True Fracture Strength Research Articles

Nano-amorphous—crystalline dual-phase design of Al80Li5Mg5Zn5Cu5 multicomponent alloy

The design of metallic materials with high strength, high ductility, and high thermal stability has always been a long-sought goal for the materials science community. However, the trade-off between strength and ductility remains a challenge. Here, we proposed a new strategy to design and fabricate bulk amorphous—crystalline dual-phase superior alloys out of the Al80Li5Mg5Zn5Cu5 multicomponent alloy. The nano-amorphous phase revealed unexpected thermal stability during fabrication and mechanical testing above the crystallization temperature. The true fracture strength of the Al80Li5Mg5Zn5Cu5 nano-amorphous-crystal dual-phase multicomponent alloy was increased from 528 to 657 MPa, and the true strain was increased from 18% to 48%. In addition, the alloy yielded a strength 1.5 times higher than that of the commonly used high-strength aluminum alloys at 250°C. This strategy provided a new approach and concept for the design of high-performance alloys to ensure strength—plasticity balance.

Dislocation-induced ultra-high strength in a novel steel fabricated using laser powder-bed-fusion

In this work, a novel advanced steel (Fe–10Mn-1.6Al-0.4C-0.7V, wt.%) with excellent mechanical properties of yield strength 1076 ± 44 MPa, true ultimate tensile strength 1626 ± 58 MPa and true fracture strength 1589 ± 52 MPa is successfully fabricated by laser powder-bed-fusion (L-PBF). The contributions of solid-solution strengthening, grain-boundary strengthening and dislocation strengthening to the yield strength and true fracture strength are well investigated. The dislocation strengthening contributes to the majority of strength, which accounts for ∼44% of the yield strength and ∼59% of the true fracture strength due to the high dislocation density in the L-PBF fabricated Fe–10Mn-1.6Al-0.4C-0.7V. This study provides insight not only for understanding the strengthening mechanisms of the Fe–10Mn-1.6Al-0.4C-0.7V, but also for the future development of advanced steels with outstanding properties for L-PBF.

Pre-strain effects on critical stress and hydrogen content for hydrogen-induced quasi-cleavage fracture in a TRIP-aided bainitic ferrite steel: Martensitic transformation, matrix damage, and strain aging

In this study, we characterized the details of the pre-strain effects on the microstructure, deformation/fracture behavior, and hydrogen content, with respect to the true fracture strength in the TRIP-aided bainitic ferrite steel. Three types of hydrogen embrittlement behavior were distinguished, based on pre-strain and hydrogen content. Pre-strain reduces the fraction of retained austenite, which in turn decreases the hydrogen embrittlement susceptibility when the hydrogen content is low. However, further pre-straining increases dislocation density, which has three main effects: an increase in hydrogen content, work hardening, and strain-age hardening. The increase in the hydrogen content that exceeds 4 mass ppm has been found to decrease the true fracture strength from approximately 1.5 to 1.2 GPa. The work hardening and strain-age hardening were found to increase until the critical fracture stress was achieved with respect to the strain; this led to a reduction in the elongation, particularly when the hydrogen content was high.

Mechanical Properties of Powder Metallurgy Nickel‐Based Superalloy Composite Reinforced by Low Content Graphene Nanosheets

This study focuses on the effects of adding low content of graphene nanosheets (GNSs) (≤0.3 wt%) on microstructure and tensile property of GNSs‐reinforced nickel‐based superalloy (FGH96). The GNSs‐reinforced FGH96 composite is fabricated by wet‐chemical mixing and hot isostatic pressing. Microstructures and mechanical properties of the composites are studied by scanning electron microscope, transmission electron microscope, and static tensile tests. Results indicate that, when the addition amount of GNSs is less than 0.1 wt%, the tensile properties of the GNSs‐reinforced FGH96 composites improve without losing their plasticity. The largest reduction of area (R/A) ratio of the composite is observed when 0.005 wt% GNSs is added, which is 81.4% higher than the unreinforced alloy. As the cross‐sectional area of tensile sample reduces during necking, the GNSs/unit area increases, leading to the delayed fracture with an increase in R/A ratio and true fracture strength. Theoretical analysis suggests that the strengthening mechanism of GNSs–FGH96 belongs to the modified shear lag model. The interfacial shear stress caused by the presence of GNSs is the main reason for the increase in yield strength.

