Small Crack Effect Research Articles

A unified prediction model for physically small crack and long crack growth based on modified CTOD

A unified model based on CTOD is proposed to calculate the growth rate of physically small crack (PSC) and long crack (LC). Firstly, an in-situ SEM testing under the single overload condition is performed to reveal that CTOD could be the unique driving parameter for fatigue crack growth (FCG). Then, a unified FCG model is established based on the modified CTOD formula which considers the main physical mechanisms of small crack effect. Additionally, a theoretical equation of the transition crack length from PSC to LC is given. Finally, several datasets of different materials are used to validate the model.

A Pavement Crack Detection Method Based on Multiscale Attention and HFS.

To solve the problem of low detection accuracy due to the loss of detailed information when extracting pavement crack features in traditional U-shaped networks, a pavement crack detection method based on multiscale attention and hesitant fuzzy set (HFS) is proposed. First, the encoding-decoding structure is used to construct a pavement crack segmentation network, ResNeXt50 is used to extract features in the encoding stage, and a multiscale feature fusion module (MFF) is designed to obtain multiscale context information. Second, in the decoding stage, a high-efficiency dual attention module (EDA) is used to enhance the ability of capturing details of the cracks while suppressing background noise. Finally, the membership degree of the crack is calculated based on the advantages of the HFS in multiattribute decision-making to obtain the similarity of the crack, and the binary image after segmentation is judged by the hesitation fuzzy measure. The experiment was conducted on the public road crack dataset Crack500. In terms of segmentation performance, the evaluation indexes Intersection over Union (IoU), Precision, and Dice coefficients of the proposed network reached 55.56%, 74.26%, and 67.43%, respectively; in terms of classification performance, for transversal and longitudinal cracks, the classification accuracy was 84% ± 0.5%, while the block and the alligator were both 78% ± 0.5%. The experimental results prove that the crack details detected by the proposed method are more abundant, and the image detection effect of complex topological structures and small cracks are better.

Ellipsoid non-probabilistic reliability analysis of the crack growth fatigue of a new titanium alloy used in deep-sea manned cabin

Reliability analysis of crack growth fatigue of titanium alloy is one of the important guarantees for the safety of deep-sea manned cabin. However, the traditional probabilistic fatigue reliability analysis methods often need accurate parameter statistics information. In this paper, under the condition of the limited test data, a first-order approximate ellipsoid non-probabilistic reliability analysis method for fatigue crack growth of a titanium alloy was proposed. A limit state equation of the crack growth correction model, considering the small crack effect, is formulated according to the fatigue life model. The uncertain parameters are constructed by the interval model, normal distribution, and Khachiyan algorithm. The different limit state equations are obtained by changing the coefficient κ. The first-order approximation and Monte Carlo methods are used to solve the reliability of the ellipsoid model. Calculations demonstrate that the results of the ellipsoid non-probabilistic reliability method are more conservative than that of the probabilistic reliability method. When κ is less than or equal to 2, the Khachiyan algorithm is preferred to build the ellipsoid model, and when κ is greater than 2, the interval model is preferred. Through parameter sensitivity analysis, the uncertain parameters KC, n, m, A, and ΔKthR are found to significantly affect the crack growth rate.

Quantitative evaluation of the flaking strength of rolling bearings with small defects as a crack problem

Rolling contact fatigue (RCF) tests were conducted on rolling bearings with drilled holes at the mid-point of the raceway. Flaking limit was determined by the shear-mode threshold of a crack that emanated at the edge of drilled hole. To quantify the RCF strength according to fracture mechanics principles, Mode II stress intensity factor (SIF) range of the crack was derived. Taking a small crack effect on the shear-mode fatigue crack into account, RCF strength was successfully evaluated as a crack problem, irrespective of the diameters and depths of the hole. The values of the threshold SIF range obtained by RCF tests were in good agreement with those obtained in the torsional fatigue tests under static compression.

Dynamic propagation of a macrocrack interacting with parallel small cracks

In this study, the effect of small cracks on the dynamic propagation of a macrocrack is investigated by using a new continuum mechanics formulation, peridynamics. Various combinations of small cracks with different number, location and density are considered. Depending on the location, density and number of small cracks, the propagation speed of macrocrack differs. Some combinations of small cracks slows down the propagation of a macrocrack by 34%. Presented results show that this analysis can be useful for the design of new microstructurally toughened materials.

