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A large number of key components are widely operated under extreme conditions of high temperature and high pressure in power, nuclear energy, aerospace and other important industrial fields. Microdefects are inevitably generated in these key components during the process of manufacturing. Due to the periodic loading and thermal stress, microcracks are generally initiated from these microdefects at the location with stress concentration. It has been found that the initiation and propagation of fatigue microcracks usually occur prior to 70%–90% of fatigue life. It is difficult to observe the evidence before the formation of macrocracks, which may result in catastrophic accidents. Thus, the risk of sudden failure of key components increases greatly. Therefore, the accurate detection of microcracks is crucial for guaranteeing the safe operation of key components. Based on the diffraction, reflection and transmission, traditional ultrasonic testing method is merely validated to effectively detect macroscopic defects within half a wavelength. The nonlinear ultrasonic testing method is found to be sensitive to microcracks, as high order harmonics, sub-harmonics, DC components and frequency mixing waves can be generated due to the strong nonlinear interaction microcracks and ultrasonic waves. With the outstanding effectiveness to detect microcracks, the nonlinear ultrasonic testing method has attracted much attention in recent years. In this work, the research on detection of microcracks using nonlinear ultrasonic testing method is systematically reviewed in terms of theoretical models, numerical simulations and experimental measurements. Firstly, according to the breathing effect and sliding mechanism of microcrack surfaces, we present several theoretical models of nonlinear interaction between ultrasonic waves and microcracks, namely contact acoustic nonlinearity (CAN), bi-linear stiffness model, nonlinear spring model, Hertz contact theory, and rough contact surface model. The advantages and disadvantages of these theoretical models are discussed comparatively. Two mechanisms are mainly considered for the nonlinear interaction of microcracks and ultrasonic waves, namely the macroscopic opening and closing of crack surfaces and the microscopic contact of rough microcracks surfaces. Secondly, we introduce numerical simulation studies on the nonlinear ultrasonic generation induced by microcracks. The applicability is analyzed for several simulation methods, such as finite element method, spectral finite element method, finite difference method, boundary element method and local interaction simulation approach, and the generation and propagation of nonlinear ultrasonic waves are analyzed as well. The effect of microcrack geometries on the nonlinear ultrasonic waves is then summarized. The microcrack geometries are considered mainly clouding the microcrack length, width, depth, density and direction. Furthermore, we present the experimental measurements on the generation of nonlinear ultrasonic waves induced by microcracks and on the localization and imaging of microcracks based on the nonlinear ultrasonic waves. The generation of high order harmonics, sub-harmonics, DC components and frequency mixing waves was observed in the specimens with microcracks. The nonlinear ultrasonic detection is presented to evaluate fatigue microcracks with various microcrack length, width, depth, density and direction. Based on this phenomenon, nonlinear ultrasonic mixing method, transducer array imaging method and nonlinear ultrasonic phased array method are introduced to localize the microcracks. Finally, further studies on evaluation of microcracks using nonlinear ultrasonic waves are proposed, including the comprehensive physical model for actual microcracks in engineering components, robust detection method, and quantitative evaluation of microcracks and service life.

Although the time-dependent deformation behaviors of microscale materials have been investigated through experiments with uniaxial loading conditions, the influence of the strain gradient has not been clearly clarified due to the lack of appropriate testing methods. In the current study, to investigate the stress relaxation behavior of microscale single-crystal copper (Cu) at room temperature, a quantitative in situ SEM bending experiment is presented using microcantilever specimens of single-crystal Cu. The microcantilever specimens were fabricated using a focused ion beam, and a tungsten (W) layer was deposited onto the front surface to eliminate the error induced by the penetration of the stiff indenter into the metallic specimen. The yield stress of microscale single-crystal Cu is determined to be 445 MPa by a monotonic loading test, showing an apparent size effect, and no strain hardening is observed due to single-slip deformation. On the other hand, the stress relaxation behavior of the microscale single-crystal Cu consists of both a continuous stress relaxation and an abrupt stress decrease due to a strain burst. The activated volume in each dwell stage is obtained by thermodynamics theory and is found to be mainly related to the abrupt stress decrease. The value of the activated volume indicates that the continuous stress drops in the 1st and 2nd dwell stages are attributed to the evolution of dislocation structures by the single slip on system B4, while the dislocation pile-up near the neutral plane leads to the dominance of cross slip on the stress relaxation behavior in the bending plateau. The proposed microcantilever bending experiment is applicable to explore the time-dependent deformation behavior of small-scale materials.

