Small Punch Test Research Articles

DBTT shift of Optifer-IX, Eurofer 97 and MA956 steels after irradiation evaluated with small punch tests

In order to analyze the irradiation-induced changes of ductile-to-brittle transition temperature (DBTT) of ferritic/martensitic (FM) steels, small punch (SP) tests were conducted on the specimens of Optifer-IX, Eurofer 97 and MA956 steels after irradiation in a target of the Swiss spallation neutron source (SINQ) at doses between 8.5 and 18.8 dpa and temperatures between 165 and 365°C. The SP test results demonstrate that the DBTT of the three steels increases with irradiation dose, which is in agreement with the previous results of SP and Charpy impact tests on FM steels irradiated at SINQ. Both SP and SEM observation results indicate that, among the three steels, Eurofer 97 possesses the highest resistance to irradiation-induced embrittlement. On the other hand, the ferritic ODS steel MA956 exhibits the poorest radiation resistance, manifested by the higher DBTT in both unirradiated and irradiated conditions and higher DBTT shift after irradiation as compared to Optifer-IX and Eurofer 97 steels.

Deviations in yield and ultimate tensile strength estimation with the Small Punch Test: Numerical analysis of pre-straining and Bauschinger effect influence

There is a growing interest on the Small Punch Test (SPT) and its many applications in a broad range of industries and sectors. The miniature volume of the SPT specimen eases the mechanical characterization of components and structures when standard tests are impracticable. But this miniature test is limited for one of its more constraining requirements: the material isotropy. Since this test subjects the material to triaxial stress and strain fields, isotropy is necessary to compare the resulted data from SPT with the standard tests, which generally show uniaxial stress fields. Another key element of this question is that initially, isotropic material can lose this property due to a cold pre-straining process and the existence of the Bauschinger effect. In this investigation, a numerical study with the finite element method was performed to understand the influence of pre-straining and the Bauschinger effect in yield strength and ultimate tensile strength estimation with the SPT. This study concludes that in the absence of isotropy, generated by a pre-straining, the SPT estimated a mean value of the principal yield strength components of the yield surface. It is also verified that presence of pre-straining in the SPT specimens invalidated their use to calculate the coefficients of the correlation equations for yield strength estimation.

Correlation of Measured Load-Displacement Curves in Small Punch Tests with Tensile Stress-Strain Curves

The Small Punch Test (SPT) has demonstrated tremendous potential for high throughput evaluation of mechanical response of metal sheets. However, its application has thus far been limited to the estimation of plastic material properties such as yield strength and ultimate tensile strength. This paper demonstrates new analysis protocols for extracting the uniaxial stress-strain curves from the SPT measured load-displacement curves. This is accomplished by first establishing a correlation between the SPT measurements and corresponding finite element (FE) simulations that take as input the uniaxial stress-strain response of the material. After establishing this correlation, it is shown that the correlation can be inverted to estimate the uniaxial stress-strain response from the SPT load-displacement measurement on a new material sample.

Threshold stress estimation in hydrogen induced cracking by Small Punch tests based on the application of the incremental step loading technique

The small punch test consists on punching a plane small specimen until it breaks. This technique, born in the 80’s, should be considered when evaluating mechanical properties in situations where materials are in shortage. In recent works, it has been used to estimate the mechanical properties of steels in aggressive environments, where characterizations usually consist on the determination of the threshold stress to avoid subcritical cracking by means of constant loading tests, which is a slow technique, and sometimes presents a considerable dispersion in the results. The standard ASTM F1624 solves these problems, by applying constant loads gradually increased, called loading steps, until the sample fails.In the present work, it is proposed to apply the incremental step loading technique from ASTM F1624 adapted to the Small Punch Test (SPT). As a novel approach, modifications on the steps durations for SPT are proposed according with the sample thickness, allowing to obtain the threshold stress in aggressive environments within a few days, by using at least 3 samples. The proposed methodology is applied to a set of two steels, of medium and high-strength, in hydrogen embrittlement environments under three different levels of cathodic polarization in an acid electrolyte. As a reference, cylindrical tensile specimens were subjected to conventional standard tests in accordance with ASTM F1624.The correlation between the threshold stresses, obtained according to ASTM F1624, and the threshold loads, obtained by the Small Punch proposal, is presented and analyzed. Finally, from the aforementioned correlation, a threshold stress estimation based just on Small Punch tests is proposed.

