Vertical Crack Propagation Research Articles

Macro/micro failure mechanism of transparent armour subjected to multiple impacts of 7.62mm bullets

Transparent armor is widely used in military and civilian impact protection fields due to its excellent light transmittance and ballistic performance. This work focused on the macro/micro failure mechanisms of transparent armor for vehicles subjected to multiple impacts. Results showed that the penetration depth after the first impact by a 7.62 mm bullet is about 14 mm, regardless of the impact position. Based on the cavity expansion theory, the penetration depth under multiple projectile impacts was predicted, relating it to the distance between the impact points, the distance from the projectile hole to the edge of the target plate, and the damage radius caused by the first impact. In the thickness direction, observation of the glass layer damage modes revealed that the interlayer adhesive could hinder the propagation of vertical cracks between different glass layers, with delamination primarily caused by insufficient shear strength. In the in-plane direction, the size of the fractured glass gradually increases outward from the impact point because circumferential cracks can prevent the propagation of radial cracks. Finally, the micro failure analysis of glass fragments showed that the radial cracks are dominated by numerous irregular microcracks and river-like textures, while the circumferential cracks consist of the mirror region, mist region, hackle region, and river-like texture region.

3D mesoscale model of steel fiber reinforced concrete based on equivalent failure of steel fibers

Steel fiber reinforced concrete (SFRC) is widely used in civil engineering. Investigating the mechanical properties and failure mechanisms of SFRC materials using mesoscopic models holds significant theoretical and practical importance. The bond-slip interaction between steel fibers and the cementitious matrix is crucial, yet current computational capabilities struggle to simulate the interactions among the numerous components within concrete containing a large number of steel fibers. This paper establishes an equivalent failure model to replace the bond-slip interaction. Based on this model, a multiphase mesoscopic model (including aggregates, the interfacial transition zone (ITZ), mortar, and steel fibers) is developed to predict the mechanical properties and failure modes of SFRC. The accuracy and validity of the model are verified by comparing the simulation results with the experimental results from four-point bending tests on SFRC beams, focusing on load-displacement curves, failure modes, and crack propagation. The results indicate that under load, the ITZ at the sharp edges of large aggregates in the SFRC beam is the first to be damaged, forming microcracks. Subsequently, these cracks bypass the large particles and propagate towards weaker regions with fewer steel fibers, forming macrocracks. At this stage, the steel fibers take over the load from the cementitious matrix and limit crack propagation. The simulated crack propagation trend and crack width during the loading process of the SFRC multiphase mesoscale model match well with the experimental observations. A higher fiber content can mitigate stress concentration within SFRC beams and enhance the fiber bridging effect, significantly improving the flexural load-bearing capacity and toughness of the beams. A smaller steel fiber inclination angle can effectively inhibit the propagation of vertical cracks, thereby increasing the load-bearing capacity and toughness of SFRC beams and extending the service life of the structure.

Composite high-entropy (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Ce2O7 thermal barrier coatings with enhanced thermal cycling performance

The emerging high-entropy (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Ce2O7 (LECO) has been verified as a potential thermal barrier material. To ensure accurate thermal cycling life prediction of LECO-based coating, double ceramic layered coatings LECO-8YSZ (8YSZ: 8 mol% yttrium stabilized zirconia) (Y-LECO) and composite (25 wt% 8YSZ + 75 wt% LECO)-8YSZ (Y-YLECO) were prepared via atmospheric plasma spraying (APS). In Y-YLECO, the toughening of 8YSZ in LECO effectively impeded the driving force of crack propagation and slowed down the delamination rate of the coating. Thus, the thermal cycling life of Y-YLECO is about 30 % higher than that of Y-LECO. Besides, both coatings exhibited mild delamination behavior. The exfoliation only appeared at the LECO-based coating with tiny flake-like delamination occurring on the surface. Such spallation pattern contributed to alleviating the stress concentration and preventing the further propagation of vertical cracks towards substrate. Moreover, the addition of 8YSZ did not sacrifice the thermal insulation performance of the composite coating, while it was 0.75 W·m−1·K−1 at 1000 °C for 25 wt% 8YSZ + 75 wt% LECO coating, close to the single-component LECO coating (0.67 W·m−1·K−1). The component design strategy could be a new pathway to further enhance the thermal protection performance of the high-entropy coatings.