Selective Laser Melting (SLM) was employed for Additive Manufacturing (AM) of nearly equiatomic CoCrFeNiMn high-entropy alloy (HEA) and its matrix composite parts reinforced by 12 wt% nano-sized TiN particles (TiNP). HEA matrix microstructure of printed TiNP/HEA was remarkably refined and isotropic, rather than anisotropic coarse-grains of printed HEA in absence of TiNP. The ceramic reinforcements achieved a relatively uniform distribution in HEA matrix to contribute to the pinning effects for strengthening. High specific strength and good sliding-wear resistance of printed TiNP/HEA were achieved. The as-printed Gyroid architecture of TiNP/HEA exerted an elevated compression-resistance. The introduced TiNP, as a large number of nanoscale nucleation sites during SLM, benefits the formation of nearly equiaxed and ultrafine grains of HEA matrix for strengthening.

Bolted flange joints are indispensable components in process industries due to the good sealing, assemble and disassemble capacities. Generally, the flange rigidity characterized by the rotation angle is a key index to evaluate the sealing tightness of flange joints. However, the rotation angle of flange is usually too small (less than 1°) to monitor during the assemble and operation stages. Accordingly, a novel flexure magnifying mechanism is designed to measure the small rotation angle of flange joints under internal pressure and external bending moment. The magnification factor and calculation approach of the flexure amplification mechanism are deduced and verified by experimental data and finite element simulation. Results indicate that the proposed measuring apparatus has good performance to monitor the maximum rotation angle. It is of great interest that the measured location of the maximum rotation angle is in good agreement with that in the experiment, and the average error is 7.3%, which is acceptable for practical application. Additionally, the leakage rate at the top of flange joints slowly and almost linearly increases with the increment of external bending moment ascribing to the decrease the gasket stress near the top of flange joints.

Air-coupled ultrasonic testing (ACUT) has many advantages over other non-destructive testing (NDT) methods, among which the most outstanding one is completely no contamination of the sample. Therefore, it can be applied in various conditions where liquid coupling media is not suitable. With the development of ACUT, it is found that mismatch between air and solid is one of the biggest difficulties confronting researchers, and electromechanical film (EMFi) with low acoustic impedance can be a promising solution when used as vibration element in air-coupled ultrasonic transducers. In this contribution, one planar and two spherically focused EMFi-based transducers were designed, fabricated, and their properties were characterized through frequency response test as well as focal length test. Then imaging experiments of PMMA samples were performed to demonstrate the possibility and feasibility of EMFi transducers in non-contact testing.

This paper presents the study of creep-fatigue interaction damage and creep rupture limit for 9–12% Cr steam turbine rotor under coupled cyclic thermal-mechanical loadings that alter in phase and out of phase. The investigation is implemented using the Linear Matching Method (LMM) and based on the newly developed creep-fatigue and creep rupture evaluation procedures. Latest experimental creep data of FB2 which belongs to 9–12% Cr heat-resistant steel family is employed to calculate creep-related damage and creep rupture bearing capacity of steam turbine rotor. Various factors that affect creep and fatigue damage of steam turbine rotor are analyzed and discussed, including dwell period and rotating speed of rotor. From the parametric studies, the damage locations and cycles to failure induced by creep and fatigue mechanism are presented respectively. Moreover, through creep rupture analyses for varying desired fracture time, the novel creep rupture curves under multi-type double-cyclic-loading conditions are given and further compared with cyclic plasticity failure curve. These surrogate-model analyses offer a deep understanding of structural responses of steam turbine rotor under long-term high temperature operation and provide critical loading conditions to be referenced for smooth running.

The rate-dependent ratchetting characteristics of 35CrMo alloy under repeated uniaxial tension considering different stress amplitudes were studied at 500 °C. It is found that the applied stress rate and stress amplitude influence the accumulated plastic deformation greatly. The existing visco-plastic ratchetting constitutive model can well predict those features considering the effect of stress rate, but not so well when the stress amplitude effect is involved, especially for the cases of relatively low stress amplitude. Accordingly, a modified rate-dependent ratchetting constitutive model has been developed, which can well predict the ratchetting deformations in a wide range of stress amplitude and stress rate. Furthermore, the modified rate-dependent ratchetting constitutive model can be used to assess the ratchet boundary of the alloy with relatively high accuracy.

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.

