Round Specimens Research Articles

Behaviour of AZ91 magnesium alloy in tension and torsion: experimental investigations and numerical modelling

Abstract This study is concerned with the evaluation of the most appropriate plasticity model for simulating the behaviour of the AZ91 magnesium alloy in terms of accuracy and analysis times, based on experimental data obtained from tensile and torsion tests. The material was supplied in ingot form, was subsequently cut using and automated saw and ISO 6892 round specimens were obtained through lathing. The test data was used to calibrate three hardening functions, namely the multi-linear hardening, Johnson-Cook power law hardening and the Ramberg Osgood plasticity model, with the former determining the best results.

Resilience analysis of stress corrosion cracking in AISI 4340 steel under varying industrial electrochemical conditions

Stress Corrosion Cracking (SCC) presents a substantial challenge within industries where materials confront both mechanical stresses and corrosive environments. This work comprehensively examines SCC, incorporating the collection and analysis of a diverse dataset. The dataset encompasses pivotal parameters, including time to failure, corrosion rates, and electrochemical data. These parameters are meticulously garnered through Slow Strain Rate Testing on carefully prepared smooth, round tension specimens. The experiments are vigilantly overseen through a condition-based data acquisition and logging system, employing various Potentiostatic loads to mirror real-world electrochemical conditions. This research elucidates the intricate interplay between mechanical stresses, electrochemical processes, and corrosive conditions, rendering crucial insights for industries dependent on material integrity amidst formidable environmental challenges. Findings show that AISI 4340 Steel specimens exposed to a +400mV potential in NS4 solution exhibit an extended time to failure of approximately 7.4 days. Conversely, a −1200mV potential in NS4 accelerates the failure to around 5.24 days. In a 3.5wt% NaCl solution at 0mV, the time to failure is approximately 5.76 days. On the other hand, an applied potential of +400mV increases the failure time to approximately 6.16 days. And an applied potential of −1200mV accelerates the failure time to approximately 4.77 days. Specifically, the study reflects on structures submerged in water and those buried underground, represented by NS4 (Near Neutral Soil Simulating Solution) and 3.5w.t.% NaCl solutions, simulating real-world corrosive conditions. Through a deeper comprehension of SCC, industries can better anticipate and mitigate the risks associated with material failure in harsh environmental conditions, advancing the safeguarding of critical infrastructures.

Mechanical Characterization of Pultruded GFRP Round Tube under Axial Loading

Abstract: Pultruded Fibre‐Reinforced Polymer profiles are widely used as structural elements in many civil infrastructure applications. However, the anisotropic elasticity and the application‐driven slenderness make these profiles prone to local buckling failure, well below their ultimate load capacity. In this paper, an experimental study was undertaken to characterise the tensile and compressive failure of GFRP round tube under axial loading. Based on the research focus of GFRP structures, this work will present a comprehensive study of the mechanical properties of GFRP tubes in terms of both tensile and compressive loads and investigate the mechanical parameters. This paper presents an investigation on the mechanical properties of pultruded Glass fiber reinforced polymer (GFRP) round tubes for structures subjected to tensile and compressive axial loading. The tensile and compressive strength of GFRP round tubes were first tested. For the stability under compression, the slenderness ratio 6 is adopted. The results show that the tensile strength of GFRP tube could reach 580 MPa, while the compressive strength has been around 72% of tensile strength. Also Experimental results showed that specimens exhibited linearly elastic up to failure. Compared with the single tensile failure mode of GFRP tubes, two types of compressive failure modes, including micro-buckling and local buckling were observed. Moreover, a finite element (FE) analysis was carried to simulate the tensile and compressive behaviours of round tube specimen. The comparison between the tensile and compressive peak load values obtained experiment and FE methods revealed that their difference is less than 5%. These indicated that FE analysis predicted reasonably the actual tensile and compressive behaviours of the pultruded GFRP round tube.

