Threshold Stress Intensity Range Research Articles

A threshold criterion of stress intensity range in a steel sheet

Fatigue fractures often originate and propagate from geometric discontinuities or material defects in equipment or components. The occurrence and propagation of cracks near the fatigue limit were observed in notched specimens of a steel sheet that simulated this situation, and analyzed using fracture mechanics parameters. The notch caused strain concentration, and local plastic strain arose even at low stress amplitudes. Transgranular fracture occurred, and arrested cracks were observed near the threshold stress amplitude Δσth. The threshold stress intensity range ΔKth had a good power function correlation with the effective notch length aef, but it depended on the length and did not reach a constant value. By identifying and applying the critical power exponent, we proposed a prediction model using the function ΔKthc=Yπ1/2Δσthaef1/κ, which is the critical threshold stress intensity range that has a constant value. κ is equal to 5 for the steel sheet under transverse bending. Since ΔKth c obtained using this critical exponent method does not depend on the crack length, it can be used as a material constant and a failure criterion, and can be applied as a new strength prediction model.

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.

The effects of thermo-mechanical processing on fatigue crack propagation in commercially pure titanium with a harmonic structure

Commercially pure (CP) titanium having a bimodal harmonic structure, in which coarse grains are enveloped by a network consisting of fine grains, was obtained by consolidating mechanically milled powders to enhance both strength and ductility. We have examined the fatigue properties of materials having harmonic structures to achieve sufficient performance for practical applications in the engineering fields. In this study, thermo-mechanical processing (TMP) was performed for the CP titanium having a bimodal harmonic structure to improve its fatigue properties. Four-point bending fatigue tests and fatigue crack propagation tests were conducted under ambient conditions to investigate the fatigue crack initiation and propagation in CP titanium having a harmonic structure following TMP. Applying TMP to CP titanium having a harmonic structure increased its fatigue life due to greater resistance to the initiation of fatigue crack as a result of grain refinement. In contrast, CP titanium having a harmonic structure shows lower threshold stress intensity range, ΔKth, values following TMP as a result of reduced the degree of crack closure and lower effective threshold stress intensity range, ΔKeff,th, values due to grain refinement.

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.

Effect of bimodal harmonic structure on fatigue properties of austenitic stainless steel under axial loading

Axial fatigue tests and fatigue crack propagation tests were conducted to investigate the fatigue properties of austenitic stainless steel (JIS-SUS304L) with a harmonic structure (HS), which is defined as a coarse-grained structure surrounded by a network of fine grains. Fatigue cracks initiated at the coarse-grained structure in the HS SUS304L, but the fatigue limit was increased by the HS design due to the grain refinement. In contrast, the crack growth rates for the HS material for long cracks were constantly higher, and its threshold stress intensity range were lower compared to a material with a homogenous coarse-grained structure.

Evaluation of Fatigue Properties under Four-point Bending and Fatigue Crack Propagation in Austenitic Stainless Steel with a Bimodal Harmonic Structure

Austenitic stainless steel (JIS-SUS304L) with a bimodal harmonic structure, which is defined as a coarse-grained structure surrounded by a network of fine grains, was fabricated using powder metallurgy to improve both the strength and ductility. Four-point bending fatigue tests and K-decreasing tests were conducted in air at room temperature under a stress ratio R of 0.1 to investigate fatigue crack propagation in SUS304L. The fatigue limit of this harmonic-structured material was higher than that of the material with a homogeneous coarse-grained structure. This is attributable to the formation of fine grains by mechanical milling and to the suppression of pore formation. In contrast, the threshold stress intensity range, DKth, for the harmonic-structured material was lower than that for the homogeneous coarse-grained material, while the crack growth rates, da/dN, were higher at comparable DK. These results can be attributed to a reduction in the effective threshold stress intensity range, DKeff,th, due to the presence of fine grains in the harmonic structure.

