Prediction of Lower Bound Fracture Toughness in the Transition Temperature Region by T33-Stress

It is well known that the fracture toughness Jc in the ductile-to-brittle transition temperature region depends highly on the specimen thickness (hereafter, TST). The TST effect on Jc, which Wallin [1] described as Jc (∝ KJc2) ∝ B(-1/2) (Jc was calculated from the equations outlined in ASTM E1820 [2], KJc was derived from Jc as KJc = (Jc·E′)1/2; E′ = E/(1−ν2), B: TST), has been reproduced by Anderson et al. [3] based on the weakest link model. However, as Anderson et al. [3] themselves admit, Jc does not decrease indefinitely with B. On the other hand, Meshii et al. [4–6] tried to explain this TST effect on Jc as a mechanical issue. They obtained the same relationship, Jc ∝ B(-1/2) from the fracture toughness test for a non-standard CT and 3PB specimen (non-standard on the point that planar configuration was identical and thickness to width ratio B/W was varied from 0.25 to 0.5) and the stress intensity factor (SIF) corresponding to fracture load Pc denoted as Kc (Kc was calculated from the equations outlined in ASTM E399 [7]), was almost constant for TST. They also reproduced the experimental tendency by large strain FEA under the assumption based on their experimental observation that Kc was independent of TST. In addition, they expressed the TST effect on Jc by correlating Jc with the out-of-plane elastic T-stress T33. We thought that if TST effect on Jc is a mechanical issue, the lower bound Jc for TST could be predicted by FEA under some assumption such as Kc = constant for TST, and the TST corresponding to the lower bound Jc could be predicted by T33. However, before proceeding to this prediction, we thought we have to understand the candidate assumption for prediction more deeply, i.e, understand why Kc was constant for TST. Thus in this work, we attempted to explain the reason why Kc was constant for TST. Our idea was to apply the well-known “planar” failure criterion to our out-of-plane TST issue. After demonstrating our idea was valid, the lower bound Jc of carbon S55C for non-standard 3PB specimen was predicted based on this planar failure criterion and the large strain elastic-plastic FEA. The results showed that Jc showed a lower bound for specimen of B/W ≥ 1.5. In addition, it was shown that this threshold B/W could be estimated by the elastic out-of-plane T33.

Study on the Stress-Strain Redistribution Caused by Inelastic Deformation in Perforated Plate

Fast Breeder Reactors and chemical plants that is operated at elevated temperature must be designed considering creep deformation in addition to elastic-plastic deformation. Especially at structural discontinuities, strain concentration induced by stress-strain redistribution reduces creep-fatigue strength. For this reason, a design method is needed for appropriately evaluating inelastic behavior at a structural discontinuity. As one of simplified methods with elastic analysis, a rational method with Stress Redistribution Locus (SRL) has been studied during recent years. Previous studies have shown that SRL does not depend on constitutive equations or on the magnitude of loading. And through the elastic-plastic-creep analysis of a one-dimensional pipe model, it was revealed that there was a relation between stress-strain redistribution and the size of elastic core. The purpose of this study is to clarify the mechanism of stress-strain redistribution in complex structures like actual components. Multi-dimensional stress-strain distribution and multiaxial stress occur in those structures. For considering those effects, inelastic analyses on perforated plate were performed and the relation between the region of elastic cores and SRL was examined. Then, it was revealed that SRL could be divided into two parts. One half is affected by the region of elastic core and the other half depends on the loading type. Furthermore, this paper proposes the new SRL method based on the mechanism and validates the method.

Thermal-Creep Induced Stress Relaxation Near a Notch in Zr-2.5Nb

In CANDU nuclear reactors, flaws on the surface of Zr-2.5Nb pressure tubes naturally cause stress concentration, and subsequently potentially lead to crack initiation through delayed hydride cracking. However, the concentrated stresses tend to relax due to creep, which is suggested to reduce the occurrences of cracks. In this study, C-shaped specimens with a small radius notch (15μm or 30μm) manufactured from Zr-2.5Nb pressure tubes were first preconditioned through an ex-situ creep process (Keff = 6MPam, 583K) for a series of hold times. Elastic lattice strains around the notch were measured using high energy X-ray diffraction at the Advanced Photon source. Results show that strains along the loading direction are tensile and concentrated in front of the notch tip, while those perpendicular to the loading direction are compressive and distributed symmetrically about the notch plane. The concentrated strains relax with creep time, and the larger-size notch has a higher rate of relaxation.

