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The instrumented indentation test (IIT) is an attractive non-destructive testing technique. Determining accurate strength properties of steel using IIT is still challenging. In this paper, a new methodology is proposed to acquire the yield and ultimate tensile strength from a single IIT. This method extracts true stress-strain curves from IIT results. Acquired stress-strain curves indicate that the initial yield stress is not repeatable. This is caused by the inhomogeneous deformation of IIT specimens. Based on the obtained true stress-stain curves, corresponding yield strength, and ultimate tensile strength are calculated through theoretical derivation. The results show that the strength has a convergent tendency. On basis of this phenomenon, the strength is determined with an extrapolating method. Finally, the strength properties of Q345R are investigated to verify the reliability of this method. It is found that the strength determined from IIT and conventional tensile tests shows good agreement. The proposed method is effective in predicting strength properties from a single instrumented indentation test.

A methodology is proposed to obtain mechanical properties of bulk materials from multiple loading-unloading curves of an instrumented indentation test (IIT). The mechanical properties, including strain-hardening exponent (n) and strain-hardening rate (K), were extracted from IIT data using iterative finite element simulation coupled with a simulated annealing swarm particle optimization. A systematical study was carried out to discuss the uniqueness of the identified properties. Results showed that the mechanical properties obtained in this way are unique and accurate adequately. It was found that the maximum errors of K and n generated from the macro indentation tests are 8.2% and 6.3% respectively. The identified mechanical properties from ten loading-unloading curves was compared with those obtained from a single loading-unloading curve. We found that ten loading-unloading cycles of an indentation test will result in more precise mechanical properties.

Small punch test (SPT) was conducted to investigate the mechanical properties of A350 steel in low temperatures. Un-notched and U-shaped specimens of the A350 steel were cut from the axial, circumferential and radial directions. And their ductile-brittle transition temperature and upper plateau impact energy were determined. No significant difference was observed in the upper plateau impact energy and ductile-brittle transition temperature of the steel according to the obtained results of the analyzed three different directions of the un-notch specimens. In contrast to the un-notch steel specimens, the upper plateau fracture energy of the three different directions of the material was determined using the U-notched specimens. This observation was in agreement with the results obtained by the Charpy impact test and confirms that the SPT using U-shaped notch specimens is a more reliable technique to evaluate the mechanical properties of anisotropy materials.

Acquiring a full range of yield curves is a long-standing challenge in material science and engineering. Such curves are extremely important for stress analysis using finite element simulation. In this article, we proposed an improved procedure integrating finite element analysis and hybrid particle swarm optimization to extract a post-necking yield curve from a smooth tensile round bar. The investigated material was 3Cr1MoV. The strain range of the yield curve was extended from 0.0681 mm/mm before necking to 1.5 mm/mm. The results revealed that curves obtained through this procedure are reliable and unique. Three notched round bars were designed to investigate the effects of stress triaxiality on the yield curves. We found that stress triaxiality has a significant influence on curves at large plastic strains (strain > 0.3 mm/mm) and has a negligible effect at low plastic strains (strain

Evaluating the strength properties of materials of an in-service pipeline without shutting down transportation has been always a challenge. A novel and non-destructive method for determining the true stress-strain curve of pipeline steel based on backpropagation artificial neural network and small punch test is proposed in this study. The elastoplastic mechanical properties of the pipeline steels could be obtained by this method. The load-displacement curves of 2261 groups of different hypothetical materials were obtained by the finite element model of small punch test within Gurson-Tvergaard-Needleman (GTN) damage parameters and used to train the neural network. The relationship between the load-displacement curve of small punch test and the true stress-strain curve of the conventional uniaxial tensile test was established based on the trained neural network. The accuracy and wide applicability of the trained neural network were verified by the experimental data of four types of materials obtained by small punch test and standard tensile test, respectively. The established relationship can be used to predict the true stress-strain curve of the pipeline steels to determine the elastoplastic mechanical properties only by the load-displacement curve of the small punch test without performing the conventional tensile test.

Developing a universal approach for acquiring yield strength and ultimate tensile strength by small punch test (SPT) is a long-standing challenge. In this paper, a methodology is proposed to obtain strength properties of bulk materials from SPT results. It involves extracting true stress–strain curves from SPT data using iterative finite element simulation. Acquired curves show that initial yield stress cannot be reproduced. This is attributed to the inhomogeneous deformation of SPT specimens. Finite element simulations were then conducted on tensile tests with the extracted true stress–strain curves to obtain strength. Results indicate that predicted strength shows a convergent tendency with the times of iterative finite element simulation. An approach is proposed to process the predicted strength, based on data analysis. The acquired strength properties are in good agreement with those obtained by standard tensile testing. This is an entirely novel procedure that predicts reliable tensile properties via a single SPT run. It can be employed with confidence to cases for which standard tensile testing is impractical.