Microstructure and mechanical anisotropy of CoCrW alloy processed by selective laser melting

We have investigated the correlation between the microstructure and mechanical anisotropy of biomedical CoCrW alloy processed by selective laser melting (SLM). A phase analysis revealed that the as-SLM CoCrW alloy consisted of mostly the γ-fcc phase and little ε-hcp phase. The as-SLM CoCrW microstructure featured the rotated cube texture, a strong <100> texture along the building direction (BD) and a weaker <110> texture along the front direction. To investigate the mechanical anisotropy, the external loading was applied either perpendicular or parallel to the BD (i.e., horizontal loading (⊥BD) or vertical loading (//BD), respectively). The rotated cube texture leads to a higher elastic modulus (~ 2.4 times), plastic flow stress, and microhardness in the horizontal loading. Macroscopic plastic anisotropy was elucidated by the Taylor factor analysis of polycrystalline aggregate. Strain-induced martensite transformation (SIMT) occurring in the horizontal loading (ef = 6.9%) is responsible for significantly reduced elongation to fracture compared to the vertical loading (ef = 32.8%). In the fatigue crack growth experiment, the threshold stress intensity range in the vertical loading is ~ 32% higher compared to the horizontal loading, leading to a retarded fatigue crack growth rate. A vertically elongated columnar granular structure due to the epitaxial grain growth along the BD results in anisotropic behavior of the fatigue crack growth.

Durability of self-healing dental composites: A comparison of performance under monotonic and cyclic loading

Durability is an important quality of dental restorative materials, and the ability to autonomously heal damage incurred during their oral function is highly desirable. ObjectiveThe objective was to evaluate the improvement in durability of self-healing dental composites (SHDCs) in terms of their resistance to fracture and capacity for healing of damage under monotonic and cyclic loading. MethodsSHDCs were prepared by incorporating dental resin composites with microcapsules containing healing liquid. Control specimens with the same mass fraction (5% and 25%) of microcapsules filled with water were also evaluated. Two sets of SHDCs were distinguished by the silane coupling agents that functionalized and bonded the microcapsules to resin network. One set used a methacrylate silane (MA-silane) that connected resin network through covalent bonds, and the other used a H-bonding forming hydroxyl silane (OH-silane). The fatigue crack growth resistance was assessed in terms of the threshold stress intensity range and the conventional Paris Law parameters. Cyclic loading was conducted at 5 Hz with maximum cyclic load ranged between approximately 1 N and 5 N. The efficiency of the autonomous healing was determined per the recovering of the fracture toughness and the extension of fatigue life. ResultsThe SHDCs with 5 wt% of healing microcapsules exhibited a larger fracture toughness than those with 25 wt% microcapsules. MA-silane SHDCs had approximately five times more responsive microcapsules triggered by fracturing of the composites. Consequently, the MA-silane SHDCs with 5 wt% of microcapsules achieved the best performance in terms of fracture toughness and healing efficiency. In regards to the fatigue crack growth behavior, there was a significant increase in the resistance to fatigue crack growth and 580 ± 15% improvement in the fatigue life. SignificanceStrong silanization is vital in SHDCs to simultaneously achieve clinically applicable mechanical performance and substantial healing capability. Moreover, the evaluation of self-healing under cyclic loading is a promising tool in quantifying the degree of fracture-induced healing.

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.

Fatigue crack propagation in UFG Ti grade 4 processed by severe plastic deformation

Fatigue crack growth was investigated in commercially pure Ti grade 4 processed by severe plastic deformation. The ultrafine-grained structure was prepared by means of equal channel angular pressing technique followed by cold drawing. The growth of fatigue cracks was investigated also in the as-received coarse-grained state of material for comparison.The fatigue crack growth rate in the ultrafine-grained material was found to be higher and the threshold stress intensity range ΔKth for crack growth lower than in the coarse-grained Ti, 2.5 and 4.7MPam1/2, respectively. A combination of transcrystaline and intercrystaline crack growth was typical for all crack growth rates in the coarse-grained material. Contrary to the combined fracture mechanism in coarse-grained Ti only transcrystalline crack growth was acharacteristic feature of crack propagation in the ultrafine-grained material.It has been found that the experimentally and theoretically determined values of closure are in reasonable agreement. Crack closure calculated according to the Newman model taking into account only plasticity induced closure was in the range from 67% to 69% of Kmax for as-received and 70% for UFG state of material. Crack closure determined experimentally was 67% of Kmax for as-received and 67% for UFG CP Ti grade 4.Investigation of fracture surfaces did not bring any evident signs of the effects of fracture surface morphology resulting in roughness induced closure or oxide induced closure under testing conditions used. This indicates that plasticity induced closure seems to be a dominant closure mechanism in both states of the investigated Ti grade 4.

