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This paper studies the feasibility of modelling the maximum stresses in a boltedjoint using analytical solutions, compared to using computed FEM solutions.The bolted joint is simplified to three basic load cases and modelled with Euler-Bernoulli and Timoshenko beam theory. Modelling is feasible for longer beams,but at aspect ratios <1 the FEM solution becomes dominated by singularitiesfrom the FEM model’s sharp corners and boundary conditions. These singular-ities mirror real-life stress concentrations at the dimensional transition betweenshank and bolt head as well as the fixed nature of the threaded section, and mustbe accounted for in an analytical model.

We study high-speed friction on soft polymer materials by measuring the amplitude dependence of cyclic lateral forces on the atomic force microscope (AFM) tip as it slides on the surface with fixed contact force. The resulting dynamic force quadrature curves separate the elastic and viscous contributions to the lateral force, revealing a transition from stick-slip to free-sliding motion as the velocity increases. We explain force quadratures and describe how they are measured, and we show results for a variety of soft materials. The results differ substantially from the measurements on hard materials, showing hysteresis in the force quadrature curves that we attribute to the finite relaxation time of viscoelastic surface deformation., QC 20220104

Studiens övergripande syfte är att undersöka hur läsart produceras i en pedagogisk miljö när ett drama ska upp till produktion. Med hjälp av ett designteoretiskt perspektiv lyfts vilka resurser som används i arbetet med etablerandet av läsart och tillsammans med performativ teori visas hur dessa resurser påverkar elevernas förhållande till läsart och teater som medium. Studien utgår från en situation där en grupp teaterelever i åk 3 på gymnasiet i staden X tillsammans med sin lärare försöker utröna ett dramas läsart inför deras medverkan i Riksteaterns LÄNK-projekt 2018. I studien redovisas deras tre första möten med texten. Uppmärksamhetsfokus ligger på de representationer av kunskap kring läsart, i förhållande till ämnet teater, som kommuniceras och transformeras genom tre modala perspektiv: högläsning, skrivande och kroppslighet. Studien visar på hur läsart uppkommer och transformeras genom elevernas positionering i förhållande till de resurser som används och som i ett performativt perspektiv visar hur elevernas kunskap om teater utvecklas genom deltagande i undervisningen.

We perform a comparative study of dynamic force measurements using an Atomic Force Microscope (AFM) on the same soft polymer blend samples in both air and liquid environments. Our quantitative analysis starts with calibration of the same cantilever in both environments. Intermodulation AFM (ImAFM) is used to measure dynamic force quadratures on the same sample. We validate the accuracy of the reconstructed dynamic force quadratures by numerical simulation of a realistic model of the cantilever in liquid. In spite of the very low quality factor of this resonance, we find excellent agreement between experiment and simulation. A recently developed moving surface model explains the measured force quadrature curves on the soft polymer, in both air and liquid., QC 20180522

The interaction between a rapidly oscillating atomic-force-microscope tip and a soft-material surface is described with use of both elastic and viscous forces in a moving-surface model. We present the simplest form of this model, motivating our derivation with the models ability to capture the impact dynamics of the tip and sample with an interaction consisting of two components: interfacial or surface force, and bulk or volumetric force. Analytic solutions to the piecewise linear model identify characteristic time constants, providing a physical explanation for the hysteresis observed in the measured dynamic-force-quadrature curves. Numerical simulation is used to fit the model to experimental data, and excellent agreement is found with a variety of different samples. The model parameters form a dimensionless impact-rheology factor, giving a quantitative physical number to characterize a viscoelastic surface that does not depend on the tip shape or cantilever frequency., QC 20181009

