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Abstract: Compression tests were conducted on a ceramic-fiber-reinforced SiO2 aerogel at high temperature. Anisotropic mechanical property was found. In-plane Young's modulus is more than 10 times higher than that of out-of-plane, but fracture strain is much lower by a factor of 100. Out-of-plane Young's modulus decreases with increasing temperature, but the in-plane modulus and fracture stress increase with temperature. The out-of-plane property does not change with loading rates. Viscous flow at high temperature is found to cause in-plane shrinkage, and both in-plane and out-of-plane properties change. Compression induced densification of aerogel matrix was also found by Scanning Electron Microscope analysis. [Copyright &y& Elsevier]

Abstract: Ambient creep tests at constant stresses of 0.05 and 0.1MPa were conducted on a ceramic fiber-reinforced SiO2 aerogel for more than 240h. Experimental result shows that there are creep deformations even at very low stress level, and the creep strains finally reach constant values. High temperature creep experiments were also investigated at constant stresses of 0.05 and 0.2MPa for 1800s. The creep strains in 1800s at 900°C are 20–69 times higher than that at 300°C. The creep behavior could be well described by the power-law creep model. Scanning Electron Microscope (SEM) analysis was also conducted to understand the micro mechanism of the creep behavior. Thermal softening of aerogel nanoparticles to generate larger ones was found to be the main reason for the change in mechanical properties at high temperature. [Copyright &y& Elsevier]

The present study investigated the potential effects of seawater acidification on the taxonomic structure and diversity of nematode communities using a microcosm experiment. Nematode samples for the microcosm experiment were collected from the low tidal zone of two sandy beaches with different sediment compositions (medium sand vs. very fine sand) in Qingdao (China). In the microcosm, nematode communities were exposed to nine experimental treatments comprising two pH levels for 56 days: 8.0 (ambient control) and 7.3. Communities were exposed for 0, 7, 14, 28, or 56 days. Results showed that the most distinguishable differences in nematode community structure and diversity indices were caused by sediment type. Reduced pH changed the taxonomic structure of nematode communities in medium sand sediments. An increase in species with higher tolerance to lowered pH occurred as a response and resulted in increased diversity in medium sand sediments. Nematode communities in finer sediments appeared less sensitive to reduced pH. • Reduced pH changed nematode community composition in medium sand sediment. • Reduced pH increased nematode diversity in medium sand sediments. • Proportion of nematodes with higher tolerance to lowered pH increased in medium sand. • Nematode communities in finer sediments appeared less sensitive to reduced pH. [ABSTRACT FROM AUTHOR]

Abstract Kinematic hardening in metals is widely correlated with various cyclic plastic behaviors. Inspired by the classical dislocation pile-up model and the finite potential well concept in quantum mechanics, a hypothetical dislocation well model is proposed to describe dislocation behavior on a slip plane. In this model, dislocations are also generated in pairs from the source, gliding along opposite directions and interacting with well sides, resulting in impeded dislocations forming kinematic hardening and tunneling dislocations passing through the well. Given the point symmetries of dislocation generation at the source, interaction with the well sides and annihilation in the sink, signed scalars are imported to quantify the impeded and tunneling dislocations and their corresponding shear strains. A common power form rule for tunneling shear strain and a linear hardening law for impeded shear strain are introduced for validation. By assigning additional axisymmetry in the model, the kinematic hardening response is proven to be consistent with the Ohno-Wang models under uniaxial loading. Moreover, the multiple discrete kinematic hardening components are expressed by decomposing the plastic strain into several parts with different tunneling attributes but identical kinematic hardening moduli, and can be further extended into a continuous distribution density form in the total shear strain components. Apart from the path-independent constant density distribution in the total shear strain components, dislocation well sides as stacks in which shear strain components can be stored in order and recovered in the reverse order are also introduced. Shakedown behaviors with constant distribution density in total shear strain components and dislocation well sides or source as stacks, are explored in detail, respectively. In addition, a general non-axisymmetric dislocation well is also proposed and shakedown behavior under the same situations is exploited accordingly. Meanwhile, ratcheting behavior in the non-axisymmetric dislocation well model with varying attributes caused by the tunneling process is compared with that in the usual axisymmetric one. Finally, the cyclic responses of polycrystals in this hypothetical dislocation well model are investigated. Grain boundary effect is also taken into account additionally by associating the impeded shear strain tolerance with the distance to nearest grain boundary. Highlights • Plastic shear strain is decomposed into the impeded and tunneling parts in the hypothetical dislocation well model. • The cyclic plastic response is consistent with the Ohno-Wang models under the uniaxial load. • The multiple discrete kinematic hardening components are extended into continuous distribution form. • Asymmetric tunneling attributes are further proposed in the dislocation well model. • Grain boundary effect is simulated by assuming the impeded strain tolerance as a function of distance to the grain boundary. [ABSTRACT FROM AUTHOR]

