Smooth Cylindrical Specimens Research Articles

Microstructure evolution of Incoloy 800H in industrial environment and correlation with creep mechanisms from literature

ABSTRACT The microstructures of 800 H samples affected by natural corrosion within industrial-like environmental conditions have been studied, and their potential effects on the creep response have been investigated based on literature information. Smooth cylindrical specimens of the alloy extracted from rolled sheet material have been placed into an industrial furnace, where they were exposed to high temperature thermal cycles within an air atmosphere. Samples spending 0 to 5 years inside the furnace underwent a microstructural characterisation campaign. Optical microscopy images reveal no grain coarsening. Macroscopic Vickers hardness shows a relatively wide surface hardness range (115≲HV10≲140). Compared to as-received material, micro-hardness profiles from specimens after years of high-temperature exposure exhibit a surface hardening trend within the vicinity of the edge exposed to the environment. Scanning electron microscopy analyses revealed two coexisting corrosion mechanisms: oxidation and nitridation. The latter is identified as the cause of the micro-indentation hardening trend.

Assessing Fatigue in Materials with Small Defects: A New Multiaxial Model Based on Principal Stress Amplitudes

A new multiaxial fatigue model for materials containing small defects proposed by the authors is presented, which one can be considered as a modification of the SWT model. This new model relates the fatigue limit of the material obtained from the √area parameter with values associated with the principal stresses. These values are the amplitude of the principal stresses and the maximum principal stress observed. The amplitude value associated with the principal stresses is defined by using the Maximum Variance Method. This amplitude only can be easily obtained under uniaxial loading conditions, but its calculation for torsion, proportional and non-proportional multiaxial loadings is not trivial. Therefore, the calculation of the principal stress amplitude posed a challenge not yet addressed by other authors. The multiaxial fatigue model was evaluated with experimental data from AISI 4140 steel, with several different loading conditions, including uniaxial loading, combined loads in in-phase and out-of-phase configurations. In addition, two types of specimens were used: smooth cylindrical specimens and specimens with a surface micro hole. Comparing the experimental data with the prediction of the new model it was observed that the predictions are slightly conservative with average error not exceeding 6%.

Qualification Approach Detailed for Offshore Hydrogen Pipeline Systems

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 32158, “Offshore Hydrogen Pipeline System Qualification: Design and Materials/Welds Testing in Hydrogen Environment,” by Angelo Santicchia, Elvira Aloigi, and Salvatore Terracina, Saipem, et al. The paper has not been peer reviewed. Copyright 2023 Offshore Technology Conference. Reproduced by permission. _ The qualification of a pipeline system for hydrogen transport, important in the transition to a decarbonized energy system even if strictly related to offshore pipelines, is a broad field that requires a systematic approach from basic material knowledge to complex physical models and fracture and fatigue assessments. The authors’ analysis of qualification requirements, including available test types and testing protocols, led to a matrix of potential tests, detailed in the complete paper, to be conducted in hydrogen and air environments for the steel base material, seam weld, and girth weld of offshore pipelines. Offshore Pipeline Materials Requirements vs. Hydrogen Transportation The authors’ work outlines many challenges, including the following: - Although hydrogen pipelines installed and operating onshore are common, at the time of writing, none exist in the offshore environment. - The blending percentage of hydrogen into natural gas in the future-transport scenario is still under discussion. - Small amounts of hydrogen can have a substantial effect on fatigue and fracture on high-strength materials. - The effect of hydrogen on pipe-material fatigue and fracture properties correlates directly to the specificity of the offshore environment, which will be very demanding in terms of longitudinal stress and fatigue. Another important aspect to consider is the effect of hydrogen on weldments both longitudinal and circumferential that are part of pipe-material fabrication and pipeline fabrication. A dedicated engineering team analyzed key standards and the available literature in terms of theoretical studies and experimental tests of materials in hydrogen environments. This activity indicated that, in addition to theoretical and design considerations, characterization of primary material and welding properties in hydrogen environments that will affect the failure modes of offshore pipeline design is a critical step. Testing Protocols and Equipment Slow Strain Rate Test (SSRT). Smooth cylindrical specimens were tested in inert gas (nitrogen), pure hydrogen, and a hydrogen/natural gas mixture for comparative purposes. This testing was aimed at evaluating the susceptibility of the pipeline materials (base material and weld metals) to hydrogen embrittlement when subjected to typical conditions envisaged for future hydrogen service. Tensile cylindrical specimens were tested at room temperature (24°C) in a high-pressure gaseous atmosphere. Tests were carried out in displacement control mode. In the initial part of the tensile test, in the plastic regime, after attainment of material yielding but before reaching the maximum tensile strength of the specimen, the strain distribution along the specimen was approximately uniform.

