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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]

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]

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]

The creep damage in high temperature resistant titanium alloys Ti60 is measured using the nonlinear effect of an ultrasonic Lamb wave. The results show that the normalised acoustic nonlinearity of a Lamb wave exhibits a variation of the “increase-decrease” tendency as a function of the creep damage. The influence of microstructure evolution on the nonlinear Lamb wave propagation has been analyzed based on metallographic studies, which reveal that the normalised acoustic nonlinearity increases due to a rising of the precipitation volume fraction and the dislocation density in the early stage, and it decreases as a combined result of dislocation change and micro-void initiation in the material. The nonlinear Lamb wave exhibits the potential for the assessment of the remaining creep life in metals. [ABSTRACT FROM AUTHOR]

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]

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: 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]

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: 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: The interaction and coalescence of multiple flaws will significantly influence the service life of components. In this paper, the interaction of two identical semi-elliptical cracks in a finite thickness plate subjected to the remote tension is investigated. The results indicated that interaction of multiple cracks is different between the time-dependent fracture characterized by C *-integral and linear elastic fracture noted by SIF. The magnifying factors of creep fracture are obviously larger than that of the linear elastic fracture cases. Therefore, the current interaction and coalescence rule developed from linear elastic fracture analysis may lead to a non-conservative result when it is used in the assessment of creep crack. At the end, an empirical equation is developed based on the numerical results. [Copyright &y& Elsevier]

Abstract: The existence of multi-axial states of stress cannot be avoided in elevated temperature components. It is essential to understand the associated failure mechanisms and to predict the lifetime in practice. Although metal creep has been studied for about 100 years, many problems are still unsolved, in particular for those involving multi-axial stresses. In this work, a state-of-the-art review of creep analysis and engineering design is carried out, with particular emphasis on the effect of multi-axial stresses. The existing theories and creep design approaches are grouped into three categories, i.e., the classical plastic theory (CPT) based approach, the cavity growth mechanism (CGM) based approach and the continuum damage mechanics (CDM) based approach. Following above arrangements, the constitutive equations and design criteria are addressed. In the end, challenges on the precise description of the multi-axial creep behavior and then improving the strength criteria in engineering design are presented. [Copyright &y& Elsevier]

Abstract: A failure case of the low pressure blades of steam turbine is presented in this paper. The suction side of blades has been quenched to improve the erosion resistance. Cracks with different lengths were found in the quenched region of final stage blades after about 13,200h service. The failure analysis of blades was performed in terms of composition analysis, microstructure and mechanical tests, etc. The yield strength and tensile strength conform to the corresponding standard, whereas the elongation, area reduction and impact toughness are lower than the criteria. From the crack morphology, fractography and composition analysis on the fracture surface, it was found that the failure mechanism of blades is the environment-assisted fatigue fracture. The location of fatigue crack initiation is related with the salient of blades due to the stress concentration. In order to decrease the blade cracking susceptibility, the increment of tempered temperature in both modified treatment and high-frequency hardening was recommended. [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]

Stress corrosion cracking (SCC) growth tests in heat affected zone (HAZ) of 25Cr2Ni2MoV steel welded joint were conducted in 3.5 wt.% NaCl solution at 180 °C. The deflection of SCC towards the hard side of HAZ was observed interestingly, which was ascribed to the combined effect of the graded mechanical and electrochemical properties in HAZ. In addition, the initiation of SCC deflection was related to the crack-tip in-plane constraint. Through the specimen thickness, higher crack-tip out-of-plane constraint on the mid-thickness plane caused a preferential SCC deflection compared to that on the surface. [ABSTRACT FROM AUTHOR]