Compressive behavior of brick masonry columns confined with composites embedded in the horizontal mortar joint

The application of composite materials embedded in the horizontal mortar joints to strengthen masonry structures with insufficient load-bearing capacity has shown beneficial effects in terms of improving mechanical behavior and preserving the aesthetics of the building. To investigate the effectiveness of this strengthening technique, concentric axial compressive tests were performed on eight masonry columns, including two unconfined columns, five specimens confined by carbon fiber reinforced polymer (CFRP) plates, and one specimen confined by engineered cementitious composite (ECC). The failure mode, strength, deformability, and energy dissipation capacity of the specimens were investigated. The effective tensile strain of the CFRP plates was also discussed. The results showed that the CFRP-confined masonry columns exhibited brittle failure, which was characterized by the debonding failure of the CFRP caused by the sudden fracture of the epoxy resin, while the failure of the ECC-confined masonry column exhibited significant ductility, the reason for which was that the propagation of vertical cracks was delayed due to the strain-hardening characteristics of the ECC. The reinforcement technique was effective for masonry columns in obtaining better compressive performance. The peak strength and energy dissipation of the confined columns were increased by a maximum of 86.31% and 115.71%, respectively. A non-uniform distribution of the effective tensile strain of the CFRP plates along the perimeter was observed. In addition, a simple strength model for masonry columns confined by embedded material in the horizontal mortar joints was developed based on the best-fit analysis of a database assembled from the existing literature and the current study. The predictive capabilities of the proposed model and an existing model for the compressive strength of confined masonry columns were analyzed.

Study on the Stress Field and Crack Propagation of Coal Mass Induced by High-Pressure Air Blasting

High-pressure air blasting (HPAB) is one type of physical blasting technique that enhances the extraction rate of coalbed methane by impacting the coal mass with high-pressure gas to create cracks within it. First, based on the physical and mechanical parameters of the simulated coal rock mass, the RHT constitutive model of the coal rock mass was established, and its parameters were determined. Then, the laws of crack propagation and stress wave decay in coal induced by high-pressure air blasting were revealed by comparing the effect with that of equivalent explosive blasting. Next, the HPAB experiment was simulated to explore the coal crack propagation law under in-situ stress conditions. Finally, the HPAB experiment was carried out and the results of this experiment were compared with the numerical simulation results. The results indicate that the crack propagation induced by high-pressure air blasting is considered as two major stages, i.e., the crack initiation and crack propagation stage induced by the stress wave and the crack stable propagation stage induced by the duration high-pressure gas. In the case of equal energy, the peak stress wave of high-pressure gas is smaller, decays more slowly and has a longer action time, compared to explosive blasting. Therefore, the number of initial random cracks in coal mass induced by high-pressure air blasting is less, and the range of crack propagation induced by high-pressure air blasting is larger. When λ = 0 (λ is the ratio of the horizontal in-situ stress to the vertical in-situ stress), the in-situ stress in the coal seam can promote the propagation of vertical cracks but inhibit the propagation of horizontal cracks. When λ = 0.5 and 1, the in-situ stress inhibits the propagation of both horizontal and vertical cracks.

