Triaxial compression deformation for artificial frozen clay with thermal gradient

In order to study the deformation characteristics of artificial frozen soil with thermal gradient, such as the stress-strain relationship, a series of triaxial compression tests for frozen clay had been conducted by K₀DCGF (K₀ consolidation, freezing with non-uniform temperature under loading) method and GFC (freezing with non-uniform temperature, isotropic consolidation) method at various consolidation pressures and thermal gradients. Stress-strain curves in K₀DCGF test present strain softening during shearing process and the elastic strain is approximately 0.001;but which present the strain hardening characteristics in GFC tests and the elastic strain is approximately 0.01. The elastic modulus and peak stress for frozen clay decrease as the thermal gradient increased at different consolidation pressure both in K₀DCGF test and GFC test. The peak stress and elastic modulus in K₀DCGF test are significant independent on the pressure melting and crushing phenomena occurring in GFC test. To describe the shear deformation characteristics for frozen clay with thermal gradient, the exponent and power equations considering the correction equation on thermal gradient and model parameters from frozen clay with uniform temperature are developed .The results indicated that the proposed equations can reproduce the shear deformation well both in K₀DCGF test and GFC test.     

Depth-sensing indentation for assessing the mechanical properties of cold-sprayed Ta

The stress-strain behaviour of cold-sprayed Ta coatings, deposited onto Al and steel substrates, was studied by depth-sensing spherical and sharp (Berkovich) indentation testing, and was compared to that of an annealed Ta sheet. The mechanical properties of the coatings, free of any scale-dependence, are insensitive to the presence of a lamellar structure, indicating strong, tight bonding between cold-sprayed Ta particles. Accordingly, the coatings are isotropic. The stress-strain curves reveal that the coating material experienced some degree of work-hardening during cold-spraying; indeed, the sprayed particles were plastically deformed at impact. This was confirmed by line profile analysis on XRD patterns. The consistency between the results from different indentation testing techniques provides a means to cross-validate the entire analysis.     

部分预应力混凝土梁相对界限受压区高度取值分析

钢筋混凝土构件和预应力混凝土构件相对界限受压区高度相差很大,同一截面尺寸构件,当由相对界限受压区高度控制时,预应力混凝土梁与普通混凝土梁相比,承载力降低约17％.本文根据预应力钢绞线拉伸应力-应变试验曲线,分析了预应力钢绞线条件屈服强度取值对相对界限受压区高度的影响.对预应力筋和非预应力筋配筋强度基本相同的部分预应力混...     

Laboratory and theoretical investigations on the deformation and strength behaviors of artificial frozen soil

In order to study the mechanical characteristic of artificial frozen soils, such as strength, and stress-strain relationship, a series of triaxial compression tests of frozen sand has been conducted under confining pressures varying from 0.0 to 14.0 MPa with different water contents at − 6 °C. Frozen sand presents strain softening during shearing process under low confining pressures; but with increasing confining pressure, the strain softening decreases, and even presents strain hardening under high confining pressures. The strength of frozen sand is affected by water content and confining pressure. The strength with low water content always increases with increasing confining pressure; however, for frozen sand with a high water content, the strength experiences an increase followed by a decrease with increasing confining pressure. To describe the strength characteristic of frozen sand, the non-linear Mohr-Coulomb criterion, in which the generalized internal friction angle and cohesion under various confining pressures are obtained from experimental results, has been presented. The result shows that the non-linear Mohr-Coulomb criterion can reflect the decrease of strength of frozen sand under high confining pressures. The stress-strain relationships of frozen sand are represented by hyperbolic functions, which can describe both the strain hardening behavior of frozen sand under high confining pressure and the strain softening behavior under a low confining pressure.     

一种非线性应力应变关系表达式及应用

把一般非线性应力应变关系分成线性部分和偏离这部分的非线性部分，前者用线性关系式即虎克定律描述，后者用幂次关系式描述作为前者的修正。此表达式避免了加载瞬时模量为无穷大，更符合实际。讨论了表达式中参数的确定，并应用于软土非线性流本构关系中。     

过载后缺口局部应变范围Δε的估算式

研究了经简单拉伸过载后，缺口局部应变范围Δε的估算方法．理论与实验结果分析表明，拉伸过载虽引起缺口局部残余应变（ε＇）但随后的局部应变范围Δε与应力比（Ｒ）也无关，仅取决于名义应力范围（ΔＳ）和缺口几何尺寸参数（Ｋσ或Ｋｔ）．此外，本文还讨论了材料的应变硬化特性对Δε的影响．     

THE INFLUENCE OF AGEING AT INTERMEDIATE TEMPERATURES ON THE MONOTONIC STRESS-STRAIN BEHAVIOUR AND FRACTURE TOUGHNESS OF A DUPLEX STAINLESS STEEL

Abstract— The mechanical behaviour of the duplex stainless steel AISI 329 has been investigated for ageing times up to 15,000 h at 475, 425, 375, 325 and 275°C. The study has concentrated on changes in the monotonic stress-strain behaviour and fracture toughness as a function of ageing temperature and time. It is shown that the tensile behaviour of the steel changes strongly due to ageing. A large increase in yield strength and reductions in ductility and fracture toughness are observed. The deformation hardening behaviour of the aged steel is explained by using a model based on a modified rule of mixtures. Finally it is shown that the higher toughness of aged duplex stainless steels, in comparison with ferritic stainless steels aged under the same ageing conditions, may be associated with the increase in crack growth resistance induced by ductile ligaments of austenite which bridge the crack faces.     

金属Al-Si复合不烧铝碳滑板材料的热机械性能及显微结构

研究了金属Al-Si复合不烧铝碳滑板材料的热态抗折强度、弯曲应力-应变关系和抗热震性,并利用XRD、SEM和EDAX对其物相组成和显微结构进行了分析。结果表明:(1)材料的热态抗折强度很高,在1200～1400℃时达到50MPa以上;(2)材料从400℃开始到1400℃一直保持塑性状态,在1400℃仍没有进入粘滞流动阶段,只是在800℃以上变形量有所增加;(3)材料具有良好的抗热震性,在经受ΔT为1000～1200℃的一次热震后,其残余抗折强度均>30MPa,其抗折强度保持率仍为70%左右;(4)材料热机械性能优良的基本原因在于加入的金属Al和Si发生碳化和氮化等反应原位生成了非氧化物增强相,其显微结构特征从以碳结合为主转化为以非氧化物结合为主。     

Energy dissipation rate: An indicator of coal deformation and failure under static and dynamic compressive loads

Dynamic disasters in Chinese coal mines pose a significant threat to coal productivity. Thus, a thorough understanding of the deformation and failure processes of coal is necessary. In this study, the energy dissipation rate is proposed as a novel indicator of coal deformation and failure under static and dynamic compressive loads. The relationship between stress-strain, uniaxial compressive strength, displacement rate, loading rate, fractal dimension, and energy dissipation rate was investigated through experiments conducted using the MTS C60 tests(static loads) and split Hopkinson pressure bar system(dynamic loads). The results show that the energy dissipation rate peaks are associated with stress drop during coal deformation, and also positively related to the uniaxial compressive strength. A higher displacement rate of quasi-static loads leads to an initial increase and then a decrease in energy dissipation rate, whereas a higher loading rate of dynamic loads results in larger energy dissipation rate. Theoretical analysis indicates that a sudden increase in energy dissipation rate suggests partial fracture occurring within coal under both quasi-static and dynamic loads. Hence, the energy dissipation rate is an essential indicator of partial fracture and final failure within coal, as well as a prospective precursor for catastrophic failure in coal mine.