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Based on fracture mechanics theory, a finite element method was used to determine the stress intensity factors of the inclined crack on the inner surface of the pipe under axial compression load and external pressure. The effects of different influencing factors on the stress intensity factor along the crack front considering crack closure were systematically explored, which were different to those under internal pressure. The effects of high aspect ratio on KII, the crack inclination asymmetry caused by curvature and the effects of the friction coefficient on the stress intensity factors of the pipe with an inclined inner surface crack under axial compression load and external pressure were explored in this paper. To be fit for defect assessment, the solutions for stress intensity factors KII and KIII were derived, and new correction factors fθ and fμ were proposed in the empirical solutions to accommodate the crack inclination asymmetry and the friction coefficient, respectively. [ABSTRACT FROM AUTHOR]

Abstract: This work used in-situ scanning electron microscopy method to investigate the small fatigue crack behavior of the aluminum alloy 2024 laminated structure produced by ultrasonic consolidation process. The influence of local microstructure on the crack propagation of the laminates after different heat treatments (T4 and T62) has been compared and discussed. It was found that both the precipitations and the interfaces of the layers could retard crack growth and alter the growth direction, and the effect of interfaces was much stronger. Due to the higher ratio of the toughness to bond strength, the barrier effect of the interfaces on impeding the fatigue crack growth in specimens after the T4 heat treatment was more evident. [Copyright &y& Elsevier]

In this paper, the effects of temperature and dwell time on the Fatigue Crack Growth (FCG) behavior of commercial pure titanium were studied under high and low load ratios. Besides, combined with the fracture surface morphology, the specific characteristics of FCG were analyzed under pure fatigue and dwell fatigue conditions. The experiment results show that the FCG rate of commercial pure titanium (CP-Ti) increases with the temperature under low load ratio, and the dwell time increases the FCG rate. Also, the enhancement of the dwell time increases as the temperature rises. The dwell effect tends to be saturated when the temperature rises to 200 °C. Under high load ratio, the FCG rate of CP-Ti also exhibits a temperature-sensitive enhancement. The enhancement effect of the dwell time on the FCG rate under high load ratio is more significant. However, the effect of the hold time on the FCG rate does not increase at 300 °C. The da/dN–ΔK/E FCG curves for CP-Ti have a tendency to approach each other under different load ratios, which indicates that the E-modulus is an important factor for the difference. The effect of dwell time on the FCG behavior of CP-Ti is dominated by the creep deformation mechanism under different load ratios from room temperature to 300 °C. At the same time, the oxidation effect gradually becomes significant as the load ratio increases to 300 °C. The fracture surface morphology shows that the secondary cracks and the roughness increase with temperature or dwell time under low load ratio condition, while, under high load ratio, the effect of creep deformation on the FCG behavior is more obviously enhanced, and plastic deformation is gradually significant with increase of the dimples. [ABSTRACT FROM AUTHOR]

Highlights • A process of inhibiting and promoting crack growth is performed. • The dimples and striations cover up the facet creep fracture characteristic. • The fatigue-creep interaction diagram based on the analyses is depicted. Abstract In this paper, the effect of the cold creep on fatigue crack growth rate are investigated by experiment considering wide ranges of load amplitudes and load ratios with or without certain degrees of dwell time. To further illustrate the cold creep influence, a series of finite element results are carried out. Finally fracture morphology analyses are also made to explain related crack growth behavior. Results show that fatigue crack growth rate either increases or decreases with certain degrees of hold load and dwell time imposed, depending on the values of the load amplitude and load ratio. This finding is the same as that in the fatigue life. Discussion shows that the effect of creep on fatigue crack growth can be attributed to the competition of the creep stress relaxation and creep strain. A process of inhibiting and promoting crack growth is performed. The fatigue-creep interaction diagram based on the analyses is depicted, by which the interaction between cold creep and fatigue can be well understood. Finally further studies and discussion are carried out by fracture morphology analyses from the dislocation pile-ups, which is in accordance with the finite element results from local creep behavior. This research will provide support to the clear understanding of the cold creep effects on the fatigue crack growth behavior. [ABSTRACT FROM AUTHOR]

Both experimental and numerical investigations are performed to study the fracture behaviors of I-II mixed mode crack for commercially pure Titanium (CP-Ti). Three different types of specimens are considered in this paper, including Compact tension shear specimen (CTS specimen), Four point bending specimen (FPB specimen) and Cracked central plate (CCP). Results show that there exist two fracture modes of these specimens, including tensile-type fracture mode and shear-type fracture mode. For single edge notched specimen, the failure mode is determined by plastic mixity parameter M p , and the critical values of M p distinguishing the fracture modes are dependent on the specimen geometry (CTS or FPB specimen). For central cracked plate, specimens are all in a tensile-type fracture mode. Furthermore, specimen in a shear-type fracture mode has a higher loading capability than that in a tensile-type fracture mode. And the loading capability of specimen in tensile-type fracture decreases almost linearly with M p . Finally, constraints of these specimens are analyzed. Discrepancies of constraint between specimens increase with the component of mode I increasing. And the difference of constraint resulted from specimen type is much greater than that from crack length. [ABSTRACT FROM AUTHOR]

