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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.

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.

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.

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 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.

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.

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.