Key Engineering Materials Vols. 345-346

Abstract: This paper deals with evaluation of fracture toughness in functionally graded materials (FGMs) consisting of partially stabilized zirconia (PSZ) and austenitic stainless steel SUS 304. FGMs and non-graded composites (non-FGMs) with fine and coarse microstructures are fabricated by powder metallurgy using PSZ and two kinds of SUS 304 powders. The fracture toughness is determined by conventional tests for several non-FGMs with each material composition and by a method utilizing stable crack growth for FGMs. Based on the experimental results, fracture mechanism, influences of microstructure on fracture toughness, and difference in fracture toughness between the FGMs and non-FGMs has been discussed.

Abstract: In this study, we introduce an approach which simulates crack propagation under mixedmode loading condition. In comparison with the conventional element removing method which eliminates any element that satisfies a prescribed failure criterion near the crack tip, the present approach selects a set of elements ahead of the crack tip on the crack growth direction and removes them one by one when the element meets a prescribed failure criterion. Compact tension shear (CTS) specimens of type 304 stainless steel were used for failure testing. Finite element simulation has been carried out to simulate crack profiles and compared with observed ones. Results showed the proposed element removing algorithm is useful for crack growth simulation under mixed mode loading condition. The experimentally measured crack growth profile is in an agreement with the predicted ones.

Abstract: In this study, stellite 6 alloy was overlaid on spheroidal graphite (SG) cast irons with various carbon contents (1.5wt%~3.8wt%) and a fixed silicon content (approximately 2.5wt%) using the plasma transferred arc (PTA) process at different overlaying current (140A~220A) while the travel speed of the PTA torch was maintained constant. Results indicated that the solidification structures of the stellite 6 overlayers were dendritic and had a large amount of interdendritic precipitates (M7C3 and M23C6 carbide) after the satellite 6 alloy had been overlaid on SG cast irons with different carbon contents (1.5wt%~3.8wt%) at a low overlaying current (I=140A). The partially melted zones of the substrates below the carbide-containing interfacial layers consisted of eutectic carbides (ledeburites) and fine pearlites. The amount of the carbide-containing interfacial layers and ledeburites increased following the increase in the overlaying current. The results of the tearing test reported that the occurrence of the carbide-containing interfacial layers was detrimental to the bonding strength between the overlayers and the substrates. The metallography of the fractured area of the tearing specimens after the bonding strength test revealed that fractures always occurred at the carbide-containing interfacial areas. On the other hand, the overlayers were rich in iron content when the overlaying currents were high (I=190A and 220A). Owing to the dilution effect, the matrices of the overlayers were α-Fe with lamellar M7C3 carbides. The results of the tearing test indicated that the bonding strength of the overlaid specimens was relatively low, and fractures always occurred in these highly diluted overlayers.

Abstract: A series of fracture tests and elastic-plastic FEM analysis were carried out for three-point bend specimens with different notch root radius in the cleavage temperature region of a RPV low alloy steel. The cleavage initiation distance (CID) from the notch tip was measured by SEM photographs on the fracture surface of each specimen. The local cleavage fracture stress σf * was defined as the stress σyy at the cleavage initiation site. The σf * increased with decreasing notch root radius. The CID’s were larger in specimens with bigger root radius. This implies that a larger volume of material and possibly bigger microcracks should be involved in the cleavage fracture process of those specimens. The σf * is considered to have a certain relationship with the level of stress-strain concentration in different geometry specimens.

Abstract: Fracture toughness data from the cleavage resistance test of structural steels often show a large scatter. Geometry dependency as well as the scatter makes it difficult to evaluate appropriate fracture integrity of cracked components. To address these restrictions, several stochastic models have been proposed by Beremin group, Mudry and other researchers while each of them employs specific estimation scheme and micro-mechanical parameters. The purpose of this paper is to investigate applicability of the Weibull stress model in transition temperature regime and to quantify constraint effect among different-sized CT and PCVN specimens. The constituting parameters m and σu are determined at three temperatures by maximum likelihood estimate (MLE) technique in use of FE analysis results and experimental data of PCVN specimens. Also, failure probabilities of PCVN and CT specimens are calculated from the Weibull parameters, which are used for derivation of a prototype of toughness scale diagram. The diagram provides a technical basis to resolve transferability issue in the same material under different temperatures and constraint conditions.

Abstract: This paper provides closed-form plastic limit load solutions for elbows with local wall thinning under in-plane bending, via three-dimensional (3-D), small strain FE limit analyses using elastic-perfectly plastic materials. Wide ranges of elbow and thinning geometries are considered.

Abstract: The present work proposes a method for elastic-plastic fracture mechanics analysis of the circumferential through-wall crack in between elbows and attached straight pipes, subject to in-plane bending. Based on small strain finite element limit analyses, closed-form limit load solutions are given first. Then applicability of the reference stress based method to approximately estimate J is proposed. One interesting finding is that a popular approach to assume that the crack locates in the straight pipe could lead to significantly different assessment results.

Abstract: Thrust bearing is one of the most important components in esPCP system, which determines the lifespan of the whole system under the restriction of some factors such as the depth of downhole unit, the viscosity of petroleum oil to be pumped. To increase the high temperature bearing ability of the bearings under the limitation of radial size, the damage reason of pre-applied bearings was analyzed. Some different modified plans were put forward from two aspects of structure and material. Contact problems were researched for different plans with finite element analysis software of ANSIS. Comparison of results shows that, in the well of 5½" tube, the optimum selection is ball thrust bearing with 8 balls of radial size of 10mm without keeper.

Abstract: Nickel Alloy 22, UNS N06022, is being evaluated for use as the material of construction in permanent containers for spent nuclear fuel in Yucca Mountain, NV. To meet nuclear design criteria, Charpy impact data were required for the N06022 plate material, but conventional full-size specimens exceed the energy capacity of typical 400 J impact test machines, which results in stopping the pendulum during the test. Half-size specimens break with about 40% of the machine energy capacity, but their use raises questions concerning energy scaling to full size equivalent data. To address this, a range of subsize specimens were tested at room temperature using a standard 400 J impact test machine, and full-size, 3/4-size, and 2/3-size specimens were tested on a high energy capacity, 950J machine. Additional tests were performed at temperatures ranging from -196 to +200°C. Impact energy and lateral expansion measurements for the various test conditions are presented, their implications are examined, and a new model for absorbed energy correlation between subsize specimens and full size conventional Charpy specimens is proposed.

Abstract: The material quality, the deformation rate, the temperature and the stress state influence mechanical behaviour and properties of different materials. Due to this great variety of the influencing factors we do not have one model of general validity describing the behaviour of materials, but we have to use a great number of material constants in order to characterize the properties. The exponents of the Manson-Coffin, the Basquin and the Paris-Erdogan laws were applied for the verification of the connection among the fatigue fracture types. Own measured values and test results can be found in the literature were used for the illustration of the connections. “Fracture surface”-s were determined for characterizing of different steel grades and their welded joints. It can be concluded that “fracture surface”-s are suitable for the describing of the fracture behaviour and the conversion of different fracture parameters of steels.