In this paper, the boriding of low-carbon steel was studied. We found that FeB phase forms on the surface of low-carbon steel at the initial stage, and then Fe2B nucleates on the FeB, with a crystallographic relation between the two phases. In the third stage, Fe2B further grows on the FeB phase, and FeB starts to diminish. A model is proposed to explain the formation of the textured FeB and Fe2B phases. It is found that the strong anisotropy of the FeB lattice structure and the lattice matching between FeB and Fe2B are the reasons for the preferred crystallographic development in the boride coating.

Standardization of small punch test (SPT) is being carried out, but the specimen thickness is not unified in different laboratories and specimen size effect on deformation mechanism still remains unclear. In this paper, SPT specimens with thicknesses of 100 μm–500 μm were used to investigate specimen size effect on strength properties and fracture morphology of 3Cr1MoV and Incoloy 800H. The grain sizes of two materials were 10 μm (3Cr1MoV) and 50 μm (Incoloy 800H). Yield and ultimate tensile strengths were extracted from SPT data based on a combined-scheme of finite element simulations and experiments. Fractography of specimens was studied using a scanning electron microscope. The results showed that the strength value of Incoloy 800H increased with the decrease of thickness, the size dependence of yield strengths was stronger than that of ultimate strengths, and the strength value of 3Cr1MoV was thickness independent. Fractographic investigations found that failure was caused by ductile fracture for all the specimens. A theoretical model considering geometrically necessary dislocations densities, available dislocation sources and grain sizes was proposed to explain the different mechanical responses of investigated materials to specimen thicknesses. This work clarifies the SPT specimen thickness and the grain size of studied materials should be taken into consideration before sampling.

Small punch creep test is a semi-destructive method which can be used for creep determination and assessment of the probability of failure. The deformation during this small punch creep test is very complicated because the accumulation of both plastic and creep deformation will lead to the failure. In order to understand the mechanism of small punch creep test from grain orientation changes during creep and fracture process, small punch creep test was carried out using P91 steel under a 500 N load at 600 °C in this study. Finite element method was used to simulate the mechanical deformation process. Electron back-scattered diffraction technique and texture analysis were used to analyze the microstructure of the small punch creep deformation and rupture. The results showed that the grain orientation evolves from the rather random orientation in the first stage to the//ND texture in the tertiary stage which caused mainly by shear strain.

Heat exchange tubes are a weak part in reboilers. This paper introduces a cavitation corrosion case of heat exchange tube bundles in an oil reboiler. A series of inspections including chemical, mechanical, energy spectroscopy and metallographic were carried out on heat exchange tube bundles to investigate the material quality. The results show that chemical compositions and mechanical properties of the material agree with requirements. Numerical simulation was performed to figure out the thermal filed of the reboiler heat exchanger. It was found that the temperature of heat exchange tube bundles near support plates was high. The localized high temperature caused boiling bubbles formed faster near support plates than other regions. The impact force induced by bubble collapse lead to cavitation corrosion on the surface of tubes and then produced corrosion pits. The factors causing cavitation corrosion were improper design and operation of the reboiler, and suggestions were given to aviod similar failure ocuured repeatedly.

Zinc was added to high-temperature borated and lithiated water and its effect on the oxide film formation, electrochemical behavior and crack growth rate of 316 L stainless steel was investigated. The actual concentrations of zinc in the coolant solution at 300 °C and 350 °C were determined by ICP-Mass spectrometry. Zinc improved the electrochemical corrosion parameters of steel and reduced the crack growth rate of the sample by more than 13 times. Zinc also reduced the thickness of the formed oxide layers on steel and modified the chemical composition of the oxides to a mainly zinc and iron rich spinel.

