Mode conversion and scattering of lamb waves at delaminations in composite laminates

Abstract not availableGnana Teja Pudipeddi, Ching-Tai Ng, and Andrei Kotouso

Determining openness and compressibility of natural fractures of carbonate reserves in the Logovskoye deposit

The large amounts of hydrocarbons and produced oil in Perm krai are found in carbonate reservoirs. Accurate determination of reservoir properties is currently one of the topical issues. Laboratory studies of core samples do not always allow to evaluate parameters of fractured environment because of the eventual core destruction (along the fractures) at the surface and tiny dimensions of the samples investigated. A method to determine parameters of natural fractures involving tracer injection into the injection wells in the Logovskoye deposit, Perm krai (Tournaisian-Famennian deposit) is presented. The data obtained by tracer studies are compared to those got by other methods of evaluating openness of natural fractures, such as a Warren-Root formula applied to process pressure build up curves in reservoirs with natural fracturing, and the relation discovered by V.D. Viktorin for carbonate reservoirs of Perm krai. An error of the results received by different methods does not exceed 5 %, which means fidelity and overall accuracy of the techniques applied and their feasibility for investigation of fracturing in carbonate reservoirs. It is established that pressure conductivity in carbonate reservoirs varies from the highest value corresponding to pressure conductivity in fractured-porous formation to the lowest value corresponding to pressure conductivity in the matrix pores. This allows us to conclude that a reservoir features natural fracturing. One of the most serious issues in 3D-simulation, especially of carbonate reserves, is accurate determination of permeability, openness and directional attitude of natural fractures. The studies with tracers coupled with hydrodynamic research (pressure interference test) in the process of carbonate reserves development permit to improve the quality of input data and fidelity of prediction calculations

3D finite element prediction of scattering and mode conversion of lamb waves at delaminations in composite laminates

Detection of damage using Lamb wave has attracted significant attention in the last decade. It has been shown that Lamb wave is sensitive to most types of damages, efficient in detecting small and subsurface damage, and able to inspect large area. However, the capability and sensitivity of Lamb wave on detecting delaminations in composite laminates, especially the mode conversion effect, have not yet been fully investigated, which limits quantitative characterization of the delaminations. This study presents a three-dimensional (3D) finite element (FE) simulation of the fundamental anti-symmetric mode (A0) Lamb wave scattering and mode conversion from A0 to fundamental symmetric mode (S0) Lamb wave at the delamination in composite laminates. The 3D FE simulation is first validated using experimentally measured data, such as group velocity and amplitude of the incident Lamb wave in different wave propagation directions. There is good agreement between the FE calculated and experimentally measured results. The 3D FE model is then employed to predict the A0 scattered Lamb wave and mode conversion from A0 to S0 Lamb wave, which provides understanding about the scattering and mode conversion phenomenon of Lamb wave at the delaminations.C. T. Ng, G. T Pudipeddi, A. Kotouso

Numerical simulation and experimental verification of lamb wave scattering and mode conversion at delaminations in composite laminates

This study presents an experimental verification of a three-dimensional (3D) finite element (FE) model for simulating Lamb wave scattering and mode conversion at delaminations in composite laminates. In this study, an explicit 3D FE model is proposed to simulate the fundamental anti-symmetric mode (A0) Lamb wave scattering and mode conversion at the delaminations in the composite laminates. The 3D FE model is first verified with the experimental results and then it is used to investigate phenomena of the A0 Lamb wave scattering and mode conversion from A0 to S0 Lamb wave at the delamination. Overall, there is good agreement between the FE simulated results and experimental measured data. The results of this study provide insights into the Lamb wave scattering and mode conversion at the delaminations in the composite laminates.C. T. Ng, G. T. Pudipeddi and A. Kotouso

Elementary mathematical theory of thermal stresses and fracture during welding and cutting

In many technological processes involving the cutting or welding of thin plates there is local thermal heating or cooling at the tip of the cut. Under particular conditions the cut may be considered as a semi-infinite crack and the thermal source as a point heat source. For cutting processes the cut is behind the moving thermal source and for welding the cut is ahead of the moving thermal source. For most processes the value of the thermal source is positive (e.g. lasers, electron beam cutting, welding and others). In this paper the authors investigate analytically the stress distribution induced by a point thermal source moving with a constant velocity in an infinite plate. The stress intensity factor due to the point thermal source at the tip of the cut is calculated. It is shown that for both welding and cutting in the case of a thermal heat source the stress intensity factors will be negative, which means that the thermal field induced by the point thermal heat source will tend to close the surfaces of the cut in the vicinity of the tip. The opposite situation occurs when the cut tip is cooled by the thermal source. In this case there are positive values for the stress intensity factors and the thermal stresses may lead to brittle fracture ahead of the cut. As an example, the applications of the theory under development to the uncontrolled fracture phenomenon during the thermal beam cutting of brittle material is considered in detail. A fracture criterion for this process is obtained, which enables choosing the manufacturing parameters of the process in order to avoid the possibility of uncontrolled fracture

