Detection of internal crack growth in polyethylene pipe using guided wave ultrasonic testing

Despite the success of guided wave ultrasonic inspection for internal defect detection in steel pipes,its application on polyethylene(PE)pipe remains relatively unexplored.The growth of internal cracks in PE pipe severely affects its pressure-holding capacity,hence the early detection of internal cracks is crucial for effective pipeline maintenance strategies.This study extends the scope of guided wave-based ultrasonic testing to detect the growth of internal cracks in a natural gas distribution PE pipe.Laboratory experiments and a finite element model were planned to study the wave-crack interaction at different stages of axially oriented internal crack growth with a piezoceramic transducer-based setup arranged in a pitch-catch configuration.Mode dispersion analysis supplemented with preliminary experiments was performed to isolate the optimal inspection frequency,leading to the selection of the T(0,1)mode at 50-kHz for the investigation.A transmission index based on the energy of the T(0,1)mode was developed to trace the extent of simulated crack growth.The findings revealed an inverse linear correlation between the transmission index and the crack depth for crack growth beyond 20%crack depth.

Experimental evaluation of asphalt mixtures with emerging additives against cracking and moisture damage

The objective of this study was to evaluate and recommend an asphalt mixture design with emerging additive technologies that would provide superior performance against asphalt concrete(AC)stripping and cracking.To achieve this objective,a laboratory test program was developed to evaluate the use of nanomaterials(nanoclay and graphene nanoplatelet),an emerging anti-stripping agent(adhere),and warm-mix asphalt technologies(ZycoTherm,Sasobit,and EvoTherm).Two mix types were evaluated,which were a stone-matrix asphalt(SMA)and a dense-graded binder mix.In addition,the modified Lottman test(AASHTO T 283)and the indirect tensile asphalt cracking test(IDEAL-CT)test were used as performance indicators of moisture damage resistance and cracking susceptibility.Results were analyzed statistically to identify and quantify the effects of the design variables and selected additives on the performance,moisture damage resistance,and durability of asphalt mixes.Based on the cracking test results,a superior cracking resistance performance was observed with ZycoTherm,irrespective of the mix type.Adhere had the lowest average cracking indices for both mix types,which suggest that it would not perform as well as the other additives in terms of cracking resistance.Overall,SMA mixes displayed greater cracking resistance than the dense-graded mixtures,which may have been the result of the reclaimed asphalt pavement(RAP)material used in the dense-graded mix and its lower asphalt binder content.In terms of moisture resistance,both nanomaterials(graphene nanoplatelet and nanoclay)did not perform well as they did not meet the minimum required tensile strength ratio(TSR)criterion(>0.80).In addition,nanomaterials showed the lowest TSR values in both mix types suggesting that their effectiveness against moisture-induced damage may not be as good as warm-mix additives.On the other hand,warm-mix additives were expected to show enhanced performance in terms of moisture resistance as compared to the other additives evaluated in this study.

Study on hydrogen assisted cracking susceptibility of HSLA steel by implant test

DMR-249 A is an indigenously developed high strength low alloy steel for Indian ship building industry for making ship-hull and is extensively used in the construction of war ships and submarines. Welding electrodes conforming to SFA 5.5 AWS E8018 C1 has been indigenously developed for welding of this steel using shielded metal arc welding process. In the present study, susceptibility to hydrogen assisted cracking of DMR-249 A steel welds made using this electrode has been assessed using implant test. Implant tests were conducted using this electrode at two different levels of diffusible hydrogen, measured using gas chromatography technique. It is observed that both the steel and the welding consumable are not susceptible to hydrogen assisted cracking even with a high diffusible hydrogen level of 9 m L/100 g of weld metal. In implant tests, specimen did not fracture even after loading to stress levels higher than the yield strength of the base metal. The good resistance of this steel and the welding consumable, even with high levels of diffusible hydrogen, is attributed to absence of a susceptible microstructure in both the weld metal and heat affected zone. Hence, this study shows that, in the absence of a susceptible microstructure, hydrogen assisted cracking is unlikely to occur even if hydrogen level is high. It also confirms that in welding of DMR-249 A with indigenously developed E8018 C1 electrode, hydrogen assisted cracking is not a concern and no preheating is required to avoid it during welding.

