Key Engineering Materials Vols. 345-346

Abstract: A series of fatigue tests has been conducted to estimate the fracture behavior depending on stacking sequences in quasi-isotropic CFRP laminates. Strips of laminate with four different directions were cut out from the same CFRP plate to prepare the specimens with the four kinds of stacking sequences. Prior to fatigue test, the specimens were impacted with lateral load in order to simulate the damage induced by the fallen tools. During the fatigue tests, the damage developed in the specimen was detected by non-destructive techniques using a scanning acoustic microscope (SAM) and a replicating method. Stacking sequences exerted an influence on fracture behavior of laminate specimens, i.e. the stacking position of the zero degree layers, in which fiber direction was parallel to the loading axis, played an important role in the fracture process. When the zero degree layers are located at the specimen surface, no remarkable damage propagation was observed from the surface. On the other hand, when the zero degree layers are located in the middle of laminates, the damage in the surface layer was found in the fatigue process. Not only the degradation of the equivalent elastic modulus was seen in all specimens, but also the degradation curve varied with the stacking sequences.

Abstract: In this paper, Experimental results on the measurement of mechanical properties of fine patterns in the MEMS structure are described. The mechanical properties of embossing patterns on metallic thin foil is measured using the nano indentation system, that is developed by Korea Institute of Industrial Technology(KITECH). These micro embossing patterns are fabricated using CIP(Cold Isostatic Press) process on micro metallic thin foils(Al-1100) that are made by rolling process. These embossing patterned metallic thin foils(Al-1100) are used in the reflecting plate of BLU(Back Light Unit) and electrical/mechanical MEMS components. If these mechanical properties of fine patterns are utilized in a design procedure, the optimal design can be achieved in aspects of reliability as well as economy.

Abstract: The cyclic behavior of type DIN 1.4460 duplex stainless steels in as-received and aged conditions was investigated at room temperature and at 500°C. At room temperature, the aged samples showed embrittlement effects such as loss of toughness and reduction of fatigue life. At 500°C, beneficial effects of the synergy between temperature and strain cycling were observed. It is proposed that at high temperature in the ferrite the strain cycling can decompose the chemical composition fluctuations, promoting a demodulation of the spinodal decomposition formed by aging.

Abstract: In this work, the formation of fatigue cracks is considered as a nucleation process due to the development of a characteristic microstructure formed just beneath the specimen surface. Strain controlled cyclic tests were carried out at room temperature at total strain ranges εt = 0.8 and 1.2% in flat specimens of SAF 2507 Duplex Stainless Steel (DSS). The results show that for this DSS, at εt = 0.8%, the correlation between phases (Kurdjumov-Sacks crystallographic relation) plays an important role in the formation of microcracks. On the other hand, at εt = 1.2%, microcracks initiate in the ferritic phase and the K-S relation does not seem to affect the formation of the cracks.

Abstract: The thermo-mechanical fatigue (TMF) behaviour of the Nimonic 90 Nickel base superalloy has been investigated within two laboratories. In-phase-tests (IP) where the maximum mechanical strain occurs at the maximum temperature (850°C), and 180°-out-of-phase-tests (180° OP) where the maximum mechanical strain coincides with the minimum temperature (400°C) have been applied. All tests were carried out at varying mechanical strain ranges with a constant strain ratio of Rε = - 1. A temperature rate of 5 K/s was used throughout the whole cycle without any additional cooling system during decreasing temperature. The fatigue life of 180° OP tests is longer compared to identical IP tests. The stress / mechanical strain hysteresis loops are completely different and some characteristic values are compared to each other. The fracture surfaces observed show that fatigue crack (or cracks) starts on the external surface and propagates inwards. The fractures of 180° OP tests are transgranular showing the presence of fatigue striations, while the fractures of IP tests are mixed transgranular and intergranular with no fatigue striations.

Abstract: In this present study, the fatigue behavior of titanium single crystals containing cracks growing in different crystallographic orientations has been investigated. To investigate fatigue fracture behavior of small single crystals, plain bending fatigue test method for thin sheet specimen was developed. One end of the sheet specimen is fixed at a voice coil of the loudspeaker and the other end is set free. A bending mode resonance occurs in the specimen due to forced vibration at the fixed end. In A-specimen, the notch plane and the direction are (11-20) and [1-100], respectively, the crack propagated parallel to {1-100} plane and striation-like markings were observed on the fatigue surfaces. In B-specimen with a notch of (1-100) and [11-20], the crack also propagated parallel to {1-100} plane as similar to A-specimen. These cracks are deduced to extend by alternating shear on two intersecting prismatic slip systems at the crack tip. In F-specimen with a notch of (0001) and [11-20], the crack propagated parallel to (0001) and twins were observed near the crack. As a result, S-N curves of each specimen showed strong orientation dependence and the fatigue strength of F-specimen was the highest in these specimens.

Abstract: In recent years, the replacement repair welding, which repairs damaged steel girder bridges by replacement of the damaged sections with new steel plates through cutting and welding under in-service conditions, is spotlighted for its brilliant features, i.e. it can be achieved without incurring traffic dislocation. However, the mechanical behavior of the welded joints under cyclic loads due to the traffic which passes along the girder bridges is not clarified. In this paper, the fatigue strength of the replacement repair welded joints was investigated in order to improve reliability in the repair welded joints. Steel plate was extracted from the old steel girder bridge; then was welded with new steel plate to construct specimen with the replacement repair welded joints. Flux Cored Arc Welding (FCAW) process was used to fabricate the double "V" butt joints. Fatigue tests were conducted using a servo hydraulic controlled 50tonf capacity UTM with a frequency of 5Hz under constant amplitude loading. The fatigue strength of the replacement repair welded joints was analyzed in detail.

Abstract: In order to establish a reasonable and integrated fatigue design criterion for the long life design of the gas welded body structure, stress distribution around the gas welded joints subjected to tensile load was numerically analyzed. And also, the
P-Nf curves were obtained by fatigue tests. By using these results,
P-Nf curves were rearranged in the
σ-Nf relation with the hot spot stress at the welded joint. And a guideline for fatigue design of gas welded joints was proposed.

Abstract: Under fatigue loading, the number of cycles to failure and its associated scatter increase when the loading level decreases. The High-Cycle Fatigue (HCF) regime is thus characterized by a large scatter in the number of cycles to failure [1]. Cracks initiation represents an important part of the lifetime of the structures. A stochastic method is used to study the fatigue crack initiation prediction in the 316L austenitic stainless steel. The present work proposes to show that this scatter can be attributed to the random orientation of individual grains, which influences the crack initiation localization. The stresses in grains are determined by finite element computations (FEM [2]), using a configuration representative of a polycrystalline aggregate. This approach takes into account the crystallographic orientations of the grains in the aggregate as well as the deformation incompatibilities between neighbouring grains due to crystalline anisotropic elasticity and elasticplasticity [3]. Then, the scatter of the number of cycles to crack initiation is derived from the FEM stress fields using two fatigue crack initiation criteria: an usual one, Mura’s criterion [4] and a more recent one [5], based on Discrete Dislocation Dynamics (DDD) simulations and taking into account plastic slips, cross slip and stress tensor components.