Fatigue damage and micro-cracks growth have been proved to be closely related to the nonlinear effect of ultrasonic Lamb waves propagation in materials. In this paper, the relationship among fatigue life, length and width of micro-cracks, and the acoustic nonlinearity parameter was established. The variation of the acoustic nonlinearity parameter caused by fatigue damage was analyzed based on micro-cracks growth. Considering the multimodal nature of nonlinear Lamb waves, we selected the low-frequency mode pairs S0–s0 and S1–s2 to evaluate fatigue damage in 7075 aluminum alloy plates. Both FE simulations and experiments show that the mode pair S1–s2 is more suitable than S0–s0 for the assessment of fatigue damage. It was found that with increasing fatigue cycles, the relative acoustic nonlinearity parameter A2/A12 firstly increases because the micro-cracks length extends while the micro-cracks width does not change much. The relative acoustic nonlinearity parameter A2/A12 reaches its peak at about 60% of the fatigue life. Then the relative acoustic nonlinearity parameter A2/A12 decreases because of the rapid increase in the micro-cracks width. Our findings clarify the mountain-shape curve between the relative acoustic nonlinearity parameter A2/A12 and the fatigue cycles.

A mechanistic based cavitation model that considers nucleation, early-stage growth and sintering under creep-fatigue interaction is proposed. The number density of cavities ρ and their evolution during multi-cycle creep-fatigue loading are predicted. Both the cavity nucleation and early-stage growth rates, controlled by grain boundary (GB) sliding mechanism during the tension phase, are formulised as a function of local normal stress σ. The cavity sintering that occurs during the compression phase is described as a function of σ, but the mechanism switches to the unconstrained GB diffusion. By examining various load waveform parameters, results provide important insights into experimental design of studying the creep-dominated cavitation process under creep-fatigue interaction. First, creep-fatigue test with initial compression will promote higher ρ value compared to that with initial tension, if the unbalanced stress hold time in favour of tension is satisfied. Second, the ρ value does not have a monotonic dependence on either the compressive hold time or stress level, because of their competing effect on nucleation and sintering. Third, the optimum value of stress variation rate exists in terms of obtaining the highest ρ value due to sintering effect.

Multifunctional sensing devices with high flexibility, high sensitivity, and scalable fabrication are inevitable components of Internet of Things (IoT) for human-machine interfaces, structural health monitoring, and soft robots. Herein, high-performance flexible sensor arrays using carboxymethylcellulose (CMC) and its composite were developed for mechanical and thermal stimuli detection by laser direct writing. CMC contains abundant carbon precursors for strain-sensitive laser-carbonized CMC (LC-CMC), while the incorporation of graphene oxide (GO) into CMC leads to the formation of thermal-sensitive laser-carbonized GO/CMC (LC-GO/CMC). The LC-CMC-based strain sensor delivers gauge factors of 487.7 (strain8.5%) and 8557 (8.5%strain14%), with long-term stability over 10 000 cycles. With 0.2 wt % GO, the LC-GO/CMC-based device provides a temperature coefficient of resistance of -0.289% °C

In this work, improved creep-fatigue life prediction models based on time fraction (TF) and ductility exhaustion (DE) rule are established for creep-fatigue life prediction of components at elevated temperatures. Cyclic softening/hardening and stress relaxation models for cyclic softening/hardening material are incorporated in the model. Materials data conducted by authors and collected from references are employed for validations. Results indicated that the improved models proposed could present more realistic evolution behavior of materials than the half-life models. The differences of the predicted lives based on the improved models and half-life models are dependent on cyclic behaviors of materials. The predicted life by the improved model is lower than that gained by the half-life model for cyclic softening steels, while it is higher than that by the half-life model for cyclic hardening steels. In addition, the DE rule could overcome the influence of cyclic softening/hardening on creep-fatigue life prediction more significantly than the TF rule.

An accurate stress estimation for creep evaluation is essential for the long-term safe operation of components at elevated temperature. In general, the section-dependent parameter Kt is defined in elastic analysis routine to account for the reduction in extreme fiber bending stress due to the effect of creep action. A constant value is provided for each cross section in ASME III-5 code, but this may induce underestimation or overestimation of the representative stress of the component due to the relevance of parameter Kt to many factors. Therefore, it is essential to re-examine the validity of the parameter Kt in ASME III-5 code for an adequate creep evaluation. In this work, numerical analyses on beam models with a rectangular section are conducted to calculate parameter Kt, and effects of related factors to the parameter Kt are included. The results reveal that the parameter Kt is closely related to the design life, loading level, operation temperature, and loading combinations. It can be higher than that for the pure bending moment (or tensile loading) with the introduction of the tensile loading (or the bending moment). Meanwhile, the parameter Kt may be higher than the value (i.e., 1.25) in ASME III-5 code for a longer design life, a higher loading level, and a higher operation temperature, inducing an overestimation of the representative stress. On the contrary, a smaller value than 1.25 can be found, inducing an underestimation of the representative stress for creep evaluations.