A Walker exponent corrected model for estimating fatigue life of metallic materials in loading with mean stress

AbstractMean stress significantly influence the fatigue life predictions of metallic materials. The Walker mean stress equation with its additional material parameter w provides good predictions for a wide range of materials. Unfortunately, additional tests are necessary to determine the Walker exponent w. In order to overcome this shortcoming, for aluminum alloys, the Walker exponent w was correlated linearly with the sum of ultimate tensile strength and true fracture strength. Then, a Walker exponent corrected effective strain energy density criterion was developed by incorporating the Walker mean stress equation into the strain life curve. The capability of fatigue life prediction for the developed model was checked against the tested data of 304 L stainless steel, SAE 1045 steel, 7075‐T651 aluminum alloy, and Incoloy 901 superalloy, and comparisons were also performed by using the Lv's Walker exponent corrected model. The developed model provides more satisfactory results, especially for the considered materials in loading with mean stress.

Predicting the multiaxial fatigue limit and the multiaxial high-cycle fatigue life based on the unified equivalent shear stress from axial strength characteristics with various waveforms

A new method for predicting high-cycle fatigue life under constant multiaxial amplitude loading is proposed using a unified equivalent shear stress amplitude proposed by the author. This criterion is an extension of the multiaxial fatigue limit criterion proposed previously by the author. This criterion uses the axial loading S-N curve and the true fracture strength as commonly available material characteristic values. The fatigue test data from the literature such as the bending and torsion combined loadings, the biaxial loadings and the biaxial loadings with sinusoidal, triangular and trapezoidal waveforms, were used to verify the criterion. The fatigue lives estimation error and fatigue limit estimation error of the proposed criterion were better than that of the other criteria used for comparison.

多軸一定応力振幅下の高サイクル疲労寿命予測

Multiaxial high cycle fatigue life prediction under constant stress amplitude is proposed. This new criterion, which extends the proposed fatigue limit criterion by the authors in the previous papers, uses the unified equivalent shear stress amplitude. The material properties for the proposed criterion are the axial fatigue limit and the true fracture strength. The multiaxial high cycle fatigue tests for life prediction accuracy verification was carried out with S45C and SCM440. The fatigue tests were conducted under combined axial and torsion stresses with mean stress and phase difference. The criterion error is better than that of the Susmel criterion. The proposed criterion predicted the experimental fatigue life to be within a factor of 3.

Effect of Austenite Grain Size on the Mechanical Properties in Air-Cooled 0.1C-5Mn Martensitic Steel

In 0.1C-5Mn steels, 5%Mn addition increases hardening ability and makes 100% martensitic transformation even in air cooling without water quenching. Their Ms and Mf temperatures are in the range of 350-250°C, and subzero treatment is not needed. This makes it possible to measure Ms and Mf temperatures accurately by dilatometry. Utilizing a newly developed experimental technique that makes it possible to examine phase transformation behavior and conduct tensile testing with the same specimen, we examined these relationships with identical specimens and obtained the following results. Ms temperature decreases as much as 40 K with a decrease in austenite grain size from 254 to 30 m. Regarding martensite structure, the packet size and the block length decrease, while the lath width does not change, with the refinement of austenite grain size by about one tenth. True stress - true strain curves obtained up to fracture elucidates that the austenite refinement substantially improves true fracture strength and greatly increases true fracture strain of martensite, potentially invalidating the conventional concept of a trade-off balance between strength and ductility.

