Near-threshold Fatigue Crack Propagation Research Articles

Near-threshold fatigue crack propagation in a Ni-based single crystal superalloy affected by crystallographic anisotropy

Fatigue crack propagation in the near-threshold regime was investigated experimentally and numerically in a Ni-based single crystal superalloy, namely, CMSX-4. Fatigue crack propagation tests were conducted at room temperature using four types of specimens with different primary and secondary crystallographic orientations. The results reveal that the crystallographic orientations strongly affect the fatigue crack propagation behavior, including the crack paths, fatigue crack propagation rates, and fatigue threshold. Crystal plasticity finite element analysis was conducted to quantify the slip activity of an octahedral slip system in front of the crack tip, considering the actual 3D geometry of the crack plane and elastic–plastic anisotropy. The fatigue damage parameter, considering the slip activities of the individual octahedral slip systems, provides reasonable explanation for the fatigue crack propagation rates and crack paths in the near-threshold regime, regardless of the crystallographic orientation. Furthermore, these damage parameters are equivalent at the fatigue thresholds for all specimens with different crystallographic orientation.

Grain size effect on near-threshold fatigue crack propagation in CrMnFeCoNi high-entropy alloy fabricated by spark plasma sintering

High-entropy alloys (HEAs), which comprise five or more elements and have an equiatomic composition, are known to exhibit superior static strength and ductility. The present study evaluated the effect of grain size on fatigue crack propagation in a CrMnFeCoNi alloy comprising a single-phase solid solution with a face-centered-cubic structure and fabricated by spark plasma sintering. Stress intensity factor-K decreasing tests were conducted with force ratios from 0.1 to 0.8, and no noticeable differences in fatigue threshold were observed between specimens having fine-grained and coarse-grained structures at relatively low force ratios. This result was attributed to fatigue crack reflection inside coarse grains. The magnitude of crack closure was high even in the fine-grained specimens and exceeded that of austenitic stainless steel having the same fine grains. Thus, the effect of grain size on the threshold stress intensity factor range was not significant in the present CrMnFeCoNi alloy, in contrast to the behavior of austenitic stainless steels.

Near‐Threshold Fatigue Crack Propagation in Sintered 304L Stainless Steel Compact with Network Structure Composed of High‐Entropy CrMnFeCoNi Alloy

High‐entropy alloys (HEAs) have several superior mechanical properties. However, for these alloys, there is a trade‐off between tensile strength and fatigue‐crack propagation resistance, as is the case for materials with a homogeneous microstructure. The purpose of the present study is to achieve both high‐strength and fatigue‐crack propagation resistance by using a bimodal structure. CrMnFeCoNi alloy and AISI304L powders with different particles diameters are mixed and then consolidated using spark plasma sintering to fabricate sintered compacts with a network structure composed of coarse‐grained 304L surrounded by fine‐grained HEA microstructure. Stress intensity factor K‐decreasing tests are conducted at various force ratios in the ambient laboratory atmosphere to examine near‐threshold fatigue‐crack propagation in the sintered compacts. The ductility and threshold stress intensity range ΔKth tested at a high force ratio for the sintered compacts are higher than that for a homogeneous fine‐grained CrMnFeCoNi alloy. Fatigue cracks in the sintered compacts mainly propagate through the coarse‐grained 304L microstructure, which has high‐fatigue‐crack propagation resistance near the threshold. Therefore, ΔKth for the sintered compacts increases because the ΔKeff,th, which is derived from the 304L without degrading the tensile strength, is higher than that for homogeneous fine‐grained CrMnFeCoNi alloy.

Anomalous fatigue crack propagation behavior in near-threshold region of L-PBF prepared austenitic stainless steel