Extensive finite element analyses of cracked pipes with different crack sizes and orientations have been conducted to investigate effects of creep properties of materials on the unified creep constraint parameter Ac. The results show that the constraint parameter Ac is independent on Norton’s coefficient A, and it is only affected by the creep exponent n of materials. For a given crack size, with increasing n, Ac decreases and constraint level increases. The Ac of lower constraint cracks is more sensitive to n. The unified correlation equations between Ac and n have been obtained for cracked pipes with a wide range of crack sizes and constraint levels. They may be used to estimate the constraint parameter Ac at different positions along the crack fronts in cracked pipes made of materials with different n values. The two-parameter C*-Ac approach for assessing creep life of cracked pipes has also been discussed.

The stress corrosion sensitivities of 25Cr2Ni2MoV, 26NiCrMoV10-10 and 30Cr2Ni4MoV low-pressure rotor steels in simulated nuclear steam turbine operation condition were investigated by slow strain rate test (SSRT), and the stress corrosion cracking (SCC) mechanisms were studied by optical microscope (OM), scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). Results revealed that the SCC sensitivity of 25Cr2Ni2MoV steel was highest in 3.5wt.%NaCl solution at 180°C, while the SCC sensitivity of 26NiCrMoV10-10 steel and 30Cr2Ni4MoV steel are similar. The SCC sensitivity of CrNiMoV steam turbine rotor steels could be decreased by the increase of Ni element and the decline of mechanical intensity. Cracks initiate from metal surface and then propagate to the inner metal, which showed a form of transgranular cracking.

Stress and strain controlled low cycle fatigue of the modified 9–12% Cr steel with the hierarchical arranged lath martensitic structure were conducted. We found that, apart from microstructure recovery in lath structure, an additional mechanism of reverse avalanche of low angle boundary, associated with the burst-like plastic deformation is responsible for the accelerated softening during stress cycling. These microstructural evolutions can be explicitly represented via deriving different dislocation evolution laws in terms of dislocation annihilation and storage events. Accordingly, a new model is proposed involving the microstructural evolution. Results indicate that the accelerated softening behavior under stress cycling can be reproduced very well by the present model with a very limited number of adjustable parameters. In addition, the model can capture the features of cyclic response and microstructural evolution under both the strain and stress cycling over a wide range of amplitudes.

In this paper, the selective laser melting (SLM) forming method was combined with the requirement of boiling heat transfer for porous structure to study the forming process and boiling heat transfer characteristics of porous structure of G-surface lattice formed by SLM. 316L stainless steel powder was used to rasterize the G-surface structure. Besides, porosity and surface morphology of G-surface lattice were analyzed. Deionized water was used for boiling heat transfer experiment to study the effect of technological parameters on boiling heat transfer enhancement. On this basis, the bubble escape modes in different stages of boiling heat transfer of G-surface lattice were considered, and the delay of the critical heat flux caused by G-surface lattice was explained. This study provides a useful reference for the design of lattice heat cooling structure.

Notched structures widely exist in steam turbine components in fossil power plant, e.g., the groove of the rotor, etc. Previous studies indicate that creep failures occur at the groove of the rotor or the adjacent regions. Thus, it is essential to study the creep failure behavior of the notched structures for the safe operation of the system. In this work, creep tests of the simulated steam turbine component have been conducted at the temperature of 605 °C, and the creep-damage constitutive equation is used to track the evolution behavior of creep strain and damage of this component. The influence of structural and loading configurations on creep failure behavior of the component is discussed. Results demonstrate that the shearing failure at the contact area between the blade root and the rotor is observed for the tested component, while the cracking at the groove of the rotor is not found. Creep-damage constitutive equation employed in this work could provide an adequate solution of the simulated component. Parametric studies indicate that creep crack initiation and creep failure of the simulated components may occur at the contact region (i.e., between the blade root and the rotor) and the groove of the rotor, which is closely related to structural and loading configurations of the components.

Low stress interrupted creep test, as an interim compromise, can provide essential data for creep deformation design. However, there are no clear guidelines on the characterization of the terminating time for interrupted low-stress creep test. To obtain a suitable terminating time in terms of economy and effectiveness, long-term creep strain data of 9%Cr steels are collected from literatures and their creep deformation characterization is analyzed. First, the variations of normalized time and strain of each creep stage with the stress level are discussed. Then, the effect of the terminating time on final fitted results of Norton–Bailey equation is estimated. Third, the relationship between demarcation points at different creep stages and minimum/steady-state creep rate is analyzed. The results indicate that when the creep rupture life is considered as an important factor for creep design, the tertiary creep stage is of greatest significance due to the largest life fraction and creep strain fraction at low stress level. However, the primary and secondary creep stages are of great significance for design due to their larger contribution to 1% limited creep strain. And the long-term secondary creep data could be extrapolated by combining the primary creep strain data obtained from interrupted creep tests with the time to onset of tertiary creep derived from a similar Monkman–Grant relationship.