Multiple potential drop measurement – A new method for crack detection and crack length measurement

The Direct Current Potential Drop Method (DCPDM) is widely used for crack detection and crack length measurement in fatigue experiments. Unfortunately, crack length measurements are only possible when a calibration for a specified crack initiation site and potential probe location is available. Investigations by Hartweg and Bär (2019) as well as Nahbein and Bär (2022) on notched round specimens equipped with three potential probes placed in a distance of 120° on the circumference have shown that the crack initiation site can be determined using a simple vector-based model. In this model the normal vector of a plane span by the three potentials is calculated in cylindrical coordinates. In the present work a more detailed investigation of crack initiation and propagation is undertaken to show the possibilities offered by this method. During fatigue tests with the simultaneous measurement of the three potential drops using amplifiers of the control electronics, overloads were introduced in fixed intervals to mark the crack front on the fracture surface. The measurement of both crack depth and fractured area allows a calibration of the potential drop measurement. The calibration was performed on the radius coordinate n of the normal vector using a function suggested by Tiedemann (2016). The multiple potential drop measurement enables an early detection of crack initiation as well as a location of the crack initiation site on the circumference of the specimen. Using the Tiedemann function fitted to the measured data, it is possible to determine the crack length independent of the crack initiation site on the circumference of the specimen.

Microstructure and mechanical behaviour study of the dissimilar weldment of ‘IN82 buttered’ P92 steel and AISI 304L steel for ultra super critical power plants

This study examines the influence of the ERNiCr-3 (Inconel 82) butter layer on the microstructure and mechanical feature of the dissimilar weld joint (DWJ) of ferritic grade P92 and austenitic grade AISI 304 L steel, manufactured using gas tungsten arc welding (GTAW) method with IN82 filler. The butter layer aims to prevent the formation of a hard martensitic layer adjacent to the fusion line by mitigating the carbon migration from the P92 steel to the welding pool. The optical microscope, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and electron probe microanalyzer (EPMA) were used to examine the microstructures of the buttered welded plate. The mechanical characteristics of the joints were assessed using the standard and sub-sized tensile and Charpy tests. The flat and round specimens underwent the tensile test to determine their relative tensile properties at room temperature and high temperature. The high-temperature tensile properties were assessed at 600 °C and 650 °C at 1 mm/min extension rates. Along the weldments, hardness profilometry was also performed. Microstructural analysis validated that the application of a buttering layer successfully eliminated the extensive martensitic layer on the P92 side. Nevertheless, in the vicinity of the interface between the buttering layer and P92, there was evidence of a narrow unmixed zone and macrosegregation. SEM/EDS and EPMA studies confirmed that the predominant phases identified in IN82 butter and weld metal are NbC and TiC. The butter layer, overall, improved elongation/ductility by 25 % without a noticeable reduction in tensile strength. During tensile testing, the specimen fractured from the IN82 butter layer, revealing the presence of dimples and voids on the fracture surface, along with the TiC/NbC phases, as confirmed by SEM analysis. Tensile specimens subjected to high-temperature testing exhibited failure at the base materials of AISI 304 L (tensile strength=404 MPa) and P92 (307 MPa), specifically at 550 °C and 650 °C, respectively, rather than at the buttering layer or weld metal. The results of the tensile tests met the minimum criteria specified for Ultra-Supercritical (USC) boilers. More importantly, the IN82 butter layer successfully serves as a protective barrier between the weld pool and the P92 HAZ. As a result, the dual hardness peaks on either side of the welding pool are eliminated. After adding the buttering layer, the peak hardness in the coarse-grained heat-affected zone (HAZ) of P92, which ranged from 450 to 500 HV in the unbuttered sample, dropped by 200 MPa. The relationships between microstructural characteristics and mechanical properties have been thoroughly examined and discussed.

Experimental study on post-fire mechanical properties and fracture behavior of Q690 steel

