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The physical phenomenon of residual stress relaxation and redistribution in the film/substrate systems due to creep deformation is focused in this work. A new analysis model to elucidate either the film or the substrate subjected to creeping deformation is developed. Specific analyses are made on the NiCrAlY coating-based system and silicon-epoxy bilayer structure. Results reveal that the proposed model can lead to an excellent agreement with the simulated results of finite element method. Furthermore, comparisons among FE results, the present model, and Zhang’s creep solution [J. Appl. Phys. 101, 083530 (2007)] have been carried out. Meanwhile, comparisons between Hsueh’s viscoelastic solution [J. Appl. Phys. 91, 2760 (2002)] and our current model in the case of creep exponent n=1 have also been conducted. Effects of thickness ratio of the film to the substrate on the stress distribution and the evolution of the accumulated creep strain have also been discussed in this paper. [ABSTRACT FROM AUTHOR]

Highlights • Heterogeneous coarsening of nanolath structure is characterized under the low plastic strain level condition. • Two regions in CSS curve are classified based on the cyclic softening behavior and the homogeneity of microstructural coarsening. • A dislocation-based viscoplastic constitutive model is developed by introducing an intergranular back-stress related to the heterogeneous coarsening. • The macroscopic mechanical behaviors and microstructural evolution of a nanolath strengthened steel at high/low plastic strain levels are reasonably reproduced. Abstract Cyclic softening behavior at elevated temperature of nanolath strengthened steel was rationalized under different strain amplitudes. Results indicated that the evolution of internal stress components and the coarsening of subgrains are significantly depending on the plastic strain level. This paper proposes a dislocation-based viscoplastic constitutive model to represent the cyclic deformation response especially at low plastic strain level. The heterogeneous coarsening of subgrains is explicitly formulated via employing different evolution laws for multiple dislocation populations. An additional back-stress hardening component is introduced to account for the retarded cyclic softening. The cyclic deformation response and microstructural evolution of a nanolath strengthened steel at high/low plastic strain levels are perfectly reproduced. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

Abstract The evolution of grain size and structure morphology across the CrMoV welded joint were investigated. The tensile properties of subzones in the weldment were measured by miniature specimens. And the measured results were compared to the results acquired from the standard round bar specimen. It is found that the strength characterized by miniature specimen is lower than that obtained by standard specimen. The deviation of strength is acceptable. However, the measured Young's modulus is not consistent with the elastic modulus of standard specimen, due to the jig deformation and clamped effects of the miniature specimen. Although HAZ is the weakest location due to the deteriorated microstructure and the reduced strength, the failure pattern is different comparing the measured results of the miniature specimen and that of the standard round bar cross-weld specimen. For the miniature specimen, fracture occurred at the HAZ region close to the base metal (BM). However, the fracture of the standard specimen occurred in the BM instead of the HAZ region. The difference is ascribed to the different constraints of specimens with various geometric dimension. [ABSTRACT FROM AUTHOR]

Abstract Developing the fiber/matrix interface with combined structural and functional performances is of tremendous importance in ensuring the service safety of fiber reinforced plastic composites (FRPs) and expanding the capabilities of FRPs to perform multiple parallel tasks. Herein, a multi-functional interface sensor that can improve the interfacial shear strength, and simultaneously monitor and heal the interfacial damage between glass fiber yarn and thermoplastic ethylene-vinyl acetate copolymer resin (GF/EVA) is reported. The basic idea is introducing multi-walled carbon nanotube into GF/EVA interface. By this proposed method, the interfacial shear strength (IFSS) of GF/EVA composites is enhanced by 48.9%. The resistance of the interface sensor is recorded during the pull-out tests, and results show that the relative resistivity change could reflect the interfacial damage information very well. Electric heating of the sensor is adopted to heal the interfacial damage. The applied electrical power is discussed to evaluate the self-healing efficiency of the interfaces at different damage degrees. Successful interfacial self-healing ability is achieved and confirmed using different characterization techniques. Moreover, to verify the feasibility of our method in GF/EVA composite laminates, in-situ monitoring of the laminates under complicated stress and healing of delamination are performed. The results show that the developed sensor could monitor and heal the GF/EVA composite laminates with high sensing/healing ability. The proposed method and the obtained results could help to get a multi-functional sensor to ensure the service safety of composite structures. [ABSTRACT FROM AUTHOR]

A micro-mechanism based void growth model involving the void shape change during the creep deformation is proposed, referring to the traditional Cocks–Ashby (C–A) model framework, for multiaxial power-law creep rupture prediction. Accordingly, an improved prediction for creep lifetime and multiaxial creep ductility are derived. Results indicate that the creep life and creep ductility are significantly affected by the void shape evolution in the low stress triaxiality range. By contrast, such an effect could be neglected under the situation of high stress triaxiality. In addition, a large fraction of creep ductility is consumed during the tertiary creep stage. A good agreement has been achieved between the proposed model and the collected experimental data. [ABSTRACT FROM AUTHOR]

