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• Determination of the highest stressed regions/locations in the closure head through detailed stress analysis under various combinations of loading conditions. • Development of high-quality hexahedral/ brick mesh around the crack fronts. • Investigation of stress intensity factors (SIFs) of the surface cracks under various combinations of loading conditions. • Effect of the thermal stress and bolt pre-load on the stress intensity factors (SIFs) of the surface cracks. The effect of the bolt pre-load and thermal stresses in addition to pressure stresses on the structural integrity of a closure head of an RPV is presented here. The trends of the stress intensity factors (SIFs) of the surface cracks have been investigated under the combined effects of the loading conditions. A contradictory behavior of the surface cracks at the inside and outside locations of the head is observed due to thermal stresses, while the bolt pre-load decreases the integrity of the head at both locations. The maximum SIFs of the analyzed cracks are also found well below the material's fracture toughness. [ABSTRACT FROM AUTHOR]

Fatigue surface crack growth and the in-plane and out-of-plane constraint effects are studied through experiments and computations for aluminium alloy D16T. Subjects for studies are cruciform specimens under different biaxial loading and bending central notched specimens with external semi-elliptical surface crack. Both the optical microscope measurements and the crack opening displacement (COD) method are used to monitor and calculate both crack depth and crack length during the tests. The variation of crack growth rate and surface crack paths behaviour is studied under cyclic pure bending and biaxial tension-compression fatigue loading. This work is centered on the relations between crack size on the free surface of specimen considered configurations, COD and aspect ratio under different fatigue loading conditions. For the experimental surface crack paths in tested specimens the T-stress, the local triaxiality parameter h, the out-of-plane TZ factor and the governing parameter for the 3D-fields of the stresses and strains at the crack tip in the form of In-integral were calculated as a function of aspect ratio by finite element analysis to characterization of the constraint effects along semi-elliptical crack front. The plastic stress intensity factor approach is applied to the fatigue crack growth on the free surface of the tested bending and cruciform specimens as well as the deepest point of the semi-elliptical surface crack front. As result fatigue surface crack paths or crack front positions as a function of accumulated number of cycle of loading are obtained.

Rotational components of aero engine turbine are the most important components. It operates at high temperature and under conditions of extreme environmental attack such as oxidation, corrosion and wear. These conditions can cause cracking of rotational components. The failure damage modes of turbine are classified in terms of main components as flow path parts, rotating such as rotor, groove, disk, and blade. Aero-engine turbine components such as discs and blades are susceptible to environmentally assisted cracking. Unlike fatigue crack growth, this involves crack growth under constant load. If the crack grows long enough, sudden failure can occur with catastrophic consequences. It is therefore desirable to identify the limiting crack size within fixings so that they can be inspected at regular intervals and removed from service before failure occurs. Three dimensional axis-symmetric finite element models were created to simulate a disc and the portion of a blade. The finite element method (FEM) allowed the prediction of the point of crack initiation and the crack propagation using the orientations of the maximum principal stresses. Stress intensity factor (SIF) is the base parameter in strength analysis regarding fracture mechanics. For a correct determination of SIF in this paper, combining J-integral approach and FEM is used. J-integral is a path independent integral around the crack tip.

The surface crack, also known as the partly through-thickness crack, is a serious threat to the structural integrity of offshore metallic pipes. In this paper, we review the research progress in regard to surface crack growth in metallic pipes subjected to cyclic loads from the fracture mechanics perspective. The purpose is to provide state-of-the-art investigations, as well as indicate the remaining challenges. First, the available studies on surface cracked metallic pipes are overviewed from experimental, numerical, and analytical perspectives, respectively. Then, we analyse state-of-the-art research and discuss the insufficiencies of the available literature from different perspectives, such as surface cracks and pipe configurations, environmental influential parameters, the girth welding effect, and numerical and analytical evaluation methods. Building on these surveys and discussions, we identify various remaining challenges and possible further research topics that are anticipated to be of significant value both for academics and practitioners.

