Round Bar Specimens Research Articles

Fatigue Strength Estimation Based on Local Mechanical Properties for Aluminum Alloy FSW Joints.

Overall fatigue strengths and hardness distributions of the aluminum alloy similar and dissimilar friction stir welding (FSW) joints were determined. The local fatigue strengths as well as local tensile strengths were also obtained by using small round bar specimens extracted from specific locations, such as the stir zone, heat affected zone, and base metal. It was found from the results that fatigue fracture of the FSW joint plate specimen occurred at the location of the lowest local fatigue strength as well as the lowest hardness, regardless of microstructural evolution. To estimate the fatigue strengths of aluminum alloy FSW joints from the hardness measurements, the relationship between fatigue strength and hardness for aluminum alloys was investigated based on the present experimental results and the available wide range of data from the references. It was found as: σa (R = −1) = 1.68 HV (σa is in MPa and HV has no unit). It was also confirmed that the estimated fatigue strengths were in good agreement with the experimental results for aluminum alloy FSW joints.

Analysis of multiaxial low cycle fatigue of notched specimens for type 316L stainless steel under non-proportional loading

The paper analyzes multiaxial low cycle fatigue tests of notched specimens under proportional and non-proportional loading conditions. Strain controlled multiaxial low cycle fatigue tests were carried out using circumferentially notched round-bar specimens of type 316L stainless steel, with different stress concentration factors. The experimental results show that the crack initiation site is shifted from the notch tip. Based on this finding, a new model for life evaluation is proposed by taking into account the strain gradient in the proximity of the notch tip and the effective maximum strain range. The new model allows evaluating the fatigue life in a narrow scatter band.

Three dimensional simulations of texture and triaxiality effects on the plasticity of magnesium alloys

We investigate the synergistic effects of texture and intrinsic plastic anisotropy (at the level of single crystal) on the deformation of polycrystalline magnesium using three-dimensional crystal plasticity finite element simulations. Using rolled plate texture as a basis, we simulate the deformation using a range of synthetic textures. Smooth and notched round bar specimens are considered to achieve different levels of stress triaxiality. Two sets of constituent single crystal properties, representative of an Mg alloy and pure Mg respectively, are adopted in order to investigate the role of intrinsic plastic anisotropy. Our results reveal that textural variations couple into the intrinsic plastic anisotropy and triaxiality to determine the active deformation mechanisms. When loaded along the rolling direction of the plate, the deformation is accommodated by prismatic slip at low triaxiality and pyramidal 〈c+a〉 slip at high triaxiality. Softer mechanisms such as basal slip and extension twinning, which are not favored by the loading orientation, are activated due to the intergranular stresses. The smooth specimens show macroscopic strain localization the onset of which depends on initial texture. The deformed textures are strongly modulated by both triaxiality and intrinsic plastic anisotropy. We also briefly discuss the potential role of twinning in damage evolution.

A Comparison of the Plastic-Flow Response of a Powder-Metallurgy Nickel-Base Superalloy Under Nominally-Isothermal and Transient-Heating Hot-Working Conditions

The flow-stress behavior at hot-working temperatures and strain rates of the powder-metallurgy superalloy LSHR was determined under nominally-isothermal and transient-heating conditions. Two conventional methods, compression of right-circular cylinders and torsion of thin-walled tubes, were used for isothermal tests. A direct-resistance-heating technique utilizing torsion of round-bar specimens in a Gleeble® machine was applied for both isothermal and transient-heating conditions. When expressed in terms of effective stress and strain, baseline data determined by the two conventional methods showed good agreement. With the aid of a flow-localization analysis to assess the confounding influence of axial (and radial) temperature gradients on deformation uniformity, the flow stresses determined from nominally-isothermal Gleeble® torsion tests were shown to be broadly similar to those from the conventional tests. With regard to transient phenomena, Gleeble® tests were also useful in quantifying the effect of rapid heating and short soak time on the observed higher flow stress associated with a metastable microstructure. The present work also introduces two new test techniques using direct-resistance-heated torsion specimens. One involves continuous heating under constant-torque conditions, and the other comprises testing an individual specimen at a series of temperatures and strain rates. Using a single specimen, the former method enabled the determination of the apparent activation energy for plastic flow, which was similar to that determined from the series of isothermal tests; the latter provided a low-cost, high-throughput approach to quantify the flow behavior.

