Collapse failure in dry clay soils caused by tine implements

The relation between forces applied to the soil and the resultant soil reaction was studied in dry clay soils under a quasi-static condition. As a tine advanced in dry compact clay soils at 5.2% dry basis moisture content, masses of soil collapsed one by one in front of the tine. The horizontal and vertical components of soil resistance measured were cyclic and in phase, with distinct peak and trough values. The peak values and trough values indicated the soil stress conditions before and immediately after each failure occurred. The frequency of failure depended on the size of the tine. The magnitude of the peak values depended on level of compaction and trough values on density of collapsed mass. The paper presents the details of observations.     

A simple cohesive zone model that generates a mode-mixity dependent toughness

A simple, mode-mixity dependent toughness cohesive zone model (MDG_c CZM) is described. This phenomenological cohesive zone model has two elements. Mode I energy dissipation is defined by a traction–separation relationship that depends only on normal separation. Mode II (III) dissipation is generated by shear yielding and slip in the cohesive surface elements that lie in front of the region where mode I separation (softening) occurs. The nature of predictions made by analyses that use the MDG_c CZM is illustrated by considering the classic problem of an elastic layer loaded by rigid grips. This geometry, which models a thin adhesive bond with a long interfacial edge crack, is similar to that which has been used to measure the dependence of interfacial toughness on crack-tip mode-mixity. The calculated effective toughness vs. applied mode-mixity relationships all display a strong dependence on applied mode-mixity with the effective toughness increasing rapidly with the magnitude of the mode-mixity. The calculated relationships also show a pronounced asymmetry with respect to the applied mode-mixity. This dependence is similar to that observed experimentally, and calculated results for a glass/epoxy interface are in good agreement with published data that was generated using a test specimen of the same type as analyzed here.     

Prediction of soil structures produced by tillage

A statistical method for describing the distribution of aggregates and voids within tilled soil is used as the basis for a prediction technique. Transforms of aggregate-void and void-void transition probabilities are used in such a way that factors can be defined which describe how the soil structure differs under a range of circumstances. A standard structure is defined for a given soil as that which is produced at the 5 cm depth by one pass of a set of tines working at 10 cm depth when tillage is done at a speed of 1.4 m/s in soil at a water content equal to the plastic limit and which has previously grown a cereal crop. Factors are defined which describe deviations from this standard structure as a function of depth in the tilled layer, implement type, previous use of the soil, number of implement passes, water content at time of tillage, and subsequent compaction of thettilled layer. Application of the inverse transform then enables estimates of the distributions of aggregates and voids in the tilled layer to be predicted for required, specified conditions.     

A cohesive model based fragmentation analysis: effects of strain rate and initial defects distribution

A one-dimensional fragmentation analysis that incorporates elastic wave propagation and a cohesive failure process is presented. An irreversible cohesive law models the internal crack nucleation and opening process, and the elastodynamic states of the intact material are calculated using the method of characteristics. Both the average fragment size and the fragment size distribution are obtained. The fragmentation of a model ceramic system is simulated over a wide range of strain rates, and the calculated results are compared to existing theoretical, numerical and experimental results. In the high strain-rate regime, the calculated average fragment size is smaller than that predicted by energy models, but at quasistatic rates the calculated average size is larger than that estimated by such models. The intrinsic mechanisms leading to these deviations are discussed. The fragment size distributions exhibit similarity under all strain-rate range. The effect of the distribution of internal defects on the fragmentation and fragment size distribution is also investigated using this methodology.     

A stress point algorithm for an elastoplastic model in unsaturated soils

Two stress fields, combination of total stresses, liquid pressure and gas pressure have to be considered to explain the deformational behaviour of unsaturated media. Elastoplastic models developed for these materials consider generally two yield surfaces, each one associated to a stress field, and whose intersection produces a corner in the space of generalised stress components. In this paper, a stress point algorithm is proposed to cope with the integration of such constitutive laws, which can be seen as non smooth multisurface plastic models in the space of the two stress fields. The basic model developed by Alonso et al. (Alonso, E.E., Gens, A., 1990. A constitutive model for partially saturated soils. Géotechnique 40 (3), 405–430), which will be used to test the algorithm, is first described. Generalised stress and strain variables are then defined. Implementation of the return mapping algorithm, based on an implicit integration scheme, is presented with special attention devoted to the problem of mixed control imposed by the F.E. formulation generally used to analyse the hydromechanical behaviour of unsaturated media. Validation results on distinct generalised stress paths are given at the end.     

