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Finite element analyses on notched beams and axis‐symmetric pipes of power‐law creep materials subjected to combined primary and secondary loads are performed in this work. In view of the characteristics of time‐dependent strain energy density at notches under combined loads, a time‐dependent stress and strain estimation method to address the primary and secondary loads has been proposed by considering the stress ratio between the reference stress devoted by primary stress and the total reference stress. The applicability of the modified method to notched components with different geometrical configurations and load conditions is discussed. Good agreement between the results of inelastic analysis and the modified method is observed. [ABSTRACT FROM AUTHOR]

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]

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]

Symmetry properties of second harmonics generated by antisymmetric primary Lamb waves are systematically studied in this work. In theory, the acoustic field of second harmonic Lamb waves is obtained by using the perturbation approximation and normal modal method, and the energy flux transfer from the primary Lamb waves to second harmonics is mainly explored. Symmetry analyses indicate that either the symmetric or antisymmetric Lamb waves can merely generate the symmetric second harmonics. Finite element simulations are performed on the nonlinear Lamb wave propagation of the antisymmetric A0 mode in the low frequency region. The signals of the second harmonics and the symmetric second harmonic s0 mode are found to be exactly equivalent in the time domain. The relative acoustic nonlinearity parameter A 2 / A 1 2 oscillates with the propagation distance, and the oscillation amplitude and spatial period are well consistent with the theoretical prediction of the A0-s0 mode pair, which means that only the second harmonic s0 mode is generated by the antisymmetric primary A0 mode. Experiments are further conducted to examine the cumulative generation of symmetric second harmonics for the antisymmetric-symmetric mode pair A3-s6. Results show that A 2 / A 1 2 increases linearly with the propagation distance, which means that the symmetric second harmonic s6 mode is generated cumulatively by the antisymmetric primary A3 mode. The present investigation systematically corroborates the proposed theory that only symmetric second harmonics can be generated accompanying the propagation of antisymmetric primary Lamb waves in a plate. [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: Considering the possible effect of the thin protein-rich sheath on the indentation deformation of an enamel rod, we analyzed the indentation response of an elastic cylinder with a compliant layer between the cylinder and rigid-surrounding material. For the film thickness much less than the characteristic dimension of the cylinder, closed-form solutions were obtained between the indentation load and the indentation depth, which depends on the film thickness and the ratio of the Young's modulus of the cylinder to the Young's modulus of the film. The finite element results supported the relationships for the ratio of the film thickness to the characteristic dimension of the cylinder less than or equal to 1/3. The indentation load required to produce the same indentation displacement decreases with increasing the ratio of the Young's modulus of the cylinder to the Young's modulus of the film for compressible-elastic films. Incompressible-elastic films have no significant effect on the indentation response of the elastic cylinder. [Copyright &y& Elsevier]

Abstract: The thermal–mechanical stress distributions and equivalent coefficient of thermal expansion (CTE) of the staggered arrangement of mineral platelets wrapped by soft matrix are analyzed, which exist in numerous natural biological and biomimetic materials. Two analytical models, ‘Stress model’ and ‘Displacement model’, were established from the ways of stress and displacement solution based on the modification of classical shear-lag model. Complementary finite element analysis (FEA) was used to verify the analytical models. Results reveal that, compared to ‘Displacement model’, ‘Stress model’ gives a better prediction of the stress distributions within the staggered structure referring to FEA. The equivalent CTE predicted by both models reach constant as the aspect ratio and volume fraction of platelets exceeding the critical values. Nevertheless, the relative error between the results from different models increases with the increase of the ratio of overlap to length of platelets. These provide a benchmark to the optimum design of micro/nano-structure in bio-inspired materials suffering to temperature fluctuation and applied loading. [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]

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: 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: In this work, a survey on the closed form solutions for the prediction of interfacial shear and peeling stresses of bonded structures is first given. And then, comparisons between the closed form solutions, elastic finite element analysis (FEA) and elastic-creep FE results are performed for the bonded structures subjected to thermal loading. To elucidate the creep effects on interfacial stresses, at last, creep damage FE analysis is carried out referring to the K–R damage creep law. Results indicated that both creep and creep-damage analysis will lead to interfacial stress redistribution and thus reduce the shear stress and peeling stress at interface. Creep damage analysis yields the lowest stresses among the linear elastic and elastic creep analysis results. The maximum damage is produced at the edge of the interface where the failure often occurs and is proportional to the modulus ratio of substrate to film. On the other hand, the damage distribution along the interface is very similar to the trend of peeling stress distribution. Compared to the FE results, the existing closed solutions from and can only give an accurate prediction of the interfacial shear stress but give a rough estimation to the interfacial peeling stress. As far as effects of creep are concerned, the existing closed form solutions based on elastic analysis are inapplicable anymore. [Copyright &y& Elsevier]