円周切欠きを有する丸棒試験片におけるひずみ速度集中係数の提案と応力集中係数の関係についての考察

円周切欠きを有する丸棒試験片におけるひずみ速度集中係数の提案と応力集中係数の関係についての考察

直径1mm丸棒引張型ミニチュア試験片を用いたCrMoV鍛鋼のクリープ損傷評価

Creep damage assessment of high temperature components in aged thermal power plants is necessary to maintain reliable operation. Although an accurate creep damage assessment result is obtained by a destructive assessment method in which creep tests are performed, it is difficult to cut a standard size specimen from actual components. Therefore, development of testing and damage assessment methods by using a miniature specimen are expected. In this study, creep tests of a CrMoV forging steel were conducted by using a solid bar tensile type miniature creep specimen with 1mm diameter at 600℃. It was demonstrated that equivalent creep deformation and rupture properties to those obtained from the standard size specimen are obtained by the miniature specimen under stress level below 200MPa. Creep damage materials were produced by interrupting creep tests performed by using a standard sized smooth bar specimen and two types of round notch bar specimen with radiuses at notch root of 0.5mm(R0.5) and 2.0mm(R2.0). Uniaxial tensile creep tests were performed by using the miniature creep specimens taken from the plain specimens and the notch roots of the notch specimens. Although accurate creep damage assessment results were obtained from the creep tests with the miniature creep specimens taken from the plain specimens, it should be noted that the results may give under estimation for higher creep damaged condition. Creep damage assessment results based on the creep tests with miniature specimens also gave under estimation for the creep damaged material of R0.5 due to lower axial creep strain accumulation under multiaxial stress states before interrupting the creep tests. On the other hand, accurate creep damage assessment results were obtained for the creep damage materials of R2.0 indicating that creep tests with the miniature specimens may be applicable to creep damage assessment of components under certain range of stress multiaxiality.

Life property of a round notch bar specimen on CrMoV forging steel under creep-fatigue loading

Life property of a round notch bar specimen on CrMoV forging steel under creep-fatigue loading

Experimental Study on the Coaxing Effect of Multi-Level Stresses with Different Sequences

The coaxing effect in fatigue refers to an increase in the fatigue strength of material imduced by prior cycling at a stress level below the fatigue limit. In this study, the coaxing effect of multi-level stresses in the fatigue process in the automotive drive shaft material 40Cr was investigated by applying three levels of stress amplitudes below the fatigue limit in different sequences prior to conducting the conventional constant amplitude fatigue test with standard round bar specimens. The existence of coaxing effect under multi-level stress with a load sequence effect was observed and confirmed by the results. The longest fatigue life is achieved through three levels of coaxing load with a step increase sequence, while a decrease sequence will result in the shortest fatigue life. The mechanisms of the coaxing effect which increased the fatigue life at the crack initiation and propagation stages were observed, through fracture morphology analysis, as the regulation of micropores, decreasing of pore density and the narrowed striation spacing. This finding is instrumental for extending the service life of mechanical components.

Anisotropic (Continuum Damage Mechanics)-based multi-mechanism model for semi-crystalline polymer

In this work, the anisotropic damage of semi-crystalline polymers is investigated. The model, developed within a thermodynamic framework, includes the following features: (i) the degree of crystallinity; (ii) the hydrostatic pressure effect; and (iii) the damage anisotropy. The adopted tensorial damage variable is based on the Continuum Damage Mechanics approach under the energy equivalence assumption. For the quantification of the anisotropy, a parameter called “shape factor” is defined as the ratio between the void mean diameter and the void mean height. This parameter is linked to the main axial and the main radial damage components. Experimental data taken from the recent literature using the tomography technique were selected to assess the model capability. Finite element simulations of notched round bar specimens subjected to tensile test stopped at three key loading stages are systematically compared with experimental data. The proposed model was able not only to accurately simulate the macroscopic response of the material, but more interestingly, to reproduce the spatial distribution of the shape factor. This demonstrates the anisotropy effects of the material under study induced at different stages of the deformation.

Thermo-Mechanical and Low Cycle Fatigue Failure Behavior Relevant to Temperature Regime in a TBCed Superalloy Specimen

Steady state and non-steady state thermo-mechanical fatigue failure is great concern in this work. At first steady state thermo-mechanical fatigue failure behavior was investigated using the round-bar TBC specimens, after getting basic data of mechanical properties of the bond/top coats and the substrate alloy. The failure behavior was compared with that during isothermal low cycle fatigue (LCF). Next non-steady state TMF tests were carried out in which non-steady state thermal stress was significant in the TBC specimen, compared with the properties under the steady state TMF. The experimental work clearly demonstrated that the TMF failure lives were significantly changed depending on the temperature regime during TMF and LCF. Of particular importance was found in the non-steady state TMF tests. The non-steady state TMF cycling promoted the delamination of ceramic top coat, resulting in a significant reduction in fatigue life.

