Fatigue Threshold Stress Intensity Research Articles

The cohesive zone crack analogue for incomplete contacts under mild wear conditions

Fretting fatigue may appear in mechanical components that are in contact under vibrational loading, inducing oscillatory frictional stresses, often combined with alternating bulk stresses. Fretting fatigue cracking is important, with its initiation being difficult to estimate. This work extends a well-known fretting fatigue model, the crack analogue model that examines fretting fatigue induced by incomplete contacts of metallic surfaces. This is done by inserting a mode II type cohesive zone at the contact edges. In our previous work on fretting fatigue of complete contacts, we proposed mixed mode I and II cohesive zones because of the nature of the stress fields around the contact edges. The proposed cohesive zones address several issues related to the mechanics of friction and wear, by combining them in the case of mild wear where asperity plastic shakedown is the dominant inelastic mechanism. Thus, the micromechanical behaviour of the surface topology and its evolution are naturally introduced into the analysis. The surface roughness becomes an internal variable that evolves with the cyclic loading, establishing a steady state friction coefficient. Alternatively, friction evolution can be established pointwise by monitoring the accumulated micro-slip. The work of friction dissipates the mechanical work and at the same time becomes the precursor of large scale wear. A predictive methodology for fretting crack nucleation is proposed through the fatigue threshold stress intensity factor used in ordinary fatigue.

Study of Creep–Fatigue Crack Growth Behavior in a Gas Turbine Casing

A creep–fatigue crack growth on the outer surfaces of a turbine casing was assessed, and the turbine casing’s overall lifetime was predicted. The crack location, size, and direction were determined using nondestructive tests. Temperature distribution, measured using thermography method, was applied as boundary conditions in FEM simulation. ABAQUS software was utilized for calculating the stress distribution of the casing in accordance with the real cycles of the turbine. Both the creep and the fatigue crack growths were predicted using the ZENCRACK code. The Paris equation was applied in order to estimate the fatigue life, and the time-dependent Paris equation was used to predict the creep life. It was shown that after a certain amount of time has passed, the crack stress intensity factor (K) and time-dependent stress intensity factor (Ct) decrease and stop in reaching the fatigue threshold stress intensity factor (Kth) and creep fracture mechanics parameter (C t * ) values, respectively.

Fatigue crack growth and threshold stress intensity factor range of a small crack in a Fe-Mn-C TWIP steel.

Fatigue crack growth and threshold stress intensity factor range of a small crack in a Fe-Mn-C TWIP steel.

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.

Fatigue Life Prediction Based on Crack Closure and Equivalent Initial Flaw Size.

Failure analysis and fatigue life prediction are necessary and critical for engineering structural materials. In this paper, a general methodology is proposed to predict fatigue life of smooth and circular-hole specimens, in which the crack closure model and equivalent initial flaw size (EIFS) concept are employed. Different effects of crack closure on small crack growth region and long crack growth region are considered in the proposed method. The EIFS is determined by the fatigue limit and fatigue threshold stress intensity factor △Kth. Fatigue limit is directly obtained from experimental data, and △Kth is calculated by using a back-extrapolation method. Experimental data for smooth and circular-hole specimens in three different alloys (Al2024-T3, Al7075-T6 and Ti-6Al-4V) under multiple stress ratios are used to validate the method. In the validation section, Semi-circular surface crack and quarter-circular corner crack are assumed to be the initial crack shapes for the smooth and circular-hole specimens, respectively. A good agreement is observed between model predictions and experimental data. The detailed analysis and discussion are performed on the proposed model. Some conclusions and future work are given.