Fracture properties and mechanisms for irradiated austenitic steels over high temperature range and formulation of fracture criterion. Part 1. Experimental results

The experimental study results of the fracture properties and mechanisms are represented over temperature range from 200 up to 600°С for austenitic 304 steel (18Cr–9Ni steel) in the following conditions: (1) after neutron irradiation at temperature of 400°С up to damage dose of 30 dpa; (2) after neutron irradiation and subsequent aging at 550°С for 3000 hours. The fracture properties and mechanisms are determined under uniaxial tension of standard smooth cylindrical specimens and notched cylindrical specimens, i.e. for various stress triaxialities. Sharp decrease in the fracture strain and transition to intergranular fracture are revealed over high temperature range forstandard smooth cylindrical specimens. Over the same temperature range the fracture strain is larger for notched cylindrical specimens than for smooth cylindrical specimens, moreover a portion of intergranular fracture is also larger. This finding is rather abnormal as the fracture strain usually decreases when stress triaxiality and intergranular fracture portion increase. The obtained results are used for formulation of fracture criterion and elaboration of fracture model for high temperature radiation embrittlement that is considered in the second part of the paper.

Isothermal high-temperature fatigue behaviour of a near-γ titanium aluminide alloy

AbstractThe high-temperature fatigue behaviour of smooth cylindrical specimens of a near-γ-TiAl alloy with duplex microstructure was studied performing total-strain-controlled low-cycle-fatigue tests in the temperature range 500 – 750 °C. The tests were run in air and high vacuum. Mean stress, environmental degradation (oxygen- and hydrogen-induced embrittlement) and plastic strain range were identified to be the most significant parameters affecting damage evolution. The cyclic stress– strain response was found to depend strongly and characteristically on temperature. Below the brittle-to-ductile-transition temperature (TBD) of about 650 °C, cyclic deformation leads to a pronounced initial hardening manifesting itself in an increase of stress amplitude and a decrease of plastic strain range. Conversely, cyclic deformation curves from tests performed aboveTBDshow a marked state of saturation which is established already after few cycles. With increasing temperature, fatigue life decreases in vacuum. However, testing in air not only decreases fatigue life tremendously as compared to vacuum, but also leads to an inverse temperature effect, i. e., the number of cycles to failure increases with temperature.

High-temperature low-cycle fatigue of a heat-resistant alloy of the Co–Cr –Ni–W–Ta system obtained by additive manufacturing

The results of fatigue tests of smooth cylindrical specimens with total deformation control under conditions of a symmetric cycle and elevated temperature are presented. The relationship between the values of the fatigue characteristics of the material with allowance for stresses, plastic deformation and the number of cycles to failure is considered. Comparison of deformation curves plotted from experimental data with deformation curves plotted by evaluative methods is presented.

Evaluating size effects on fatigue life of 42CrMo4+QT steel using a statistical S-N model with highly-stressed volume and surface

A statistical S-N model based on the highly-stressed volume/surface concept is proposed for evaluating the size effects on fatigue strength of specimens from 42CrMo4+QT steel. The proposed model extends an existing physically valid model (Castillo et al. 2009) and allows the S-N curve to be extrapolated from small-scale to full-scale components. The model is validated against experimental fatigue data obtained by testing solid and hollow smooth cylindrical specimens under push-pull constant amplitude loadings. After checking that the proposed model agrees with experimental fatigue data, it is possible to apply the model to estimate the S-N curve of specimens of the highly-stressed volume up to 1802 mm3. For this case, an S-N expression is provided that can directly be used in fatigue analysis without the necessity of running a non-linear optimization.

