-healing efficiency of up to 57% after 6 days with just 1 vol% microcapsule concentration in a high glass transition temperature (179 °C) carbon fiber-reinforced composite.

Repeated healing of low velocity impact induced damage in orthogrid-stiffened sandwich panel

Herein, we present a new sandwich panel composed of a carbon fiber grid-stiffened shape memory vitrimer (SMV) core. The sandwich panels were fabricated via a pin-guided dry-weaving technology, and their impact responses were evaluated via low-velocity impact testing. The main failure mode observed after the first round of impact was the transverse cracking of the SMV matrix in the sandwich core. The healing efficiency according to the crack initiation energy (CIE) was found to be 76.5% after the first healing cycle. Even after the second healing cycle, the healing efficiency was greater than 72%. From the low-velocity impact tests, reinforcing the pure SMV core with a grid-skeleton enhanced the impact resistance significantly, that is, the crack initiation energy and peak load were increased by 64.0% and 169.0%, respectively. The results also show that smaller bay area leads to higher impact resistance. With the repeated crack healing, increased impact tolerance, and shape memory effect, it is expected that the sandwich panels will have a good possibility for usage in aerospace and automotive applications.

Effects of Transverse Cracks on the Backcalculated Layer Properties of Asphalt Pavements from Non-destructive Testing Data

Effects of Transverse Cracks on the Backcalculated Layer Properties of Asphalt Pavements from Non-destructive Testing Data

Automatic identification of pavement cracks in public roads using an optimized deep convolutional neural network model.

The current study aims to improve the efficiency of automatic identification of pavement distress and improve the status quo of difficult identification and detection of pavement distress. First, the identification method of pavement distress and the types of pavement distress are analysed. Then, the design concept of deep learning in pavement distress recognition is described. Finally, the mask region-based convolutional neural network (Mask R-CNN) model is designed and applied in the recognition of road crack distress. The results show that in the evaluation of the model's comprehensive recognition performance, the highest accuracy is 99%, and the lowest accuracy is 95% after the test and evaluation of the designed model in different datasets. In the evaluation of different crack identification and detection methods, the highest accuracy of transverse crack detection is 98% and the lowest accuracy is 95%. In longitudinal crack detection, the highest accuracy is 98% and the lowest accuracy is 92%. In mesh crack detection, the highest accuracy is 98% and the lowest accuracy is 92%. This work not only provides an in-depth reference for the application of deep CNNs in pavement distress recognition but also promotes the improvement of road traffic conditions, thus contributing to the progression of smart cities in the future. This article is part of the theme issue 'Artificial intelligence in failure analysis of transportation infrastructure and materials'.

Mechanical responses of anchoring structure under triaxial cyclic loading

Dynamic load on anchoring structures (AS) within deep roadways can result in cumulative damage and failure. This study develops an experimental device designed to test AS under triaxial loads. The device enables the investigation of the mechanical response, failure mode, instability assessment criteria, and anchorage effect of AS subjected to combined cyclic dynamic-static triaxial stress paths. The results show that the peak bearing strength is positively correlated with the anchoring matrix strength, anchorage length, and edgewise compressive strength. The bearing capacity decreases significantly when the anchorage direction is severely inclined. The free face failure modes are typically transverse cracking, concave fracturing, V-shaped slipping and detachment, and spallation detachment. Besides, when the anchoring matrix strength and the anchorage length decrease while the edgewise compressive strength, loading rate, and anchorage inclination angle increase, the failure intensity rises. Instability is determined by a negative tangent modulus of the displacement-strength curve or the continued deformation increase against the general downward trend. Under cyclic loads, the driving force that breaks the rock mass along the normal vector and the rigidity of the AS are the two factors that determine roadway stability. Finally, a control measure for surrounding rock stability is proposed to reduce the internal driving force via a pressure relief method and improve the rigidity of the AS by full-length anchorage and grouting modification.

Study on rubbing characteristics of blade-casing model considering transverse cracks

The study focuses on the nonlinear vibration response of a blade system with cracks subjected to coupled rub-impact failure. Initially, the cracked blade system is mathematically modeled based on the Lagrangian theorem, and its motion is derived using Hamilton's principle. Secondly, a revised rubbing model is proposed with consideration of varying depths and locations of the cracks. Subsequently, the influences of parameters related to crack failure and rub-impact are comprehensively investigated. Finally, a novel crack influence index is also proposed to study the impact of resonance effect on crack failure. Different dynamic characteristics, along with the resonance coupling effect, were compared to demonstrate the working mechanism. The results indicate that severe rubbing occurred as the crack depth increased and the crack position decreased. The impact of crack depth on rubbing deformation than that of crack location. In addition, larger cracking depths and smaller cracking locations result in larger cracking influence coefficients, while the resonance effect disrupts this influence law in some cases.

