Based on the precedent design fatigue curves and recent fatigue data obtained from materials with different mechanical properties, new design fatigue curves with high general versatility in air have been developed by The Japan Welding Engineering Society (JWES). Structural materials with different tensile strength are utilized in fatigue tests to verify the validity of these design fatigue curves and discuss the mean stress effect. The materials employed in this study are austenitic stainless steel (SS) SUS316LTP, carbon steel (CS) STPT370, low-alloy steels (LASs) SQV2A and SCM435H, all of which are used in the structural components of nuclear power plants of Japan. The best-fit curves (BFCs) are formulated by using the parameter of tensile strength to describe the relationship between strain (stress) amplitude and fatigue life [1]. The results of fully reversed axial fatigue tests conducted with small-scale test specimens of those materials in air at ambient temperature show good agreement with the developed BFCs. The results of fatigue tests also indicate that the mean stress effect is remarkable in materials with higher tensile strength. The applicability of Modified Goodman and Smith-Watson-Topper (SWT) approaches to the design fatigue curves is compared and discussed when considering mean stress effect. The correction of mean stress effect with SWT approach shows a good agreement with the developed BFCs.

2.25Cr-1Mo-0.25V steel, which is a high strength low alloy (HSLA) steel, has been widely used for structural material of hydrogenation reactor due to its excellent combination of mechanical properties and resistance to hydrogen embrittlement (HE). However, it still suffers serious hydrogen damage during the aggressive service environment. When sufficient hydrogen concentrates in the reactor steel, the ductility and strength of the steel will be greatly decreased. Such a phenomenon of reduction of toughness is known as HE, and it can significantly weaken the safety and reliability of equipment. Therefore, the aim of this investigation is to focus on the effect of hydrogen on fracture toughness behavior of 2.25Cr-1Mo-0.25V steel. The fracture mechanics specimens in geometry of single edge notch bending were used. The immersion charging method was used to pre-charge hydrogen inside the specimens. Moreover, the fracture toughness of specimens with and without hydrogen pre-charging were measured following the ASTM E1820 standard. Finally, the fracture morphology was observed by scanning electron microscopy (SEM) to identify the HE mechanisms. The results of the present investigation showed that the pre-charged hydrogen resulted in significant reduction of fracture toughness of 2.25Cr-1Mo-0.25V steel, indicating a reduced crack growth resistance of specimens in the presence of hydrogen. Furthermore, the uncharged specimens failed in a ductile manner, whereas the fracture of pre-charged specimens is a mixed ductile and brittle fracture mode. It was believed that the hydrogen-induced decohesion (HEDE) mechanism contributed to the HE in hydrogen pre-charged specimens.

In the framework of a pressurized water reactor primary loop replacement, elbows of different types were produced in cast austenitic stainless steel grade Z3CN 20-09 M. For that type of component, acceptance tests to check the sufficient mechanical properties include room and hot temperature tensile tests, following the RCC-M CMS – 1040 and EN 10002 specifications. A large test campaign on standard 10mm diameter specimens was performed and exhibited a high scattering in yield stress and ultimate tensile strength values. As a consequence, some acceptance tensile tests failed to meet the required minimal values, especially the ultimate tensile strength. Optical and electronic microscopy revealed that the low values were due to the presence of very large grain compared to the specimen gage diameter. However, tensile tests strongly rely on the hypothesis that the specimen gage part can be considered as a representative volume element containing a number of grains large enough so that their variation in size and orientation gives a homogeneous response. To confirm the origin of the scattering, a huge experimental tensile test campaign with specimens of different diameters was conducted. In parallel, FE calculations were also performed. From all those results, it was concluded that it was necessary to improve the RCC-M code for that type of test for cast stainless steel: to do so, a modification sheet was sent and is being investigated by AFCEN.

In order to determine the target flaw depths for performance demonstration (PD) of nondestructive testing of cast austenitic stainless steel (CASS) pipes, probabilistic fracture mechanics (PFM) code, “PREFACE”, was developed to evaluate ductile fracture and plastic collapse considering change in mechanical properties due to thermal aging and uncertainties of the mechanical properties. In the previous study [1] , the tables of the target flaw depths for the highest ferrite content material of the Japanese CASS pipes at the fully saturated thermal aging condition were calculated for several flaw lengths and several stress levels by the PREFACE code. However, the sensitivity study of the ferrite content on the target flaw depth revealed that the fully saturated thermal aging condition may not be the most severe condition for the target flaw depth. In this study, the PREFACE code was modified to improve the dependency of ferrite content on true stress-strain curve and to apply limit load method directly at the failure mode of plastic collapse. To confirm of the correct relation between ferrite content and aging time, deterministic sensitivity analyses were performed. For validation of the PFM function, the results of the deterministic analysis and PFM analysis were compared.