Rotor steels crack resistance and fracture behavior for hydrogen targeted materials ever-widening database

It has been investigated the influence of hydrogen at pressure up to 10 MPa (and absorbed hydrogen with concentration up to 8.1 ppm) on the strength, ductility, crack resistance of 35NiCrMoV12-5 (38KhN3MFA) rotor steel specimens with different chemical composition and tempering modes. Due to hydrogen embrittlement, the high-pressure gaseous hydrogen (up to 10 MPa) has the most significant influence on rotor steel properties under static tensions, namely relative transverse elongation, reduction of area in smooth cylindrical specimens, fracture toughness in compact samples, and less - on the critical stress intensity factor (SIF) Kfc at cyclic loading. The hydrogen influences the plasticity, static, and cyclic fracture toughness parameters reach the maximum for the ambient temperature. The hydrogen impact on the listed parameters becomes negligible at 530 K. With the increase of the annealing temperature from 923 up to 1023 K structure with larger structural components has formed, lower local stresses increases, the flow boundary decreases and the sensitivity to hydrogen embrittlement under all types of the load is reduced. The SIF diagrams (at static and cyclic loading) were constructed, the crack growth rates on plateau sections were determined. The Paris equation parameters were obtained; all above are valuable tools for the rotors' lifetime prediction. As deducted from stress and strain-controlled S-N curves, hydrogen decreases the high-cycle durability of rotor steel. Hydrogen also decreases the low-cycle durability of rotor steel. The degree of hydrogen effect increases with strain amplitude decrease.

The influence of initial radial deformations on the residual stresses distribution in the hardened layer

The study of the initial radial deformations influence on the residual strained state components distribution through the hardened layer thickness under various types of surface hardening has been conducted. Chemical thermal machining (CTM) and shot blasting (SB) have been examined on the example of smooth solid steel cylindrical specimens with diameters D=10 mm and D=25 mm under a linear distribution of initial deformations through the hardened layer thickness (the maximum value is on the surface and zero on the maximum depth of hardening). The calculation has been conducted with use of the program complex PATRAN/NASTRAN. The solution has been carried out in axial symmetric positing by thermo elasticity method. On study results its been stated that the radial stresses through the hardened layer thickness under CTM and SB are positive. Incidentally the radial component of initial deformations under chemical thermal machining and shot blasting doesnt exercise essential influence on the radial and axial residual stresses distribution through the hardened layer thickness. Hence the initial radial deformations havent much significance at the evaluation of the residual strained stress state influence on the hardened parts endurance limit increment calculated by the average integral residual stresses criterion.

Comparative investigation of ductile fracture with 316L austenitic stainless steel in small punch tests: Experiments and simulations

This paper deals with a small punch test and ductile failure prediction of austenitic stainless steel 316L. Comprehensive investigation of fracture surfaces was conducted as well as examination of δ-ferrite and σ-phase present in the material in order to reveal the reason of occurrence of nonstandard type of fracture. Three phenomenological ductile fracture criteria were selected for comparative simulations regarding the prediction of crack initiation and propagation during the SPT. These criteria were calibrated by using the fracture tests of smooth cylindrical specimens, notched cylindrical specimens, notched tube specimens and cylinders with spherical recess. The whole material model including the fracture criteria with von Mises plasticity and element deletion technique for simulation of the crack initiation was implemented by using the user subroutine Vectorised User MATerial into the commercial finite element code of Abaqus. Finally, the simulation results with three ductile fracture criteria are compared and discussed based on the experimental results. Only one of the failure criteria captured the displacement at fracture correctly.