A unified potential-based cohesive model of mixed-mode fracture

A generalized potential-based constitutive model for mixed-mode cohesive fracture is presented in conjunction with physical parameters such as fracture energy, cohesive strength and shape of cohesive interactions. It characterizes different fracture energies in each fracture mode, and can be applied to various material failure behavior (e.g. quasi-brittle). The unified potential leads to both intrinsic (with initial slope indicators to control elastic behavior) and extrinsic cohesive zone models. Path dependence of work-of-separation is investigated with respect to proportional and non-proportional paths—this investigation demonstrates consistency of the cohesive constitutive model. The potential-based model is verified by simulating a mixed-mode bending test. The actual potential is named PPR (Park-Paulino-Roesler), after the first initials of the authors’ last names.     

A thermodynamics-based cohesive model for interface debonding and friction

A constitutive model for interface debonding is proposed which is able to account for mixed-mode coupled debonding and plasticity, as well as further coupling between debonding and friction including post-delamination friction. The work is an extension of a previous model which focuses on the coupling between mixed-mode delamination and plasticity. By distinguishing the interface into two parts, a cracked one where friction can occur and an integral one where further damage takes place, the coupling between frictional dissipation and energy loss through damage is seamlessly achieved. A simple framework for coupled dissipative processes is utilised to derive a single yield function which accurately captures the evolution of interface strength with increasing damage, for both tensile and compressive regimes. The new material model is implemented as a user-defined interface element in the commercial package ABAQUS and is used to predict delamination under compressive loads in several test cases.     

Embedded digital display and warning system of velocity ratio and wheel slip for tractor operated active tillage implements

A microcontroller-based embedded digital display and warning system was developed for measuring wheel slippage, velocity ratio, PTO torque, and draft requirement of active tillage machinery. The hardware system included magnetic pickup sensor for measuring the engine speed, load cells and amplifiers to measure and amplify the sensing unit signals of the draft, proximity sensors for wheel slip, and PTO torque transducer for measuring the torque requirement. It was provided with buzzers and LEDs to warn the operator, whenever slip and velocity ratio were not in the desired range based on the algorithm, for maximum fuel efficiency and tractive performance. It measured slippage, velocity ratio, torque and draft with a maximum absolute variation of 12.90%, 7.92%, 8.99% and 11.57%, respectively. The developed system can be easily adaptable to any combination of tractor and tillage implements, and guide the operator for better soil tilth with lesser energy input.     

A refined cohesive zone model that accounts for inertia of cohesive zone of a moving crack

Existing cohesive zone models assume that actual fracture zone of non-zero mass can be modeled by a line segment (cohesive zone) with no mass and inertia. In the present work, a simplified mass-spring model is presented to study inertia effect of cohesive zone on a mode-I steady-state moving crack. It is showed that fracture energy predicted by the present model increases dramatically when a finite limiting crack speed is approached. Reasonable agreement with known experiments indicates that the present model has the potential to catch the inertia effect of cohesive zone which has been ignored in existing cohesive zone models and better simulate dynamic fracture at high crack speed.     

A simple model for the flow and temperature distribution analysis in a manifold-shaped direct cooling system

A fluid dynamic and thermal analysis has been carried out for the calculation of pressure, velocity and temperature distribution in a manifold-shaped cooling system of varying geometrical characteristics.A physical model of the general problem has been developed and solved by a numerical procedure. The results obtained are compared with some available experimental data on a box-shaped manifold, arranged with straight and inclined branches.     

Electro-hydraulic tillage depth control system for rotary implements mounted on agricultural tractor Design and response experiments of control system

A tillage depth control system for rotary implements mounted on an agricultural tractor was designed and constructed to improve accuracy of tillage depth. The control system was composed of five main units: (1) a detecting unit to measure the tilting angle (position) of the lift arm, the pitching angle of the tractor and heights of sensors from ground surface, (2) a controlling unit, (3) a hydraulic unit to operate a three-point hitch linkage by a lift arm cylinder, (4) a three-point hitch linkage and rotary implements, and (5) a setting unit to put the reference tillage depth and a dead zone into the control circuit. The tillage depth was controlled by an on/off operation of a solenoid valve, of which time was proportional to the controlling time. Experiments to evaluate the response characteristics of the control system were conducted under various engine speeds, i.e. various flow rates of hydraulic oil, various tillage depths and some input frequencies. The results of the response experiments of the control system are discussed in this paper.     

