Stress Intensity Factor Range ΔK Research Articles

Fatigue Crack Growth of Alloy 7050-T7451 Plate in L-S Orientation

Experiments have been conducted to investigate the crack growth characteristics of 7050-T7451 aluminium plate in L-S orientation. Two loading conditions are selected, i.e. constant amplitude and constant stress intensity factor range (ΔK). The effects of ΔK-levels and stress ratios (R) on crack splitting are studied. Test data shows that crack splitting could result in the reverse of crack growth rate trend with the increasing R ratio at high ΔK-level. The appearance of crack splitting depends on both ΔK and R.

Describing fatigue crack growth and load ratio effects in Al 2524 T3 alloy with an enhanced exponential model

The fatigue crack behavior in metals and alloys under constant amplitude test conditions is usually described by relationships between the crack growth rate da/dN and the stress intensity factor range ΔK. In the present work, an enhanced two-parameter exponential equation of fatigue crack growth was introduced in order to describe sub-critical crack propagation behavior of Al 2524-T3 alloy, commonly used in aircraft engineering applications. It was demonstrated that besides adequately correlating the load ratio effects, the exponential model also accounts for the slight deviations from linearity shown by the experimental curves. A comparison with Elber, Kujawski and “Unified Approach” models allowed for verifying the better performance, when confronted to the other tested models, presented by the exponential model.

An experimental study of loading parameters effect on fatigue crack closure in 12NC6 steel

In this paper, the fatigue crack closure (FCC) phenomenon was experimentally revealed using the back face strain gauges technique. The effects of the loading parameters on FCC have been investigated on 12NC6 heat treated steel. Four yield stresses values have been tested. The loading conditions were chosen to have the stress intensity factor range ΔK between 17 and 30MPam with four load ratio values: 0.05–0.1–0.3 and 0.5. In this study, the closure stress intensity factor Kcl is approximately constant for all load ratio values (R). The expression describing the evolution of the closure parameter U is obtained as a function of both the R and Kmax.

Influence of defects on fatigue strength and failure mechanisms in the VHCF-region for quenched and tempered steel and nodular cast iron

Investigations are presented in this paper on quenched and tempered steel 42CrMoS4 from two batches, with two different tensile strengths (Rm=1100MPa, 1350MPa) but with similar microstructure, and a nodular cast iron EN-GJS-900-2 (Rm=930MPa). Fatigue tests with smooth (Kt=1) and notched (Kt=1.75) specimens were performed at R=−1 and R=0 up to the number of cycles N=2·109 in order to determine the fatigue strength behaviour and failure mechanisms, especially in the VHCF-region. Failure in smooth specimens often initiated at material defects such as oxides in the quenched and tempered steel and shrinkage holes in the nodular cast iron. Firstly, a fatigue strength analysis was performed that did not consider these defects. A possibility of analysis of experimental data including VHCF-results has been discussed. Next, a linear elastic fracture mechanics analysis was performed in order to describe the defect behaviour, assuming that the defects act like cracks. The results showed that there are lower limit or threshold values of the stress intensity factor range ΔK for crack propagation in both materials. Analysis of defects and defect distribution in run-out specimens confirmed this conclusion. From the comparison of the results with an S–N curve from the design code FKM-Guideline Analytical strength assessment of components, recommendations for design and assessment of components have been derived.

Fatigue Crack Behavior of Welds in Q460NH Steel

The fatigue crack growth behavior of weld joint of Q460NH steel is very important for estimating life-time in terms of a crack length in weld joint. In this paper , fatigue crack growth behavior of weld metal (WM),heat affected zone(HAZ) and mother metal(BM) have been researched at room temperature. The variation of fatigue crack growth rate(da/dN) with stress intensity factor range (ΔK) for WM, HAZ and BM is discussed within the Paris region. It is shown that fatigue crack growth rate of WM is the slowest in tree parts .

OS0506 切欠き底に発生した微小疲労き裂の進展挙動に及ぼす切欠き半径および応力比の影響(OS5-2 き裂進展・計測,OS-5 材料の疲労挙動と損傷評価1)

It has been recognized that the threshold stress intensity factor range ΔK_<th> of a short crack is lower than that of a long crack. This behavior is unsafe in mechanical design if only ΔK_<th> of long crack is considered. Therefore many researchers have studied short crack behavior using plane fatigue test specimen. These studies provided quantitative understanding of the short crack behavior in relation to crack closure. However, for a short crack at notch root, the behavior has not been clarified yet. In this study, crack propagation tests using various kinds of notched specimens with a short pre-crack were done to consider the effect of notch root radius and stress ratios on the crack propagation behavior of the short crack at notch root. The crack closure was different depending on the stress ratio as well as notch root radius. This resulted difference in the ΔK_<th> of the short crack. When high stress ratio was applied, since there was no crack closure the ΔK_<th> was relatively low regardless of notch root radius. The effect of notch root radius on the ΔK_<th> of the short crack at notch root was significant when the stress ratio was low. The change in ΔK_<th> with the decrease in notch root radius could be understood by development of crack closure.

