Key Engineering Materials Vols. 340-341

Abstract: The transition from SSC to LSC ahead of a crack tip (notch root) or an ordinary interface edge (bad pair) causes stress relaxation and the decrease of stress intensity in general. However, the good pair bi-material and the bad pair bi-material with weak elastic stress singularity in this study show the inverse phenomenon under the transition. The results indicate that the stress near the interface edges of both cases, which have no or low stress singularity at the loading instant, increases and brings about the stress concentration during the transition. In addition, the creep strain distribution in the early stage is different from that occurred in the transition of an ordinary interface edge (bad pair) or a crack tip (notch root).

Abstract: Structural steel is widely used in engineering even in nowadays and many new materials have been developed or are under developing. As one of the most common type, torsional fatigue properties are normally calculated from the rotating bending fatigue properties. In this study, the torsional fatigue properties of a structural carbon steel (S45C) with different tensile pre-strain ratio is investigated, especially on the effects of Mode II crack before it branches under different stress amplitudes. Based on the experimental results and microstructure observation, the main results obtained are: (1)Torsional fatigue strength is increased after tensile pre-strain deformation, and the fatigue limits increase with increasing of tensile pre-strain ratio, the highest improvement can be up to 125% of that of the plain specimen. (2) Torsioanl fatigue cracks initiate from the torsional slip lines in ferrite grains, and there is no obvious effect by the tensile pre-strain. With increasing of tensile pre-strain ratio, the fatigue crack initiation life ratio becomes later and the growth rate becomes faster. (3) The mode II crack length along the axial direction becomes longer with increasing of tensile pre-strain ratio, and the crack branch direction does not affected by the tensile pre-strain. Moreover, the length along axial direction is not affected by the stress amplitude change for specimens with the same tensile pre-strain ratio. (4) The length of non-propagating crack becomes shorter with increasing of tensile pre-strain ratio.

Abstract: Experimental characteristics of complementary plastic energy produced by a stress-strain hysteresis curve at a saturated stage in low cycle fatigue are investigated for some steels, and mechanical models for analyzing microstructures of fatigued metals are discussed. As a result, it is found that volume of a cell is varied in inverse proportion to plastic strain range: the density of cells is in proportion to plastic strain range. Consequently, the total number of cells is proportional to plastic strain range. This final conclusion is similar to Winter's opinion concerning persistent slip band structures in high cycle fatigue [1] where, although wall spacing of a cell is invariable and inde-pendent of plastic strain range, a region occupied by persistent slip bands increases proportionally to plastic strain range and consequently the number of cells is in proportion to plastic strain range.

Abstract: In this study, the authors have investigated and compared the effects of roller working and ion nitriding on fatigue properties of eutectoid steel. Five kinds of roller worked and two kinds of ion nitrided specimens were used in this test. The fatigue test was performed using a rotating bending fatigue testing machine to evaluate the fatigue strength of roller worked and ion nitrided eutectoid steel. The fatigue test result shows that roller working is more effective on improving the fatigue strength of the material than ion nitriding. On the other hand, the ion nitriding can much greatly increase the surface hardness than the roller working. In the case of roller working, the fatigue properties are improved by the three main factors which are compressive residual stress, work hardening and fiberized micro-structure.

Abstract: The mechanical and electrical applications of fine wires (D = 0.1 mm) has become more widely spread. In general, it is well known that fine drawn wires have high tensile strength while maintaining ductility. It has been determined that a hardened layer of around 0.04 mm in depth, referred to as the “additional shear strain layer,” is generated beneath the surface layer of the wire, and this additional shear strain layer affected the tensile strength of the fine wire. As an origin of this phenomenon, it was ascertained that the microstructure of surface layer was finer than that of center layer. The purpose of this paper is to investigate the effect of die angle on the microstructure and the tensile strength of the additional shear strain layer. The tensile test was performed as the surface layer was thinned by electro-polishing, and the crystal orientation and the crystal grain were measured via EBSD. As a result, it was ascertained that die angle affected the tensile strength and crystal grain refinement of the additional shear stray layer.

