Charpy Specimens Research Articles

Near-surface microstructures convolute mechanical properties in additively manufactured metals

Kovar specimens were additively manufactured with 180 variations in process conditions. Three distinct geometries (two tensile geometries and a Charpy specimen) were evaluated for each set of process conditions. Tensile specimens additively manufactured to net shape had less porosity, more uniform material properties, higher average ductility, and they consistently failed via ductile rupture. Tensile specimens harvested via electric discharge machining from larger additively manufactured blocks often contained lack of fusion voids throughout the cross section - those pre-existing pores drove pre-mature failure. As-printed specimens, on the other hand, were more representative of the outer border properties rather than the interior of large printed parts. The properties of the specimens cut from the larger block of additively manufactured material were more representative of the inner hatch properties but were stochastic, depending on the size and location of present voids. Using a high-throughput methodology, the results from over 800 tensile tests are reported here. This extensive statistical sampling allows the effects of specimen type and location to be clearly distinguished from other intentional variables (process parameter variations) and stochastic material variability.

Determination of metal crack resistance on half-size Charpy specimens at cryogenic temperatures

The work consists in the manufacture of experimental equipment for fracture toughness tests at cryogenic temperatures in the range from -196 °Cto -80 °Cand the analysis of the efficiency of determining the reference temperature of a metal by the maker curve for static crack resistance in accordance with ASTM E 1921 and ASTM E399. Liquid nitrogen and its vapours were used to cool the samples to the required temperature. The temperature stability during sample loading was ensured within ±0.5°C around the required temperature. Pre-cracking of the samples was carried out at room temperature in accordance with ASTM E647. Experimental studies on static crack resistance are carried out for three-point bending using half-size Charpy specimens (55×10×5 mm) for the material - 10GN2MFA steel, which is used as the main metal for the main circulation pipelines of VVER-1000. The reference temperature is determined by processing a set of relevant experimental data.

Modelling the Charpy Impact Ductile-Brittle Transition of a Ship Plate Steel with CAFE Modelling

In the present work, a cellular automata finite element model (CAFE) was developed to model the ductile-brittle transition of a Grade A ship plate steel. Therefore, ductile and brittle cellular automata (CA) arrays of cells were created in the model to integrate material data at microstructural level, along with the ductile and brittle fracture processes. Microstructural data was analysed with Weibull distributions and incorporated in CAFE model using random number generators, along with ductile and brittle fracture parameters. Ductile fracture was modelled with Rousselier damage model; hence damage model parameters were calibrated with experimental data. Brittle fracture was modelled with Beremin model, and four different cleavage particles, found in a Grade A ship plate steel, were incorporated in CAFE model in order to model a competition of particles nucleating microcracks of critical size in the damage regions of Impact Charpy tests and four-point double-notch bend tests performed at low temperature. The mechanical properties the plate steel was measured in the transition region and incorporated in CAFE model, along with ductile-brittle transition rules. The present CAFE model was able to simulate distributions of microcracks in the notch region of four-point double-notch bend models (in the transition region), which correlated with experimental data. CAFE model was also able to simulate microvoids in the notch region of Charpy specimens along with the load-displacement Charpy curve for room test temperature, with very good agreement with experimental data. Once CAFE model was validated at micro and structural level, it was applied to model the typical scatter of impact Charpy energy values in the transition region of Grade A ship plate steel with good agreement with the measured ductile-brittle transition curved of the plate steel. Keywords: cellular automata, finite element modelling, ductile-brittle transition, damage modelling.

Effect of duplex surface treatment on the impact properties of maraging steel produced by laser powder bed fusion

The impact properties of an L-PBF 18Ni-300 maraging steel were investigated after different heat and duplex (nitriding and PVD coating) surface treatments. Impact energy is almost isotropic. Contrary to expectations, impact energy is not decreased by surface treatments, since the effect of surface embrittlement of nitriding is compensated by the increase in the austenite content. An important role is played by the geometry of the notch where the surface hardening is lower than in the flat surface of the Charpy specimens.

