Decoding the chemomechanics of friction and scratch in complex granular hydrated oxides

Calcium-Silicate-Hydrate (C–S-–H) is a structurally complex material that is the primary binding phase of all Portland cement concrete materials. C–S-–H is typically considered as an assemblage of discrete nanoscale particles whose interactions are governed by nanoscale friction and interfacial cohesion. Despite the critical role of interparticle forces on mechanics of granular materials, such as C–S-–H, there is currently no unified understanding on the atomistic mechanisms governing the nanoscale friction, scratch, and cohesion in C–S-–H; a lack of knowledge, which presents a major barrier to decode the interplay between chemistry, structure, and mechanics of various C–S-–H systems. Here, we develop a molecular dynamics framework, coupled to a set of redefined classical continuum relations, aimed at filling this gap. The normal force is calculated by relaxation of two C–S-–H particles at various distances and crystallographic orientations. The effect of watery environment is studied by immersing the particles in water molecules, and the friction and scratch forces are calculated by sliding the tip over the substrate in various directions. Our study identifies the distinct contribution of van der Waals and electrostatic forces to the interfacial behavior of C–S-–H particles. Although the electrostatic forces govern the interaction of particles at short and large distances, the van der Waals forces are responsible for variations in the normal force at intermediate distances. We find that normal force varies sublinearly with nanoscale contact area between the tip and substrate, whereas the friction force shows a linear trend. Our results demonstrate to what extent atoms of particular type contribute to the total interfacial forces. Because of large electronegativity, Si atoms (vs. Ca, O, and H) contribute most to the friction and normal forces. Finally, we probe the effect of pile-up and show that the nanoscale scratch test does not follow the classical laws of size-effect in fracture of quasi-brittle materials. Our findings, for the first time, provide an “atomistic lens” on nanoscale friction and contact phenomena in particulate C–S-–H systems. This has a significant impact on fundamental understating of C–S-–H and modulation of its behavior, thus impacting the mechanics of granular cementitious material

On the estimation of foundation damping of mono pile-supported offshore wind turbines

This paper investigates the estimation of foundation damping in a monopile supported offshore wind turbine. The soil-structure interaction is modelled using the commercial geotechnical Finite Element (FE) software, Plaxis 3D. This allows for a more rigorous consideration of the soil response and its effect on the overall dynamic behaviour of the system. A free vibration test is simulated by applying and removing a constant horizontal static load at the top of the tower. The structure starts free vibration when the load is removed. The free decay displacement response is measured at the point of loading. The well-known logarithmic decrement method is used for the estimation of overall damping from the free decay response. The damping is estimated at different time steps along the signal to provide an instantaneous damping. It is shown that the damping varies with the amplitude of the decaying displacement response

An integrated dynamic analysis of a 5MW monopile-supported offshore wind turbine under environmental loads

The 2022 Civil Engineering Research in Ireland (CERI) and Irish Transportation Research Network (ITRN) Conference, Dublin, Ireland, 25-26th August 2022This paper investigates the dynamic behaviour of an offshore wind turbine (OWT) supported on medium dense sand using an integrated load assessment. An integrated modelling approach is introduced to allow for considering aerodynamic and hydrodynamic loading and also soil-structure interaction. A numerical model of the NREL 5MW monopile-supported OWT is initially developed in OpenFAST software. The foundation of the structure is modelled using SESAM software where soilstructure interaction is adopted using API curves and integrated to the OpenFAST model using the stiffness values at mudline level. The integrated model provides a better understanding of the structural behaviour of OWT under various environmental conditions.Irish Research Counci

Molecular Mechanistic Origin of Nanoscale Contact, Friction, and Scratch in Complex Particulate Systems

Nanoscale contact mechanisms, such as friction, scratch, and wear, have a profound impact on physics of technologically important particulate systems. Determining the key underlying interparticle interactions that govern the properties of the particulate systems has been long an engineering challenge. Here, we focus on particulate calcium–silicate–hydrate (C–S–H) as a model system and use atomistic simulations to decode the interplay between crystallographic directions, structural defects, and atomic species on normal and frictional forces. By exhibiting high material inhomogeneity and low structural symmetry, C–S–H provides an excellent system to explore various contact-induced nanoscale deformation mechanisms in complex particulate systems. Our findings provide a deep fundamental understanding of the role of inherent material features, such as van der Waals versus Coulombic interactions and the role of atomic species, in controlling the nanoscale normal contact, friction, and scratch mechanisms, thereby providing de novo insight and strategies for intelligent modulation of the physics of the particulate systems. This work is the first report on atomic-scale investigation of the contact-induced nanoscale mechanisms in structurally complex C–S–H materials and can potentially open new opportunities for knowledge-based engineering of several other particulate systems such as ceramics, sands, and powders and self-assembly of colloidal systems in general