Selective Laser Melting (SLM) was applied to Additive Manufacturing (AM) of carbide ceramic particulate reinforced 316 L stainless steel matrix composites. The dense and ultrafine-grained Austenite matrix with uniformly distributed and nano-sized VCx particulate reinforcements were obtained according to the tailored SLM process parameters and 3 wt% submicron-sized V8C7 ceramic particle addition into 97 wt% 316 L powder via ball-milling treatment. The influences of SLM process parameters on the SLM printed composites in terms of density, surface roughness, phase constitution, microstructure characteristics and mechanical tensile performance were investigated in detail, comparing with the SLM printed 316 L in absence of vanadium carbides. Based on the electron back-scattered diffraction analyses, the number of grains with grain size less than 2 µm accounted for more than 90% in the printed composites. The formation mechanisms of the printed composites with their unique microstructure characteristics were elaborated. The room-temperature ultimate tensile strength of the printed 316 L matrix composites achieved higher than 1400 MPa, which was more than twice as high as that of printed 316 L stainless steel. It contributed substantially to the strengthening effect on the printed composites of the ultrafast laser-induced and highly nonequilibrium melting-solidification nature of SLM process, the dispersed carbide ceramic particulate reinforcements, the nanoscaled nucleation sites of VCx for the equiaxed Austenite grains, and the inhibition against Austenite matrix growth by particulate dispersedly distributed along Austenite grain boundaries.

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. Keywords: Fatigue, Life prediction, Stress field intensity, Multi-axial stress state

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.

Geometrical discontinuities (such as holes and grooves) widely exist in many components, which operate at elevated temperatures. Creep assessment of the geometrical discontinuities is essential for the safe operation of the system. In general, creep evaluations are conducted from the stress-based and strain-based strategies, but comparative studies on the extent of conservatism of these two design strategies are rarely included. In this work, creep test data of the notched components made of 9–12% Cr steel conducted by authors and that of 9Cr–1Mo steel collected from published works are employed for comparative evaluations. Results indicate that strain-based and stress-based strategies are both relatively conservative for notches in metal materials, and the conservatism of the assessment strategies increases with the notch acuity ratio of the component. The differences of the conservatism for strain-based and stress-based strategies are dependent on the notch acuity ratio and the ductility of the materials. For a blunt notch, the strain-based strategy is more conservative than the stress-based strategy for materials mentioned, while the dominant assessment strategy is dependent on stress redistribution performances of components with a sharp notch.

Ratcheting and low cycle fatigue are failure mechanisms observed in components subjected to cyclic temperature and mechanical loads. Ratcheting is a global failure mechanism which leads to an incremental plastic collapse of the component whereas low cycle fatigue is a localized mechanism which leads to crack initiation. It is exacerbated by grooves, notches and changes in the geometry of the component. To estimate the remaining life of the component and predict its failure mechanism, it is important to understand how it responds to various combinations of cyclic loads. This paper includes investigation of the ratchet limit and the plastic strain range, which is associated with the low cycle fatigue, of a circumferential butt-welded pipe by using the ratchet analysis method which includes Direct Steady Cycle Analysis (DSCA) within the Linear Matching Method Framework (LMMF). The pipe is subjected to a constant internal pressure and a cyclic thermal load. The investigation is carried out by varying 1) material properties of the weld metal (WM); 2) ratio of inner radius to wall thickness; 3) weld geometry. Within the specified ranges, yield stress and the ratio of inner radius to wall thickness affect the ratchet limit curve. The cyclic thermal load plays a crucial role compared to the internal pressure in influencing the ratchet limit curve. It is observed that the pipe experiences thermal ratcheting in the absence of pressure load at lower yield stress values of the WM. The results obtained are combined to create a limit load envelope, which can be used for the design of welded pipes within the specified ranges.

High-performance flexible pressure sensors are highly demanded in wearable electronics for various applications. Herein, a flexible pressure sensor was designed using carbon nanotube (CNT)-wrapped polydimethylsiloxane (PDMS) microspheres (MSs). Compared to the device with flat configuration, the sensor with CNT-wrapped PDMS MSs has an enhanced sensitivity of −0.111 kPa−1, with a lowest detectable pressure of 0.02 kPa, response time approximately 0.1 s, as well as a long-term dynamic stability more than 10,000 cycles. The sensors with CNT-wrapped PDMS MSs show the potential to be integrated into wearable electronics for real-timely monitoring the physiological signals.