The small punch test (SPT) can be recognized as an alternative to uniaxial tensile testing. Many attempts have been made to extract tensile strength properties from SPT results, but few works on estimating true stress-plastic strain curves. This paper aims to propose an approach for deriving a true stress-plastic strain curve from the force-deflection curve of SPT. Engineering stresses at different plastic strains are extracted from a force-deflection curve using a database method. These engineering stress and plastic strain points are employed to determine the parameters of Ludwik hardening law, then a true stress-plastic strain curve before necking is generated using those parameters. To verify the accuracy in estimating true stress-plastic strain curves in this way, the procedure was applied to three hypothetical materials. It is found that true stress-plastic strain curves before necking are accurately extracted from SPT data. The proposed procedure was also used to process the force-deflection curves of 3Cr1MoV, 2.25Cr1Mo and 16MnR. The results show that the obtained stress and strain relation can predict the force responses of SPT specimens well. Using a database to evaluate true stress-plastic strain curves is effective and with the support of theory that could promote the application of SPT in characterizing material mechanical properties.

Evaluating the mechanical properties of materials of in-service pipelines without shutting down transportation has been always a challenge. A semi-destructive method for determining the true stress-strain curve of in-service natural gas pipeline steel based on backpropagation (BP) artificial neural network and small punch test (SPT) was proposed in this study. The load-displacement curves of 457 groups of different hypothetical materials were obtained by the verified finite element model of SPT within Gurson-Tvergaard-Needleman (GTN) damage parameters and used to train the neural network. The relationship between the load-displacement curve of the SPT and the true stress-strain curve of the conventional tensile test was established based on the trained neural network. The elastoplastic properties of in-service natural gas X80 pipeline steel were obtained by this method. The accuracy and wide applicability of the trained neural network were verified by the experimental results of four types of materials obtained by the SPT and conventional tensile test. This work demonstrates a semi-destructive method, which can be applied to derive the true stress-strain curve of the in-service pipeline steels to determine the elastoplastic properties only by the load-displacement curve of the SPT without performing conventional tensile test.

Residual creep life evaluation is an essential part of Fitness-For-Service (FFS) assessment for engineering components operating at elevated temperatures. This paper elaborates a new evaluation approach based on macro-pillar compression testing (MPCT) and Omega method. The MPCT creep test was performed at the same stress level as the tensile creep test by adjusting the testing load using an algorithm, which is also provided in this paper. The barreling effect of MPCT creep test was corrected by two proposed parameters, which were calibrated using finite element method with 6 types of Cr-Mo alloys from API 579. Finally, a 1.25Cr-0.5Mo-Si steel was prepared for both MPCT and round bar tensile creep tests at 550 ℃ and different initial stress levels. The predicted residual creep life by MPCT has an error of 2.2% at 140 MPa, 6.4% at 160 MPa and 13.5% at 180 MPa.

The GTN model proposed by Gurson, Tvergaard and Needleman has been widely applied to predict ductile fracture. However, the evaluation of the GTN model under high stress triaxiality has only been reported in a few studies. In this paper, a series of tensile tests on round notched specimens were performed to evaluate the applicability of the GTN model parameters under high stress triaxiality. The evaluation was carried out by comparing the predicted load-displacement curves with experimental results. It was observed the GTN model parameters only depend on the material except the critical void volume fraction. The influence of stress triaxiality on the critical void volume fraction was discussed. A further discussion about the construction of a new void coalescence criterion for the GTN model was also presented in this paper.

The instrumented indentation test is a promising nondestructive technique for evaluating mechanical properties of metallic materials. In this study, the localised mechanical properties of welded joint of 304 austenitic stainless steel were characterised with the instrumented indentation test. The single V-groove welded joint was produced using the electric arc welding. A series of instrumented indentation tests were carried out at different regions, including base material, weld zone and heat-affected zone (HAZ). A soft zone regarding strength properties was found in the coarse-grain HAZ. The results show that the HAZ has the lowest yield strength and tensile strength (263.6 MPa, 652.5 MPa) compared with the base material (307.4 MPa, 807.9 MPa) and the weld zone (285.6 MPa, 702.1 MPa). In addition, characterisations of microstructure, microhardness and conventional tensile tests have been performed for comparison. The results reveal that the localised mechanical properties of welded joint of auste...

The indentation technique provides a non-destructive characterization method for in-service equipment. Recent years have seen an increased interest in determination of tensile properties from spherical indentation test (SIT) through inverse optimization theory, while this approach is computational costing and time consuming. This paper aims to propose a novel method for estimation of strength properties based on the application of a database. Firstly, systematically varied Ludwik constitutive parameter vectors were used to establish a database that contains indentation force-depth curves (extracted from the simulated results of indentation test) and corresponding strength values (calculated with the parameter vectors). The indentation force-depth curve of a concerned material was then processed by the database, as a result of which the yield and ultimate strengths can be obtained. A series of virtual materials were used to validate the accuracy of this method. The results show that the database method provides an impressive accuracy and stability in estimating strengths. The database method was subsequently applied to eight real materials. It is found that the predicted strengths agree well with the values from uniaxial tests for most materials, while the database method may be not suitable for materials that show obvious tension-compression asymmetry.