Study on spalling mechanism of APS thermal barrier coatings considering surface vertical crack evolution affected by surrounding cracks

The propagation of vertical cracks on the surface of thermal barrier coating (TBC) may be affected by the surrounding cracks. In this study, a TBC model with multiple cracks is developed to study the effects of surrounding cracks on surface vertical crack evolution and coating spalling. Dynamic TGO growth, continuous ceramic sintering, and dynamic crack propagation are considered in the model. The general characteristics of stress state and crack evolution in the coating are first examined. Then, the effects of the surrounding horizontal and vertical cracks on the overall crack growth are investigated. The results show that the vertical crack expands to the interface and merges with the horizontal crack rapidly. The crack length variation by simulation is consistent with the reported results by experiment. The surrounding horizontal crack close to the coating surface can prevent the surface vertical crack from propagating to the interface, which is independent of the surrounding crack length. A certain density of vertical cracks with equal spacing and initial length is conducive to delaying the horizontal crack propagation near the interface. Some optimized design methods proposed in this work provide another option for developing TBC with high durability.

Study on the arresting mechanism of two arrest-holes on moving crack in brittle material under impacts

To develop the technique of arresting running cracks is essential because it can be used to protect cracked structures being further damaged. In this paper, a specimen of large semicircular edge crack with two arrest-holes (LSECTH) was proposed. The specimens were made of cement, river sand, fly ash and water. Impact experiments were carried out in the LSECTH specimens with different two-hole spacing (35 mm, 40 mm, 45 mm, 50 mm, 55 mm, 60 mm and 70 mm) under drop-weight impacts. Crack propagation gauges (CPGs) were used to monitor the fracturing time and crack speed. The interaction characteristic between the crack and two arrest-holes was investigated numerically by the AUTODYN code. In the numerical models, the failure criterion of maximum tensile stress and shear stress were employed for the brittle material. The crack propagation path, the circumferential stress of crack tip and the stress state between the two arrest-holes were analyzed. The calculation results indicate that compressive stresses between the two arrest-holes induced by the shape change from circle to ellipse play a key role in confining the vertical crack propagation. Both experimental and numerical results indicate that the two arrest-holes have suppressing action on moving crack; as the two arrest-hole spacing decreases, the suppressing action intensifies.

Effect of Holes on Dynamic Crack Propagation under Impact Loading

In civil, geotechnical, and mining engineering, the investigation of the holes’ effect on dynamic crack propagation is essential because it can be used to predict possible fracture and protect cracked structures being further damaged. In this paper, a specimen made from polymethyl methacrylate (PMMA) with a pre-crack and two holes was proposed, and the Split-Hopkinson pressure bar was employed to investigate the effect of holes on dynamic crack propagation under impact loading. Notably, the locations of the holes were well designed with different two-hole spacing (12 mm, 16 mm, and 20 mm) and crack-hole distance (15 mm, 30 mm, and 45 mm). Crack propagation gauges were applied to monitor the fracturing time and crack extending velocity. The interaction characteristic between the crack and two holes was studied numerically using the AUTODYN code. In the numerical models, the failure criteria of maximum tensile stress and softening damage were employed for brittle material. The crack path, the propagating velocity, the particle velocity vector, and the stress state between the holes were analyzed. The calculation results indicate that compressive stresses between the two holes induced by the deformation of the holes play a crucial role in confining the vertical crack propagation. Both experimental and numerical results demonstrate that the holes have a suppressing action on the moving crack; as the two-hole spacing decreases, the suppressing action intensifies.

Effect of Mg concentration on interfacial strength and corrosion fatigue behavior of thermal-sprayed Al-Mg coating layers