Cr-Mo and Ni-Cr-Mo high-strength low-alloy steels are candidate materials for the storage of high-pressure hydrogen gas. Forging materials of these steels have been used for such an environment, while there has been a strong demand for a higher performance material with high resistance to hydrogen embrittlement at lower cost. Thus, mechanical properties of Cr-Mo and Ni-Cr-Mo steels made of quenched and tempered seamless pipes in high-pressure hydrogen gas up to 105 MPa were examined in this study. The mechanical properties were deteriorated in the presence of hydrogen that appeared in reduction in local elongation, decrease in fracture toughness and accelerated fatigue-crack growth rate, although the presence of hydrogen did not affect yield and ultimate tensile strengths and made little difference to the fatigue endurance limit. It is proposed that pressure vessels for the storage of gaseous hydrogen made of these seamless line pipe steels can be designed.

The effect of simulated post-weld heat treatment (PWHT) on the mechanical properties and microstructure of P-No. 1 materials was investigated and the suitability of the exemption requirements of simulated PWHT as specified in ASME Section III Division 1 Subsection NX-2211 was evaluated. SA-516 Gr. 60 and 70 carbon steel plates and SA-106 Gr. B carbon steel pipe were employed, and the materials were subjected to the simulated PWHT with holding temperatures of 610, 650, 690, and 730 °C for 8 hours. Both the tensile strength and lateral expansion of the materials tested in this study showed a tendency to decrease with increasing holding temperature of the simulated PWHT. In some cases, the materials after the simulated PWHT cannot satisfy those mechanical requirements.

Evaluation of mechanical properties from a small amount of test material is essential in a process of development of new alloys and monitoring of degradation and damage of in-service components. Miniaturization of test specimen together with a specimen reconstitution technique enable direct method of evaluation of mechanical properties if only a limited amount of material is available. Modern types of construction steels and alloys possess improved metallurgical and mechanical properties and also demonstrate a high level of fracture toughness. In these cases, correlation and correction of size effect on fracture toughness between different specimen sizes are needed. Within this contribution the size effect of J-R curve for two sizes of three-point-bend specimens is solved. The J-R curves of three-point-bend specimens with cross-section 10 × 10 mm and 3 × 4 mm of P91 and Eurofer97 steel were determined at room temperature. The existence of size effect is proven as larger specimens demonstrate higher J-R curves than smaller specimens for both steels. Simplified mechanical model based on fracture energies initially proposed by Schindler et al. is applied to describe experimentally determined J-R curves and to predict the observed size effect between used specimen sizes.

The use of correlations is common in the research and development arena of the nuclear industry with the realization that some applications with direct implications to safety demand a more rigorous approach. Most correlations involve the relationship between two experimental properties, such as that between hardness and tensile strength. There are others that are much more complicated and are often designated models because they incorporate physically-based knowledge; examples of this are predictive correlations for irradiation-induced embrittlement of reactor pressure vessels (RPV). The objective of this paper is to collect and discuss many of the commonly used correlations for applications to nuclear RPVs. This paper identifies and discusses various correlations that relate easily measured properties to properties that are more difficult, more time consuming, or more expensive to measure. In the case of irradiated RPV materials, irradiation-induced changes in easily measured properties are related to the changes in those more difficult to measure. It is noted that recognition and understanding of the uncertainties associated with all correlations is highly important.

A simple testing method to evaluate the influence of high pressure hydrogen gas up to 100 MPa on mechanical properties at the temperature between 20 K and up to 800 K had been developed. In this method, instead of using high-pressure gas vessels, high pressure gas was filled into a small hole in the hollow-type test piece from a hydrogen gas cylinder or a compressor for 10 MPa or 100 MPa test. A small inner diameter of the hole enables to evaluate also the reduction of area in the slow strain-rate tensile (SSRT) tests. The temperature of the test piece with the high pressure gas can be changed simply by surrounding coolant or heater between 20 K and 800 K. Lots of test results by this method proved that almost the same results were obtained between this method and the conventional method with high-pressure gas vessels where test piece is installed. The great advantages of this method are not only the less cost for the facilities of high-pressure vessels but also the ability of tests at lower or higher temperatures than those with the vessels. So, this method is proposed to be used world-widely to evaluate the mechanical properties of structural materials for extremely severe environments, such as high-pressure hydrogen applications and also to study the mechanism of the influence of high-pressure hydrogen for design and reliability of those facilities. In this paper, the details of testing procedure of this method and results of tensile and fatigue tests in up to 70 MPa hydrogen gas on several kinds of stainless steels obtained by this method are presented.

