Key Engineering Materials Vols. 324-325

Abstract: Using the double-edge notched geometry proposed by Xu and Reinhardt recently, the dimension of 200 mm×200 mm×100mm concrete cube specimens, of which the crack length are 10 mm, 20 mm, 30mm, 40mm, 50mm respectively, are designed to experimentally measure mode II fracture toughness KIIC of concrete. For almost all specimens, typical shear fracture features i.e. approximately 0º initial cracking angle as well the following crack forwards propagation along the direction of ligament is phenomenally observed. This fact strongly confirms that this double-edge notched geometry is validly and capable of being utilized as a mode II fracture geometry to evaluate mode II fracture behavior. Then, from the discontinuity point of the measured load-displacement plot, the critical shear fracture load Pc is determined and the corresponding mode II fracture toughness KIIC is also calculated using the formula developed by Xu and Reinhardt. The computed results show that KIIC has no dependency on initial crack length, about 3.36MPa·m1/2 for the tested specimens.

Abstract: The article deals with the behaviour of a fatigue crack propagating across a bi-material interface. A stability criterion for the crack touching the bi-material interface, taking into account the residual stresses closing the crack faces is formulated. The linear elastic fracture mechanics is assumed to apply and the finite element method is used in the calculations. The criterion proposed is applied to determine the fatigue threshold stress for crack propagation across the interface. It is shown that the threshold values for crack propagation are influenced by the residual stresses closing the crack and by the specific combination of the elastic constants of the materials used. The results contribute to a better understanding of the failure of structures with bi-material interfaces (protective layers, composite materials, etc.).

Abstract: Wear is often of definite influence in the service life of mechanical components and has been recognised as one of the major causes of failure in engineering practice. It is noted that although extensive attention has been paid to phenomenological studies like surface morphology analysis for wear assessment, the physical mechanism of wear particle formation remains unclear. This paper proposes a micro damage and fracture model to simulate the process of wear particle generation. An explicit finite element (FE) formulation is employed to capture the nonlinearities involved. Unlike existing FE analysis (FEA), any initial sub-fractures underlying the wear surface are no longer required. Crack initiation and propagation as well as the corresponding mesh updating are implemented in an automatic fashion associated with the explicit FE framework. The results presented are in good agreement with experimental observation and the reports in existing literature.

Abstract: Generalized fatigue damage is derived based on S-N curve under constant amplitude loads with S characterizes the load cycle by the combination of load peak and load amplitude. For random fatigue loads, a natural choice is to use half cycle (Range) counting method. Rain Flow counting is popular due to its useful nature to pick-out full cycles among random loads. It is found in the present research through analysis and verified by experimental results that Rain Flow counting seriously overestimate the damage for the actual true random loads owing to its inherent nature and the Range counting method is more reliable. As the filtering threshold increases gradually, the overestimation effect by Rain Flow counting method becomes less, and the calculated damages by Rain Flow and Range counting begin to converge. The filtering threshold damage corresponding to the “fatigue limits” for the concerned case was calculated. According to equal damage principle, the filtering threshold amplitudes were derived and found to vary with the load peaks. In practice, an acceptable solution is to use quasi-variable filtering thresholds.

Abstract: This paper presents a fracture mechanics analysis in continuously non-homogeneous, isotropic, linear elastic and functionally graded materials (FGMs). A meshless boundary element method (BEM) is developed for this purpose. Young’s modulus of the FGMs is assumed to have an exponential variation, while Poisson’s ratio is taken as constant. Since no simple fundamental solutions are available for general FGMs, fundamental solutions for homogeneous, isotropic and linear elastic solids are used in the present BEM, which contains a domain-integral due to the material non-homogeneity. Normalized displacements are introduced to avoid displacement gradients in the domain-integral. The domain-integral is transformed into a boundary integral along the global boundary by using the radial integration method (RIM). To approximate the normalized displacements arising in the domain-integral, basis functions consisting of radial basis functions and polynomials in terms of global coordinates are applied. Numerical results are presented and discussed to show the accuracy and the efficiency of the present meshless BEM.