The subject for studies is an aluminium cylindrical hollow specimen with external axial and part circumferential semi-elliptical surface crack undergoing fatigue loads. Both the optical microscope measurements and the crack opening displacement (COD) method are used to monitor and calculate both crack depth and crack length during the tests. The variation of crack growth behaviour is studied under cyclic axial tension, pure torsion and combined tension+torsion fatigue loading. For the particular surface flaw geometries considered, the elastic and plastic in-plane and out-of-plane constraint parameters, as well as the governing parameter for stress fields in the form of In-integral and plastic stress intensity factor, are obtained as a function of the aspect ratio, dimensionless crack length and crack depth. The combined effect of tension and torsion loading and initial surface flaw orientation on the crack growth for two type of aluminium alloys is made explicit. The experimental and numerical results of the present study provided the opportunity to explore the suggestion that fatigue crack propagation may be governed more strongly by the plastic stress intensity factor rather than the magnitude of the elastic SIFs alone. One advantage of the plastic SIF is its sensitivity to combined loading due to accounting for the plastic properties of the material.

Highlights • Ion chelating agent improves the pore size distribution and mechanical property of cement-based materials. • Self-healing performance of mortar on freeze-thaw damage is obviously enhanced by ion chelating agent. • Surface crack of width more than 0.3 mm of the mortar with ion chelating agent can be self-healed. Abstract Ion chelating agent, a novel crystalline additive, is used to improve the self-healing performance of cement-based materials on freeze-thaw damage and surface crack for prolonging service life of the construction. The pore size distribution, internal structure and compressive strength of mortar with ion chelating agent before and after freeze-thaw as well as self-healing were investigated by nuclear magnetic resonance, scanning electron microscope and mechanical property test. The width of surface crack of the mortar during self-healing was measured. The results show that the microstructure and mechanical properties of mortars are improved by ion chelating agent. After 28 days of curing, harmful pores (larger than 0.1 μm) proportion of mortar with 0.5 wt% ion chelating agent of the cement (C2) declines by 42.3%, compressive strength increases by 26.8%, and much needle-like crystals exists in pores comparing with the control mortar (C0). After 100 freeze-thaw cycles, the compressive strength loss, mass loss and harmful pores proportion of C2 reduce by 38.0%, 29.2% and 28.7% relative to C0. After curing in water for 28 days, the compressive strength recovery ratio of C2 is 51.8%, and lots of needle-like crystals are formed in pores and micro-cracks. Energy dispersive spectrometer analysis indicates that the main components of these needle-like crystals are calcium carbonate, CSH and ettringite. Besides, the surface crack of 0.32 mm width for C2 is self-healed and X-ray diffraction analysis shows that the self-healed product in the crack is calcium carbonate. [ABSTRACT FROM AUTHOR]

Highlights • A FE mesh generator is developed to generate FE models of misaligned pipes. • An experimental test is conducted to verify the validity of the mesh generator. • Stress intensity factor equations are proposed for the misaligned pipes. • The equations can be used for fitness-for-service assessment of misaligned pipes. Abstract In this study, fatigue assessment of cracked pipes with weld misalignment is carried out by using fracture mechanics approach. Two types of weld misalignment, viz., centreline offset weld misalignment with equal thickness but unequal diameters, and centreline offset weld misalignment with unequal thickness but aligned inside surface, are considered. A finite element (FE) mesh generator for cracked plain pipes is developed first, and then it is used for the misaligned pipes by incorporating a coordinate transformation method. A small-scale experimental test on the stress concentration curve along the longitudinal direction of misaligned pipes is conducted to ascertain the accuracy of the transformation method. Then, extensive FE analyses are performed to determine the stress intensity magnification factor (M k) at the crack deepest point and the crack ends of a surface crack in misaligned pipes. A new set of M k equations for the misaligned pipes are built up using multiple regression analysis, and statistical analyses are carried out to validate the goodness of fit of these equations. The proposed equations can be employed in the fatigue strength calculation and fatigue life prediction based on the fatigue assessment procedure provided by BS 7910:2013. [ABSTRACT FROM AUTHOR]