Complex hydraulic studies of welded pipes with different visco-plastic characteristics

For the first time, complex hydraulic studies of pipes welded according to three different schemes were carried out, which made it possible to combine pipes of different viscosities, which affects the performance of welding joints. It was found that in the experimental temperature range, the zone of thermal influence of the second welding variant has the greatest resistance to the initiation of fractures under shock loading, and the lowest - the first variant. The static strength and ductility of the welded pipe joints of the second and third pipe welding options are approximately equivalent, and the resistance to the initiation of the thermally affected zone in all options is almost the same and not lower than that of the base metal. The results of the performed experimental studies indicate a weak correlation in the area of small values of impact viscosity with the characteristics of resistance to fracture propagation of welding joints in the conditions of full-scale pipe tests. The characteristics obtained from tests of full-thickness DWTT specimens are closer than the Charpy specimens to the actual performance characteristics of the weld joints in the pipe. In particular, it was established that the values of Az (-150C)=0.75 kJ and Ar (-150C)=0.45 kJ provide satisfactory resistance to the initiation and propagation of fractures in welding joints (at the base metal level). These characteristics correspond to KSU-60 = 0.5 MJ/m2, which is close to the impact toughness requirements for the base metal (0.55 MJ/m2). The use of optimal structural materials, that is, materials with high resistance to hydrogen destruction of both the base metal and zones of welding joints subjected to a thermo-deformation cycle of welding. A significant effect can be achieved by technological measures that will reduce residual welding stresses, as well as significantly reduce stress concentrators due to structural improvement of the shape of the welding unit. Application of such heat treatment regimes that restore the stability of metal with a coarse-grained and defective structure. Use improved pipe steels and welding materials, such as 06G2BAand 08 KhMCHA steels, which are characterized by increased resistance to hydrogen embrittlement and high crack resistance in aggressive environments, for the construction of pipelines of responsible purpose.

Fatigue crack growth simulation by extended finite element method: A review of case studies

AbstractNumerical simulation of fatigue crack growth (FCG) is presented, based on the application of the extended finite element method (XFEM). Couple of case studies are presented, as a review of recent experience of the Belgrade school of XFEM simulations, established at the Faculty of Mechanical Engineering. Case studies comprised Charpy specimen with initial crack at the notch tip, oil rig pipe with an axial surface crack, pressure vessels with a crack at the welded joint with connecting pipe, turbine shaft with a crack at the transition radius, optimization of a wing spar in respect to FCG, the effect of mesh size on fatigue life estimation of a stringer panel, fatigue life estimation of wing‐to‐fuselage connecting lug, geometry effect on fatigue life of orthopedic plates and fatigue life of hip implant. Based on these case studies, XFEM is discussed in respect to its accuracy and efficiency. It is concluded that XFEM is a versatile tool for simulation of FCG, providing an excellent option for precise and reliable fatigue life of a cracked component.

Impact Strain Signal Characteristics of Al and Mg under Instrumented Charpy Test

Impact strain signal is used to examine strain signal patterns under various parameters. Impact is a complicated phenomenon that occurs within a millisecond timeframe. Material toughness is measured by the energy absorption recorded by the Charpy machine and closely related to the specimen fracture deformation. By utilizing the strain gauge and data acquisition, the impact strain signal provides additional data regarding impact duration, maximum strain value and the area under curve for a deeper understanding of the impact problem. A material with high toughness has great energy absorption and the capability to withstand high impact load. Although magnesium is lighter in weight compared to aluminium, aluminium is a better corrosion-resistant material and is stronger, which makes it more suitable to be fabricated as automotive structural components. Tensile test is typically used for investigating a material’s mechanical properties. In the automotive industry, materials are required to have good crashworthiness. This study investigates the relationship between the energy absorbed with the power spectral density and the area under strain–time graph for different materials, impact speeds, and material thicknesses. Furthermore, the relationship between the stress–strain curve and impact strain signal were examined. In this study, the behaviour of two materials, namely Aluminium 6061-T6 and Magnesium AM60, was investigated using instrumented Charpy test, by referring to the impact strain signal pattern result. For the experiment, strain gauge attached to the Charpy machine striker was employed and linked to the data acquisition system. Charpy impact specimen has three different thicknesses; 10 mm, 7.5 mm and 5 mm. Impact speed is at 3.35 m/s and 5.18 m/s. Results show a correlation between energy absorbed with strain energy. Strain energy obtained is directly proportional to the energy absorbed. Aluminium 6061-T6 has the highest energy absorption, maximum strain, and strain energy under power spectral density graph compared to Magnesium AM60. Relation of strain signal from Charpy test and stress–strain curve from tensile test shows a significant finding where the material deforms and fracture points are identified through the signal pattern and curve. Thus, the strain signal pattern can be used to predict material behaviour.