Damage Mechanism of Trivalent Chromium Coatings under Tensile Stress

Due to new environmental regulations, hexavalent chromium electrolytes can no longer be used for thick, hard chromium plating. In response to this industrial and environmental challenge, trivalent chromium electrolyte plating has been developed. In this paper, we propose a study of the adhesion of CrIII coatings based on the implementation of numerical models in comparison with an identified experimental scenario. The aim is to dissociate the influence of coating and substrate behaviours from the adhesion work by describing the intrinsic damage of the chromium layer and the coating–substrate interface. Two types of cracking were studied: transverse cracking and delamination. For the former, the crack density was higher for CrIII than for CrVI and increased with deformation and coating thickness. Microtensile tests with scanning electron microscopy (SEM) observations allowed us to highlight the cracking process in the coating (transverse cracking) and at the coating–substrate interface (delamination). The numerical simulation of the test allowed us to estimate a damage-initiation threshold normal stress of 1900 MPa, which occurred at an average applied strain of 2.5%. Delamination of the coating was complete at an average strain of 13.6% and an interfacial normal stress of 2600 MPa.

Evaluation of Pavement Surface Distress Using Image Processing and Artificial Neural Network

Abstract Pavement evaluation helps assess the structural and functional condition of roads. Traditional pavement evaluation methods cannot efficiently and accurately assess the state of road conditions at the network level. Though the pavement evaluation techniques based on high-resolution cameras and laser sensors assess the road conditions efficiently at traffic speeds, there are a few limitations in respect of automotive detection and quantification of pavement surface distress. Though artificial intelligence is a broad area, machine learning applications in highway engineering were proven to be successful. This paper presents the automated pavement distress classification using a convolutional neural network (NN) from the Keras library. VGG-16, the deep convolutional NN (DCNN) model, was deployed by necessary modification to get the desired output. The model is trained on a big data set of images with a wide range of pavement defects and irregularities. A DCNN classifier trained with an “Adam” optimizer was used to change the features of the NN to minimize the loss. As an output, the model classifies the pavement surface distresses as alligator cracks, longitudinal cracks, transverse cracks, pothole, and no crack portion of the pavement with maximum accuracy using a DCNN classifier.

A Fracture Criterion with Biaxial Constraints of Deformations along the Front of a Normal Rupture Crack

In this article, a new fracture criterion is formulated for a normal rupture crack, the most common in practice, based on the assumption that the tangential stresses in the prefracture zone are equal to the local strength of the material. In this case, the size of the prefracture area and the local strength are determined taking into account the nonsingular Тхx and Тzz stresses included in the asymptotic stress distribution according to Williams and characterizing the two-dimensional local constraint of deformation along the crack front in three-dimensional bodies. An expression for the effective stress intensity factor is obtained. In addition to the classical stress intensity factor, it includes the ratios of the Тxx and Тzz stresses to the yield strength. This makes it possible to take into account the restriction of deformations in the transverse (due to Тxx stresses) and longitudinal (due to Тzz stresses) directions in the vicinity of the crack front. Verification of the developed software tools and the proposed fracture criterion has been carried out. Examples of the implementation of the developed criterion for assessing crack resistance of a plate stretched in one or two directions with a coaxial transverse crack are given.

Effect of aged material properties on transverse crack performance with two-round field observations

This paper is aimed to validate the trend between thermal-related parameters and field-measured thermal cracking. Field thermal cracking is collected from four projects with twelve pavement sections. The material properties include creep compliance m-value and tensile strength of mixture, and asphalt low-temperature performance grade (PG). A lower value of m-value and tensile strength leads to a higher thermal cracking, while the low-temperature PG has the contrary trend. One or even several material properties cannot fully characterize field thermal cracking for all projects. Then the change of material properties, pavement structures, and climatic conditions are considered in three-dimensional finite element models (FEM). The aging effect of material properties attributes to the increase of thermal stress. The placement of chip seal could greatly reduce the aging rate, and the thicker pavement result in less thermal stress. Both material property and thickness are important design factors for thermal cracking resistance.

Non-uniform spacing of transverse cracks in symmetric composite laminates

Non-uniform spacing of transverse cracks in symmetric composite laminates

In Situ Tensile Testing under High-Speed Optical Recording to Determine Hierarchical Damage Kinetics in Polymer Layers of Flax Fibre Elements.