The local creep properties of a CrMoV weldment were acquired by using miniature specimens of in-situ creep test. The creep performances of heat-affected zone (HAZ), weld pass metal in the welding direction (WP) and multiple weld passes through the thickness direction (MWP) were compared with each other. It is shown that the magnitudes of creep strength order satisfy WP > MWP > HAZ, while the creep ductility of these three samples exhibit an inverse order: WP < MWP < HAZ. In addition, the typical three-stage creep deformation curves are observed in both WP and MWP samples. However, no significant difference between the primary and the secondary stages is detected for HAZ sample. [ABSTRACT FROM AUTHOR]

Strain controlled fatigue of P92 steel with various strain hold dwells introduced at the peak loading point were conducted at 625 °C. Two features which depend on the cycle and strain range level were observed under the fatigue-creep condition for the viscous and cyclic softening material. The first one is the accelerated cyclic softening response which is ascribed to the accumulated inelastic strain transformation from the creep mechanism during the strain dwell period and becomes more significant with the decrease of strain ranges. The second one is the decelerated stress relaxation behavior which is caused by the reduced viscous stress related to the continuous cyclic softening and fades with the decrease of cyclic strain ranges. Accordingly, a new unified viscoplastic constitutive model within the framework of Chaboche model was developed by improving the nonlinear isotropic hardening rule and the kinematic hardening rule with a cyclic softening parameter. As a result, the accelerated cyclic softening and decelerated stress relaxation response of fatigue-creep interaction was finely reproduced by the proposed model. [ABSTRACT FROM AUTHOR]

Cyclic softening behavior of modified 9–12%Cr steel was investigated at 873 K under different loading modes and stress levels. Results indicated that two domains were identified depending on the plastic strain level, in which cyclic deformations and control modes were different. Significant different cyclic softening was only observed in the macroplasticity domain at high loading levels between stress and strain controlled fatigue, companied with a relatively homogenous coarsening of subgrain. By contrast, in the microplasticity domain at low loading levels, slight cyclic softening was attributed to the fact that back stress remained a relative stable value (or decreased very slightly). An additional back stress component was produced due to the heterogeneous subgrain coarsening at the inter-packet scale. As a result, the cyclic softening was retarded due to the competition between the hardening of inter-packet back stress and the softening of intragranular back stress. [ABSTRACT FROM AUTHOR]

Effect of inhomogeneous microstructure of a CrMoV multi-pass weld metal on creep behavior was investigated experimentally. Four subzones in the multi-pass weld metal were identified in terms of the grain size and morphology. Compared to the micro-hardness and tensile strength, a remarkable different creep behavior was achieved for different subzones. Especially, the heterogeneous creep deformation was observed during the tertiary creep stage. Such a heterogeneous creep behavior of multi-pass weld metal was ascribed to the coupling effect of inhomogeneous microstructure and loading direction. [ABSTRACT FROM AUTHOR]

Lamb wave propagating behavior in woven glass fiber reinforced epoxy (WGF/epoxy) composite laminates was studied by simulation and experiment. A 3D finite element (FE) model was built in Abaqus/Explicit software with the help of user material subroutine to capture the wave propagating features in WGF/epoxy composite plate. The Lamb wave tests were carried out by a non-circular array with 8 piezoelectric (PZT) wafers. A defect imaging algorithm was developed using MATLAB based on the ellipse theory in time-dependent domains. Two defect imaging cases were carried out in the WGF/epoxy composite plate to verify the feasibility of the proposed algorithm. A good agreement between the simulated waveforms and experimental data was achieved. This indicates a great potential of the proposed algorithm in the field of damage localization and identification for WGF/epoxy thin-walled laminates. [ABSTRACT FROM AUTHOR]

Fatigue tests of 9–12% Cr ferrite steel with various hold times at both peak and valley points were conducted under stress control at 898 K. The role of anelastic recovery in the creep-fatigue interaction was discussed systematically. Results indicate that anelastic strain increases primarily with the cycle increase and then decreases slowly until the specimen failed during the unloading process. By contrast, the kinetics of anelastic recovery decreases during the unloading process while is insensitive to the peak stress and hold time at the early stage. In addition, the creep-fatigue (cyclic creep) interaction is much decelerated due to the anelastic recovery compared to the static creep state. A modified life prediction model is developed accordingly under the frame of ductility exhaustion concept by incorporating the effect of anelastic recovery. A good agreement with the experimental data is achieved and thus provides us confidence in the use of the modified model in practice. [ABSTRACT FROM AUTHOR]