Abstract: Near-equiatomic Ni-Ti alloys are known to exhibit shape memory effect associated with a B2↔B19’ martensitic transformation. These alloys are often used in various cyclic modes in application, typically as actuators and sensors. The B2↔B19’ martensitic transformation in Ni-Ti is accompanied with a large lattice distortion. Cycling through this transformation, induced thermally, mechanically or by the combination of the two, is found to cause structural damage to the alloys, hence changes their functional properties. This study investigates the effect of transformation cycling and heat treatment on the property stability of near-equiatomic Ti-Ni. It was found that in the case of thermally induced transformation cycling, incomplete transformation cycles caused less structural damage to the matrix than full transformation cycles whereas in the case of mechanically induced transformation cycling via pseudoelasticity in tension, partial or full transformation cycling caused similar property changes. The indifference of the case of pseudoelastic cycling is attributed to the localisation of the deformation, commonly known as the Lüders-type deformation.

Abstract: SiC materials have excellent high temperature strength, low coefficient of thermal expansion, good resistance to oxidation and good thermal and chemical stability etc. In this study, monolithic liquid phase sintered SiC (LPS-SiC) was made by hot pressing method with nano-SiC powder an average particle size is 30nm and less. Alumina (Al2O3), yttria (Y2O3) and silica (SiO2) particles were used for sintering additives. To investigate effects of SiO2, Al2O3/Y2O3 composition was fixed and then ratios of SiO2 were changed as seven kinds. Materials have been sintered for 1 hour at 1760
, 1780
and 1800
under the pressure of 20MPa. The system of sintering additives which affects a property of sintering as well as the influence depending on compositions of sintering additives were investigated by measurement of density, mechanical properties such as Vickers hardness and sliding wear resistance were investigated to make sure of the optimum condition which is about matrix of SiCf/SiC composites. The abrasion test condition applies to load of 20N at 100rpm for 20min. Sintered density, abrasion property of fabricated LPS-SiC increased with increasing the sintering temperature. In case of LPS-SiC with low SiO2 content, has very excellent wear resistance.

Abstract: Recently, the steel parts used at the aerospace and automobile industries are required to be used high stress more than ever before in need of the weight down. Therefore, used material steel have to be a high stress, which is an indispensable condition in this field. At the consideration of parts design, high hardness of the lightweight parts has an benefit of saving fuel and material. In this study, the surface conditions are measured to know the influence on fatigue properties by shot peening. In this study, the optimum shot peening condition is investigated. Fatigue test was accomplished to investigate the effects of compressive residual stress by shot peening on fatigue characteristics. Test results show the fatigue characteristics were increased by optimum peening condition. But the fatigue characteristics were decreased by under peening or over peening condition.

Abstract: By using a continuum modeling approach based on the equivalent elliptical crack representation of a local damage and the strain energy equivalence principle, the effective elastic compliances and the effective engineering constants are derived in closed forms in terms of the virgin (undamaged) elastic properties and a scalar damage variable for damaged two- and threedimensional isotropic solids. It is shown that the effective Young’s modulus in the direction normal to the crack surfaces is always smaller than its intact value.

Abstract: Degradation of reactor pressure vessel (RPV) steels due to neutron irradiation embrittlement is directly related to safety and life of the nuclear power plant (NPP). In order to ensure structural integrity and safe operation of NPP, surveillance programs are conducted to monitor and predict the changes in RPV materials. Availability of irradiated specimen from RPV or irradiation of specimens under simulated conditions of RPV for conducting fracture toughness tests remains a major problem in surveillance programs. In order to resolve this problem, various methods are adopted to experimentally simulate the effect of neutron irradiation on mechanical behavior of RPV steels using electron irradiation, thermal aging, strain hardening, combined quenching and hardening and pre-straining combined with heat treatment. This paper presents a review of the existing research on experimental simulation of neutron irradiation damage through various methodologies and discusses the future scope of their application in plant safety and life assessment of RPV’s.