Fire is one of the most dangerous disasters for high-strength steel structures. After a fire event, the deterioration of mechanical properties of high-strength steel together with the complex stress state generated by external loading may lead to an unexpected failure or collapse of high-strength steel structures. Therefore, it is very critical to evaluate the post-fire behavior of high-strength steel considering stress state effects. This study conducted a series of tests on the Q690 steel specimens after being exposed to high temperatures ranging from 200 to 1000°C with different cooling methods including air-cooling and water-cooling methods. The tested specimens including smooth round specimens, notched round specimens, and grooved plate specimens were designed to consider the failure mode in axisymmetric stress state and plane strain state. To validate the accuracy of the test results, the mechanical properties such as yield strength, ultimate strength, elastic modulus, and ultimate strain were first compared with other test results reported in the literature. The post-fire ultimate strength under different stress states was obtained directly from the test results. Then, the post-fire equivalent plastic strain at fracture of different specimens was obtained from test data together with the finite element analysis results and analyzed under different stress triaxialities and Lode angle parameters. According to this study, an increase in stress triaxiality can result in an increase in ultimate strength but a decrease in the equivalent plastic strain at fracture for Q690 steel after fire. When the exposed temperature was not higher than 800°C, the post-fire strength of the plane strain specimen (Grooved Plane Specimen) was higher than that of the axisymmetric tension specimen (Notched Round Specimen) under a similar stress triaxiality; however, for their post-fire equivalent plastic strain at fracture, the results were opposite.

A local stress criterion to assess the effects of hydrogen embrittlement on the fracture strength of notched tensile specimens

This work addresses the applicability of a local criterion incorporating the coupling of critical stress and a critical hydrogen concentration to predict hydrogen embrittlement effects on the fracture strength of high strength steels using notched round specimens with different notch root radii. The numerical simulations incorporating a relatively simple hydrogen transport model provide strong support for the adoption of a failure criterion in terms of achieving a critical level of tensile stress coupled to the local hydrogen concentration, which, in turn, enable the construction of a failure locus for the degraded material. For the cases analyzed here, construction of such a failure locus based on a critical combination of maximum principal stress and hydrogen concentration enabled predictions of fracture strength for hydrogen-charged tensile specimens which are in very good agreement with experimental data. Overall, the results presented here lend additional support for further developments of a local stress-based criterion to predict hydrogen embrittlement effects on the fracture strength of high strength steels.

Chaboche viscoplastic material model for process simulation of additively manufactured Ti-6Al-4 V parts

For the estimation and further optimization of the residual stress and distortion state in additively manufactured structures during and after the wire arc additive manufacturing (WAAM) process, thermomechanical simulation can be applied as a numerical tool. In addition to the detailed modelling of key process parameters, the used material model and material data have a major influence on the accuracy of the numerical analysis. The material behaviour, in particular the viscoplastic behaviour of the titanium alloy Ti-6Al-4 V which is commonly used in aerospace, is investigated within this work. An extensive material characterization of the viscoplastic material behaviour of the WAAM round specimen is carried out conducting low cycle fatigue (LCF) and complex low cycle fatigue (CLCF) tests in a wide temperature range. An elasto-viscoplastic Chaboche material model is parameterised, fitted, and validated to the experimental data in the investigated temperature range. Subsequently, the material model is implemented in the thermomechanical simulation of a representative, linear ten-layer WAAM structure. To finally determine the effect of the fitted material model on the estimation accuracy of residual stress and distortion, simulation results using the standard material model and the elaborated Chaboche model from this study are compared to experimental data in the substrate. The thermomechanical simulation with the Chaboche model reveals a better agreement with the experimental distortion and residual stress state, whereby the standard material model tends to an overestimation. The estimation accuracy with respect to the maximum distortion is improved from an error of 60% with the standard model to an acceptable error of about 6% using the elaborated model. Additionally, the estimated residual stress state shows a sound agreement to the experimental residual stress in the substrate.

Numerical Investigation on the Residual Ultimate Strength of Central-Cracked Stiffened Plates under Tensile and Bending Loads Using XFEM

The present paper aims to study the crack propagating behavior of a stiffened plate under tensile and bending displacement load loads. The extended finite element method (XFEM) is used to analyze the residual ultimate strength of stiffened plates with a central crack. The quasi-static crack growth process is simulated by software ABAQUS. The validity of the grid is validated by the plate with a central crack. The numerical method is validated by comparing the fatigue crack growth rate of the round compact tension specimen (RCT) results of the extended finite element with experiment values. Influential parameters, including the size of the stiffened plates, heights of the stiffeners is varied, and uniaxial tensile and four-point bending models are analyzed. The results show that ultimate strength is reduced by the action of tensile and bending loads. The bottom plate and stiffener are destroyed with crack propagation, successively. With the increase in stiffener height, the crack resistance will also increase, thus restraining the central crack growth of stiffened plates.

