Xijia Wu

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I am the author of the book--Deformation and Evolution of Life in Crystalline Materials, CRC Press, 2019, which offers methodologies and working examples on constitutive modeling, life prediction and fracture mechanics addressing fatigue, creep, thermomechanical fatigue, fatigue and creep crack growth problems in design and life cycle management.

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Papers by Xijia Wu

Grain boundary sliding in the presence of grain boundary precipitates during transient creep

Metallurgical and Materials Transactions, Apr 1, 1995

The Integrated Creep-Fatigue Theory

Structural Integrity and Creep-fatigue Assessment by ASME-NH for Hydrogenation Equipment at 454 ̊C

Procedia Engineering, 2015

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Material Life Prediction for Sustainable Development

2018-Sustainable Industrial Processing Summit, Apr 19, 2018

Sustainable development demands advanced materials to transform or conserve energy, which have to... more Sustainable development demands advanced materials to transform or conserve energy, which have to satisfy the durability requirements under working conditions with enhanced energy performance. Constitutive modeling and life prediction are core tasks for durability design, especially with thin margins at increasing operating temperature and lowering weight. Traditionally, these two tasks were handled separately, via phenomenological approaches that employ too many empirical correlations, with parameters that do not physically connect to each other. This is not the way to go, when facing the challenges of sustainable development. Recently, an integrated creep-fatigue theory (ICFT) has been developed, which constructs the material constitutive law based on physical decomposition of the total deformation as the sum of elastic strain, rate-independent plastic strain, intragranular dislocation glide, climb, as well as grain boundary sliding [1]. This mechanism-based approach allows delineation of inelastic deformation in creep, fatigue, and thermo-mechanical fatigue into mechanism strains, which in turn describes the various forms of physical damage, such as persistent slip bands, grain boundary cavitation and cracking in relation to the responsible mechanism(s), respectively. Furthermore, the holistic damage accumulation in a material under random thermomechanical loading generally consists of nucleation of surface/subsurface cracks by fatigue [2], environmental effects, and their propagation in coalescence with internally distributed damage such as cavities promoted by creep. Application of ICFT to creep has led to a deformation mechanism-based true-stress (DMTS) model that describes the three-stage creep behaviour with the influence of oxidation [3]. The DMTS model can be used for long-term creep life prediction. Application of ICFT to low-cycle fatigue and thermomechanical fatigue of ductile cast iron and austenitic cast steels has demonstrated that the complicated hysteresis behaviors are governed by fundamental deformation mechanisms operating under the corresponding loading profiles [4,5,6]. In general, ICFT provides an integrated approach to constitutive modeling and life prediction with the physics-based "genome" to include all effects of environment and material internal damage, enabling a holistic durability analysis for materials under user-specified loading profiles.

Materials Modeling for Optimized Material Performance and Life Prediction-An Overview (II)

中國航空太空學會學刊, Mar 1, 2003

ABSTRACT

$Research paper thumbnail of Constitutive laws of plastic deformation and fracture$

Constitutive laws of plastic deformation and fracture

Environment assisted fatigue involves plastic deformation and degrading chemical reactions, which... more Environment assisted fatigue involves plastic deformation and degrading chemical reactions, which occur in a localized region ahead of the crack tip. Basically, transgranular crack growth proceeds by alternating slip processes. In this study, a transgranular fatigue crack growth rate model is developed on the basis of restricted slip reversibility (RSR), where transgranular fatigue crack growth rate is related to plastic deformation accommodation ahead of the crack tip (the product of the cyclic plastic strain range and the plastic zone size). The model is shown to take the form of the Paris equation with a power law exponent of 3 at positive R values. Fatigue crack growth behavior of a 8090 aluminum-lithium alloy has been examined by a series of tests using compact tension (C(T)) specimens with the load axis (a) parallel to the rolling direction (LT specimen), (b) inclined at 15$\sp\circ$ (L + 15$\sp\circ$), (c) inclined at 30$\sp\circ$ (L + 30$\sp\circ$), (d) inclined at 45$\sp\circ$ (L + 45$\sp\circ$) to the rolling direction. It has been found that in the LT, L + 15$\sp\circ$ and L + 30$\sp\circ$ specimens macroscopic cracks propagate along the plane normal to the rolling direction regardless of the deviation of loading directions and the fatigue crack in the L + 45$\sp\circ$ specimen propagates along the plane of specimen symmetry. Fatigue crack growth rate has been found to vary with the specimen orientation with the LT direction exhibiting the best fatigue crack growth resistance. These phenomena are discussed in terms of the crystallographic texture and the highly planar slip behavior of this ally. A revised RSR model is developed for the description of transgranular fatigue crack growth in aluminum-lithium alloys, where the effect of texture is related to a geometric factor for the favorable slip planes. Extension of the RSR model to environment assisted fatigue is also discussed. It is recognized that environmental effects contribute to crack propagation by the formation and rupture of an embrittlement [...]