Small internal fatigue crack growth rate measured by beach marks

Internal fatigue crack growth rates were measured using the beach marks created by repeated two-step fatigue tests on a high-strength steel. This material revealed internal fractures originating from oxide-type inclusions whose sizes ranged from 14 to 40µm. Large and small beach marks were observed depending on the conditions under which the repeated two-step fatigue tests were carried out. Small beach marks indicated small internal cracks just after crack initiation, while large beach marks indicated internal cracks in the final stages. The small internal cracks showed extremely slow growth rates that were much smaller than the lattice length: the measured growth rates were very close to those calculated using fracture mechanics. The large internal cracks showed conventional growth rates, larger than the lattice length, and the border between the large and small internal cracks was almost equal to the threshold stress intensity range ΔKth. These results show the validity of evaluating the fatigue lives by calculating the crack propagation lives of the small internal cracks as in the Tanaka-Akiniwa model. However, it has been suggested that the Tanaka-Akiniwa model overestimates the effects of inclusion sizes, meaning that the Tanaka-Akiniwa model has room for improvement. To correct this problem, we propose a new model, based on a new crack growth law, in which the crack growth rate also depends on crack size. The new model generates more realistic predictions.

Synergy of crack closure, near-tip residual stress and crack-tip blunting in crack growth under periodic overloads – A fractographic study

Fatigue crack growth tests were performed on an Al-Cu alloy under specially designed load sequences with a periodic overload cycle. These overloads were either immediately followed by or preceded by underloads of controlled magnitude. Baseline loading magnitude was designed to induce near-threshold conditions well below the Paris Regime. The fatigue fracture surfaces obtained from the tests were subject to fractographic analysis. The study shows that irrespective of their magnitude, underloads preceding tensile overloads do not affect crack growth rate after the overload. However, if the underload follows an overload, it carries a deleterious effect on retardation, whose magnitude is determined by the extent to which the underload drops below baseline cycling level. In either case, observed effects do not seem to have anything to with crack closure, but rather, are attributable to the effect of underloads on near-tip residual stress during baseline cycling that in turn controls threshold stress intensity range. Post-overload crack closure is reduced by crack-tip blunting, but rapidly recovers.

コンプライアンスによるナノ結晶電着ニッケル薄膜の疲労き裂長さの自動計測とΔ&lt;i&gt;K&lt;/i&gt;漸減試験

In fatigue crack propagation tests of thin films, crack length has been measured by an optical microscope. In the present study, a fatigue crack propagation testing system was developed for thin film by the automatic measurement for crack length with compliance. Nickel thin film used for this test was prepared by electrodeposition method. Two types of electrodeposited nickel thin films with different grain sizes were prepared by changing the concentration of brightener (0.0 g/L and 0.8 g/L). As results of EBSD analysis, the grain size of nickel thin film tended to decrease with increasing the additive amount of brightener. ΔK-decreasing tests were conducted on nickel thin films under stress the ratio R of 0.3 in a laboratory atmosphere. The automatic measurement of crack length with compliance was effective in fatigue crack propagation tests for nickel thin film, and the threshold stress intensity range, ΔKth, which obtained in our developed tensing system, showed a tendency to decrease with decreasing grain size.

調和組織を有するTi-6Al-4V合金の微小疲労き裂発生および伝ぱ挙動

Titanium alloy (Ti-6Al-4V) with bimodal “harmonic structure”, which consists of a coarse-grained structure surrounded by a network structure of fine-grains, was produced by sintering mechanically milled powder to achieve high strength and good plasticity. In this study, we examine the behavior of small fatigue crack initiation and propagation of Ti-6Al-4V alloy with harmonic structure. In the material with harmonic structure, fatigue crack growth rate (da/dN) was high and threshold stress intensity range (ΔKth) was low compared to the material with coarse acicular structure. On the other hand, the material with harmonic structure showed higher fatigue limit. This is because the fine-grained microstructure, which shows high fatigue strength, is formed in the harmonic structure. However, their fatigue crack growth resistance was lower compared to the coarse-grained microstructure.

Damage tolerance and classic fatigue life prediction of a helicopter main rotor blade

Fatigue life of the main rotor blade of a helicopter is investigated from two different approaches. The blade is comprised of many different parts such as spar, skin, abrasive strip, doublers, box beam doublers, etc. and it undergoes complex random load spectra. A finite element analysis is carried out to assess the stress distribution in the blade parts. Then, using the Miner’s rule the fatigue life of the blade is calculated following a classic life estimation method under random loading. At the next step, damage tolerance approach is used for life estimation of the blade. Fatigue crack growth properties and threshold stress intensity range of the spar is obtained based on ASTM E647 test method. Then, crack growth analysis is done using Zencrack software. In order to calculate the length of the smallest growing cracks and their fatigue crack growth life, no crack growth and slow crack growth approaches are used. A comprehensive investigation is carried out on the effect of initial crack length and aspect ratio on growth or no growth of cracks. Comparing the obtained fatigue life from two different approaches, some conclusive results are drawn.