Abstract This study aims at observing the effect of Mg concentration on the interfacial strength and corrosion fatigue behavior of Al-Mg coating layers on structural steel (SS400) substrates. Al-Mg coating has been applied to components of bridges as sacrifice coating layers. Although Al or Al-Mg coating layers are typically applied to the components located in severely corrosive environment, a mechanism behind the effective protection provided by increasing the concentration of Mg has not yet been clarified. The interfacial strength test using four-point bending revealed that a Al-Mg coating layer of higher Mg concentration showed a higher interfacial strength only before immersion in 3.5-wt% NaCl aq.. After immersion in 3.5-wt% NaCl aq. for 30 days, such the difference in interfacial strength was lost due to rapid dissolution of Mg in the coating layers. As regards the fatigue crack growth behavior, the Al-Mg coating with higher Mg concentration exhibited a lower resistance to vertical crack propagation and interfacial delamination. A fracture mechanics model, which includes both effects of corrosion and delamination/cracking was proposed. Numerical simulation based on the fracture mechanics model successfully predicted exposure lives of substrates due to fatigue failure of the Al-Mg coating layers. These results could provide a selection policy for Al-Mg coating layers in which a loading level and the severity of corrosive environment were considered.

Effect of fracture toughness on vertical split rim failure in railway wheels

Vertical split rim (VSR) cracks are the most common cause for the removal of broken or cracked heavy haul car wheels in North America. This failure mode is characterized by rapid, unstable crack propagation in the hoop or in the tangential direction. To investigate the methods to prevent VSRs, an understanding of how the service loads correlate to their occurrence is necessary. In this study, a finite element analysis is conducted to evaluate the rim stresses below the tread surface during rolling contact between the wheel tread and the rail. Then, the stress intensity factor is calculated using the stress distribution in the wheel rim obtained using the finite element analysis. The results of the analysis show that axial residual tensile stress, which contributes to unstable vertical crack propagation, increases at deeper positions below the tread surface owing to cyclic rolling contact. These results indicate that a vertical crack can propagate rapidly below the tread surface. The fracture toughness of the wheel steel is measured using Charpy impact tests. The results of the test show that Weibull distribution enables the approximation of the relationship between fracture probability and fracture toughness. To evaluate the effect of the fracture toughness of wheel steel on the differences in VSR crack probability, a comparison between high fracture toughness wheels and conventional Class-C wheels is made. The VSR rate can be predicted from the stress intensity factor and Weibull equation for fracture toughness. The prediction results show that the VSR rate of high fracture toughness wheels is only 5% of that of conventional Class-C wheels. Therefore, high fracture toughness steel wheels are considered to have a higher resistance to VSR cracks compared with conventional Class-C wheels.

Cyclic delamination behavior of plasma-sprayed hydroxyapatite coating on Ti–6Al–4V substrates in simulated body fluid

This study aimed to clarify the effect of a simulated body fluid (SBF) on the cyclic delamination behavior of a plasma-sprayed hydroxapatite (HAp) coating. A HAp coating is deposited on the surfaces of surgical metallic materials in order to enhance the bond between human bone and such surfaces. However, the HAp coating is susceptible to delamination by cyclic loading from the patient's gait. Although hip joints are subjected to both positive and negative moments, only the effects of tensile bending stresses on vertical crack propagation behavior have been investigated. Thus, the cyclic delamination behavior of a HAp coating was observed at the stress ratio R=−1 in order to determine the effects of tensile/compressive loading on the delamination behavior. The delamination growth rate increased with SBF immersion, which decreased the delamination life. Raman spectroscopy analysis revealed that the selective phase dissolution in the HAp coating was promoted at interfaces. Finite element analysis revealed that the energy release rate Gmax showed a positive value even in cases with compressive loading, which is a driving force for the delamination of a HAp coating. A prediction model for the delamination growth life was developed that combines a fracture mechanics parameter with the assumed stress-dependent dissolution rate. The predicted delamination life matched the experimental data well in cases of lower stress amplitudes with SBF.