Prediction of the critical stress to crack initiation associated to the investigation of fatigue small crack

ABSTRACT. Fatigue design is of vital importance to avoid fatigue small crack growth in engineering structures. This study shows that the critical fatigue design stress can be defined below the usual endurance limit, considered in rules and codes. The material constitutive behaviour is using linear isotropic elasticity. Lassere and Pallin-Luc [1-2] use the elastic energy and over-energy under uniaxial load (tension and rotating bending). The authors deduce the influencing critical stress value corresponding to ?*. It’s a linear approach. We propose an over-energy under dissymmetrical rotating bending and expressed in the ellipse axes. An asymptotic approach is transformed the over-energy in polynomial function of critical stress. Unknown depend on experimental service conditions, endurance limit of tension and rotating bending of specimen. The fatigue database of 30NCD16 steel studied by Froustey and Dubar [3-13] is used. Critical stresses are evaluated (Fig. 2). The research done by Manning and all [4] has shown the small crack effect to be as large as 0.3 mm. Small crack and critical stress are illustrated here in as resulting from pure bending approach expressed by Bazant law [7]. It’s reproduces well the Kitagawa diagram [6] (Fig. 3). When the short cracks are hidden in the material, we shows that the number cycles during small crack growth be significantly higher (Fig. 4) than the corresponding cycles of large cracks growth (ONI’s fatigue test) for the same physically crack size. Indeed its evolution can be blocked by a microstructural barrier (grain boundary, for example). Hence, the considerations of small crack growth are strongly influencing the fatigue life of a component or structure. KEYWORDS. Tensile; Dissymmetrical rotating bending; Over-energy; Critical stress; Small crack; Fatigue cycle.

An investigation of small-crack effects in various aircraft engine rotor materials

Multiple sets of growth rate data for small fatigue cracks in seven different aircraft gas turbine engine rotor materials, including both titanium alloys and nickel-based superalloys, have been collected and are critically compared against corresponding large-crack data. The database includes a wide range of microstructures as well as multiple stress ratios and temperatures. The ability of the simple El Haddad small-crack model and its length parameter a0 to correlate the small-crack and large-crack data by adjusting the small-crack driving force is critically evaluated. Different methods of estimating a0, including the traditional calculation from large-crack threshold and endurance limit properties, as well as a new approach based on empirical scaling from microstructural dimensions, are explored and compared. Strengths and limitations of the simple El Haddad approach for engineering applications are discussed, as well as the practical significance of small-crack effects for life prediction.

マルチ・マイクロプローブ直流電位差法によるオーステナイト系ステンレス鋼の高温疲労余寿命予測：分布微小き裂の成長が電位差に及ぼす影響

マルチ・マイクロプローブ直流電位差法によるオーステナイト系ステンレス鋼の高温疲労余寿命予測：分布微小き裂の成長が電位差に及ぼす影響

Effect of Mean Stress on Small Fatigue Crack Growth Rate on Low Carbon Steel with Several Simulated HAZ Heat Treatment

In this study, to clarify the effect of microstructure of Heat Affected Zone (HAZ) and residual stress of weld on small fatigue crack growth behavior, uniaxial fatigue testing with surface small fatigue crack growth observation were carried out for low carbon steel with three simulated HAZ heat treatment under several mean stress conditions. As a results, εna type small crack growth law was also effective to HAZ microstructure under zero mean stress. Under tensile mean stress condition, fatigue fracture life and small fatigue crack growth life were decreased. In addition, it is clarified that small fatigue crack growth rate acceleration under tensile mean stress was able to be evaluated by using Smith-Watson-Topper equivalent strain εeq which is obtained from mean stress dependency of fatigue fracture life for smooth specimen and εeqna type modified small crack growth law was proposed. Furthermore, it is clarified that εeq was proportional to εeff which was evaluated by crack opening point measured with Digital Image Correlation technique. It is considered that equivalent strain εeq implicitly includes small crack growth life and effect of crack closure.

Study of small cracks on nanocomposite ceramics cut by WEDM

We studied and analyzed the phenomenon of small cracks on the surface of the TiN/Si3N4 nanocomposite ceramics when cut by wire electrical discharge machining (WEDM). When power density of plasma in the discharge channel reaches a certain value, gasification cracks will appear. The effect of small cracks on heat transfer process when the TiN/Si3N4 nanocomposite ceramics are machined by WEDM was also analyzed. Results show that these small cracks make the surface heat source change toward the body heat source, so thermal stress damage happens and the crack propagation depth increases rapidly when the TiN/Si3N4 nanocomposite ceramics are machined by WEDM.