Microstructurally-dependent model for predicting the kinetics of physically small and long fatigue crack growth

Based on the proposed concept of the fatigue threshold stress intensity factor ranges, a model has been developed that describes the kinetics of physically small fatigue crack and long fatigue crack growth. The model allows the calculation of the crack growth rate under the regular fully-reversed uniaxial loading from the data on the static characteristics of mechanical properties and the microstructure of the initial material. The crack depth at which the cyclic plastic zone size ahead of the crack tip will exceed the grain size should be considered as a criterion of the small-to-long crack transition. Under high-cycle fatigue conditions physically small fatigue crack growth will be divided into two phases of growth: the first phase is when the crack propagates along the slip planes of individual grains, and the second one is when the crack changes the mechanism of growth and propagates in the plane perpendicular to the loading direction. The model validity has been tested using the experimental data on the growth of the long cracks in specimens of titanium alloy VT3-1 in seven microstructural states and the small cracks in specimens of titanium alloy Ti–6Al–4V and aluminum alloy 2024-T3. Good agreement between the calculated and experimental results is obtained.

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.

Effective notch stress and critical distance method to estimate the fatigue life of T and overlap friction stir welded joints

The interface notch is a general characteristic of friction stir welded joints and appears in many types of the welds. Indeed interface notch most likely is a location that fatigue failure can occur there. This paper is concerned with the fatigue of friction stir welded joints. In the present paper fatigue of T and over lap joints were modelled base on critical distance method. Two material properties ΔKth, the fatigue threshold stress intensity factor and Δσ0, the fatigue limit were used in order for estimating the fatigue life at interface location. This is shown that assuming homogeneous material throughout the weld and the base material in FE analysis still can provide a reasonable fatigue prediction for friction stir joints when using critical distance method. This suggests the same procedure can be used for complex component when failure appears in the interface notch. In order to verify it the critical distance prediction method was also used for actual component where the failure occurred in the weld. The predicted fatigue limit deviated only 15% from the experimental result.

Fatigue Crack Growth in Ultrafine Grained Aluminium Alloy

The microstructure, fatigue crack growth behaviour and hardness of ultra fine grained 6061 aluminium alloy obtained by equal angle channel processing was studied. ECAP resulted in significant grain refinement down to the sub micron level and corresponding increase in hardness. Results point to a similar fatigue threshold stress intensity range and fatigue crack growth rates for 1, 2, 4 and 6 passes of ECAP.

A fracture mechanics approach to fatigue of heavily drawn steel wires

Heavily drawn steel wires, used in applications such as radial tires, have to withstand a high number of fatigue cycles, typically > 108. Such steel wires have diameters as small as 175μm. High cycle fatigue failure in such wires is initiated by either surface defects or internal non-metallic inclusions. The former leads to shorter fatigue lives than the latter. A methodology has been developed to measure the fatigue threshold Stress Intensity Factor (SIF) for long cracks in these heavily drawn steel wires. Then, the SIF's of the non-metallic inclusions, at which the high cycle fatigue cracks initiate, were compared to the threshold value for long crack growth. It was shown that the defect size that should be used to calculate the SIF is larger than the inclusion size itself. This is validated using Scanning Electron Microscopy in combination with a Focused Ion Beam, proving that the region around non-metallic inclusions is characterized by alterations in the microstructure. In this intermediate region between the inclusion and the start of the long cracks, the crack growth rate is very slow and can be described by a Paris’ law like relationship. Hence, a bi-linear Paris law has been proposed, enabling to predict the fatigue life of heavily drawn steel wires containing any size of inclusions.

G030052 過大荷重負荷形式がステンレス鋼の疲労き裂進展下限界に及ぼす影響([G03005]材料力学部門一般セッション(5):ステンレス鋼の疲労)

The effects of overloading condition on the fatigue threshold stress intensity factor range (AKth) in type 316 austenitic stainless steel (SUS316) were studied. Overloading was applied to a compact tension specimen of SUS316, then fatigue tests were carried out to determine the resultant zIKth. In the previous study, it was found that the apparent value of the AKth increased with increasing stress intensity factor by single tensile overloading. In this study, it was found that if a compressive overloading was applied after tensile overloading, the apparent z1Kth showed no increase. However, if a tensile overloading was further applied after compressive overloading, the apparent z11(th increased as high as the results obtained by a single tensile overloading.

