Volume 100, Issue 7

We report a detailed study of the shear interaction between two tropocollagen molecules, a major mechanism that contributes to the fibril mechanical behavior. Using steered molecular dynamics simulations in explicit solvent, we model the slip of two tropocollagen molecules at varying pulling rates. We find that the adhesion strength is highly sensitive to the pulling rate, and that it converges to a value of 10.12 pN Å – 1 for vanishing loading rates. We find that intermolecular H-bonds play a key role in determining the resistance against slip. Our results provide quantitative details on this mechanism of load transmission inside collagen fibrils and fibers, which is crucial for the development of constitutive models of collagenous tissues at larger hierarchical levels. Such constitutive models of collagenous tissue mechanics have many applications, ranging from development of bio-inspired materials to studies in tissue engineering. By incorporating pathological collagen mutations, our studies could advance our knowledge of mechanisms underlying important collagen-related diseases like Osteogenesis Imperfecta or Ehlers – Danlos Syndrome.

Spherical indentation is commonly used to study the elastic and visco-elastic response of materials at different length scales. However, depending on the strength properties of the investigated material and the geometric properties, such as sphere radius and penetration depth, plastic material response might occur, making parameter identification from load – penetration curves a difficult task. In this paper, the influence of the aforementioned parameters is investigated by means of numerical simulations, considering elasto-plastic material response. The numerical results obtained are presented in dimensionless format, finally providing (i) general rules for the specification of test parameters in the case of spherical indentation, on the one hand, and – in the case plastic deformation takes place – (ii) relations for the determination of plastic model parameters (such as the deviatoric strength) from the loading phase of the indentation test.

A simple model of elastic – plastic penetration of a Berkovich pyramid into a multilayer coating has been developed. The model of a simply deformable body was used. It generalizes Winkler's foundation to the case of elastic – plastic deformation of each layer. The elastic – plastic deformation of a layer was modeled by Prandtl's bilinear approximation of a stress – strain curve. The homogenized coefficients of the elastic and plastic properties of the layer set and an analytical solution of the problem were found. The distribution of contact stress and depth of a pyramid indenter penetration were obtained for different angles.

Nanoindentation allows local characterization of the microstructural processes associated with pseudoelastic recovery in NiTi shape memory alloys. We discuss nanoindentation and tensile testing results, both carried out at different temperatures on a NiTi alloy with three different microstructures (containing no, small and large Ni-rich precipitates). Maximum shape recovery in indentation experiments and during tensile testing occurs in microstructures with fine precipitates. Thermomechanical constraints affect pseudoelasticity differently in micro- and macro-scale testing: Indentation at temperatures as close as possible to the transformation start temperatures is associated with better shape recovery than testing at higher temperatures, whereas maximum shape recovery is only observed at slightly higher temperatures in macroscopic testing. These results are rationalized by considering the effect of temperature on stress-induced martensitic transformation and dislocation plasticity.

Load versus displacement curves obtained from nanoindentation experiments on polymers are particularly difficult to analyze due to their complex mechanical and especially time-dependent behavior. The indentation curves obtained on two amorphous polymers (polycarbonate (PC) and polymethyl methacrylate (PMMA)) show an increase in the indentation depth during load hold time. A non-linear time-dependent model shows that this phenomenon could be decomposed in a first part due to a viscoelastic response and a second part assigned both to a viscoelastic and a viscoplastic response. If the first part is not achieved, the values of the elastic modulus computed using the Oliver and Pharr method are abnormally high. If it is achieved, the beginning of the unloading curve presents an abnormality that is linked to the increase in indentation depth during hold load time. Nevertheless, if the load hold time is high enough and if the contact stiffness is calculated from the initial slope of the power fit of the unloading curve, the values of the elastic modulus computed using the Oliver and Pharr method are close to those obtained by tensile tests.

The indentation method is a useful tool for studying the onset of plastic flow and work hardening in small volumes. Several approaches based on either single and dual indenters have been reported to characterise a wide range of materials. In this work, the single and dual indenters approaches, with different indenter types, have been comparatively studied using an inverse finite element modelling program developed based on the Kalman filter method. The effect of indenter shapes on the accuracy and uniqueness of the predicted results, as well as convergence time of the program are systematically analysed by automatically mapping the results with a wide range of initial values.

Analytical and numerical methods for the identification of creep properties of two polymers and indium from indentation tests are compared. Creep indentation experiments are performed at fixed peak force following ramp loading. The experiments are modeled both by analytical techniques and by finite elements and the models are integrated in an identification routine. The assumed mechanical behavior allows for a satisfactory fit of the experimental data. For the considered viscoelastic materials analytical and numerical modeling are equivalent whereas for indium finite element modeling is to be preferred since the analytical model is not representative at the large levels of plastic strain experimentally obtained.

