Initial Defect Research Articles

Numerical simulation and experimental verification of fatigue crack propagation in high-strength bolts based on fracture mechanics.

To investigate the fatigue crack propagation behavior of high-strength bolts for high-speed train brake discs, the fatigue crack propagation of high-strength bolts with initial defects under various load ratios was numerically simulated and experimentally verified based on fracture mechanics in this paper. Firstly, the fracture mechanics model of a three-dimensional hexahedral mesh with initial root defects was established using ABAQUS-FRANC3D interactive technology. Then the stress intensity factor (SIF) of the crack front was calculated by the stress superposition of the crack surface to simulate the coupling effect of preload and axial cyclic load. Based on it, fatigue crack propagation was simulated. Finally, the corresponding fatigue experiments on prefabricated crack bolts were carried out. The results show that mode I cracks dominate in the process of crack propagation. The stable crack propagation zones of the fractured high-strength bolts all show a semi-elliptical cross-section. The SIF of the crack front decreases with the increase of the load ratio, thus making the crack propagation life increase with the increase of the load ratio. The experimental outcomes are in great agreement with the simulation results, which verify that the numerical simulation method can effectively and accurately evaluate the fatigue life of high-strength bolts with initial defects and provides an effective means for predicting the fatigue crack propagation life of the same type high-strength bolts in engineering applications.

ПРОГНОЗИРОВАНИЕ СРОКА СЛУЖБЫ АНТИКОРРОЗИОННЫХ ПОКРЫТИЙ МЕТАЛЛИЧЕСКИХ КОНСТРУКЦИЙ ОБЪЕКТОВ ПЕРЕРАБОТКИ, ХРАНЕНИЯ И ТРАНСПОРТИРОВКИ УГЛЕВОДОРОДОВ

Leaks of liquid and gaseous hydrocarbons from pipelines and equipment of transportation, storage and processing facilities lead to fires and explosions. Often the cause of hydrocarbon leaks is corrosion of metal structures, leading to their depressurization. To increase the durability of metal structures, they are protected with anticorrosive coatings. The protective ability of the anticorrosive coating is determined by its continuity (defect). The prediction of the continuity (defectiveness) of the anticorrosive coating is carried out using empirical exponential or power expressions. The disadvantage of these expressions is a large number of hard-to-determine factors and empirical coefficients. An empirical expression is given to assess the defectiveness of the anticorrosive coating at the current time. An empirical expression is proposed for predicting the operational continuity (defectiveness) of an anticorrosive coating, taking into account the production (initial) defectiveness, the permissible service life and the aggressiveness of the environment. It is shown that the aggressiveness of the environment significantly affects the continuity (defectiveness) anticorrosive coating. The results of forecasting the continuity (defectiveness) of the anticorrosive coating are compared according to the currently used expressions and according to the proposed expression.
 The obtained results of forecasting the continuity (defectiveness) of the anticorrosive coating are close. The empirical expression proposed for predicting the continuity (defectiveness) of the anticorrosive coating is promising for practical application.

Reduction of the initial defects generated during casting of quasi-single crystalline silicon by reserving gaps between seed crystals

The presence of dislocation is a significant impediment to enhancing the quality of quasi-single crystalline silicon cast by seed-assisted directional solidification. A novel approach was proposed to reduce dislocations in G7 166-type silicon ingots, especially in the region of seed crystal seams, by reserving seed crystal gaps during loading. We calculated that the seed crystal's linear thermal expansion after heating from 298.15 to 1683.15 K was 0.9 mm. However, in consideration of the actual deviation in seed processing and measurement, we increased the gap between seeds to 1.2 mm to prevent compressive stress on all 49 seed crystals. The actual residual seed gap measured after ingot casting was approximately 0.325 mm, which is relatively close to the calculated value. It was discovered that the average defect ratio in the silicon blocks decreased from 26.95% (without gaps) to 6.79% (with 1.2 mm seed gaps) for the proposed method. This indicates that a portion of the dislocation source was reduced by reducing compressive stress caused by linear expansion between adjacent unmelted seeds. The average conversion efficiency of the solar cells prepared using the gapless seed method for p-type polycrystalline cells is 22.03%, whereas the novel approach resulted in an average cell efficiency of 22.22%, featuring a higher proportion of high-efficiency sections.