SPT analysis of hydrogen embrittlement in CrMoV welds

In this paper Small Punch Tests (SPT) are used to determine the mechanical properties of the heat affected zone (HAZ) of CrMoV welded joints and to evaluate the embrittlement caused by the presence of internal hydrogen. Hydrogen permeation tests were carried out to determine the effective diffusion coefficients of the different regions of welded joints, whose hydrogen contents were measured using a hydrogen analyser.The paper demonstrates that SPTs can be used to estimate the tensile properties and the toughness of small areas, such as the subzones of a welded joint which are not thick enough for machining standard specimens. In cases where it is not possible to determine the fracture toughness of brittle regions using standard SPT samples, tests using longitudinally notched SPT samples were conducted. The normalized energy at failure, W/t2, was selected as the most appropriate SPT parameter to estimate the fracture toughness regardless of the fracture behaviour of the material (ductile or brittle). It was seen that the fracture toughness of the fine grained HAZ (FG-HAZ) is similar to that of the Base Metal (BM), while the fracture toughness of the coarse grained HAZ (CG-HAZ) is more similar to that of the Weld Metal (WM). The effect of hydrogen on BM and FG-HAZ is low, with embrittlement indexes of about 20%. However, a strong effect of hydrogen was observed in CG-HAZs and WMs, with embrittlement indexes of 60–80%. and a clear change from ductile to brittle behaviour in their fracture surfaces.

Construction of True Stress-Strain Curve of Metallic Material by Artificial Neural Network and Small Punch Test

Small punch test (SPT) is used to evaluate mechanical properties of metallic materials by a miniature specimen. A method combining SPT and artificial backpropagation neural network for determining the true stress-strain curve of metallic materials is proposed. The load-displacement curves of different hypothetical materials were obtained by the finite element model of SPT with considering Gurson-Tvergaard-Needleman (GTN) damage parameters and used to train a backpropagation neural network. The relationship between the load-displacement curve of SPT and the true stress-strain curve of the conventional uniaxial tensile test was established based on the trained neural network, which is validated by the experimental results of X80 pipeline steel. The results demonstrate that the established relationship can be used to predict the true stress-strain curve of the metallic materials and then to determine their elastoplastic properties by SPT.

The Effect of Microstructure and Welding-Induced Plasticity on the Strength of Ni–Mo–Cr Alloy Welds

The mechanical performance of a Ni–Mo–Cr (GH3535) alloy weldment, produced using a matching filler metal, was assessed and compared to the surrounding parent metal. Ambient-temperature mechanical characterisation included hardness testing, small punch testing and uniaxial tensile testing, while a crystal plasticity finite element model was used to assess the impact of crystallographic texture on the mechanical properties. Despite the similar chemical composition, the weld metal exhibited superior strength and ductility to that of the parent metal. The higher strength was primarily attributed to the high dislocation density in the weld metal imbued by the welding-induced thermo-mechanical loading. In contrast, the ductility difference was attributed to M6C carbide stringers in the parent metal that initiated fracture at lower strains than for the weld metal, with the latter containing much finer, well-dispersed M6C carbides.

