Key Engineering Materials Vol. 795

Abstract: Overage service pressure vessels are widespread in China. These pressure vessels may be suffered from some forms of material deterioration and damage with an increasing possibility of failure. However, some of the potential damage mechanism couldn’t be found if the inspection strategies were carried out in according with traditional inspection regulations. Through identifying the damage mechanism of these pressure vessels, the risk evaluation would be carried out by use of RBI technology in company with the structure integrity assessment. The procedure of inspection for overage service pressure vessels was developed, and some of the reasonable inspection and testing requirements and basic principles also were proposed. The three levels evaluation methods of residual safe-service life for these pressure vessels were proposed. At the same time, the risk supervision method of these pressure vessels was established by use on the risk basic theory.

Abstract: Regenerated Fiber Bragg grating (RFBG) sensors exhibit tremendous growth in recent years and could be widely used for measuring strains and for structural integrity assessment of components operated at high-temperature if critical issues (fatigue life, interface integrity, etc.) are addressed. For the determination of fatigue life of metal-packaged RFBG strain sensors at high temperatures, tensile fatigue tests are performed on the sensors. The preliminary results show that the sensors exhibit good linearity and repeatability, and rare sensitivity change under cyclic tension loading when exposed to constant high temperature up to 500 °C, without obvious variations in the spectral response. The rupture of the optical fibers is the major cause of fatigue failure of the sensors. The sensors subjected to lower test strain have greater fatigue life. A rapid decrease in Bragg wavelength is observed at the beginning of the fatigue tests followed by stabilization of the Bragg wavelength. These results indicate that the sensors are the promising candidates for health monitoring of structures subjected to cyclic loads at high temperatures.

Abstract: The study attempts to present a new finite element method to implement the direction prediction of crack extension for 2D and 3D cracks based on the maximum tensile stress (MTS) criterion by identifying the first principal stress direction. Especially in the 3D crack propagation, this paper gave a detailed and innovative research method to overcome the challenge of direction prediction on crack extension. The maximum principal stress of the node of crack tip and its neighboring nodes were calculated to determine the increment and the direction of crack tip respectively. Some examples of complex crack propagation were performed under a variety of mixed fracture modes based on linear elastic analysis by ANSYS software. A reasonable evaluation was executed and the advantages of this method were analyzed in detail.

Abstract: A modified normalization (NM) method to determine J-R curves using clamped single edge notched tension (SENT) specimens was proposed. To validate and quantify the modified NM method, the J-R curves of X80 pipeline steel obtained by NM method are compared with those determined by the unloading compliance (UC) method for SENT specimens. The comparison shows that modified NM method is obvious better than unmodified NM method for SENT specimens. The modified NM method has great agreements with UC method, and is a valid and cost-effective tool to be applied to obtain J-R curves of API X80 steel using SENT specimens with shallow cracked depth to deep cracked depth.

Abstract: The application of small specimen testing techniques in the evaluation of creep properties of materials in-service arise. In order to acquire the creep data accurately and conveniently, the bending test with small beam specimens has been proposed and validated for the metal materials. Initially, the fact that material behaves different creep rates under tension and compression is ignored for simplification. Thus, the effect of material property difference on the creep behavior of bending specimen is analyzed in the present paper. On the basis of Norton creep law, the deformation behavior of three type’s specimens under tension and compression is theoretically described. Assumed different creep exponents and constants, finite element models of these beam bending specimens are established. The creep response is simulated. Meanwhile, the effect of material property under different stress state is further investigated. The results show that the stress exponent has a significant effect on the creep curves. Usually, the stress exponent can be evaluated based on the displacement rate or strain rate. However, if large discrepancy of creep properties under tension and compression exits, it will yield disparate results for the steady-state stress exponent. It is suggested that the stress exponent determined solely by bending test should be accepted with a certain degree of reliability, especially for the non-metal materials.