On Ausferrite Produced in Thin Sections: Stability Assessment through Round and Flat Tensile Specimen Testing

Ductile irons were produced into different casting wall sections, that is, 25 mm, 5 mm and 3 mm. The alloys were then austenitized with the same conditions at 875 °C for 2 h and austempered for three different combinations of temperatures and times: 250 °C for 6 h, 310 °C for 3 h and 380 °C for 1 h. The aim of the investigation was to study the ausferrite stability of austempered ductile irons with three different nominal contents of nickel produced in thin sections through tensile testing. So, strain hardening analysis of tensile flow curves was carried out since it has been found to be a reliable support to ductility analysis in assessing the optimal austempering conditions. Because of different wall sections, round and flat tensile specimens with geometries complying with ASTM E8/E8M-11 were tested. Austempered ductile irons from 5 and 3 mm wall sections were tested through flat geometry specimens only, while 25 mm wall sections were tested through both round and flat geometries. Though the ausferrite was affected by Ni content and the graphite morphology was improved with reduced thin sections, the ausferrite stability and the tensile mechanical behavior were insensitive to Ni content and section thickness below 25 mm. Furthermore, it resulted that the tensile plastic behavior was sensitive to the specimen geometry in a consistent way, increasing the instability of ausferrite and indicating that a proper analysis and comparison of tensile properties of austempered ductile irons must take into account the tensile specimen geometry.

Influence of hydrogen environment on fatigue fracture morphology of X80 pipeline steel

The rapid development of hydrogen energy requires the use of natural gas infrastructure for hydrogen transportation. It is very important to study hydrogen-added natural gas transportation technology, which is a key way to rapidly develop coal-based gas and renewable energy. This study aims to study the influence of X80 pipeline steel's fatigue performance in hydrogen environment, and perform fatigue tests on notched round rod specimens under different hydrogen concentration. The experimental results show that hydrogen seriously affects the fatigue life of pipeline steel. After reaching a certain hydrogen concentration, as the hydrogen concentration continues to increase, the fatigue life decreases gradually. Combined with SEM analysis of fracture morphology, the decrease in the size and density of the dimples reduces the displacement amplitude, while the increase in the planar area increases the displacement during fatigue fracture due to accelerated crack propagation. From this study, we can know the influence of hydrogen concentration on the fracture morphology of pipeline steel, which provides an understanding of the effect of hydrogen on fatigue fracture morphology and a broader safety analysis.

Thermomechanical fatigue of round tube specimens manufactured by precision directional solidification casting method

AbstractIn the present study, a superalloy round tube specimen prepared by directional precision casting without mechanical turning is experimentally investigated under the thermomechanical fatigue conditions of the in‐phase and out‐of‐phase thermomechanical angels from 500°C to 1000°C. The deformation behavior and damage mechanism of the round tube specimens under TMF load are discussed. It was confirmed that the tension–compression asymmetry under TMF cycling is affected by temperature–load phasing and mechanical strain range. Moreover, the damage to round tube specimens includes creep, fatigue and oxidation. Due to the concentrated stress, the rough surface of the specimen provides suitable locations for crack initiation. A novel life prediction approach that combines the Neu–Sehitoglu model and modified Manson–Coffin model was proposed in response to the traditional life model's insufficient prediction performance. This method may explain the TMF life of precision cast specimens.

Use of modified Madou–Leblond model to predict crack initiation in low alloy steel specimens with different stress states

This paper aims to predict the ductile fracture in low alloy steel specimens with pure tension, pure shear and combined shear–tension type loading using modified Madou–Leblond (MML) porous plasticity model, proposed by the present authors in a recent work. This model includes the effect of localised mode of plastic deformation in a narrow band through the definition of localised porosity level under shear dominated loading conditions. The damage parameter includes contributions due to void growth process under high stress triaxiality level as well as that due to shear localisation process under low stress triaxiality conditions. Experiments have been performed on the round tensile specimen, flat type shear specimen and combined shear–tension specimen. The numerical simulations results obtained using MML model has been compared with those of experiments. It has been shown that the MML model performs well in the prediction of the material softening behaviour and critical fracture location along with the fracture profile in all three types of specimens.