Modelling of crack propagation in miniaturized and normal SENB specimens based on local failure criterion

The use of miniaturized specimen testing methods is a promising way to solve the problem of limited materials in RPV monitoring programs. The use of miniature specimens allows the evaluation of fracture toughness from other specimen materials used. In particular, the small-size compact tensile test specimen (0.16T CT) is promising for the determination of fracture toughness, as it can be produced from the standard size Charpy specimen that has already been tested. However, if we have only 0.16T CT test, we cannot investigate the dimensional response and also have only one restricted deformation state, which may pose problems in verifying geometry independence and determining local parameters for state-of-the-art analyses. It is therefore recommended to prepare at least two tests with two different restricted deformation specimens. Therefore, the testing of mini single edge notched bending (SENB) is also required and can be worked out from the Charpy specimens. The paper presents the determination of fracture toughness for these miniaturized specimens by modifying the virtual crack closure technique (VCCT) simulation method using GTN parameters instead of energy release as the driving force. This allows the calculation of the J-integral to proceed in parallel with the crack propagation.

Influence of the heat treatment on the mechanical and tribological properties of cryogenically treated AISI M2 steel

Cryogenic treatments are known to increase the wear resistance and toughness of steel. However, the literature reports some contradictory results. This work aims to investigate and determine the optimum cryogenic parameters combined with cycles of quenching and tempering of AISI M2 tool steel to increase wear resistance and toughness. Charpy specimens were austenitized (1185 °C, 1200 °C or 1215 °C), quenched to room temperature, cryogenically treated to −196 °C (20 h or 28 h), and subjected to double tempering (520 °C, 535 °C or 550 °C). A Taguchi L9 DoE approach was used to determine the optimum set of heat treatment cycles. Mechanical and tribological properties were investigated in terms of wear, impact, hardness, and microhardness tests. The results show that austenitization temperature (AT), tempering temperature (TT), and cryogenic treatment (CT) time are interdependent. Depending on the AT and TT parameters used, CT can improve, have little influence, or even deteriorate the mechanical and tribological properties of AISI M2 steel. The amount of retained austenite after CT ranged from 4.3 to 7.6%. Cryogenic treatment improves wear resistance and impact toughness simultaneously.

Assessment of Different Approaches for Measuring Shear Fracture Appearance in Charpy Tests

Abstract Different approaches for establishing shear fracture appearance (SFA) on tested Charpy specimens were assessed and compared, namely (in order of increasing expected accuracy) estimates based on characteristic forces from instrumented Charpy tests, optical measurements using image analysis software, and measurements performed with the aid of a scanning electron microscope. Comparisons between different SFA values for 129 instrumented Charpy tests currently included in the National Institute of Standards and Technology database allowed us to establish the relationships between the different available approaches and formulate recommendations for the revision of the standards ASTM E2298-18, Standard Test Method for Instrumented Impact Testing of Metallic Materials, and ISO 14556:2015, Metallic Materials — Charpy V-Notch Pendulum Impact Test — Instrumented Test Method, which currently provide four empirical correlations that can be used to estimate SFA.

