S. Kucharski

Finite-elements model for the contact of rough surfaces

Abstract In this paper the elastoplastic asperity-based model for the contact of rough surfaces is presented. The model adopts most of the basic asperity-based models assumptions, introducing, however, a more realistic elastoplastic deformation law for the analysis of individual asperity deformation. The elastoplastic deformation of an individual asperity is numerically investigated by means of the Finite Elements Method, and evaluated relations are combined with statistical description of the surface. The results were contrasted and discussed together with results obtained from other models, as well as with experimental results.

Identification of plastic hardening parameters of metals from spherical indentation tests

Abstract Using the two-parameter power hardening rule σ=kepm, the parameters k and m are identified from spherical indentation loading–unloading tests which account for the variation of the indentation profile during elastic unloading and sphere deformation. The predicted and measured stress–strain curves are compared for several materials. Both experimental and actual data for 18G2A low-alloy steel are used to assess the accuracy of the identification procedure. Finally, identification of the stress–strain curve of an aluminium alloy is demonstrated.

Identification of Hardening Parameters of Metals From Spherical Indentation Tests

The identification method of hardening parameters specifying stress-strain curve is proposed by applying spherical indentation test and measuring the penetration depth during loading and unloading. The loading program is composed of a geometric sequence of loading and partial unloading steps from which the variation of permanent penetration with load level is determined. This data is used for specification of two parameters k and m occurring in the plastic hardening curve e p = (σ/k) l/m , where e p denotes the plastic strain.

Are trichomes involved in the biomechanical systems of Cucurbita leaf petioles

AbstractMain conclusionTrichomes are involved in petiole movement and likely function as a part of the plant biomechanical system serving as an additional reservoir of hydrostatic pressure. The large, non-glandular trichomes on Cucurbita petioles occur across collenchyma strands. Time-lapse imaging was used to study the leaf reorientation of Cucurbita maxima ‘Bambino’ plants placed in horizontal position. The experiment comprised four variants of the large non-glandular petiole trichomes: (1) intact, (2) mechanically removed, (3) dehydrated, and (4) intact but with longitudinally injured petioles. Isolated strands of collenchyma with intact epidermis or epidermis mechanically removed from the abaxial and adaxial sides of the petiole were subjected to breaking test. The stiffness of the non-isolated tissue with intact epidermis was measured using the micro-indentation method. Petioles without trichomes did not exhibit tropic response, and the dehydration of trichomes slowed and prevented complete leaf reorientation. Isolated strands of collenchyma showed no correlation between strength values and position on the petiole. However, strands of collenchyma with epidermis exhibited a significantly greater strength regardless of their position on the petiole. The indentation test showed that non-isolated collenchyma is stiffer on the abaxial side of the petiole. Trichomes from the abaxial side of the petiole were larger at their base. The application of the ‘tensile triangles method’ revealed that these trichomes had a biomechanically optimized shape in comparison to the adaxial side. We conclude that trichomes can be involved in plant biomechanical system and serve as an additional reservoir of hydrostatic pressure that is necessary for maintaining petioles in the prestressed state.

Ni-W Electrodeposited Coatings on Low Carbon Steel Substrate: Fatigue Observations

The fatigue response of electrodeposited Ni-W on low carbon steel substrate was studied. The considered Ni-W coatings are a promising substitute for toxic hard chromium coatings which, according to EU directives (2000/53/WE, 2011/37/UE), must be eliminated. Specimens of pure substrate and those covered by coating were compared. Coated specimens exhibited lower fatigue resistance under higher stress amplitudes than their non-coated counterparts in stress-controlled fatigue experiments. However, at lower stress amplitudes, the fatigue strength of both specimen types was similar. The cyclic softening of the coatings was demonstrated using indentation techniques.

Morphometric and mechanical characteristics of Equisetum hyemale stem enhance its vibration

Main conclusionThe order of the internodes, and their geometry and mechanical characteristics influence the capability of theEquisetumstem to vibrate, potentially stimulating spore liberation at the optimum stress setting along the stem.Equisetum hyemale L. plants represent a special example of cellular solid construction with mechanical stability achieved by a high second moment of area and relatively high resistance against local buckling. We proposed the hypothesis that the order of E. hyemale L. stem internodes, their geometry and mechanical characteristics influence the capability of the stem to vibrate, stimulating spore liberation at the minimum stress setting value along the stem. An analysis of apex vibration was done based on videos presenting the behavior of an Equisetum clump filmed in a wind tunnel and also as a result of excitation by bending the stem by 20°. We compared these data with the vibrations of stems of the same size but deprived of the three topmost internodes. Also, we created a finite element model (FEM), upon which we have based the ‘natural’ stem vibration as a copy of the real object, ‘random’ with reshuffled internodes and ‘uniform’, created as one tube with the characters averaged from all internodes. The natural internode arrangement influences the frequency and amplitude of the apex vibration, maintaining an equal stress distribution in the stem, which may influence the capability for efficient spore spreading.

Influence of Nitriding and Laser Remelting on Properties of Austenitic Stainless Steel Type X10CrNi18-8 and Cavitation Erosion Resistance

Abstract The paper presents properties of surface layers. Surface layers were obtained by using low temperature glow–discharge nitriding process and laser remelting carried out on austenitic stainless steel type X10CrNi18-8. Investigations were done by using an Ultra Nanoindentation Tester (UNHT) in the Warsaw Institute of Fundamental Technological Research. The influence of the above mentioned treatments on obtained surface layers is shown. The values of the Vickers hardness (HV), the irreversible indentation work (Wir), the reversible work (We) and the maximum depth (hmax) during indentation were determined using the method proposed by Oliver and Pharr [1]. On the basis of mechanical properties, the elasticity (Ie) and ductility (Iir) indexes were calculated. Moreover, microstructure cross-section of the austenitic stainless steel after nitriding process and laser remelting was observed using a scanning electron microscope. Cavitation test was performed at a vibratory rig with stationary specimen. On the basis of erosion curves the cavitation resistance was evaluated.

Simulation of Evolution of the Residual Stress in the Surface Layer under Tension and Torsion

It is attempted to combine the method of approaching the problem of surface layer from the point of view of mechanics with that of production technology. The real layer formed by production engineers is described by considering the continuous elastic-plastic model with mixed type strain hardening. The theoretical results thus obtained make it possible to construct a computation program for the state of stress in an axially symmetric element with a surface layer under tension and torsion loading. An application of the model for the computation of the residual stress state produced in a real layer as a result of induction hardening and burnishing, for a stabilized loop of cyclic strain, has been shown.