In situ observation of rolling contact fatigue cracks by laminography using ultrabright synchrotron radiation

In rolling contact fatigue (RCF), cracks usually initiate from inclusions beneath the surface and propagate to the contact surface. In the present study, synchrotron radiation computed laminography (SRCL) imaging was performed to observe flaking defects during the RCF of a high-strength steel. Specially fabricated inclusion-rich steel plate specimens were employed in the experiments. For the in situ observation of crack propagation, a compact RCF testing machine was developed, and a 4D analysis scheme was applied to the data obtained by SRCL. RCF tests were carried out near the measurement hatch of the beam line used SRCL to enable the successive observation of crack initiation and growth behaviors. Specimens before and after the occurrence of flaking were observed by SRCL, and flaking defects and cracks under the surface were successfully detected. As a result, details of the crack initiation and flaking process in RCF could be discussed. Shear-type horizontal cracks were found to initiate after the initiation and propagation of tensile-type vertical cracks along inclusions, where the face of the vertical cracks was perpendicular to the rolling direction and rolling surface. Therefore, the formation of vertical cracks is considered to affect shear-type crack formation and flaking, where the shape and length of inclusions also affect the initiation and propagation of vertical cracks.

Theoretical Research of Vertical Crack Propagation in Overlaid Cement Concrete Pavement

In this paper, overlaid cement concrete pavement with crack is considered based on fracture mechanics. Cement concrete pavement is reduced to an elastic plate on Winkler foundation. Fourier integral transform, residue theorem and Lobatto-Chebyshev integration formula are used to obtain the analytical solutions of this problem. To analyze the effect of overlay for the old pavement, stress intensity factors ofItype crack and II type crack are calculated respectively for various overlay thicknesses, elastic moduli of the overlay. The stress intensity factors of the crack tips in the pavement with 10cm thickness overlay are reduced heavily. However, stress intensity factors are less affected by elastic modulus of overlay material.

Evolution of the crack network in glass samples submitted to brittle creep conditions

A crack network is introduced in glass by quenching heated samples. The sharp variation of temperature at the sample boundaries leads to tensile stresses that nucleate cracks. Then, they propagate in the entire sample. Quenching has been performed at 100, 200 and \(300\,^\circ \hbox {C}\). Cracks have been imaged with a scanning electron microscope. A transverse isotropic crack network is observed. Crack length and orientation have been measured. Obtained crack density has been compared to that inferred from elastic wave velocity measurements using effective medium theory. Cracked samples have been then submitted to creep tests. Two samples have been recovered, one before its failure and another after. Our observations show that vertical crack propagation takes place during brittle creep and that tertiary creep leads to a localized failure in a shear plane. The damaged and post-mortem microstructural networks have been documented.

High temperature behavior of nanolayered CrAlTiN coating: Thermal stability, oxidation, and tribological properties

Hard protective nitride coatings are often applied to cutting tools operating at high temperature. To further develop and optimize their performance, in-situ investigation of structure, oxidation, mechanical and tribological properties at elevated temperature is required. In this study we focus on the high temperature behavior of a nanolayered CrAlTiN coating deposited on WC substrates by cathodic arc evaporation. The coating's chemical composition and the bonding state were evaluated by electron probe microanalysis and by X-ray photoelectron spectroscopy (XPS). The structure of as-deposited and annealed samples was analyzed using X-ray diffraction. The adhesion was investigated by scratch test and the mechanical properties were studied by depth sensing nanoindentation. The main objective of this work was to have a detailed analysis of friction and wear properties tested by high temperature tribometer (pin-on-disc) with alumina balls as counterparts in the temperature range of 20–800°C. Selected wear track cross-sections were prepared by focused ion beam and analyzed by transmission electron microscopy; the wear track was investigated as well by XPS (chemical depth profile) and by Raman spectroscopy. The coating showed an excellent thermal stability and wear resistance. The friction reached a maximum at 500°C and then decreased, whereas the wear rate was negligible up to 600°C and then increased significantly for higher temperatures. Oxidation of the worn surfaces was surprisingly low even at the highest temperature corroborating results of oxidation tests. The main identified wear mechanism was polishing combined with a nanoscale delamination of thin coating layers; nanoscale multilayer proved to be a vital factor blocking vertical crack propagation.