Multiaxial high cycle fatigue threshold with small defects and cracks

In this study, the effects of small surface defects and small cracks on the fatigue limit of metallic materials were investigated. As a theoretical framework, the McEvily model was adopted to account for the Kitagawa effect, the elastoplastic behavior in the vicinity of the crack tip, and the crack closure development in a unified manner. Based on the comparisons between the theoretical predictions and the experimental data, the predictive capability of the present method was examined by focusing on the influence of defect size and biaxial stress. Finally, in view of the area -parameter model, our effort was devoted to propose a simple yet reasonably accurate equation to predict the fatigue limit.

Effect of Hydrogen Gas on the Growth of Small Fatigue Crack in JIS-SCM435

The fatigue crack growth (FCG) from a small hole in a low alloy steel JIS-SCM435 round bar was investigated using tension-compression fatigue tests in 0.7 MPa hydrogen gas and ambient air. In the higher FCG rate regime (e.g. da/dN > 108 m/cycle), FCG was accelerated in hydrogen gas compared to in air. On the other hand, in the lower FCG rate regime (e.g. da/dN < 108 m/cycle), FCG in hydrogen was rather slower than that in air. There was no noticeable difference in fatigue limits between these two atmospheres. The FCG in the respective atmospheres showed a typical small crack behavior, i.e. the da/dN for small cracks were much greater than those for large cracks obtained by compact tension (CT) specimen when they were compared at the same ΔK level. In order to unify such a discrepancy of FCG behavior between small crack and large crack, the strain intensity factor range ΔKε was adopted. As a result, the da/dN data for various crack sizes was gathered in a narrow band, i.e. the small crack effect was successfully evaluated with the strain intensity. Moreover, the crack growth life was predicted based on the da/dN-ΔKε relation. The reproduced S-N curve showed a conservative agreement with the fatigue life obtained by experiments.

Stress ratio effects on small fatigue crack growth in Ti–6Al–4V

A systematic study of the effects of stress ratio on small fatigue crack growth in Ti–6Al–4V was conducted. Cylindrical fatigue specimens were tested axially at room temperature under a maximum stress of 690 MPa and with stress ratios ( R) of 0.5, 0.1, and −1. Tests were periodically interrupted and a standard replication technique was used to monitor the growth of cracks artificially initiated from 30 to 40 μm micro-notches, which were milled into the specimen surface with a focused ion beam (FIB). Measurement of striation spacing from fracture surfaces was evaluated for determining small crack growth rates and showed good agreement with replication data, but is only possible for relatively high stress intensity factor ranges, Δ K, on the order of 10 MPa√m or greater. A significant small crack effect is observed in this alloy, consistent with previous observations, where small cracks grew at stress intensity factor ranges below the long crack threshold and at higher rates than long cracks for equivalent Δ K levels. While a modest effect of stress ratio is seen on small crack growth rates when plotted as a function of crack size (faster growth at lower mean stresses for a given maximum stress), no discernable effect of R is seen when plotting as a function of Δ K. Significant scatter is observed in the small crack growth rates, and the implications of data reduction methods are discussed.

Fatigue threshold R-curves predict small crack fatigue behavior of bridging toughened materials

Small crack fatigue is a widely recognized problem in the fatigue of materials; however, there has been limited progress in developing methods to predict small crack fatigue behavior. In this paper small crack effects due to crack bridging are addressed. A fatigue threshold R-curve was measured for a 99.5% pure polycrystalline alumina using standard compact tension specimens and was used (i) to determine the bridging stress profile for the material and (ii) to make fatigue endurance strength predictions for realistic semi-elliptical surface cracks. Furthermore, it has been shown that the fatigue threshold R-curve can equivalently be determined by measuring the bridging stress distribution, in this case using fluorescence spectroscopy, using only a long crack compact tension specimen without the need for difficult small crack experiments. It is expected that this method will be applicable to a wide range of bridging toughened materials, including composites, toughened ceramics, intermetallics, and multi-phase materials.

Characterization of high cycle fatigue behavior of a new generation aluminum lithium alloy

The high cycle fatigue life characteristics of an Al–Li alloy were studied as a function of microstructure. While for the parent microstructure fatigue life decreased as grain size increased, no such effect was noted at high stresses. This decrease in fatigue life was correlated with lower crack initiation life due to small crack effect. Under multiaxial stress conditions, the alloy exhibited intergranular cracking. The cross-linking of intergranular cracks (in the T8 condition) caused a further deterioration in fatigue life. Additionally, planar slip movements (in the T3 condition) in stage I crack propagation were observed. Slip planarity depended on both the sample texture and the nature of the precipitates. Fractographic and textural evidence is presented.