A review on evaluation of crack resistance of asphalt mixture by semi-circular bending test

Although there are many kinds of fracture tests to choose from in evaluating the crack resistance of asphalt mixture,the semi-circular bending(SCB)test has attracted a lot of attention in the academic road engineering community because of its simplicity,stability,and flexibility in testing and evaluation.The SCB test has become a common method to study the cracking resistance of asphalt mixture in recent years.This paper mainly summarizes the overview of the SCB test,summarizes some research results and common characterization parameters of the SCB test method in monotone test and fatigue test in recent years,and predicts and suggests the research direction of the SCB test in the future.It is found that the research on the monotonic SCB test is more comprehensive,and the research on the SCB fatigue test needs to be further improved in the aspects of loading mode,characterization parameter selection,and so on.Researchers can flexibly adjust the geometric dimensions and the test parameters of semi-cylindrical specimens,and conduct comprehensive analysis combined with the results of numerical simulation.The crack resistance of asphalt mixture can be comprehensively evaluated by fracture energy,fracture toughness,stiffness,flexibility index and other fracture indicators,combined with the crack propagation of the specimen.The analysis of numerical simulation can confirm the test results.In order to standardize the setting of fatigue parameters for future application,it is necessary to standardize the setting of bending performance.

Oblique Y-groove cracking test of the welding cold cracking susceptibility of domestic X-70 pipeline steel

Gas Transmitting From West to East Project' is significant. It should ensure the welding quality and safety of pipeline. The task is very arduous to guarantee the quality of the project in the condition of long line, complex weather and geology features. In this paper, the welding cold cracking susceptibility of domestic X 70 pipeline steel adopted by the project, which is one of the most interesting questions of welding quality about petrol pipeline, was studied by means of oblique Y groove cracking test. The crack ratio of surface and section was tested under the conditions of different welding materials and preheat temperature .The thickness of plate steel was 14.7 mm and 10.3 mm . The results reveal that X 70 pipeline steel has good crack resistance. The research has important value for the construction of large scale pipeline engineering and the application of domestic X 70 pipeline steel.

Determining rock crack stress thresholds using ultrasonic through-transmission measurements

The crack initiation stress threshold is widely used in excavation industries as rock spalling strength when designing deep underground structures to avoid unwanted brittle failures.While various strain-based methods have been developed for the estimation of this critical design parameter,such methods are destructive and often requires subjective interpretations of the stress–strain curves,particularly in rocks with pre-existing microcracks or high porosity.This study explore the applicability of non-destructive ultrasonic through-transmission methods for determining rock damage levels by assessing the changes in transmitted signal characteristics during loading.The change in velocity,amplitude,dominant frequency,and root-mean-square voltage are investigated with four different rock types including marble,sandstone,granite,and basalt under various stress levels.Results suggest the rate of signal variations can be reliably used to estimate crack closure and crack initiation stress levels across the tested rocks before failure.Comparison of the results between the conventional techniques and the new proposed methods based on ultrasonic monitoring are further discussed.

Crack mechanism of ground fissures in loess layer of Fenwei Basin, China

The Fenwei Basin, covered by loess, experiences severe ground fissure disasters. These disasters disrupt the continuity of the loess and pose significant threats to engineering construction safety along transportation routes. Nevertheless, the crack characteristics and the influence zone of ground fissures in the loess layer remain inadequately investigated. To effectively prevent and control ground fissure disasters, physical model tests and the PFC(particle flow code) numerical simulation method are used to investigate the crack mechanism of buried ground fissures in the loess layer. The results show that there are two main cracks in the layer profile, which have a Y-shape morphology. As the dip angle of the preset cracks increased from 60° to 90°, the main deformation zone at the surface gradually shifted towards the footwall. The process of crack propagation from depth to surface is divided into five stages. Additionally, the results confirm the accuracy of the width of the rupture zone d2in the footwall calculated by the cantilever beam theory. These findings can offer theoretical guidance for determining the avoidance distance of ground fissures in loess regions, as well as for implementing disaster prevention and corresponding control measures for various stages of buried ground fissure propagation.