Based on the comprehensive finite element (FE) creep analyses, the influence of free surface on the time dependent fracture mechanics parameter of a crack near the free surface in plates under tension has been investigated. It is found that the time dependent fracture parameter C* increases as the crack tip closes to the free surface. Such an increment is related not only to the crack configurations but also to the material properties, especially the creep exponent n of power creep law. In addition, more pronounced interaction is observed between the C* of subsurface crack and that of a single isolated crack compared to that denoted by SIF under the linear elastic fracture condition. Under the framework of reference stress method, we also developed a closed form solution for creep interaction factor. Overall good agreement is achieved between the proposed method for the C* of subsurface crack and the FE results which provides us confidence in practical application. [ABSTRACT FROM AUTHOR]

Abstract: For the compact tension (CT) specimen with a mismatched weld in creep properties, an equivalent homogeneity model is presented based on the limit load analysis. In use of the proposed equivalent material model and existing limit load solutions, the modified experimental C ∗ integral estimation equations for the CT specimens with weld centre crack, fusion line crack and asymmetric crack were derived. Finite element (FE) analysis for C ∗ integral of CT specimens with a mismatched weld was performed by using the ABAQUS software. Results show that, compared with the η o factor for homogeneous CT specimens, a soft under-mismatched weld will increase the non-dimensional factor η w while a hard over-mismatched weld will decrease the η w value. It indicates that the current C ∗ estimation equation in ASTM E1457 may underestimate C ∗ for creep soft weld but overestimate it for creep-hard weld. The discrepancies between the C ∗ from ASTM E1457 and FE analysis are sensitive to the crack location in weld, to the mismatch ratio and to the weld width. Whether for under-mismatched weld (M <1) or for over-mismatched weld considered herein, the predicted C ∗ parameters from the presented formulae are very close to FE results and thus can be used to interpret the creep crack growth behaviour of welded specimens. [Copyright &y& Elsevier]

Abstract: This paper is concerned with the prediction of limit load of the piping branch junctions with circumferential crack under internal pressure. Recently, we have developed a new approach for predicting the limit load of two-cylinder intersection structures with diameter ratio larger than 0.5, which has been successfully applied to defect free cases under various loading conditions. In the present work, we consider the extension of the approach to cover cracked piping branch junctions. On the basis of stress analysis in the vicinity of intersection line, a closed form of limit load solution for piping branch junctions with circumferential crack was developed. Then, 36 finite element (FE) models of piping branch junction with various dimensions of structure and crack were analyzed by using nonlinear finite element software. The limit loads from FE analysis and the proposed solution are compared with each other. Overall good agreement between the estimated solutions and the FE results provides confidence in the use of the proposed formulae for defect assessment of piping branch junctions in practice. [Copyright &y& Elsevier]

The limit load is an important input parameter in engineering defect-assessment procedures and strength design. In the present work, a total of 100 different piping branch junction models for the limit load calculation were performed under combined internal pressure and moments in use of non-linear finite element (FE) method. Three different existing accumulation rules for limit load, i.e., linear equation, parabolic equation and quadratic equation were discussed on the basis of FE results. A novel limit load solution was developed based on detailed three-dimensional FE limit analyses which accommodated the geometrical parameter influence, together with analytical solutions based on equilibrium stress fields. Finally, six experimental results were provided to justify the presented equation. According to the FE limit analysis, limit load interaction of the piping tees under combined pressure and moments has a relationship with the geometrical parameters, especially with the diameter ratio d/D. The predicted limit loads from the presented formula are very close to the experimental data. The resulting limit load solution is given in a closed form, and thus can be easily used in practice. [Copyright &y& Elsevier]

Multiaxial corrosion fatigue tests with interruption were carried out on 304 stainless steel in 6 wt.% FeCl 3 solution at room temperature. Pit initiation, growth and pit to crack transition behavior under proportional and non-proportional loading were investigated. Under proportional loading, significant sharper pits, higher initial pit density and growth rate were observed in comparison with non-proportional loading. The pit depth growth rate under proportional loading was larger than those under non-proportional loading and without loading. Pit to crack transition mechanism was mainly controlled by mechanical assisted local dissolution under proportional loading while additional hardening under non-proportional loading. [ABSTRACT FROM AUTHOR]

Abstract: A mathematical model for simulating the active dissolution of a pit on stressed metal surface had been developed. Based on active dissolution mechanism, dissolution current density on the pit surface was assumed and extended through accounting for the thermal activation energy and the multiaxial stress state in pit bottom. The influence of applied tensile stress, pit radius and temperature was addressed. The distribution of solution potential and species concentration was predicted for different applied tensile stresses based on finite element calculations. [Copyright &y& Elsevier]