Effect of TIG remelting on microstructure, corrosion and wear resistance of coating on surface of 4Cr5MoSiV1 (AISI H13)

This study aims at surface modification of the 4Cr5MoSiV1 (AISI H13) steel through the TIG (tungsten inert gas) remelting process. TIG remelting process was conducted using the major parameters, i.e. 250 A welding current and 120 mm/min welding speed (3.0 mm electrode diameter), under the protection of Argon. After remelting, X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) were used to analyze phases, microstructure and element distribution, respectively. After TIG remelting on the surface of 4Cr5MoSiV1 steel, the phase transition from body-centred cubic (BCC) to face-centred cubic (FCC) has occurred, and a great amount of Cr3C2 segregated along the grain boundaries, which resulted in eutectic structures. The results illustrated that the TIG remelting layer has better corrosion resistance than the original steel, which is related to the grain boundaries with Cr3C2, and result in wider passivation. Further, the microhardness of the TIG remelting layer has been increased to a great extent, which has leaded to lower friction coefficient and better wear resistance. And then, the small punch tests exhibit the morphology of intergranular fracture at the remelting layer, and this situation testified the role of grain boundaries in this study. To summarize, in the study, including the phase transition, precipitation, and change of grain boundary, have improved the performance of the TIG remelt-ed 4Cr5MoSiV1 greatly, which can develop its innovative applications.

Development of New 14 Cr ODS Steels by Using New Oxides Formers and B as an Inhibitor of the Grain Growth

In this work, new oxide dispersion strengthened (ODS) ferritic steels have been produced by powder metallurgy using an alternative processing route and characterized afterwards by comparing them with a base ODS steel with Y2O3 and Ti additions. Different alloying elements like boron (B), which is known as an inhibitor of grain growth obtained by pinning grain boundaries, and complex oxide compounds (Y-Ti-Zr-O) have been introduced to the 14Cr prealloyed powder by using mechanical alloying (MA) and were further consolidated by spark employing plasma sintering (SPS). Techniques such as x-ray diffraction (XRD), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM) were used to study the obtained microstructures. Micro-tensile tests and microhardness measurements were carried out at room temperature to analyze the mechanical properties of the differently developed microstructures, which was considered to result in a better strength in the ODS steels containing the complex oxide Y-Ti-Zr-O. In addition, small punch (SP) tests were performed to evaluate the response of the material under high temperatures conditions, under which promising mechanical properties were attained by the materials containing Y-Ti-Zr-O (14Al-X-ODS and 14Al-X-ODS-B) in comparison with the other commercial steel, GETMAT. The differences in mechanical strength can be attributed to the precipitate’s density, nature, size, and to the density of dislocations in each ODS steel.

An application of FEM in the determination of tensile properties for work-hardened carbon steel by means of small punch test

The determination of tensile properties from a small punch (SP) test by using a finite element method (FEM) has been studied. First, a finite element (FE) simulation of the SP test using true stress-strain curves including post-necking strain data of work-hardened SM490A carbon steel specimens was conducted. The simulation was validated with actual experimental results, which showed that the FE model can effectively reproduce the load-displacement curves without using a damage modeling technique. The FE model was then used to investigate an empirical correlation between the punch loads and the material parameters, including yield strength and ultimate tensile strength. The results indicated that the empirical constants are dependent on specimen thickness and may be valid for only a limited range of test configurations and materials. To expand the applicability of the SP test in the assessment of tensile properties, an inverse finite element method (iFEM) consisting of the iterative finite element calculation, bisection method, and Ludwik’s constitutive model was developed. Through the procedure, true stress-strain curves were constructed from the simulated load-displacement curves of hypothetical materials based on the SM490A steel specimens. Finally, yield strengths and ultimate tensile strengths of the carbon steel specimens were determined and found to be within 5.2% and 2.8% discrepancies, respectively, from the uniaxial tensile test results. iFEM provides consistent estimations of tensile properties with almost no influence of specimen thickness.

