Key Engineering Materials Vol. 741

Abstract: Many studies have been conducted to characterize the fracture toughness of structural steels and their welded joints. However, most studies focus on newly developed steels, and the number of studies on the fracture toughness of long-term used steels in structural components is rather limited. Furthermore, a lack of data on the fracture toughness causes difficulties in evaluating the structural integrity of existing steel structures. In this study, CTOD tests were performed to characterize the fracture toughness of penstock that has been in service for 50 years. By measuring the critical crack tip opening displacement in conjunction with analysis for chemical compositions, the characteristics of fracture toughness were investigated.

Abstract: A laminar flaw is a planar subsurface flaw parallel to the rolling direction of the plate, where the applied stress is typically parallel to the rolling direction. The laminar flaw oriented within 10 degree of a plane parallel to the component surface is defined as a laminar flaw, in accordance with the definition of the American Society of Mechanical Engineers (ASME) Boiler & Pressure Vessel (B&PV) Code Section XI. The ASME Code provides combination criterion for multiple laminar flaws. If there are two or more laminations, these laminations are projected to a single plane and, if the separation distance of the projected laminations is less than or equal to 25.4 mm, the laminations shall be combined into a single large laminar flaw in the assessment. The combination criterion was established on the basis of the non-destructive examination capabilities in the 1970’s. However, this methodology did not consider the offset distance of the laminations nor the mechanical interaction between the flaws. Therefore that combination methodology is not suited in case of a large number of laminar flaws. This may occur e.g. in case of hydrogen flaking in steel forging components. Actually, when multiple discrete laminar flaws are close to each other, interaction between the flaws has to be taken into account and these flaws shall be combined to a single laminar flaw for assessment. Stress intensity factor interactions for inclined laminar flaws were analyzed in the frame of hydrogen flaking issue in reactor pressure vessels of Doel 3 and Tihange 2 Belgian nuclear power plants. Based on the mechanical interaction between flaws, new combination criterion was developed and was presented in this paper.

Abstract: In this paper was studied dynamic behavior of the armor steels Armox 500T and Secure 500 by testing specimens in quasi-static tensile test with strain rate 1∙10^-3 s^-1 and high-speed tensile test within range of intermediate strain rates from 100 s^-1 to 400s^-1 at the room temperature. Hardness test and quasi-static tensile test confirmed material properties specified by the manufacturer. Stress-strain diagrams showed very low strain-rate hardening effect at investigated strain rates. Total elongation at fracture was larger in case of Armox 500T for the whole strain rate range. Deformation energy density was calculated from the stress-strain curve and temperature rise due to adiabatic heating was estimated. Because of higher total elongation, Armox 500T was able to withstand higher deformation energy.

Abstract: The characterization of subsurface fatigue crack initiate sites of near α and α-β types titanium alloys and their cracking models proposed were reviewed. The crack initiation sites consisted of facets mostly on near basal plane of α grain, although the crystallographic orientation and surface topography of the facets presented a subtle difference. The crack initiation mechanisms were a quasi-cleavage accompanying high normal stress on the plane, a combination of basal slip and normal stress across the basal plane, and a pure slip on facet plane inclined near 45 degree to loading axis.

Abstract: In this paper, fatigue crack growth simulation of interacting subsurface cracks using the s-version finite element method (SFEM) is presented. In order to evaluate the accuracy and reliability of the proximity rules published by the ASME, during the fatigue crack growth simulations, the subsurface cracks are approximated to either a single elliptical crack or semi-elliptical surface crack in accordance with the proximity rules. Then, the proximity rules are slightly modified for improving the accuracy and reliability. The results of crack depth evolution calculated by the SFEM with the use of the new proximity rules suggest that the approximation to deep cracks drastically improves the accuracy of the fatigue crack growth evaluation. Thus, the approximation to deep cracks must be a promising approach for having better evaluation of fatigue crack growth of subsurface cracks.

Abstract: The fatigue crack growth threshold ΔK_th is an important characteristic of crack growth assessment for the integrity of structural components. However, the accurate threshold ΔK_th values for austenitic stainless steels in air environment are lacking in many fitness-for-service (FFS) codes, although fatigue crack growth tests have been performed and many test data had been published. This paper focuses on fatigue crack growth threshold ΔK_th values for austentic stainless steel in air environment. The paper introduces the current ΔK_th values provided by four major FFS codes and summarizes the available test data based on the literature survey. The paper then discusses the applicability of the existing ΔK_th for stainless steels and proposes a new relation as a function of the stress ratio (the R ratio) for use by FFS codes.

Abstract: Damage and microcracks formed by rolling contact fatigue (RCF) were characterized for carburized SCM420 steel. A large number of microcracks were detected beneath the contact surfaces after RCF. The microcrack generation and strain distribution beneath the contact trail depended on the slip ratios of 0 %, -20% and -40 % in the roller pitting test. Such severe slip increased shear strain in the region higher than 160 µm in depth from the contact surface. Compressive stress also gave rise to strain in the region near the surface up to 100 µm in the depth. Those strain gradients may cause a strain incompatibility at the transition layer in which a crack branching was detected.

Abstract: Creep characteristics of alloys and compounds have been evaluated mainly by the minimum creep rate or the steady-state creep rate, and by its stress and temperature dependences. In some cases, however, direct comparison of the minimum creep rate or the steady-state creep rate are not practically easy due to difficulties of experiment, i.e., a long duration of primary stage of creep deformation. The minimum creep rates are not always precise representative value, which is directly evaluated from experiments. It should be valuable, if one could estimate the minimum creep rate from creep curve in primary stage. I have proposed a method of quantitative evaluation of creep curve based on the evaluation of strain rate change and its strain dependence during creep [1-3]. The value that reflects a shape of creep curve is named “Strain Acceleration and Transition Objective-Index (SATO-Index)” [4]. SATO-Index and related differential equation show a strain dependence of strain rate and lead entre creep curve by numerical integration. This concept provides quantitative information of shape of each creep curve, and information of the entire creep curve. In this paper, examples of evaluation and extrapolation of creep rate from primary stage in compression are presented. It is concluded that the extrapolation with the concept of SATO-Index reasonably provides imaginal minimum creep rate. Usability of extrapolation of creep curve by the concept is presented.

Abstract: Non-destructive stress measurement methods have been developed. However, there are a few approaches into the effect of stress direction on the nondestructive physical factors. In the present work, a non-destructive and non-contact method using three-dimensional magnet microscopy was applied to stress evaluation of an as-received tool steel (JIS, SKS93). Three-dimensional components of magnetic fields were observed using a scanning Hall probe microscope in order to find the important component which was related to the tensile stress. The observations were carried out under the tensile stress that was less than the yielding stress of the material. It was found that the magnetic field component that was parallel to a tensile loading direction was strongly correlated to stress values.

Abstract: The contribution is focused on characterization of methods enabling to apply small/subsized specimens for fracture resistance characterization. The applied methods are divided into transition region and upper shelf region. The approaches used in the upper shelf region represent at the same time methods applicable for ductile materials without transition. Relating to subsized samples two basic approaches are applicable: (i) miniaturized samples based on common standard ones and (ii) specific specimens/methods, e.g. small punch test etc. The results described in the paper belong to the first group. For interpretation of data generated under low constraint conditions toughness scaling models and master curve approached are commented. In ductile region, either the sample used generate valid toughness characteristics, or, if not, there is no way how to correct measured data except damage quantification through micromechanical models.