Small Artificial Defects Research Articles

A coupled critical plane-[formula omitted] methodology to estimate fatigue life for an AISI 1045 steel with small artificial defects

The presence of notches, tears, holes, cavities, inclusions, and scratches is common in structural designs and mechanical components, that in the presence of cyclic loading can influence their resistance and fatigue life. The objective of this research is to propose and validate an approach for fatigue life estimation for materials with small defects subjected to combined loading. The approach of the project is formulated based on a calibration strategy for Fatemi-Socie, Smith-Watson-Topper and Susmel and Lazarin critical plane models, using Murakami criteria and notch factor to compute the defect effect. The results predicted by the model were compared and validated with experimental data, showing it to be a promising strategy. For further extension and a more generalized application of the proposed model, it is necessary to perform life prediction comparisons for different materials, loadings, and defect geometries.

Adapted multiaxial fatigue models based on the critical plane approach to consider the presence of small defects in steel

Non-metallic inclusions can often be seen in the fatigue crack initiation site in high-strength steels. These inclusions are known to be detrimental to the fatigue strength, as they behave as stress concentration elements. The same is valid for small defects such as notches, holes and scratches. The goal of this research is to investigate not only the effect of non-metallic inclusions but also of small artificial defects on the fatigue strength of the AISI 4140 steel under multiaxial loading conditions. In order to do so, two critical plane based multiaxial fatigue models are coupled with the √area parameter. This is an empirical parameter, which is used to estimate the uniaxial fatigue strength of metals containing small defects or inclusions. One of the major merits of √area parameter is the fact that the material fatigue limits (under push-pull or torsion) can be estimated by considering only the material hardness and the geometry (area) of the small defect, i.e no costly and lengthy fatigue tests are required. To validate the analysis, experiments under pure and combined push-pull and torsional loading in and out-of-phase have been conducted in smooth and also in specimens containing an artificially produced micro hole. Comparison between the estimates provided by the coupled critical plane-√area parameter multiaxial criterion and data provided very good results, with errors around 20%.

Analysis on Fatigue Life and its Scatter of Specimens Containing Small Artificial Defects with Various Sizes Based on the Essential Structure of <i>S-N</i> Curve

Analysis on Fatigue Life and its Scatter of Specimens Containing Small Artificial Defects with Various Sizes Based on the Essential Structure of <i>S-N</i> Curve

Effect of defects on the fatigue limit of Ni‐based superalloy 718 with different grain sizes

AbstractTension‐compression fatigue tests were performed on two types of Ni‐based superalloy 718 with different microstructures in which small artificial defects of various sizes and shapes were introduced. The susceptibility of the fatigue strength to the defects varied significantly with the microstructure morphology, ie, a smaller grain size made the alloy more susceptible to defects. The fatigue limit as a small‐crack threshold was successfully predicted using the parameter model. The evaluation of the fatigue limit was classified into the following three stages depending on the defect size: (a) harmless defect regime, (b) small‐crack regime, and (c) large‐crack regime. Such a classification enabled comprehensive evaluation of the fatigue limit in a wide range of defect size, considering (a) defect size over a range of small crack to large crack and (b) characteristics of matrix represented by grain size and hardness.

Effect of defects and hydrogen on the fatigue limit of Ni-based superalloy 718

Tension-compression fatigue tests were performed on two types of Ni-based superalloy 718 with different microstructures, to which small artificial defects of various shapes and sizes were introduced. Similar tests were also conducted on hydrogen-charged specimens with defects, with a solute hydrogen content ranging from 26.3 to 91.0 mass ppm. In the non-charged specimens in particular, the fatigue strength susceptibility to defects varied significantly according to the type of microstructural morphology, i.e., a smaller grain size made the alloy more vulnerable to defects. The fatigue limit as a small-crack threshold was successfully predicted using the √area parameter model. Depending on the size of defects, the fatigue limit was calculated in relation to three phases: (i) harmless-defect regime, (ii) small-crack regime and (iii) large-crack regime. Such a classification enabled comprehensive fatigue limit evaluation in a wide array of defects, taking into consideration (a) the defect size over a range of small crack and large crack and (b) the characteristics of the matrix represented by grain size and hardness. In addition, the effect of defects and hydrogen on fatigue strength will be comprehensively discussed, based on a series of experimental results.