Laser powder bed fusion (L-PBF) process produces a specific non-equilibrium microstructure with properties significantly different from those of conventionally processed materials. In the present study, the near-threshold fatigue crack propagation in austenitic stainless steel 304L processed by L-PBF was investigated. Three series of specimens with different orientation of initial notch with respect to build direction were manufactured in order to evaluate the effect of specimen orientation on the near-threshold fatigue crack propagation behavior. The results showed absence of the orientation dependence of the fatigue crack propagation behavior. In addition, abnormally low threshold stress intensity factor values were recorded, which were attributed to the absence of crack closure even at low load ratio (R = 0.1). In order to explain observed behavior, the specimens of selected orientation were heat treated to relieve build-in residual stresses (HT1) and to create recrystallized microstructure (HT2) comparable to conventionally processed (wrought) stainless steels. It was found that the characteristic microstructure produced by L-PBF is the main reason for the absence of crack closure and the low threshold values at low load ratios. As-built microstructure containing sub-micron dislocation cell-substructure is prone to cyclic instability. In the crack tip region, cyclic plasticity results in strain-induced phase transformation and continuous thin martensitic layer is formed in the crack vicinity. The induced martensite phase is softer compared to the austenite matrix strengthened by cell-substructure. Together with cyclic instability of the matrix, the macroscopic cyclic softening occurs as the result within the crack tip region. Under such conditions, the formation of the plasticity-induced and roughness-induced crack closure is significantly reduced and macroscopic resistance to the fatigue crack propagation is low.

Development of Periodic TiB-Reinforced Titanium via Powder Metallurgy and Its Near-Threshold Fatigue Crack Propagation

Development of Periodic TiB-Reinforced Titanium via Powder Metallurgy and Its Near-Threshold Fatigue Crack Propagation

Near-threshold fatigue crack propagation without oxide-induced crack closure

An accurate value for the threshold stress intensity factor range, ΔKth, is a key parameter for studying crack-like defects. However, it is difficult to obtain accurate ΔKth values due to oxide-induced crack closure. In this study, we report conditions for minimizing the effects of oxide-induced crack closure near the threshold region, where a concave curve of the effect on the loading frequency on oxide-induced crack closure is achieved. The resulting conditions allow for an accurate determination of ΔKth, which is a key material parameter relating to the pertinent loading ratio.

Effect of TiB Orientation on Near-Threshold Fatigue Crack Propagation in TiB-Reinforced Ti-3Al-2.5V Matrix Composites Treated with Heat Extrusion.

TiB-reinforced Ti-3Al-2.5V matrix composites, in which TiB whiskers are oriented parallel to the direction of heat extrusion, were fabricated via mechanical alloying and hot isostatic pressing (HIP). To investigate the near-threshold fatigue crack propagation in TiB-reinforced Ti-3Al-2.5V matrix composites, stress intensity factor K-decreasing tests were conducted for disk-shaped compact specimens having two different orientations of TiB whiskers at force ratios from 0.1 to 0.8 under ambient conditions. The crack growth rates, da/dN, for the composites incorporating TiB whiskers oriented perpendicular to the direction of crack growth were constantly lower than those obtained in the case where the orientation was parallel at the same stress intensity range ΔK, while the threshold stress intensity range, ΔKth, was higher. This effect can be explained by the increase in the degree of roughness-induced crack closure resulting from the perpendicular TiB, because fatigue cracks preferentially propagated across the boundaries between the matrix and the TiB in certain regions. In contrast, the effective threshold stress intensity range, ΔKeff,th, for composites was unaffected by the TiB orientation at low force ratios.

Mechanisms of fatigue crack propagation in a Q&P-processed steel

The fatigue crack propagation (FCP) behavior of a quenched and partitioned (Q&P), low-alloyed CSiCrMn steel containing 12% of retained austenite is investigated in ΔK-controlled tests at different R-ratios and compared with its quenched and tempered (QT) counterpart, containing 4% of retained austenite. Characteristic features of the crack wake and FCP surfaces are examined by scanning electron microscopy and related to the presence or absence of martensitic transformation. Comparing the FCP behavior of the Q&P condition with its QT counterpart, the fatigue threshold ΔKth is higher, and fatigue crack growth rates in the Paris-regime are lower, for Q&P. With an increase of ΔK, Paris-lines intersect and, at high ΔK, the fatigue crack growth resistance of the Q&P condition becomes inferior. Such FCG behavior is attributed to the amount and stability of retained austenite: at low ΔK, contributions of the martensitic transformation are negligible, and thus a higher fraction of retained austenite accounts for an increased FCG resistance. With increasing ΔK, an increasing percentage of retained austenite in the cyclic plastic zone at the crack tip is transformed, providing brittle pathways for fast crack propagation and thus lowering FCG resistance. Analysing the crack surface, cleavage-like intergranular crack propagation can be directly related to martensitic transformation: in the threshold region, FCP occurs predominantly in a transgranular mode and the percentage of cleavage planes increases as ΔK increases. This study is the first one that examines the FCP behavior in a Q&P-processed steel and one of very few which deal with the near-threshold FCP behavior in TRIP steels.