Instrumented indentation testing (IIT) is extensively and successfullyused as an alternative technique for investigations of the mechanical properties of steels, where traditional specimens, such as round bar specimens for tensile tests, are not acquirable. The IIT is based on an assumption that the true stress-strain curve of the analyzed sample follows one of the known constitutive equations, such as the Hollomon, Ludwik, Swift, or other predefined equations. However, steel samples exposed to extreme conditions, such as media with high-temperature or neutron irradiation may exhibit a novel stress-strain relation that follows an unknown constitutive equation. In this situation, the accuracy of the IIT may be doubtful. This article proposes a constitutive model independent technique, called macro-pillar compression testing (MPCT), where constitutive equations are not necessary. The MPCT was conducted on a specimen with pillars and established on the theory of the compression testing. In this study, a modified imaginary radius function was proposed for the pillar to consider its sinking and barreling effect during the compression. Finally, three different types of steel (austenitic stainless, low-alloy and low-carbon steel) were used to verify the accuracy of the MPCT. Both MPCT and round bar tensile tests were performed on each type. The test results show that the true strain-stress curves obtained by MPCT have a good agreement with the corresponding curves by tensile tests.

Failure of spiral wound gaskets rarely occurs due to its outer ring. This paper presented a failure case of a spiral wound gasket owing to the rupture of the outer ring. A series of inspections including chemical, mechanical, metallographic and fractographic analysis were carried out to explore the failure mechanism of that ring. The results show that chemical compositions and mechanical properties of material agree with standard requirements and failure is related to multi-source fatigue fracture. Serious plastic deformation is found near the fracture region. It is owing to squeeze between the bolt and the outer ring, causing a high local circumferential stress. Fatigue fracture occurs when the outer ring is subjected to alternating loads under the high stress. The factors causing fracture of the outer ring were discussed and suggestions were also given for avoiding this failure.

Various notched small punch test (SPT) specimens have been designed to determine ductile–brittle transition temperature (DBTT) of materials, but the use of notched specimens is still lacking an in-depth understanding. This paper aims to study the effects of notch depths on SPT results. U-notched specimens made from 3Cr1MoV were tested at different temperatures. It is found that the specimen with a notch of depth-to-thickness ratio a/t = 0.3 generates a clear lower shelf and higher ductile-brittle transition area as compared to the specimens with notches of a/t = 0 and 0.5. Finite element simulations reveal that the lower shelf and higher transition part are related to the geometric constraint of specimens. Fracture behaviors of notched specimens were also thoroughly studied using simulations. It suggests that a/t = 0.3 can be regarded as a reference depth for U-notched specimens. This finding is confirmed with metallic alloys 2.25Cr1MoV, A106 and 16MnR. The present work gives a new insight into the design of notched SPT specimens for estimating DBTT.

The 17–4 precipitation hardening (PH) stainless steel (SS) is being widely used in high-pressure corrosive environments due to its high strength and excellent corrosion resistance. Some stems of valves employed in petrochemical pipeline are manufactured from 17-4 PH SS in order to ensure strength, impact toughness and reduce corrosion. Here we presented a rare failure case of 17–4 PH SS. In the present failure analysis, an instantaneous fracture of a valve stem in a pipeline system occurred. The stem was made of 17-4 PH SS and ruptured after a short-term use. A systematic study including metallographic, fractographic, chemical and mechanical analysis was carried out to explore the failure mechanism. The results indicate that incorrect peak-aging treatment and phase segregation induce high hardness and high hydrogen embrittlement (HE) tendency of 17-4 PH SS. It was concluded that, the failure of the valve stem is related to HE at the stress concentration region.

Damage models are widely used to describe the ductile damage and fracture of metals. This paper proposes a new approach to determine the damage parameters of the Gurson-Tveergard-Needleman model, which uses a load–displacement curve coupled with finite element method. The load–displacement curve was obtained from a tensile test of a smooth tensile specimen and contained information about the damage and fracture behavior of the tested material. The principle of damage parameters identification is to minimize the deviation between experimental and simulated load–displacement curves by a hybrid particle swarm optimization. As a combination of particle swarm optimization and simulated annealing, the hybrid particle swarm optimization is an economical and effective algorithm to identify damage parameters. The identified damage parameters are also verified by testing and simulating deformation shapes of the smooth tensile specimen which is used for parameters determination. Tests and simulations of notched tensile specimens were carried out to discuss the transformability of the identified damage parameters. It is observed that the value of critical void volume fraction of the Gurson-Tveergard-Needleman model decreases with the increase of stress triaxiality.