Axially loaded push-pull tests of a low strength Cr−Ni−Mo−V steel welds using cross-weld specimens with different sizes were conducted up to the very high cycle fatigue regime under ultrasonic frequency. Results showed that smaller samples had higher fatigue strength with interior cracking predominant at the weld metal. Fine granular area characteristic of polycrystalline features close to micro-defects was indicative of higher fatigue strength with implications of cyclic hardening, and was related to localization of plastic deformation. A physical criterion for formation of fine granular area was proposed as the ratio of accumulation to release rates of cyclic plastic energy was higher than one. It was found that the size effect on fatigue strength of the low strength welds was coupled with frequency effect, an issue that both current deterministic and probabilistic approaches were unable to rationalize, was actually dependent on tensile strength as well as extrinsic factors such as cyclic hardening and micro-defect distribution. [ABSTRACT FROM AUTHOR]

The targeted concentration of acoustic waves has significant implications for industrial nondestructive testing, ultrasound diagnosis, and medical treatment. Most conventional sound-focusing metasurfaces suffer from an untunable focus, narrow bandwidth, and fixed geometric configurations, which severely constrain their practical utility. In this paper, we propose a reconfigurable acoustic coding metasurface composed of two coding units with high transmittance and transmitted phases of 0 and π for realizing tunable and broadband sound focusing. Through the straightforward manipulation of each unit structure and alterations in the coding sequences, precise control of the focus position across the entire working plane is attainable, enabling both tunable axial-axis and off-axis sound-focusing effects. Moreover, the sound-focusing performance of the proposed metasurface is excellent within a broad frequency range from 3000 to 5500 Hz. The experimental results are consistent with theoretical expectations and numerical simulations. This work lays a practical foundation for the design of acoustic devices for tunable and broadband sound focusing. [ABSTRACT FROM AUTHOR]

Using the reactive molecular dynamic simulation, the surface roughness and oxide layer morphology are studied during the early oxidation of polycrystalline copper under applied tensile strains at 900 K. We find that the surface roughness of oxide layer increases with the formation and growth of oxide islands. The transformation of oxide wetting layer to the island structure occurs as a result of atom diffusion triggered by the oxidation-induced growth stress. Such a transformation process is enhanced by promoting the free energy of copper atoms, especially along the grain boundary when the external tensile strain is applied. This new insight provides a fundamental interpretation of the instability of oxide layer and the origin of surface defects under applied tensile loadings. [ABSTRACT FROM AUTHOR]

Symmetrical and asymmetrical strain cycling tests of X12CrMoWVNbN10-1-1 steel were conducted at 873 K. Significant rate dependence, mean stress relaxation and continuously cyclic softening behavior were observed during the fatigue process. In addition, a strain-amplitude dependent competition between cyclic softening and mean stress relaxation was revealed. When the strain amplitudes were larger than the initial plastic point, the cyclic softening was dominated with a rapidly relaxed mean stress and a significantly altered hysteresis loop during the primary cycles. Whilst for the strain amplitudes being less than the initial plastic point, the continuously relaxed mean stress with unclearly altered hysteresis loops was observed during the whole lifetime. Accordingly, a new cyclic viscoplastic constitutive is proposed through the combination of a new nonlinear kinematic hardening rule and the Abdel-Karim–Ohno model. The strain-amplitude dependent cyclic softening and mean stress relaxation behavior were finely reproduced by the proposed model, which was achieved by introducing a mean stress relaxation parameter as a function of the maximum plastic strain and the accumulated cyclic plastic strain. [ABSTRACT FROM AUTHOR]

Welding residual stress relaxation and migration behavior in the welded turbine rotor was studied by using thermal elastic–plastic finite element method under the fatigue and creep loadings. The effect of solid-state phase transformation on the generation of residual stress during welding process was involved. Results reveal that the increase of welding pass number will reduce the peak of residual stress and the width of heat affected zone (HAZ). Under the repeated loadings, the welding residual stress is largely released after the first cycle of start-up and shut-down with a residence time. In addition, such a relaxation mainly depends on the temperature, the initial level of residual stress and the residence time of steady operation. Compared to the contribution of alternating load induced during the start-up and shut-down operation, the relaxation of residual stress is mainly ascribed to the creep mechanism. [ABSTRACT FROM AUTHOR]

Cyclic microindentations were performed on the occlusal surface and axial section of tooth enamel, using the Berkovich indenter. Under the action of a cyclic indentation load, the indenter continuously penetrated into the tooth enamel and reached a quasi-steady state at which the penetration depth per cycle was a constant. At the quasi-steady state, both the amplitude of the indentation depth and the penetration depth per cycle for the cyclic indentation of the axial section are larger than those for the indentation of the occlusal section under the same loading condition. The energy dissipation per cycle consists of two contributions; one is the plastic energy dissipated per cycle due to the propagation of the plastic zone underneath the indentation and the other is the energy dissipation due to the viscous flow during the cyclic indentation. Both the penetration depth and the plastic energy dissipated per cycle at the quasi-steady state are independent of the maximum applied load and increase with increasing the amplitude of the cyclic indentation load. [ABSTRACT FROM AUTHOR]

