Untersuchung der tribologischen Eigenschaften von Nutzfahrzeugmotoren unter besonderer Berücksichtigung der Luftfiltration

Es wurde eine unter Betriebsverhältnissen anwendbare Prüfmethode für die Bestimmung der Verschleβeigenschaften von Verbrennungsmotoren mit Hilfe der Gammaspektrometrie der neutronenaktivierten Verschleiβprodukte ausgearbeitet und die die Lebensdauer begrenzende Wirkung des abrasiven Verschleiβes festgestellt. Durch Verbesserung der Luftfilterung konnte die potentielle Lebensdauer der Motoren gut angenähert, und es konnten die Verunreinigung des Schmieröls vermindert und die Dauer der Ölwechselperiode verdoppelt werden.     

Short sisal fiber reinforced polypropylene composites: the role of interface modification on ultimate properties

Sisal fibers have been used for the reinforcement of polypropylene matrix. The compatibilization between the hydrophilic cellulose fiber and hydrophobic PP has been achieved through treatment of cellulose fibers with sodium hydroxide, isocyanates, maleic anhydride modified polypropylene (MAPP), benzyl chloride and by using permanganate. Various fiber treatments enhanced the tensile properties of the composites considerably, but to varying degrees. The SEM photomicrographs of fracture surfaces of the treated composites clearly indicated the extent of fiber–matrix interface adhesion, fiber pullout and fiber surface topography. Surface fibrillation is found to occur during alkali treatment which improves interfacial adhesion between the fiber and PP matrix. The grafting of the fibers by MAPP enhances the tensile strength of the resulting composite. It has been found that the urethane derivative of polypropylene glycol and cardanol treatments reduced the hydrophilic nature of sisal fiber and thereby enhanced the tensile properties of the sisal–PP composites, as evident from the SEM photomicrographs of the fracture surface. The IR spectrum of the urethane derivative of polypropylene glycol gave evidence for the existence of a urethane linkage. Benzoylation of the fiber improves the adhesion of the fiber to the PP matrix. The benzoylated fiber was analyzed by IR spectroscopy. Experimental results indicated a better compatibility between benzoylated fiber and PP. The observed enhancement in tensile properties of permanganate-treated composites at a low concentration is due to the permanganate-induced grafting of PP on to sisal fibers. Among the various treatments, MAPP treatment gave superior mechanical properties. Finally, experimental results of the mechanical properties of the composite have been compared with theoretical predictions.     

Dynamic mechanical characterization of PMR polyimide/carbon fiber composites modified by fiber coating with silicones

Viscous and elastomeric silicones have been applied as interlayers to carbon fibers in order to develop a tougher, micro-crack resistant, thermally stable polyimide (PMR-15) composite. Carbon fiber is continuously coated with very high molecular weight polydimethylsiloxane (PDMS) and polyvinyl-methylsiloxane (PVMS). Dynamic mechanical properties of the composites have been determined and compared with uncoated carbon fiber reinforced PMR-15 polyimide composites. The presence of the interlayer is shown by the appearance of a new relaxation peak. The peak temperature is found to be a good indication of the degree of the cure of the silicone elastomer. Comparison of the storage moduli of uncoated and coated carbon fiber composites at the service temperature range of the composites indicates that the presence of the silicone interlayer affects the shear moduli of the composites. Apparent activation energy of the α transition of the matrix in the modified composites varies with the amount of interlayer and composition in concert with the impact strength.     

High-pressure torsion applied to nickel single crystals

Nickel single crystals with different crystallographic orientations were deformed by high-pressure torsion. Special attention is devoted to examining the evolution of the micro-texture and microstructure. The initial crystal orientation was found to have a significant effect on the mechanical hardening and evolution of micro-texture at low and medium equivalent strains, whereas at very high strains no effect of the initial orientation was observed and the behaviour was very similar to a polycrystal. The evolution of micro-texture is in good qualitative agreement with the full constrained Taylor model. At very high equivalent strains the initial crystal orientation has no influence on micro-texture. At such strains, the hardening, the refinement of the structure and the texture reaches a saturation. The final micro-texture is explained by the change from one preferred crystallographic orientation to another.     

Numerical simulation of flat-tip micro-indentation of glassy polymers: Influence of loading speed and thermodynamic state

Flat-tip micro-indentation tests were performed on quenched and annealed polymer glasses at various loading speeds. The results were analyzed using an elasto-viscoplastic constitutive model that captures the intrinsic deformation characteristics of a polymer glass: a strain-rate dependent yield stress, strain softening and strain hardening. The advantage of this model is that changes in yield stress due to physical aging are captured in a single parameter. The two materials studied (polycarbonate (PC) and poly(methyl methacrylate) (PMMA)) were both selected for the specific rate-dependence of the yield stress that they display at room temperature. Within the range of strain rates experimentally covered, the yield stress of PC increases linearly with the logarithm of strain rate, whereas, for PMMA, a characteristic change in slope can be observed at higher strain rates. We demonstrate that, given the proper definition of the viscosity function, the flat-tip indentation response at different indentation speeds can be described accurately for both materials. Moreover, it is shown that the model captures the mechanical response on the microscopic scale (indentation) as well as on the macroscopic scale with the same parameter set. This offers promising possibilities of extracting mechanical properties of polymer glasses directly from indentation experiments.     