Behavior of Short Fatigue Crack at Notch Root

It has been recognized that the threshold stress intensity factor range Kth of a short crack is lower than that of a long crack. The short crack behavior in plain specimen has been studied by many researchers. However, the behavior of a short crack at the root of a long notch is not yet clear. The crack closure behavior is considered to be affected by the constraint at notch root and it is dependent on the length and the root radius of notch. In this study, fatigue tests on specimens with short pre-crack at long notch were done and the difference in crack closure behavior was studied. As a result, short crack effect appeared in any notch root radius. In a sharp notch, the crack opening point easily reached its stable condition after a small amount of crack extension. On the contrary in a dull notch, the opening point was lower than the stable condition and consequently short crack effect lasted in relatively wide range of crack extension. The small crack effect of notched specimen was discussed based on crack closure behavior.

Fatigue strength for small shallow defects/cracks in torsion

Fatigue strength in non-homogenous materials is controlled by the threshold condition of small cracks which nucleate at the defect tip. Therefore, the fatigue limit is not a limit stress for crack “nucleation”, but rather a threshold condition of small cracks emanating from the defect. In order to investigate fatigue crack propagation under Mode III, torsional fatigue tests with shallow micro-notches (√area < 1000 μm) were carried out on a SAE 5135 gear steel. An initial series of tests showed the formation of Mode III non-propagating cracks ahead of the micro-notches on run-out specimens. A second series of tests were then carried out after a Mode I pre-cracking (at stress ratio R = −2) procedure: results show a Mode III co-planar crack growth for Δ K III > Δ K th,I, even on specimens whose failure is controlled by Mode I branches. Fatigue tests at different Δ K III levels showed two types of crack advance: a ‘discontinuous’ growth in the near-threshold region and a stable co-planar growth at higher Δ K levels. The Δ K III levels which control the formation of discontinuous crack (Δ K th,III) and the onset of stable co-planar growth (Δ K III > Δ K th,III) clearly show a ‘small crack effect’.

Fretting fatigue strength of 12% Cr steel under high local contact pressure and its fracture mechanics analysis

Fretting fatigue fractures of industrial machines often occur at the point where high contact pressure occurs due to uneven contact. In this study, fretting fatigue tests were performed under high contact pressure applied in line-contact conditions using 12% Cr steel with parameters of the mean stress, contact pressure, and material strength. The fretting fatigue strength was shown to decrease as contact pressure increased, and minimized when Hertz’s average contact pressure was about 1.5 times 0.2% proof stress σ0.2, and increased again at higher contact pressure.Crack propagation behavior was examined using fracture mechanics by observing the detailed crack propagation profile of non-propagating cracks and performing finite element analysis with an inclined elliptical surface crack. Cracks were found to propagate in stage II at the angle where the maximum stress intensity factor range ΔKθmax occurred. Also, test results concerning the fretting fatigue strength could be successfully explained by the micro-crack propagation model in which a micro-crack can propagate when its stress intensity factor range ΔK is greater than the threshold value ΔKth when considering small crack effects and mean stress effects. This model also confirmed the experimental results that showed the length of non-propagating cracks decreased as the mean stress and the material strength increased.

Small fatigue crack growth and failure mode transitions in a Ni-base superalloy at elevated temperature

A study of the long and small fatigue crack growth behavior in IN100 tested at 650 °C both with and without dwell periods is summarized. A significant small crack effect is evident in this alloy, and it is observed that the influence of loading variables on small crack behavior is profoundly different from that on long cracks. While a 6 s dwell has negligible effect on long crack growth rates, it results in more than an order of magnitude faster growth for small cracks (∼30 μm to 1 mm). Long crack growth is dominated by intergranular cracking both with and without 6 s dwell. Small crack growth mode depends on numerous factors including crack size, dwell time, exposure to environment, and character of initiation site. Transitions in small crack growth modes and the operative crack growth mechanisms are discussed.

Fatigue Strength Characteristics of Non-Combustible Mg Alloy

An ignition point of a non-combustible Mg alloy is about 300K higher than that of a Mg alloy currently in use. It is expected that the non-combustible Mg alloy can be used as a substitute for Al alloys as structure components. In this paper, the fundamentals, the notch effect and the long or small crack effect on fatigue strength of the non-combustible Mg alloy, are investigated. Moreover, the fatigue strength characteristics of the non-combustible Mg alloy are compared with that of an Al alloy to clarify the differences in the fatigue strength characteristics between the non-combustible Mg alloy and Al alloy.