Shuai Wang

I. The dissimilar metal weld joint (DMWJ) between low-alloy steel SA508 and 316L austenitic stainless steel (SS) piping is a critical component extensively utilized in the primary coolant circuits ...

Local variations in mechanical properties are commonly observed in engineering structures, driven by complex manufacturing histories and harsh service environments. The evaluation of mechanical properties accurately constitutes a fundamental requirement for structural integrity assessment. The Instrumented Indentation Technique (IIT) can play a critical role in the in-site testing of local properties. However, it could be often a challenge to correlate indentation characteristics with uniaxial stress–strain relationships. In this study, we investigated quantitatively the connection between the indentation responses of commonly used metals and their plastic properties using the finite element inversion method. Materials typically exhibit plastic deformation mechanisms characterized by either linear or power-law hardening behaviors. Consequently, conventional prediction methods based on a single constitutive model may no longer be universally applicable. Hence, this study developed methods for acquiring mechanical properties suitable for either the power-law and linear hardening model, or combined, respectively, based on analyses of microstructures of materials exhibiting different hardening behaviors. We proposed a novel integrated IIT incorporating microstructures and material-specific constitutive models. Moreover, the inter-dependency between microstructural evolution and hardening behaviors was systematically investigated. The proposed method was validated on representative engineering steels, including austenitic stainless steel, structural steel, and low-alloy steel. The predicted deviations in yield strength and strain hardening exponent are broadly within 10%, with the maximum error at 12%. This study is expected to provide a fundamental framework for the advancement of IIT and structural integrity assessment.

Accurate mechanical property parameters constitute an indispensable guarantee for the accuracy of finite element simulations. Traditionally, uniaxial tensile tests are instrumental in acquiring the stress–strain data of materials during elongation, thereby facilitating the determination of the materials’ mechanical property parameters. By capitalizing on the digital image correlation (DIC) non-contact optical measurement technique, the entire test can be comprehensively documented using high-speed cameras. Subsequently, through in-depth analysis and meticulous numerical computations enabled by computer vision technology, the complete strain evolution of the specimen throughout the test can be precisely obtained. In this study, a comparison was made between the application of strain gauges and DIC testing systems for measuring the strain alterations during the tensile testing of 316L stainless steel, which serves as the material for the primary circuit pipelines of pressurized water reactor (PWR) nuclear power plants (NPPs). The data procured from these two methods were utilized as material mechanical parameters for finite element simulations, and a numerical simulation of the uniaxial tensile test was executed. The results reveal that, within the measuring range of the strain gauge, the DIC method generates measurement outcomes that are virtually identical to those obtained by strain gauges. Given its wider measurement range, the DIC method can be effectively adopted in the process of obtaining material mechanical parameters for finite element simulations.

Direct current potential drop (DCPD) technique is a crack growth monitoring technique widely used in the crack growth test of specimens in laboratory environment. Deformation of the specimen during test is one of the important factors that affect the accuracy and resolution. Because the effect of deformation on measurement is usually mixed with other factors, quantitative evaluation is difficult through experiments method only. In this work a novel quantitative estimation method combining experiment and finite element analysis has been proposed, and the effect of deformation on measurement for 316 L stainless steel compact tension specimen has been quantitatively evaluated. Through the tensile test and DCPD monitoring test of the non-cracked 316 L stainless steel plate-shaped specimen, the material mechanical property parameters and the change of potential drop caused by deformation were obtained. Then the slow tensile tests have been performed on compact tension specimens. Combined with the FE simulation of the experimental process, the effect of deformation on the potential drop under different crack length has been evaluated. The results show that the influence of deformation on the potential drop is a factor that must be considered for the crack growth monitoring experiment with high crack growth resolution. The proposed method can be used to quantitatively evaluate the influence of deformation, a factor that was usually ignored in the past, on the measurement of crack growth, which provides a method and basis for correction of crack measurement errors caused by deformation. In addition, the proposed method can be used to obtain a more accurate calibration function, which is of great significance to the improvement of the accuracy and resolution of crack growth monitoring.