To develop safer and more cost-effective high-pressure hydrogen tanks used in fuel cell vehicles (FCVs), the metallic materials with the following three key properties, i.e. lightweight, high strength and excellent resistance to hydrogen embrittlement should be explored. In this study, the compatibility of high-strength, precipitation-hardened aluminum alloy 7075-T6 was evaluated according to the four types of mechanical testing including slow-strain rate tensile (SSRT), fatigue life, fatigue crack growth (FCG) and fracture toughness tests in high-pressure gaseous hydrogen environments (95 ∼ 115 MPa) at room temperature. Even though numerous publications have previously reported significant degradation of the mechanical properties of 7075-T6 in some hydrogenating environments, such as moist atmosphere, the understanding with regards to the performance of this alloy in high-pressure gaseous hydrogen environments is still lacking. In SSRT tests, the alloy showed no degradation of tensile strength and ductility. Furthermore, fatigue life, fatigue crack growth and fracture toughness properties were also not degraded in hydrogen gas. Namely, it was first demonstrated that the material has big potential to be used for hydrogen storage tanks for FCVs, according to its excellent resistance to high-pressure gaseous hydrogen.

In the last decades, the use of adhesives has rapidly increased in many industrial fields. Adhesive joints are often preferred to traditional fasteners due to the many advantages that they offer. For instance, adhesive joints show a better stress distribution compared to the traditional fasteners and high mechanical properties under different loading conditions. Furthermore, they are usually preferred for joining components made of different materials. A wide variety of adhesives is currently available: thermoset adhesives are generally employed for structural joints but recently there has been a significant increment in the use of thermoplastic adhesives, in particular of the hot-melt adhesives (HMAs). HMAs permit to bond a large number of materials, including metal and plastics (e.g., polypropylene, PP), which can be hardly bonded with traditional adhesives. Furthermore, HMAs are characterized by a short open time and, therefore, permit for a quick and easy assembly process since they can be easily spread on the adherend surfaces by means of a hot-melt gun and they offer the opportunity of an ease disassembling process for repair and recycle. For all these reasons, HMAs are employed in many industrial applications and are currently used also for bonding polypropylene and polyolefin piping systems. In the present paper, the dynamic response of single lap joints (SLJ) obtained by bonding together with a polyolefin HMA two polypropylene substrates was experimentally assessed. Quasi-static tests and dynamic tests were carried out to investigate the strain rate effect: dynamic tests were carried out with a modified instrumented impact pendulum. Relevant changes in the joint performance have been put in evidence. Failure modes were finally analysed and compared. A change in the failure mode is experimentally found: in quasi-static tests SLJ failed due to a cohesive failure of the adhesive, whereas in dynamic tests the SLJ failed due to an interfacial failure, with a low energy absorption.

The rigorous analysis of tube-to-tubesheet joints requires a particular attention to the transition zone of the expanded tube because of its impact on joint integrity. This transition zone is the weakest part of the joint due to the presence of high tensile residual stresses produced during the expansion process which coupled to other in-service loadings and harsh corrosive fluids results in joint failure. In fact, this zone is often subjected to stress corrosion cracking caused by intergranular attack leading to plant shutdown. Therefore, the evaluation of the residual stresses in the transition zone is of major concern during the design phase and its accurate assessment is necessary in order to achieve a reliable joint in service. In this study, a new analytical model to evaluate the residual axial and hoop stresses in the transition zone of hydraulically expanded tubes based on an elastic perfectly plastic material behavior has been developed. The model is capable of predicting the stress state under the maximum expansion pressure and after the expansion process has been completed. Three main regions are identified in the transition zone: the fully plastic region, the partially plastic region and the elastic region. Therefore, various theories have been applied to analyze the stresses and deformations neglecting the partial plastic region because of simplicity. The validation of analytical model is conducted by comparison of the results with the ones of 3D finite element models representing typical geometry and mechanical properties. The effect of reverse yielding of the expansion zone is also investigated.