Behavior of Fatigue Cracks Generated from a Small Artificial-Defect in Plain Specimen of Copper Processed by Equal Channel Angular Pressing

Fatigue tests of ultrafine-grained copper processed by equal channel angular pressing were conducted on the round-bar specimens with a small artificial-defect. The fatigue crack initiated from the defect at an early fatigue stage. After the crack initiation, the crack grew with a 45° inclination to the loading axis at stress amplitudes above 180 MPa. At the stress less than 160 MPa, however, the crack grew perpendicular to the loading axis. The physical background of deferent crack path directions between high-and low-stresses was discussed from the viewpoint of a morphological feature of damaged traces along the crack path.

Crystallographic orientation-dependent growth mode of microstructurally fatigue small crack in a laminated Ti–6Al–4V alloy

The fatigue life of Ti–6Al–4V alloy exhibits significant scatter, which is derived from the statistical scatter of uncontrollable material factors. In particular, crack propagation behaviors in microstructurally small cracks strongly depend on material factors. It is necessary to understand the mechanisms underlying the scatter of fatigue properties. In this study, the dominant material/mechanical factors of microstructurally small fatigue crack growth behaviors are extracted, and the mechanisms of fatigue crack propagation life scatter derived from them are clarified. A Ti–6Al–4V billet was studied in which the microstructure was fully laminated with α and β phases. The major constituent phase was α. Microstructurally small artificial defects were introduced at a center of prior β grains by a focused ion beam (FIB). A fatigue test was carried out at a stress ratio R = 0, stress amplitude σa = 400 MPa, and frequency 1.3 × 10-2 Hz in a vacuum using scanning electron microscopy (SEM). In situ SEM observation was carried out around each FIB notch. After the fatigue test, the dependence of the crack growth path on crystallographic orientation was investigated by using electron backscattered diffraction. It was found that microstructurally small crack growth behaviors depended on crystallographic orientation. The dominant crack growth mechanism was the mode II crack growth mechanism by shear stress along the basal plane as a driving force. The dominant factor that causes large scatter of fatigue life in a α- Ti–6Al–4V alloy is the difference in resolved shear stress along the basal plane. Specifically, the difference in the angle between the basal plane and loading direction, the low reproducibility of the mode II crack growth mechanism, and the random position of pre-existing damage cause the scatter of fatigue crack growth rate.

The influence of various types of small defects on the fatigue limit of precipitation-hardened 17-4PH stainless steel

Fatigue tests were performed on 17-4PH stainless steel specimens containing small artificial defects, with area ranging from 30 to 900μm at stress ratios, R, of −1, 0.05 and 0.4. An investigation was thus conducted to examine the influence of various types of small artificial defects on fatigue strength, including circumferential notches, corrosion pits, drilled holes and pre-cracked holes. The fatigue limit was determined by the threshold condition for the propagation of a crack emanating from the defects. The threshold stress intensity factor range, ΔKth, exhibited a defect size dependency for area≤80µm , and it became a constant value for area>80µm independent of R. Based on the area parameter model and a material constant of ΔKth for long cracks, the fatigue limit could be predicted as a function of R, with the exception of drilled holes with relatively large diameters of 100 and 300μm, for which the fatigue limit was determined by the critical condition for crack initiation. When artificial defects were absent or non-detrimental, intrinsic defects, such as non-metallic inclusions, were found to control the fatigue strength and, in addition, were responsible for the scatter in the fatigue limit. The proposed method enables the quantitative evaluation of the lower bound of the scatter as a function of the number of test specimens, or the overall control volume of fatigue-loaded components.

ステンレス鋼 SUS304 の微小疲労き裂の進展下限界に及ぼす水素の影響

For the safety use of hydrogen utilization systems, it is necessary to understand properly the effect of hydrogen on the fatigue properties of structural materials. In the present study, fatigue life tests of 304 stainless steel with small artificial defects were carried out under the presence of hydrogen; the tests in hydrogen gas environment and the tests using hydrogen-charged specimens, and the effect of hydrogen on the fatigue limit of the material was investigated by paying attention to the behavior of short fatigue cracks. As a result, the fatigue limit was not degraded in hydrogen gas environment, whereas there was slight degradation of fatigue life in short life regime (i.e. Nf < 105 ). On the other hand, fatigue limit of hydrogen-charged specimens was increased compared with that of non-charged specimens. It is noteworthy that non-propagating cracks existed in the unbroken specimens in all environmental conditions. As is well known, fatigue limit of steels with low or moderate strength is determined by the non-propagation limit of small fatigue cracks. Consequently, it was revealed that the fatigue limit of the specimens was determined by the arresting limit of fatigue cracks, which was not degraded in the tests in hydrogen gas as well as in the tests using hydrogen-charged specimen.