Improvement fatigue limit of steel containing a small crack‐like surface defect by overload effect

PurposeSurface defects reduce fatigue strength and may greatly reduce component reliability, particularly in pressure vessel weld regions, springs, and other applications. The fatigue strength of components, and thus their reliability, can be substantially increased by tensile overloading prior to use. The purpose of this paper is to investigate the effect of tensile overload on small cracks by applying a tensile overload to steel plates containing semicircular slits that simulate small surface cracks and by determining the degree of increase in the fatigue strength. The effect of tensile overload on the apparent fatigue threshold stress intensity factor range (ΔKth) was also investigated.Design/methodology/approachA tensile overload stress of 1,000 or 1,200 MPa was applied once to all test pieces. Then, bending fatigue tests were conducted with a stress ratio R=0.1. The slit region was subjected to applied cyclic tensile stress by four‐point bending throughout the fatigue test. A test specimen to which no overload stress was applied was tested for comparison.FindingsThe improvement in ΔKth by tensile overload is observed in the specimen with a small crack like surface defect. However, in the specimen with a small crack like surface defect, the improvement in ΔKth by tensile overload is saturated as increasing tensile overload. The improvement rate of ΔKth by tensile overload and the upper limits of improvement in ΔKth were predicted. The predicted values of the improvement rate of K were well in agreement with the experimental results.Practical implicationsThe proposed method can be applied to pressure vessels and springs.Originality/valueThe overload effects on fatigue strength are studied for large cracks. However, the effect is not understood at all for small cracks. This study focused the over load effects for small cracks. This is the original point of the present study.

Overloading effect on the fatigue threshold stress intensity factor range (ΔKth) as a function of crack length in SUS316

PurposeThe purpose of this paper is to study the effects of overload on the threshold stress intensity factor range (ΔKth) in SUS316.Design/methodology/approachThe fatigue tests are carried out to determine the resultant threshold stress intensity factor range (ΔKth). The mechanism of the improvement of ΔKth by the tensile overloading is analyzed using the Dugdale model.FindingsIt is clarified that the value of ΔKth increases as increasing the overloading.Research limitations/implicationsThe apparent value of ΔKth of stainless steel can be improved by a tensile overload, the fatigue strength of structural members that have a surface crack can be increased by a tensile overload.Originality/valueAs a result, the reliability and safety of structures, such as energy plants, can also be improved.

Ti-6Al-4V合金の超高真空環境における疲労き裂進展特性(<小特集>M&M2009材料力学カンファレンス)

To determine the effects of the vacuum environment on fatigue crack propagations, K-decreasing tests with compact tension (CT) specimens were conducted in air and vacuum environments. The test data clarified the fatigue crack propagation rate (da/dN) and threshold stress intensity factor range (ΔK_<th>) for both environments. The da/dN becomes lower and ΔK_<th> becomes larger with decreasing vacuum pressure. However, this tendency was not explained only by the crack closure. Based on fracture surface observations, the crack propagation properties in vacuum were compared with those in air. A few micrometer size granular region was observed on the fracture surface only in the high vacuum (〜10^<-6>Pa) and ultra high vacuum (〜10^<-7>Pa), but not in the air and medium vacuum (〜10^<-1>Pa). The high vacuum environment is one of the necessary conditions for the formation of the granular region, and the fraction of surface coverage of fracture surfaces probably relates to the phenomenon. The formation of the granular region represents the difference of the crack propagation mechanism between in vacuum and in air environments, and the tendency observed in da/dN and ΔK_<th> can be accounted for by the different mechanism.