Featured Application: This paper presents a method to identify the location and depth of surface cracks in cylindrical components based on scanning laser line source (SLLS) techniques. The research results are conducive to improving the accuracy and efficiency of laser ultrasonic testing for complex structures such as curved surface structures. Cylindrical structures play an important role in industrial fields. The surface crack is a typical defect in the cylindrical structures. Non-destructive surface crack detection of these structures is critical to the safe operation of the equipment. In this study, the signal enhancement of the scanning laser line source (SLLS) method is investigated by a numerical simulation method to identify the location and depth of the surface crack in the aluminum cylinder. A fully coupled explicit finite element model is established to study the signal enhancement of cylindrical surface waves on the aluminum cylinder. The simulation results indicate that the signal enhancement of the SLLS is more sensitive to the surface crack of a cylinder than that of the scanning laser detection (SLD) because of the wider span and higher peak. Due to the phase shift characteristics of surface waves on the cylinder, the maximum peak-to-peak amplitude of signal enhancement in the SLLS method (the SLLS peak) is affected by the detection position and diameter of the cylinder. Therefore, an optimization approach for detection position in SLLS is proposed for the location of surface crack on the cylinder. The locations of the surface crack on the solid cylinders with different diameters are investigated using simulated laser ultrasonic field data. Moreover, we find that the SLLS peak for signal enhancement can effectively respond to the crack depth within a limited scope which is dependent on the directivity pattern of the longitudinal waves. [ABSTRACT FROM AUTHOR]

To understand the effect of temperature on the rate of fatigue crack growth in D16T and B95AT aluminum alloys, fatigue crack growth experiments using a constant load amplitude were carried out on hollow cylindrical specimens with induced external semi-elliptical surface cracks. The rates of fatigue crack growth in the specimens were determined as a function of temperature, ranging from low (−60 °C) to elevated (+250 °C), and were analyzed as a function of the elastic and plastic stress intensity factors (SIF). The process of numerical calculations includes the analysis of the elastic constraint parameters in the form of the non-singular T- stress and T Z –factor, as well as the elastic-plastic constraint parameters in the form of the local stress triaxiality h and In- factors for the various tested materials at the different tested temperatures. The interpretation of the fatigue crack growth data in terms of the plastic SIFs results in a single general curve with a small scatter band over a wide range of temperatures. [ABSTRACT FROM AUTHOR]

It is always impractical to measure the fatigue crack growth rate in the thickness direction of a plate using a standard specimen because of the limited plate thickness. To address this issue, we proposed a method for crack growth rate measurements utilizing a semi-elliptical surface crack, the size of which is determined via marker bands. This method allows the crack growth rates both in the surface direction and the thickness direction to be measured in a single test. The core of the method involves the load spectrum development and the crack growth data analyses. In the load spectrum development, the number of marker load cycles in a load block is estimated through a crack growth equation including the crack closure effect, while the interval of marker load blocks is determined using the crack length measured in a pilot test. The estimated parameters can be amended using the crack growth data acquired by fractographic analyses, should unclear marker bands or unreasonable band spacing occur. In the crack growth data analyses, a closed stress intensity factor solution for a surface-cracked plate with clamped ends was developed, facilitating stress intensity factor calculations. Finally, we demonstrated an application of the proposed method, i.e. fatigue crack growth rate measurements for Ti-6Al-4 V plates. Fine and legible marker bands were observed by fractography, and the resulting d a / d N - Δ K curve was consistent with the curves obtained by using standard specimens, justifying the proposed method. [ABSTRACT FROM AUTHOR]

Various mechanisms of transverse crack formation within the surface of continuously-cast billets in the pouring temperature range are analyzed. It is shown that low mechanical properties of peritectic steels in the high-temperature region are due to the high temperature for completion of the δ→γ transformation (Tγ). The effect is considered individually of grain size on metal mechanical properties. Conditions are analyzed for the formation of coarse grains during continuous casting of peritectic steels. It is shown that in the low-temperature region key effects are stress concentration conditions at grain boundaries and intergranular bond forces. On the basis of this analysis, a series of recommendations is proposed for reducing the probability of forming surface cracks in billets. [ABSTRACT FROM AUTHOR]

Abstract: An outline of a newly proposed methodology for evaluating creep crack growth (CCG) parameters using cracked small‐punch (SP) specimens is explained. Three‐dimensional finite element analyses were performed to calculate the stress intensity factor along the crack front for a surface crack formed at the centre of a SP specimen. Effects of crack ratio, (a/t); crack aspect ratio, (a/c); and thickness of the specimen, (t), on the fracture parameters were studied. It was observed that the minimum variation of K‐value along the crack front can be achieved when a/c was 0.50 except the location very near the intersection of the crack and free surface. This condition is similar to the case of constant K‐values along the crack front of the conventional compact tension specimen. Thus, it can be argued that the SP specimen with a surface crack is a suitable specimen geometry for CCG testing. The proposed CCG test method was found to be practically applicable for the crack geometry of 0.10 to 0.30 of a/t with constant aspect ratio of 0.50. An estimation of the K and Ct‐parameter under the small scale creep condition was derived. Future work for further development of the suggested CCG testing is discussed. [ABSTRACT FROM AUTHOR]