A micromechanical model of elastoplastic and damage behavior of a cohesive geomaterial

The present study is devoted to the development and validation of a nonlinear homogenization approach of the mechanical behavior of Callovo-Oxfordian argillites. The material is modeled as an heterogeneous composite composed of an elastoplastic clay matrix and of linear elastic or elastic damage inclusions. The macroscopic constitutive law is obtained by adapting the incremental method proposed by Hill [Hill, R., 1965. Continuum micro-mechanics of elastoplastic polycrystals. J. Mech. Phys. Solids 13, 89–101]. The approach consists in formulating the macroscopic tangent operator of the material by considering the nonlinear local behavior of each phase. Due to the matrix/inclusion morphology of the microstructure of the argillite, a Mori–Tanaka scheme is considered for the localization step. The developed model is first compared to Finite Element calculations and then validated and applied for the prediction of the macroscopic stress–strain responses of argillites.     

Shear band formation analysis in soils by the subloading surface model with tangential stress rate effect

The generalized elastoplastic constitutive equation for soils is proposed based on the subloading surface model extended so as to describe the dependence of both the magnitude and the direction of inelastic stretching on the stress rate tangential to the subloading surface [Int J Plasticity 17 (2001) 117]. It would be applicable to the analysis of deformation of soils in both normal-yield and subyield states for not only lower but also higher stress ratio than that in the critical state. Then, the shear band formation in the rectangular specimen subjected to the biaxial compression under the undrained plane strain condition is analyzed by the generalized equation, and thus the condition for shear band formation and the shear band inclination are discussed in relation to material properties and the state of stress, i.e. the stress-ratio and the normal-yield ratio. These results reveal that the tangential stretching term makes easy to fulfill the necessary condition of shear band formation for not only normal-yield but also subyield states, and further the formation is affected by the material parameter prescribing the approaching degree of the stress to the normal-yield state.     

确定应力函数的一种简单方法

在归纳弹性力学平面问题各种选择应力函数方法的基础上,对于狭长矩形截面梁,提出了一种新的确定应力函数的方法. 该方法简单、实用,克服了选择应力函数的盲目性.在弹性力学教学中有一定参考价值.     

A harmonic model of hydrodynamic forces produced by a flapping fin

The hydrodynamic control laws of unsteady fins inspired by swimming and flying animals are considered. A controller based on cycle-averaged forces requires a bandwidth lower than the flapping frequency, with correspondingly slow reactions to disturbances or commands in order to avoid undesirable feedback from the oscillating fins. A harmonic model of the periodic thruster forces was empirically found using a mechanical fin flapping in roll and pitch in hover, in uniform flow, and under various kinematic conditions. A multi-fin vehicle could use this model to account for the dominant non-linearities and minimize undesirable motions through coordinated control of individual fins.     

A cohesive plastic and damage zone model for dynamic crack growth in rate-dependent materials

Mode I steady-state dynamic crack growth in rate-dependent viscoplastic solids containing damage, under small scale yielding conditions, is analyzed based on a modified cohesive zone model. A multi-scale approach is used to describe the entire non-linear zone consisting of a plastic region and a damage region, each of which has its own constitutive law. Traction in the damage region is characterized by a softening power-law, in terms of the ultimate strength, a softening index and a rate sensitivity factor. In the plastic region, the cohesive law is assumed to be both strain hardening and rate dependent. The critical crack opening displacement at the physical crack-tip controls crack growth. The governing integral equations are derived and solved by a collocation method combined with associated boundary conditions. Numerical results are presented for the traction and opening profiles along the cohesive zone, the fracture energy and lengths of the damage and non-linear zones at different crack speeds and for different material parameters. The importance of factors, such as material softening, plastic deformation, crack speed and viscosity, is identified by parametric studies. In addition, the competition of plastic flow and material damage, and its effect on crack growth, are discussed.     

Deformation produced by a simple tensile load in an isotropic elastic body

It is shown that a simple tensile load produces a simple extension provided the empirical inequalities (Truesdell and Noll [1], eqn. 51.27) hold.