Small Scale Experiments on Boiling Liquid Expanding Vapor Explosions: Supercritical BLEVE

The most dangerous accident that can occur in LPG storage is the boiling liquid expanding vapor explosion (BLEVE). To better understand the rupture of the reservoir and the blast wave characteristics, small scale BLEVE experiments are performed with cylinders of 95 ml, filled at 86% with propane, laid horizontally and heated from below. A weakening of the reservoirs on the upper part allows better reproducibility of the rupture. High speed visualization, blast overpressure and surface reservoir temperature are measured. Internal pressure measurement shows that the rupture pressure and temperature are well above the critical point. The fluid is then supercritical and there is no distinction anymore between liquid and gas prior rupture. This kind of reservoir rupture is significant of a new type of BLEVE, a supercritical BLEVE. The experiments also show that the fluid behavior during rupture differs with the size of the weakened part and therefore with the rupture pressure. Finally, the measured peak overpressures are compared with literature models.

Investigation on On-Board High-Pressure Composite Tanks Subjected to Localized and Engulfing Fire

Safety performance of on-board high-pressure composite tanks under fire exposure has drawn extensive attention. Vehicle fires usually lead to such tanks experiencing a term of localized and engulfing fire. During this period, the composite tank would be degraded and even burst before pressure relief device (PRD) opens to release internal high-pressure gas. In this paper, experimental investigation for the tanks subjected to localized and engulfing fire was conducted on an Aluminum liner composite tank filled with hydrogen. Based on the temperature distribution and pressure rise measured in the experiment, a three-dimensional computational fluid dynamics model is developed to study the key factors influencing PRD activation time. The effects of filling medium, tank pressure and localized fire exposure time are analyzed in detail. The experimental results showed that pressure and temperature of internal gas changed little during the localized fire. In addition, filling medium and tank pressure have weak influence on the activation time of thermally-activated PRD (TPRD), but have significant effect on the activation time of pressure-activated PRD. TPRD can respond more quickly to protect the hydrogen composite tank than pressure-activated PRD. PRD activation time increases as the localized fire exposure time extends.

Isothermal Fatigue Responses and Constitutive Modeling of Haynes 230

Constitutive models are an integral part of a lifing system because it allows for accurate estimation of stresses and strains at failure locations of interest. Constitutive models can be properly defined in a material subroutine of a finite element code. The computational capabilities of today are far higher, allowing for more comprehensive models that can provide more accurate results. Macroscopic models that are physically based, phenomenological models characterize the material behavior on a larger scale that provides invaluable insights even at such length scales which are compatible for industrial application. A unified viscoplastic model based on nonlinear kinematic hardening (Chaboche type) with several added features such as nonproportionality, multiaxiality, strain range dependence, and thermal recovery is being implemented in ANSYS through the User Programmable Features. The simulation capability of the model will be experimentally validated on a nickel based superalloy, HA230. The experimental database encompasses a broad set of low cycle fatigue, symmetric, uniaxial strain-controlled loading histories which include isothermal with and without hold times, with and without a mean strain, at temperatures ranging from 75°F to 1800°F. Simulations from the modified model compared to the experimental responses will be presented to demonstrate the strengths and weaknesses.