Carbon in Solution and the Charpy Impact Performance of Medium Mn Steels

Carbon is a well known austenite stabiliser and can be used to alter the stacking fault energy and stability against martensitic transformation in medium Mn steels, producing a range of deformation mechanisms such as the Transformation Induced Plasticity (TRIP) or combined Twinning and Transformation Induced Plasticity (TWIP + TRIP) effects. However, the effect of C beyond quasi-static tensile behaviour is less well known. Therefore, two medium Mn steels with 0.2 and 0.5 wt pct C were designed to produce similar austenite fractions and stability and therefore tensile behaviour. These were processed to form lamellar and mixed equiaxed + lamellar microstructures. The low C steel had a corrected Charpy impact energy (KV_{10}) of 320 J cm^{-2} compared to 66 J cm^{-2} in the high C steel despite both having a ductility of over 35 pct. Interface segregation, e.g., of tramp elements, was investigated as a potential cause and none was found. Only a small amount of Mn rejection from partitioning was observed at the interface. The fracture surfaces were investigated and the TRIP effect was found to occur more readily in the Low C Charpy specimen. Therefore it is concluded that the use of C to promote TWIP + TRIP behaviour should be avoided in alloy design but the Charpy impact performance can be understood purely in terms of C in solution.

Effect of Temperature on S32750 Duplex Steel Welded Joint Impact Toughness.

The search for alternative materials that can be used for parts of aircraft hydraulic systems has led to the idea of applying S32750 duplex steel for this purpose. This steel is mainly used in the oil and gas, chemical, and food industries. The reasons for this lie in this material's exceptional welding, mechanical, and corrosion resistance properties. In order to verify this material's suitability for aircraft engineering applications, it is necessary to investigate its behaviour at various temperatures since aircrafts operate at a wide range of temperatures. For this reason, the effect of temperatures in the range from +20 °C to -80 °C on impact toughness was investigated in the case of S32750 duplex steel and its welded joints. Testing was performed using an instrumented pendulum to obtain force-time and energy-time diagrams, which allowed for more detailed assessment of the effect of testing temperature on total impact energy and its components of crack initiation energy and crack propagation energy. Testing was performed on standard Charpy specimens extracted from base metal (BM), welded metal (WM), and the heat-affected zone (HAZ). The results of these tests indicated high values of both crack initiation and propagation energies at room temperature for all the zones (BM, WM, and HAZ) and sufficient levels of crack propagation and total impact energies above -50 °C. In addition, fractography was conducted through optical microscopy (OM) and scanning electron microscopy (SEM), indicating ductile vs. cleavage fracture surface areas, which corresponded well with the impact toughness values. The results of this research confirm that the use of S32750 duplex steel in the manufacturing of aircraft hydraulic systems has considerable potential, and future work should confirm this.

Machine learning for the classification of macroscale fracture surfaces

Abstract The characterization of fractographic surfaces typically requires experts to evaluate the characteristics of fracture surfaces. However, these evaluations are influenced by human factors, such as subjectivity, and suffer from a lack of reproducibility. In this context, machine learning (ML), which has been established in various disciplines within materials science over the past few years, is a promising field enabling a more objective and reproducible evaluation. This study will evaluate the use of ML for the evaluation of fracture surfaces of notched Charpy specimens based on digital camera images. Image sections of the two reference regions “upper shelf” (ductile) and “lower shelf” (brittle) will serve as the database. In a first step, data visualization will be performed and data separability will be verified using unsupervised ML. On this basis, supervised ML will be used to train models to distinguish brittle and ductile fractures. These models will then be applied to determine ductile und brittle portions in mixed fracture modes, with the results being in good agreement with the expert consensus achieved in the round robin test.

Temperature-dependent deformation and fracture properties of low-carbon martensitic steel in different stress states

The temperature-dependent ductile-brittle transition (DBT) in low-carbon martensitic steel is investigated in different stress states. To capture the effects of the stress state, tensile tests have been performed with flat specimens and three-point bending tests have been conducted with Charpy specimens at dynamic (impact) and quasi-static loading rates, covering a broad temperature range of 77 K–423 K. The results reveal that ductile-brittle transition temperature (DBTT) is significantly affected by stress triaxiality, wherein the transition temperature in uniaxial tension is ∼142 K lower than that in plane strain condition of Charpy specimens. The energy absorption capacity in the ductile resistance domain during three-point bending tests is sensitive to the loading rate. Moreover, dynamic strain aging poses a decrease in ductile resistance upon quasi-static loading when the temperature exceeds 323 K and thus without a specific upper shelf plateau. Tensile strength and ductility tend to increase with decreasing temperature, and the associated failure mode changes from shear-dominated to nearly pure tension due to the specific discrepancy of temperature influence on the critical normal and shear strengths, wherein the formation of shear bands that dominate the tensile deformation could be strictly inhibited by the formation of nanoscale dislocation cells based on dislocation analysis. The failure mechanisms in tensile tests correlate with the tensile specimens' macroscopic fracture angle. In the end, a new local parameter is proposed for temperature-dependent toughness estimation in tensile tests.