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.

Damage tolerance approach for analyzing a helicopter main rotor blade

In this article, damage tolerance approach is used for life estimation of the main rotor blade of a helicopter under random loading. At first, fatigue crack growth properties and threshold stress intensity range of the spar is obtained based on ASTM E647 test method. Then, crack growth analysis is done using Zencrack software. In order to calculate the length of the smallest growing cracks and their fatigue crack growth life, no crack growth and slow crack growth approaches were used. A comprehensive investigation was carried out on the effect of initial crack length on growth or no growth of cracks.

Effect of water vapor pressure on fatigue crack growth in Al–Zn–Cu–Mg over wide-range stress intensity factor loading

Investigation of the deleterious effect of high purity water vapor on fatigue crack growth rate (da/dN) in 7075-T651 shows that growth rate declines with decreasing water vapor pressure (PH2O) over a wide range of stress intensity range (ΔK). Decreasing ΔK experiments exhibit a novel minimum in da/dN for intermediate PH2O and low ΔK. This minimum and testing protocol dependent environmental-fatigue behavior are explained by crack wake fracture morphology and R-dependent crack opening displacement effects on molecular flow to the crack tip. This environmentally-based mechanism leads to the onset and reversal of a non-conservative and false high threshold stress intensity range.

Differences in the microstructure and fatigue properties of dentine between residents of North and South America

Spatial variations in the microstructure of dentine contribute to its mechanical behaviour. ObjectiveThe objective of this investigation was to compare the microstructure and fatigue behaviour of dentine from donors of two different countries. MethodsCaries-free third molars were obtained from dental practices in Colombia, South America and the US to assemble two age-matched samples. The microstructure of the coronal dentine was evaluated at three characteristic depths (i.e. deep, middle and superficial dentine) using scanning electron microscopy and image processing techniques. The mechanical behaviour of dentine in these three regions was evaluated by the fatigue crack growth resistance. Cyclic crack growth was achieved in-plane with the dentine tubules and the fatigue crack growth behaviour was characterized in terms of the stress intensity threshold and the Paris Law parameters. ResultsThere was no difference in the tubule density between the dentine of patients from the two countries. However, there were significant differences (p≤0.05) in the tubule lumen diameters between the two groups in the deep and peripheral regions. In regards to the fatigue resistance, there was a significant increase (p≤0.05) in threshold stress intensity range, and a significant decrease in fatigue crack growth coefficient with increasing distance from the pulp in teeth from the US donors. In contrast, these properties were independent of location for the dentine of teeth from the Colombian donors. ConclusionsThe microstructure of dentine and its mechanical behaviour appear to be a function of patient background, which may include environmental factors and/or ethnicity.

An Outrun Competition of Corrosion Fatigue and Stress Corrosion Cracking on Crack Initiation in a Compressor Blade

The first row rotating blades of four axial-flow compressors fractured prematurely. Previous investigations had shown that the atmosphere of the site contained corrosive compounds which lead to an increase in possibility of pitting of the blades. It was also revealed that the crack was originated from two corrosion pits. Thus, this work is conducted to ascertain which of the fatigue or stress corrosion cracking (SCC) mechanisms is predominant in transition of pits into initial cracks. To this end, experimental, numerical and theoretical studies are considered. Replica testing, Scanning Electron Microscope (SEM) and fractography of the broken blade indicate that the pits join together and make one bigger pit under SCC mechanism which reduces the failure time. 3-D models of the pitting on the blade under existing forces are analyzed by ABAQUS software. The simulation results show the location of maximum stress concentration inside one of the pits which is compatible with the location of initial SCC crack. Theoretical and numerical analysis show that stress intensity range (ΔK ) around the pits is smaller than threshold stress intensity range ( ΔKth ); thus, cracks initiate and propagate at the mouth of the pits under SCC mechanism. Then, the pits join together and make one equivalent pit, so that ΔK reaches the ΔKth and finally crack propagates under fatigue mechanism.