Key Engineering Materials Vol. 795

Abstract: The type IV cracking is one of the main reasons for service failure. The repair welding is implemented to solve this problem. The residual stress in P91 repair welding incorporating martensitic transformation (MT) is acquired through finite element simulation. In the simulation, the ABAQUS and the user subroutines FILM, DFLUX, HETVAL, USDFLD, UEXPAN and UHARD are adopted. The effect of MT latent heat on temperature and the effects of the volume expansion, the yield strength change and the transformation plasticity on stress are considered. The results show that there is full MT in the repair welding area and the old welding area. The MT latent heat makes the temperature of the repair welding area increase. The residual stress of the repair welding area decreases because MT relieves the thermal stress. Before repair welding, the residual stress distribution is M-shape, which is consistent with the experimental results. After repair welding, the residual stress of repair welding area decreases and the residual stress of old welding area increases.

Abstract: High Pressure Institute of Japan (HPI) has instituted the industrial standard HPIS Z101-2, “Assessment Procedure for Crack-Like Flaws in Pressure Equipment ‒ Level 2,” which addresses the methodology for the evaluation of fitness-for-service for detected flaws in industrial structural components such as pressure vessels, pipes, storage tanks, etc. during in-service inspection. In the standard, reference stress solutions for fundamental structures with surface flaws are prepared for the assessment of the acceptability of the detected flaws. From the viewpoint of the comprehensiveness of available solutions for excessive conditions, additional reference stress solutions for long surface flaw in plate, axially through-wall flaw in cylinder, circumferentially through-wall flaw in cylinder, and through-wall flaw in sphere were required. In this paper, the appropriate reference stress solutions to be adopted for the above structures were investigated by comparing the existing solutions in referable codes/standards.

Abstract: J_R-curve is one of an important index to characterize the fracture properties of metal materials. It is usually determined by multi-sample unloading method and single-sample unloading compliance method recommended by standard methods; In consideration of the similarity between the continuous ball indentation method and the single-sample unloading compliance method, The J_R-curve is obtained by using the empirical correlation between effective elastic modulus E_D of the continuous ball indentation and the unloading compliance C for single simple in this article. The calculation conducted by the correlation formula of the extension length of crack with unloading compliance C. However, results indicate that the fracture toughness J_0.05 obtained by empirical correlation of load-depth curve of continuous spherical indentation is still quite different from those given by the standard test.

Abstract: Since the industrial revolution when a German mining engineer August Wohler first studied the frequent breaking of chains causing several casualties and developed the concept of what we now know as the S-N curve, many experimental, theoretical and software-aided simulation techniques have been developed to study ageing material behaviour and to design new materials. Over time the demands placed on new materials have required operation under more severe temperatures and loads in order to conserve natural resources and minimise emissions. Fracture mechanics based finite element algorithms to simulate 3D cracks in components / structures have proved very useful in assessing the residual life and developing repair and maintenance strategies as mandatorily required by various licensing authorities for the continuous operation of infrastructure projects in Aerospace, Power, Transportation, Oil and Chemical industries under the ever more demanding operating conditions. Here one such software tool for crack simulation of industrial applications is presented with examples including combined fatigue and time dependent crack growth under thermo-mechanical loading including hold-time and weld defect assessment with inclusion of dis-similar materials.

Abstract: An typical mode of a structural integrity failure in dissimilar steel welded joints. This paper aims at studying crack tip stress of a steam generator dissimilar welded joint under residual stress field with the method of interaction integral and XFEM. Firstly, the corresponding weak form is obtained where the initial stress field is involved, which is the key step for the XFEM. Then, the interaction integral is applying to calculate the stress intensity factor. In addition, two simple benchmark problems are simulated in order to verify the precision of this numerical method. Finally, this numerical method is applying to calculate the crack tip SIF of the addressed problem. This study finds that the stress intensity factor increases firstly then decreases with the deepening of the crack. The main preponderance of this method concerns avoiding mesh update by take advantage of XFEM when simulating crack propagation, which could avoid double counting. In addition, our obtained results will contribute to the safe assessment of the nuclear power plant steam generator.

Abstract: A new direct method for calculation of lower bound shakedown limits based on Melan’s theorem and a novel, non-smooth multi-surface plasticity model is proposed and implemented in a Finite Element environment. The load history is defined by a finite number of extreme points defining the load-envelope of a periodic load set. The shakedown problem is stated as a plasticity problem in terms of a finite number of independent yield conditions, solved for a residual stress field that satisfies a piecewise, non-smooth yield surface defined by the intersection of multiple yield surfaces. The implemented Finite Element procedure is applied to two shakedown problems and the results compared with lower and upper bound elastic shakedown solutions given by the Linear Matching Method, LMM. The example analyses show that the proposed Elastic-Shakedown Multi Surface Plasticity (EMSP) method defines robust lower bound shakedown limits between the LMM lower and upper bound limits, close to the LMM upper bound.

Abstract: Small specimen testing techniques are developed mainly for characterization of the structural materials with a limited volume of material. One of the most used miniature specimen testing techniques is the small punch (SP) test which is based on the testing of thin disc shaped samples. From the operational point of view SP test can be considered as a non-destructive method, because the material volume cut from industrial component is not diminishing its structural integrity. This benefit can be significant for the use in nuclear power generation sector, as many components are considered as irreplaceable and conventional testing techniques are not applicable for the determination of current mechanical properties. Although SP testing has been used for more than 40 years, there are still many research and development subjects. The article describes the SP test method as a suitable supplement to classical methods for evaluating mechanical properties that are fundamental for the nuclear power plant integrity and operational lifetime assessment processes. Main part of the article is focused on the current status of ASTM and EN standardization processes that are essential for possible usage in nuclear power plant integrity assessment and other industrial areas as well. Paper also describes results of the ongoing interlaboratory study which task is to determine repeatability and reproducibility of results obtained from material testing.

Abstract: There are few research on bearing fault detection and diagnosis which combine constant speed with variable speed. In this paper, a hybrid method is proposed to realize the nondestructive testing and fault diagnosis of roller bearing faults. First, adopts computed order tracking (COT) and variational mode decomposition (VMD) based order spectrum method to detect bearing faults under variable speed condition. Then, by using the method of VMD-based envelope spectrum analysis, we analyze a certain speed signal which is consistent with the velocity in the condition of variable speed, and verify the analysis under the condition of variable speed. The experimental results show that the method can realize the effective diagnosis of the fault bearing.

Abstract: Many effective image-based imaging algorithms are available for detecting and locating the defects for plate-like structures. The discrete ellipse imaging algorithm is widely applied in plate-like structures. However, the scattering-wave extracted by the baseline subtraction may easily be impacted by noise and the reflect wave from boundary, which significantly reduces the signal-to-noise (STN) of the scattering wave, and then leads to the degrading of imaging accuracy. To enhance the STN and improve imaging accuracy, a novel method of zero-signal filling is used in the discrete ellipse imaging in this paper. To verify its effectiveness, the imaging experiment with a circular sparse array has been conducted. The experimental results show that the method can effectively resolve the signal delay and enhance the STN of the scattering wave. The imaging accuracy can be improved obviously by this method and the defect can be located accurately.