Improving the fatigue strength of 7075 alloy through aging Leng, L.; Zhang, Z.J.; Duan, Q.Q. ...
Materials science & engineering. A, Structural materials : properties, microstructure and processing,
12/2018, Letnik:
738
Journal Article
Recenzirano
In order to optimize the fatigue strength of 7075 Al alloy, the hardness, tensile and fatigue tests as well as the microstructure observation under different aging conditions were investigated. The ...result indicates that, with the aging time increasing, the precipitation spacing first decreases and then increases, which leads to the hardness and tensile strength first increasing and then decreasing. With the increase of yield strength, the fatigue strength increases first and then decreases, resulting in an optimization of fatigue strength. Especially, the specimen aged at 120 °C for 48 h with strength and elongation in middle of the three specimens exhibits the highest fatigue strength of 165 MPa. Considering the effect of yield strength on fatigue strength and fatigue damage, the causes for the parabolic variation of fatigue strength were analyzed.
Changes in tropical (30 S–30 N) land hydroclimate following CO2‐induced global warming are organized according to climatological aridity index (AI) and daily soil moisture (SM) percentiles. The ...transform from geographical space to this novel process‐oriented phase space allows for interpretation of local, daily mechanistic relationships between key hydroclimatic variables in the context of time‐mean and/or global‐mean energetic constraints and the wet‐get‐wetter/dry‐get‐drier paradigm. Results from 16 CMIP models show coherent patterns of change in the AI/SM phase space that are aligned with the established soil‐moisture/evapotranspiration regimes. We introduce an active‐rain regime as a special case of the energy‐limited regime. Rainfall shifts toward larger rain totals in this active‐rain regime, with less rain on other days, resulting in an overall SM reduction. Consequently, the regimes where SM constrains evapotranspiration become more frequently occupied, and corresponding hydroclimatic changes align with the position of the critical SM value in the AI/SM phase space.
Plain Language Summary
Predictions of terrestrial hydroclimate changes (temperature, precipitation, evaporation, etc.) in a warming world rely largely on model simulations with often diverging results when presented in map view. Here, we introduce a process‐based phase space that organizes the spatial complexity by climatological aridity, and organizes the temporal complexity by daily soil moisture (SM). This allows for the analysis of model predictions in a comprehensive yet compact display which clearly reveals the connections between variables and mechanisms responsible for changes. Key results include the impact of SM limitation on elevated temperature extremes and the repartitioning of rainfall toward fewer, stronger events. This compact display is an efficient new tool for intercomparisons between models. The remarkably clean results suggest quantitative theoretical advances are possible despite the complexity of the system.
Key Points
A novel process‐oriented phase space reveals coherent patterns of terrestrial hydroclimate change
Patterns emphasize the impact of soil moisture on temperature extremes and the redistribution of rainfall toward more intense events
Patterns of P − E changes reveal how land differs from the wet‐get‐wetter/dry‐get‐drier paradigm
In this study, the effect of strain rate on the tensile deformation behavior of Fe–22Mn–0.6C–(1.5Al) (wt%) twinning-induced plasticity (TWIP) steel was investigated. The experimental results ...indicated that the work hardening exponent (n), ultimate tensile strength (σu) and the uniform elongation (δu) decreased with increasing strain rates (from 10−4 to 100s−1). This phenomenon exhibited negative strain rate sensitivity (NSRS), and the strain rate sensitivity value (m) was observed to be higher in the aluminum added FeMnC TWIP steel. In order to gain an in-depth understanding of this sensitivity and the subsequent effect of aluminum, the present research focused on the deformation twins and conducted comparative studies on their influence in terms of fractions (F), thickness (t) and spacing (s). Additionally, a twin boundary affected zone (TBAZ) model was proposed, where the relation between higher strain rates were directly applicable to the reduction of interfaces between the matrix and deformation twins. These result indicated that fewer sessile dislocations could be accommodated at high strain rates, thus weakening the work hardening ability. Finally, model calculations were performed to validate the findings, where TBAZ region fractions in FeMnC–Al were observed to be higher than that in FeMnC, corresponding to the increased strain rate sensitivity.