The current state of influence of load ratio, R , on fatigue crack growth (FCG) behavior is thoroughly reviewed. Two main approaches, including crack closure concept and driving force parameter method, when used independently, are found to be short of accuracy during load-shedding procedure in the near-threshold regime. By combining an equivalent driving force model and crack closure approach, a new crack opening equation and a general FCG law is established for the load-shedding technique. This merging method does not involve crack closure measurement, and is capable to predict R dependence of FCG curves and fatigue thresholds fairly well. [ABSTRACT FROM AUTHOR]

Using reactive molecular dynamic simulation, the diffusion behavior of oxygen ions is studied during the oxidation of Fe (1 0 0) under applied tensile strains at room temperature. We find that the strain induced changes of the oxide structure lead to the change of the oxygen diffusion coefficient. This is supported by the comparative analysis of diffusion energy barrier in the corresponding oxide structure. Compared with the effect of the diffusant concentration (composition) and oxidation growth stresses, the evolution of oxide structure plays a dominant role on the change of diffusion coefficient in the strain-reaction engineering. [ABSTRACT FROM AUTHOR]

Micro-tensile tests based on small-scale specimens were carried out to investigate the microstructure dependence of strain hardening behavior along a Ni–Cr–Mo–V steel welded joint. Results indicated the weld metal (WM) had higher tensile strength but lower strain hardening exponent than base metal (BM), whereas a gradient distribution of strain hardening exponents was observed within the heat-affected zone (HAZ). Bainitic microstructures showed weaker strain hardening capacity than lathy martensites. A Kocks–Mecking type plot of strain hardening rate versus true stress presented at least two hardening stages (stages III and IV) with varied duration along the whole welds. Larger grain size and more lathy martensites were beneficial to prolong the hardening period. A new strength prediction model was developed by considering the size effect on tensile strength. The predicted strength distributions along the welds were in good agreement with experimental data. [ABSTRACT FROM AUTHOR]

Crack closure and opening behavior of physically short fatigue crack tip was investigated in microstructures with different grain sizes by using the in-situ SEM fatigue test. A gradually decreased crack closure zone near the crack tip was observed which depended on the microstructures, crack size and stress levels. A new parameter involving the load level, grain size, specimen dimensions and crack length was proposed to interpret the physically short crack closure development. The contribution of the gradient crack closure zone on the physically short fatigue crack growth behavior was evaluated by using the modified partial crack closure model. A generalized fatigue crack growth driving force was finally determined and correlated well with the physically short crack growth rate. [ABSTRACT FROM AUTHOR]

Tooth enamel is often subjected to repeated contact and often experiences contact deformation in daily life. The mechanical strength of the enamel determines the biofunctionality of the tooth. Considering the variation of the rod arrangement in outer and inner enamel, we approximate enamel as a bilayer structure and perform finite element analysis of the cyclic indentation of the bilayer structure, to mimic the repeated contact of enamel during mastication. The dynamic deformation behaviour of both the inner enamel and the bilayer enamel is examined. The material parameters of the inner and outer enamel used in the analysis are obtained by fitting the finite element results with the experimental nanoindentation results. The penetration depth per cycle at the quasi-steady state is used to describe the depth propagation speed, which exhibits a two-stage power-law dependence on the maximum indentation load and the amplitude of the cyclic load, respectively. The continuous penetration of the indenter reflects the propagation of the plastic zone during cyclic indentation, which is related to the energy dissipation. The outer enamel serves as a protective layer due to its great resistance to contact deformation in comparison to the inner enamel. The larger equivalent plastic strain and lower stresses in the inner enamel during cyclic indentation, as calculated from the finite element analysis, indicate better crack/fracture resistance of the inner enamel. [ABSTRACT FROM AUTHOR]

Abstract: Tooth enamel is a complex structure, consisting of numerous enamel rods surrounded by a protein-rich sheath. Considering the possible effect of the protein-rich sheath on the indentation deformation of an enamel rod and the limitation of the Oliver–Pharr method in measuring the elastic modulus of the enamel rod, we used a finite element method to analyze the indentation deformation of an elastic-perfectly plastic cylinder surrounded by an elastic-perfectly plastic film. A concept of the threshold indentation depth was proposed, at which the percentage error of the measured modulus of the cylinder is ±10%. For the indentation depth less than the threshold indentation depth, the elastic modulus measured from the indentation test can be approximated as the intrinsic elastic modulus of the cylinder. The normalized threshold indentation depth strongly depends on the modulus ratio of the film to the cylinder and the ratio of the film thickness to the cylinder radius. The results can be used to guide the use of the Oliver–Pharr method in characterizing the mechanical properties of tooth enamel and bio-composites with core-shell structures. [Copyright &y& Elsevier]