In situ identification of elastic–plastic strain distribution in a microalloyed transformation induced plasticity steel using digital image correlation

A non-contact strain measurement technique, based on an in-situ digital image correlation (DIC) method in association with magnetic martensite point measurement (Feritoscopy testing) was applied to study inhomogeneous deformation corresponding to martensitic transformation of a microalloyed low carbon transformation induced plasticity steel during tensile straining. The progress of inhomogeneous deformation is traced by the strain maps. The microstructural observation is used to validate the DIC results. The experimental steel shows continuous yielding with a high true fracture strength of 1410±10MPa at 25°C along with the lack of tensile necking. The DIC results show that the yield point is controlled by stress-assisted martensite transformation, which in turn induces the strain inhomogeneity. The latter starts prior to the yield point after straining to 0.016. The microstructural evolution reveals the ε-martensite is obtained through stress-assisted martensite formation. After yielding, thanks to the strain-induced martensite transformation, the deformation inhomogeneity in strain maps is increased with strain, corresponding to increasing the volume fraction of martensite. The results suggest that the continuous yielding and initial strain hardening is controlled by stress-assisted martensite formation while the higher total elongation to fracture (80%) and the tensile necking behavior is mainly influenced by the strain-induced martensite transformation.

Effect of Austenite Grain Size on Transformation Behavior, Microstructure and Mechanical Properties of 0.1C–5Mn Martensitic Steel

The effect of austenite grain size on martensitic transformation, particularly with regard to martensite structure, Ms/Mf temperatures, and mechanical properties was investigated in 0.1C–5Mn martensitic steel. Utilizing a newly developed experimental technique that makes it possible to examine phase transformation behavior and conduct tensile testing with the same specimen, we examined these relationships and obtained the following results. Ms temperature decreases as much as 40 K with a decrease in austenite grain size from 254 to 30 μm. Regarding martensite structure, the packet size and the block length decrease, while the lath width does not change, with the refinement of austenite grain size by about one tenth. Grain boundary density, especially high-angle grain boundary density, increases with decreasing austenite grain size. Tensile strength slightly increases though austenite grain size decreases about one tenth. However, reduction in area significantly improves particularly at refined grain sizes of 30 μm. True stress - true strain curves obtained up to fracture elucidates that the austenite refinement substantially improves true fracture strength and greatly increases true fracture strain of martensite, potentially invalidating the conventional concept of a trade-off between strength and ductility. Low C–5Mn martensitic steel produced from fine austenite shows a great possibility having an excellent total balance of strength, ductility and toughness.

Multiaxial high-cycle fatigue criterion considering crack initiation and non-propagation

A new criterion for multiaxial high-cycle fatigue limit is proposed using two parameters: one governing crack initiation and the other governing initial crack growth. The axial fatigue limit and the true fracture strength are used as the material properties required in estimations. The prediction accuracies using conventional criteria and the proposed criterion were compared using known data about fatigue limits under different loading conditions. The comparison was made for the effects of phase difference, mean stress, and the combination of phase difference and mean stress. The estimation error of the proposed criterion was approximately 10%, which is better than that of the other criteria used for comparison.

High‐Interstitial Stainless Austenitic Steel Castings

AbstractInterstitial atoms are most effective in strengthening austenitic steels. In stainless grades, chromium strongly reduces the solubility limit of carbon. High‐nitrogen contents require costly pressure or powder metallurgy to dissolve N in the melt. The combination of both elements comes with a high‐interstitial solubility at normal pressure of air. Sand casting with 18 mass% Cr and Mn each and 0.85 mass% (C + N) were industrially produced. The investigation revealed: proof strength Rp0.2 = 457 [MPa], true fracture strength R = 1714 [MPa], fracture elongation A = 44%, notch impact toughness KV = 290 J combined with a DBTT of −94°C, an impact wear resistance comparable to Hadfield steel X120Mn12 but combined with a good corrosion resistance. Deep freezing and cold working does not effect the low relative magnetic permeability. This unique combination of properties offers advantages in application.