Simple inverse FEM based procedure of flow curve extraction applied to unirradiated and irradiated 73 W weld material

The H2020 FRACTESUS project is aimed to validate miniaturized compact tension specimens usage for fracture toughness characterization of irradiated materials. One of the tasks in the project is devoted to the numerical simulations of the 0.16T and 1T-CT specimens and comparison of their stress/strain fields in. For this purpose accurate material flow curves are necessary to derive comparable mechanical response between the numerical models and experiments. This paper presents a simple technique of flow curve extraction from the tensile test using engineering stress–strain data. The proposed method utilizes a coarse meshed Finite Element Model and a manual flow curve data optimization procedure without additional scripting. The robustness of the method was shown on the example of the 73 W weld material in the unirradiated and irradiated states, tested at various temperatures from −100 °C to 125 °C. The obtained numerical flow curves are in a very good agreement with the experimental results. Moreover, the advantages of the proposed method were shown over either the methods based on extrapolation of the predictions of popular models with parameters fitted to prior necking or the method based on fracture diameter measurements. The final validation of the proposed methodology was demonstrated by numerical simulations of fracture toughness tests on precracked Charpy specimens.

Manufacturing and metrological characterization of reference charpy specimens: first evaluations

The first evaluations of the manufacture and metrological characterization of reference specimens for indirect calibration of Charpy impact testing machines by the Institute for Technological Research (IPT) are presented, seeking through the laboratory resources and expertise of the Institute, the accreditation of the laboratory as a producer of this reference material. The specimens are manufactured with 300 M steel by conventional machining. Heat treatments of normalizing, quenching, and tempering were used to control the mechanical properties. To obtain greater homogeneity in the specimens, it was decided to use tubular laboratory ovens and manufacture small batches with 32 specimens each. Initially, 2 batches were manufactured, one target at a nominal Charpy energy of 30 J and another 54 J. The pilot batches of 30 J and 54 J showed, respectively, standard deviations of 0.89 HRC and 0.96 HRC between samples and mean hardness with errors of −0.22 HRC and 0.83 HRC. The microstructures obtained after the heat treatments are tempered martensite and in the second batch, it is possible to observe that the microstructure is a little more spheroidized due to the higher tempering temperature.

Cleavage Micromechanisms Study in a Ship Plate Steel through Four-Point Double-Notch Bend and Charpy Tests

In this work instrumented impact Charpy tests were performed to analyze the ductile-brittle transition of Grade A ship plate steels, which are composed of ferrite matrix with pearlite bands in the rolling direction (RD) and pearlite lands in the transverse direction (TD). Observations on fracture surfaces of Charpy specimens showed regions of cleavage from room to down temperatures, therefore blunt four-point double–notch bend specimens (4PBT), were also tested at 25°C, 0°C, –60°C and –196°C in RD and TD, to study cleavage nucleation and propagation mechanisms. In the RD the following four different cleavage microfeatures nucleated microcracks in the notch region of 4PBT in all test temperatures: lamellar-pearlite microstructure, pearlite-boundary, ferrite-grains boundary and grains ferrite-inclusions. However, in the TD at –60°C, close to the lower shelf, only lamellar-pearlite and pearlite-boundary microfeatures were found nucleating microcracks. Nevertheless, the higher density of microcracks was found in the RD. The biggest microcracks were developed in TD in pearlite lands of critical size, in all test temperature, which are the link for the development of microcracks of critical size, which can propagate and fracture steel plates. In RD most microcracks nucleated in a critical region of about 1mm from notch root for all test temperatures, however for the TD the critical region decreases with test temperature, from about 450 µm (+25°C), 370 µm (0°C) and 225µm (–60°C). The analysis also showed the effect of microstructure orientation on the number and size of microcracks, and microcracks arrest.