Abstract: Using nonlinear Rayleigh waves, the effect of surface scratches on the propagation of nonlinear ultrasonic waves in a glass plate was investigated. The results showed that the amplitudes of the fundamental and second harmonics of the nonlinear Rayleigh waves decreased with increasing average width of surface scratches due to the interaction between scratch-induced cracks and wave propagation. The increase of the nonlinearity with the average scratch width suggests that the surface scratches caused the decrease of apparent Young's modulus near the glass surface. Nonlinear Rayleigh surface waves could be a potential method for the quantitative and rapid measurement of surface and subsurface damage in brittle materials. [Copyright &y& Elsevier]

Abstract: It was still short of generalized fatigue crack growth (FCG) models in the near-threshold regime due to its complex influencing factors. The near-threshold FCG behaviour of a rotor steel 25Cr2Ni2MoV at different load ratios was investigated experimentally, and the FCG driving mechanism was theoretically analysed based on equivalent driving force model. It was found that the crack growth process was determined by combined effects of equivalent driving force at constant amplitude loading and crack closure. A new crack closure model was proposed by considering the influences of load ratio and FCG rate, which could successfully interpret the effect of load ratio on FCG. The correlation of the crack closure model with the transition of driving forces in crack advance was beneficial to unify crack closure theory and crack growth driving parameters in the near-threshold regime. [Copyright &y& Elsevier]

Abstract: Shakedown behavior of multilayered beams under cyclic thermo-mechanical loadings was analyzed based on Armstrong–Fredrick nonlinear kinematic hardening model coupled with ductile damage. The shakedown limit loads were determined by the accumulation of plastic straining after enough cycles under various loading combinations and presented by the Bree-like diagram. The effects of the thickness ratio of film/substrate as well as damage parameters on shakedown behavior were also addressed. The calculated results indicated that the twice yield stress limit for shakedown evaluation is considerable conservative compared with the shakedown domain obtained by nonlinear kinematic hardening model. However, shakedown limit loads decreases remarkably considering the ductile damage and the twice yield stress limit for shakedown estimation may be unsafe for engineering designs, especially when the thermal load is prominent. [Copyright &y& Elsevier]

Abstract: Very high cycle fatigue (VHCF) behavior of a low strength weldment was investigated by fully reversed axial tests in air at room temperature and 370°C. The role of non-metallic inclusions in the VHCF was addressed in terms of experimental results and finite element simulations. The higher potential for interior crack nucleation at higher temperature was ascribed to matrix softening, surface oxidation and surface compressive residual stress. A new model for interpretation of the role of inclusion in the transition of crack initiation modes was developed. [Copyright &y& Elsevier]

It is known that fine granular area morphology is often formed around interior micro‐defect in the case of very high cycle fatigue failure of high strength steel. The mechanism of "fragmentation of martensitic laths and formation of dislocation cells" (Zhu's model), which revealed a microstructure‐dependent crack initiation and early growth for micro‐defect induced internal fatigue cracking, was discussed in several lathy martensitic or similar steels, verified under environmental fatigue, and applicable to white etching crack in rotating contact fatigue. A machine learning‐Z parameter integrated approach was developed for fatigue life, which is promising for design against fatigue of engineering structures for long life. Highlights: The FGA model of martensitic steels in air (Zhu's model) was illustrated.Zhu's model was confirmed in environmental media and similar steels.Zhu's model was applicable to white etching crack in rotating contact fatigue.Machine learning‐Z parameter was integrated for fatigue life prediction. [ABSTRACT FROM AUTHOR]

Abstract: In-situ nanoscratch experiments were carried out on occlusal surface and axial section (on which axis is perpendicular to rods and axis parallels to rods, respectively) of human enamel using a sphero-conical indenter (∼1μm tip radius) at different normal loads. The elastic recovery and inelastic deformation during scratching process was evaluated in terms of three curves along the track: surface roughness, scratch depth, and post depth. Coefficient of friction (COF) and wear rate was addressed for the single rod. Results reveal that the amount of elastic recovery and inelastic deformation of a single rod on the axial section is more than that on the occlusal surface. At the enamel rod scale, COF depends on loads and locations. The wear resistance of a single rod on the occlusal surface is greater than that at direction of axis perpendicular to rods and axis parallel to rods. The load-dependence is attributed to the ploughing and pile-up. The location-dependence is ascribed to the angle change of normal loading direction, hydroxyapatite crystals orientation and the scratching direction. In addition, viscoelastic protein layers between the apatite crystals play an important role in the anisotropic wear behavior at microscale. [Copyright &y& Elsevier]