Suitability of small punch test (SPT) was studied by determining the mechanical behaviors of AISI 316L austenitic stainless steel samples with different crystallographic textures and similar grain size prepared by different modes of rolling and annealing treatment. It was demonstrated that SPT can capture differences in mechanical properties that arises from different crystallographic textures. Such capacity can be attributed to higher length scales involved in SPT which enables it to capture the effect of texture unlike routine micro-hardness measurements. It is also shown that important additional information like rate of strain hardening and mode of fracture that can be obtained from SPT is consistent with results expected from conventional tensile test.

In a Metal-to-Metal Contact (MMC) bolted flange joint the stress of sealing surfaces is constant. It canwithstand higherbolt load. So, under the operating conditions of high temperature, high pressure or their fluctuations, the sealing effect of MMC bolted flange joints is better than of floating (FLT) bolted flange joints. According to the structure characteristics of MMC bolted flange joints, a new tightening method (SH-Method) was recommended in this work. The bolt forces during the tightening process of a MMC bolted flange joint with SH-Method were calculated and analyzed with the finite element analysis Software ANSYS. The calculating model and results were experimentally verified. Both calculating and experimentally results showed, the new tightening method ‘SH-Method’has the advantages of fewer steps, simpler operation, more uniform bolt force, and better sealing effect, compared to the star pattern and the alternative pattern #3 of ASME PCC-1, in which only the pattern methods for FLT flange joints are recommended.

Metal to Metal contact (MMC) bolted flange joint, which appears between gasket centering ring and flange facing, is widely used in nuclear and petrochemical industry for severe conditions. Compared to the floating type joints, the gasket outer metal ring is compressed after bolt-up and the stored energy can compensate the loss of residual gasket stress due to internal pressure, bending moment and creep, resulting in a low leakage. As it is difficult to assess the flanged joint integrity affected by creep analytically, a three-dimensional nonlinear finite element model is developed to highlight the creep behaviour of MMC flanged joint under different load cases at elevated temperature. The results show that MMC flanged joint can reduce the unloading effects due to creep and a suitable preload is recommended.

The study on temper embrittlement and hydrogen embrittlement of a test block from a 3Cr1Mo1/4V hydrogenation reactor after ten years of service was carried out by small punch test (SPT) at different temperatures. The SPT fracture energy E-sp (derived from integrating the load-displacement curve) divided by the maximum load (F-m) of SPT was used to fit the E-sp/F-m versus-temperature curve to determine the energy transition temperature (T-sp) which corresponded to the ductile-brittle transition temperature of the Charpy impact test. The results indicated that the ratio of E-sp/F-m could better represent the energy of transition in SPT compared with E-sp. The ductile-to-brittle transition temperature of the four different types of materials was measured using the hydrogen charging test by SPT. These four types of materials included the base metal and the weld metal in the as-received state, and the base metal and the weld metal in the de-embrittled state. The results showed that there was a degree of temper embrittlement in the base metal and the weld metal after ten years of service at 390 degrees C. The specimens became slightly more brittle but this was not obvious after hydrogen charging. Because the toughness of the material of the hydrogenation reactor was very good, the flat samples of SPT could not characterize the energy transition temperature within the liquid nitrogen temperature. Additionally, there was no synergetic effect of temper embrittlement and hydrogen embrittlement found in 3Cr1Mo1/4V steel. Web of Science 10 6 art. no. 671

Extracting accurate strength properties from small punch test (SPT) results is still a challenge. Previous studies on determining the strength of materials via SPT employed empirical correlations, but these correlations are not generally accurate enough. Here we developed a database method to efficiently evaluate the yield strength and ultimate tensile strength. First, a database containing force-deflection curves of SPT and corresponding strengths of materials is created using finite element simulations coupled with a large amount of hypothetical materials. The database is then used to process the force-deflection curve of an investigated material for estimating strength values. A series of hypothetical materials that are not in the database were adopted to explore the accuracy in determining strengths by this method. The results show the theoretical accuracy values of the yield strength and ultimate tensile strength achieve 92.0% and 99.0%, respectively. The proposed method was also applied to estimate the strengths of Cr-Mo steels. It is found that the obtained strengths agree well with those acquired by uniaxial tensile testing. Using a database to evaluate strength properties is a novel and simple approach that could be widely spread in the study of SPT.