Low alloy steel SA508 autogenously welded with Type 309L/308L austenitic stainless steel cladding is one of the common forms of the dissimilar metal weld joint (DMWJ) in the primary water of a pressurized water reactor (PWR). Accurate evaluation of the inhomogeneous mechanical propriety and crack growth driving force at the corresponding place on the DMWJ is important for integrity analyses of a PWR. In this study, the mechanical propriety of the DMWJ was obtained using a combined Vickers hardness test and the stress-hardness relationship. And, a finite element (FE) model for the DMWJ in a PWR with continuous transition mechanical propriety was built using a predefined temperature field method. Based on consideration of the heterogeneity mechanical properties on the DMWJ with the continuous transition mechanical propriety, the Mises stress distribution and J integral on the crack tip with different crack lengths was analyzed using elastoplastic FE analysis. As shown by the distribution profile of the Mises stress distribution and J integral on the crack tip, the inhomogeneous mechanical propriety distribution is found to significantly affect the crack driving force when the crack tip is close to the fusion boundary.

The mechanical properties of dissimilar metal-welded joint materials are heterogeneous, which is an obstacle to the safety evaluation of key welded structures. The variation of stress–strain conditions at the crack tip caused by mismatch of material mechanical properties in dissimilar metal-welded joints is an important factor affecting crack propagation behavior. To understand the influence of uneven distribution of ultimate strength of the base metal and the welded metal on the crack propagation path, fracture toughness, as well as the mechanical field at the crack tip in the small-scale yield range, the user-defined field variable subroutine method is used to express continuous variation characteristics of welded joint ultimate strength in finite element software. In addition, the J-integral during crack propagation is calculated, and the effect of the ultimate strength on the J-integral and the stress field at the crack tip are analyzed. The results show that as the crack propagation direction is perpendicular to the direction of ultimate strength, the gradient of ultimate strength increases from |G_y|= 50 to |G_y|= 100 MPa/mm, the crack deflection angle increases by 0.018%, and the crack length increases by 1.46%. The fracture toughness of the material decreased slightly during crack propagation. Under the condition that the crack propagation direction is the same as the direction of ultimate strength, the crack propagation path is a straight line. As the gradient of ultimate strength increases from G_x = 50 to G_x = 100 MPa/mm, the crack propagation length decreases by 5.17%, and the slope of fracture toughness curve increases by 51.63%. On the contrary, as the crack propagates to the low ultimate strength side, the crack propagation resistance decreases, the ultimate strength gradient increases from G_x = −100 to G_x = −50 MPa/mm, and the slope of the fracture toughness curve decreases by 51.01%. It is suggested to consider the relationship between crack growth behavior and ultimate strength when designing and evaluating the structural integrity of cracks at the material interface of dissimilar metal-welded joints.

The interaction between the mechanical heterogeneity and the residual stress in dissimilar metal welded joints (DMWJs) leads to a complex mechanical field of crack tips, which strongly affects stress corrosion cracking (SCC) behaviors. A dual-field coupling model was established by using the user-defined field (USDFLD) and the predefined stress field method based on the elastoplastic finite element method in this study. Thus, the mechanical heterogeneity and the residual stress of the DMWJ are realized. The influence of the interaction between the mechanical heterogeneity and the residual stress on the mechanical field of crack tips at different locations was investigated. The results show that the mechanical heterogeneity causes the stress and strain distribution on both sides of the crack tip asymmetry. And the residual stress affects the magnitude of the stress and strain around the crack tip. The variation trend of the stress and strain along the crack propagation with crack length is basically the same as that of the residual stress. However, the stress and strain distributions are slightly lagging behind the residual stress distribution due to the redistribution of the residual stress caused by the crack propagation. In addition, the stress and strain range of cracks at different positions with crack length are also different.