Ligand-mediated adhesive mechanics of two static, deformed spheres

Originally submitted as "Ligand mediated adhesive mechanics of two deformed spheres"A self-consistent model is developed to investigate attachment/detachment kinetics of two static, deformable microspheres with irregular surface and coated with flexible binding ligands. The model highlights how the microscale binding kinetics of these ligands as well as the attractive/repulsive potential of the charged surface affects the macroscale static deformed configuration of the spheres. It is shown that in the limit of smooth, neutrally charged surface (i.e., the dimensionless inverse Debye length, κD→∞), interacting via elastic binders (i.e., the dimensionless stiffness coefficient, γ → 0) the adhesion mechanics approaches the regime of application of the JKR theory, and in this particular limit, the contact radius, Rc, scales with the particle radius, R, according to the scaling law, Rc ∝ R²⁄³. We show that static, deformed, highly charged, ligand-coated surface of micro-spheres exhibit strong adhesion. Normal stress distribution within the contact area adjusts with the binder stiffness coefficient, from a maximum at the center to a maximum at the periphery of the region. Although reported in some in vitro experiments involving particle adhesion, until now a physical interpretation for this variation of the stress distribution for deformable, charged, ligand-coated microspheres is missing. Surface roughness results in a diminished adhesion with a distinct reduction in the pull-off force, larger separation gap, weaker normal stress and limited area of adhesion. These results are in agreement with the published experimental findings.Sarthok Sircar, Giang Nguyen, Andrei Kotousov, and Anthony J. Robert

Localised failure mechanism as the basis for constitutive modelling of geomaterials

Localised failure of geomaterials in the form of cracks or shear bands always requires special attention in constitutive modelling of solids and structures. This is because the validity of classical constitutive models based on continuum mechanics is questionable once localised inelastic deformation has occurred. In such cases, due to the fact that the macro inelastic responses are mainly governed by the deformation and microstructural changes inside the localisation zone, internal variables, representing these microstructural changes, should be defined inside this zone. In this paper, the localised failure mechanism is identified and employed as an intrinsic characteristic upon which a constitutive model is based on at the first place, instead of being dealt with after developing the model using various regularisation techniques. As a result, inelastic responses of the model are correctly associated with the localisation bands, and not smeared out over the whole volume element as in classical continuum constitutive models. It is shown that this inbuilt localisation mechanism in a constitutive model can naturally capture important features of the material and possess intrinsic regularisation effects while minimising the use of additional phenomenological treatments, and also possessing intrinsic regularisation effects. The development of the proposed model is based on an additional kinematic enhancement to account for high gradient of deformation across the localisation band. This enrichment allows the introduction of an additional constitutive relationship for the localisation band, which is represented in the form of a cohesive-frictional model describing traction-displacement jump relationship across two sides of the localisation band. The model, formulated within a thermodynamically consistent approach, possesses constitutive responses of the bulk material and two localisation bands connected through internal equilibrium conditions. Its key characteristics are demonstrated and validated against experimental data from different types of geomaterials under different loading conditions at both material and structural levels.Linh A. Le, Giang D Nguyen, Ha H. Bui, Abdul Hamid Sheikh, Andrei Kotouso

Modelling jointed rock mass as a continuum with an embedded cohesive-frictional model

The mechanical and hydraulic properties of a jointed rock mass are strongly affected by the characteristics of joints within the intact rock mass. In this study, a constitutive model for jointed rock masses is developed by incorporating the contributions of both the joint and its surrounding rock mass. The behaviour of the joint is represented by a new coupled damage-plasticity cohesive-frictional model taking into account its dilation evolution and the reduction of both strength and stiffness, while the surrounding rock behaviour is assumed to behave elastically. The interactions between the joint and the surrounding rock are described by a set of kinematic enhancements and internal equilibrium equations across the interface of the joint. The formulation of the proposed model is presented along with its implementation algorithms and validation with experimental data. The enhanced kinematics facilitates the incorporation of both behaviour and orientation of the joint, together with the size and behaviour of the surrounding rock, allowing capturing key characteristics of jointed rock mass responses under mixed-mode loading conditions at different spatial scales.Linh A. Le, Giang D. Nguyen, Ha H. Bui, Abdul H. Sheikh, Andrei Kotousov, Aditya Khann