Influence of the material anisotropy in the estimation of the yield strength with the Small Punch Test

The Small Punch Test (SPT) was developed in the 80’s as an alternative miniature test for the characterization of mechanical properties in the nuclear industry. One of the key aspects that materials must fulfill to be used with this miniature test is that of homogeneity and isotropy. The origin of the isotropy requirement comes from the fact that the estimation of mechanical properties using the SPT requires empirical correlations with standard tests, which generally show uniaxial stress fields. By contrast, the SPT shows a multiaxial stress field. There are few publications related with the influence of material anisotropy in the yield strength estimation using the SPT, and most of them are empirical studies. This research was intended to address, with a systematic finite element analysis, the influence that different anisotropy combinations could show in the yield strength estimation using the SPT. Thirty-six anisotropic hypothetical materials were evaluated with SPT simulations using the Hill’48 yield criterion. The yield strength of each material was estimated with the SPT using four correlation methods: Mao’s, CEN’s, the t/10 offset, and the optimized t/10. This study concluded that the SPT was not an appropriate test to evaluate or quantify the material anisotropy, but it was a valuable experimental test to estimate a mean yield strength of the six yielding stress components of the anisotropic material.

Understanding the Role of the Constituting Elements of the AlCoCrFeNi High Entropy Alloy through the Investigation of Quaternary Alloys

Quinary AlCoCrFeNi high entropy alloy (HEA) is one of the most studied alloys in the recent decade due to its outstanding properties. However, it is still far from becoming an applicable industrial alloy. To our understanding, in order to promote this, the role of elements, constituting the quinary alloy, needs to be defined. Knowing the role of each element, modification of the quinary alloy toward minimization of its disadvantages will be possible. In the current research, we shed some light on this subject, presenting a thorough investigation of the microstructure (carried out using scanning and transmission electron microscopy) and mechanical properties, performed by microhardness and fractography post small punch test (SPT), of five equiatomic quaternary alloys, constituting the quinary system, namely: CoCrFeNi, AlCoFeNi, AlCoCrNi, AlCoCrFe, and AlCrFeNi. CoCrFeNi (i.e., w/o Al) was found to be Face Centered Cubic (FCC) solid solution, exhibiting relatively low micro-hardness and ductile fracture post SPT measurement. AlCoFeNi (i.e., w/o Cr) was essentially single phase B2. Other alloys had a mixed BCC + B2 dual phase content with variable microstructures and sizes of particles. The fine microstructure of the alloy without Ni implies eutectic solidification or spinodal decomposition. This fine microstructure imposed remarkable high hardness though the alloy was too brittle and unmachinable. Among the BCC/B2 mixture alloys, Fe and Co-less ones resembled the most quinary AlCoCrFeNi in terms of microstructure and mechanical properties.

Creep lifetime prediction of 9Cr-1Mo (grade T91) steel via small punch creep tests and hierarchical multiscale analysis

ABSTRACT In this study, the creep deformation of ferritic/martensitic 9Cr-1Mo (Grade T91) steel boiler tubes that occurs during the operation of thermal power plants is studied using crystal elasto-viscoplasticity finite element analysis (CEV-FEA). The material properties and creep behaviour of Grade T91 were obtained through experimentation and computational analysis at different scales, e.g., molecular dynamics simulation, scanning electron microscopy, small punch (SP) tests. A triple junction of grains with various combinations of crystallographic orientations of polycrystalline Grade T91 was also simulated by CEV-FEA to investigate the stress concentration evolution during creep deformation. The stress concentration at the triple junction was then used as an input for the creep cavitation model, i.e., as the representative stress driving creep cavitation. The combination of CEV-FEA and SP test experiments can improve the evaluation of creep lifetime for as-received boiler tubes with minimum requirements for the amount of material and testing time.