Experimental and numerical research on the static behavior of locally corroded OSBD

Corrosion can accelerate the deterioration of the mechanical properties of orthotropic steel bridge decks (OSBDs), seriously affecting the service safety of steel bridges. In this paper, thirty Q345C steel and four OSBD specimens with various corrosion levels are designed to investigate the effect of corrosion on the static behavior of OSBDs. Then, corrosion damage and its influence on mechanical properties are explored by static loading experiments. The constitutive model for Q345C steel is built considering the impact of corrosion. Finally, a numerical analysis considering geometric initial defects is employed to compare the effect of corrosion location on bearing capacity degradation. It is observed from the tested results that the corrosion led to the failure mode of Q345C steel changing from plastic failure to brittle failure. The ultimate bearing capacity of specimens with a local corrosion rate of 7.92%, 12.05%, and 16.24% decreased by 11.08%, 14.45%, and 17.82%, respectively. Corrosion leads to stress concentration in the top plate area, causing local instability of the structure. The degradation degree of the bearing capacity of the top plate is greater than that of the U-rib corrosion corresponding to the same corrosion conditions.

Research on compressive damage mechanism of concrete based on material heterogeneity

To reveal the influence of material heterogeneity on the mechanical properties of concrete, computed tomography (CT) tests of concrete under uniaxial compression were carried out. A CT scan test of concrete under uniaxial compressive stress was carried out, and CT scan images of a loading stage were obtained. Based on the theory of damage evolution, a mapping relationship between elastic modulus and CT number was constructed, andthe “numerical concrete model of material heterogeneity” was established. The equation for damage evolution and the constitutive equation for subsection damage based on porosity were constructed. Then, the influence of heterogeneity on the damage mechanical properties of concrete under uniaxial compressive stress was studied. The results show that material heterogeneity has a more significant effect on the mechanical properties of concrete due to damage. In addition, the elastic modulus of the material heterogeneity samples is smaller than that of the structural heterogeneity samples, and the peak strength of the material heterogeneity samples is 10.7% lower than that of the structural heterogeneity samples. The numerical concrete model of material heterogeneity can effectively reflect the initial defects of the concrete. Furthermore, the crack propagation of the sample is orderly. The crack is evident around the aggregate and has good directivity. The shear crack characteristics are also noticeable. Meanwhile, the main crack is roughly at a 45-degree angle with the sample, which is closer to the actual physical CT test.

Buckling behavior of Formed-in-Place-Pipe (FIPP) liners under groundwater pressure: An experimental investigation for buried municipal pipelines

Specifications and models for FIPP lining structures are conservative, with the El-Sawy model providing the most accurate prediction of critical buckling pressure. Initial elliptical defects significantly reduce the liner's resistance to external groundwater pressure, and these defects, along with pipeline constraints, influence the deformation processes. This study emphasizes the necessity of strict quality control measures, the consideration of initial ovality during design, and the exploration of reinforcement strategies for FIPP liners. Future research should investigate the long-term creep properties of the material, assess the impact of various types and sizes of initial defects, and develop effective reinforcement technologies for FIPP liners to advance pipeline rehabilitation technologies and practices.