Abstract: Very high cycle fatigue (VHCF) tests at 370°C were carried out for a Cr–Mo–V steel to investigate the influence of microstructure and microdefects on fatigue behavior. Duplex S–N curves were obtained with the crack initiation switched from specimen surface to interior. A new elliptical type of fish-eye pattern originated from microstructure heterogeneities was observed. In the high cycle fatigue regime, the influence of microstructures on fatigue performance was greater than that of microdefects. In the VHCF regime, by contrast, the fatigue performance was dominated by inclusions and could be interpreted by using the proposed life controlling parameter Z. [Copyright &y& Elsevier]

Abstract: In order to reveal the physical mechanisms of ratchetting process under creep–fatigue loadings, following ratchetting tests in advanced 9–12% chromium ferrite steel, a study of associated fracture modes and dislocation patterns explored by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations is presented in this paper. Two main domains were observed depending both on the peak hold time and on the stress ratio, in which the ratchetting deformation and failure mechanisms were different. These two damage domains correspond to two distinct creep–ratchetting interaction mechanisms. Particular attention was paid to the dependence of ratchetting damage behavior on the stability of dislocation substructure. In addition, an attempt is made to correlate the results of the microstructural investigations with the variations of internal stress. [Copyright &y& Elsevier]

Abstract: Synthesis creep test of Sn63Pb37 alloy was conducted under the combination of tensile stress and direct electric current. Results indicate that the steady-state creep strain rate of Sn63Pb37 alloy increases with increasing the electric current and changes more drastically at low stress level than that at high stress level. The stress exponent in power law decreases linearly with increasing the electric current, while the creep activation energy increases with the square of electric current. To illuminate the electromechanical interaction in creep behavior, a modified Arrhenius type model involving the current effect was proposed. [Copyright &y& Elsevier]

Abstract: The closed-form solutions of bending curvature and stress distribution in film/substrate system with the synthesis surface effect are proposed by minimizing the total potential energy. Effects of the roughness and the residual surface stress on stress in film are addressed. Results reveal that, at a given thickness of the substrate, effects of roughness and residual surface stress on the bending curvature become significant with decreasing the film thickness. The roughing surface will enlarge the magnitudes of bending curvature and film stress. The direction change of residual surface stresses can lead to a reversed bending of film/substrate system. [Copyright &y& Elsevier]

Abstract: The ratcheting-creep interaction of advanced 9–12% chromium ferrite steel was tested and simulated under various hold times at 600°C. A constitutive model involving the influence of anelastic relaxation was proposed and agreed very well with the experimental data. Results indicated that when the dwell time at peak load was less than 5-min, the total strain was mainly produced by the ratcheting strain due to the anelastic creep recovery and eventually failed due to ductile-fatigue damage, while for a longer holding time the accumulated creep strain and the fatigue-creep damage were the controlling mechanism of deformation and failure. [Copyright &y& Elsevier]

Abstract: The ratchetting behavior of advanced 9–12% chromium ferrite steel was investigated by cyclic loading tests with various hold times and stress ratios at elevated temperature of 873K. Particular attention was paid to the effect of hold time on the whole-life ratchetting deformation and failure mechanism. Results indicate that the total ratchetting strains under creep-fatigue loading can be decomposed into two parts, i.e., cyclic accumulated creep strain produced during the peak stress hold time , cyclic accumulated inelastic strain produced during the stress change process (). A transition in ratchetting components and rupture behavior with the increase of hold time was observed. In the long hold time domain, a quick shakedown of ratchetting strain occurs after the very first few cycles and the rupture behavior is fully controlled by the time-dependent creep damage. In the short hold time domain, ratchetting strain increases till the specimens fails and a mixed damage mode is responsible for the failure. An attempt is made to explain the existence of these two domains in terms of the evolutions of three internal stress components (back stress, isotropic stress and viscous stress) measured at the end of the holding period. [Copyright &y& Elsevier]

Abstract: The mechanochemical effects between dissolved hydrogen and deformation on active dissolution of 304 stainless steel in acid chloride solution were investigated by use of constant loading test method. A theoretical model was proposed in terms of the stored strain energy and electron work function. Results indicated that active dissolution was affected mainly by Gibbs free energy produced by the part of plastic deformation and surface dislocation structure. A good agreement between the prediction and test results revealed that the synergistic effect of stress and dissolved hydrogen only promoted the process of active dissolution. [Copyright &y& Elsevier]