In the study, a new triple-helix polysaccharide with favorable stability was purified from C. cornucopioides. Its structural characterization, stability and solution behavior were investigated by the ...GC–MS, periodate oxidation-smith degradation, FT-IR, 1D and 2D NMR spectroscopy, methylation analysis, Scanning electron microscope, Congo-red, CD, TGA and DSC analysis. The results showed that Craterellus cornucopioide polysaccharide (CCP) possessed the molecular weight of 1.97 × 103 kDa, is mainly composed of mannose (48.73%), galactose (17.37%), glucose (15.97%) and xylose (17.93%), respectively. It was a heteroglycan with (1 → 3)‑linked‑β‑d‑Manp‑(1 → 6)‑linked α‑d‑Galp backbone distributed by (1 → 4)‑linked‑α‑d‑Xylp‑t‑α‑d‑Manp and t‑β‑d‑Glup units at O-6. The result of TGA and DSC assay indicated that CCP has a favorable thermal stability. MTT and Scanning electro microscopy (SEM) assay showed that CCP could significantly improve the proliferation activity and induce cells activation of RAW264.7 in a certain range of concentrations and period.
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•We found and investigated a novel polysaccharide from Craterellus cornucopioides.•The Conformation studies of this polysaccharide solution was studied.•The structure stability of this polysaccharide was studied.•The morphological changes of RAW264.7 cells under CCP activation was observed.
We present a community data set of daily forcing and hydrologic response data for 671 small- to medium-sized basins across the contiguous United States (median basin size of 336 km2) that spans a ...very wide range of hydroclimatic conditions. Area-averaged forcing data for the period 1980-2010 was generated for three basin spatial configurations - basin mean, hydrologic response units (HRUs) and elevation bands - by mapping daily, gridded meteorological data sets to the subbasin (Daymet) and basin polygons (Daymet, Maurer and NLDAS). Daily streamflow data was compiled from the United States Geological Survey National Water Information System. The focus of this paper is to (1) present the data set for community use and (2) provide a model performance benchmark using the coupled Snow-17 snow model and the Sacramento Soil Moisture Accounting Model, calibrated using the shuffled complex evolution global optimization routine. After optimization minimizing daily root mean squared error, 90% of the basins have Nash-Sutcliffe efficiency scores greater than or equal to 0.55 for the calibration period and 34% greater than or equal to 0.8. This benchmark provides a reference level of hydrologic model performance for a commonly used model and calibration system, and highlights some regional variations in model performance. For example, basins with a more pronounced seasonal cycle generally have a negative low flow bias, while basins with a smaller seasonal cycle have a positive low flow bias. Finally, we find that data points with extreme error (defined as individual days with a high fraction of total error) are more common in arid basins with limited snow and, for a given aridity, fewer extreme error days are present as the basin snow water equivalent increases.
Higher fluoride concentrations in water have inconsistently been associated with the levels of intelligence in children. The following study summarizes the available evidence regarding the strength ...of association between fluoridated water and children's intelligence.
Meta-analysis.
PubMed, Embase, and Cochrane Library databases were systematically analyzed from November 2016. Observational studies that have reported on intelligence levels in relation to high and low water fluoride contents, with 95% confidence intervals (CIs) were included. Further, the results were pooled using inverse variance methods. The correlation between water fluoride concentration and intelligence level was assessed by a dose–response meta-analysis.
Twenty-six studies reporting data on 7258 children were included. The summary results indicated that high water fluoride exposure was associated with lower intelligence levels (standardized mean difference : −0.52; 95% CI: −0.62 to −0.42; P < 0.001). The findings from subgroup analyses were consistent with those from overall analysis. The dose–response meta-analysis suggested a significant association between water fluoride dosage and intelligence (P < 0.001), while increased water fluoride exposure was associated with reduced intelligence levels.
Greater exposure to high levels of fluoride in water was significantly associated with reduced levels of intelligence in children. Therefore, water quality and exposure to fluoride in water should be controlled in areas with high fluoride levels in water.
•High fluoride content in drinking water could affect the intellectual development of children.•High water fluoride exposure is significantly associated with a reduced level of intelligence.•Water quality and water fluoride exposure should be improved in areas with high water fluoride content.
By replacing a Newtonian dashpot in the classical Nishihara model with the fractional derivative Abel dashpot, a new creep constitutive model is proposed on the basis of time-based fractional ...derivative. The analytic solution for the fractional derivative time-dependent constitutive model is given. The parameters of the fractional derivative model and the Nishihara model are determined by fitting to existing experimental results of time-dependent deformation of salt rock. The results estimated by the fractional derivative model proposed in the paper are in better agreement with the experimental data than the results estimated by the Nishihara model. A sensitivity study for the analytic solution of the time-based fractional derivative model is carried out, showing the effects of fractional derivative order and stress level on creep strain of salt rock. It is shown that the time-based fractional derivative model can be simplified to the Nishihara model for the special case of fractional derivative order equal to 1.0.