Corrosion Fatigue Assessment of Bridge Cables Based on Equivalent Initial Flaw Size Model

Bridge cables under traffic loads are more prone to failure during the service life due to the corrosion–fatigue coupling effect. In this study, a novel lifespan model based on the equivalent initial flaw size (EIFS) theory is established to analyze the various stages of the lifespan of steel wires. Additionally, a comprehensive corrosion-fatigue lifespan calculation method for parallel steel wire cable is established based on the series–parallel model. A case study of the Runyang Suspension Bridge is conducted to evaluate the evolution of corrosion-fatigue damage in bridge cables during the service life. The results indicate that under the action of corrosion-fatigue, steel wires are more prone to crack initiation, and their fracture toughness is further reduced. In cases where the corrosion level is relatively low, the steel wires of the bridge cables experience no corrosion-fatigue fracture. When the steel wires have initial defects and are subject to corrosion-fatigue conditions, their fracture lifespan is dependent on the severity of the corrosive medium. The reduction in the service life of the cables under the corrosion environment is much greater than that under heavy loads. This research may contribute to the understanding of corrosion-fatigue damage in bridge cables, involving assessment, maintenance, and replacement for bridge cables.

Experimental and numerical investigation on fatigue behaviors of Q690D butt welds

With the improvement of steel production technology, different grades of high-strength steel have been developed and widely applied in engineering, however the fatigue properties of high-strength steel need to be further studied. In this research, regarding the fatigue properties of Q690D high strength steel butt welds, experimental investigation and numerical simulation were carried out. Experimental investigations of Q690D high strength steel butt welds were performed, and properties including the basic mechanical behaviors, metallographic structure, fatigue crack growth rate and fatigue performance were obtained. The fatigue test results were compared with fatigue data from existing studies of butt welds and the Eurocode3 fatigue design S-N curve. The fatigue life prediction was performed and the fatigue crack propagation process was simulated based on the platform combining ABAQUS and Franc3D. The results show that the tested butt-welded joints specimen includes more initial defects, its fatigue properties are comparable to those of 355 MPa grade steel butt-welded joints, and the fatigue resistance advantages of high strength steels are not obvious. The simulated results are in good agreement with the fatigue test results, indicating the proposed simulation method is applicable in the fatigue life assessment of engineering structures with known initial defects. The proposed simulation method provided a numerical analysis approach to the fatigue assessment of engineering structures, thereby reducing the cost of fatigue tests and bringing significant economic and social benefits.

Crack propagation analysis and fatigue life assessment of high-strength bolts based on fracture mechanics

To investigate the effect of initial cracks on the fatigue performance of high-strength bolts for high-speed train brake discs, the fatigue crack propagation behavior of high-strength bolts under the coupling action of preload and dynamic fatigue load was investigated experimentally and numerically based on the theory of linear elastic fracture mechanics. Firstly, fatigue tests of high-strength bolts with initial crack defects were carried out, and then a three-dimensional accurate numerical model with the hexahedral mesh for a bolt-nut was established by MATLAB, and the fatigue crack propagation behaviors were investigated using ABAQUS-FRANC3D interactive technology. In this paper, the effects of the initial crack state, the bolt preload, the axial excitation load, and the friction coefficient of the screw pair on crack propagation life were emphatically studied, and the simulated crack propagation trajectory and crack propagation life agreed well with the experimental results. The findings indicated that 0°-oriented cracks beginning at the maximum principal stress were predicted to have the shortest fatigue life. The crack propagation life was sensitive to the initial crack size, the coefficient of initial crack geometry, and the bolt preload, but not to the friction coefficient of the screw pair. Furthermore, when evaluating the effect of fatigue load on crack propagation, the load ratio, the mean load, and the load range should all be considered.