Molecular dynamics simulation of the interaction between a mixed dislocation and a stacking fault tetrahedron

A high number-density of nanometer-sized stacking fault tetrahedra are commonly found during irradiation of low stacking fault energy metals. The stacking fault tetrahedra act as obstacles to dislocation motion leading to increased yield strength and decreased ductility. Thus, an improved understanding of the interaction between gliding dislocations and stacking fault tetrahedra are critical to reliably predict the mechanical properties of irradiated materials. Many studies have investigated the interaction of a screw or edge dislocation with a stacking fault tetrahedron (SFT). However, atomistic studies of a mixed dislocation interaction with an SFT are not available, even though mixed dislocations are the most common. In this paper, molecular dynamics simulation results of the interaction between a mixed dislocation and an SFT in face-centered cubic copper are presented. The interaction results in shearing, partial absorption, destabilization or simple bypass of the SFT, depending on the interaction geometry. However, the SFT was not completely annihilated, absorbed or collapsed during a single interaction with a mixed dislocation. These observations, combined with simulation results of edge or screw dislocations, suggest that defect-free channel formation in irradiated copper is not likely by a single dislocation sweeping or destruction process, but rather by a complex mix of multiple shearing, partial absorption and defect cluster transportation that ultimately reduces the size of stacking fault tetrahedra within a localized region.     

Fundamental relationships between deformation-induced surface roughness,critical strain localisation and failure in AA5754-O

This research employs two approaches to characterise the apparent structure observed in localised strain maps constructed from surface topography data acquired from AA5754-O sheet stock that was deformed in three in-plane stretching modes. The first uses a conventional two-point autocorrelation function (ACF), while the second uses the combination of the eigenvalue spectrum associated with each map and information theory. The results from the ACF analysis are inconclusive, implying that this technique lacks the sensitivity necessary to quantify the relationships between multi-point clustering and strain localisation. The information theory-based approach reveals that the relative spectral entropy increases monotonically, attains a maximum and then decreases sharply to the failure strain. This behaviour occurs in all three strain modes and results from two competing processes: one where the formation of structure is favourable and one where it is not. The crossover point is a clear indicator of the onset of critical strain localisation and, therefore, can be regarded as a precursor to failure because once the dominant process shifts, additional strain results in the precipitate formation of a critical strain localisation event.     

Microstructure and mechanical properties of nano-Y2O3 dispersed ferritic steel synthesized by mechanical alloying and consolidated by pulse plasma sintering

Ferritic steel with compositions 83.0Fe–13.5Cr–2.0Al–0.5Ti (alloy A), 79.0Fe–17.5Cr–2.0Al–0.5Ti (alloy B), 75.0Fe–21.5Cr–2.0Al–0.5Ti (alloy C) and 71.0Fe–25.5Cr–2.0Al–0.5Ti (alloy D) (all in wt%) each with a 1.0?wt% nano-Y₂O₃ dispersion were synthesized by mechanical alloying and consolidated by pulse plasma sintering at 600, 800 and 1000°C using a 75-MPa uniaxial pressure applied for 5?min and a 70-kA pulse current at 3?Hz pulse frequency. X-ray diffraction, scanning and transmission electron microscopy and energy disperse spectroscopy techniques have been used to characterize the microstructural and phase evolution of all the alloys at different stages of mechano-chemical synthesis and consolidation. Mechanical properties in terms of hardness, compressive strength, yield strength and Young's modulus were determined using a micro/nano-indenter and universal testing machine. All ferritic alloys recorded very high levels of compressive strength (850–2850?MPa), yield strength (500–1556?MPa), Young's modulus (175–250?GPa) and nanoindentation hardness (9.5–15.5?GPa), with up to 1–1.5 times greater strength than other oxide dispersion-strengthened ferritic steels (<1200?MPa). These extraordinary levels of mechanical properties can be attributed to the typical microstructure of uniform dispersion of 10–20-nm Y₂Ti₂O₇ or Y₂O₃ particles in a high-alloy ferritic matrix.     

Quasi-static deformation and final fracture behaviour of aluminium alloy 5083: influence of cryomilling

The commercial aluminium alloy 5083 was processed via cryomilling to produce nanocrystalline (NC) powders with an average grain size of ～25–50?nm. The powders were subsequently degassed at 723 K (450°C), pre-heated and immediately quasi-isostatic (QI)-forged to produce a thermally stable bulk ultrafine grain (UFG) material having average grain size values ranging from 190 to 350?nm, depending on the processing conditions used. In this paper, the tensile properties and fracture behaviour of the bulk UFG material are presented and compared with the tensile properties of its conventionally processed counterpart. The specific influence of preheat temperature on strength and ductility of the alloy is briefly discussed. Three different pre-heat temperatures of 523, 623 and 723?K (250, 350 and 450°C) were chosen and used with the primary objective of controlling grain growth during forging. The influence of preheat temperature on tensile deformation and final fracture behaviour is highlighted. The macroscopic fracture modes of the bulk nanostructured material (BNM) prepared following three pre-heat temperatures are investigated. The microscopic mechanisms controlling tensile deformation and final fracture behaviour are discussed with regards to the intrinsic microstructural effects in the UFG alloy, nature of loading, and the kinetics and mechanisms of deformation.