Environmentally Assisted Fatigue Evaluation Model of Alloy 690 Steam Generator Tube in High-Temperature Water

Based on corrosion fatigue tests using a boat-shaped specimen in borated and lithiated high-temperature water, the corrosion fatigue behavior of Alloy 690 tubes was investigated. An Institute of Metal Research fatigue model for the nickel-based alloys is proposed, which incorporates fatigue life data of Alloy 690 tubes, as well as round-bar specimen data from the open literature. The proposed environmental fatigue life correction factor Fen addresses the effects of temperature, strain rate, and dissolved oxygen concentration. An evaluation methodology for environmental fatigue damage of structural materials in nuclear power plants is proposed.

Evaluation of multiaxial low cycle fatigue life for type 316L stainless steel notched specimen under non-proportional loading

This study discusses a multiaxial low cycle fatigue (LCF) life of notched specimen under proportional and non-proportional loading conditions at room temperature. Multiaxial LCF tests controlled by strain were carried out using a smooth and a circumferentially notched round-bar specimens of type 316L stainless steel (316LSS). Four kinds of notched specimens of which elastic stress concentration factors Kt are 1.5, 2.5, 4.2 and 6.0 were employed. The strain paths employed were proportional loading and non-proportional loading. The former is push–pull loading and reversed torsion loading and the latter is circular loading achieved by the strain path of which axial and shear strains are loaded by 90° sinusoidal out-of-phase. Total axial strain and total shear strain ranges were the same ranges based on von Mises. An evaluation of the fatigue life for the notched specimen is discussed based on the experimental results and also by employing an inelastic finite element analysis (FEA). The fatigue life is decreased with increase in Kt at each strain path and is underestimated by a maximum local strain range evaluated by FEA result. A proposed strain range defined by a local mean strain value near the notch calculated from FEA by taking into account the non-proportional effect is an appropriate parameter for life evaluation of 316LSS notched specimen under non-proportional loading.

Formation mechanism of inclined fatigue-cracks in ultrafine-grained Cu processed by equal channel angular pressing

The formation mechanism of inclined fatigue-cracks in ultrafine-grained Cu processed by equal channel angular pressing was studied by using a partially notched specimen in which a fatal natural crack was introduced, to a specific site of the smooth surface was feasible regardless of microstructural inhomogeneity caused by the processing. The crack growth direction depended on the location along the circumferential direction of the round bar specimen and on the applied stress amplitudes. The role of the microstructure and deformation mode at the crack-tip areas on the formation behavior of fatal cracks is discussed in terms of the microstructural evolution caused by cyclic stressing and the mixed-mode stress intensity factor. The in-plane shear mode deformation at the crack tip assisted the formation of the inclined crack paths and the unique crack face profile.

Role of microstructure on initiation and propagation of fatigue cracks in precipitate strengthened Cu–Ni–Si alloy

Fatigue tests were conducted on round-bar specimens to understand the fatigue behavior of precipitate-strengthened Cu–6Ni–1.5Si alloy. Aging at 500°C for 0.5h produced δ-Ni2Si precipitates in the matrix, homogeneously and heterogeneously precipitated δ-Ni2Si particles, and a precipitate-free zone around the grain boundaries. The cracks were initiated at the grain boundaries, followed by growth along the crystallographic slip planes in the adjacent grains. Crack propagation from the crack origin along the grain boundaries was occasionally observed. The physical background of fatigue damage is discussed in light of the role of microstructure on the behavior of fatigue cracks.

GS0410 JIS規格外の微小試験片によるクリープ破断特性

Some destructive techniques are required to evaluate the degradation of components for thermal power plants. However, conventional uniaxial creep specimens were sometimes too large to collect a sample from local area of target components. Miniature Creep (MC) test that employs smaller size of the specimen can be collected from the local site is well known as a better semi-destructive testing technique. Although the MC specimen of round bar needed a sample of 4 mm in thickness, that of thin plate shape can be collected under 2 mm in thickness. In this study, MC tests using thin plate and round bar specimens were performed in Argon gas and in vacuum to evaluate their creep properties. Test results were compared with uniaxial ones employing 2.25Cr-1Mo steel. It was shown that creep rupture lives of the MC plate specimens in vacuum were a similar trend to those of uniaxial creep, and that the MC test using the thin plate is able to substitute for the conventional uniaxial creep test.

多軸応力条件下におけるGRADE 92鋼のクリープ寿命評価

Creep tests on two kinds of circumferentially notched round bar specimens as well as plain bar specimen of ASME grade 92 steels were performed to obtain the multiaxial and uniaxial creep data. Then creep damage models according to strain fraction rule and energy fraction rule were developed based on the results of these uniaxial and multiaxial creep tests. Additionally, creep damage analyses using a finite element software, MSC Marc, were carried out on notched specimens and creep failure lives were predicted using the classical time fraction rule, as well as the two approaches developed here. Experimental failure lives of all the conditions of notched specimens were compared with analytical results. As a result, creep failure lives obtained by time fraction rule with von Mises equivalent stress were underestimated in the short term region and overestimated in the long term region. On the other hands, the majority of creep failure lives obtained by strain and energy fraction rule showed an agreement within a factor of two with experimental lives.

