Large-Scale Triaxial Tests on Dilatancy Characteristics of Lean Cemented Sand and Gravel

The dilatancy equation, which describes the plastic strain increment ratio and its dependence on the stress state, is an important component of the elastoplastic constitutive model of geotechnical materials. In order to reveal their differences of the dilatancy value determined by the total volume strain increment ratio and the real value of lean cemented sand and gravel (LCSG) materials, in this study, a series of triaxial compression tests, equiaxial loading and unloading tests, and triaxial loading and unloading tests are conducted under different cement contents and confining pressures. The results reveal that hysteretic loops appear in the stress–strain curves of equiaxial loading and unloading tests, and triaxial loading and unloading tests and that the elastic strain is an important component of the total strain. The hysteretic loop size increases with an increase in the stress level or consolidation stress, whereas the shape remains unchanged. Furthermore, with an increase in the cement content, the dilatancy value determined by the total volume strain increment ratio becomes smaller than that determined by the plastic strain increment ratio, and the influence of the elastic deformation cannot be ignored. Thus, in practical engineering scenarios, especially in the calculation of LCSG dam structures, the dilatancy equation of LCSG materials should be expressed by the plastic strain increment ratio, rather than the total volume strain increment rati.

Non-orthogonal elastoplastic constitutive model with the critical state for clay

A non-orthogonal elastoplastic model for clay is proposed by combining the non-orthogonal plastic flow rule with the critical state concept, and the model framework is presented from the perspective of the magnitude and direction of the plastic strain increment. The magnitude is obtained based on the improved elliptical yield function and the plastic volumetric strain dependent hardening parameter. The direction is determined by ap-plying the non-orthogonal plastic flow rule with the Riemann-Liouville fractional derivative to the yield function without the necessity of additional plastic potential function. The presented approach gives rise to a simple model for soil with five parameters. All parameters have clear physical meaning and can be easily identified by triaxial tests. The model performance is shown by analyzing the evolution process of the yield surface, the hardening rule and the plastic flow direction. The capability of the proposed model to capture the mechanical behaviours of clay with different stiffness is also confirmed by predicting test results from the literature.

Experimental Investigation and Micromechanical Modeling of Elastoplastic Damage Behavior of Sandstone

The mechanical behavior of the sandstone at the dam site is important to the stability of the hydropower station to be built in Southwest China. A series of triaxial compression tests under different confining pressures were conducted in the laboratory. The critical stresses were determined and the relationship between the critical stress and confining pressure were analyzed. The Young’s modulus increases non-linearly with the confining pressure while the plastic strain increment Nϕ and the dilation angle ϕ showed a negative response. Scanning electron microscope (SEM) tests showed that the failure of the sandstone under compression is a coupled process of crack growth and frictional sliding. Based on the experimental results, a coupled elastoplastic damage model was proposed within the irreversible thermodynamic framework. The plastic deformation and damage evolution were described by using the micromechanical homogenization method. The plastic flow is inherently driven by the damage evolution. Furthermore, a numerical integration algorithm was developed to simulate the coupled elastoplastic damage behavior of sandstone. The main inelastic properties of the sandstone were well captured. The model will be implemented into the finite element method (FEM) to estimate the excavation damaged zones (EDZs) which can provide a reference for the design and construction of such a huge hydropower project.