過大予荷重効果によるき裂状微小欠陥を有する鋼の疲労下限界応力拡大係数範囲の向上(&lt;小特集&gt;M&amp;M2009材料力学カンファレンス)

過大予荷重効果によるき裂状微小欠陥を有する鋼の疲労下限界応力拡大係数範囲の向上(&lt;小特集&gt;M&amp;M2009材料力学カンファレンス)

Effect of overload on the fatigue threshold stress intensity factor range (ΔKth) of SUS316

Effect of overload on the fatigue threshold stress intensity factor range (ΔKth) of SUS316

Effect of Nitriding on Fatigue Crack Initiation and Fatigue Crack Propagation Retardation in Ni-Cr-Mo Steel

Effect of nitriding on fatigue crack initiation and growth rate has been studied on Ni-Cr-Mo steel. Specimens were nitrided for 15hr at 680°C. The fatigue limit of nitrided specimens were superior to those of annealed(680°C, 15 hr) specimens. Based on detailed observations of slip band and micro crack initiation, it is concluded that the excellent fatigue limit of nitrided specimens is attributed to improved slip initiation resistance by nitriding. The characteristic of fatigue crack growth rate of nitrided specimens was investigated by comparing with those of annealed specimens. It was found that by nitriding the crack growth rate was markedly decreased and the threshold stress intensity factor range was improved. It is concluded that the excellent fatigue limit of nitrided specimens is also attributed to improved fatigue crack growth rate and threshold stress intensity factor range by nitriding.

高強度鋼の疲労き裂進展下限界条件に及ぼすき裂寸法，応力比および水素の影響

The effects of crack size, stress ratio and absorbed hydrogen on the fatigue crack propagation characteristics of high strength steel were studied near threshold regime. The test material was SCM440H low alloy steel. The material was quenched in vacuum and tempered at low temperature to achieve high strength and to lower the initial content of hydrogen of the material. The effects of crack size and stress ratio on the fatigue crack propagation rate and threshold stress intensity factor were investigated in air at first. These data offered a baseline as a reference. The effect of absorbed hydrogen was examined using a material in which hydrogen was introduced by cathodic charging. It was verified that the absorbed hydrogen in high strength steel affected the high cycle fatigue crack propagation characteristics. The fatigue crack propagation rate near threshold region was accelerated and ΔKth was lowered. The concentration of about 1ppm hydrogen caused 20% reduction of ΔKth. Since absorbed hydrogen has an influence even on high cycle fatigue crack propagation, it should be taken into account in the evaluation of defect of hydrogen utilization machine.

Equivalent stress approach to predicting the effect of stress ratio on fatigue threshold stress intensity range

A previously proposed exponential stress function for predicting the effect of mean stress on fatigue life of smooth specimens is employed to account for the effect of stress ratio on the fatigue stress intensity range for crack-like members. By means of equivalent stress that is expected to yield the same damaging effect as combined cyclic and mean stresses, a relation between fatigue threshold stress intensity range Δ K th and stress ratio R is derived. It is indicated that the R-sensitivity on Δ K th is controlled by a single parameter, which appears to depend on the degree of crack closure exhibited by the material under the loading conditions. Predictions of the relation are compared with published experimental data and agreement is found to be very good. The R-effect on Δ K th is briefly discussed in terms of crack closure.

Effect of Grain Size on the Fatigue Crack Growth in Steels at Temperatures 295 and 77 K

Fatigue crack growth (FCG) and threshold stress intensity in low carbon and HSLA steels with remarkably different ferrite grain sizes (d f ) at room temperature and temperature of liquid nitrogen were investigated. The FCG rates were found to decrease with decreasing temperature for all investigated microstructural states and the influence of temperature decrease was the most significant in coarse grain microstructures. The threshold values of the stress intensity factor, (ΔK th ), below which cracks do not propagate, decreased by between 29.1 % (d f =2.7μm) and 49.3 % (d f =88.4μm) when specimens were tested at the cryogenic temperature. A general relationship between the FCG rate, the effects of grain boundary blocking on the plastic zone size and/or the crack-tip opening displacement and the effect of changing temperature, is discussed. Furthermore, the concept of a functional relation between tensile and fatigue data at cryogenic temperatures was also investigated. It was shown that the ratio of ΔK th at 77 K to ΔK th at 295 K is proportional to the second root of the ratio of the tensile strength values at these temperatures i.e. ΔK th77K /ΔK th295K (σ u77K /σ u295K ) 1/2 .