The Spherical Indentation Approach for Fracture Toughness Evaluation: A Study Based on the Energy Release Rate

In this study, we investigated the drawbacks of previous studies regarding the evaluation of fracture toughness from spherical indentation tests (SITs). This was achieved by an examination of the material damage mechanism during indentation tests, uniaxial tensile tests, and Mode I/II fracture tests. A new approach based on the energy release rate was proposed in this study to evaluate the fracture toughness of ductile metals. Scanning electron microscope (SEM) observations revealed that the mechanism for material damage during an indentation test was different with the material damage in uniaxial tensile tests and Mode I fracture tests, but similar to that in Mode II fracture tests. Thus, the energy release rate during SITs should be correlated with JIIC. Compared with previous studies, this new proposed method was more consistent with the actual damage mechanism and did not rely on the specific critical damage values. Experiments on SA508, SA533, 15CrMoR, and S30408 revealed that the maximum error from this energy release rate-based approach was no more than 13% when compared with their conventional counterparts (compact tension tests), and thus can meet the precision requirement of engineering applications.

Investigation Into Applications of Local Failure Criterion for X70 Pipeline With Corrosion Defect

In this paper, the local failure criterion using stress modified critical strain method based on annex B of API 579 is applied to evaluate the ductile failure of API X70 pipelines with a volumetric corrosion defect. Ductile failure is quantified in terms of strain, representing the tensile strain capacity (TSC) which is commonly used in strain-based assessment for fitness-for-service of pipelines installed in frozen area where large-scale ground movement can arise due to earthquakes, freezing and thawing. Based on the local failure criterion suggested for API X70 steel material, the TSCs of the corroded pipelines are evaluated by using the detailed finite element (FE) analyses. The effects of internal pressure and defect size (such as longitudinal length, circumferential width and depth in the direction of thickness) on TSC of pipelines subjected to axial displacement are systematically investigated. In addition, TSCs based on local failure criterion are compared with those based on net-section limit load. The TSCs from the present FE analyses for various defect geometries and internal pressure can be used to predict ductile failure of corroded pipelines and to build the framework for a strain-based assessment for in-service pipelines.

A Multidimensional and Multiscale Model for Pressure Analysis in a Reservoir-Pipe-Valve System

Reservoir-pipe-valve (RPV) systems are widely used in many industrial process. The pressure in an RPV system plays an important role in the safe operation of the system, especially during the sudden operation such as rapid valve opening/closing. To investigate the pressure especially the pressure fluctuation in an RPV system, a multidimensional and multiscale model combining the method of characteristics (MOC) and computational fluid dynamics (CFD) method is proposed. In the model, the reservoir is modeled by a zero-dimensional virtual point, the pipe is modeled by a one-dimensional MOC, and the valve is modeled by a three-dimensional CFD model. An interface model is used to connect the multidimensional and multiscale model. Based on the model, a transient simulation of the turbulent flow in an RPV system is conducted, in which not only the pressure fluctuation in the pipe but also the detailed pressure distribution in the valve are obtained. The results show that the proposed model is in good agreement with the full CFD model in both large-scale and small-scale spaces. Moreover, the proposed model is more computationally efficient than the CFD model, which provides a feasibility in the analysis of complex RPV system within an affordable computational time.

Defect Localization for Pressure Vessel Based on Circumferential Guided Waves: An Experimental Study

Pressure vessels are normally employed under extreme environments with high temperature and high pressure. Inevitably, the defects like crack and corrosion that easily occur in the equipment and can significantly influence the normal operation. Guided wave-based method is a cost-effective means to measure the utility of pressure vessel. In this paper, finite element (FE) simulation is used to explore the propagation characteristics of circumferential guided waves in pressure vessel. Based on the propagation characteristics, the experiments with different configurations of piezoelectric transducers (PETs), which contain a sparse array and a dense array, have been conducted on pressure vessel respectively. Different imaging methods, including discrete ellipse imaging algorithm and probability damage imaging algorithm have been applied to locate the defect based on the configurations above. Furthermore, a multi-channel ultrasonic guided wave detection system has been set up for pressure vessel inspection. The experimental results show that the sparse array with the discrete ellipse imaging algorithm can locate the defect effectively. The imaging results based on probability damage imaging algorithm show that the dense array presents the better localization result.