Investigating visco-elastic forces at the nanometer-scale is important to thecharacterization of soft materials. A quantitative force measurement can be ob-tained using an atomic force microscope (AFM) with a calibrated force transducer(the AFM cantilever). In this thesis, we discuss and evaluate simple methods ofcalibration and we use these calibrations to measure dynamic force quadraturecurves for both normal and in-plane tip-surface forces using Intermodulation AFM(ImAFM). ImAFM utilizes the nonlinearity of the tip-surface force by measuring the mix-ing between two or more drive frequencies placed close to a resonance of the AFMcantilever. The intermodulation response at many mixing frequencies provides ad-ditional observables, useful for characterization of materials. We use ImAFM nearthe first flexural resonance to measure visco-elastic materials and we show that sur-face motion plays an important role in the analysis of soft samples. To explain ourmeasurements we derive a simple model where the surface position is described byan exponential relaxation when perturbed from its equilibrium. Through numericalsimulations of this model we explain experiments for many different soft sampleswith varying properties. We further apply the intermodulation technique to softsamples in liquid. ImAFM at the first torsional resonance frequency induces motion of the tip in-plane with the surface, enabling friction measurements between the tip and sample.Due to the high torsional resonance frequency, the tip velocity can reach severalcm/s, many orders of magnitude higher than typical AFM friction measurements.By measuring the amplitude dependence of the dynamic force quadrature curves,we can resolve the transition between the tip sticking to the surface, through stick-slip to free sliding motion., QC 20180522

Surface science, which spans the fields of chemistry, physics, biology and materials science, requires information to be obtained on the local properties and property variations across a surface. This has resulted in the development of different scanning probe methods that allow the measurement of local chemical composition and local electrical and mechanical properties. These techniques have led to rapid advancement in fundamental science with applications in areas such as composite materials, corrosion protection and wear resistance. In this perspective article, we focussed on the branch of scanning probe methods that allows the determination of surface nanomechanical properties. We discussed some different AFM-based modes that were used for these measurements and provided illustrative examples of the type of information that could be obtained. We also discussed some of the difficulties encountered during such studies., QC 20171009

The mechanical properties of polymeric nanocomposites are strongly affected by the nature of the interphase between filler and matrix, which can be controlled by means of surface chemistry. In this report, we utilize intermodulation atomic force microscopy (ImAFM) to probe local mechanical response with nanometer-scale resolution of poly(dimethylsiloxane) (PDMS) coatings with and without 20 wt% of hydrophobic silica nanoparticles. The data evaluation is carried out without inferring any contact mechanics model, and is thus model-independent. ImAFM imaging reveals a small but readily measurable inhomogeneous mechanical response of the pure PDMS surface layer. The analysis of energy dissipation measured with ImAFM showed a lowering of the viscous response due to the presence of the hydrophobic silica nanoparticles in the polymer matrix. An enhanced elastic response was also evident from the in-phase stiffness of the matrix, which was found to increase by a factor of 1.5 in presence of the nanoparticles. Analysis of dissipation energy and stiffness in the immediate vicinity of the nanoparticles provides an estimate of the interphase thickness. Because the local stiffness varies significantly near the nanoparticle, AFM height images contain artifacts that must be corrected in order to reveal the true surface topography. Without such a correction the AFM height images erroneously show that the stiff particles protrude from the surface, whereas corrected images show that they are actually embedded in the matrix and likely covered with a thin layer of polymer., QC 20170818

Atomic force microscope (AFM) users often calibrate the spring constants of cantilevers using functionality built into individual instruments. This calibration is performed without reference to a global standard, hindering the robust comparison of force measurements reported by different laboratories. Here, we describe a virtual instrument (an internet-based initiative) whereby users from all laboratories can instantly and quantitatively compare their calibration measurements to those of others-standardising AFM force measurements-and simultaneously enabling non-invasive calibration of AFM cantilevers of any geometry. This global calibration initiative requires no additional instrumentation or data processing on the part of the user. It utilises a single website where users upload currently available data. A proof-of-principle demonstration of this initiative is presented using measured data from five independent laboratories across three countries, which also allows for an assessment of current calibration.

QC 20180522

QC 20180522

We perform a comparative study of dynamic force measurements using an Atomic Force Microscope (AFM) on the same soft polymer blend samples in both air and liquid environments. Our quantitative analysis starts with calibration of the same cantilever in both environments. Intermodulation AFM (ImAFM) is used to measure dynamic force quadratures on the same sample. We validate the accuracy of the reconstructed dynamic force quadratures by numerical simulation of a realistic model of the cantilever in liquid. In spite of the very low quality factor of this resonance, we find excellent agreement between experiment and simulation. A recently developed moving surface model explains the measured force quadrature curves on the soft polymer, in both air and liquid.