On low-temperature environment-assisted fatigue crack propagation

Metallurgical and Materials Transactions, Mar 1, 1994

The determination of environmental contributions would follow the formulation of reaction kinetic... more The determination of environmental contributions would follow the formulation of reaction kinetics. The extent of corrosion damage, I, depends on the rate of dissolution or on surface, transport, diffusion and embrittlement reactions, which are all thermally activated processes. Several models have been proposed for the description of these processes. Physically based constitutive laws of environmental-assisted fatigue are therefore expected to be developed within one physical framework on the premise of thermal activation theory. 8 ref. The orientation dependence of fatigue-crack growth in 8090 aluminum-lithium plate.

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An Integrated Creep-Fatigue Theory for Material Damage Modeling

Key Engineering Materials, Sep 1, 2014

This paper presents an integrated creep-fatigue (ICF) theory to describe the non-linear creep-fat... more This paper presents an integrated creep-fatigue (ICF) theory to describe the non-linear creep-fatigue interaction during thermomechanical loading. The ICF theory recognizes the damage evolution as a holistic process consisting of nucleation and propagation of surface or subsurface cracks in coalescence with internally distributed damage, leading to final fracture. In a polycrystalline material under combined cyclic and dwell loading, crack nucleation and propagation occurs by fatigue or oxidation mechanisms, whereas internally distributed damage often occurs in the form of grain boundary cavities or microcracks due to creep or dwell effects, particularly at high temperatures. Based on the above mechanism, a damage evolution equation is mathematically derived, and the generality of the above physical mechanisms warrants the applicability of the ICF theory over a wide range of stresses and temperatures. This paper uses Mar-M 509, a cobalt base superalloy, as an example to illustrate how the ICF theory describes creep and low cycle fatigue (LCF).

Material Life Prediction for Sustainable Development

An intergranular creep crack growth model based on grain boundary sliding

Metallurgical and Materials Transactions A, 1999

An intergranular crack growth model is developed to describe the effect of microstructural featur... more An intergranular crack growth model is developed to describe the effect of microstructural features such as grain size, grain boundary precipitates, and serrated grain boundaries on creep crack growth under grain boundary sliding (GBS) conditions. The model considers quantitatively that several deformation mechanisms contribute to the stress redistribution ahead of the crack tip through a stress relaxation process. The crack tip region is divided into three zones: (a) the intragranular-deformationcontrolled stress relaxation zone, (b) the GBS-controlled stress relaxation zone, and (c) the elastic region. Intergranular creep crack growth is considered to occur as a result of the GBS-controlled process in all cases. The derived creep crack growth model shows a complex dependence of the creep crack growth rate (CCGR) on fracture mechanics quantities, such as C(t) (the path-independent energy integral with its steady-state value as C*) and K (the stress intensity factor). For creep-brittle materials, the model predicts that the CCGR depends on K to the power of 2 and this is verified experimentally; however, when environmental effects contribute to the crack growth process, the power exponent will increase. A semiempirical factor is introduced to account for the effects of oxidation on CCGR.