• The stress transfer mechanism for overload effect was verified successfully. • The level of crack closure after overload was found varied periodically. • Overload plastic zone and overload affected zone were linearly correlated with overload ratio. Cyclic plastic finite element modeling was carried out to investigate effect of overload on fatigue crack growth under blunting and no-blunting mechanisms at different overload ratios. The stress transfer mechanism was verified and was found more obvious in the case of crack blunting though its effect was time-limited. The level of crack closure varied periodically due to gradual relaxation of compressive residual stress during crack propagation. Both the overload plastic zone and the overload affected zone were linearly correlated with overload ratio, and an overload ratio value of 1.1 was the best choice for studying overload effect. [ABSTRACT FROM AUTHOR]

Creep-cyclic plasticity of a benchmarked holed plate subjected to thermo-mechanical loading is investigated by means of nonlinear finite element analysis. From the analyses, a structural creep recovery response is found within a dwell period, which has serious repercussions on structural integrity. The structural creep recovery can take place by reversing the creep stress in sign during the stress relaxation due to the creep stress redistribution, consequently enhancing unloading plasticity which causes a substantial increase of total strain range within a cycle. Based on this critical observation, further analyses and discussions are provided to investigate the root cause of this precautious structural response. Various cyclic loadings with a dwell at the peak thermal load are analysed to define factors influencing the structural creep recovery mechanism, and to investigate how the mechanism affects the lifetime of the structure. To show the effectiveness of the structural creep recovery mechanism under cyclic loading, Chaboche nonlinear kinematic hardening model is adopted. Limitations of applying elastic follow-up in predicting creep strains and appropriate creep-fatigue damage calculation methods are discussed in the presence of this structural creep recovery mechanism. This research work confirms that when a structure experiences the structural creep recovery it can reduce creep damage, nevertheless the structure may experience significant fatigue damage due to creep enhanced plasticity. [ABSTRACT FROM AUTHOR]

• A probabilistic framework of creep life assessment for components at elevated temperature is provided. • Effect of standard derivation of material parameters on creep reliability assessment is discussed. • Comparisons between probabilistic and deterministic creep design methods are conducted. Current creep life assessment methods of components at elevated temperatures are mainly based on deterministic analysis strategies, which could not achieve the goal of probabilistic evaluation on creep failure. Based on this, a probabilistic framework of creep life assessment for components at elevated temperatures was provided. A method of determining distribution characteristics of material parameters was provided by randomly selecting a group of results at each stress level. Monte Carlo simulation was combined with finite element analysis technology to capture the distribution characteristic of creep rupture life of one typical structural component. Effect of standard derivation of material parameters on creep reliability assessment was discussed. Comparisons between probabilistic and deterministic creep design methods were made. Results indicated that the probabilistic analysis strategy can calculate the specific value of failure probability at various loading conditions, not two values of failure probability (i.e. 100% and 0%) by deterministic analysis strategy. The effect of standard derivation on mean values of effective stress and creep rupture life of the component is dependent on distribution characteristics of material parameters and related variables. A small standard derivation reduces the data scatter of effective stress and creep rupture life of the component. [ABSTRACT FROM AUTHOR]

Abstract: In this paper the creep parameters for a Norton law material are determined from the creep bending tests data by an elliptical arc shape specimen. According to the ideal time-dependent beam theory, the creep coefficient B and creep exponent are derived as a function of the applied load and displacement rate. Finite element simulations of the elliptical arc specimen bending creep are also carried out to validate the feasibility and accuracy of the proposed approach. Our work will contribute to a better understanding to derive creep parameters from creep bending test and extend its application for the milli/micro scale miniature specimen. [Copyright &y& Elsevier]

In the present paper, a modified fatigue life model is proposed based on stress field intensity (SFI) approach, where the average weight function is introduced as the denominator of the traditional model and a modified definition of the fatigue zone is given using the concept of characteristic stress. Combined with finite element analysis, the fatigue life predictions of a central hole specimen from the literature and the notched plate specimens/a simulated engineering component conducted by the authors are carried out by the new model. Results indicate that the predicted lives are in good agreement with the experimental results and the modified stress field intensity approach is convenient for practical use. Unlabelled Image • Fatigue tests of notched plate specimens and a simulated engineering component are conducted. • A modified stress field intensity approach is proposed for fatigue life prediction of the component. • The proposed model could provide an adequate prediction solution and reduce computation efforts. [ABSTRACT FROM AUTHOR]