Papers by Keyword: Internal Stress

Abstract: The development of internal stress in injection molded parts is analyzed. Different from other researches, this study uses a new modified Maxwell model to calculate the internal stress. On the basis of the creep experiments of injection molded parts, a non-linear constitutive equation is proposed. Non-linear finite element equation to calculate the internal stress is derived. By means of this model, the internal stress of an injection molded polystyrene plate is simulated. The effects of mold wall temperature, cooling time and packing pressure on the development of internal stress are investigated. The predicting results are in good agreement with experimental data.

Abstract: The internal stress in solid-oxide fuel cells (SOFCs) was evaluated during the thermal, reduction and re-oxidation cycles by using high-energy X-ray synchrotron radiation of about 70 keV at Beam line BL02B1 of SPring-8. The oxidized cell has a compression of about 400 MPa in the c-ScSZ electrolyte and a tension of 50-100 MPa in the NiO-YSZ anode at room temperature. In-situ measurement during the thermal cycle in an air atmosphere, the internal stress decreased with increasing temperature, becoming approximately zero at 1000 K. After the thermal cycle, the internal stress returned to its initial value. In the measurement during the reduction cycle, the internal stress was smaller than that measured during the cooling cycle after the anode was reduced from NiO-YSZ to Ni-YSZ. In the re-oxidation cycle of a reduced cell, the internal stress in the electrolyte went into tension above 800 K when the anode was re-oxidized from Ni-YSZ to NiO-YSZ. This tensile stress is responsible for possible fracture of unit cells in SOFCs.

Abstract: To overcome the low adhesion of hydrogen-free DLC films on metal substrates, in this studying, DLC films (0.9μm) were prepared with 3 types of interlayer (2 types of Ti/TiCx/DLC interlayer and 1 type of Ti/TiNx/TiNxCy/DLC interlayer) on different substrates (W18Cr4V, Cr12, GCr15, TC4, 40Cr, 9Cr18 and Cr18Ni9Ti). The internal stress of the films was calculated by the bending of substrate (Si(100)). It was found that it is as high as 3.9GPa, and part of the high residual stress of the DLC films was dissipated through a compound interlayers, and the thick films achieved, and the pull tests found that the adhesion has been highly strengthened with a proper interlayer. The films structure qualities were investigated by Raman spectroscopy and the results indicate that the films have the same structure properties which suggests that the properties of the DLC films can not been affected by substrates. The hardness is about 5000HV, defined by nanoindentation techniques. The frictional properties were investigated by reciprocal sliding tests and the friction coefficient was about 0.1, independent of their substrates. Thus our DLC films may have great potential applications in precision parts tribological application.

Abstract: Internal stresses are very important for the performance of protective hard coatings. Tensile stresses favour the formation and propagation of cracks, inducing fracture and corrosion. Medium compressive stresses hinder fatigue. But high compressive stresses, typically for hard coatings produced by PVD (physical vapour deposition) processes, support delamination in order to relax the stored elastic energy. However notwithstanding its relevance, the internal stresses are only seldom used for the optimisation and quality control of hard coatings in industry. This unsatisfying situation is caused by the deficit in efficient measuring methods. The results of thin sheets, where the stresses can be simply measured by their curvature, are not necessarily representative for the coating of thicker parts. The conventional XRD (X-ray Diffraction), based on angle-dispersive evaluation needs expensive devices and is rather time consuming. The energy-dispersive technique opens new possibilities. It is based on polychromatic radiation. The interference of the lattice plane reflections corresponding to the Bragg-equation is investigated by the diffraction intensity of the different wavelength (or photon energies), not by varying the Bragg-angle as in conventional XRD. Hence, the whole diffraction pattern can be obtained in one shoot without the use of any goniometer. This allows the construction of small and compact measuring devices and the reduction of measuring time to a few minutes. The capability of the ED-XRD (Energy Dispersive X-ray Diffraction) is demonstrated for titanium nitride and chromium nitride films deposited by cathodic vacuum arc with varying parameters. Comparisons were made with the much more time-consuming AD-XRD (Angle Dispersive X-ray Diffraction) for residual stress analysis. The results of both methods are in good agreement.

Abstract: The Mg micropowder was mixed and ball milled with ceramic nanoparticles. The material was compacted and then hot extruded. Samples have been deformed in tensile as well as compression tests at temperatures between 20 and 300 °C at a constant initial strain rate. The flow stress is significantly influenced by temperature. The yield stress and maximum stress decrease with increasing temperature. Stress strain curves obtained at lower temperatures in tensile tests substantially differ from ones estimated in compression. Stress relaxation tests were conducted in order to find the internal stress as well as to identify possible thermally activated process(-es).

Abstract: The relation of the internal stress and the parameters of the heterogeneous dislocation structure was suggested in the form of the classical Taylor formula relating the internal stress to the total dislocation density stored in the subgrain interior and in the subgrain boundaries. The other formula combines linearly the stress contribution generated by network dislocations and the stress contribution of the subgrain structure semiempirically related to the subgrain size. The formulas can evaluate the ratio of internal stress components due to sub-boundaries and free dislocations.

Abstract: In this study, a Ni-P alloy electroforming nanostructure material with low surface roughness and low internal stress was developed by using a pulse current. Square-wave cathodic current modulation was employed to electrodeposit ultrafine-grained Ni-P films from an additivefree Sulfamate nickel bath. The effect of various factors, such as peak current density, duty cycle and pulse frequency on the roughness and internal stress were investigated. Pulse current significantly influences the microstructure of Ni-P alloys. The internal stress and roughness of Ni-P alloys increased as peak current density increased, but the internal stress of Ni-P alloys decreased as duty cycle decreased.

Abstract: 15%Cr ferritic stainless steel was machined in rectangular samples and then processed by multiple forging to a total cumulative strain of 7.2 at an ambient temperature. The large strain deformation resulted in almost equiaxed submicrocrystalline structure with a mean grain/subgrain size of 230 nm and about 2.2×1014 m-2 dislocation density in grain/subgrain interiors. The annealing at a relatively low temperature of 500oC did not lead to any discontinuous recrystallizations. The grain/subgrain size and the interior dislocation density slightly changed to 240 nm and 2.1×1014 m-2, respectively, after annealing for 30 min, while the Vickers hardness decreased from 3140 MPa in the as-processed state to 2900 MPa. This annealing softening was attributed to remarkable release (by 50%) of internal stresses, which are associated with a non-equilibrium character of strain-induced grain/subgrain boundaries.

Abstract: The main purpose of this work is to reveal the structural changes occured after the impact test on ball bearing steel samples, relative to their ferrite-pearlite phase. The XRD analysis has been used to investigate the level of first and second order internal stresses, the dimensions of mosaic blocks as well as the dislocation density in crystal lattice. The influence of the impact velocity, material hardness and surface roughness on fine structural parameters, mentioned above, is also analysed. On the basis of structural changes it is possible to control the material response during the impact loading.