Evaluation of near-threshold fatigue crack propagation in harmonic-structured CP titanium with a bimodal grain size distribution

To examine near-threshold fatigue crack propagation in commercially pure (CP) titanium with a bimodal harmonic structure, which is defined as a coarse grained structure surrounded by a network of fine grains, K-decreasing tests were conducted. The crack growth rates for harmonic-structured CP titanium were higher than those for homogeneous material with coarse grains at comparable stress intensity range, while the threshold stress intensity range was lower. This phenomenon was attributed to a reduction in the extent of crack closure and the effective threshold stress intensity range, resulting from the presence of fine grains in the harmonic structure.

Crack closure in near-threshold fatigue crack propagation in railway axle steel EA4T

The paper is focused on fatigue crack propagation in the threshold area of the EA4T railway axle steel. Railway axles represent one of the most critical components for safety of trains. The main aim of this paper is to describe fatigue crack closure mechanisms in EA4T, which significantly influence the threshold value – a crucial parameter for determination of residual fatigue lifetime of railway axles. Investigated fatigue crack closure mechanisms are: plasticity induced crack closure, roughness-induced crack closure and oxide-induced crack closure. One part of this paper is focused on comparison of crack closure determined by NASGRO relationship (including plasticity induced crack closure mechanism only) with the experimentally obtained one. Difference between these two approaches is explained by presence of roughness and oxide induced crack closure. Based on theoretical models different sources of fatigue crack closure mechanisms was separated and evaluated. The significance of oxide-induced crack closure was confirmed based on experimental measurement of fatigue crack propagation in low humidity air. Finally, the effect of a reduced threshold value in low-humidity air on residual fatigue lifetime of railway axles is demonstrated.

Near-threshold fatigue crack propagation behavior of austenitic high-Mn steels

High-Mn austenitic steels utilizing TWIP (twinning induced plasticity) effect have excellent combination of tensile strength and ductility. The near-threshold fatigue crack propagation (FCP) behavior, as represented by the ΔKth value, of high-Mn steels was examined with the emphasis on the effect of stacking fault energy (SFE), grain size, twinning and tensile properties. Even though no predominant parameter determining the near-threshold FCP behavior of high-Mn steels was found, the SFE showed the most reasonable correlation to the ΔKth values among the variables examined. It was also suggested that the slip reversibility as determined by SFE could not solely explain the near-threshold FCP behavior of high-Mn steels. The presence of twin boundaries appeared to be not beneficial in improving the resistance to FCP of high-Mn steels in low ΔK regime. The near-threshold FCP characteristics of high-Mn austenitic steels were discussed and correlated with the well-known parameters presumably affecting FCP.

Effect of harmonic structure design with bimodal grain size distribution on near-threshold fatigue crack propagation in Ti–6Al–4V alloy

Titanium alloy (Ti–6Al–4V) with a bimodal harmonic structure, which consists of a coarse-grained structure surrounded by a network structure of fine equiaxed grains, has been fabricated by sintering mechanically-milled powders to achieve high strength and good plasticity. To investigate the near-threshold fatigue crack propagation in the harmonic structured Ti–6Al–4V alloy, K-decreasing tests are conducted on disk-shaped compact specimens (ASTM standard) under stress ratios R from 0.1 to 0.8 with a constant-R loading regime in a laboratory atmosphere. The fracture surfaces are observed using scanning electron microscopy (SEM), and crack profiles are analysed using electron backscatter diffraction (EBSD) to discuss the mechanism of the fatigue crack propagation. The crack growth rates da/dN in the harmonic structured material are constantly higher than those in a material with coarse acicular microstructure under comparable stress intensity range ΔK, while the fatigue thresholds ΔKth are lower. This is attributed to a decrease in the magnitude of roughness-induced crack closure and the effective stress intensity range ΔKeff,th in the harmonic structured Ti–6Al–4V alloy due to the presence of fine grains. Furthermore, in some areas, fatigue cracks do not propagate in the coarse-grained structure with higher fatigue crack growth resistance, but they preferentially propagate across the network structure of fine grains in the harmonic structure.