A comparative study of hydrogen embrittlement of 20SiMn2CrNiMo, PSB1080 and PH13-8Mo high strength steels

The present study was attempted to evaluate the hydrogen embrittlement (HE) of commercial 20SiMn2CrNiMo, PSB1080 and PH13-8Mo high strength steels using slow strain rate tensile tests with smooth cylindrical tensile specimens. The results reveal that three steels exhibit the superior strength and plasticity in the absence of hydrogen. However, the mechanical properties are deteriorated after hydrogen charging and the resistance to HE is 20SiMn2CrNiMo steel, PH13-8Mo steel and PSB1080 steel, which is ranked in a decreasing order. Fine lath boundaries in 20SiMn2CrNiMo steel disperse hydrogen atoms, reduce local hydrogen accumulation and then result in the lowest HE. Coherent NiAl precipitates in PH13-8Mo steel and O-Mg-Al-Si-Ca type inclusions in PSB1080 steel are responsible for the high HE susceptibility due to the concurrent effect of the stress concentration and hydrogen-enhanced decohesion mechanism. It is thus suggested that the inclusions in steels should be eliminated and the efforts concerning to obtain low HE susceptibility in steels strengthened by the coherent precipitates should be conducted to guarantee the safe service of engineering components fabricated from high strength steels.

ДИАГРАММА ПРЕДЕЛЬНОЙ ПЛАСТИЧНОСТИ АЛЮМИНИЙ-ГРАФЕНОВОГО МЕТАЛЛОМАТРИЧНОГО КОМПОЗИТА С СОДЕРЖАНИЕМ ГРАФЕНА 2 МАС. % ПРИ ТЕМПЕРАТУРЕ 300 °С

Deformation behavior of a 2 wt% aluminum-graphene metal matrix composite (MMC) at 300 °C is studied. The MMC under study is synthesized at the Institute of High-Temperature Electrochemistry, Ural Branch of RAS. Data on ultimate ductility of the MMC is obtained. The value of shear strain to fracture is used as the characteristic of ultimate ductility. The shear strain to fracture is the function of the stress triaxiality coefficient and the Lode-Nadai coefficient. Tensile tests of smooth cylindrical specimens, notched cylindrical specimens, tensile and compressive tests of bell-shaped specimens, and tests of thick-walled cups with thinned bottoms are used to study ultimate ductility. The fracture locus of the composite at 300 °C is identified by the results of the study. It has been determined that graphene increases the ductility of aluminum, even under conditions of prevailing tensile stresses. However, the influence of graphene is significantly dependent on the form of the stress state of the metal under deformation. The composite under conditions of tension of smooth cylindrical specimens manifests practically unlimited ductility. The specimens underwent plastic deformation till the moment of the physical separation of specimen parts in the fracture region, while the cross section of the specimen in the neck tended to zero. However, when thick-walled cups with thinned bottoms are tested, the influence of graphene on ductile properties of the MMC can be neglected. Under conditions of prevailing compressive stresses, the ductility of the MMC significantly increases for all the test types. The obtained fracture locus is compared with the fracture loci of commercially pure aluminum and the 1 wt% aluminum-graphene MMC. It has been determined that the ductility of the material increases with graphene content.

A Fatigue Life Prediction Method Based on Strain Intensity Factor.

In this paper, a strain-intensity-factor-based method is proposed to calculate the fatigue crack growth under the fully reversed loading condition. A theoretical analysis is conducted in detail to demonstrate that the strain intensity factor is likely to be a better driving parameter correlated with the fatigue crack growth rate than the stress intensity factor (SIF), especially for some metallic materials (such as 316 austenitic stainless steel) in the low cycle fatigue region with negative stress ratios R (typically R = −1). For fully reversed cyclic loading, the constitutive relation between stress and strain should follow the cyclic stress-strain curve rather than the monotonic one (it is a nonlinear function even within the elastic region). Based on that, a transformation algorithm between the SIF and the strain intensity factor is developed, and the fatigue crack growth rate testing data of 316 austenitic stainless steel and AZ31 magnesium alloy are employed to validate the proposed model. It is clearly observed that the scatter band width of crack growth rate vs. strain intensity factor is narrower than that vs. the SIF for different load ranges (which indicates that the strain intensity factor is a better parameter than the stress intensity factor under the fully reversed load condition). It is also shown that the crack growth rate is not uniquely determined by the SIF range even under the same R, but is also influenced by the maximum loading. Additionally, the fatigue life data (strain-life curve) of smooth cylindrical specimens are also used for further comparison, where a modified Paris equation and the equivalent initial flaw size (EIFS) are involved. The results of the proposed method have a better agreement with the experimental data compared to the stress intensity factor based method. Overall, the strain intensity factor method shows a fairly good ability in calculating the fatigue crack propagation, especially for the fully reversed cyclic loading condition.