Comparison of Constant Load, SSRT and CSRT Methods for Hydrogen Embrittlement Evaluation Using Round Bar Specimens of High Strength Steels

Comparison of Constant Load, SSRT and CSRT Methods for Hydrogen Embrittlement Evaluation Using Round Bar Specimens of High Strength Steels

Growth behavior of fatigue cracks in ultrafine grained Cu smooth specimens with a small hole

In order to study the growth mechanism of fatigue cracks in ultrafine grained copper, stresscontrolled fatigue tests of round-bar specimens with a small blind hole as a crack starter were conducted. The hole was drilled on the surface where an intersection between the shear plane of the final ECAP processing and the specimen surface makes an angle of 45° or 90° with respect to the loading axis. At a low stress ( ? a = 90 MPa), the direction of crack paths was nearly perpendicular to the loading direction regardless of the location of the hole. Profile of crack face was examined, showing the aspect ratio (b/a) of b/a = 0.82. At a high stress ( ? a = 240 MPa), although the growth directions inclined 45° and 90° to the loading-axis were observed depending on the location of the drilling hole, crack faces in these cracks were extended along one set of maximum shear stress planes, corresponding to the final ECAP shear plane. The value of aspect ratios was b/a = 0.38 and 1.10 for the cracks with 45° and 90° inclined path directions, respectively. The role of deformation mode at the crack tip areas on crack growth behavior were discussed in terms of the mixed-mode stress intensity factor. The crack path formation at high stress amplitudes was affected by the in-plane shear-mode deformation at the crack tip.

Determination of fracture toughness and fatigue crack growth rate using circumferentially cracked round bar specimens of Al2014T651

Fracture toughness (KIC) and fatigue crack growth rate (FCGR) are the important material properties in fracture mechanics. ASTM-E399 and ASTM-E647 are the standards for determination of KIC and FCGR of metallic materials. These standards recommend the use of compact tension (CT) or single edge notched bend (SENB) test specimens. Literature review indicates that CT or SENB specimens are complex in nature, difficult to manufacture, require typical fixtures for loading during experimentation and the test procedures using CT or SENB are time consuming and cumbersome. An alternate specimen geometry which can overcome the above said drawbacks is needed by the industry which can be used as standard test specimen geometry. This research work explains use of circumferentially cracked round bar (CCRB) specimens of high strength Al2014T651 alloy for determination of KIC and FCGR.The pre-cracked round bar specimen was loaded in tensile in a universal testing machine and pulled till failure. Using suitable stress intensity factor equations the fracture toughness can be calculated. In case of crack growth test, the pre-cracked round bar specimen is allowed to rotate under fatigue load. The ratio of length of crack propagated to the number of cycles to failure was the crack growth rate. The SEM analysis of fractured surfaces was also done.The results are comparable with the values reported in the literature obtained by using standard test specimens. There are numerous advantages of using round bar specimen in KIC and FCGR tests. It is concluded that, the methodology of determination of fracture toughness and fatigue crack growth rate using CCRB specimens is relatively simple, reliable, fast and economical. CCRB specimen may be recommended as a standard test specimen for fracture toughness as well as crack growth tests.

Ductile fracture prediction for welded steel connections under monotonic loading based on micromechanical fracture criteria

Micromechanics-based fracture criteria for structural steels can be used to predict ductile fracture initiation with large-scale yielding and no initial flaws. High-fidelity finite-element analyses were carried out on ten welded connection specimens between the square steel tube column and H-beam flange under monotonic tensile loading. The calibrated micromechanics-based fracture criteria, including the Stress Modified Critical Strain (SMCS) model and the Void Growth Model (VGM), were used to predict fracture initiation for each specimen. The predicted results demonstrated high accuracy compared to test results. Sensitivity analyses of fracture toughness parameters in the SMCS model and VGM to the predicted fracture results were conducted. The results show that, when the fracture toughness parameters were increased or reduced by twenty percent, fluctuations in the predicted fracture results were within the acceptable range. Therefore, calibrated micromechanics-based fracture criteria can be used to predict ductile fracture initiation of connections under monotonic loading. Subsequently, the user subroutine VUMAT, from ABAQUS software edited by the authors, was employed. Using the SMCS model and VGM as fracture criteria, the post-fracture load–displacement curves of eighteen notched round bar specimens and two welded connection specimens (between the square steel tube column and H-beam flange) that fractured at different locations were traced by deleting the failure elements one by one. The predicted results agree well with the test results. Consequently, these calibrated micromechanics-based fracture criteria and associated simulating techniques can be used for collapse analysis of steel structures during extreme events.