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The topic of fatigue strength of joints of flat and honeycomb-structured thin sheets has not yet been adequately studied. This concerns, for example,the development of an appropriate test method. In many cases, there is still a lack of information, how effective joints of flat and structured sheet metals are in omparison with reference joints of flat sheets. Combinations of resistance spot-welded specimens of flat and honeycomb-structured thin sheets of deep-drawing steel DC04 with a thickness of 0.5 mm were examined. Adopted clamping plates were applied for the testing. Joined smooth specimens were used as a reference material. The analysis of the frequency [1] was applied for comparison. The fatigue limit of a joint of a flat and a structured sheet metal is not significantly affected by spot welding. The fatigue limit of these spot-welded joints is comparable to that of an non joined honeycomb-structured sheet. By means of frequency or stiffness analysis, stability and damage mechanisms of differently shaped joints were explained. The joining of bridges of the structure shows a better stability than the combination of honeycombs. The stiffness analysis is a favored method for the clarification of the damage mechanisms in a spot-welded joint. The reasons for the low fatigue strength values of the joined flat-structured specimens compared to flat-flat joints are strong notch effects in the bridge area and momentum effects by the structure height of the studied honeycomb structure. The spot weld with a pronounced indentation also affected the fatigue strength of both flat and structured joined sheet metals.
Thin sheet metals from deep drawing steel DC04 are very often used in the production of car body and case parts. Quality improvement of sheet
metal components by new constructive solutions (structuring) as well as
adapted joining technology is going on. Structured sheet metals differ from each other by their high bending stiffness. At the same time, they show certain anisotropy due to the structure. Therefore a typical testing
method of structured semi-finished parts (single sheet metals, sandwiches)is the bending test. The literature review revealed that in many
studies no special demands on tests of structured materials were made. This concerns particularly the structure arrangement, structure direction and structure location of the specimen relative to the mandrel position during bending tests, i. e., the direction of the fixed load relative to the structure. The aim of this study was to determine the influence of the test
specification on flexural behavior. In the present paper, honeycomb-structured
sheet metals were examined using 3-point bending tests. Bending stiffness and lightweight potential were calculated with respect to the location of load application and compared for different structure arrangements,directions and locations. The influence of the anisotropy on flexural
behavior of the honeycomb-patterned sheet metals was moderate.
This paper serves as an overview of the ongoing research in the field of multiscale and multidisciplinary analysis of deformation and damage in the case of oligocrystalline structures. The research focuses on experimental measurement and numerical calculation of ductile failure in the X2CrNiMo18-15-3 (AISI 316L) stainless steel. An embedding numerical technique is employed where crystal plasticity theory is used to represent plastic deformation in the material and element removal technique based on Rice&Tracey damage model for ductile void growth to simulate damage initiation inside the material, which is observed in the experiments. Additionally, the crystal plasticity model is supported by a hierarchical multiscale approach connecting nano-, micro- and meso-scales.
Role of alloying elements during thermocyclic oxidation of β/γ-TiAl alloys at high temperatures
(2016)
Gamma titanium aluminides are promising alloys known for their good mechanical properties and low densities, but their low oxidation resistance at high temperatures limits their
application. This work discusses the thermocyclic oxidation behavior of newly developed β/γ-TiAl alloys at temperatures between 600 °C and 900 °C. An influence of β-stabilizing alloying elements like Nb and V on the oxidation of these alloys has been investigated here.
The selected alloys are tested in an in-house developed thermocyclic furnace. The oxidation
study is supported by gravimetric measurements along with Scanning Electron Microscopy (SEM) and Electron Diffraction Spectroscopy (EDS) mapping of the oxide layers. Additionally, phase formation after oxidation has been determined using X-Ray Diffraction (XRD). Results show that the Nb containing alloys are more oxidation resistant as compared
to V containing alloys. The formation of a mixture of Al2O3 and TiO2 layers was found for all the alloys. The oxide kinetics controlled oxide growth and formation of
various phases at different testing temperatures.
Coating growth and mechanical properties of nanolamellar Cr2AlC coatings at various sputtering power were investigated in the present study. Cr2AlC coating was deposited on the IN 718 superalloy and (100) Si wafers by DC magnetron sputtering at different sputtering powers.
The structure and properties were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and nanoindentation. It was found that coatings had columnar structure with nanocrystalline substructure. Deposition rate increased with the sputtering power. XRD results showed the presence of the Cr2AlC MAX phase, intermetallic
AlCr2 and Cr7C3 carbide phases, along with the change in preferential coating growth orientation. TEM observations confirmed the occurrence of these phases, and the SAED patterns demonstrated significant texture of the coatings. Hardness values were measured in the range between 11–14 GPa, showing a slight increase with the sputtering power.
Controlled ion bombardment is a popular method to fabricate desirable coating structures and modify their properties. Substrate biasing at high frequencies is a possible technique, which allows higher ion density at the substrate compared with DC current bias. Moreover, high ion energy along with controlled adatom mobility would lead to improved coating growth. This paper focuses on a similar type of study, where effects of coating growth and properties of DC magnetron-sputtered chromium nitride (CrxN) coatings at various substrate bias frequencies are discussed. CrxN coatings were deposited by pulsed DC magnetron sputtering on Inconel 718 and (100) silicon substrates at 110, 160 and 280 kHz frequency at low duty cycle. Coating microstructure and morphology were studied by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), scratch adhesion testing and nanoindentation. Results indicate a transformation of columnar into glassy structure of CrxN coatings with the substrate bias frequency increase. This transformation is attributed to preferential formation of the Cr2N phase at high frequencies compared with CrN at low frequencies. Increase in frequency leads to an increase in deposition rate, which is believed to be due to increase in plasma ion density and energy of the incident adatoms. An increase in coating hardness along with decrease in elastic modulus was observed at high frequencies. Scratch tests show a slight increase in coating adhesion, whereas no clear increase in coating roughness can be found with the substrate bias frequency.
Hydrogen degradation is a serious problem in industrial applications like power plants (boilers, turbines), marine structures, car and aircraft components, as it leads to failures as well as to deterioration of properties. Inconel 718 is one of the most commonly used materials for these applications. Different metal nitrides like TiN coatings have been deposited in past to prevent hydrogen degradation, which are also known for their high hardness and good wear resistance [1, 2]. However, reports on hydrogen degradation of CrN coatings, which shows better oxidation and corrosion resistance, higher temperature stability and lower friction coefficient than TiN [3, 4] has not been reported till now. Despite a lot of publications about CrN films, up to now the effect of hydrogenation on mechanical and tribological properties of CrN coatings is still not completely understood. In the current work CrxN coatings were deposited by Direct Current Magnetron Sputtering (dcMS) on Inconel 718 substrate at different chamber pressures and substrate voltages. Substrate voltage is one of the most important process parameters which determines the structure of the coating and the adhesion between substrate and coating. Simultaneously a study of the chamber pressure is also needed to understand the deposited structure and growth rate because at higher pressures the high number of argon atoms reduce the number of ionized ions available for the deposition leading to low deposition rates [5]. Gas-phase hydrogenation of the samples was performed at a temperature of 600° C and hydrogen pressure of 2 atm. It was found that CrxN coatings are resistant against hydrogen exposure as compared to uncoated surfaces. The results of changes in the mechanical, tribological properties and phase composition of the coatings after hydrogenation are discussed. Coating microstructure was studied by scanning electron microscopy (SEM). The mechanical properties of the coatings were characterized by means of nanoindentation and scratch test.