No empirical solution for the stress intensity factors (SIFs) of a surface crack in a solid cylinder under torsion was reported in literature. In this paper, empirical solutions for Modes I, II, and III stress intensity factors of a semi-elliptical surface crack in a solid cylinder under pure torsion are proposed. The dual boundary element method (DBEM) based software was used to generate the SIFs for different crack size, crack aspect ratio, and different inclination angles. Next, the solutions for the SIFs were produced through a systematic curve fitting approach. [ABSTRACT FROM AUTHOR]

The through-thickness distribution of welding residual stress (RS) in a 30-mm-thick butt-welded Q345qD steel plate is investigated through experimental measurements and finite-element simulations. The simulated RS fields agree reasonably with the measured results for both surface and internal stresses. In addition, the weight function and finite-element methods are used to investigate the stress intensity factors (SIFs) at the surface and deepest points of semi-elliptical surface cracks subjected to a combination of external tensile load and through-thickness welding RS. Different crack aspect ratios and relative depths are analyzed. The results reveal that the longitudinal RS is always tensile through the plate thickness, making the SIFs of the surface and deepest points larger than those without considering the longitudinal RS. However, the transverse RS through the thickness presents tension–compression–tension, with the tensile transverse RS causing the SIFs to increase. When the crack tip enters the compressive stress region, the compressive stress offsets the external load and causes the SIFs to decrease. [ABSTRACT FROM AUTHOR]

In this work, the performances of circular single- and double-sided composite patch repairs are compared by computing the maximum stress intensity factor of a repaired surface crack. The three-dimensional finite-element method is used to calculate the stress intensity factor along the crack. The effects of the crack depth, composite patch thickness and patch material on the stress intensity factor variation are highlighted. The obtained results show that the selection of single- or double-sided patches depends on both the crack depth and patch thickness. [ABSTRACT FROM AUTHOR]

This paper concerns the fracture assessment of a clad pipeline subjected to large bending moment, and identifies different parameters influencing the fracture behaviors. The evolution of Crack Tip Opening Displacement (CTOD) of a pipeline containing a canoe shape crack on the external surface of the girth weld is studied under pure bending and combined pure bending and internal pressure through 3D elastic-plastic Finite Element (FE) simulations. Various parameters affecting the evolution of CTOD like crack depth, crack length to perimeter diameter ratio, internal pressure and the weld geometrical configurations have been investigated. It is observed that only the average width of the weld has a significant influence on the fracture response under pure bending, while the weld reinforcement height and fusion line slope have a moderate influence on the fracture behavior under biaxial loading. The crack at the interface between the weld and back steel has a similar fracture response to that of a crack located in the weld with equivalent effective crack depth. Finally, a strain based Failure Assessment Diagram (FAD) is proposed and compared with the fracture assessments produced by BS7910:2013, and 3D elastic-plastic FE simulations suggest that the British Standard is more conservative especially for deep cracks. [ABSTRACT FROM AUTHOR]

In order to investigate the coupling control of reducing coal pillar pressure concentration and realizing uniform surface subsidence to reduce surface cracks in shallow buried coal seams mining,the physical simulation and numerical calculation are employed.The interval strata breaking form,the concentrated stress distribution of coal pillars,the overburden and surface cracks and the ground surface subsidence in different coal pillar stagger distances are obtained.The relationship of the coal pillar stagger with the evolution of coal pillar stress concentration and of overburden fracture is discovered.According to the study,the determination of the reasonable upper and lower coal pillars stagger by interval layer breaking form can avoid the section coal pillar concentrated stress superposition and non-uniform strata subsidence in coal pillar supporting influence area,and realize the coupling control of stress and cracks in shallow coal seam group mining.In this paper,the control model of concentrated stress and ground crack of coal pillar group is established,and the coupling criterion to avoid the coal pillar concentrated stress and realize the uniform surface subsidence is put forward,so the mechanism of reducing the stress concentration of coal pillar and realizing the uniform subsidence of the ground surface is revealed. [ABSTRACT FROM AUTHOR]