Failure criteria and microstructure evolution mechanism of the alkali–silica reaction of concrete

Abstract In this study, the influence of two distinct reactive aggregate dosages on the alkali–silica reaction (ASR) of concrete was investigated. The expansion ratio, axial compressive strength, splitting tensile strength, and relative dynamic elastic modulus (RDEM) of concrete were considered as the main parameters to study the failure criteria of concrete induced by ASR at 40°C. Microscopic experiments, such as scanning electron microscopy and X-ray computed microtomography, were applied to analyze the damage induced by ASR on the concrete prisms and the propagation of internal cracks of the reactive aggregates in nanospace. The results showed that the two ranges can be used as the failure criteria produced by the ASR of concrete containing 3 and 6% dosages of reactive aggregates at 40°C, respectively, when the RDEM of concrete was reduced to 70‒75% and 85‒90%. Additionally, the results of CT indicated that microcracks tended to extend from the initial defects of aggregate to the cement slurry. Meanwhile, ASR gel packed with the pores, which reduced the porosity. It was noteworthy that the adhesive force between the ASR gel and the cement matrix after filling pores could not make up for the loss of the mechanical properties of concrete.

Performance Test of Weld Zone of Rotary Friction Welding of Titanium Alloy Drill Pipe

With the development of complex oil and gas reservoirs, it is difficult for traditional carbon steel to meet the requirements of drill pipe materials in drilling process. Titanium alloy has gradually become a new choice of drill pipe material because of its low density and good corrosion resistance. Titanium alloy has poor weldability and weld zone of titanium alloy drill string is easy to become initial defect and lead to drill pipe failure. Therefore, it is essential to evaluate the performance of titanium alloy drill pipe weld zone. In this paper, the weld zone of titanium alloy rotary friction welding is tested. Metallographic structure, hardness, tension and impact mechanical tests, fracture toughness evaluation and bending fatigue test are carried out. The test results show that the metallographic structure of weld zone changes under the influence of temperature, resulting in different mechanical properties of weld zone from that of pipe body zone. The yield strength and tensile strength in the weld zone are higher than those in the pipe zone, but the fracture toughness is lower than that in the pipe zone. The fatigue life of specimens in weld zone is more than 107 in drilling mud environment. In general, the mechanical properties of weld zone meet drilling requirements.

Fluorination-mitigated high-current degradation of amorphous InGaZnO thin-film transistors

As growing applications demand higher driving currents of oxide semiconductor thin-film transistors (TFTs), severe instabilities and even hard breakdown under high-current stress (HCS) become critical challenges. In this work, the triggering voltage of HCS-induced self-heating (SH) degradation is defined in the output characteristics of amorphous indium-gallium-zinc oxide (a-IGZO) TFTs, and used to quantitatively evaluate the thermal generation process of channel donor defects. The fluorinated a-IGZO (a-IGZO:F) was adopted to effectively retard the triggering of the self-heating (SH) effect, and was supposed to originate from the less population of initial deep-state defects and a slower rate of thermal defect transition in a-IGZO:F. The proposed scheme noticeably enhances the high-current applications of oxide TFTs.

Novel fatigue life prediction method of a Ni-based superalloy welded joint considering defect and temperature

Defects are the primary cause of fatigue failure of welded structures, resulting in high dispersion in fatigue test results. In this study, a rapid fatigue life evaluation model is established, which considered both the effects of initial welding defects and service temperature. Firstly, the mathematical relationship between temperature and Vickers hardness is obtained by fitting a series results of high temperature Vickers hardness tests. Then, based on the SEM fracture morphology analysis of fatigue failure samples, the concept and formula of modified stress considering the information of defect size and position are proposed. Finally, a new life prediction model is established by combining the temperature-hardness relationship model with Murakami's model. The fatigue life of GH4169 electron beam welding joints can be evaluated by this model at any temperature, and most of the predicted results are within a scatter band of ±2.