Proposal of New Upper Limit of Hydrostatic Test Pressure in KT-3 of ASME Section VIII Division 3

The current upper limit of hydrostatic test pressure in KT-3 of ASME Sec. VIII Division 3 is determined by general yielding through the thickness obtained by Nadai’s equation with a design factor of 0.866 (= 1.732/2). On the other hand, the upper limit of hydrostatic test pressure in 4.1.6 of the ASME Sec. VIII Division 2 is determined by general yielding through the thickness with a design factor of 0.95. In cases where a ratio of hydrostatic test pressure to design pressure of 1.43 similar to PED (Pressure Equipment Directive) is requested, the upper limit of hydrostatic test pressure may be critical for vessel design when material with a ratio of yield strength to tensile strength less than 0.7 is used. In order to satisfy the requirements in KT-3, it is necessary to decrease design pressure or increase wall thickness. Therefore, it is proposed to change the design factor of intermediate strength materials to obtain the upper limit of hydrostatic test pressure. In this paper, a new design factor to obtain the upper limit of hydrostatic test pressure is proposed and the validity of this proposal was investigated by burst test results and elastic-plastic analysis.

Finite-Element Analysis of Crack Growth in Austenitic Stainless Steel Under Equibiaxial Loading

The lifetime extension of the nuclear power plants is considered as an energy challenge worldwide. That is why, the risk analysis and the study of various effects of different factors that could potentially represent a hazard to a safe long term operation are necessary. These structures, often of great dimensions, are subjected during their life to complex loading combining varying mechanical loads, multiaxial, with non-zero mean values associated with temperature fluctuations and also PWR environment. Based on more recent fatigue data (including tests at 300°C in air and PWR environment, etc...), some international codes (RCC-M [2], ASME and others [3][4][5]) have introduced a modification of the austenitic stainless steels fatigue curve combined with a calculation of an environmental penalty factor, namely Fen, which has to be multiplied by the usual fatigue usage factor [6]. Unfortunately, experimental data on this issue are rare. In order to obtain fatigue strength data under structural loading, biaxial test means with and without PWR environment were developed at LISN in collaboration with EDF and AREVA [6]. Two kinds of fatigue device have been developed. Within the same specimen geometry, structural loads can be applied in varying only the PWR environment. The first device (FABIME2) is devoted to study the effect of biaxiality and mean strain/stress on the fatigue life [9]. A second and new device called FABIME2e is for the study of the environmental effect. With these new experimental results, the PWR environment effect on the fatigue life of stainless austenitic steels will be quantified accurately on semi-structure specimen. This device combines the structural effect like equi-biaxiality and mean strain and the environmental penalty effect with the use of PWR environment during the fatigue tests. The aim of this paper is to present the numerical interpretation of the results obtained with these two devices “FABIME2” and “FABIME2e”. Two important aspects will be addressed. The first concerns the mechanical behavior of austenitic stainless steel and the capabilities of the numerical model to reproduce the hardening of the material. And the second concerns the study of the crack growth during the equibiaxial fatigue test.

Experimental Determination of the Static Equivalent Pressures of Detonative Explosions of Ethylene/O2/N2-Mixtures and Cyclohexane/O2/N2-Mixtures in Long and Short Pipes

