2D Spring-block model to study the transition from static to kinetic friction of complex-micro-textured contact surfaces

The capability of complex micro-texturing technique for tuning the transition from static to kinetic friction is investigated based on a two-dimensional (2D) lattice spring block model. Results reveal that implementation of micro-texturing remarkably decreases the static friction coefficient even for a small amount of covering percentage, however this effect gets slight after covering percentage of about 10%. It is observed that elongation of micro-texturing cavities perpendicular to the sliding direction can improve its reducing effect on static friction coefficient. Furthermore, as simulations prove, using complex shapes of micro-texturing cavities with sharp vertexes slightly modifies the frictional response

The analysis and identification of friction joint parameters in the dynamic response of structures.

Imperial Users onl

Laboratory earthquake forecasting. A machine learning competition

Earthquake prediction, the long-sought holy grail of earthquake science, continues to confound Earth scientists. Could we make advances by crowdsourcing, drawing from the vast knowledge and creativity of the machine learning (ML) community? We used Google’s ML competition platform, Kaggle, to engage the worldwide ML community with a competition to develop and improve data analysis approaches on a forecasting problem that uses laboratory earthquake data. The competitors were tasked with predicting the time remaining before the next earthquake of successive laboratory quake events, based on only a small portion of the laboratory seismic data. The more than 4,500 participating teams created and shared more than 400 computer programs in openly accessible notebooks. Complementing the now well-known features of seismic data that map to fault criticality in the laboratory, the winning teams employed unexpected strategies based on rescaling failure times as a fraction of the seismic cycle and comparing input distribution of training and testing data. In addition to yielding scientific insights into fault processes in the laboratory and their relation with the evolution of the statistical properties of the associated seismic data, the competition serves as a pedagogical tool for teaching ML in geophysics. The approach may provide a model for other competitions in geosciences or other domains of study to help engage the ML community on problems of significance

Harmonic excitation of bolted joints

Bolted joints provide one of the most common means of joining two structural components together. The joints themselves use friction to transmit force, torque and motion across a common interface from one component to another. In many cases a pretensioned bolt, running through a common hole at the joint interface, provides the clamping force. The friction force at a joint interface is highly nonlinear. This makes the analysis of dynamic systelTIS with joints unrealistic with conventional linear techniques. It has also been shown that the contact pressure at a joint interface is not necessarily uniform. A variable contact pressure results in a variable limiting friction load. Where the contact pressure can be shown to be smallest on an interface, local microslip can take place whilst the joint maintains its sticking contact elsewhere. Microslip is responsible for the dissipation of energy from within bolted joints that otherwise maintain their integrity. The level of energy dissipation caused by microslip can be significantly larger than that provided by other dissipative mechanisms within a structure. This provides an incentive to be able to describe and predict the energy losses and overall joint behaviour accurately. Difficulties arise when considering 3-Dimensional contact, changing contact conditions during dynamic loading and the nonlinear nature of friction phenomena. To investigate microslip behaviour in bolted joints a detailed finite element model of an isolated lap joint interface was constructed. The joint interface was subjected to a variety of preloads and applied torque. Output from the joint is in the form of hysteresis loops that reveal information about the energy dissipated and overall joint stiffness during a loading cycle. Representative models are presented that reduce the complexity of the joint, yet still maintain the defining characteristics of the hysteretic behaviour. The first representative model uses Jenkins elements that match the physical response of the joint at a number of discrete points during the loading cycle. Good agreement between the finite element model and the Jenkins element model is illustrated. The Jenkins element model is also capable of predicting the response of the finite element model when different magnitudes of preload and applied torque are applied. The second representative model is the Bouc-Wen representation of hysteresis. This model offers significant gains in efficiency when approximating the smooth transition from a fully sticking interface to the onset of joint failure. All of the hysteresis can be described using just four parameters, and matching with the finite element model is demonstrated. To demonstrate microslip behaviour physically an individual joint was experimentally analysed. A cantilever beam with a single lap joint near the clamped end is resonated to generate the dynamic joint hysteresis. The joint behaviour is monitored by local time domain measurements at a number of different preloads and excitation amplitudes. Microslip is demonstrated in the joint when the preload is reduced from a maximum "rigid" clamping value. Notably at low preloads the spectral content of the response reveals a large contribution from the superharmonics of the excitation frequency. Both the Jenkins element model and the Bouc-Wen model are successfully matched to the hysteresis output of the experimental joint

The concept of nonlinear modes applied to friction-damped systems

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Some aspects of human performance in a Human Adaptive Mechatronics (HAM) system