Axially loaded cyclic tests of a nickel-based alloy were conducted at 550 °C, 630 °C and 650 °C and stress ratios of −1, −0.3 and 0.5 up to high cycle fatigue regime. Multiscale characterizations and quantitative analyses of microstructural damage were carried out to illustrate the physics and mechanics of interior microstructure induced fatigue strength degradation and cracking. Results showed that the fatigue strength increased with the decreases of twin spacing, the size and aspect ratio of carbide, and γ′ phase size. Twin collapse due to slip band-twin interaction was found for the first time and confirmed as a form of crack initiation process, followed by short crack growth forming facets with significant contribution from shear stress. A new mechanistic fatigue life model of interior microstructure induced cracking was established with sound agreement with experimental data. All these combined into a design-manufacturing integrated approach which are capable of ensuring the safe operation and maintenance of service structures in long-life regime. • A shorter twin spacing, a reduced size and aspect ratio of carbide, and a smaller γ′ phase size were found beneficial for higher fatigue strength. • Slip bands-twin interaction induced twin collapse was found for the first time, which was the first step to interior facet induced cracking. • Dislocation slip, the size and location of facet was combined into a fatigue life model to evaluate interior facet induced fatigue failure. • A design-manufacturing integrated approach to anti-fatigue for long-life service structures was established. [ABSTRACT FROM AUTHOR]

Abstract: The aim of this paper was to address the effect of WC content on the tensile properties of WC/Ni composite coatings deposited by laser cladding on 45# steel substrates. The tensile tests were performed in the chamber of a scanning electron microscope. Experimental results showed that the mechanical properties and fracture behavior of WC/Ni coatings were strongly dependent on the volume fraction of WC reinforcing particles in the coatings. The tensile properties of the coating decreased with increasing content of WC particle. When the WC content was relatively high, multiple through-width cracks (TWC) in the coating can be found. The number of through-width cracks in the coating as a function of applied strain was measured. From the in-situ observation of coating surfaces, most of cracks were initiated from the WC particle, and then propagated throughout the coatings. This phenomenon was attributed to the stress concentration in the WC particle or near the WC particle, which was confirmed by using finite element analysis. The present research may provide some insights on the development of designing methodology of composite coatings. [Copyright &y& Elsevier]

• 3D synchrotron and 2D in-situ SEM were used for crack closure determination. • FCG retardation was correlated with crack-tip straining, plastic zone and COD. • Crack-tip shielding effect and strain transfer was observed for the first time. • Load history dependence of FCG and residual compressive strain relaxation were evidenced directly. • Load transfer was proposed as the physics of near-tip straining, crack opening and FCG. • FCG retardation was mainly governed by residual compressive stress. Fatigue crack growth retardation of AZ31 alloy under a single tensile overload was investigated by 3D synchrotron radiation, 2D in-situ SEM and digital image correlation techniques. Results showed a consistent evolution of crack closure level by both 3D and 2D methodologies. The straining and plastic zone ahead of crack-tip, and crack opening displacement behind, which were quantitative indicators of crack-tip shielding, residual stress formation and relaxation, were correlated well with the retardation behavior. Mechanistically, compressive residual stress played a dominant but time-limited role while crack closure was secondary, based on an intrinsic load transfer concept during crack growth. [ABSTRACT FROM AUTHOR]

Highlights: • Different loading mode results in different crack initiation mechanism. • The mismatch between galvanic corrosion and strength affects fracture location. • Stress corrosion cracking sensitivity must be assessed by multiple test methods. Abstract Stress corrosion cracking (SCC) behaviors of 30Cr2Ni4MoV welded joint in simulated environment of low pressure nuclear steam turbine were addressed with different fracture positions in the welded joint by constant loading test (CLT) and slow strain rate test (SSRT). The fracture of the welded joint happened near the fusion line which was ascribed to the interaction between stress and galvanic effect for CLT, while in welded metal with the weakest strength for SSRT. The mismatch between galvanic corrosion and strength of welded joint determined the fracture location and SCC sensitivity of the welded joint when the two different loading modes were applied. [ABSTRACT FROM AUTHOR]