A floating-type joint applies the entire bolt load to the gasket to generate sealing contact stress, and is prone to leak due to the bolt load loss. Different with the floating type one, a metal-to-metal contact type joint applies only part of the bolt load to the gasket to achieve seating stress, and applies the additional bolt load to the metal-to-metal contact to compensate for the unloading effects due to internal and external loadings. Due to this advantage, metal-to-metal contact type joints are gradually used in chemical industries, nuclear power industries, etc. A flange joint together with a metal-to-metal contact type gasket forms the metal-to-metal contact type joint studied in this article. A three-dimensional nonlinear finite element model is developed to highlight the complex behavior of the flange joint under bolting-up, pressure loading and thermal loading conditions. Despite the common perception that gasket stress in a metal-to-metal contact type joint stays constant, reduced gasket stress is concluded due to flange rotation and joint thermal expansion.

In this paper, conventional small punch (SP) tests are carried out to determine the SP ductile-to-brittle transition temperature, T SP . The center holed small punch (CHSP) specimens are tested at T SP to determine the Weibull parameters with two different Weibull stress model. The relationship between temperature and the Weibull stress scale parameter, σ u , is investigated. The result showed that a unique set of Weibull parameters for CHSP specimens and 0.4T SENB specimens is available by using the modified model not the original model. And the Weibull stress scale parameter, σ u , is a power function of temperature.

The miniature specimen test technique has been extensively studied for quantifying the properties of bulk materials with greatly reduced volume recently. In this paper small punch test (SPT) is used to evaluate effects of specimen thickness, grain size and thickness-to-grain size ratio (TGR) on mechanical properties of 25 samples of type 316L austenitic stainless steel with nearly the same crystallographic texture. Effective SPT yield and maximum loads were measured and correlated with yield and ultimate strengths of conventional tensile test. The results show that SPT is sensitive not only to grain size, but also to thickness and TGR. Size effects exist between SPT and standard-sized samples. The present work gives a size effect criterion of SPT for engineering application. In addition, the underlying mechanisms of these size effects are provided.

Despite the fact that spring energized C-rings are widely used in nuclear power plants for flange seal, few studies are carried out to highlight the complex mechanical behaviors of the C-ring. A three dimensional nonlinear finite element analysis is developed to examine the compression recovery performance of the spring energized C-ring. Through the analysis, it is found that the compression recovery curve of the spring energized C-ring can be simplified to three straight lines. Two dimensional parameters, including the inner lining thickness and spring wire diameter, are studied to determine their impacts on the spring energized C-ring. Relations between the compression recovery curve and spring wire diameter are concluded and expressed by specific equations. Moreover, the deformation and contact stress distribution of the C-ring are also studied. The validation of the finite element model is confirmed by the experimental method.

A small punch test (SPT) was carried out to evaluate the stress corrosion cracking (SCC) susceptibility of nanocrystallized stainless steel (SS) 304L welded joint in 1 mol/L NaCl + 0.5 mol/L HCl aq. The surface mechanical attrition treatment (SMAT) was applied to realize the surface naocrystallization (SNC). Mechanical properties and micro-structural evolutions of S nanocrystallized SS 304L welded joint were investigated through optical microscopy (OM), X-ray diffraction (XRD), micro-Vickers hardness test and transmission electron microscopy (TEM). The SPT was conducted in both ambient air and corrosive solution at the loading rate 3 × 10 −3 mm/min, and SNC phenomena were observed at the welded seam (WS) and heat affected zone (HAZ) after SMAT. The results show that for both HAZ and WS materials, specimens with 30 min SMAT have higher hardness, higher yield strength, and lower ductility than the as-received counterparts in ambient air. After SPT in the corrosive solution, the as-received HAZ specimen showed a mixed type of both the intergranular SCC (IGSCC) and transgranular cleavage fracture; The SMAT sample shows a typical intergranular stress corrosion cracking (IGSCC) mode with a higher SCC susceptibility. Neither the as-received nor the SMAT WS specimen showed significant SCC susceptibility, they had the similar fractures with their counterparts in ambient air.