• Failure mechanisms of a stitched ceramic sandwich structure were investigated using DIC. • Progressive damage analysis was conducted based on micro mechanisms and macro behaviors. • Several simplified full models were proposed for simulations under complex loads. • Sutures can be ignored in bending simulations, while they are necessary for vibration analysis. Stitched ceramic sandwich structures attract much attention in the aerospace field due to their advantage of heat insulation/load-bearing integration. In this study, the industrial camera and strain gauges were applied to perform out-of-plane tensile, compressive, interlaminar shear and 4-point bending tests of a stitched sandwich structure. Different specimens were fixed on the corresponding fixtures of a universal testing machine, and the loading rates referred to related standards. The images were captured at 1 Hz and analyzed by digital image correlation method. Based on macroscopic properties and microscopic damage mechanisms, progressive damage analysis was conducted to simulate mechanical behaviors of the structure. The representative volume element model was established for tensile, compressive and shear simulations, and several full models were proposed for bending simulations based on different degrees of simplification. The experimental results indicate that: The stitched sandwich structure shows approximately linear macroscopic mechanical behaviors under out-of-plane tensile and interlaminar shear loads, but exhibits bilinear behaviors under out-of-plane compression and 4-point bending loads. Core-related damage typically occurs first under external load, including interface debonding between the core and panel, shear delamination, collapse of holes, and densification of matrix; the sutures is the main load-bearing constituent. The precise model can describe nonlinear and bilinear behaviors, and characterize the damage evolution of the sandwich structure. The simplified model without considering the sutures is able to simulate 4-point bending behavior efficiently and accurately. The research demonstrates that, sutures can be ignored when simulating 4-point bending behavior, while they (especially out of plane) are necessary for vibration analysis. The mechanical effect of constituents during loading should be fully considered in the finite element model simplification. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ag/4-MBA/PAN nanocomposite was designed and exhibited an excellent surface-enhanced Raman scattering (SERS) activity. • The degree of charge transfer (CT) has a positive correlation with the PAN thickness. • The CT direction in the nanocomposite system was successfully verified by 2D correlation spectroscopy (2D-COS). The purpose of this paper is to study the charge transfer (CT) process of a Ag/MBA/PAN system with different thicknesses of PAN and explore its SERS enhancement mechanism. It is found that as the PAN thickness is increased, the frequency and intensity of the characteristic MBA band gradually increases. The intensity of the b 2 mode increases significantly, which is attributed to the CT mechanism. As the PAN thickness is increased, the degree of CT (ρ CT) gradually increases, which is due to the excited SPR from Ag. The electromagnetic enhancement mechanism, which occurs at the interface between PAN and Ag, generates hot electrons and rearranges the interface between PAN and Ag, thereby promoting electron transfer for the whole CT in Ag/MBA/PAN. These results show that CT occurs from the Fermi level of Ag to the LUMO level of MBA and then to the LUMO level of PAN. Additionally, the direction of CT is verified by 2D-COS. Therefore, this study not only reveals the ultrahigh SERS activity of the substrate but also explains the detailed process of regulating the substrate through the PAN thickness, which affects the SERS mechanism. This conductive polymer will open up a new horizon of SERS research and applications. [ABSTRACT FROM AUTHOR]

Abstract: Experiments are carried out at elevated temperature to investigate the effect of brazing itself and post-brazing heat treatment on the lifetime of directionally solidified (DS) superalloy. Meanwhile, a new lifetime prediction methodology based on continuum damage mechanics (CDM) is proposed to predict the lifetime of DS superalloy and its brazed joints. After creep and fatigue testing, the fractured specimens are observed by means of Optical Microscope (OM) and Scanning Electron Microscope (SEM) with the Energy Dispersive X-ray Spectrometer (EDXS) to understand their failure mechanisms. The results showed that: (1) under the same test condition, all the brazed joints exhibited a lower creep and fatigue lifetime compared with a typical DS superalloy; (2) the creep and fatigue lifetime of heat treated brazed joint was longer than that of non heat treated one; (3) the developed CDM method can be used to predict the lifetime of all the specimens; (4) all the brazed joints fractured in the brazing seam; and (5) there were many distinctive differences between the failure mechanics of DS superalloy and its brazed joints. [Copyright &y& Elsevier]

In this paper, creep-fatigue behaviors of the thin-walled tubular specimen with film holes were experimentally studied to address the influence of cyclic stress interruptions on creep strength and creep-fatigue life of a directionally solidified superalloy. The specimens with and without multiple holes were designed and manufactured to consider the effect of stress concentration produced by film cooling holes. The creep life of the specimen with holes has reduced significantly with the introduction of fatigue. An interesting result was found that the creep strength and life was enhanced on the stress concentrated specimens even if the creep loading was interruption by fatigue loadings, while creep fatigue life will be commonly shortened for notched specimen. Failure mechanisms related with cooling holes were analyzed on the basis of microscope observation. • Creep-fatigu tests of specimens with film-cooling holes were carried out. • Fatigue creep interaction is reflected in the formation of dimples. • High-temperature deformation and stress concentration is a key factor leading to damage. • The creep life of the specimen with holes has reduced with the introduction of fatigue. • The relationship between rafting type and stress state around film cooling hole was found. [ABSTRACT FROM AUTHOR]

In this paper, the thermal-cycling creep of Ni-based superalloy DZ125 is investigated at very high temperature. Experiments were conducted with various parameters including overheating duration, applied stress, temperature interval and heating/cooling frequency. Microscopic observation has revealed a modified thermodynamic equilibrium with lower γ' content in comparison to isothermal creep situation. This phenomenon is discussed on the basis of Johnson-Mehl-Avrami-Kolmogorov law and pre-existing theoretical assumptions of dislocation density evolution during thermal transients. A linear relationship between overheating numbers and equilibrium γ' content is obtained to develop a phenomenological life model of thermal-cycling creep. Life prediction results of proposed method are in good agreement with experimental ones. In this work, the decrease of γ' content at thermodynamic equilibrium of Ni-based superalloys during thermal-cycling creep(TCC) process is quantitatively analyzed to obtain a microstructure-based failure criterion. A modification on JMAK law is proposed to illustrate why the γ' content at thermodynamic equilibrium differs with the length of overheating duration. A creep life prediction method is developed to evaluate TCC lives based on proposed failure criterion. The predicted results are well consistent to the experimental results. Image, graphical abstract [ABSTRACT FROM AUTHOR]