Effect of mean stress on fatigue properties of 1800 MPa-class spring steels

Fatigue tests were conducted for 1800 MPa-class spring steels at various stress ratios. For comparison, similar fatigue tests were conducted for conventional steels whose tensile strength was lower than 1200 MPa. The spring steels exhibited fish-eye fractures, and the origins of these fractures were oxide, TiN and the matrix itself. In contrast, the conventional steels never exhibited fish-eye fractures. The fatigue strength of these steels decreased monotonously as the stress ratio increased, when the fatigue strength was evaluated in terms of stress amplitude. However, the fatigue strength degradation was less than that expected from a modified-Goodman line, and the best fit line was obtained by connecting the fatigue limit at zero mean stress to true fracture strength instead of tensile strength. This research also reviewed application of a power low to the stress ratio effect evaluation. In these results, the difference between the spring and conventional steels was negligible.

Mean stress effects in stress‐life fatigue and the Walker equation

ABSTRACTMean stress effects in finite‐life fatigue are studied for a number of sets of experimental data for steels, aluminium alloys and one titanium alloy. Specifically, the agreement with these data is examined for the Goodman, Morrow, Smith–Watson–Topper and Walker equations. The Goodman relationship is found to be highly inaccurate. Reasonable accuracy is provided by the Morrow and by the Smith–Watson–Topper equations. But the Morrow method should not be used for aluminium alloys unless the true fracture strength is employed, instead of the more usual use of the stress‐life intercept constant. The Walker equation with its adjustable fitting parameter γ gives superior results. For steels, γ is found to correlate with the ultimate tensile strength, and a linear relationship permits γ to be estimated for cases where non‐zero mean stress data are not available. Relatively high‐strength aluminium alloys have γ≈ 0.5, which corresponds with the SWT method, but higher values of γ apply for relatively low‐strength aluminium alloys. For both steels and aluminium alloys, there is a trend of decreasing γ with increasing strength, indicating an increasing sensitivity to mean stress.

DYNAMIC TENSILE PROPERTIES OF IRON AND STEELS FOR A WIDE RANGE OF STRAIN RATES AND STRAIN

The tensile stress-strain curves of iron and a variety of steels, covering a wide range of strength level, over a wide strain rate range on the order of 10−3 ~ 103 s −1, were obtained systematically by using the Sensing Block Type High Speed Material Testing System (SBTS, Saginomiya). Through intensive analysis of these results, the strain rate sensitivity of the flow stress for the large strain region, including the viscous term at high strain rates, the true fracture strength and the true fracture strain were cleared for the material group of the ferrous metals. These systematical data may be useful to develop a practical constitutive model for computer codes, including a fracture criterion for simulations of the dynamic behavior in crash worthiness studies and of work-pieces subjected to dynamic plastic working for a wide strain rate range.

An investigation of high-cycle fatigue models for metallic structures exhibiting snap-through response

A study is undertaken to develop a methodology for determining the suitability of various high-cycle fatigue models for metallic structures subjected to combined thermal-acoustic loadings. Two features of this problem differentiate it from the fatigue of structures subject to acoustic loading alone. Potentially large mean stresses associated with the thermally pre- and post-buckled states require models capable of handling those conditions. Additionally, snap-through motion between multiple post-buckled equilibrium positions introduces very high alternating stress. An aluminum beam structure is chosen as the computational test article, with its geometric and material nonlinear response determined via numerical simulation. A cumulative damage model is employed using a rainflow cycle counting scheme and fatigue life estimates are made for 2024-T3 aluminum using various non-zero mean stress fatigue models, including Walker, Morrow, Morrow with true fracture strength, and MMPDS. A baseline zero-mean stress model is additionally considered. It is shown that for this material, the Walker model produces the most conservative fatigue life estimates when the stress response has a tensile mean introduced by geometric nonlinearity, but remains in the linear elastic range. However, when the loading level is sufficiently high to produce plasticity, the response becomes more fully reversed and the baseline, Morrow, and Morrow with true fracture strength models produce the most conservative fatigue life estimates.