Ultra-slow Fatigue Crack Propagation in Metallic Alloys

The influence of frequency (20kHz ultrasonic tests and conventional 20 to 35Hz tests) and environment (air and vacuum) on near-threshold fatigue crack propagation of three metallic alloys, Ti-6Al-4V, 2024-T351 and 12% Cr stainless steel is compared experimentally. The effective stress-intensity factor which is considered as the propagation driving force is determined from closure measurements or tests run at high R-ratio. Based on microfractographic observations, the results are discussed in terms of a preexisting model for intrinsic and environmentally assisted fatigue crack propagation.

Evaluation of near-threshold fatigue crack propagation in Ti-6Al-4V Alloy with harmonic structure created by Mechanical Milling and Spark Plasma Sintering

Titanium alloy (Ti-6Al-4V) having a bimodal “harmonic structure”, which consists of coarsegrained structure surrounded by a network structure of fine grains, was fabricated by mechanical milling (MM) and spark plasma sintering (SPS) to achieve high strength and good plasticity. The aim of this study is to investigate the near-threshold fatigue crack propagation in Ti-6Al-4V alloy with harmonic structure. Ti-6Al-4V alloy powders were mechanically milled in a planetary ball mill to create fine grains at powder’s surface and the MM-processed powders were consolidated by SPS. K-decreasing fatigue crack propagation tests were conducted using the DC(T) specimen (ASTM standard) with harmonic structure under the stress ratios, R, from 0.1 to 0.8 in ambient laboratory atmosphere. After testing, fracture surfaces were observed using scanning electron microscope (SEM), and crack profiles were analyzed using electron backscatter diffraction (EBSD) to discuss the mechanism of fatigue crack propagation. Threshold stress intensity range, ?Kth, of the material with harmonic structure decreased with stress ratio, R, whereas the effective stress intensity range, ?Keff, showed constant value for R lower than 0.5. This result indicates that the influence of the stress ratio, R, on ?Kth of Ti- 6Al-4V with harmonic structure can be concluded to be that on crack closure. Compared to the compact prepared from as-received powders with coarse acicular microstructure, ?Kth value of the material with harmonic structure was low. This was because the closure stress intensity, Kcl, in the material with harmonic structure was lower than that of the coarse-grained material due to the existence of fine grains. In addition, the effects of the grain size on the fatigue crack propagation behaviors of Ti-6Al-4V alloy were investigated for the bulk homogeneous material. The effects of the stress ratio and the grain size on the fatigue crack propagation of the material with harmonic structure were quantified.

Comparative studies on near-threshold fatigue crack propagation behavior of high manganese steels at room and cryogenic temperatures

The near-threshold fatigue crack propagation behavior of high manganese steels, including 24Mn4Cr, 22Mn3Cr, 20Mn, 18Mn and 12Mn steels, were examined at 298, 173 and 113K, and the results were compared to those of widely used cryogenic alloys, including STS304, 9% Ni and 5% Ni steels. The near-threshold FCP behavior of high manganese steels was in general excellent at both room and cryogenic temperatures as compared to the reference cryogenic steels. The near-threshold ∆K values tended to be inversely proportional to the stacking fault energy values of high manganese steels. The fractographic analysis suggested that the SFE mostly affected the slip reversibility of high manganese steels in near-threshold ∆K regime.

Factors influencing fatigue crack propagation behavior of austenitic steels

In the present study, the fatigue crack propagation (FCP) behaviors of austenitic single phase steels, including STS304, Fe18Mn and Fe22Mn with different grain sizes ranging from 12 μm to 98 μm were investigated. The FCP tests were conducted in air at an R ratio of 0.1 using compact tension specimens and the crack paths and fracture surfaces were documented by using an SEM. The highest ΔKth value of 9.9MPa·m1/2 was observed for the Fe18Mn specimen, followed by 5.2MPa·m1/2 for the Fe22Mn specimen and 4.6MPa·m1/2 for the STS304 specimen, showing a substantial difference in the near-threshold FCP resistance for each microstructure. The crack path and fractographic analyses suggested that the near-threshold FCP behavior of these austenitic steels was largely influenced by the degree of slip planarity, as determined by stacking fault energy and grain size, rather than the tensile properties. In the Paris’ regime, the slip planarity still played an important role while the tensile properties began to affect the FCP. The FCP behavior of austenitic steels with different microstructural features are discussed based on detailed fractographic and micrographic observations.