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A kinetics formulation for low-temperature plasticity

Journal of Materials Engineering and Performance, Feb 1, 1994

The mechanism of low-temperature plastic deformation is controlled by thermally activated disloca... more The mechanism of low-temperature plastic deformation is controlled by thermally activated dislocation movements. An evolutionary constitutive law based on the principles of deformation kinetics is described in this article. The constitutive law is expressed with a sinh function designed for computational efficiency. It is derived from rigorously defined kinetics principles. The approximation involved in the sinh function is defined so that in applications an exact evaluation can be made of the validity limits. The system of the constitutive law and the external constraints lead to the operational equations. Applications are developed for constant strain-rate loading, constant stress-rate loading, stress relaxation, creep, and ratchetting processes. The analysis provides a unified treatment for low-temperature plastic deformation.

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Creep behavior and life prediction of a reactor pressure vessel steel above phase‐transformation temperature via a deformation mechanism‐based creep model

Fatigue & Fracture of Engineering Materials & Structures, Jun 21, 2023

Creep Performance Characterization of Haynes 282 Alloy Based on Microstructural Deformation Mechanisms

Social Science Research Network, 2022

Deformation-mechanism-based modeling of creep behavior of modified 9Cr-1Mo steel

Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, Mar 1, 2017

A deformation-mechanism-based true-stress creep model is proposed for studying the creep behavior... more A deformation-mechanism-based true-stress creep model is proposed for studying the creep behavior of modified 9Cr-1Mo steel in this research. Constant-load creep test is conducted on modified 9Cr-1Mo steel in forged form (F91). The creep data obtained in the present study and those reported from the National Institute for Materials Science (NIMS, Japan) on modified 9Cr-1Mo steels processed by different means are analyzed. It is revealed that the relationship of minimum creep rate versus applied engineering stress exhibits distinct power exponent in three stress regions, which are associated with different deformation mechanisms. The proposed model considers three well recognized deformation mechanisms: dislocation glide, dislocation climb, and grain boundary sliding. The analyses of the experimental data show that this deformation-mechanism-based model can describe fairly well the entire creep deformation process consisting of primary, steady-state, and tertiary creep.

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Effects of oxidation-resistant coating on creep behavior of modified 9Cr-1Mo steels

Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2019

Oxidation is a life-limiting factor for material durability. In this study, constant-load creep t... more Oxidation is a life-limiting factor for material durability. In this study, constant-load creep tests are conducted on modified 9Cr-1Mo steel (F91) with an oxidation-resistant coating, MCrAlY, to evaluate the coating effect on the creep behavior. The creep strain vs. time curves are compared with that of the uncoated material under constant-load conditions. The previously developed deformation-mechanism based true-stress (DMTS) model and the composite stress rule are applied to analyze the creep data for the coated material coupons. This way, the effect of coating and the contributions from material-intrinsic deformation mechanisms, i.e., intragranular dislocation glide and climb, as well as grain boundary sliding, are all delineated quantitatively. The model is shown to describe well the entire creep deformation process consisting of primary, steady-state, and tertiary creep of the F91 with MCrAlY coating.

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Creep Performance Characterization for Haynes 282tm Using the Deformation-Mechanism-Based True Stress Model

Social Science Research Network, 2022

Creep Performance Modeling of Modified 9Cr-1Mo Steels with Oxidation Effect

Social Science Research Network, 2019

In this study, the deformation-mechanism-based true-stress (DMTS) creep model is modified to incl... more In this study, the deformation-mechanism-based true-stress (DMTS) creep model is modified to include oxidation influence on the long-term creep performance of modified 9Cr-1Mo steels. An area-deduction method is introduced to evaluate oxide scale formation on the creep coupons, which is incorporated into the DMTS model formulated based on intragranular dislocation glide (IDG), intragranular dislocation climb (IDC), and grain boundary sliding (GBS) mechanisms, in modifying the true stress. Thus, the modified DMTS model can not only describe the creep curve, but also predict the long-term creep life and failure mode, which is shown to be in good agreement with the creep data generated in the authors' laboratory as well as by the National Institute for Materials Science (NIMS) of Japan for long-term (> 10 4 hours) creep life prediction on Grade 91 steels. In particular, the predictability of the model is demonstrated in comparison with the Larson-Miller parameter method. In addition, the modified DMTS model provides quantitative information of mechanism partitioning, insinuating the failure mode via intragranular/intergranular deformation. Therefore, it has advantages over the empirical models in providing physical insights of creep failure, which can be useful to material design for performance optimization.