Molecular Mechanistic Origin of Nanoscale Contact, Friction, and Scratch in Complex Particulate Systems

Nanoscale contact mechanisms, such as friction, scratch, and wear, have a profound impact on physics of technologically important particulate systems. Determining the key underlying interparticle interactions that govern the properties of the particulate systems has been long an engineering challenge. Here, we focus on particulate calcium–silicate–hydrate (C–S–H) as a model system and use atomistic simulations to decode the interplay between crystallographic directions, structural defects, and atomic species on normal and frictional forces. By exhibiting high material inhomogeneity and low structural symmetry, C–S–H provides an excellent system to explore various contact-induced nanoscale deformation mechanisms in complex particulate systems. Our findings provide a deep fundamental understanding of the role of inherent material features, such as van der Waals versus Coulombic interactions and the role of atomic species, in controlling the nanoscale normal contact, friction, and scratch mechanisms, thereby providing de novo insight and strategies for intelligent modulation of the physics of the particulate systems. This work is the first report on atomic-scale investigation of the contact-induced nanoscale mechanisms in structurally complex C–S–H materials and can potentially open new opportunities for knowledge-based engineering of several other particulate systems such as ceramics, sands, and powders and self-assembly of colloidal systems in general

Effect of scour erosion on mode shapes of a 5 MW monopile-supported offshore wind turbine

This paper investigates the influence of scour erosion on the mode shapes of the National Renewable Energy Laboratory (NREL) 5 MW offshore wind turbine (OWT) in varying soil conditions, where modes are derived from wind and wave-induced accelerations. A numerical model, which considers aerodynamic and hydrodynamic loading with soil-structure interaction (SSI) and servo-dynamics, is developed. The superstructure is modelled using OpenFAST and the foundation is modelled with SESAM, where SSI is incorporated using Winkler (p-y) springs. The foundation stiffness is subsequently integrated in OpenFAST using mudline interface springs. The influence of soil density, scour hole depth, and loading on the derived mode shapes is studied. Accelerations are extracted at nine locations along the tower. The first two system natural frequencies and mode shapes are estimated using frequency domain decomposition (FDD) applied to the acceleration time-histories for each scour depth and soil condition. Modal Assurance Criterion (MAC) values are calculated to evaluate the changes in mode shapes that occur due to scour. It is shown that the mode shapes present consistent changes as scour depth increases, and the second mode shape exhibits more sensitivity to scour. The sensitivity of mode shape changes to scour under variations in wind speed and measurement noise are also investigated

Numerical modelling of Cone Penetration Test in Clay using Coupled Eulerian Lagrangian Method

The Cone Penetration Test (CPT) has been extensively used in geotechnical engineering, to evaluate the properties of wide range of soils. Numerical simulations of CPT involves large deformations in the soil domain which causes numerical difficulties in traditional finite element analysis. Early numerical studies used simplifying assumptions such as wished-in-place condition and in-situ stress distribution as the initial stress state of the soil domain. Recent developments in finite element analysis allow large deformation analyses to be performed. This paper presents the results of the continuous penetration of cone in single-layer and double-layer clay using the Coupled Eulerian Lagrangian (CEL) method. Performance of the CEL technique was in modelling the cone penetration test was validated against the existing studies. Then, a parametric study was performed to develop a correlation between the cone bearing factor and rigidity index of the clay. The proposed correlation showed a very good agreement with the existing experimental and numerical correlations. The cone penetration test in double-layer clay was simulated. The results suggested that CEL analysis is a reliable technique in modelling large deformation problem of cone penetration test.Geo-engineerin

A data-driven approach for scour detection around monopile-supported offshore wind turbines using Naive Bayes classification

This paper proposes a novel data-driven framework for scour detection around offshore wind turbines (OWTs), where damage features are derived from wind and wave-induced acceleration signals collected along the tower. A numerical model of the NREL 5 MW wind turbine, which considers aerodynamic and hydrodynamic loading with soil-structure interaction (SSI) and servo-dynamics, is developed. The model is used to simulate the acceleration responses along the tower for a healthy structure, and a structure affected by progressive scour. A data segmentation process is initially performed on the collected data, which is followed by a feature selection scheme based on the analysis-of-variance (ANOVA) algorithm, to eliminate irrelevant characteristics from the time domain feature set of responses. The proposed framework consists of two main components: (a) offline training, and (b) real-time classification. The acceleration responses collected from the healthy structure and the structure subjected to three different damage scenarios (different scour depths) and under various load conditions, are used in the offline training mode. The selected feature vector from the feature extraction process is used as input to a Naive Bayes classifier (NBC) algorithm to train the model. In the real-time classification, a prediction of the scour depth affecting the structure is performed using a new dataset simulated from unseen load cases and scour conditions of the OWT. The results show that the model trained in the offline stage can predict the scour depth in the real-time monitoring stage with performance measures over approximately 94%