Due to the non-negligible role of T -stress on fracture parameter, T -stresses of three typical specimens, including central cracked plate (CCP), compact tension specimen (CTS) and four point bending specimen (FPB), are calculated using 3D finite element method. From the results, the tensile dominant specimen like CCP is in a lower-constraint state when the component of mode I is significant, and in a higher-constraint state when the component of mode II is significant, compared with bending dominant specimen like CTS and FPB specimens. Further, the crack initiation angles and plastic zones are assessed for the three specimens based on modified maximum tangential stress (MMTS) and modified R-criterion (MR). From the comparison, the effect of T -stresses or constraints induced by specimen geometries and loading modes play a significant role on crack initiation angle and plastic zone around the crack tip, which in turn can affect the fracture of specimen. [ABSTRACT FROM AUTHOR]

In this work, finite element (FE) method is used to determine plastic limit load solutions for pipe bends with circumferential through-wall crack under torsion moment. Both extrados and intrados crack are considered regarding of crack location. Based on FE results estimated solutions are proposed for application, and are also compared with corresponding solutions in previous literatures showing that existing solutions are unfit for prediction of torsion load. The weakening factor W decreases with crack length increasing, and the decreasing rate exhibits three typical stages which performs a similar trend to that with bending moment. Finally a detailed analysis for the mechanical behavior of pipe bends has been conducted which provides some evidence to explain the weaken effect caused by the presence of crack. In torsion mode results show that the non-uniform distributed shear stress is performing symmetric to the profile plane and may have a crucial influence on the weakening factor, while in bending mode the distribution in axial stress and circumferential stress is symmetric to the profile plane and has a decisive influence. Therefore crack is pulled open by in-plane bending load while torn open by torsion load. Some evidence also can be obtained from the Mises stress distribution, where similar variation rule will provide another argument to support such description. [ABSTRACT FROM AUTHOR]

• Mechanism of overload effect of mixed mode I-II crack growth at negative load ratio is revealed. • Effect of loading angle and loading ratio on crack growth with overload loading is discussed. • Stress–strain field and plastic deformation at crack tip are revealed by finite element method. Fatigue crack growth (FCG) behavior under mixed mode loading has been received extensive attention, including overload condition. The mixed mode I-II fatigue crack growth behavior of commercial pure titanium (CP-Ti) with tensile overload at negative load ratios was investigated by experiment and finite element method (FEM). The effects of loading angle, load ratio and overload on mixed mode I-II fatigue crack growth were discussed with a modified compact-tensile-shear (CTS) specimen. The mechanism of mixed mode crack growth with overload was revealed. Fatigue crack growth path and fatigue crack growth rate (FCGR) with/without overload were compared experimentally, and overload retardation effect was investigated. The variation of plastic zone and the distribution of stress and strain were simulated through finite element method. The mechanism of overload retardation effect of mixed mode I-II crack growth under compressive load was illustrated by microscopic perspective, crack closure and residual compressive stress at crack tip, and the weakening effect of mode II shear component and compressive load effect on overload retardation effect was demonstrated. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, based on the finite element (FE) simulation and the analytical net-section criterion (NSC) solution, the influences of the off-centered angle and the dimension of local wall thinning (LWT) on limit loads of pipes with LWT are systematically investigated. It is interesting that the off-centered angle commonly affects the limit load for all dimensions of LWT. Moreover, the axial length of LWT also plays an important role in the load capacity, which cannot be considered in the analytical NSC solution. In order to improve the prediction accuracy of the NSC analytical solution of pipes with the off-centered LWT, the effects of the off-centered angle and the axial length are considered together in the establishment of the improved NSC solution. It is confirmed by the statistical analysis that the prediction accuracy of the NSC solution can be significantly improved by addressing the effects of two factors together. [Copyright &y& Elsevier]

Abstract: The safety assessment of pipes with local wall thinning defects is highly important in engineering. Most attention has been paid on the safety assessment of pipe with single local wall thinning defect, while the studies about multiple local wall thinning defects are not nearly enough. However, the interaction of multiple local wall thinning defects in some conditions is great, and may have a great impact on the safety assessment. In the present standard API 579/ASME FFS, the safety assessment of pipes with multiple local wall thinning defects is given, while as well as the influence of load condition, the influences of arrangement and relative depth of defects are ignored, which may influence the safety assessment considerably. In this paper, the influence of the interaction between multiple local wall thinning defects on the remaining strength of pipes at different arrangements and depths of defects under different load conditions (pressure, tension-bending moment and compression-bending moment) are studied. A quantified index is defined to describe the interaction between defects quantitatively. For different arrangements and relative depths of defects, based on a limit value 0.05 of the quantified index of the interaction between defects, a relatively systematic safety assessment of pipes with multiple local wall thinning defects under different load conditions has been proposed. [Copyright &y& Elsevier]