Experiments of vertical fracture propagation based on the digital speckle technology

Abstract The vertical crack propagation behavior for sandstone/mudstone layered specimen made by the similar material was analyzed. The stress and strain field was obtained using Digital Speckle Correlation Method to analyze the propagation characteristics of vertical cracks. The test shows that the higher the strength of the interface layer, the more susceptible the crack to deflect from the side of the low-strength than from the side of the high-strength of the material. It is easy for the interface to unglue when the interfacial adhesion strength is weak, which results in the dislocation propagation after the crack passing the interface whether crack extending from the side of high-strength material or the side of low-strength material. The longer the expansion path is before the crack passing the interface, the more likely it deflects after passing the interface. Similarly, the shorter the initial prefabricated crack is, the more susceptible to deflect after the crack passing the interface. The main factor of the deflection is the difference in the properties of the composite materials, resulting in a larger shear strain of the interfacial layer. The shear strain can induce type II shear fracture, so it is the main factor leading to crack deflection. The bigger the shear deformation is, the more intensive the crack deflection is.

Cortical bone failure mechanisms during screw pullout

An experimental and computational study of screw pullout from cortical bone has been conducted. A novel modification of standard pullout tests providing real time image capture of damage mechanisms during screw pullout was developed. Pullout forces, measured using the novel test rig, have been validated against standard pullout tests. Pullout tests were conducted, considering osteon alignment, to investigate the effect of osteons aligned parallel to the axis of the orthopaedic screw (longitudinal pullout) as well as the effect of osteons aligned perpendicular to the axis of the screw (transverse pullout). Distinctive alternate failure mechanisms, for longitudinally and transversely orientated cortical bone during screw pullout, were uncovered. Vertical crack propagation, parallel to the axis of the screw, was observed for a longitudinal pullout. Horizontal crack propagation, perpendicular to the axis of the screw, was observed for a transverse pullout. Finite element simulation of screw pullout, incorporating material damage and crack propagation, was also performed. Simulations revealed that a homogenous material model for cortical bone predicts vertical crack propagation patterns for both longitudinal and transverse screw pullout. A bi-layered composite model representing cortical bone microstructure was developed. A unique set of material and damage properties was used for both transverse and longitudinal pullout simulations, with only layer orientations being changed. Simulations predicted: (i) higher pullout forces for transverse pullout; (ii) horizontal crack paths perpendicular to screw axis for transverse pullout, whereas vertical crack paths were computed for longitudinal pullout. Computed results agreed closely with experimental observations in terms of pullout force and crack propagation.

Influence of thermal and mechanical cracks on permeability and elastic wave velocities in a basalt from Mt. Etna volcano subjected to elevated pressure

We report simultaneous laboratory measurements of seismic velocities and fluid permeability on lava flow basalt from Etna (Italy). Results were obtained for dry and saturated samples deformed under triaxial compression. During each test, the effective pressure was first increased up to 190 MPa to investigate the effect of pre-existing crack closure on seismic properties. Then, the effective pressure was unloaded down to 20 MPa, a pressure which mirrors the stress field acting under a lava pile of approximately 1.5–2 km thick, and deviatoric stress was increased until failure of the specimens. Using an effective medium model, the measured elastic wave velocities were inverted in terms of two crack densities: ρ i the crack density of the pre-existing thermal cracks and ρ v the crack density of the stress-induced cracks. In addition a link was established between elastic properties (elastic wave velocities V p and V s ) and permeability using a statistical permeability model. Our results show that the velocities increase with increasing hydrostatic pressure up to 190 MPa, due to the closure of the pre-existing thermal cracks. This is interpreted by a decrease of the crack density ρ i from ~ 1 to 0.2. The effect of pre-existing cracks closure is also highlighted by the permeability evolution which decreases of more than two orders of magnitude. Under deviatoric loading, the velocities signature is interpreted, in the first stage of the loading, by the closure of the pre-existing thermal cracks. However, with increasing deviatoric loading newly-formed vertical cracks nucleate and propagate. This is clearly seen from the velocity signature and its interpretation in term of crack density, from the location of the acoustic emission sources, and from microstructural observations. This competition between pre-existing cracks closure and propagation of vertical cracks is also seen from the permeability evolution, and our study shows that mechanically-induced cracks has lesser influence on permeability change than pre-existing thermal cracks.