Fatigue crack growth behavior of Inconel 740H and Haynes 282 nickel-based alloys at 20 °C, 700 °C and 750 °C was studied. Results showed increased fatigue crack growth rate for both the two alloys, while increased fatigue threshold in Inconel 740H at high temperatures. The size and distribution of MC carbides, serrated grain boundaries and twins deflected crack growth path. The strengthening γ' phase was interacted with dislocations by Orowan bypass and shear mechanisms at room temperature, while γ' was sheared in Haynes 282 and Orowan bypassed in Inconel 740H alloy at elevated temperatures. The cross slip, dislocation climbing and entanglement reduced the resistance of dislocation movement and accelerated the accumulation of damage. The higher content of Cr in Haynes 282 alloy promoted formation of M 23 C 6 carbide while the lower content in Inconel 740H alloy promoted thicker oxide layer. The Cr content needed careful tuning as it was superior to microstructural factors for higher fatigue threshold at elevated temperatures. [ABSTRACT FROM AUTHOR]

Abstract: The temperature dependence of Beremin model parameters in the ductile-to-brittle transition region was addressed by employing the Master Curve. Monte Carlo simulation was performed to produce a large number of 1T fracture toughness data randomly drawn from the scatter band at a temperature of interest and thus to determine Beremin model parameters. In terms of the experimental data of a C–Mn steel (the 16MnR steel in China), results revealed that the Weibull modulus, m, decreases with temperature over the lower transition range and remains a constant in the lower-to-mid transition region. The Weibull scale parameter, σ u, increases with temperature over the temperature range of investigated. A small sample may lead to a considerable uncertainty in estimates of the Weibull stress parameters. However, no significant difference was observed for the average of Weibull stress parameters from different sample sizes. [ABSTRACT FROM AUTHOR]

Creep damage is governed by different microstructural features (e.g. precipitate coarsening, substructure growth, cavity/crack nucleation and growth) during long term high temperature exposure in 9–12%Cr steels. The dependences of stress states (e.g. equivalent stress and stress tri-axiality) on these damage mechanisms are clarified in this work. Various stress states are produced in FB2 steel notched components with different root radii for creep tests at 605 °C. Results indicate that equivalent stress and stress tri-axiality present different influences on creep damage behavior of notched components at elevated temperatures. The equivalent stress and stress tri-axiality both enhance the precipitation of the carbide, indicating larger mean diameter and higher area of the precipitates. Meanwhile, the equivalent stress plays a more significant role on the coarsening of the substructure of the material than the stress tri-axiality. Creep cavities are mainly found at several hundred micrometers away from the notch root, which seems to be induced by the combining action of equivalent stress and stress tri-axiality. These microstructural degradation can be proved by the distribution of the hardness in the notch region of component accompanied by the variation of stress states. • Dependences of stress states on microstructural evolution and creep damage behavior are clarified. • Equivalent stress and stress tri-axiality present different influences on creep damage behavior of notched components. • Equivalent stress and stress tri-axiality both enhance the precipitation of the carbide. • Equivalent stress plays a more significant role on the coarsening of the substructure than stress tri-axiality. [ABSTRACT FROM AUTHOR]

Physically short fatigue crack growth behavior in the heat-affected zone of a Ni–Cr–Mo–V steel welded joint was investigated in-situ in SEM to reveal effects of crack closure and tortuous crack growth path on crack-tip straining behavior with the help of high resolution digital image correlation technique. The crack growth path and associated fatigue damage at microstructural scale was characterized by electron backscatter diffraction and electron channeling contrast imaging techniques. Results showed that the short crack growth was influenced by both local microstructure and global strength gradient. The short crack could deflect intergranularly when meeting with one large grain due to misfit deformation and strain localization. The crack closure, which was accompanied during loading and unloading in terms of crack surface contact, shielded the real crack-tip and manifested itself well in both compressive strain at crack wake and strain development at crack-tip. It is indicated that strain localization at lathy boundaries and formation of sub-grains was responsible for the fatigue of short cracks. • Physically short fatigue crack grew within HAZ of a welded joint. • Crack closure at crack wake could affect crack-tip straining and evolution. • Crack growth path deflection and intergranular cracking were due to misfit deformation and strain localization. • Fatigue crack growth was related with strain localization at martensitic lath boundaries. [ABSTRACT FROM AUTHOR]