Fully plastic J-integral and C* equations for small punch test specimen with a surface crack

Small punch (SP) creep test specimen with an elliptical surface crack was suggested for use with creep crack growth testing. Estimation equations for fracture parameters are needed. K-equation was derived previously for the crack in the SP specimen.In this study, estimation equations for J and C* were derived. 3D elastic-plastic finite element analyses were carried out to determine J value along the front of the surface cracks of the same geometries as were employed for the K-estimation. The dimensionless function, h1 was determined for various crack locations according to the EPRI scheme. The plastic limit load solution was also determined. The C*-equation of the cracked SP specimen was derived based on the Jp solution using the analogy between the secondary creep constitutive law and the Ramberg-Osgood plasticity law. The C*-equation may be able to be used to characterize the creep crack growth behavior under the extensive creep condition of the cracked SP specimen.

Evaluation of Transition Temperature in Reactor Pressure Vessel Steels 6using the Fracture Energy Transition Curve from a Small Punch Test

The small punch (SP) test is one of the small specimen test techniques, and standardization of the SP test method for evaluating the mechanical properties of metallic materials is in progress. In this study, the impact transition temperature of reactor pressure vessel steels (RPV) in nuclear power plants was estimated using the draft standard SP test method. The SP fracture energy (ESP) and normalized SP fracture energy (ENSP) of the RPV steels were evaluated at various temperatures, and their transition curves were derived and compared to the transition curve in the Charpy V notch (CVN) test. The SP transition region appeared at a much lower temperature range than that of the CVN owing to the size and notch effect. Ductile brittle transition temperature (DBTT) in the SP transition curve showed a linear relationship with DBTT and T41J in the CVN transition curve. The ductile to brittle transition behaviors of SP specimens were analyzed using fractographs and compared with the transition curves in ESP and ENSP. ENSP started to decrease at the temperature at which the SP ductile to brittle transition behavior occurred, and this means that the ENSP transition curves were in good agreement with transition behavior in the SP test. However, the ESP transition curves did not match transition behavior. Using DBTT in the ENSP transition curve is appropriate to estimate the CVNDBTT and T41J.

Effects of process parameters on the high temperature strength of 17-4PH stainless steel produced by selective laser melting

In this study, the effects of process parameters on the high temperature strength of 17-4PH stainless steel manufactured by selective laser melting (SLM) were investigated. Nine rectangular block specimens were fabricated with various process parameters. Small punch (SP) tests were conducted at 425 °C four times for each rectangular block specimens. The average maximum SP loads were measured from the tests. The fracture surfaces of SP-tested specimens were also examined. Un-melted powder was observed on the fracture surface of the specimen with the lowest average maximum SP load value, which could give rise to local cracking. Analysis of the microstructures showed that the retained austenite and amount of pore defects were the main factors that could affect SP test results. Regression surface methodology (RSM) models was applied to predict the maximum strength as a function of laser scan speed and energy density. Results showed that the energy density level of 64.29 J/mm3 and a scan speed higher than 1884 mm/s are recommended for fabricating SLM parts in the shortest time without losing material strength and with minimum metallurgical defects.

Brittle-ductile transition temperature of recrystallized tungsten following exposure to fusion relevant cyclic high heat load

The lifetime of tungsten (W) monoblocks under fusion conditions is ambivalent. In this work, the microstructure dependent mechanical behaviour of pulsed high heat flux (HHF) exposed W monoblock is investigated. Two different microstructural states, i.e. initial (deformed) and recrystallized, both machined from HHF exposed monoblocks are tested using tensile and small punch tests. The initial microstructural state reveals a higher fraction of low angle boundaries along with a preferred orientation of crystals. Following HHF exposure, the recrystallized state exhibits weakening of initial texture along with a higher fraction of high angle boundaries. Irrespective of the testing methodology, both the microstructural states display brittle failure for temperatures lower than 400∘C. For higher temperatures (>400∘C), the recrystallized microstructure exhibits more ductile behaviour as compared to the initial state. The observed microstructural state-dependent mechanical behaviour is further discussed in terms of different microstructural features. The estimated brittle-to-ductile transition temperature (BDTT) range is noticed to be lower for the recrystallized state as compared to the initial state. The lower BDTT in the recrystallized state is attributed to the high purity of the W in combination with its low defect density, thereby preventing segregation of impurities at the recrystallized boundaries and the related premature failure. Based on this observation, it is concluded that the common opinion of the aggravation of BDTT in W due to recrystallization is not unerring, and as a matter of fact, recrystallization in W could be instrumental for preventing the self-castellation of the monoblocks.