The Standard Technical Documentation of the Association of Mechanical Engineers is elaborated by a team of experts on the base of an actual knowledge and practice as a part of a row of recommendations for an assessment of strength and reliability, choice of materials and solution of service problems of the Czech nuclear power plants. It was elaborated by the Association of Mechanical Engineers as an important help for a practice. This issue is published from the following main reasons: 1. The criteria for an assessment of reliability and safety of technical tasks have changed in a last decades by influence of development of fracture mechanics and a diagnostics of an actual components of the nuclear power plants, exposed to a static and dynamic strength in a service and to effects of an aggressive environment, leading to an origin of integrity failure. The access is changed now from an assessment of an original strength to a long-term operation and service reliability. A conception of a choice of suitable materials has changed, especially about an influence of production technology on barriers against a degradation of material mechanical characteristics. Imaginations also changed about increasing of a resistance against a damage of material components in complex service conditions. New scientific knowledge it is necessary to apply responsibly just in a construction of as important equipment as nuclear power plants are. 2. A social responsibility is increasing for economic and ecological behavior and hence manners of service control are always innovated. A problematic of a residual lifetime of those tasks is connected with. A prolongation of a lifetime without a risk of equipment failure hereto is a principal demand of capital-intensive units. 3. In a last decades a possibility of efficient use either an automatic service diagnostics or an inspection of component state in service brakes has deepened. It is necessary to enable a right engagement of this instrumentation and to interpret in a right way obtained information. 4. An international cooperation in this important region is developed, an elaboration of information and their common deepening has a substantial importance for a future of a nuclear energetic. A dissembling of service obstacles, an objective analysis of failure situations and a method of their control is today’s obvious duty of an international cooperation. Programs of all variants of a possible damage and its control must be prepared. It is impossible at once to apply all those aspects in provisions and recommendations. Issued provisions however must be constantly improved and complemented.

A cracking incident of a 304 stainless steel elbow serving in the synthesis gas purification device occurred during running. In order to get an understanding of the failure mechanism, a failure analysis was performed on the cracked elbow in this paper. The chemical composition, mechanical properties of strength, toughness and hardness, hydrogen content were identified and determined. The metallographical structure was observed and analyzed by optical microscope (OM) and X-Ray Diffraction (XRD), while the fracture morphology was observed by scanning electron microscope (SEM). The results showed that the chemical composition of the cracked elbow meet the requirements for China standard, while comparing with GB/T 14976-2012 standards, the strength and elongation of the leaked elbow are higher and lower respectively, and the hardness of the leaked elbow was higher than quality certificate documents that of HB ⩽ 187. Large quantities of martensite and δ-ferrite were observed in elbow, which indicated that the elbow was not well solid solution heat treated required by specification (1050°C,30min). The fracture morphology presents typical brittle fracture. The hydrogen content of cracked elbow was significant higher than that of other 304 stainless steel elbow serving in the environment without hydrogen. It is acknowledged that martensite showed higher sensitivity of hydrogen embrittlement compared with austenite. Furthermore, the operating temperature of cracked elbow was in the range of high hydrogen embrittlement sensitivity. Depending on the metallographical structure, strength, service environment, hydrogen content and fracture morphology, it can be concluded that hydrogen induced delayed cracking was the dominant mechanism of the failure.

In this paper, the mechanical properties and microstructural changes of 2.25Cr1Mo0.25V steel under different heat treatment and welding process were investigated. The heat treatment of steel during practical processing is taken as a reference. Different heat treatment time are used to obtain samples with different condition. Automatic submerged arc welding was used to obtain welding sample. The mechanical properties of different samples are obtained by tensile test; the evolution of microstructure and precipitates of different sample with heat treatment and welding was studied on scanning electron microscopy. The experimental results show that with the increase of heat treatment time, the strength of the samples decreases and the plasticity remains nearly constant. Heat treatment also affects the precipitation of carbides; the longer the heat treatment time is, the more precipitates are. Compared with the base metal, the welding metal sample has higher strength. The amount of precipitates in welding metal is much larger than it in base metal. The research on precipitation shows that there are different kinds of precipitates which have different morphologies in welding metal.

Quenched and tempered high strength steel 07MnMoVR with a better combination of mechanical properties and low susceptibility to welding crack has attracted attention for applications in engineering fields. Exposure to fire will subject steel to thermally induced environmental conditions that may alter the material’s properties. The residual strength after a fire event is important to assess the extent of the fire damage and the potential reusability of the vessel. This paper presents the details of an experimental investigation on the post-fire mechanical properties of 07MnMoVR steel. Uniaxial tension tests and Charpy impact tests were performed on coupons exposed to elevated temperatures varying from 550°C to 850°C for half an hour to 8 hours and then naturally cooled in air or cooled by water. The post-fire stress-strain curves, strength, ductility and impact toughness of 07MnMoVR steel are discussed. The results show that the yield plateau in post-fire stress-strain curves disappears when the exposure temperature is higher than 700°C. The residual yield strength and ultimate strength decrease firstly and increase afterward with increasing exposure temperature. The influences of duration time on the residual strength are considerable for exposure at 650°C. The post-fire impact toughness of 07MnMoVR steel at −20°C degrades drastically with increasing duration time when the exposure temperature reaches 700°C. The effects of cooling methods on strength and toughness become significant when the exposure temperature exceeds 750°C. The critical tempetature for the mechanical properties deterioration is 650°C. This study can provide basis data and guidelines for the fitness for service assessment of 07MnMoVR steel suffered from fire accident.