Numerical simulation of plate rigid restraint cracking tests based on cohesive element model

Cohesive element is developed from the Dugdal-Barenblatt model in the field of fracture mechanics. The mechanical constitutive relation of cohesive element can be artificially assumed depending on the specific applications. It has been successfully applied in the study of crystal plasticity/brittle fracture process and decohesion between delaminations. In this paper, tensile experiments of large steel plate with different length of pre-existing cracks are conducted. Based on commercial software ABAQUS, cohesive element is adopted to simulate the tensile tests, and appropriate parameter values are obtained by fitting displacement-force curves. Using these parameters, a numerical method is presented by applying cohesive element to thermo-elastic-plastic finite element method （TEP-FEM） to simulate plate rigid restraint cracking （PRRC） tests. By changing constitutive relation of cohesive element, dimensions of the model and welding conditions, the influence of welding restraint intensity and welding conditions on the crack propagation are discussed, respectively. Three types of welding cold cracking are simulated. Significant influence of welding cold cracking on resistant stress in welding line is captured by this numerical method.

The Dry Cracking of Cementitious Materials in the Eccentric Ring Test and the Concentric Ring Test

In order to reduce the randomness of the occurrence of cracks and shorten the long cracking time in the traditional concentric ring tests,the elliptical ring test,the square-eccentric ring test,and the eccentric ring test have been gradually developed.In this paper,we reported experiments on the eccentric ring test and concentric ring test that were carried out to compare the differences between the two methods.It is found that an increase in the water-cement ratio and the amount of aggregate will increase the cracking time.However,a more obvious cracking tendency of cement-based materials can be seen in the eccentric ring test.The correlation between humidity and strain was established by the use of the Kelvin equation and the Laplace equation so that the coupling analysis of humidity and strain during the drying process of cement-based materials could be determined.The experimental results show that the external surface humidity will decrease rapidly in the early stage of drying,while the interior areas of the cement-based materials decrease more slowly.The closer to the inner circle will decrease the humidity slowly.

Catalytic cracking of light diesel over Au/ZSM-5 catalyst for increasing propylene production

The catalytic cracking of light diesel oil （235–337 ＆#176;C） over gold‐modified ZSM‐5 was investigated in a small confined fluidized bed at 460 ＆#176;C and ambient pressure. Different Au/ZSM‐5 catalysts were prepared by a modified deposition‐precipitation method by changing the preparation procedure and the amount of gold loading and were characterized by X‐ray diffraction, N2 adsorp‐tion‐desorption, temperature‐programmed desorption of NH3, transmission electron microscopy and inductively coupled plasma spectrometer. It was found that a small amount of gold had a posi‐tive effect on the catalytic cracking of light diesel oil and increased propylene production at a rela‐tively low temperature. The maintenance of the ZSM‐5 MFI structure, pore size distribution and the density of weak and strong acid sites of the Au/ZSM‐5 catalysts depended on the preparation pa‐rameters and the Au loading. Simultaneous enhancement of the micro‐activity and propylene pro‐duction relies on a synergy between the pore size distribution and the relative intensity of the weak and strong acid sites. A significant improvement in the micro‐activity index with an increase of 4.5 units and in the propylene selectivity with an increase of 23.2 units was obtained over the Au/ZSM‐5 catalyst with an actual Au loading of 0.17 wt%.

Cryo-Cracking方法及其在焊接接头蠕变断裂失效分析中的应用

介绍了 Cryo- Cracking方法及其在分析和评定高温下长期服役的焊管焊接接头蠕变失效中的应用特点。这种方法使用的试样易于截取 ,制样工艺过程简单 ,对制样者的技艺无特殊要求 ,不易产生因试样的制备而引起的误解。通过扫描电镜断口形貌的分析 ,结合金相定量分析方法 ,从分析断口的二维图象获得的数据 ,可以定量地反映在役构件蠕变失效的程度。实践表明 ,Cryo- Cracking方法是一种简单实用的焊接接头蠕变失效分析方法。

Experimental study on cracking behaviour of moulded gypsum containing two non-parallel overlapping flaws under uniaxial compression

Failure of rock mass that is subjected to compressive loads occurs from initiation, propagation, and linkage of new cracks from preexisting fissures. Our research investigates the cracking behaviour and coalescence process in a brittle material with two non-parallel overlapping flaws using a high-speed camera. The coalescence tensile crack and tensile wing cracks were the first cracks to occur from the preexisting flaws. The initiation stresses of the primary cracks at the two tips of each flaw were simultaneous and decreased with reduced flaw inclination angle. The following types of coalescence cracks were identified between the flaws: primary tensile coalescence crack, tensile crack linkage, shear crack linkage, mixed tensile-shear crack, and indirect crack coalescence. Coalescence through tensile linkage occurred mostly at pre-peak stress. In contrast, coalescence through shear or mixed tensile-shear cracks occurred at higher stress. Overall, this study indicates that the geometry of preexisting flaws affect crack initiation and coalescence behaviour.