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.

In this study, a laser-microengineered wearable pressure sensor was developed. The sensor offers a sensing range from 0.005 to 50.0 kPa, a sensitivity approximately -0.107 kPa−1, and high durability (>10,000 cycles). The finite element simulation shows that the external forces are amplified and transmitted by the laser-engineered microstructures, contributing to the enhanced sensitivity of the device. The laser-microengineered sensors are promising to be applied in the electronic skin for detecting various external stimuli.

Wearable pressure sensors fabricated with sandpaper-molded carbon nanotube-polydimethylsiloxane (CNT-PDMS) electrodes are developed for electronic skins. Compared to the lithography method, widely adopted to prepare pressure sensors based on microstructures, the sandpaper molding method is much more cost-effective and scalable. The microhumps templated from the sandpaper are suggested to amplify the external stimuli during device deformation, leading to an improved sensitivity that is ∼20 times of the one without microstructures. Moreover, the detectable range of the sandpaper –molded device covers pressures from 5.0 Pa to 50.0 kPa, with a stimuli-response time around 0.19 s and a cycling stability more than 5000 cycles. The sandpaper-molded devices have been explored for external stimuli detection such as pressing, bending and twisting, and human health monitoring including phonation and pulsing.

This paper presents an effective technology that could simultaneously enhance the interfacial shear strength (IFSS) and monitor the interfacial stress state between glass fiber and epoxy vinyl ester resin (GF/epoxy). Muiti-walled carbon nanotube (MWCNT) was added to aqueous surfactant solution and dispersed by ultrasonic. Subsequently, MWCNT was deposited on GF surface by physical vapor deposition. The results show that the sensing performance of the developed sensor was dependence on MWCNT solution concentration, interface length, ultrasonic dispersion duration, and immersion cycles. Fiber-bundle pull-out tests show that IFSS of GF/epoxy was enhanced by incorporating MWCNT into the interphase. By measuring resistance change of the MWCNT sensor, the interfacial evolution behaviour was monitored during the pull-out test. The results indicate that the presented technology can be successfully used for in-situ sensing the accumulated interfacial damage and simultaneously enhancing the IFSS of GF/epoxy composites.

A double-lap bolted composite joint was designed. In the joint, the base material was woven glass fibre–reinforced epoxy (WGF/Epoxy) composite, and the outer plates were WGF/Epoxy, woven carbon fibre–reinforced epoxy (WCF/Epoxy) and WGF/WCF/Epoxy hybrid composite laminates, respectively. Fundamental mechanical properties of the composite panels were determined. Flexural testing of the designed bolted joints was performed. Scanning electron microscopy was used to compare the damage modes. The interlaminar curves could be divided into different stages. The flexural load–displacement curves had progressive damage characteristics. The strengths of joints with hybrid composite panel as outer plates were between those of pure specimens, and hybrid composites with WGF as contact surface had a positive hybrid effect.

Axially loaded push-pull cyclic tests of a precipitation-hardened stainless steel with different sampling orientations were conducted in high cycle and very high cycle fatigue regimes. Results showed apparent fatigue anisotropy with non-metallic inclusions dominating crack initiation behavior. A fatigue lifing model was developed by combining size, location and shape of inclusions into a new form of Z parameter to rationalize the orientation effect. Using a multi-scale and full-field approach, the inclusion-induced interior cracking mechanisms were found to be associated with inclusion-microstructure interaction resulted plasticity. Micro-hardness at the cracking site was the lowest on the fracture surface, and surrounding microstructures showed formation of small grains with clear interfaces. The fine granular area was characteristic of several nano-scale fine grains formed in terms of dislocation cell structures by martensitic laths breakdown. The coalescence of interfaces or micro-crackings finally became interior early fatigue cracks. The mechanistic modeling of “fragmentation of martensitic laths and formation of dislocation cells” revealed a microstructure-dependent crack initiation and stage I growth for interior fatigue cracking. All these inform the significance of combining metallurgical and processing factors in designing against fatigue of engineering materials.

To evaluate the fracture life of steam turbine HP-IP rotors more accurately, an understanding of the creep damage effect on creep cracking performance of service-exposed rotor steel, and a proper creep crack growth (CCG) model considering the creep damage effect, is essential. In the present work comparative CCG tests were carried out on virgin and rotor steel that had been in service for 16 years, to examine the differences in creep crack microstructure and CCG rate between these two specimen types. Test results showed that the CCG rate of service-exposed steel is accelerated by creep damage due to metallurgical deterioration. Based on the CCG model derived by Webster, an improved CCG model is proposed by replacing the constant exponent with a creep-damage-related variable. To validate the improved model, a comparison of da/dt vs. C* between experiment data and the CCG models was conducted. The predictive result of the improved model is in better agreement with the experiment results than the classical CCG and Webster models for the service-exposed rotor steel. With decrement of C* value within the improved CCG model, the remaining life of HP-IP rotors may be predicted more accurately.