Abstract: The traditional Low Cycle Fatigue (LCF) evaluation method is based on elastic analysis with Neuber’s rule which is usually considered to be over conservative. However, the effective strain range at the steady cycle should be calculated by detailed cycle-by-cycle analysis for the alternative elastic-plastic method in ASME VIII-2, which is obviously time-consuming. A Direct Steady Cycle Analysis (DSCA) method within the Linear Matching Method (LMM) framework is proposed to assess the fatigue life accurately and efficiently for components with arbitrary geometries and cyclic loads. Temperature-dependent stress-strain relationships considering the strain hardening described by the Ramberg-Osgood (RO) formula are discussed and compared with those results obtained by the Elastic-Perfectly Plastic (EPP) model. Additionally, a Reversed Plasticity Domain Method (RPDM) based on the shakedown and ratchet limit analysis method and the DSCA approach within the LMM framework (LMM DSCA) is recommended to design cyclic load levels of LCF experiments with predefined fatigue life ranges.

Abstract: Traditionally, the stress intensity factor (SIF) is solely focused on in the fracture analyses of stiffened plate, while the T-stress is usually ignored. However, the fracture toughness is influenced by T-stress and the effects need to be studied. By means of three-dimensional finite element method, the characteristics of mode I SIF and in-plane T-stress are investigated in the double symmetrical central stiffened plate (DSC-SP). Then the fracture toughness is corrected with in-plane T-stress based on the two-parameter model in the R6 assessment procedure. Lastly, the curves of corrected fracture toughness and crack arrest effect are obtained in the stiffened plate. The results show that the T-stress is dependent on the size and integrity of the stiffener, and has significant effects on the fracture toughness of stiffened plate. Compared with the traditional estimation of the crack arrest effect of the stiffener, the evaluation is more reasonable when the influences of in-plane T-stress are taken into account.

Abstract: The effects of biaxial loading and creep mismatch on creep fracture parameter C(t) and constraint effect are investigated for the plane strain central crack plate. The studied biaxialities in this paper are limited to the range of [-1,1]. Based on the three-term asymptotic solution for creep crack tip field, the higher-order terms coefficient parameter A₂(t) is introduced to characterize the creep mismatch constraint effect. The impacts of loading level on the parameter A₂(t) for different biaxialities and creep mismatch factors are also examined in this paper. The result shows that the steady creep fracture parameter C^* decreases with the increments of mismatch factor and biaxiality B. The creep constraint parameter A₂(t) is insensitive to loading level for the creep mismatch crack plate under biaxial loading. The steady creep constraint parameter increases with the increments of biaxialities and decreases as m increases, which indicates the higher under-match plate with higher biaxiality has higher creep constraint effect.

Abstract: At present the orthotropic pressurized metal structure is generally used as the isotropic one, ignoring the anisotropic characteristics of material caused during rolling process. At the same time, the elastic stress analysis design method is commonly used in pressure vessel, and the load capacity coming from plasticity of material has not been utilized. Therefore, elastic-plastic analysis of orthotropic pressurized structure is of great theoretical significance and engineering value. In present paper the limit load of orthotropic titanium cylindrical shell under internal pressure was studied. By finite element method with twice elastic slope criterion the variations of limit load for orthotropic and isotropic titanium cylindrical shells under different diameter-thickness ratios were investigated. The effect of orthotropic mechanical property on limit load of titanium cylindrical shell was discussed. At the same time, the difference of limit loads between orthotropic and isotropic titanium cylindrical shells was compared. The calculation results show that the limit loads of orthotropic and isotropic titanium cylindrical shell increase with the diameter-thickness ratio, and the limit load of orthotropic titanium cylindrical shell increases more obviously. Additionally, if the yield strength of isotropic cylindrical shell is the same as or close to the yield strength of circumferential direction for orthotropic titanium cylindrical shell, the difference of limit load is smaller. While the yield strength of isotropic cylindrical shell is much different from the yield strength of circumferential direction for orthotropic titanium cylindrical shell, the difference of the limit load is higher.

Abstract: This paper presents a method to allow the measurement of residual stress (), yield strength () and strain hardening exponent () of metal materials through continuous spherical indentation test. Based on the analysis of finite element simulation results, a new fitting equation between the indentation response parameter and the material properties was given. The influence of friction on finite element model simulation was also discussed when the residual stress is included in the material. Only three load values are required at each different indentation depths, and the three unknown parameters (, and ) can be obtained through corresponding dimensionless functions. Finally, the validation of the method presented in this paper was verified by simulating the material (SUS304) with the known material parameters in the literature.