Stress Corrosion Cracking of Nitrogen-containing Stainless Steel 316LN in High Temperature Water Environments

Stress corrosion cracking （SCC） of stainless steels and Ni-based alloys in high temperature water coolant is one of the key problems affecting the safe operation of nuclear power plants （NPPs）. The nitrogen-added stainless steel is a kind of possible candidate materials for mitigating SCC since reducing the carbon content and adding nitrogen to offset the loss in strength caused by the decrease in carbon content can mitigate the problem of sensitization. However, the reports of SCC of nitrogen-added stainless steels in high temperature water are few available. The effects of applied potential and sensitization treatment on the SCC of a newly developed nitrogen-containing stainless steel （SS） 316LN in high temperature water doped with chloride at 250 ℃ were studied by using slow strain rate tests （SSRTs）. The SSRT results are compared with our data previously published for 316 SS without nitrogen and 304NG SS with nitrogen, and the possible mechanism affecting the SCC behaviors of the studied steels is also discussed based on SSRT and microstucture analysis results. The susceptibility to cracking of 316LN SS normally increases with increasing potential. The susceptibility to SCC of 316LN SS was less than that of 316 SS and 304NG SS. Sensitization treatment at 700℃ for 30 h showed little effect on the S CC of 316LN S S and significant effect on the S CC of 316 S S. The predominant cracking mode for the 316LN S S in both annealed state and the state after the sensitization treatment was transgranular. The presented conditions of mitigating stress corrosion cracking are some useful information for the safe use of 316LN SS in NPPs.

Cold Cracking of Flux Cored Arc Welded Armour Grade High Strength Steel Weldments

In this investigation, an attempt has been made to study the influence of welding consumables on the factors that influence cold cracking of armour grade quenched and tempered （Q＆.T） steel welds. Flux cored arc welding （FCAW） process were used making welds using austenitic stainless steel （ASS） and low hydrogen ferritic steel （LHF） consumables. The diffusible hydrogen levels in the weld metal of the ASS and LHF consumables were determined by mercury method. Residual stresses were evaluated using X-ray stress analyzer and implant test was carried out to study the cold cracking of the welds. Results indicate that ASS welds offer a greater resistance to cold cracking of armour grade Q＆T steel welds.

Test of subcritical crack growth and fracture toughness under water-rock interaction in three types of rocks

The subcritical crack growth and fracture toughness in peridotite, lherzolite and amphibolite were investigated with double torsion test. The results show that water-rock interaction has a significant influence on subcritical crack growth. With water-rock interaction, the crack velocity increases, while the stress intensity factor declines, which illustrates that water-rock interaction can decrease the strength of rocks and accelerate the subcritical crack growth. Based on Charlse theory and Hilling & Charlse theory, the test data were analyzed by regression and the correlation coefficients were all higher than 0.7, which shows the correlation is significant. This illustrates that both theories can explain the results of tests very well. Therefore, it is believed that the subcritical crack growth attributes to the breaking of chemical bond, which is caused by the combined effect of the tensile stress and the chemical reaction between the material at crack tip and the corrosive agent. Meanwhile, water-rock interaction has a vital effect on fracture toughness. The fracture toughness of samples under atmospheric environment is higher than that of samples immersed in water. And water-rock interaction has larger influence on fracture toughness in amphibolite than that in peridotite and lherzolite.