Cross-weld and pure base metal specimens machined from dissimilar welded joints of 9%Cr and CrMoV steels were cyclically loaded up to very high cycle fatigue at 500 °C under load ratio of zero. Results showed fatigue strength of the welds was greatly weakened, lower than base metals. A transition of failure mechanisms from geometrical soft zone in heat-affected zone of CrMoV steel to micro-defects induced cracking at weld metal was responsible for decreasing fatigue strength reduction factor with the increasing of fatigue lifetime. A sound fatigue design requires careful selection of filler metal and consideration of soft zone and micro-defects.

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.

High performance antireflection (AR) coating is essential to the conversion efficiency ( η ) enhancement of PV devices. In this work, an AR structure based on syringe-shaped ZnO nanorod arrays was obtained by two-step aqueous solution method. The morphology of the syringe-shaped nanorods can be easily regulated by changing experiment parameters of each growth step. With this structure, the surface reflection of multi-crystalline silicon solar cell was effectively suppressed below 5% over a wide range of wavelengths. Meanwhile, the short-circuit current density and η of the solar cell were increased up to 34.98 mA/cm 2 and 17.16%, enhancements of ~37% and ~41% from those on bare surface. These results are attributed to the ultrasharp tips on the tops of nanorods can make the effective refractive index gradual increasing from air to the bottom of ZnO nanorods, which leads to enhanced light trapping on the solar surface.

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

The combination of air-coupled ultrasonic testing (ACUT) and ultrasonic Lamb wave is featured with long-distance propagation and high sensitivity to discontinuities, which is a promising method for rapid and accurate inspection of plate-like materials and lightweighted structures. However, dispersive nature of Lamb wave, signal attenuation plus inevitable noises would lead to low signal-to-noise ratio (SNR). To address this problem, phase coded excitation and pulse compression technique are proposed in this paper to achieve higher SNR by over 10 dB in received signals. 13-bit and 1-carrier-period Barker code is employed as both main lobe peak and Peak Side-lobe Level (PSL) are relatively high. It is demonstrated that A0 mode Lamb wave has good localization ability for defects based on these SNR-enhanced signals. Furthermore, Damage Index (DI) and modified Reconstruction Algorithm for the Probabilistic Inspection of Damage (RAPID) are applied to realize ultrasonic imaging based defect evaluation. Results show that the imaging results agree well with the actual artificial defects in terms of size and shape. Lamb-wave-based air-coupled ultrasonic testing, combined with DI and ultrasonic imaging algorithm, could be a potential way in the NDT of lightweighted structures.

Ultrasonic guided waves are desirable technologies for damage monitoring in large structures such as pipes and pressure vessels since they could propagate over a long distance with low attenuation. However, due to the complex scattering of guided waves, it is a great challenge to extract the damage information when using guided wave-based techniques to monitor the pressure vessels equipped with accessories. As an attempt, we investigated the interactive behavior between guided waves and pressure vessel nozzles. Ensemble empirical mode decomposition, fast Fourier transform and ellipse-based damage localization algorithm are adopted to perform the signal processing and damage localization analysis, respectively. A sensor array configuration optimization method is presented to mitigate the nozzle effects on the propagation of guided waves. The optimal number of sensors is determined experimentally, and the sensor array placement is optimized by a Particle Swarm Optimization algorithm. The accurate experimental damage localization results in the pressure vessel and curved plate indicate the effectiveness of our methods.

Molecular dynamics simulation (MD) of the observed stepwise oxidation of core-shell structured Al/Al2O3 nanoparticles is presented. Different from the metal ion hopping process in the Cabrera-Mott model, which is assumed to occur only at a certain distance from the oxide layer, the MD simulation shows that Al atoms jump over various interfacial gaps directly under the thermal driving force. The energy barrier for Al ionization is found to be increased along with the enlargement of interfacial gap. A mechanism of competition between thermal driving force and ionization potential barrier is proposed in the interpretation of stepwise oxidation behavior.