Prediction of Fatigue Cracks Using Gamma Function

Abstract In the present study it has been endeavored to estimate the fatigue crack propagation in V-notch Charpy specimens of 2024 T351 Al-alloy. For this purpose, a new application of fatigue crack growth (FCG) is developed based on the “Gamma function.” Experimental fatigue tests are conducted for stress ratios from 0.1 to 0.5 under constant amplitude loading. The empiric model depends principally on physical parameters and materials’ properties in non-dimensional form. Deviation percentage, prediction ratio, and band error are used for validation of the performance of the fatigue life. The results determined from Gamma application are in good agreement with experimental FCG rates and those obtained from using Paris law.

Investigation of Potential Material Inhomogeneity in the Magnetically Detected Neutron-Irradiation-Generated Structural Degradation of Nuclear Reactor Pressure Vessel Steel

A novel nondestructive method called magnetic adaptive testing has been previously applied to detect the neutron-irradiation-generated structural changes in reactor pressure vessel steel material. This method has been found to be a useful tool for this purpose, and good correlation—as a tendency—has been found between the estimated ductile-to-brittle transition temperature and magnetic parameters. However, a significant scattering of measured points was also observed for the investigated set of Charpy specimens. The main result of the work was that by magnetic selection of samples, the scatter can be notably reduced. As a conclusion, the magnetically measured parameters seemed to be precise and reliable for the detection of embrittlement of the reactor pressure vessel steel, with lower scattering of points than in the conventionally used destructive mechanical characteristics (ductile-to-brittle transition temperature). This result is surprising and needs further verification. The purpose of the present work is to repeat the measurement on irradiated reactor steel blocks. In this work, instead of the DBTT transition temperature, individually measured Vickers hardness (VH) data were used to help characterize the mechanical properties of the material. The so-called “property transformation” is a known and applied technique in the nuclear industry. The mechanical property characterized by the transition temperature cannot be determined individually for each specimen; instead, it can be obtained only on a set of samples by statistical fitting. Therefore, the individually measured Vickers hardness values can be utilized in order to predict the individual transition temperature values by the help of the property transformation technique. In this paper, however, not these derived transition temperature values, but their origins, the Vickers hardness values, are studied in a direct manner. The same behavior of blocks was observed as in the case of Charpy specimens, which is considered to validate the previously published results. As a possible reason for the scattering of points, large magnetic inhomogeneity of samples cut even from the same block was also proved. The magnetic parameters and Vickers hardness correlate well with each other. This result justifies the potential future application of magnetic techniques in practice aimed at the regular inspection of nuclear reactors.

A unified fracture criterion considering stress state dependent transition of failure mechanisms in bcc steels at –196 °C

The fracture properties of a high-strength steel with a body-centered cubic (bcc) crystal structure have been characterized at –196 °C by performing tensile tests with different specimen geometries, three-point bending tests using Charpy specimens, and fracture mechanics tests, covering a broad range of stress states under quasi-static conditions. Both strength and ductility of the bcc steel are significantly increased when the temperature is decreased from room temperature to –196 °C. Enormous plasticity occurs in the material during tensile tests using various specimens at –196 °C, while macroscopic brittle fracture takes place in high triaxiality scenarios. A stress state dependence of ductile to brittle transition properties is observed, as the failure mechanisms at –196 °C change from cleavage fracture to shear failure with decreasing stress triaxiality. A unified stress-state-dependent fracture criterion, which considers the transition of failure mechanisms, is proposed to describe the fracture properties of similar bcc materials at cryogenic temperatures. The threshold triaxiality at which the transition of failure mechanisms takes place is a material property that is determined by the strain hardening capacity and fracture strength. In addition, a probabilistic formulation relying on the extreme value distribution has been incorporated into the model to render the statistical nature of cleavage fracture.