The determination of mechanical properties of thin films is of crucial interest for applications of protective layers. Nanoindentation and nanoscratch experiments of diamond-like carbon coated (100) silicon surfaces under ambient conditions have been performed with a conical indenter on a Hysitron system, mounted on an atomic force microscope system. Single layers and multilayers with hard soft stacking were tested as well. Microhardness, friction and wear behaviour were determined with multi-indentations, single scratches and oscillating multicycles scratch. The indentation experiments revealed that critical force plastic deformation takes place. The oscillating multicycles scratch experiments determined the high wear resistance of the diamond-like carbon coatings. Multilayer systems were compared with single layer coatings.

To a considerable degree hardness is an empirical value which depends on many details involved in the measuring process. In the present work, hardness values were measured in the depth range 2 – 800 nm using microhardness, nanoindentation and scratch hardness tests on fused silica, steel and sapphire. It is shown that overestimation of hardness values for steel and sapphire with the nanoindentation method can be directly related to the relative height of pile-ups observed on the topography image of the indentation imprint. It is also shown that the scratch hardness test is a promising technique for nanoscaled hardness measurements.

Brushes made of carbon nanotubes have been tested by use of atomic force microscopy. Two different cantilevers were used: a silicon cantilever with a spherical tip made of SiO 2 and a beryllium bronze cantilever with a tip made of steel. Sliding in the lateral direction was studied to measure the friction coefficient and to examine nanotribological properties. Nanoindentation was used to measure the pull-off force. Finally, a model of adhesion was created and experimental results were compared with it.

In industrial biotechnology microbial cells are utilized to produce diverse products (e. g., enzymes, food and animal feed additives, pharmaceutical agents). Whilst relationships on genetic and molecular levels are reasonably understood there are still numerous unsolved questions in matters of the mechanical properties of those cells. Recently, a compression test in liquid environment using a nanoindentation device was established to address the cell mechanical behavior of Saccharomyces cerevisiae (baker's yeast) cells. At higher deformations compressive stressing yielded failure of the cell wall. In the present work, the influence of the extracellular osmotic conditions on the failure characteristics (bursting force, bursting energy, and relative deformation at bursting) was studied. Moreover, a simple mechanical model was applied to characterize the mechanical properties of the cell wall.

Aluminium, Al-4Cu-0.5Mg-1Mn aluminium alloy (AA2017), copper, iron and nickel in the form of a bundle of wires placed in appropriate capsules were subjected to cumulative hydrostatic extrusion to form 3 mm wires with total true strain above 10 in Al, the aluminium alloy and Cu and above 7 in Ni and Fe. The deformation was first induced in single wires which were then assembled and further deformed to form a multifilament wire. The final product had a nanocrystalline (grain size below 100 nm in Fe and the 2017 Al alloy) and ultra-fine grain (below 130 nm in Ni and below 270 nm in Cu and Al) microstructure. The grain refinement resulted in the microhardness being increased, with the greatest increase to 3.5 GPa being achieved in iron. During hydrostatic extrusion, the severe deformation was possible due to the high applied pressures of up to 1.7 GPa. The hydrostatic extrusion of multifilament wires broadens the application range of this process extending it to the fabrication of ultra-fine grain and nanocrystalline microstructures.

In the present work, investigations of the microstructural changes were carried out during hardening and tempering of 42CrMo4 (1.7225) steel. Using transmission electron microscopy, the hardened state was examined initially. Following this, the material was incrementally heated in a heated specimen holder to 580 °C. Diffraction patterns were recorded at regular intervals in order to analyse and document the changes from a martensitic structure to a ferritic structure. The characteristic tempering stages were identified for the material and assigned to a temperature range. Moreover, thermal activated diffusion processes were detected above 500 °C.

The enthalpies of mixing in liquid Ag – Ca, Ag – Eu and Ag – Yb alloys were measured by isoperibolic calorimetry at 1300 K. The experimental data were used to calculate the mixing enthalpies within the total composition region. The results of the investigation show exothermic effect of alloy formation. The minimum integral enthalpies of mixing are – 23.5 ± 1.0 kJ · mol – 1 (the Ag – Ca system); –16.5 ± 1.5 kJ · mol – 1 (the Ag – Eu system) and – 21.5 ± 1.9 kJ · mol – 1 (the Ag – Yb system) and are located in the silver-rich composition regions.

The precipitation kinetics of Si after different thermal treatments in AlSi-alloys have been studied by means of isothermal calorimetry, differential scanning calorimetry and dilatometry tests. The measured exothermic and endothermic effects produced by the precipitation and dissolution of Si are correlated with the increase and reduction in the coefficient of thermal expansion CTE(T), respectively. The deviation of the CTE(T) of the studied AlSi1.1 – 1.7 alloys with respect to pure Al correspond to the volume fraction of precipitating Si according to thermodynamic solubility limits. Two different kinetics in the growth of Si precipitates are distinguished: while Si precipitates in the range of 200 – 300 °C during heating of solution quenched samples supersaturated with vacancies, retardation into the temperature range of 300 – 420 °C is observed in samples slowly cooled after solution treatment. The presence of the eutectic Si increases the quench sensitivity of the precipitation kinetics of Si as observed in CTE(T) curves. The ripening of the eutectic Si during prolonged solution treatment retards the Si precipitation slightly.