The introduction of dense vertical cracks in ceramic top coat has been recognized as an efficient way to improve the strain tolerance and service durability of thermal barrier coatings (TBCs). However, further growth of these pre-existing cracks may cause the delamination of the coatings to form various failure modes. The motivation of this work is to numerically study the mechanisms that govern the formation of failure modes in dense vertically cracked TBCs. The effects of vertical crack density and material properties on the fracture behaviors are discussed. An energy-based criterion is employed to deal with the surface crack deflection/penetration behavior at a bimaterial interface, and an interaction integral method for discontinuous materials is utilized to evaluate the fracture parameters. The failure maps of TBCs are constructed by examining the continuous variations of crack tip driving forces over the whole fracture process. It is verified that the interfacial fracture resistance could be enhanced by introducing high-density vertical cracks. At the same time, these cracks tend to penetrate into the bond coat that results in debonding at bond coat/substrate interface or at both interfaces. In addition to benefit in alleviating the interfacial mismatch stress, it is found that the high-density vertical cracks give toughening increases to the coating system by releasing more energy from multiple paths. Moreover, the effects of mixed-mode interface toughness and initially residual stress on the formations of failure mode are discussed. [ABSTRACT FROM AUTHOR]

This paper provides an engineering assessment of an oval-shaped clad pipes with a circumferential part-through surface crack subjected to bending moment based upon equivalent stress-strain relationship method (ESSRM) in conjunction with EPRI J -estimation procedure. Plastic limit load equations are developed specially to determine the equivalent stress-strain relationship of the welded clad pipe. Then, the conventional EPRI J -estimation procedure is extended to calculate J values of the equivalent configuration with oval cross-section converted from the welded clad pipe using the ESSRM. The validation of the ESSRM/EPRI estimation framework mentioned in the above is carried out by comparing with the results obtained from 3-D elastic-plastic finite element (FE) analysis. [ABSTRACT FROM AUTHOR]

The fracture prediction for materials containing surface cracks is not well established. In this paper, a new two-parameter fracture criterion which can predict the location of maximum crack extension along a surface crack front was presented. The surface-cracked specimens with different crack sizes and loads were selected, and the finite element method (FEM) was used to model the J -integral and the equivalent plastic strain ( ε p ) distributions along the crack front for different specimens. And then, in succession, the areas A PEEQ surrounded by the ε p isoline ahead of crack tips under different J -integrals and the unified constraint parameter A p were calculated. Based on the parameter A p , J -integral and the fracture toughness J ref measured in a standard test, the J / ( J ref A p ) , which considered the combined effects of crack driving force and material resistance force, was determined. The critical fracture location is the location for which the J / ( J ref A p ) is a maximum. The results show that for all the specimens, the prediction results obtained by two-parameter criterion are consistent with the measured results obtained by experiments. The J / ( J ref A p ) is a suitable prediction criterion to model the location of the maximum crack extension of surface crack. For comparison, similar predictions were also made using a single-parameter J -integral and A p . [ABSTRACT FROM AUTHOR]

Crack self-healing is the result of a combination of multiple effects and involves many factors, such as mechanics, thermodynamics, physics, and chemistry. In this paper, a finite element simulation model and a modified crack-length-prediction model for crack self-healing in BK7 glass were proposed and verified experimentally. The simulation results showed a stress concentration at the tip of the crack at the initial stage of the crack self-healing process. The crack length decreased gradually, and the stress concentration area moved to the surface. The stress concentration area almost disappeared when the crack was healed completely. When the relative humidity was 64% and the compression was 5 MPa, under variable-temperature heating, the required time for a 19.8 µm pre-crack complete healing was 7.5 h. As the temperature increased, the required time for complete healing decreased, the contact state between the crack boundaries was improved, and thus the cracks were connected and healed. [ABSTRACT FROM AUTHOR]