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Fracture Mechanics Analysis of a Single Crystal Turbine Blade

Single crystal superalloy turbine blades exhibit anisotropic behaviors, and the stress at the fir... more Single crystal superalloy turbine blades exhibit anisotropic behaviors, and the stress at the fir-tree root often reaches the yield stress of the material when the turbine operates at the peak rotational speed and at the maximum temperature. The nonlinear behavior of the material character at these operating conditions poses a significant challenge to prediction of the blade behavior using the conventional linear elastic fracture mechanics approach. In this paper a fracture mechanics analysis was performed for a single crystal turbine blade using the J-integral concept. First of all, the elastic-perfectly plastic J-integral and CTOD was used to correlate with the fatigue crack growth rates obtained in a single crystal blade in [100] and [110] directions, with the [001] direction as the loading direction under typical service conditions. The weight function method was used to evaluate the stress intensity factor for a crack growing along the serration bottom of the blade fir-tree root under small-scale yielding conditions and the crack growth analysis was performed using the correlated fatigue crack growth data. In addition, crack growth simulations were also performed using the Zencrack software. The simulated crack growth profile was compared with the actual crack profile on the component.

Residual Life Assessment of a Critical Component of a Gas Turbine: Achievements and Challenges

The residual life assessment of a turbine spacer from a gas turbine engine is presented. The spac... more The residual life assessment of a turbine spacer from a gas turbine engine is presented. The spacer has been identified as one of the safety critical components of the engine, therefore the useful life of this component significantly affects economic operation of the fleet. Numerical analyses of fatigue crack propagation at one critical location of the spacer were performed using both three dimensional (3D) finite element based method and the weight function method. These results combined with the material data allowed for basic assessment of the damage tolerance of this component. Experimental validation of the spacer life was performed in a spin rig facility. During this validation, two sets of spacers were tested and the number of cycles to appearance of a detectable crack was recorded. Moreover, a fractographic study was conducted on the fracture surfaces of two spin rig tested spacers using scanning electronic microscopy techniques. It was found that crack nucleation occurred at multiple sites and crack propagation occurred by a mixed mode of striation formation and faceted fracture. Therefore it was concluded that the mixed mode interaction should be considered in predicting the fatigue life of the spacer. Finally, the paper describes the challenges and pitfalls encountered during preparation and execution of the analyses and tests, including availability of engine and operational data and also uncertainties in interpretation of the results.

Determination of Stress Intensity Factors for a Cracked Shell under Bending with Improved Shell Theories

Journal of Aerospace Engineering, 2006

A formulation based on the linearized shallow shell theory has been developed by Delale and Erdog... more A formulation based on the linearized shallow shell theory has been developed by Delale and Erdogan to determine the stress intensity factors of a shell with a through-the-thickness crack. The drawback in the formulation is that the crack-face closure at the compressive edges when a cracked shell is subjected to a bending load is not taken into account, which results in the penetration of material at the crack faces. The present research is aimed to improve the formulation by including the effect of the crack-face closure on the stress intensity factors. Simulation of the closure is achieved by a line contact at the compressive edges of the crack faces. The unknown contact force is then computed by solving a mixed-boundary value problem iteratively to ensure that either the normal displacement of the crack face at the compressive edges equals zero or the contact pressure equals zero along the crack length. The results have shown that the crack-face closure significantly influences the magnitude of the stress intensity factors and tends to reduce the maximum stress intensity factor. The magnitude of the reduction varies with the curvatures of the shell and the ratio of the two curvatures as well.