INCREASING THE DURABILITY OF CENTRIFUGAL PUMP SHAFTS BY COMBINED METHODS

The article analyzes the literature, which showed that in recent years there have been new technological methods of strengthening the surface layers of the centrifugal pumps shafts necks. They are based on the use of environmentally friendly method of electrospark alloying (ESA) (cementation, application of combined coatings with molybdenum disulfide (MoS2), coatings of Al-C-B) and without comparative tests for wear resistance is difficult to prefer one method or another. Studies on the friction machine СМЦ -2 found that the weight wear of round specimens of polished steel 40X, in contact with lubrication of turbine oil T-22 with babbitt inserts of the sliding bearing according to the scheme "disk-insert", 1.15 times greater than steel 30X13 and, respectively, 2.50 and 4.41 times than 40X steel after cementation by ESA (CESA) + non-abrasive ultrasonic finishing (NAUF) and ESA (Mo + S) + NAUF. In turn, the linear wear of polished steel 40X round samples is greater than steel 30X13 1.09 times and, respectively, 2.83 and 5.85 times than steel 40X after CESA + NAUF and ESA (Mo + S) + NAUF. The lowest coefficient of friction was recognized as 0.025 and 0.030, which belongs to the coating of MoS2, formed, respectively, on samples of steel 30X13 and 40X and treated with NAUF.
 Analysis of the linear wear change nature of steel 40X and 30X13 round samples, both hardened and not hardened, showed that all samples first have more intense wear, which after 2 hours of abrasion slows down, becomes more stable and at first glance resembles exponential dependence. Samples reinforced in the sequence ESA (Mo + S) + NAUF have the best wear resistance, the wear of which after 8 h of tests is 0.9 and 1.1 μm, respectively, of steel 30X13 and 40X.
 Weight and linear wear of 40X steel specimens without hardening is higher compared to non-reinforced 30X13 steel specimens tested in pair with BK8 counterbody under conditions of abundant lubrication with I40A industrial oil, by 21.7 and 14.2%, respectively; in comparison with samples of 40X steel with a coating applied by the method of ESA composition (Al-C-B) followed by NAUF, respectively, 77.8 and 84.7%; and 30X13 steel with similar treatment of ESA (Al-C-B) + NAUF, 98.0 and 71.8%, respectively.

Influence of the stress history and of the Lode angle on the determination of the ductile fracture locus for two steel alloys

The fracture locus of ductile materials is primarily related to the stress triaxiality. Recently, the Lode angle was also found to be relevant in the fracture locus determination. In order to characterize a material in terms of fracture locus, usually several tests, spanning a wide range of stress triaxiality and Lode angle values, have to be performed using different specimen geometries, obtained both from round bars and sheets.In this paper the fracture loci of two steel alloys (S500MC and 22MnB4) are obtained using a simplified procedure, based on the Bai–Wierzbicki model. The procedure considers, for experimental tests, only flat specimens, avoiding the use of round specimen of the same material which were not available. Experimental tests were performed on differently shaped specimens and the results were used to train finite element (FE) models. The triaxiality and Lode angle histories, computed for each test by the FE model, were used to obtain the fracture loci following two different approaches: the proportional loading approach and the non-proportional loading approach. The former does not consider how the stress triaxiality and Lode angle vary during the test, while the latter considers their history in the computation of the fracture locus.The results show how the Lode angle influences the fracture locus, especially for 22MnB4, and how non-proportional loading approach is more accurate to compute the damage for the different specimen geometries.

Comparison of Conventional and Robotic Fused Filament Fabrication on Silicone Build Plates.

The objective of this study is the investigation of the transferability of the material extrusion process from conventional to robotic fabrication on silicone build plates for use in Enhanced Multipoint Moulding with Additive Attachments. Therefore, the study is based on two series of experiments. The first series of tests used a conventional plant extended by a silicone construction platform. In comparison, a six-axis industrial robot was chosen to produce the test specimens in the second series of tests. The comparisons of adhesion strengths and relative shape deviations are used to validate the transferability. The results of the tests show a very good transferability of the process from conventional to robotic production. Whilst angular specimen geometries can be transferred directly, for round specimen geometries, the results show a need for further adaptation to the robot kinematics. The round specimen geometries showed deviations in the surface quality caused by an over-extrusion in the robotic manufacturing. This over-extrusion results from the slicing process in combination with the robot control and may be avoided through further optimisation of the process parameters. Overall, to the best of our knowledge, this study is the first that successfully demonstrates the transfer of Fused Filament Fabrication (FFF) from a conventional system to manufacturing using robots on silicone build plates for the use in Enhanced Multipoint Moulding with Additive Attachments.