Effect of Grain Size and Load Ratio on the near Threshold Stress Intensity Factor during Fatigue Crack Propagation of Dual Phase Steel

Fatigue crack growth near-threshold stress intensity factor is affected by the microstructure of the material. A large portion of microstructural influence is due to the change in grain size of the material. Grain size in the dual phase steel was varied and found that the near-threshold stress intensity factor (rKth) increased as the grain size increased. Influence of load ratio nearthreshold fatigue crack propagation was also studied. It was observed that the near-threshold stress intensity range, rKth for fatigue growth decreased with increasing load ratio.

Near-Threshold Fatigue Crack Propagation in an ECAP-Processed Ultrafine-Grained Aluminium Alloy

In the present work, the near-threshold fatigue crack propagation (FCP) at different load ratios is studied for an aluminium alloy processed by equal-channel angular pressing (ECAP). The conditions under investigation represent different stages of microstructural refinement as well as a ductility-optimized condition with superior crack growth properties, obtained by a combination of ECAP and aging. The results show a strong dependency of the threshold and its load ratio sensitivity on the grain size and grain size distribution. These observations can be rationalized on the basis of crack path tortuosity and the contribution of (roughness-induced) crack closure. Moreover, the experimental data is evaluated using the two-parametric concept of Vasudevan and Sadananda, which employs two necessary minimum conditions for crack growth, namely a critical cyclic K*th, and a critical maximum stress intensity K*max. The application of this concept shows a strong interaction of both parameters for all ECAP-processed conditions, where the ductility-optimized condition reveals superior FCP properties compared to the “as-processed” conditions.

Near-threshold fatigue crack propagation and internal cracks in steel

The existence or non-existence of fatigue crack growth thresholds is of practical importance and scientific interest, as is knowledge of the stresses enabling crack formation at the surface and the interior of a specimen. Near-threshold measurements have been performed on 12% chromium steels with an ultrasound testing technique at different mean loads in ambient air and vacuum at very high numbers of cycles. Microstructural features and fracture surfaces were investigated in an SEM. The fracture appearance after cycling in vacuum in the near threshold regime is compared with that of the fine granular and smooth areas of internal “fish-eyes”.

Influence of the remelting process on the fatigue behavior of electron beam irradiated UHMWPE

Electron beam irradiation at doses below 150 kGy is a widely used technique to obtain highly crosslinked ultra-high-molecular-weight polyethylene (UHMWPE). Its current use in total joint replacement components may improve wear resistance and decrease UHMWPE particle debris. However, currently used post-irradiation thermal treatments, which aim to decrease the free radicals within the material, introduce microstructural changes that affect UHMWPE mechanical properties, particularly the fatigue strength. This influence may be crucial in total knee replacements, where fatigue-related damage limits the lifespan of the prosthesis. Therefore, more studies are required to understand UHMWPE fatigue after current crosslinking protocols. This study was planned to evaluate the influence of UHMWPE remelting after irradiation on the material fatigue resistance. The remelting was achieved at 150 degrees C for 2 h on UHMWPE previously irradiated at 50, 100, and 150 kGy. Fatigue evaluation included short-term tests under cyclic tensile stress with zero load ratio, R = 0, and 1 Hz. In addition, stress-life testing was performed using 12% yield as the criterion for failure. Near-threshold fatigue crack propagation experiments were also performed at a frequency of 5 Hz, and crack length was measured in nonthermally treated and remelted irradiated UHMWPE. Crystallinity percentage was calculated from DSC measurements. The results pointed out that irradiation positively contributed to total life analysis, but the further remelting process decreased the flaw initiation resistance. On the other hand, both processes negatively affected the fatigue resistance of notched components. From a clinical point of view, the results suggest that the material fatigue behavior should be carefully studied in new UHMWPE to avoid changes related to material processing.