Equivalent biaxial strain evaluation in small punch testing using acoustic emission

The small punch test initially used mainly in the nuclear sector proved to be an efficient method of estimating the mechanical properties of materials, including fracture toughness. In general, the fracture toughness is estimated based on a correlation with equivalent biaxial fracture strain. However, the latter depends on knowing the onset of the unstable fracture. In this article, the acoustic emission technique is used within the small punch test for estimating the unstable fracture initiation and determining the corresponding punch displacement from the load–displacement curve. Although, the acoustic emission signal can be slightly altered due to the location of the sensor, the results confirm the potential of the acoustic emission technique for detecting the onset of the unstable cracking and thus accurately estimating the equivalent biaxial fracture strain. The accumulated energy during the small punch test is one of the relevant acoustic emission parameters in estimating the fracture onset since it can be associated with the accumulated strain energy released by the deformed material.

Effect of N addition on microstructure and high temperature mechanical properties of Ti-48Al-2Cr-2Nb (at. %) intermetallic alloy

Microstructures and high temperature mechanical properties of the Ti-48Al-2Cr-2Nb (4822) intermetallic alloy containing low content of N (up to 2 at. %) were investigated. The alloys were fabricated by vacuum arc re-melting followed by hot isostatic pressing and homogenization treatment. Phase diagram and critical temperature data of the alloys were evaluated by differential thermal analysis and Thermo-Calc™ calculations. Microstructures were characterized by optical microscopy, secondary electron microscopy and X-ray diffraction. High temperature (800 ℃) small punch testing (SPT) was used to investigate mechanical properties of the homogenized samples. Results showed that alpha transus transformation temperature increased by N addition. A fully lamellar microstructure with nitride precipitates was formed in the furnace cooled (FC) microstructures, while a complex microstructure of massive, feathery, and widmanstätten gamma and lamellar structure with some nitride precipitates was achieved in the air cooled (AC) alloys. Increasing N resulted in a finer lamellar structure in the FC alloys and more lamellar structure in the AC ones. The SPT results of the FC alloys revealed that the maximum force and mean displacement of 4822 alloy decreased by N addition. However, despite different N content and various microstructures of the N-bearing alloys, their SPT curves were approximately similar. Precipitation of the coarse, blocky primary Ti2AlN precipitates at colonies boundaries and the fine, elongated secondary Ti2AlN precipitates at the α/γ lamella interface were suggested as the main cause for premature failure of the N-bearing alloys. The 4822 alloy containing 1 at. % N showed more uniform deformation behavior during SPT. Inter-lamellar and trans-lamellar fracture and delamination of lamellas were found the most involved fracture mechanisms in the N-containing alloys.

Characterisation of deformation process and fracture mechanisms of P91 steel at 600 °C in small punch tensile testing

This paper investigates the microstructural evolution and fracture mechanism of the P91 steel during small punch tensile tests. Disc specimens, 8 mm in diameter and 0.5 mm in thickness, were tested in a small punch test rig at 600 °C using a constant displacement rate of 2 μm/s. Interrupted small punch tensile tests were performed to investigate the microstructural evolution in different deformation regimes. Deformed specimens were characterised by scanning electron microscopy and electron backscatter diffraction. Microstructure characterisation showed that block boundary alignment occurred in the plastic deformation direction, before achieving the maximum punch load. It was revealed that the martensitic structure was recovered when the tests progressed. The elongation of recovered grains occurred after achieving the maximum punch load. In addition, it was shown that the voids tended to nucleate along the recovered grain boundaries and became elongated to align with the plastic flow within the specimen.