Composites can undergo micro damage leading to variations of the mechanical properties. The micro damage can be the result of high internal pressures and/or exposure to physical trauma. Modeling techniques, such as homogenization, are effective outside of these critical regions. However they break down in regions where the micro damage is occurring. To account for this, an algorithm has been developed to detect and implement a multiscale method on elements that observe a critical strain. The remaining model uses the homogenization method to simulate the composite. The elements surrounding the multiscale elements are adapted to act as a transition from multiscale to the homogenization. The multiscale approach is based on the partition of unity paradigm, allowing macro-scale computations to be performed with the micro-structural features explicitly considered. The model has been validated by comparing it to experimental results of a composite overwrapped steel tube section subjected to high internal pressures.

The sealing performance prediction of bolted pipe flange connections with gaskets is important factor. However, it is known that the sealing performance of the larger nominal diameter connection is worse than that with smaller nominal diameter connection due to the flange rotation. Furthermore, recently PTFE blended gaskets were developed newly and the excellent sealing performance in the bolted pipe flange connection with smaller nominal diameter is found. So, it is necessary to examine the sealing performance and the mechanical characteristics of pipe flange connections with larger nominal diameter under internal pressure. The objectives of present study are to examine the mechanical characteristics of the pipe flange connection with PTFE blended gasket under internal pressure such as the load factor, the contact gasket stress distribution and the sealing performance using FEM and experiments. Using the obtained contact gasket stress distribution and the fundamental leak rate for smaller PTFE gasket, the leak rate of the connection is predicted under internal pressure. In the FEM calculation, the effects of the nominal diameter of pip flange connections on the mechanical characteristics are shown. In the experiments, ASME class 300 24” pipe flange connections is used and the gasket is chosen as No.GF300 in PTFE blended gaskets. The FEM results of the axial bolt forces are in a fairly good agreement with the experimental results. In addition, the leak rate obtained from the FEM calculations are fairly coincided with the measured results. The mechanical characteristics of pipe flange connection with PTFE blended gasket are compared with those with spiral wound gasket.

In this work, the mechanical properties of two different adhesives compositions have been investigated both experimentally and numerically. The studied thermoplastic adhesives are Hot-Melt Adhesive (HMA). In particular, a pristine and a nanomodified adhesive with 10% in weight of iron oxide have been considered. The adhesives have been subjected to a series of single lap joint (SLJ) tests using adherends made of polypropylene copolymer. As it is well-known, the structural-mechanical behavior of adhesive joints is mostly influenced by the bonding process: thickness of adhesive as well as its application procedures and the surface preparation of adherends are among the most influencing factors. In addition, the mechanical behavior of SLJ test is particularly influenced by the correct alignment of adherends and applied load. These aspects have been investigated, analyzing the experimental results. Moreover, the experimental results have been used to develop a numerical model of the two adhesives. The numerical analysis has been carried out using the commercial software LS-DYNA. Transient nonlinear finite element analysis has been performed to simulate the mechanical behavior of the thermoplastic adhesives. In particular, the cohesive formulations of the elements have been taken into consideration after a careful literature review. In order to set-up and to validate the mechanical properties of the adhesives, the experimental SLJ tests have been simulated. The developed finite element models enable to investigate more complex joint structures where these types of adhesives are used, such as plastic piping systems and automotive applications. Further, the numerical models allow to investigate with higher accuracy and lower time different aspects such as manufacturing and non-linear effects.

In this paper, the results of evaluations of the stress intensity factor and of natural SCC (stress corrosion cracking) growth simulations under weld residual stresses are presented. The results reveal some characteristic SCC propagation behaviors. A surface crack is assumed at the welded joint of a mock-up of a pressurized water reactor (PWR) pressurizer nozzle. SCC propagation law for the weld metal is assumed to be anisotropic. SCC propagation in the surface direction is assumed to be 10 times slower than that in the thickness direction of the welded joint. The anisotropy is caused by different solidification rates of the weld metal in these directions. The material in terms of mechanical properties is set to be homogeneous as differences in the Young’s moduli and the Poisson’s ratios are not large between the weld and the mother metal. The analytical procedures based on the finite element method with an automatic mesh generation scheme for cracked structures, the results of analyses under a various residual stress distributions and some discussions are presented in this paper.