Abstract: This paper deals with a new model for life estimation of creep derived from the law of energy conservation and the momentum conservation principle. By mathematical technique, the expressions of these two rules are transformed into a simple function, which can describe the relationship between the internal energy and creep life. This model supposes that the internal energy change of a specimen is a constant from the initial state to the failure one under certain conditions. Moreover, owing to its simple expression, it is very easy to use this model to predict the creep life. To check the validity of the new model proposed in this paper, the published data of 2.25Cr–1Mo, 1Cr–0.5Mo, 18Cr–12Ni–Mo and 1Cr–1Mo–0.25V steels have been studied and analyzed. A well agreement is noted between the model and the results. [ABSTRACT FROM AUTHOR]

Abstract: Tensile and impact behavior of dissimilar weld joints of newly developed rotor steels 23CrMoNiWV88 and 26NiCrMoV145 were conducted at various temperatures below 350°C. Inhomogeneous microstructures and asymmetrical micro-hardness along the dissimilar welding joint were observed. With the increase of temperature, strength decreased which was associated with the increased plasticity, and fracture location changed from weld metal (WM) to intermediate pressure (IP) base metal (BM) at around 300°C. Compared to the homogeneous impact specimen with two fracture zones at fracture surface, a combined quasi-cleavage and ductile fracture mode with three zones is observed at the fracture surface of the dissimilar weld joint when the testing temperature is in the range of 0–40°C. The occurrence of separated zones are mainly ascribed to the multi-layer welding process and thus improved the impact toughness of the welding joint. [Copyright &y& Elsevier]

Abstract: Correlation between microstructure, hardness and strength was investigated in heat affected zone (HAZ) of dissimilar welding joints of newly developed rotor steels in terms of both the traditional micro-hardness testing and the nanoindentation technique. Relationships between micro-hardness and nano-hardness were obtained for HAZ of welds, where the mechanical properties were microstructure dependent. Lath width of the tempered martensites was selected as the characteristic microstructure size for correlating the micro-hardness. The detailed strength distribution in HAZ and the correlation with the characteristic microstructure size were discussed. A good agreement was observed for the correlated strength and the experimental results in both weld metal (WM) and base metal (BM), and therefore gave us confidence for the application of the proposed relationship between the hardness and the mechanical properties in HAZ of the dissimilar weld. [Copyright &y& Elsevier]

Abstract: A self-consistent diffusion model has been employed to simulate the atomic nitrogen transport in titanium alloy with the coupling effect of both initial activated nitrogen concentration and high temperature gradient. Transport equations are solved through coupling the transient heat transfer equation and incorporating the surface physical phenomena in the governing equations as boundary conditions. For comparisons, laser gas nitriding of Ti6Al4V alloy has been performed by means of a pulsed Nd:YAG laser irradiation in a controlled nitrogen chamber. Results indicate that the simulated nitriding molten pool geometry and nitrogen concentration profiles are broadly consistent with the experimental results for various laser intensities and nitrogen pressures. Due to the combined driving of both concentration and temperature gradient, a rapid increase of nitrogen concentration profile during the heating stage is observed. While in the cooling stage, the redistribution of nitrogen in substrate and the local maximum of nitrogen close to the surface are mainly caused by the transient temperature gradient which is confirmed by both the experimental and simulated results. In addition, the influence of laser intensity on nitrogen transport is strongly related to the initial nitrogen concentration at surface. Especially, when the initial nitrogen concentration at surface approaches to the maximum concentration limit of nitrogen in titanium nitrided layer, slight drops of nitrogen concentration in molten zone are observed with the increase of laser intensity. This indicates that the earlier formed diffusion barrier due to the maximum concentration limit will impede the further penetration of atomic nitrogen and thus lead to the relatively lower concentration. [Copyright &y& Elsevier]

Abstract: The influence of residual stress on diffusion-induced bending in bilayered microcantilever sensors has been analyzed under the framework of thermodynamic theory and Fick''s second law. A self-consistent diffusion equation involving the coupling effects of residual stress and diffusion-induced stress is developed. Effects of thickness ratio, modulus ratio, diffusivity ratio and residual stress gradient of film and substrate on the curvature of bilayered cantilever are then discussed with the help of finite difference method. Results reveal that the curvature of bilayered cantilever increases with decreasing the diffusivity ratio and modulus ratio of substrate to film at a given time. Case study of the polysilicon/palladium hydrogen sensor has been finally carried out using the above developed bending theory. [Copyright &y& Elsevier]

Near-threshold fatigue crack growth behavior in 25Cr2NiMo1V steel with different microstructures was investigated by utilizing the load-shedding technique at ambient temperature. Crack surface morphology was observed by SEM with special emphases on the incidence of intergranular fracture and the influence on crack growth rates. Results show that the maximum intergranularity occurs at the ΔK corresponding to the cyclic plastic zone size being equivalent to the prior austenitic grain size. Two types of crack growth mode were observed in the near-threshold regime, i.e., the crystallographic mode of crack growth and the striation mode of crack advance. The incidence of faceted fracture was mainly rationalized by comparing the cyclic plastic zone size with the grain size. It is concluded that, in the crystallographic mode, lower crack growth rates in samples with higher heat treatment temperatures are caused by a greater degree of roughness-induced crack closure (RICC), faceted fracture induced crack closure (FFICC), and oxide-induced crack closure (OICC). The faceted fracture shows negligible influence on crack growth rates when cracks grow in a striation controlled mode. [ABSTRACT FROM AUTHOR]