The fracture toughness obtained by pre-notched (pre-cracked) small punch specimens is much higher than that obtained by conventional single edge notch bending (SENB) or by compact tension specimens. In this paper, an equivalent Weibull stress method has been introduced. The tests with 0.4 T SENB specimens and linear notched small punch specimens were carried out at 77 K. At this temperature (on lower shelf), the results of both test methods show scatter but not significant. The loads at fracture of both types of specimen were used to determine the Weibull stress parameter, m, and the Weibull stress, σw, with finite element method. Base on the assumption that Weibull stress parameters are material constants, the m which could meet a condition that σ w 0.4 TSENB = σ w LNSP was regarded as Weibull stress parameter, m. The critical J integral of 1 T SENB J c ITSENB at 77 K has been estimated with m and σw deduced by 0.4 T SENB and LNSP specimens.

The interplay of the mechanical and electrochemical phenomena has been a subject of active research. In this paper, corrosion resistance studies about SAF2205 and SAF2507 duplex stainless steel were carried out under elastic stress applied (100 MPa, 300 MPa, 500 MPa) and pre-strain (5%, 10%, 15%) in 3.5% NaCl and 2 mol/L HCl solution. Potentiodynamic anodic polarization study revealed that corrosion resistance of SAF2205 duplex stainless steel decreases slightly with increasing of elastic stress level and noticeably with increasing of pre-strained level. Scanning electron microscopy investigation on surface of the electrochemical tested SAF2205 duplex stainless steel samples indicated that pitting is always located in austenite grains when pre-strain level is below 5% (including different elastic stress level) and located on intersection of ferrite and austenite grain when pre-strain level is above 5%. For SAF2507 duplex stainless steel, elastic stress and pre-strain have no effect on general corrosion and pitting corrosion. Based on deformation mechanism of duplex structure and the relationship of mechanical load and corrosion potential, Pitting corrosion behavior of duplex stainless steel is explained and discussed.

In this study, the small punch test (SPT) was conducted to evaluate the stress corrosion cracking (SCC) susceptibility of stainless steel (SS) 304L with surface nanocrystallization (SNC) in 1 mol/L NaCl+0.5 mol/L HCl aq. The surface mechanical attrition treatment (SMAT) was applied to realize the SNC. The mechanical property and micro-structural evolutions of SS 304L induced by SMAT were investigated through optical microscope (OM), X-ray diffraction (XRD), micro-Vickers hardness and transmission electron microscopy (TEM). The grain size on the surface of the material was reduced to 30–100 nm. The SPT was conducted in both ambient air and corrosive solution. The results were investigated by OM and scanning electron microscopy (SEM), showing that in ambient air, the specimen with 30 min SMAT performed a higher yield strength and lower ductility than the solution annealed (SA) counterpart. The SS 304L without SMAT presented a transgranular SCC (TGSCC) mode in chloride solution. In contrast, the SNC 304L SS showed a higher SCC susceptibility with a typical intergranular SCC (IGSCC).

The Gurson-Tveergard-Needleman (GTN) model has been widely used to describe ductile fracture. In this paper, a series of tensile tests were carried out on notched specimens to assess the GTN model. The GTN model parameters were calibrated from a smooth tensile specimen by a hybrid particle swarm optimization, and the reliability of the calibrated parameters was verified by the profile of the smooth tensile specimen. The calibrated parameters were used to predict the ductile fracture of notched specimens. A comparison of fracture initiation sites between simulations and experiments indicates that the GTN model has a good performance on predicting fracture initiation site but fails at predicting fracture moment. The assessment of the transformability of the GTN model parameters was performed by comparing the load-displacement curves between simulations and experiments. It is observed that the GTN model parameters are material constant, except the critical void volume fraction fc. The influence of stress triaxiality on the critical void volume fraction fc is also discussed.

In the recent years, small punch testing (SPT) techniques has made great progress in China. The SPT was studied to estimate the tensile properties, the fracture toughness and ductile-to brittle transition temperature, and the creep behavior. In 2012, a standard of small punch testing was issued in China and the application of SPT in power generation and petrochemical industry has become a prime candidate. The present paper concentrates on progress of technique and standardization and industrial acceptance in assessing the structure integrity in China. China has carried out close cooperation with Material & Metallurgical research Ltd in Czech to compare Standards between China and EU code. The size of specimen and jig was researched and compared each other. The influence of jig and test machine was researched and improved the specific requirement of jig and test machine. The evolution of stress state of deformation process of specimen in SPT was clarified. The results showed that at initial stage the elastic bending stress is predominant and then the stress state dominated by membrane stress with the decreasing of elastic bending stress and the increasing of punch displacement. The reason for introducing the specimen thickness h2 to the equation for correlating yield load of SPT with yield strength, and the reason for introducing specimen thickness h to equation for correlating the maximum load of SPT with tensile strength were provided respectively. The correlation equation of ductile to brittle transition temperature and SPT energy transition temperature TSP was established and it was successfully used to evaluate embrittlement of hydrogenation reactor. Small punch creep testing by reverse finite element simulation was carried out and used to evaluate the creep life in power generation industry. Fracture toughness and Master curve using SPT by reverse finite element simulation combined with local approach was studied.

