Erman Evgin

Elasto‐plastic modelling of stress‐path‐dependent behaviour of interfaces

A single-surface elasto-plastic model developed by Desai and his coworkers is used to predict the behaviour of an interface between sand and a steel plate. The loading in the experiments and in their predictions followed various stress and displacement paths. The results of predictions of the two- and three-dimensional behaviour of the interface under both constant normal stress and constant normal stiffness conditions are presented. The predictions are compared with their corresponding experimental results. The model parameters were determined on the basis of 2-D conventional experiments under the condition of constant normal stress and they were used in the prediction of the interface behaviour in various stress paths. There is, in general, a good agreement between the predicted and experimental results. Copyright

An Automated Apparatus for Three-Dimensional Monotonic and Cyclic Testing of Interfaces

A new computer-controlled apparatus has been developed to study the behavior of interfaces between two materials under three-dimensional monotonic and cyclic loading conditions. The interface can be subjected to a normal stress, σn, and two shear stresses, τx, and τy, acting simultaneously on the interface plane. The normal stress is applied by a pneumatic actuator that is operated by a motorized regulator. Shear stresses, which are perpendicular to each other, are applied by two stepper motors. Tests can be either displacement or load controlled. The results of an experimental program that includes two series of tests to study the behavior of an interface between a dry coarse sand and a rough surface are presented. In the first series, two-dimensional monotonic and cyclic tests were carried out to verify the performance of the apparatus. The effects of surface roughness, initial relative density of sand, and the magnitude of normal stress on the shear stress-shear displacement and volume change behavior of the interface are presented. In the second series, the loading was three-dimensional and monotonic. First, the interface was sheared in one tangential direction up to a shear stress level less than the peak value. Then, the interface was sheared in a perpendicular direction, while the shear stress in the previous direction was maintained at a constant level. Shear stress and shear displacement increments experienced different paths, while the resultant shear stress-shear displacement curves remained the same irrespective of stress paths.

A three dimensional anisotropic constitutive model for ductile behaviour of columnar grained ice

Abstract A three dimensional constitutive model for the ductile behaviour of fresh water columnar grained ice is developed. The model is capable of describing the inherent anisotropy, the crack evolution, the opening and sliding of cracks, the grain size effect on material behaviour, and the effect of cracks on viscous strain. The model is used to predict the dependence of strength on the angle between the mean c-axis and loading direction for S3 ice and the magnitude of the anisotropic deformation of transversely isotropic S2 ice. The model predictions are compared with experimental results on S2 ice. Good agreement is achieved for (1) time dependence of axial and lateral strains in constant load tests, (2) strain rate dependence of strength in the ductile range, and (3) the effect of grain size on creep behaviour of ice.

Evaluation of Permeability of Tire Shreds Under Vertical Loading

A concern in the use of tire shreds as drainage media in landfill leachate collection systems is the impact of compression strains on the permeability of the waste material, as earlier work has reported that these materials experience large (25 to 50%) axial strains when subjected to vertical loading. This study examines the changes in permeability of a tire shred sample after being subjected to 30 to 50% axial strain from average vertical stresses of 75 to 330 kPa. The maximum vertical stress of 330 kPa approximated 40m of waste overburden. A constant-head permeability apparatus was fabricated to measure the permeability of the tire shred sample under different axial strains. Further, the fabricated assembly was capable of measuring permeability of the sample at various sample locations at a given strain level. Experimental results showed that despite experiencing large axial strains, the average permeability of the tire shred sample consistently remained two to three orders of magnitude higher than the design performance criterion of 0.01 cm/s for landfill drainage layers, suggesting that the compressible nature of tire shreds will not interfere in their use as a leachate collection drainage layer in municipal solid waste landfills.