Thermal Fatigue Evaluation Method of Pipes by Equivalent Stress Amplitude

In recent years, reports have increased which are about failure cases caused by high cycle thermal fatigue both at light water reactors and fast breeder reactors. One of the biggest reasons of the cases is a turbulent mixing at a Tee-junction, where hot and cold temperature fluids are mixed, in a coolant system. In order to prevent thermal fatigue failures at Tee-junctions, The Japan Society of Mechanical Engineers (JSME) published the guideline S017-2003 (or JSME guideline) which is an evaluation method of high cycle thermal fatigue damage at a nuclear piping. It has some limitations in terms of its inconstant safety margin and its complexity in evaluation procedure, however. In order to solve these limitations, this paper proposes a new evaluation method of thermal fatigue damage with use of the “equivalent stress amplitude” which represents random temperature fluctuation effects on thermal fatigue damage. Because this new method makes methodology of evaluation clear and concise, it will contribute to improving the guideline for thermal fatigue evaluation.

Improvement of Fatigue Limit by Shot Peening for High-Strength Steel Containing a Crack-Like Surface Defect: Influence of Stress Ratio

It is well known that shot peening (SP) prevents the initiation and propagation of fatigue cracks and improves the fatigue limit of metals. It was recently reported that a surface crack can be rendered harmless by SP. Actual pressure vessels are generally operated under a positive stress ratio (R > 0). However, the effects of SP on the improvement of fatigue limits under R > 0 remain unclear. If these effects were clarified, the structural integrity of pressure vessels could be advanced. Thus, in this study, we investigated the influence of SP on fatigue limits and on the crack size that could be rendered harmless by SP under R > 0. High-strength steel specimens containing a semi-circular slit were subjected to SP. Bending fatigue tests were carried out under R = 0.4. The fatigue limits of the SP specimens increased 50∼100% compared with Non-SP specimens. Slits under 0.2 mm in depth were successfully rendered harmless by SP. The increasing ratios of the fatigue limit under R = 0.4 were lower than those under R = 0. However, the size of semi-circular slit that can be rendered harmless by SP remained the same. The size of the slit that can be rendered harmless can be predicted by calculating stress intensity factors.

A Study of Formularization of Uplift Behavior at the Bottom Plate of Large LNG Storage Tanks During Earthquake

In recent years, it has been found that seismic demand tends to be higher for LNG storage tanks and it requires detail estimations about the tank uplift. Generally FE analysis is performed for this aim, since the most of present design standards do not specify the design formula to calculate the stress of the bottom plate. In the case of establishing the bottom plate model with the uplift, several properties have to be considered, such as tank dimension and plate thickness, magnitude and distribution of dynamic liquid pressure that is affected by width and height of the uplift, elasticity of the bottom insulation, and bulging displacement of the tank sidewall. As an another approach for detail estimations including these properties, this study presents the mathematical solution of the bottom plate for estimating uplift height and stress distribution based on a theory of elastic bearing beam. To increase accuracy but maintain practicality, the thin cylindrical theory is also coupled to the bottom plate model for considering the influence of bulging displacement of the tank sidewall on the bottom plate. From the parameter study by the proposed model, it is found that the bulging displacement of the sidewall has significant effect to the uplift height of the bottom plate.

Constitutive Model Development for Thermo-Mechanical Fatigue Response Simulation of Haynes 230

Turbine engine combustor components are subject to thermo-mechanical fatigue (TMF) during service. The combustor liner temperatures can sometimes reach as high as 1800°F. An accurate estimate of the strains at critical locations in the combustor liner is required for reliable lifing predictions. This demands the need for a detailed analysis of the TMF responses and a robust constitutive model capable of predicting the same. A large set of experiments have been carried out on the liner material, a nickel based alloy, HA 230, in an effort to understand its thermo-mechanical fatigue constitutive response. The out-of-phase strain-controlled TMF experiments with a negative mean strain show a positive mean stress response, while the in-phase TMF experiments with a positive mean strain show a negative mean stress response. A Chaboche based viscoplastic constitutive model is under development. It will have several essential features such as nonlinear kinematic hardening, isotropic hardening, strain range dependence, rate dependence, temperature dependence and static recovery. The constitutive model being developed for accurately calculating the stress-strain response is being carried out with the final objective of predicting the strains in an actual combustor liner in service through finite element simulation for fatigue lifing.