The plastic collapse loads of cracked circular hollow section (CHS) T/Y-joints under in-plane and out-of-plane bending are investigated using finite element (FE) analyses. An in-house 3D FE mesh generator is specifically developed to create the mesh models of the cracked joints containing a semi-elliptical surface crack located at the weld toe of the chord. Mesh convergence tests are then carried out to check the accuracy of the generated mesh models. The in-plane and out-of-plane plastic collapse moments of uncracked CHS T/Y-joints are calculated and compared with results obtained from three codes and experimental data. The comparison shows that the plastic collapse moments of uncracked CHS T/Y-joints obtained in this study are accurate and reliable. Subsequently, an extensive parametric study is carried out to investigate the plastic collapse moments of cracked CHS T/Y-joints under in-plane and out-of-plane bending. It is found that the in-plane and out-of-plane plastic collapse moments may decrease by up to 35.1% and 33.6% respectively when the crack area A nc reaches 25% of the intersectional area, t 0 * l w . Finally, two lower bound strength reduction factor F AR equations for cracked CHS T/Y-joints under in-plane and out-of-plane bending are proposed. [ABSTRACT FROM AUTHOR]

In this paper, an electromagnetic bandgap inspired scanning-probe sensor and its measurement platform is presented for the characterization of high-frequency dielectric laminates. In the proposed sensing technique, the material under test (MUT) is placed between the electromagnetic bandgap structure and a substrate comprising of 50- \Omega microstrip lines. Transverse electric excitation is given from the microstrip line to the suspended electromagnetic bandgap structure via MUT, which allows the characterization of different types of damage and defect severities in the MUT. By tapping the material surface with the proposed sensor, a shift in the effective relative permittivity is calculated. The sensor structure consists of 1 \times 3 unit-cell elements. Each of the unit-cell structure has two asymmetrical H-shaped slots and a metal via. The sensor system is modeled and fabricated using Rogers isotropic thermoset microwave material of relative permittivity \varepsilon r = 12.85 , and loss tangent \tan \delta = 0.0018 . The dispersion analysis of the electromagnetic bandgap structure shows frequency bandgap between 1.93 to 3.18 GHz, which defines the operating range of the sensor. The fit effective permittivity values obtained from simulation and measured results for nondefective samples are in close agreement with the literature. A deviation in the effective permittivity is obtained for the defective material which indicates the severity of the defect. The proposed method can be effectively utilized for fault monitoring and testing of larger dielectric laminates. [ABSTRACT FROM PUBLISHER]

In this work, a fiber-guided laser array spots thermography (LAST) system with good flexibility is proposed for the inspection of surface cracks in metallic structural objects with difficult access. To generate an array of laser spots on the target surface, an optical head with fiber delivery is designed and fabricated at first, which has good adaptability and robustness. Second, a numerical simulation model is developed to optimize the spatial distribution of the laser spots. In addition, an improved image processing method with use of multiple background free images is developed to enhance the crack detectability from the LAST images. Finally, cracks of different types in stainless steel specimens are successfully detected from the thermal images obtained using the fiber-guided LAST system. [ABSTRACT FROM AUTHOR]

Offshore pipelines are subjected to large plastic deformation during the reeling stage or in-service operation. These pipelines are joined together by a welding process, and defects and weld misalignment are frequently introduced into the pipelines, posing tremendous challenges to the integrity of these pipelines. In this study, the fracture responses of misaligned clad pipeline containing a canoe shape surface crack located in weld centre line (WCL) and fusion line (FL) were investigated using 3-D elastic-plastic finite element (FE) analyses. The influences of crack depth ratio, crack length to perimeter diameter ratio and centerline offset ratio on the crack tip opening displacement (CTOD) were analyzed in detail. The relationship between the CTOD and the global strain (ε g ) was built up, and a linear relationship was observed for ε g ranging from 0.6% to 2%. The fracture assessment equation of British Standard (BS) 7910 predicted an over-conservative result in comparison with that obtained by FE analyses. Therefore, a new strain-based failure assessment curve was developed to assess the fracture behaviour of misaligned clad pipeline subjected to large plastic deformation. [ABSTRACT FROM AUTHOR]