Mathematical modelling of monotonic and cyclic behaviour of fresh water columnar grained S-2 ice

Abstract A three-dimensional, time-dependent, elasto-plastic constitutive model for fresh water transversally isotropic columnar grained S-2 ice is presented in this paper. The model describes the stress–strain behaviour of ice subjected to any loading scenario, including cyclic loading. It takes into consideration the material non-linearity, time dependency, plasticity, material anisotropy, and hysteresis behaviour of ice when subjected to cyclic loading. It accounts for the effect of grain size, confining pressure, and micro-cracking on the mechanical behaviour of the material. The model can be implemented into an explicit dynamic finite element code, which can be used to compute impact loads exerted by ice on ships and offshore structures. The capability of the model to predict the results of laboratory tests is demonstrated. Comparisons between predicted and measured stress–strain curves are presented.

A three dimensional anisotropic constitutive model for ductile behaviour of columnar grained sea ice

Abstract A three dimensional constitutive model is developed for the ductile behaviour of columnar grained sea ice. Based on the sea ice microstructure, a geometric pattern for the distribution of brine pockets in a single grain of sea ice is idealized. An initial defect tensor for a single grain of sea ice is defined by using the geometric pattern. Subsequently, the initial defect tensor for polycrystalline ice is obtained from the initial defect tensor for a single grain of sea ice and the probability distribution of thec-axis. The effect of the brine pockets on the response of sea ice to different types of loading is expressed by introducing two additional tensors. One of these tensors is called the influence tensor for deviatoric stresses, and the other one is the influence tensor for hydrostatic stresses. With the two influence tensors, an effective stress tensor is defined. The effective stress is used in the sea ice constitutive equations. Comparisons between model predictions and laboratory test results under both uniaxial and multiaxial loading conditions show that the model predictions agree with the test data.

Uniaxial Constant Compressive Stress Creep Tests on Sea Ice

First-year columnar-grained sea ice from Resolute Passage (74° 42′ N, 94° 50′ W), off Barrow Strait in the Canadian High Arctic, was tested under constant uniaxial compressive stress applied normal to the length of the columns. Creep tests were performed at 263 K, 253 K, and 243 K in the stress range of 0.7 to 2.5 MPa, using prismatic samples with dimensions of 50 mm × 100 mm × 250 mm. Because three-dimensional creep data are extremely useful for developing constitutive equations, axial strain was measured in conjunction with the measurements of two lateral strains and acoustic emissions. The deformations were measured using displacement gages mounted on the samples. A description of the experimental procedures and the observations are presented here. One-to-one correspondence has been obtained between the present results on the dependence of minimum creep rate on stress and previous data on the dependence of uniaxial compressive strength on strain rate under constant strain rate. The strain-rate sensitivity of compressive strength can, therefore, be obtained from creep tests which can be performed by using simple dead-load test systems.

Finite element validation of a bounding surface plasticity model

Abstract A bounding surface plasticity model for cohesionless soils is validated with respect to two boundary value problems. In the first boundary value problem, the behavior of a model-scale footing resting on the surface of a sandy silt is analyzed. Whereas, in the second boundary value problem, the finite element simulations are compared with the behavior of the Leighton Buzzard Sand in the Cambridge Simple Shear Device. The validation of the constitutive model also included the predictions of the behavior of a sandy silt in a true triaxial apparatus following stress paths different from those used in the calibration of the constitutive model. Comparisons of the model predictions with the test results considered here show the validity of the constitutive model and its ability to simulate the soil behavior in a variety of loading conditions.

Finite element applications of a bounding surface plasticity model

Bardets bounding surface plasticity model was used to analyze two different boundary value problems: the load-displacement responce of a model-scale footing resting on the sur-face of a sandy silt, and the behaviour of the Leighton Buzzard Sand in the Cambridge Simple Shear Device. Comparisons between experimental observations and the results of the finite element analyses proved the ability of the constitutive model to simulate accurately the soil behaviour in boundary value problems.

A nonlinear soil model in ADINA

Abstract A nonlinear stress-strain relationship for soils has been implemented into ADINA. The reliability of this development is verified. The behaviour of a bottom-founded oil exploration platform is analysed for horizontal ice forces and gravity. The influence of different types of grounded rubble ice on the load-displacement response of the structure is examined. Both the distribution of contact pressure at the soil-structure interface and the stress paths followed by the soil elements beneath the structure are determined.