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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.

Sex differentiation and hormones are essential for the development of sexual signals in animals, and the regulation of sexual signals involves complex gene networks. However, it is unknown whether a core gene is able to connect the upstream regulators for controlling sexual signal outputs and behavioural consequences. Here, we identify a single gene that integrates both sex differentiation and hormone signalling with sexual attractiveness in an insect model. CYP4PC1 in the German cockroach, Blattella germanica, controls the rate-limiting step in producing female-specific contact sex pheromone (CSP) that stimulates male courtship. As revealed by behavioural, biochemical, molecular, genetic and bioinformatic approaches, in sexually mature females, CYP4PC1 expression and CSP production are coordinately induced by sex differentiation genes and juvenile hormone (JH) signalling. In adult males, direct inhibition of CYP4PC1 expression by doublesex

The spherical indentation test has been successfully applied to inversely derive the tensile properties of small regions in a non-destructive way. Current inverse methods mainly rely on extensive iterative calculations, which yield a considerable computational costs. In this paper, a database method is proposed to determine tensile flow properties from a single indentation force-depth curves to avoid iterative simulations. Firstly, a database that contain numerous indentation force-depth curves is established by inputting varied Ludwic material parameters into the indentation finite elements model. Secondly, for a given experimental indentation curve, a mean square error (MSE) is designated to evaluate the deviation between the experimental curve and each curve in the database. Finally, the true stresses at a series of plastic strain can be acquired by analyzing these deviations. To validate this new method, three different steels, i.e. A508, 2.25Cr1Mo and 316L are selected. Both simulated indentation curves and experimental indentation curves are used as inputs of the database to inversely acquire the flow properties. The result indicates that the proposed approach provides impressive accuracy when simulated indentation curves are used, but is less accurate when experimental curves are used. This new method can derive tensile properties in a much higher efficiency compared with traditional inverse method and are therefore more adaptive to engineering application.

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

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.

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.

Hydrogenation reactor services as key equipment in chemical and energy industries. Manufacturing processes of hydrogenation reactor changes its performance before long-term service but impact of manufacturing residual influence remains unclear. In this work, actual material strength distribution (MSD) in hydrogenation reactor shell was investigated. First, a hydrogenation reactor shell made from 2.25Cr1Mo0.25V was dissected to measure MSD in thickness. Then, a numerical model was proposed to predict actual material strength in hydrogenation reactor shell. The model employs both data-driven and finite element techniques to simulate material evolution during manufacturing. Third, the predict results were discussed with respect to accuracy based on experiment result. Results exhibit good agreement between predicted value and experiment outcomes. At last, impact of manufacturing residual influence on load capacity of hydrogenation reactor shell was investigated. Results indicate that fit for service (FFS) evaluation of hydrogenation reactor based on heat treatment material properties is not conservative. This work will contribute to the accurate description of hydrogenation reactor's performance.

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.

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.

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.

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 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.

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.

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.

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.

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.

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.