The landing gear strut is mainly subjected to eccentric compression load during service, and the calculation of its stress intensity factor is often considered as a challenge to calculate the surface crack growth life. Based on the bulging effect, we modify the stress intensity factor of the surface cracked cube, and obtain the stress intensity factor formula of the surface cracked strut. It calculates the stress intensity factor by using geometric characteristics and load characteristics, and calculates the crack growth life without meshing finite element model. In the study of surface fatigue crack growth of steel strut under eccentric compression load, the proposed approach to calculate the crack growth life of surface cracked strut is proved to be feasible. The method is simple and conservative. [Display omitted] • The stress intensity factor formula of surface cracked strut is deduced considering bulging effect. • The deduced formula is used to calculated the surface crack growth life of 30CrMnSiNi2A landing gear strut. • The rationality of using the wall thickness of cracked strut as the crack growth termination condition is discussed. • The differences between the formula and the finite element method for calculating the crack growth life are discussed. • The deduced formula is easy to apply and conservative, which is conducive to solve the surface cracked engineering structure. [ABSTRACT FROM AUTHOR]

Ultrasonic is one of the most common uses of a non-destructive evaluation method for crack detection and characterization. The effectiveness of the acoustic-ultrasound Structural Health Monitoring (SHM) technique for the determination of the depth of the surface crack was presented. A method for ultrasonic sizing of surface cracks combined with the time domain and frequency spectrum was adopted. The ultrasonic frequency spectrum was obtained by Fourier transform technique. A series of test specimens with various depths of surface crack ranging from 1 mm to 8 mm was fabricated. The depth of the surface crack was evaluated using the pulse-echo technique. In this work, three different longitudinal waves with frequencies of 2.25 MHz, 5 MHz and 10 MHz were employed to investigate the effect of frequency on the sizing detection of surface cracks. Reasonable accuracies were achieved with measurement errors less than 7%.

The conventional method for predicting the shape change of a surface crack in a round bar simply utilizes the Paris-Erdogan law with the least squares method using a certain shape assumption with excessive constraints. In this paper, a three-parameter model for a round bar subjected to tension is developed with fewer shape assumption restraints by employing a fatigue crack growth circles method. The equivalent stress intensity factor Δ K e based on both stress intensity factors along the current and new crack front is used to reduce the total number of increments. The results show that the proposed method has a good convergence speed and accurate prediction of crack shapes. The present method is validated by comparing the solution with other simulation solutions and experimental data.

A larger breakage ratio occurs with the decrease of wafer thickness due to the decrease of fracture strength for as sawn silicon wafers, which is a severe problem to limit the production yield of silicon wafers. It is necessary to understand the fracture behavior of as swan silicon wafers for increasing the wafer yield. In this paper, a predictive model for wafer fracture strength is proposed according to the linear-elastic fracture mechanics. The simulation results are comparable with the experimental results from references, indicating the correctness of this proposed model. Besides that, influences of surface crack parameters on the fracture strength of silicon wafer are discussed. Results indicate that the fracture strength of silicon wafer changes less when the surface crack inclination angle between the crack plane and saw mark is between 0 and 25°. However, the fracture strength increases with the increase of the surface crack inclination angle when the angle is larger than 25°. This proposed model can predict the strength distribution of silicon wafers, and it is very helpful to understand the fracture behavior of silicon wafers. [ABSTRACT FROM AUTHOR]

Single crystal 6H-SiC wafers with 4° off-axis [0001] orientation were irradiated with carbon ions and then annealed at 900 °C for different time periods. The microstructure and surface morphology of these samples were investigated by grazing incidence X-ray diffraction (GIXRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Ion irradiation induced SiC amorphization, but the surface was smooth and did not have special structures. During the annealing process, the amorphous SiC was recrystallized to form columnar crystals that had a large amount of twin structures. The longer the annealing time was, the greater the amount of recrystallized SiC would be. The recrystallization volume fraction was accorded with the law of the Johnson-Mehl-Avrami equation. The surface morphology consisted of tiny pieces with an average width of approximately 30 nm in the annealed SiC. The volume shrinkage of irradiated SiC layer and the anisotropy of newly born crystals during annealing process produced internal stress and then induced not only a large number of dislocation walls in the non-irradiated layer but also the initiation and propagation of the cracks. The direction of dislocation walls was perpendicular to the growth direction of the columnar crystal. The longer the annealing time was, the larger the length and width of the formed crack would be. A quantitative model of the crack growth was provided to calculate the length and width of the cracks at a given annealing time. [ABSTRACT FROM AUTHOR]