NONLINEAR BUCKLING SENSITIVITY ANALYSIS OF THIN-WALLED LINED COMPOSITE PIPE LINER

In order to study the buckling failure of lining delamination of thin-walled lined composite pipe, a numerical analysis model was established by using bilinear cohesion relationship, the buckling mode and critical load of the composite pipe obtained by linear buckling are taken as reference values, the interlayer initial defects were introduced to carry out the nonlinear buckling analysis on the composite pipe lining structure with thin wall lining. The relative displacement curve and deformation morphology of the relative displacement of the lining with the change of external pressure were obtained, and the results are consistent with the existing test results. Based on this model, the buckling sensitivity of lining pipe was analyzed. The results show that the size of interlayer initial defect is the main factor that affects the critical buckling pressure of liner, but it has little influence on the propagation pressure after buckling; However, the increase of interlayer bonding effect significantly improves the buckling resistance and propagation pressure of lining pipe; The ratio of outer tube wall diameter to thickness and liner thickness have significant effects on the critical buckling pressure of liner. The research results provide a reference for determining the interlayer bonding effect and the optimal design of the minimum thickness of the inner liner.

Understanding the effect of nano‐zirconia on the compressive fatigue performance of cementitious composites

AbstractThe fatigue performance of cementitious composites that affects the service life of structures is particularly sensitive to the initial defects in cementitious matrix and interfacial transition zone. Nano‐ZrO2 (NZ), characterized by high strength and toughness as well as good dispersibility, can improve the compactness of cementitious composites. This paper investigated the modifying effect and mechanisms of NZ on the compressive fatigue performance of cementitious composites, including fatigue life, strength, and deformation behaviors. The experimental results show that adding NZ extends the fatigue life of cementitious composites by an order of magnitude and increases the fatigue failure strain by 11.9%. The microstructural analysis verifies that NZ can deflect the fatigue micro‐cracks, fill the micro‐pores and reduce the growth space of CH crystal size. The modified microstructures potentially lead to fatigue of cementitious composites starting from generating more nano‐cracks in both cementitious matrix and interfacial transition zone, thereby improving the fatigue performance of the composites.

A dynamic condition assessment model of aging subsea pipelines subject to corrosion-fatigue degradation

Extension of operating lifetime of aging subsea pipelines is of great interest to oil and gas sectors. Corrosion and fatigue are the main causations of the condition degradation of subsea pipelines. A dynamic probabilistic condition assessment model based on continuous dynamic Bayesian network (DBN) is developed to support the life extension decision-making of aging subsea pipeline subjected to the corrosion-fatigue degradation. The methodology is built based on equivalent initial flaw size (EIFS) concept and a time-dependent prediction model implemented using DBN. The complex corrosion-fatigue degradation process is simplified by EIFS concept, and the crack propagation due to corrosion-fatigue is modelled using fracture mechanics model and DBN. A limit state function (LSF) is used to express the failure condition of subsea pipeline due to crack propagation. The dynamic reliability of subsea pipeline is estimated using Monte Carlo (MC) method with the probability distributions of the predicted crack sizes at different time slices. The estimated reliability is compared with the acceptable threshold to decide whether any measures are required to extend the life of subsea pipeline. The methodology is tested by a case study, and it is observed that it can be a useful tool to support life extension decision-making of aging subsea pipelines.

Kink band orientation of 3D printed continuous carbon fiber composites under compressive loading

For continuous carbon fiber-reinforced plastic (cCFRP), in the case of loading in the longitudinal direction, occurrence of kink band failure causes compression strength to be lower than the tensile strength. Furthermore, 3D-printed cCFRP may exhibit intrinsic compressive behavior depending on the printing orientation. In this study, two types of rectangular specimens were fabricated using the Markforged Mark Two > 3D printer. The first was the in-plane specimen, whose width direction was parallel to the print bed and thickness direction was along the layup direction. The second was the out-of-plane specimen, whose width direction was the layup direction and thickness direction was parallel to the print bed. Compression tests were performed, and the experimental results were compared. As a result, the compressive strengths of both types were similar, and the kink bands always grew in the lay-up direction. For the out-of-plane specimens, nonlinear stress–strain curves were observed. This may be due to the delamination crack growth caused by the low strength in the lay-up direction and the existence of initial defects. This study experimentally investigated the effect of two different printing orientations of 3D printed cCFRP to its strength and failure mode, and this work shows the importance of considering structural deformation due to compression loading in 3D printed cCFRP.