Analytical Solution for Fracture Analysis of CFRP Sheet–Strengthened Cracked Concrete Beams

Fiber-reinforced polymer (FRP) composite materials have been widely used in the field of retrofitting. Theoretical analysis of FRP plate- or sheet-strengthened cracked concrete beams is necessary for estimating service reliability of the structural members. In previous studies, the effect of a perfectly bonded FRP plate or sheet was equivalent to a cohesive force acting at the bottom of crack to delay the crack propagation in concrete and reduce the crack width. However, delamination between FRP and cracked beam is inevitable due to interfacial shear stress concentration at the bottom of crack. The intention of this paper is to present an analytical solution for fracture analysis of carbon FRP (CFRP) sheet–strengthened cracked concrete beams by considering both vertical crack propagation in concrete and interfacial debonding at CFRP-concrete interface. The interfacial debonding is modeled as the interfacial shear crack propagation in this paper. Four different stages are discussed after initial cracking state of the concrete. At the first stage, only fictitious crack propagation occurs in the concrete. At the second stage, macrocrack propagates in the concrete without interfacial debonding. At the third stage, both vertical macrocrack propagation in the concrete and horizontal shear crack propagation at the CFRP-concrete interface occur in the strengthened beam. The tensile stress in the CFRP sheet and interfacial shear stress along the span are formulated based on the deformation compatibility condition at the CFRP-concrete interface at this stage. Finally, macroshear crack propagates at the interface until the CFRP sheet is completely peeled out from the beam, and then the member is fractured. The applied load is determined as a function of the referred two crack lengths at different stages. At the beginning, the applied load increases to one peak value with the full propagation of fictitious crack at the first stage. At the third stage, the applied load is improved to another peak value due to the relatively high cohesive effect of the CFRP sheet. Then the two peak values are determined by the Lagrange multiplier method. The validity of the proposed analytical solution is verified with the experimental results and numerical simulations. It can be concluded that the proposed analytical solution can predict the load-bearing capacity of CFRP sheet-strengthened cracked concrete beams with reasonable accuracy.

STRESS ORIENTED HYDROGEN INDUCED CRACKING BEHAVIOR OF HEAT AFFECTED ZONE OF L360MCS STEEL IN WET H<SUB>2</SUB>S ENVIRONMENT

Stress oriented hydrogen induced cracking (SOHIC) behavior of heat affected zone (HAZ) of L360MCS steel in wet H2S environment was studied by separating crack, SEM and EDS. The cracks are formed by four-point bending test in wet H2S environment. The microstructure of HAZ is obviously different from the base metal, and the hardness of HAZ is lower than those of the base metal and welding joint. The fracture surface of the cracks was separated and the hy- drogen blisters were torn off. The cross section and fracture surface of the cracks are analyzed by SEM and EDS. Results show that the cracks of SOHIC appeared as axial crack and vertical crack, and the former is preferential. The appearance of cracks is associated with the low hardness value of HAZ. The dimension of hydrogen blasters in tension stress zone of bended specimen is bigger than that in unstressed specimen, which is due to hydrogen concentration in bended specimen is higher. Both axial and vertical cracks nucleate independently and propagate in the quasi-cleavage manner. Tension stress not only increases the supersaturated hydrogen concentration in tension stress zone but directly promotes vertical crack propagation. Axial crack nucleates at MnS, CaS and precipitated phase, and propagates under internal hydrogen pressure. Vertical crack mainly nu- cleates on precipitated phase and propagates under both internal hydrogen pressure and tensile stress.