Interdiffusion behavior between Cr and Zr and its effect on the microcracking behavior in the Cr-coated Zr-4 alloy

High-temperature chromium(Cr)-zirconium(Zr)interdiffusion commonly occurs in Cr-coated zircaloys applied for enhanced accident-tolerant fuel(ATF)claddings.Such interdiffusion changes the interfacial microstructure and thus the fracture mechanism of the coating under external loading.In this study,the interdiffusion behavior in a magnetron sputtered Cr coating deposited on a Zr-4 alloy was studied in a vacuum environment at 1160C.In addition,the effect of interdiffusion on the microcracking behavior of the Cr coating was determined by in situ three-point bending tests.The experimental results show that the interdiffusion behavior resulted in the formation of a ZrCr2 layer,accompanied by the consumption of Cr coating and interfacial roughening.The growth of the diffusion layer followed a nearly parabolic law with respect to annealing time,and the residual stress of the annealed coating decreased with increasing annealing time.Under external loading,a large number of cracks were generated in the brittle interlayer,and some interfacial cracks were formed and grew at the ZrCr2/Zr-4 interface.Despite the remarkable microcracks in the ZrCr2 layer,the vacuum-annealed Cr coating has significantly fewer cracks than the original coating,mainly because of the recrystallization of the coating during annealing.

HAZ CRACKING AND HOT DUCTILITY OF Ni_3AI CONTAINING Zr ALLOY

The HAZ cracking test in EB welding condition and hot ductility test show that the cracking mechanism is the formation of Ni - Ni5Zr eutectic which is resulting from the enrichment of Zr in grain boundaries during welding.In order to eliminate HAZ cracks the diffusion and recrystalization anealing treatment after cast and during cold rolling should be conducted in vacuum or protecting atmosphere and at the same time a slow welding speed is necessary.

Behavior of Stress Corrosion Cracking in a Magnesium Alloy

Slow strain rate testing （SSRT） was employed to study the stress corrosion cracking （SCC） behavior of ZE41 magnesium alloy in 0.01 M NaCl solution. Smooth tensile specimens with different thicknesses were strained dynamically in both longitudinal and transverse direction under permanent immersions at a strain rate of 10-6 s-1. It is found that ZE41 magnesium alloy is susceptible to SCC in 0.01 M NaCl solution. The SCC susceptibility of the thinner specimen is lower than that of the thicker specimen. Also, the longitudinal specimens are slightly more susceptible to SCC than the transverse specimens. The SCC mechanism of magnesium alloy is attributed to the combination of anodic dissolution with hydrogen embrittlement.

Crack deflection occurs by constrained microcracking in nacre

For decades, nacre has inspired researchers because of its sophisticated hierarchical structure and remarkable mechanical properties, especially its extreme fracture toughness compared with that of its predominant constituent,CaCO3, in the form of aragonite. Crack deflection has been extensively reported and regarded as the principal toughening mechanism for nacre. In this paper, our attention is focused on crack evolution in nacre under a quasi-static state. We use the notched three-point bending test of dehydrated nacre in situ in a scanning electron microscope(SEM) to monitor the evolution of damage mechanisms ahead of the crack tip. The observations show that the crack deflection actually occurs by constrained microcracking. On the basis of our findings, a crack propagation model is proposed, which will contribute to uncovering the underlying mechanisms of nacre’s fracture toughness and its damage evolution. These investigations would be of great value to the design and synthesis of novel biomimetic materials.

Assessment of early-age cracking of high-performance concrete in restrained ring specimens

High-performance concrete （HPC） is stronger and more durable than conventional concrete. However, shrinkage and shrinkage cracking are common phenomena in HPC, especially early-age cracking. This study assessed early-age cracking of HPC for two mixtures using restrained ring tests. The two mixtures were produced with water/binder mass ratio （mw/mB） of 0.22 and 0.40, respectively. The results show that, with greater steel thickness, the higher degree of restraint resulted in a higher interface pressure and earlier cracking. With steel thickness of 6 mm, 19 mm, and 30 mm, the age of cracking were, respectively, 12 days, 8 days, and 5.4 days with the mw/mB = 0.22 mixture; and 22.5 days, 12.6 days, and 7.1 days with the mw/mB= 0.40 mixture. Cases of the same steel thickness show that the ring specimens with a thicker concrete wall crack later. With the mw/mB = 0.22 mixture, concrete walls with thicknesses of 37.5 mm, 75 mm, and 112.5 mm cracked at 3.4 days, 8.0 days, and 9.8 days, respectively; with the mw/mB = 0.40 mixture, the ages of cracking were 7.1 days, 12.6 days, and 16.0 days, respectively.