Study of Hot Cracking on Automatic Tandem Butt Welding

In recent years, ship hulls have become larger. And the welding has become more important technology in ship building. In other word, welding has become more important technology in shipbuilding. In Japanese shipbuilding, multiple electrode butt welding has been performed with large current and high speed to increase construction efficiency. However, the multiple electrode welding method may cause a hot cracking. X-ray or ultra sonic wave inspection and repair welding are necessary due to the generation of hot cracking. These will increase the production cost. Therefore, it is important to prevent hot cracking. It is known that hot cracking is generated by both metallurgical and mechanical factors. The authors propose a new evaluation method of hot cracking based on the modeling of both mechanical and metallurgical behaviors. In the developed method, from the metallurgical point of view, solidification growth direction is determined from the temperature gradient obtained by the heat conduction analysis to predict the position of hot cracking. Moreover, from the mechanical point of view, the possibility of the generation of hot cracking is assessed using plastic strain increment in Brittleness Temperature Range (BTR). In order to show the validity of the developed method, the developed method is applied to the analysis of hot cracking on automatic tandem butt welding through the comparison with experimental results. As a result, it is demonstrated that the hot cracking can be evaluated by using the developed method.

Analysis of Factors Affecting the Direction of Plastic Strain Increment in Sand

Taking the standard sand of Fujian as the test material, this paper concentrates on studying the influence of different stress increment directions on the direction of plastic strain increment of sand materials under different stress states and the underlying mechanism. The test results show that the plastic strain increment angle rotates counterclockwise with the increase of stress increment direction angle, but the two angles do not coincide; the higher the stress state parameters is, the larger the stress increment direction angle range corresponding to sand dilatation is, the smaller the plastic strain increment direction angle range is, and the plastic volumetric strain still increases in critical state.The plastic flow mechanism of sand is explained from the average stress increment, generalized shear stress increment and stress state, which may provide theoretical reference and numerical support for the related research of plastic strain increment direction of sand.

An Improved Thermo-Ratcheting Boundary of Pressure Pipeline

The thermal ratcheting boundary of pressure pipeline is a popular topic in nuclear power engineering. The existed thermal ratcheting boundary based on the Bree diagram is conservative for structures subjected to the thermo-mechanically coupled loadings since it was obtained only from an elastic-perfectly plastic model. Therefore, it is necessary to improve the existed thermal ratcheting boundary based on a reasonable constitutive model. The Bree diagram was validated firstly by the linear relationship between the plastic strain increment and mechanical stress by finite element method. And then the influences of different constitutive models, such as elastic-perfectly plastic, multi-linear kinematic hardening, Chaboche and Abdel Karim-Ohno models, on the thermal ratcheting boundary of pressure pipeline were investigated numerically. It is found that the elastic-perfectly plastic and multi-linear kinematic hardening models provide the lower and upper bounds for the thermal ratcheting boundary, respectively. Finally, an improved thermal ratcheting boundary by introducing the dimensionless axial tensile stress was proposed based on the Bree diagram, the improved thermal ratcheting boundary covered the present cases with different ratios of mechanical stress over thermal stress.

A Three Dimensional Plasticity Model for Sands Based on State Concept

A three dimensional plasticity model for shear deformation in sand is developed based on the plane strain model of Cubrinovski and Ishihara [1]. The model uses a circular bounding surface. The model is hypoplastic in nature, rendering the principal directions of plastic strain increment different from those of the stress, and thus, allows for accumulation of plastic strain with principal stress rotation. The model uses the multi-surface based mixed hardening rule. This paper presents results from simulations under bi-directional (torsional-stress path) uniform stress cycles, as well as bi-directional stress-controlled simulation results, where two orthogonal horizontal components of a ground acceleration time series are converted to bi-directional stress history, providing a rigorous verification of the algorithm.

Non-Coaxial Plasticity Constitutive Modeling of Sands

Classical coaxial plasticity constitutive models implicate an inevitable limitation that directions for principal stress and that for principal plastic strain increment are always coaxial. They are not capable of simulating non-coaxial phenomena during the rotation of principal stress axis. In this paper, a three-dimensional, non-coaxial plasticity constitutive model for sands with a modification of Lade angle dependent shape function is introduced to describe the non-coaxial behavior under principal axes rotation. A series of numerical simulations of hollow cylindrical torsional shear tests are performed. The results show that the proposed constitutive model can predict the variations of principal plastic strain increment directions with principal stress directions reasonably.