• Changes of fracture criterion and mechanism of I-II mixed mode FCG are revealed. • Competition of crack closure mechanisms of I-II mixed mode FCG is explained. • A new FCGR model is proposed by considering crack closure and fracture criterion. With the increase of design and application parameters for pressurized structures, the mixed mode fatigue crack growth (FCG) behavior has become a concerned problem in the field of structural integrity. In order to understand FCG behavior of I-II mixed mode crack growth at negative load ratios, an improved compact tensile shear (CTS) specimen of commercial pure titanium is taken to study the effects of load ratio and loading angle on I-II mixed mode FCG behavior through experiments and finite element method (FEM), which mechanism is discussed in depth as well. The contribution of mode I and mode II crack growth driving forces to I-II mixed mode FCG is analyzed. The changes of crack fracture criterion and the mechanism of crack growth in I-II mixed mode FCG process are revealed, and the competition relationship between two crack closure mechanisms of I-II mixed mode crack at negative load ratios is explained. An improved fatigue crack growth rate (FCGR) model that can make I-II mixed mode FCGR normalized is proposed by considering both the crack closure and the crack fracture criterion eventually. [ABSTRACT FROM AUTHOR]

• Plastic deformation and stress-strain response of crack tip under mode I and mixed-mode I-II loading are discussed. • Variation of stress-strain field and monotonic plastic zone at crack tip are revealed. • Effects of evolution of strain energy on fatigue crack growth behavior are analyzed. The mechanical behavior of crack tip under mode I and mixed-mode I-II fatigue loading for commercial pure titanium at negative load ratios are carried out by finite element method in this paper. The effects of maximum fatigue load, load ratio, loading angle and normalized crack length on the mechanical behavior of crack tip are discussed. The change of mechanical behavior of crack tip is analyzed through the evolution of strain energy. The results show that the increase of maximum fatigue load and crack length increase the plastic strain accumulation at crack tip in mode I fatigue crack growth (FCG), which causes the crack tip release more fracture energy to drive crack growth and affect the distribution of stress-strain field. The decrease of load ratio also increases the plastic strain accumulation and the plastic energy dissipation at crack tip. In mixed-mode I-II FCG, the larger loading angle increase the resistance of crack deflection and cause more plastic strain accumulation at crack tip. After the crack deflects, the crack growth increases the crack tip plastic zone and the morphology of plastic zone changes continuously, which cause the crack growth path move away from the mode I crack growth direction. Different loading angles produce different crack growth paths, which deviates from mode I crack growth direction with the increase of loading angle. [ABSTRACT FROM AUTHOR]

• New driving force parameter and crack closure models are proposed respectively. • Proposed crack closure model can reflect the variation of crack closure level. • The merits and demerits of models are analyzed by the dispersion of each model. The fatigue crack growth rate (FCGR) is strongly dependent on the load ratio R and most FCGR models are established on the basis of R. At present, there are more FCGR models with positive load ratio. In this paper, fatigue crack growth (FCG) behavior of commercial pure titanium at negative load ratios is studied. The FCGRs at various load ratios are fitted by driving force parameter model and crack closure model. The advantages and disadvantages of different models are analyzed by calculating the dispersion of FCGR curves which are condensed by each model. Improved two-parameter driving force model and proposed crack closure model are proposed by two FCGR modeling methods, respectively. One is to modify the FCGR by improving the stress intensity factor (SIF) K with the two-parameter driving force method. Another is established by correlating the crack closure ratio U and load ratio R , positive SIF range Δ K +, maximum SIF K max based on the variation of crack closure level in stable crack growth stage. It is found that proposed crack closure model is superior to the former in both the condensation effect and the physical significance. Under different loading conditions, proposed crack closure model can reliably and accurately condense the FCGR curves and reflect the variation of crack closure level in the whole stable crack growth stage. The integrity and validity of proposed crack closure model is also further verified by FCGR data of other researchers. [ABSTRACT FROM AUTHOR]

• FCG behavior is explained by compressive load effect and crack closure. • Variation of crack closure level at negative load ratio is obtained by experiment. • Influence of crack closure effect is described to explain anomaly in pre-stage. In this paper, the effect of compressive load and crack closure on mode-I fatigue crack growth (FCG) behavior at negative load ratios is studied with compact-tensile-shear (CTS) specimens made from commercial pure titanium (CP-Ti). The FCG behavior at negative load ratios under constant maximum load controlled has been explained by the interaction of compressive load effect and crack closure. The variation of crack closure level at negative load ratios within stable crack growth stage is obtained by the experiment. The crack closure level keeps constant which makes the crack closure effect dominate the FCG behavior in pre-stage. While in post-stage, the crack closure level drops gradually. The crack closure effect can be neglected currently and the compressive load effect plays a dominate role in post-stage. Meanwhile, the influence of crack closure effect on mechanical behavior at crack tip is described by finite element method (FEM) to explain the anomaly in pre-stage. It is confirmed that there exists a maximum reversed plastic zone induced by the crack closure effect during unloading process in pre-stage at negative load ratios. [ABSTRACT FROM AUTHOR]