Closely-spaced surface cracks in structures interact with each other when subjected to load. The degree of interaction depends strongly on the distribution of stress that is applied. In pressure boundary components, thermal shock, residual stress and global bending can all cause load distributions that are non-uniform through the wall thickness. A wide range of crack pairs subject to various non-uniform stress distributions have been modelled using finite element analysis. Cracks sometimes interact more strongly under non-uniform loading than when loaded in uniform tension. Consequently, interaction criteria developed by considering uniform tension may not be inherently conservative for all loading conditions. A simple weight function method for determining the interaction of twin cracks under an arbitrary through-wall stress is presented, and weight function coefficients for a wide range of crack sizes and aspect ratios are given. [ABSTRACT FROM AUTHOR]

In this paper, we propose a method for monitoring surface defects like cracks in highly loaded structures. This method is based on the ability of surface cracks to open under the influence of external loads, thus causing the appearance of stresses and tears in the sensitive element—thin films that were deposited on the surface of the test object. We developed a system for monitoring surface defects based on a distributed crack sensor, the functional scheme of the sensor and its design, the structural scheme and algorithm of the system operation, and its model as a VHDL (VHSIC (Very high speed integrated circuits) Hardware Description Language)-description. [ABSTRACT FROM AUTHOR]

Analysis of determining linear fracture mechanics parameters for a surface crack loaded with Hertzian pressure without the presence of a lubricant in the crack is presented. For this purpose, the elements of Williams equation for describing stresses near the crack tip are determined: Stress intensity factors K and K, and T-stress, which were taken into account when determing the direction of crack propagation by means of Strain energy density (SED) criterion. The elements are determined based on the case of a gear tooth flank, with various friction coefficients on the free surface and friction in the crack being taken into account. The results show that friction on the free surface and, consequently, tangential forces have an impact on whether the crack will propagate along the free surface or into the depth. [ABSTRACT FROM AUTHOR]

In this study, the fatigue behavior of filament winding pipes was determined experimentally under internal pressure. Samples were produced by ultrasonication and the filament wound methods. In order to produce the CNTs/E Glass/Epoxy nanocomposite, CNTs and E-glass reinforcements and Bisphenol-A epoxy polymer matrix were used. Test specimens had antisymmetric six layers. All specimens had ±55° winding angle. During all the experiments, the surface crack, fatigue crack growth rate and reinforcement condition of carbon nanotube were investigated. Two different rates of CNT (0.5% and 1%) were compared with the pure epoxy material. Effect of CNT on fatigue life was investigated experimentally. Cracks on the pipes for the fatigue tests had the aspect ratio of a/c = 0.2 and crack depth range between a/t = 0,25 and 0,50. Each test was repeated at least 3 times. Three different maximum stress levels were determined for the tests, which were 40%, 50% and 60% of the ultimate hoop stress. Fatigue life of nanocomposite pipes was found to increase with enhanced inter-laminar adhesion via CNTs. Stopping crack progression, bridging mechanism and mechanical interlocking between fibers of CNTs had been justified by SEM images. As a conclusion, material properties of GRP pipes were discussed by determining the effect of the reinforcement of carbon nanotubes. [ABSTRACT FROM AUTHOR]

This study aims to examine the implications of amorphous metallic fibers on the mechanical and long-term properties of concrete pavement. Two different amounts of amorphous metallic fibers were incorporated into concrete, and plain concrete without fibers was also adopted as comparison. Test results indicated that the overall mechanical properties of concrete were improved by including the fibers, and the improvement increased when a higher amount of fibers was used. In particular, the equivalent flexural strength and flexural strength ratio were substantially improved by incorporating the amorphous metallic fibers. This may enable the thickness of airfield concrete pavement to decrease. The resistance to surface cracking of concrete pavement by repeated wheel loading was also improved with the addition of amorphous metallic fibers. In addition, by adding 5 kg/m 3 and 10 kg/m 3 amorphous metallic fibers in concrete pavement, roughly 1.2 times and 3.2 times longer service life was expected, respectively, as compared to their counterpart (plain concrete). Based on a life cycle cost analysis, the use of amorphous metallic fibers in concrete pavement was effective at decreasing the life cycle cost compared to plain concrete pavement, especially for severe traffic conditions. [ABSTRACT FROM AUTHOR]

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