The low-cycle fatigue (including extremely-low-cycle fatigue) behaviors of the FeMnC TWIP steels are comprehensively investigated focusing on the effects of the imposed strain amplitude and Mn ...content on the deformation and damage characteristics. It is found that fatigue performance varies obviously with increasing Mn content, though their tensile properties do not change much. With the increase of strain amplitude or Mn content, the dislocation slip mode changes from planar slip to wavy slip. In order to evaluate the low-cycle fatigue properties, more impartial and reasonable, a hysteresis energy-based fatigue life prediction model is applied and developed. According to the experimental results and model analysis, unlike tensile properties, the low-cycle fatigue performance of the TWIP steel is extremely sensitive to their slip mode. A remarkable improvement of fatigue resistance is found to be related to the increase of Mn content, which may originate from the relatively low damage accumulation rate of planar slip. Specifically, planar slip caused by lowering the stacking fault energy is much more uniformly distributed than which caused by disordering short-range order. Such a variation in fatigue damage micromechanisms is believed to be the major reason for the occurrence of a special bilinear relationship for the Coffin-Manson curve.
Fe-Mn-C TWIP steels manifest a similar tensile deformation behavior with increasing Mn content, while their low-cycle fatigue performances vary significantly. Unlike tensile property, the fatigue performance is extremely sensitive to the slip mode, which has its origin from material’s inherent attributions, e.g. stacking fault energy and short range ordering. Display omitted
The tensile and high-cycle fatigue tests of Fe-30Mn-0.9C twinning-induced plasticity (TWIP) steel after 30%, 60% and 70% pre-straining were performed. Meanwhile, the surface damage morphologies of ...post-fatigue specimens and microstructure evolutions of pre-strained and post-fatigue pre-strained specimens were also investigated. It is found that the fatigue properties of the TWIP steel can be effectively improved through pre-straining, because the pre-straining can change the fatigue strength coefficient and exponent, respectively. The improvement of fatigue strength coefficient may be attributed to the strengthening mechanisms induced by both twin boundaries and dislocations; while the variation of fatigue strength exponent should be resulted from the combined effects of deformation homogeneity and slip reversibility, as well as the internal damages. Furthermore, the detailed mechanisms associated with the variations of fatigue strength coefficient and exponent were discussed. This study may enrich the fundamental knowledge about how to improve the high-cycle fatigue properties of TWIP steels.
An accurate evaluation of the World Health Organization grade and cellular proliferation is particularly important in meningiomas. Our aim was to prospectively evaluate and compare diffusional ...kurtosis imaging, DTI, and DWI metrics in determining the grade and cellular proliferation of meningiomas.
Ninety-six consecutive patients with histopathologically confirmed meningiomas were included in this study. Mean kurtosis, radial kurtosis, axial kurtosis, fractional anisotropy, mean diffusivity, and ADC were semiautomatically obtained in the solid components of tumors. Each normalized diffusion value was compared between high-grade meningiomas and low-grade meningiomas using the Mann-Whitney
test. Receiver operating characteristic, multiple logistic regression, and Pearson correlation analysis were used for statistical evaluations.
Diffusional kurtosis imaging metrics (mean kurtosis, radial kurtosis, and axial kurtosis) were significantly higher in high-grade meningiomas than in low-grade meningiomas (
≤ .001). Mean diffusivity and ADC were significantly lower in high-grade meningiomas than in low-grade meningiomas (
= .003 and .002). Mean kurtosis had significantly greater area the under curve values than mean diffusivity and fractional anisotropy in differentiating high-grade meningiomas from low-grade meningiomas (
= .038 and .002). Mean kurtosis was the only variable that could be used to independently differentiate high-grade meningiomas and low-grade meningiomas (
< .001). Significant correlations were found between the Ki-67 labeling index and kurtosis metrics (
< .001), as well as for mean diffusivity and ADC (
= .004, and .007).
Compared with other diffusion metrics, mean kurtosis may serve as an optimal parameter for evaluating and predicting the meningioma grade. Moreover, diffusion metrics may potentially reflect cellular proliferation.
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