Influence of intrinsic and artificial defects on the VHCF properties of 17-4PH stainless steel

In the present work, fatigue test results in the very high cycle fatigue (VHCF) regime are discussed. Experiments were performed with smooth specimens in which crack initiation mostly originated at non-metallic inclusions. Additionally, the influence of small artificial defect, i.e. circumferential notches, drilled holes and corrosion pits, was investigated. Fatigue tests in the VHCF regime were conducted using ultrasonic fatigue testing technique. Further experiments in the high cycle fatigue regime were performed using rotating bending and servo-hydraulic testing machines.For smooth specimens, fatigue fracture was observed even at more than 1010 number of cycles. In contrast, no fatigue failure occurred above approximately 107 cycles in the presence of artificial defects, i.e., a pronounced fatigue limit was determined. Furthermore, it was found that the fatigue limit is not only depending on the defect size but also on the notch root radius of a defect. Consequently, different approaches were used to predict the fatigue limit in the presence of different defects.

Development of in-situ fatigue crack observing system for rotating bending fatigue testing machine

To substitute for a traditional replication technique, an in-situ fatigue crack observing system for rotating bending testing machine has been newly developed. For verifying performance of this observing system, fatigue tests were carried out by using fatigue specimen having a small artificial defect. It is proved that this system can be detect a small fatigue crack and its propagation behavior.

Synchrotron 3D characterization of arrested fatigue cracks initiated from small tilted notches in steel

High resolution synchrotron X-ray tomography has been used to obtain 3D images of arrested cracks initiated at small artificial defects located on the surface of cylindrical steel specimens subjected to mode I fatigue loading. These defects consist in small semi-circular slits tilted at 0¢X, 30¢X or 60¢X with respect to the plane normal to the loading axis; all of them had the same defect size, area = 188 £gm, where the area denotes the area of the domain defined by projecting the defect on a plane normal to the loading axis. Arrested cracks initiated from the notch were observed for all tilt angles at the surface of samples cycled at the fatigue limit (stress amplitude at which the specimen does not fail after 1¡Ñ107 cycles). High resolution synchrotron X-ray tomography has been used to obtain 3D images of those small defects and non-propagating cracks (NPC). Despite the fact that steel is a highly attenuating material for X rays, high resolution 3D images of the cracks and of the initiating defects were obtained (0.65 ƒÝm voxel size). The values of surface crack length measured by tomography are the same as those obtained by optical microscope measurements. The area values present the same tendency as the surface length of NPC, i.e. larger non-propagating cracks areas were observed in the softer steel. In the extreme case of 60o tilted defect, the crack fronts appear much more discontinuous with several cracks propagating in mode I until arrest.

Evaluation of cathodic disbondment of epoxy coating containing azole compounds

Cathodic disbonding of the epoxy coatings on the mild steel was evaluated in 3.5% NaCl solution, where benzimidazole, benzothiazole and benzotriazole were used as additives in the coatings. Also, the electrochemical behavior of coatings delaminating in NaCl solution from a small artificial defect and the bare mild steel samples in the 3.5% NaCl solution containing the azole compounds was evaluated by electrochemical impedance spectroscopy at cathodic polarization condition. Results showed that the azole compounds were not immobilized in the coating as they could leach out and act as cathodic inhibitor at defect site and disbonding front.

OS2120 微小欠陥を有する高強度鋼の疲労特性に及ぼす真空環境の影響

High cycle fatigue tests under push-pull axial loading were carried out on SNCM439 in vacuum and air environments to clarify the mechanism of sub-surface fatigue fracture. To reveal the Effect of vacuum environment on defect size dependency of fatigue limit σ_w, specimens with small artificial defects were tested. As a result, the following conclusions were obtained: (1)The vacuum environment increased the fatigue lives and lowered the fatigue limits. (2)The S-N curves in vacuum showed a gentle sweep although those in air indicated a clear knee point with a level line. (3)Defect size dependency of σ_w, is considered different in each environment. (4)Sub-surface fractures can be quantitatively simulated by vacuum fatigue tests using specimens with an artificial small defect.

A micromechanical approach to fatigue in small notches*

ABSTRACTThe present work shows the application to small notches of a micromechanical model which describes the growth of a short crack across the steep stress gradient generated at the root of a notch. The model, based on the theory of distributed dislocations, takes into account the interaction between short cracks and material barriers such as grain boundaries. The term ‘small notches’ refers in this paper to stress raisers the size of which is of the same order as the characteristic microstructural unit of the material. Typical examples are superficial scratches, corrosion pits, inclusions or pores. Comparisons between predicted fatigue limits and experimental results reported in the literature for different materials containing small artificial defects are shown and discussed.