This study aims at better understanding the damage and fracture kinetics in flax fibre elements at both the unitary and bundle scales, using an experimental setup allowing optical observation at high recording rate in the course of tensile loading. Defects and issues from flax unitary fibre extraction are quantitated using polarized light microscopy. Tensile loading is conducted according to a particular setup, adapted to fibres of 10 to 20 µm in diameter and 10 mm in length. Optical recording using a high-speed camera is performed during loading up to the failure at acquisition, with speed ranging from 108,000 to 270,000 frames per second. Crack initiation in polymer layers of fibre elements, propagation as well as damage mechanisms are captured. The results show different failure scenarios depending on the fibre element's nature. In particular, fractured fibres underline either a fully transverse failure propagation or a combination of transverse and longitudinal cracking with different balances. Image recordings with high time resolution of down to 3.7 μs suggest an unstable system and transverse crack speed higher than 4 m/s and a slower propagation for longitudinal crack deviation. Failure propagation monitoring and fracture mechanism studies in individual natural fibre or bundles, using tensile load with optical observation, showed contrasted behaviour and the importance of the structural scale exanimated. This study can help in tailoring the eco-design of flax-based composites, in terms of toughness and mechanical performances, for both replacement of synthetic fibre materials and innovative composites with advanced properties.

Dynamic laser welding hot cracking behavior and mechanism of new structural material Ni–28W–6Cr alloy for molten salt reactor

The newly-developed Ni–28W–6Cr alloy is the structural material for the next-generation molten salt reactor (MSR). Due to the detrimental effects of welding hot cracks on the quality of welded structures, it is essential to study the hot cracking behavior and mechanism to achieve the qualified Ni–28W–6Cr alloy welded joint. This paper analyzes the characteristics of the dynamic cracking behavior of welding hot cracks using a high-speed camera. The microstructure, morphology, and cracking mechanism of hot cracks are studied. The results show that solidification cracks and liquation cracks occur in the welded joint. According to the propagation direction of the hot crack, there are longitudinal cracks and transverse cracks, respectively. The quantity and length of hot cracks increase with the laser power. The laser welded joint's hardness value fluctuates from 200 HV to 260 HV. The width of heat affected zone (HAZ) is approximately 0.5–1 mm, and there is a slight softening zone in the HAZ. Meanwhile, the hardness of the weld metal is the highest in the welded joint. The solidification cracks tend to initiate and crack along the solidification grain boundary, which is mainly related to the thermal stress caused by the welding process and the weak bonding strength of grain boundaries caused by the element segregation and precipitation at the grain boundary. Liquation cracking is mainly affected by welding thermal stress and element segregation at the grain boundary in the HAZ. However, the eutectic carbides play the role of inhibiting the propagation of liquation cracks.

Highway Crack Detection and Classification Using UAV Remote Sensing Images Based on CrackNet and CrackClassification

Cracks are a common type of road distress. However, the traditional manual and vehicle-borne methods of detecting road cracks are inefficient, with a high rate of missed inspections. The development of unmanned aerial vehicles (UAVs) and deep learning has led to their use in crack detection and classification becoming an increasingly popular topic. In this paper, an aerial drone is used to efficiently and safely collect road data. However, this also brings many challenges. For example, flying too high or too fast may produce poor quality images, with unclear cracks that may be ignored or misjudged as other features and increased environmental noise that may make it difficult to distinguish between cracks and other noise features. To address the above challenges, this paper proposes the CrackNet model and CrackClassification algorithm. The CrackNet network is an encoder–decoder architecture. Low- and high-level semantic information are combined through the skip feature fusion layers between the encoder and decoder to enhance the model’s expression and ability to recover image details. Additionally, the MHDC module at the bottom of the network can significantly increase the receptive field without reducing the feature map resolution. The MHSA module can simultaneously capture features from multiple subspaces. The average precision (AP) scores of the CrackNet network on three datasets, namely UAVRoadCrack, CRKWH100, and CrackLS315, were 0.665, 0.942, and 0.895, respectively. In addition, values of the other two evaluation metrics, ODS and OIS, were the highest among the compared methods. Meanwhile, the proposed CrackClassification algorithm in this paper achieves 85% classification accuracy for transverse and longitudinal cracks and 78% classification accuracy for block cracks and reticulated cracks. Overall, the CrackNet algorithm provides a new baseline model for crack detection in UAV remote sensing image scenes. The CrackClassification algorithm provides a new approach for batch classification of highway cracks. The detection and classification algorithm proposed in this paper were applied to 108 km of road sections.