Conventional tensile test (CTT) and small punch test (SPT) was carried out on a failed ASTM A350 forging flange to study specimen size effect on mechanical behavior. Specimens were machined from the failed flange along Circumferential and longitudinal directions to study the effect of anisotropy on mechanical properties. Our results show that an optimum sampling has to be used for correlation between SPT and conventional tensile test (CTT). Fractography studies carried out on the failed samples indicate that the fracture modes of SPT samples and the CTT samples were different with the former showing better toughness than the latter. This is attributed to smaller size of the specimen and constraints imposed by the experimental setup. Due to the anisotropy of the forged material, the sampling plane in the SPT should be perpendicular to the load direction for reliable correlation between the results of both methods of testing.

The occurrence of cracks at the electron beam (EB) welds of ethylene tubes in radiant chambers after 1–2 years in service has led to serious concerns. Failure analysis of the sample using optical microscopy, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) clearly reveals a centerline with dendritic crystals and blocky precipitates present at the welds, where the cracks occurred. Niobium-depletion at the EB weld centerline is mainly responsible for the carbide precipitates and cracks. Operation processes such as frequent start–stop cycling accelerated the cracks.

Small punch test (SPT) is a miniature sample test technique which can evaluate in-service material properties with an almost non-destructive method. In this paper, the 2.25Cr1Mo steel samples serviced for 10 years in hydrogenation reactor (with temper embrittlement), 1.25Cr0.5Mo supper-pressure vapor pipe serviced for 14 years at 520 °C and several other low alloy steels have been studied by JIC fracture toughness and SPT. The linear relationship between the small punch (SP) equivalent fracture strain and the fracture toughness of JIC was created. The correlations applied to the experimental data indicated advantages of using SPT for the determining fracture toughness of in-serviced low alloy steels. Additionally, size affects the fracture pattern. Small punch samples of small size show dimple fractures whereas large fracture toughness samples show quasi-cleavage fractures.

Miniature and standard specimens, were cut from the anisotropy materials with axial, central and radial directions to study the mechanical property. In the paper, main research focused on the small punch test (SPT) with un-notched and U-shaped notched specimen in low temperature. Through the small punch energy variation with temperature, the ductile-brittle transition temperature by the small punch test (T SP ) can be determined. The results indicated that there was no obvious difference among three different directions in transformation temperature of SPT with un-notch specimens, and it cannot represent upper plateau impact energy of three different directions. And the SPT with U-shaped notched specimens can determine the differences of upper plateau fracture energy of three different directions. Therefore, SPT with U-shaped notch specimens was more useful to evaluate the material anisotropy.

A detailed knowledge of changes in microstructures and mechanical behaviour that occur in austenitic stainless steels with or without Nb/Ti-stabilized weld during heat treatment is of great interest, since the ductility and toughness of the material may change drastically after long aging times. Two kinds of materials, i.e. AISI 321 base and without Ti-stabilized weld steel and AISI 347 base with Nb-stabilized weld steel, were compared during aging at 700 °C up to 6000 h. Both materials present increased amount of precipitate and decreased impact energy as the aging time increases. The decreased extent of impact energy with aging is almost the same for both base materials. However, it presents differences for 347 and 321 weld samples. The latter shows a more drastic decrease of impact energy than the former due to the different amount of precipitates. 321 weld sample precipitates more numerously than 347 weld sample due to the absence of stabilized Ti/Nb on the former. Large amount of carbides is formed on 321 weld sample immediately after welding. The carbides are transformed to sigma phase, which is mainly responsible for the much more sigma phase precipitation compared with other samples, after high-temperature aging. The fractographs showed, in general, brittle fracture mode in 321 weld impact-fractured specimens after aging at 700 °C for 6000 h. However, other samples show ductile fracture mode in general. Several approaches should be employed to control sigma phase precipitation in weld material. These approaches include: decreasing content of ferrite and M 23 C 6 carbide in weld and selecting Nb added weld wire during welding.