Abstract: Both EBSD and AFM methods were used to investigate the active slip systems and fatigue crack initiation behavior in face-centered cubic polycrystalline metal, austenitic stainless steel, SUS316NG, under cyclic torsional loading. Most active slip planes are the primary slip planes having the largest Schmid factor. Grains with slip band cracks or transcrystalline cracks have larger Taylor's factors. On the basis of EBSD and AFM observations, h, the depth of intrusion vertical to the surface, S, and the component of the slip displacement perpendicular to the surface trace, SB, showed a sharp increase at the onset of crack initiation. The critical value of SB at crack initiation was 170 nm.

Abstract: Tension-compression fatigue tests under various mean stress conditions were conducted with round bar specimens of short glass fiber reinforced polybuthyleneterephthalate made by injection molding. Under cyclic loading with high mean stresses, the creep phenomenon became predominant and the ratcheting deformation increased with the number of cycles. This phenomenon is characteristic of plastics including short glass fiber reinforced plastics. The experimental data of the fatigue strength at the stress ratios above 0.7 were lower than the prediction based on the modified Goodman diagram. We propose to use the creep rupture strength, σc, instead of the tensile strength, σB, as the strength without mean stress and the parabolic equation for a constant life in the amplitude-mean stress (σa-σm) diagram. Our new design equation for the mean-stress effect on the fatigue strength on plastics is as follows: σa = σw – (σw / σc 2) σm 2, where σw is the fatigue strength at the stress ratio R=-1 and σa is the stress amplitude under a mean stress of σm. We also proposed a method to obtain the constant-life relation from limited experimental data.

Abstract: In order to study the effect of plastic working on fatigue strength of notched specimen, pulsating fatigue tests had been performed on notched deformed stainless steel specimens including on notched non-deformed specimens in order to evaluate the influence of mean stress on fatigue strength. The test results showed that the fatigue limits of plastic worked specimens are higher than that of non-worked one. This difference value would be caused by residual stress, work hardening and fiber texture due to plastic working. When degree of plastic deformation equal zero ("t=0), the fatigue limit ratio (σw /σB) of SUS430 is the higher than that of SUS304. On the other hand, the fatigue limit of worked specimen for SUS304 increases as the plastic deformation value increases to 0.5 mm and then it does not significantly increase from 0.5 mm to 1 mm. It is necessary to investigate an optimal deformation value.

Abstract: In this study, we propose a new technique to evaluate some properties during fracture propagation, such as stress at the crack surface and the propagating-route by measuring distributions of leaked magnetic flux vector from the residual magnetization in the vicinity of the fracture surface. The technique involves the application of an inverse-magnetostrictive effect in ferromagnetic materials, such as tempered 11/4Cr-1/2Mo steels below the ductile-brittle transition temperature. The maximum magnetic flux density was increased with impact absorption energy measured by Charpy impact test. The highest magnetic flux density located at the crack starting point, where a fish-eye type surface morphology was observed in fractographic analysis. It indicates the highest stress for fracture initiation at this point. According to the analysis, the change in the magnetic flux vectors corresponds with the direction of crack propagation, which was well explained from the magnetostrictive properties of iron. The measurement of magnetic flux density distribution will be useful for the fractographic analysis to discuss the in-situ phenomena that are difficult to obtain in previous methods.

Abstract: A novel optimization approach is proposed to extract mechanical properties of a power law material from its given experimental nano-indentation P-h curves. A set of equations have been established to relate the P-h curve to mechanical properties, E, σ y and n, of a material. Using the proposed optimization approach, convergence studies were carried out for the determination of the mechanical properties of materials. It was found that the mechanical properties of an elastic-plastic material usually cannot be uniquely determined using a single loading and unloading P-h curve. Thus a technique has also been developed to determine the material properties from indentation p-h curves using indenters with two different angles. This enables the mechanical properties of materials to be uniquely determined.