Fatigue limit evaluation of notches, small cracks and defects: an engineering approach

ABSTRACTIn this paper, the average stress method for the fatigue limit evaluation of stress raising geometrical features is revised and extended. In particular, an analytical close‐form approach was used and the linear elastic stress equations were modified by taking into account the effect of nominal stress on the local stress distribution. Hence, the average tangential stress was correctly evaluated over a distance of 2a0, where a0 was El Haddad's short crack constant, for long and small notches as well as for crack‐like notches. When this model is applied to a wide range of geometrical features subjected to mode I fatigue loading, the classical shape of the curves of the Kitagawa–Takahashi diagram was obtained for changes in crack‐like notch size. Similarly, notch sensitivity was estimated by reducing the notch tip radius. The accuracy of the proposed method in predicting fatigue limits was then checked by using experimental data taken from the literature and generated on testing specimens weakened by rounded and sharp notches as well as by small artificial defects.

Ti-6Al-4Vの疲労き裂伝播限界に及ぼす高真空環境の影響(J10-1 チタン系材料の力学・強度特性,J10 チタン系材料の力学・強度特性と応用技術)

Uniaxial tension fatigue tests were carried out in air and in high vacuum environments using test pieces made of Ti-6Al-4V with small artificial defects. We examined environmental effects of high vacuum on S-N properties and fatigue crack growth limit. As a result, the following were obtained, (i) Regardless of defect sizes, fatigue lives in high vacuum were much longer than those in air. The slope of S-N curve showed different tendencies between in air and in high vacuum corresponding to defect sizes, (ii) The downward trend of ΔK_<th> with decreasing crack size existed not only in air but also in high vacuum, and the tendency was stronger in high vacuum than in air.

Electrochemical evaluation of defect growth in primers applied on aluminium alloy 2024T3

In the present work, the protective properties of inhibitive pigments in two epoxy‐primers against corrosion of the aluminium alloy 2024T3 in marine atmosphere were investigated, the first containing SrCrO4 and the second Zn3(PO4)2. Potentiostatic polarisation and impedance measurement methods were utilised to evaluate, both the spontaneous onset of defects on coated samples and the propagation of a small artificial defect of known dimension applied since the beginning of the test on each sample, during 24 months of exposure to the marine atmosphere. These techniques enabled a quantitative evaluation of the protective efficiency of the two primers to be made, and for the effects of the surface pre‐treatments of the metallic substrate to be investigated.

Fatigue Strength of Ti-6Al-4V Alloys Containing Small Artificial Defects

The effects of small defects on the fatigue strength of Ti-6Al-4V were investigated in tension-compression fatigue tests. To simulate the defects, holes having 50, 200, 400, 760 and 1000μm diameters were introduced onto the surface of a series of specimens (Series A). Another series of specimens (Series B) were prepared to investigate the effect of a burr or a pre-crack which was introduced at the edge of the artificial hole. For Series A specimens, the fatigue limit was defined as the threshold for crack initiation from the edge of the hole. For Series B specimens, the fatigue limit was defined as the threshold for crack propagation from the burr or the tip of the pre-crack. The results of the fatigue tests for R=-1 indicated that the fatigue limits of Series B specimens were 20∼60MPa lower than those of Series A specimens. In contrast to steels, non-propagating cracks did not form at the holes, a fact associated with the relatively high stress level for crack initiation from hole in Ti-6Al-4V. It was also found that the area parameter model underestimated the fatigue limit of Series A specimens, but the model accurately predicted the fatigue limit defined by crack-growth threshold for Series B specimens. In addition it was noted that the presence of a burr resulted in a decrease in fatigue life, since the burr facilitated the crack initiation process.

Evaluation of eddy current probes based on local field excitation

The paper deals with the eddy current testing by means of a technique based on local field excitation, with the objective to enhance sensitivity and spatial resolution of flaw detection. Two modifications of innovative eddy current probes are presented. Results of the application of the probes for an examination of thin Inconel plate specimens with small artificial defects on the opposite surface are compared. The criteria for comparison are signal-to-noise ratios and spatial resolution of the responses obtained. Performance of the probes is analyzed on the basis of the study of the distribution of their excitation fields with application of magneto-optic sensors and finite element modeling. The decrease of the local extent of the excitation field has been found to be the main cause of sensitivity decrease of the probes to detect opposite-surface defects.