Key Engineering Materials Vols. 353-358

Abstract: This study is developed to investigate the effect of monotonic plastic deformation on the torsional fatigue properties of a structural steel. Five different kinds of tensile pre-strain, 2%, 5%, 8%, 12% and 22%, were applied to the specimens, respectively. And the maximum pre-strain value is near to the necking strain of the test material. The effects of tensile pre-strain on surface hardness, fatigue crack initiation and propagation behaviors, and the behavior of non-propagating cracks. The main results obtained are: The fatigue limits are 145, 160,175, 200 and 215MPa for specimens with tensile pre-strain of 2% 5%, 8%, 12% and 22%, which are improved to 104%, 114%, 125%, 143% and 153% of the fatigue limit for non-pre-strained specimens, respectively. The torsional fatigue limit increases with the tensile pre-strain increasing, until the pre-strain value being near to the necking strain ratio. However, the fatigue limit increase becomes more slowly for high tensile pre-strained specimens than the lower ones. The fatigue cracks of the tensile pre-strained specimens initiated earlier than that of the non-pre-strained specimens, and the propagation is also accelerated, but there is no effect on the fatigue crack initiation point and the branch point. Non-propagating crack length becomes shorter with increasing of tensile pre-strain until the value near the necking strain, and the quantity of non-propagating crack increases at the same time.

Abstract: For accurate failure assessment, a second parameter like T-stress describing the constraint is needed in addition to the single parameter J-integral. In this work, selecting the structures of surface-cracked plate and pipe, we perform line-spring finite element modeling, and accompanying elastic-plastic finite element analyses. We then present a framework, which includes the constraint effects in the R6 FAD approach for failure assessment of cracked-structures.

Abstract: A study is made of the damage resistance and strength degradation of nitrided pressureless sintered (NPS) silicon nitride ceramics. The silicon nitride is prepared by cost-effective NPS process combining by nitridation and consecutive pressureless sintering. Contact testing with spherical indenters is used to characterize the damage response. Examination of the indentation sites indicates a quasi-plastic damage modes are observed. Bend tests on specimens containing quasi-plastic contact damages reveal those materials to be not susceptible to strength degradation.

Abstract: Creep crack growth (CCG) rate has been organized frequently by C* or Ct parameter However, crack behavior of early stage under unsteady state condition has not been explained. Crack energy density (CED), which has been proposed as a parameter that can provide a unified description of crack behavior with no restriction on constitutive equation, can give the general expression about creep crack growth rate. By applying Ct and the concept of CED to the results, we showed that creep crack growth rate for all ranges of creep can be explained in a unified way by CED and its derivatives. Moreover, the physical meaning of the Ct is clarified in the discussion.

Abstract: Static and dynamic loading fracture experiments were conducted on magnesium alloy under equal and unequal biaxial stress. Cross shape specimens with a crack on their center were used for tests. Fracture behavior was observed by two methods; the caustic and the one point gauge method. From the observation, the stress intensity factors and the fracture toughness values were calculated. It was found that the one point gauge method was not suitable for calculating the stress intensity factor with a strain gauge widely available. The fracture toughness values obtained under equal biaxial stress were 1.2~2.3 times greater than those of magnesium alloy naturally inherited. Moreover, plate thickness was negatively related to the fracture toughness value.

Abstract: Fatigue tests of nanometer-thick Cu films as deposited and annealed in vacuum were conducted under constant load ranges at room temperature. Fatigue strengths of the Cu films, which is defined as the critical load range being able to cause crack initiation within 106 cycles, are determined. The experimental results show that fatigue strength increases with decreasing film thickness. Fatigue cracking behaviors were characterized by electron microscope. It is also found that fatigue cracking resistance is dependent on film thickness and increases with decreasing film thickness. Size effects on fatigue properties of the nanometer-thick Cu films are discussed.

Abstract: Dyneema composite is used in lightweight armour applications, because of its high specific material properties such as strength and stiffness. In armour applications, Dyneema composite is used to protect people or vehicles from projectile impact. In order to be able to guarantee a certain protection level, an accurate prediction of fracture phenomena that are caused by projectile impact is required. Currently, fracture phenomena such as delamination and fibre fracture are not accurately described. This is because a good understanding of fracture phenomena in Dyneema composite lacks. Therefore, both Dyneema fibre and Dyneema composite are analysed by different (impact) experiments to gain more insight in both the fracture phenomena as well as in the material properties. Parallel to these experiments, a start is made with the development of a new material model in ABAQUS\Explicit using cohesive zone techniques that is able to predict the fracture phenomena due to projectile impact.

Abstract: Fatigue properties of Inconel 718 at 500°C were investigated under rotating bending. Fatigue strength was higher at 500°C than at room temperature. Fracture occurred by the initiation and propagation of a surface crack in short life region. The early propagation of a surface crack smaller than 20-30 μm was suppressed by oxidation at 500°C. This is a main reason for the higher fatigue strength at 500°C. However, the crack growth after the retardation of a small crack growth at 500°C was accelerated reflecting the lower static strength. On the other hand, in long life region, though a surface crack initiated at the early stage of fatigue life similar to the case in short life region, the crack did not propagate over 20-30 μm, and an internal fracture occurred. Consequently S-N curve at 500°C showed a duplex property and the fatigue limit was not recognized. The subsurface damage was initiated at the early stage of fatigue life.

Abstract: The objective of this study is to examine the feasibility of the X-ray diffraction method for the fatigue life assessment of high-temperature steel pipes used for main steam pipelines, re-heater pipelines and headers etc. in power plants. In this study, X-ray diffraction tests were performed on the specimens simulated for low cycle fatigue damage, in order to estimate fatigue properties at the various stages of fatigue life. As a result of X-ray diffraction tests, it was confirmed that the full width at the half maximum (FWHM) decreased with an increase in the fatigue life ratio, and that the FWHM and the residual stress due to fatigue damage were algebraically linearly related to the fatigue life ratio. From this relationship, a direct assessment of the remaining fatigue life was feasible.

Abstract: The effect of contact pressure on fretting fatigue in quenched and tempered 45-carbon steel is studied. With an increase in contact pressure, fretting fatigue life is decreased quickly at low contact pressures; however it almost unchanged at high contact pressures. With an increase in cyclic stress amplitude, fretting fatigue life decreased. In the test, concavity is formed at the fretted area accompanying wear. The main crack is initiated at the outer edge corner of the concavity at high contact pressures, and initiated at the middle portion of the fretted area at low contact pressures.