Non-collinear wave mixing recently has been proposed to detect and localize micro-damage in materials. It is proved sensitive to the interaction angle α of the incident waves. In this work, the relationship between the acoustic nonlinearity parameter χ and the α is studied by numerical simulations and experimental measurements based on the nonlinear interaction of two shear waves. A single-peak change in the normalized acoustic nonlinearity parameter χ ′ of the mixing wave versus the incident angle is observed from numerical simulations and then is verified by experiments. The results show a -6 dB decrease of χ ′ corresponds to a deviation of about 4° in the incident angle. Meanwhile, the detection and localization of plastic deformation is also conducted on an aluminum alloy based on the scanning of non-collinear wave mixing, in which the distribution of the acoustic nonlinearity is similar to that of the plastic strain.

This paper presents physical and experimental studies of the static component of Lamb waves (SCLWs) induced by partially closed cracks. The generation mechanism of SCLWs was investigated according to the nonlinear interaction between Lamb waves and partially closed cracks with various lengths, widths and orientation angles in finite element simulations. It was found that the SCLWs are more sensitive to the crack length, width and orientation angle as compared with the second harmonic Lamb waves. Fatigue crack evolution was further experimentally detected in aluminum alloy plates using SCLWs. Consistent with the simulation results, the acoustic nonlinearity parameter increases monotonically with increasing fatigue crack length. We show that the SCLWs are sensitive to partially closed cracks and can serve as a potential method to quantitatively evaluate the evolution of partially closed crack in metal plates. The findings of this study pave the way for the detection of partially closed crack using SCLWs in the field of nondestructive evaluation and structural health monitoring.

Limit load is an important parameter in structural integrity assessment of cracked components. In this work, the limit loads of an actual dissimilar metal welded joint (DMWJ) for joining safe end to pipe-nozzle of a reactor pressure vessel have been investigated based on extensive three-dimensional finite element analyses. The effects of a wide range of crack sizes and locations on limit loads have been analyzed. The results show that for shorter and shallower cracks, the limit loads are mainly determined by global plastic yielding of safe end material with the lowest yield strength, and the crack sizes have nearly no effect on them. For longer and deeper cracks, the limit loads are mainly determined by general yielding of the ligament area ahead of a crack. With increasing crack length or depth, the limit load decreases. With moving crack locations from the pipe-nozzle through buttering and weld metal to safe end, the limit load decreases. The normalized limit load solutions for the DMWJ are given.

Axially loaded cyclic tests of a precipitation-hardened martensitic steel at different stress ratios in 3% NaCl solution and steam environment were conducted up to very high cycle fatigue regime. In-depth fracture surface observation, quantitative characterization of microstructural damage, and theoretical modeling were carried out to illustrate the physics and mechanics of micro-defect induced interior cracking. Results showed apparent environmental effect on fatigue strength and crack initiation morphology. The fine granular area was observed for the first time in environmental media, heterogeneously distributed around micro-defect, and was found dependent on local fracture mode with a less probability in the case of faceted failure. The formation of fine granular area was confirmed as a result of microstructural damage with significant contribution from cyclic stress amplitude, and could be assisted by hydrogen speeding up lath martensites breakdown. A chemo-mechanical model of interior cracking was finally established based on the concept of interaction of inclusion, matrix, and environment induced plasticity. All these underpin a general fatigue damage law in micro-defects assessment for long-life structural integrity.

The bending stress relaxation of microscale single-crystal copper was studied by in situ SEM bending experiments to elucidate the time-dependent plasticity involving the effect of the strain gradient and the sample size. Three specimens with varied heights of square cross sections were fabricated using a focused ion beam . Interestingly, an obvious bending plateau was observed in the load-displacement curves. The calculated flow stress on the cross section indicated an inverse relationship between the specimen size and strength. In addition, the proportion of decay stress is approximately 2.7% for the beam with a height of h = 1.82 μm and 13.9% for that of h = 0.94 μm in the bending plateau. Moreover, during the dwell period , a sudden stress drop occurred, which is ascribed to the strain burst or stress jerky during the deformation of single-crystal metals. Microbending is usually related to a significant increase in geometrically necessary dislocations (GNDs), which is supposed to improve the resistance for further deformation. However, the bending plateau implied perfect plastic flow and an evolving balance in the microstructures . On the other hand, thermal activation theory was employed to interpret the time-dependent plasticity. The apparent activation volume of the dwell periods in the bending plateau suggested that cross slip dominates the inelastic strain rate evolution. Finally, the inelastic strain rate for stress relaxation was derived from the curve of decay stress, and it was inversely proportional to the height of the microbeam, which is in complete contrast with the trend observed in the previous compression tests of micropillars. In current microscale bending specimens, the thinner beam containing fewer initial dislocation sources exhibits a larger creep ductility with the assistance of GNDs.