Abstract: A closed form solution for C* integral of two interacting cracks in plates under tension is developed on the basis of reference stress method. Comprehensive finite element (FE) creep analyses are carried out to provide the benchmark of the interaction evaluation of multiple cracks. Results indicate that more pronounced interaction is observed between the C* of double cracks and that of a single crack compared to that denoted by stress intensity factor (SIF). Overall good agreement is achieved between the proposed method for C* of multiple crack interaction and the FE results which provides confidence in practical application. [Copyright &y& Elsevier]

Abstract: The present work reports several recent research activities on the time-dependent deformation and fracture performance of multi-material system and structures at elevated temperature. A micro region deformation measuring technique is developed to achieve the full creep strain fields of multi-material systems at high temperature in light of the long-distance microscope and digital speckle correlation method. The mean field approach based on the self-consistent method and the generalized self-consistent method are introduced to predict the time-dependent deformation of the dual-components material system under the creep condition. For the pressure vessels with functionally graded materials under both internal and external pressures, an asymptotic solution for creep stress and strain is derived based on the Taylor expansion series. For the time-dependent fracture issue of multi-material structures, a modified creep crack fracture parameter prediction method for C ∗ integral in the mismatched weldments and the particle-reinforced composite is proposed based on the generalized equivalent homogenous model. Finally, time-dependent failure assessment diagram (TDFAD) for multi-material system is derived for defect assessment of structures with various combinations of materials. [Copyright &y& Elsevier]

Abstract: Fatigue crack growth behavior was investigated in a newly developed steel 25Cr2NiMo1V with different heat treatments to meet different property requirements of high-pressure (HP) and low-pressure (LP) parts in the combined steam turbine rotor. The load-shedding method and constant load amplitude techniques were adopted. Fractography including crack surface observation and fatigue crack growth path analyses was carried out. Results show that in the threshold regime, fatigue crack growth resistance of HP is clearly superior to that of LP. The distributed bainitic microstructures and larger prior austenite grain size in HP result in more tortuous crack propagation path than that in LP. Compared with ferritic blocks in HP, the tempered martensitic laths in LP do not play a dominant role in stopping the fatigue crack advance. Nevertheless, no clear difference in fatigue crack growth in HP and LP is observed in the Paris regime. [Copyright &y& Elsevier]

Abstract: A general theoretical model was developed to predict the creep deformation and its effect on the stress relaxation and distribution in the multilayer systems under residual stress and external bending. Based on the proposed solution, a simplified solution for the special case of one film layer on a substrate is also presented. Finite element analysis was carried out to validate the presented model. Good agreements were observed between the finite element simulation and the prediction of the proposed model. In addition, the effects of film thickness on creep strain and stress distribution, the creep effect on neural axis location in the bilayer assembly subjected to the combination of residual stress and external bending were also discussed. [Copyright &y& Elsevier]

Abstract: In terms of the representative unit cell (RUC) with tri-layers, the time-dependent fracture performance of interfacial crack in multilayered structures was studied by using the finite element method (FEM). The feasibility of integral-type crack driving force parameter C ∗ for interpretation of interfacial creep crack growth (CCG) of multilayered systems were reexamined. Results indicate that the magnitude of the interfacial creep crack growth driving force was related to the mismatch factor of multi-layers and was significantly affected by the thickness of interlayer. [Copyright &y& Elsevier]

Abstract: Measurement and evaluation of the prestress losses in the reinforced structures are essential issues for structural design and healthy assessment. In the present work, an optical-fiber-sensor-based monitoring technique was introduced to evaluate the prestress loss quantitatively in the steel-strand reinforced structures. A total of 14 fiber-optic sensors were employed and fixed on the steel-strand through the pre-designed windows on a sewage treating tank. The output signals were collected and analyzed via the secondary meters and computer. Results indicate that the optical-fiber-sensor-based system is feasible for on-line monitoring of the prestress losses of the reinforced concrete structures in practice. The value of the short-term prestress loss is apparently related to the arrangement of steel strand in concrete. For the measured sewage treating tank, the value of prestress loss due to deformation of anchor is greater than that due to relaxation of steel–strand during ten days after tensioning. The whole loss of prestress due to relaxation, deformation and creep at point A is only 1.4% after ten days since tensioning. [Copyright &y& Elsevier]