Failure Locus of an ABS-Based Compound Manufactured through Photopolymerization.

This work investigates the critical plastic strain variation with stress triaxiality and the Lode angle parameter for an Acrylonitrile butadiene styrene (ABS)-based proprietary blend compound (commercial name VeroWhitePlus™ RGD835) manufactured through photopolymerization. Various triaxial states of stress and Lode angles were obtained with the help of notched flat specimens used in tensile loadings, notched round specimens used in compression (upsetting) tests and butterfly specimens used in Arcan tests. The tests were replicated using finite element analyses in order to evaluate the aforementioned parameters.

A Nonlinear Ultrasonic Method for Detection and Characterization of Dewetting Damage in Solid Propellant

AbstractThe dewetting damage of solid propellant under loading is a key factor affecting the constitutive properties of solid propellant, and it will weaken structural integrity of solid rocket motor charge. The quantitative characterization of its evolution process has always been a very active research field. In this study, a nonlinear ultrasonic (NLU) method is proposed to investigate and monitor in real‐time the nonlinear dynamic response of solid propellant. The NLU parameter, which can evolve dewetting damage, is expressed by some physical quantities such as the frequency peak, the propagation distance and so on, and is recorded at different strain in the tensile tests of the round rod solid propellant specimens. Then, the initial dewetting point, characteristic dewetting point and characteristic dewetting rate are defined to characterize the dewetting performance. The results show that NLU parameter changes continuously with the evolution of tensile process and increases sharply at the initial dewetting damage point. By analyzing the data, an empirical formula characterizing the variation law of NLU parameter with strain is proposed, which is well fitted to the experimental data under different strain rates. In addition, relevant numerical simulation is carried out, and the simulation results verify the feasibility of the NLU method.

Fatigue performance of notched and hot-dip galvanized laser and mechanically cut S960 steel components considering local defects with the theory of critical distances

Experimental fatigue tests were performed for a S960 steel grade, including hot-dip galvanized (HDG) round base material specimens, and laser cut, and machined notched component-sized specimens made of t = 6 mm S960 ultra-high-strength steel (UHSS) plates. Cracking after the HDG was found to have a major influence on fatigue strength and thus reducing the effect of surface quality on the fatigue performance. Design guidelines for notched HDG components are proposed, and HDG with UHSSs was found suitable for structures with geometrical notches. Multiparametric TCD-based 4R method application was introduced, and it was found to be applicable for the fatigue strength assessment of structural details with initial cracks.

Study of the Creep Behavior of Nickel-Based Single Crystal Superalloy Micro Specimens with Dimensional Effects

Nickel-based single-crystal superalloys are widely used in aeroengine hot-end components, owing to their unique crystal structure and outstanding high-temperature mechanical properties. In the present study, round rod specimens of different sizes were subjected to high temperature creep tests at 980 °C/300 MPa of a second-generation nickel-based single crystal superalloy. The effect of size on the creep behavior of nickel-based single-crystal superalloys was studied with reference to the creep curves and microstructure morphologies. Creep interruption tests of 3-mm micro-round rod specimens were performed for 30, 60, and 90 h until creep fracture occurred. It was found that for nickel-based single crystal superalloys, the smaller the diameter of the specimen, the longer its creep life. Furthermore, the creep fracture morphology showed obvious creep cavitation in the fracture region. The law of organization evolution was used to analyze the rafting phenomena during the creep process. A typical “N”-type drifting strip structure was found during the creep process. Meanwhile, the width of the γ-phase channel increases continuously with creep, and the rate of change of the width of the matrix phase was fastest at the earliest stage of creep, slowing significantly during the middle and late stages of creep with the completion and appearance the rafting phenomenon.