In the recent past (PVP2013-97677, PVP2014-28197, PVP2015-45286, PVP2016-63223) we had started to determine the static equivalent pressures (pstat) of the eight detonative pressure scenarios in long and short pipes for different detonable gas mixtures. The pstat-values are of vital importance for process design: by assigning static equivalent pressures to the highly dynamic detonative pressure peaks it is possible to apply the established pressure vessel guidelines, which can only cope with static loads, in the design of detonation pressure resistant pipes. In the previous publications the parameter R was defined as the ratio between pstat at the location where transition from deflagration to detonation occurs and pstat in the region of the stable detonation. One important finding was that R depends on the reactivity of the gas mixture. So far, R cannot be predicted from first principles or from combustion parameters, but can only be determined experimentally. The ratio R has a special significance, because it not only determines pstat for the Deflagration to Detonation Transition (DDT) in long pipes (first detonative pressure scenario), but also gives a good estimate for two of the three scenarios relevant in the design of short pipes: DDT and the coalescence of DDT and reflection. The present paper concludes the test series conducted at BASF during the last 4 years. It presents additional experimental data showing the variation of R over the entire detonative range of Ethylene/O2/N2 mixtures and along the stoichiometric line of Cyclohexane/O2/N2 mixtures. Based on the variation of R for these ternary mixtures and for the mixtures presented in the preceding publications, a typical variation of R for a general combustible/O2/N2-mixture is estimated over the entire explosive range. By means of this estimation the static equivalent pressures of the six design-relevant detonative pressure scenarios of any combustible/O2/N2-mixture can now be derived combining the parameter R with the Chapman-Jouguet pressure ratio, which can be calculated in a straightforward manner from thermodynamic properties.

Effect of Helix Angle on the Bending Capacity of a 5 Inch API-Style Threaded Flange

This paper investigates the effect of helix angle on the structural bending capacity of an API 5-1/8 inch 15,000 psi rated threaded flange under internal pressure and external tension and bending. The three-dimensional finite element flange connection is analyzed with and without the helix angle, and API 17TR8 criteria is selected as a standard to calculate capacity. It was found that the helix angle model predicts a more conservative bending capacity with low internal pressure when compared to the model without a helix angle. At higher bore pressures, the roles reverse and the bending capacity predicted by the helix angle model is less conservative than the non-helix angle model. This disparity is diminished as external tension is increased.

Prediction of the Leakage Threshold for Hertzian Contact Seals: An Experimental Approach

The prediction and evaluation of leakage and leak tightness is an important issue in a multitude of high-pressure applications, such as valves, flanges and threaded pipe connections that are used under extreme service conditions that occur in oil and gas exploration and production. Using Hertzian contact theory or finite element techniques it is possible to determine the local contact conditions at the seal on a macroscopic level (to wit the extent of the contact area and the contact pressure in this area). However, the leak tightness of such a contact depends also on the surface topology, which is a microscopic characteristic. Therefore, the assessment of leak tightness requires an evaluation criterion relating both scales. Empirical evaluation criteria have been postulated in the past, each with their own application domain. More recently the Persson method has been developed that models the contact area microscopically using contact models developed in the field of tribology. However, in its current form this model is limited to flat surfaces while in many applications, such as valves, O-ring seals or metal-to-metal seals of threaded pipe connections, the contact is Hertzian and the contact pressure distribution is not uniform but parabolic. This paper provides the experimental results that will be used to validate an extension of the Persson model to Hertzian contact seals. A set of samples for leakage experiments was produced with varying surface topology. The surface roughness of these samples is measured and the leakage behaviour under high pressure is evaluated. This paper focusses on the experimental evaluation of the influence of surface topology on leakage.

Advanced High Strength Martensitic Stainless Steels for High Pressure Equipment

Maraging stainless steels offer a large panel of high strength materials with good ductility and stress corrosion cracking resistance. Their mechanical properties compared to conventional 15-5 PH and 17-4 PH martensitic stainless steels show much better yield strength / toughness compromise for yield strength exceeding 1300 MPa. In the same time, fatigue resistance is significantly increased at high strength stress levels and material keeps good resistance to stress corrosion. These properties make them particularly suitable for ultra-high pressure equipment or high pressure rotating components submitted to high cyclic stresses. Their application for Pascalisation pressure vessels and ultra-high pressure compressors for ethylene gas is briefly presented.