Crack growth-based life prediction for additively manufactured metallic materials considering surface roughness

Additively Manufactured (AM) components are prone to fatigue damage due to defects accompanied by the manufacturing process, such as surface roughness, internal porosity, and anisotropy due to differences in build directions. This is especially true for as-built components with significant surface roughness, where the surface polishing and treatments are not possible due to resource or space limitations. This paper proposes a crack growth-based methodology for the fatigue life assessment of AM components subjected to uniaxial and multiaxial, constant and variable loading conditions. The work is based on a previously developed subcycle fatigue crack growth model. The developed FCG model is extended with the stress concentration factor due to surface roughness and an asymptotic stress intensity factor (SIF) interpolation method for notched specimens. The proposed model approximates the surface roughness as an equivalent notch having the same stress concentration as that posed by the irregularities on the surface. Fatigue life assessment is performed based on the concept of equivalent initial flaw size (EIFS) and FCG analysis. The proposed methodology is validated against in-house as well as experimental data from the available literature.

Influence of carbon black particle size on fatigue life of rubber compound by varying strain and temperature

AbstractFatigue failure is one of the most critical failure modes in engineering structures. The fracture process proceeds through two major phases: crack nucleation and subsequent crack propagation leading to the ultimate failure. With special reference to rubber products especially tyre, the fatigue crack growth characteristics have been well understood with good approximation by many researchers. But the experimental research on crack nucleation is in the nascent stage. In the present study, we have focused on the role of filler particle size, temperature and strain on crack nucleation and fracture surface morphology of unfilled and carbon black (CB)‐filled natural rubber (NR) compounds. Three different kinds of ASTM grade carbon blacks, namely, N134, N234 and N339 have been used to determine the effect of filler particle size on crack initiation. All compounds have been tested at room temperature (i.e., RT 25°C), 70°C and 100°C with different strain amplitudes (50%, 75%, 100%, and 125%) to understand its effect on damage mechanics. It is observed that at a given strain and temperature, compound with N134 has better fatigue life in comparison to the N234 and N339. It is also found that with increasing temperature, fatigue life is being reduced for all the compounds. Optical microscope and field emission scanning electron microscope (FESEM) along with micro computational tomography (CT) have been utilized to understand the failure mechanism which reveal the presence of initial flaws as filler protrusions that creates micro cracks during fatigue loading and thus leads to crack nucleation.

Experimental study on seismic performance of precast concrete shear walls with defective and repaired grouted sleeve

Grouted sleeve connections (GSCs) have been widely used in precast concrete (PC) shear wall structures. It is critical to reveal the effect of insufficient grouting on the seismic performance of PC shear walls (PSWs), and to recommend as well as validate the repair method for insufficient grouting defects. To achieve this, three full-scale PSWs were tested under cyclic loads: one PSW without grouting defects, one PSW with grouting defect, and one repaired PSW with initial grouting defect. The effects of defect ratio and repair action were analyzed and the test results indicated: 1) bar slip dominated the failure of the specimen with the defect ratio as expected, rather than the flexural failure mode for the specimen without defects. After the occurrence of bar slip, a significant rocking phenomenon was observed, and the damage to concrete was alleviated. The pinching phenomenon was observed for the hysteretic curves, and the strength, deformation, and energy dissipation capacities were significantly lower than those of the specimen without defects, particularly the ductility. 2) The bar-slip behavior was effectively prevented after the repair of the grouting defect, and the repaired specimen exhibited the expected flexural failure mode. The damage evolution mode, strength, deformation capacity, and energy-dissipation capacity were generally comparable with those of the specimen without defects. Rocking behavior and pinching phenomenon were not observed after repair. The negative influences of insufficient grouting were approximately eliminated, thus validating the feasibility and reliability of the repair method.