A fracture locus for a 50 volume-percent Al/SiC metal matrix composite at high temperature

The effect of the stress state on the fracture locus function of the 50 vol.% Al/SiC metal matrix composite at high temperature is studied. The value of fracture locus function is quantitatively characterized by the amount of shear strain accumulated prior to the moment of failure. Nondimensional invariant parameters are used as characteristics of the stress state, namely, the stress triaxiality k and the Lode-Nadai coefficient μ σ showing the form of the stress state. Besides conventional testing for tension, compression and torsion of smooth cylindrical specimens, the complex of mechanical tests includes a new type of testing, namely, that for bell-shaped specimens. These kinds of testing enable one to study fracture strain under monotonic deformation in the ranges μ σ = 0 … + 1 and k = − 1.08...0 without using high-pressure technologies. The stress–strain state during specimen testing is here evaluated from the finite element simulation of testing in ANSYS. The tests were performed at a temperature of 300 °C and shear strain rate intensity Η = 0.1; 0.3; 0.5 1/s. The test results have offered a fracture locus, which can be used in models of damage mechanics to predict fracture of the material in die forging processes.

Assessment of Rupture Behaviour of 9Cr-Steels Under Multiaxial State of Stress

In this investigation, detailed finite element analysis has been carried out to assess the creep behaviour of 9Cr-1Mo and modified 9Cr-1Mo steels under multiaxial state of stress. The multiaxial state of stress was introduced by incorporating the circumferential notches of different notch radii in smooth cylindrical specimens. The creep experiments were carried out on smooth and notched specimens of the steels at 873 K and in the stress range of 110–230 MPa. The modified 9Cr-1Mo steel possessed significantly higher creep strength than the 9Cr-1Mo steel. The rupture life was found to increase in presence of notch for both the steels. However, the extent of strengthening was found to be significantly higher for modified 9Cr-1Mo steel compared to 9Cr-1Mo steel. The fracture appearance was found to be dependent on the applied stress and the notch geometry. Typical transgranular ductile fracture was observed for shallow notches and mixed mode fracture consisting of ductile and creep cavitation for relatively sharper notches, which was more prevalent at lower applied stress. Finite element analysis was carried out to understand the distribution of stresses across the notch throat plane. The stresses were found to relax in the notch throat plane. The stress relaxation at the notch root was found to be more for modified 9Cr-1Mo steel than that of 9Cr-1Mo steel, resulting in higher extent of strengthening in modified 9Cr-1Mo steel.

The characterization and ballistic evaluation of mild steel

The manuscript presents a systematic study for the characterization of mild steel under varying stress-triaxiality, strain rate and temperature. The effect of stress triaxiality was studied by performing tension tests on notched cylindrical specimens. The radius of the notch has been varied as 0.2, 0.4, 0.6, 0.8, 1, 2, 3, 4, 5 and 10 mm. The smooth cylindrical specimens were tested under tension at varying strain rate and temperature. The strain rate in the range, 0.0006 s−1–1500 s−1, was obtained on a universal testing machine and the Hopkinson pressure bar apparatus. The tests at elevated temperature were carried out using a portable furnace which enabled the variation of temperature from 100 °C to 750 °C. All the material parameters for the Johnson–Cook elasto-viscoplastic material model have been calibrated. The parameters thus obtained were validated by numerically simulating the material characterization under high strain rate using ABAQUS/Explicit finite element code. The numerical simulations were also carried out for the ballistic evaluation of 12 and 16 mm thick mild steel plates impacted by 7.62 AP projectiles at varying incidence velocities in order to obtain the ballistic limit. The angle of incidence has also been varied until the occurrence of critical ricochet. The numerical results have been validated by the experiments reported in earlier studies, Gupta and Madhu [1,2].