An interest in developing the intelligent machine system that works in conjunction with human has been growing rapidly in recent years. A number of studies were conducted to shed light on how to design an interactive, adaptive and assistive machine system to serve a wide range of purposes including commonly seen ones like training, manufacturing and rehabilitation. In the year 2003, Human Adaptive Mechatronics (HAM) was proposed to resolve these issues. According to past research, the focus is predominantly on evaluation of human skill rather than human performance and that is the reason why intensive training and selection of suitable human subjects for those experiments were required. As a result, the pattern and state of control motion are of critical concern for these works. In this research, a focus on human skill is shifted to human performance instead due to its proneness to negligence and lack of reflection on actual work quality. Human performance or Human Performance Index (HPI) is defined to consist of speed and accuracy characteristics according to a well-renowned speed-accuracy trade-off or Fitts’ Law. Speed and accuracy characteristics are collectively referred to as speed and accuracy criteria with corresponding contributors referred to as speed and accuracy variables respectively. This research aims at proving a validity of the HPI concept for the systems with different architecture or the one with and without hardware elements. A direct use of system output logged from the operating field is considered the main method of HPI computation, which is referred to as a non-model approach in this thesis. To ensure the validity of these results, they are compared against a model-based approach based on System Identification theory. Its name is due to being involved with a derivation of mathematical equation for human operator and extraction of performance variables. Certain steps are required to match the processing outlined in that of non-model approach. Some human operators with complicated output patterns are inaccurately derived and explained by the ARX models

Automated Post-Earthquake Building Damage Assessment Using Smart Devices

Conventional practices to evaluate post-earthquake damage to buildings rely on reconnaissance teams deployed to the affected areas to carry out visual inspections of buildings. These inspections are done manually and are therefore time consuming and error prone. Motivated by these drawbacks, this dissertation explores the possibility and means for conducting automated inspections using smart devices, which are ubiquitous in modern communities. The premise is that smart devices can record acceleration data using their onboard sensors. The records can then be double integrated and processed to yield interstory drift ratios (IDRs), which are key indicators of building damage. The dynamic behavior of a smart device on an underlying surface subjected to seismic motion is studied first. The smart device and its frictional interactions with the underlying surface are represented using a modified friction model. The conditions under which the smart device slides on or sticks to the surface for a given earthquake intensity are investigated. The concept of a ‘probability of exceeding the slip limit curve’ is introduced to relate the probability of exceeding a given slip limit for a given structure and location. The presence of sliding motions in an acceleration record can contaminate the recorded data and make it impossible to estimate the motion of the underlying floor from smartphone measurements. To resolve this problem, stick-slip motion identification methods are studied based on two approaches. The first method relies on the theoretical observation that acceleration is constant during sliding. The second method employs two different types of machine learning algorithms to differentiate between sticking and slipping motions. It is shown that the developed techniques can yield reasonably high classification accuracies. Computation of IDR requires multiple steps, each of which is theoretically investigated and experimentally validated by using a shake table and multiple types of smart devices with different types of protective shells. The needed steps include record synchronization and warping, data fusion, and compensation for errors that are magnified by double integration (needed to compute IDR). The abilities of different types of smart devices to estimate displacement were compared and the error in displacement was shown to have a strong relationship to their mean square amplitude of stationary noise. The proposed IDR estimation process is validated using the results from previously published shake table experiments of a four-story steel frame structure. It is shown that reasonable estimates of IDR can be achieved by using the developed methods.PHDCivil EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/149867/1/yunsu_1.pd

A study of layered contact problems with particular application to tyre-wheel interfaces

This thesis is concerned with the characterization of layered contact problems. The research project has been prompted by an investigation into creep, and ultimately, gross sliding, of rubber tyres fitted around steel wheels in earth moving equipment. In general, slippage between tyre and rim is experienced in common engineering applications employing tyred systems. A general and systematic approach for investigating the interfacial behaviour of tyred systems has consequently been proposed. Classical techniques together with novel numerical approaches based on advanced mathematical programming have been implemented to support the investigation. Creep between mating surfaces, frictional shakedown and measuring friction in partial slip condition are the main objects of investigation. The analytical and numerical models developed by the author have been complemented by experimental work, whose detailed description is also included in this thesis. Finally, further studies have been performed to shed light on some of the design issues offered by the threedimensional full-scale engineering application. The numerical approach based on finite element modelling used to tackle these aspects of the project and the experimental work carried out by the author to corroborate the numerical findings are also presented