High Strength Stainless Austenitic CrMnCN Steels – Part I: Alloy Design and Properties

Generally the strength of stainless austenitic steels does not live up to their good corrosion resistance. Solid solution hardening by interstitial elements is a means of raising the strength, but is used only moderately because of poor weldability, which, however, is not required in various applications. The solubility of nitrogen is high in stainless austenite of steels with 18 mass% of Cr and Mn each, but low in the melt. Carbon reveals the opposite behaviour. Instead of producing high nitrogen steels by pressure metallurgy, about 1 mass% of C+N is dissolved in the melt at ambient pressure. The new cost‐effective C+N steel reaches a yield strength of 600 MPa, a true fracture strength above 2500 MPa and an elongation above 70 %. Conduction electron spin resonance revealed a high concentration of free electrons. Thus, the ductile metallic character of the C+N steel is enhanced, explaining the high product of strength times toughness. The high interstitial content requires rapid quenching to avoid an embrittling precipitation and respective intercrystalline corrosion.

Stainless Austenitic CrMnCN Steels of Superior Strength Part I: Alloy Design and Properties

The solubility of nitrogen is high in stainless austenite of steels with 18 mass% of Cr and Mn each, but low in the melt. Carbon reveals the opposite behaviour. Instead of producing high nitrogen steels by pressure metallurgy (PHNS), about 1 mass% of C+N is dissolved in the melt at ambient pressure. The new cost-effective C+N steel reaches a yield strength of 600 MPa, a true fracture strength above 2500 MPa and an elongation above 70 %. Conduction electron spin resonance revealed a high concentration of free electrons. Thus, the ductile metallic character of the C+N steel is enhanced, explaining the high product of strength and toughness.

Evaluation of Rolling Contact Fatigue Properties of White Layer in Railway Wheel Steel

Spalling is one of the typical rolling contact fatigue (RCF) failure of railway wheels, initiating at white layers, as the result of unexpected wheel slides. As for spalling, evaluation methods of fatigue strength have not been tried and tested. The purpose of this study is to propose a prediction method of RCF strength for crack initiation at white layers. In this study, 1) RCF tests in order to evaluate spalling behavior of wheel steels, 2) axial fatigue tests to obtain fatigue strength of white layer itself, and 3) elastic-plastic FEM analyses simulating the RCF tests to calculate stress distribution in/around white layers were conducted. As the results, the following facts were clarified. 1) In RCF tests, cracks were initiated at the leading edge of white layers particularly under slip condition. 2) Fatigue strength of the white layer can be evaluated accurately using the endurance limit diagram estimated by a straight line passing the true fracture strength and fully reversed fatigue strength. 3) According to FEM analysis results, the maximum stress at the leading edge of white layer is larger than at the trailing edge, and the maximum stress of the white layer increases by taking into consideration of small surface profile change during RCF test. Moreover, Dang Van model, which is one of the multiaxial fatigue strength evaluation methods, was applied to predict the white layer's fatigue strength in RCF tests. It can be concluded that the present prediction method is reasonably acceptable for crack initiation assessment of spalling and that in case of the existence of relatively apparent surface profile change, stress evaluation has to be done by taking it into consideration.

Deformation behavior of a Ti 66Cu 8Ni 4.8Sn 7.2Nb 14 nanostructured composite containing ductile dendrites

The deformation behavior of multicomponent Ti 66Cu 8Ni 4.8Sn 7.2Nb 14 composite containing primary dendrites embedded in a nanoeutectic matrix was studied under uniaxial compression. The evolution of slip and shear bands has been investigated at various stages of deformation. This type of advanced composite not only displays a high compressive plasticity of 31.8%, but also exhibits high fracture strength of ≈2.02 GPa (true fracture strength 1.38 GPa). Pronounced work hardening was observed after yielding. The surface deformation morphology reveals that the work hardening behavior of the composite is related to the plastic deformation of the dendritic phase through slip bands and the interaction of shear bands in the nanostructured matrix with the hardened dendrites. The nucleation of shear band occurs at a certain amount (8%) of plastic deformation at the dendrite–nanoeutectic matrix interface. The final stage of compression proceeds through passing of the shear bands through the work-hardened dendrites into the nanoeutectic matrix, whereby the propagation of the shear bands in the matrix is retarded by the nanoscale phases.