Environmental effect on the fatigue performance of bare and oxide coated 7075-T6 alloy

Fatigue behaviors of bare and anodic oxide coated 7075-T6 alloy have been investigated in laboratory air and 3.5%NaCI solution environment by using smooth cylindrical specimens. Presence of corrosive attack during fatigue test drastically reduced fatigue performance of the alloy. The deleterious effect was observed to be pronounced at high-cycles fatigue region, where the fatigue strength of the bare specimen was lowered by a factor of 2.9. However, the oxide coated specimens having a thickness of 23μm showed a modest reduction in fatigue strength. Corrosion fatigue (CF) strength of the bare specimens was predominantly controlled by pitting-induced crack nucleation. Examinations on the surfaces of the corrosion-fatigued and immersed test specimens revealed that cyclic loading stimulated corrosion pit formation during CF tests. Also, corrosion behaviors of both the coated and bare specimen shave been investigated by potentiodynamic test. Despite superior corrosion resistance of coated specimens, fatigue performance was adversely affected under the combined action of corrosion attack and cyclic loading.

Dynamic Constitutive Relation and Fracture Model of Q235A Steel

Strength and ductility data for Q235A steel from 20 oC to 950 oC was obtained from a series of experimental tests. The stress rate sensitivity was studied by conducting Split-Hopkinson Tension Bar (SHTB) test and uniaxial tension test on smooth cylindrical specimens while the influence of stress triaxiality on ductility was revealed by conducting upsetting tests, tension tests on pre-notched cylinder specimens and torsion tests on SASs. Slightly modified versions of the two Johnson–Cook (J–C) models describing flow stress and fracture strain are presented to characterize the properties of Q235A steel as function of strain rate, temperature and stress triaxiality. Corresponding model parameters were calibrated based on the test data and with the help of finite element calculation. It was found that the modified Johnson–Cook (MJC) models give more close predictive results compared with the original J–C models.

Discussion of models for LCF small crack growth

The life assessment for a component subjected to high strain concentration in critical regions can be treated as a crack growth estimation starting from the first cycle of the component life. This approach assumes that the presence of same small surface defects or slip bands causes a quickly crack nucleation in critical regions where Low Cycle Fatigue (LCF) design is adopted. The work is focused on the analysis of LCF tests carried out by the aim of smooth cylindrical specimens and specimens with semi-circular notches made of a quenched and tempered steel. The crack growth has been monitored using a thin acetate-foil based replica technique. Two different approaches have been analyzed in order to define a model able to predict the crack growth rate for high constant strain amplitudes. Such a tool is useful to assess the residual life of a component in presence of a crack in critical regions where high stress concentrations cause cyclic yielding of the material. Finally a new analytical model able to predict the crack growth rates is presented.

Low cycle lifetime assessment of Al2024 alloy

The 2024-T351 aluminum alloy is extensively used for fabricating aircraft parts. This alloy shows a relatively low ductility at room temperature and is generally heat treated in various conditions to suit particular applications. The present study experimentally and numerically analyzes the damage mechanism of an Al2024-T351 plate (short transverse direction) subjected to multi-axial stress states. The purpose of this work is to predict the cyclic lifetime of the considered alloy, based on the local approach of damage evolution using continuum damage modeling (CDM). The experimental program involves different kinds of specimens and loading conditions. Monotonic and cyclic tests have been conducted in order to measure the mechanical response and also to perform micromechanical characterization of damage and fracture processes. The cyclic plasticity behavior has been characterized by means of smooth cylindrical specimens. For analyzing the evolution of plastic deformation and damage under multi-axial stress conditions, cyclic loading tests in the low cycle regime have been conducted on different round notched bars. The predictions of the CDM were compared to the experimentally observed mechanical response and to the micromechanical characterization of damage. Emphasis was placed on the prediction of the number of cycles to failure.