OS1610 ECCI法による疲労き裂近傍の転位構造観察(OS16-2 微視的な視点からの変形と強度,OS-16 ナノ・マイクロの視点からの変形と破壊の力学)

Dislocation structures formed around a fatigue crack in copper were observed with electron channelling contrast imaging (ECCI) technique. The ECCI technique enables us to observe dislocations lying near surface using a scanning electron microscope (SEM). For a cyclically-deformed copper, self-organized dislocation structures such as vein structure were successfully observed with the ECCI technique. To investigate relationship between dislocation structure around a fatigue crack and stress concentration, a fatigue crack growth test was conducted on a center-cracked tensile (CCT) copper specimen. The ECCI observation along the fracture surface revealed that cell structures were generated at the close vicinity of the fracture surface and the vein structure existed around outer edge of the cell structure. The formation of the cell structure was affected by the degree of stress concentration: measured size of the cell-structured area l_c was a power function of stress intensity factor range ΔK_I. To estimate distribution of ΔK_I value along a fracture surface, a lot of ECCI observations of the subsurface microstructure were performed on a smooth-shape copper specimen which was fractured under push-pull fatigue. The distribution of ΔK_I value, which was calculated from the above l_c-ΔK_I relation, was consistent with crack growth direction that was predicted from striation.

Evaluation of Fatigue Properties and Study on the Fatigue Crack Propagation Behaviour in Welding Material SUS309 for Hydraulic Turbine Runner

In hydraulic turbine runner, accidents due to fatigue often occur. In this study, cyclic tension-compression tests and fatigue crack growth tests using compact tension specimens under the stress ratio R-constant were carried out for welding material SUS309, in order to clarify fatigue properties, fracture mechanisms, fatigue crack growth and closure properties. The test results are summarized as follows : (1)The S-N curve of welding material SUS309 used in this study has narrow scatter and the tension-compression fatigue strength was lower than the test data of the stainless cast steel SCS6 used as hydraulic turbine runner material. (2)From SEM observation of the fracture surfaces, crack initiation sites were found to be surface slip, and the casting defect in the bond region. (3) Fatigue crack growth rates of the specimens which is the vertical and the parallel to welded direction could be correlated with the stress intensity factor range ΔK in the Paris's law. (4)It is considered that threshold stress intensity factor range ΔKth is influenced by plastic zone and grain size, and the ΔKth of parallel direction specimen is lower than that of vertical direction specimen.

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.

Wake history effect on closure of short and long fatigue crack in 304L stainless steel

The effect of crack length and of crack wake history on crack closure is investigated in 304L austenitic steel. Tests are run on CT specimens initially pre-cracked at constant applied stress intensity factor range ΔK, at a load ratio R=0.1. After precracking, the crack wakes are machined to obtain 2D physically short cracks. Then propagation of these short cracks is still performed at constant ΔK for length ranging from 0.1 mm up to 4 mm. A numerical analysis of load-displacement data allows improved closure detection for the smaller short cracks. The ratio γ=Kop/Kmax is shown to decrease with decreasing crack length a for a <1.2mm; for a >1.2mm, the long crack domain is reached and γ is shown constant and equal to 0.352 independently of ΔK. For tests usually run at increasing ΔK or decreasing ΔK, γ is dependent on the variation of ΔK with a, and the relation da/dN vs ΔK is shown dependent on the loading history. But in all cases, the crack propagation rates are rationalized in term of ΔKeff.

Gigacycle fatigue behavior of high strength aluminum alloys

Fatigue in the very long life regime has assumed great significance in recent years particularly after it has been established that a fatigue limit doesn’t exist in most cases. The paper deals with investigations of gigacycle fatigue and near threshold fatigue crack growth behaviors of AA7075-T6, AA6061-T6 and AA2024-T3 Al alloys using piezoelectric accelerated fatigue at 20KHz. Experiments were conducted in air at room temperature on hourglass-shaped specimens under completely reversed loading conditions (R=−1). It was noticed that failure continues to take place right up to the gigacycle (109 cycles) range, at nominal cyclic stresses. The high cycle fatigue fractures are generally rough in appearance and seen to initiate predominantly from the surface. Furthermore, they contain a significant number of voids, exhibit crystallographic faceting, and discernible fatigue striations. Gigacycle fatigue fracture of AA2024-T3, however, shows features of ductile tearing instead of striations. The studies also show that fatigue crack growth rates of small cracks are greater than those of large cracks for almost the same stress intensity factor range ΔK, and that some small cracks may grow at ΔK values below the large crack threshold in the near-threshold crack growth regime.

PS31 微小き裂のΔK_を用いた平滑材及び鈍い切欠き材の疲労限度評価(フェロー賞対象ポスターセッション)

The fatigue limit of blunt-notched specimen is decided of the crack initiation limit σ_<w1> However , the evaluation of σ_<w1> which is based on fatigue crack properties is not proposed. When there is an initial crack at notch root, the threshold stress intensity factor range ΔK_<th> is calculated from the fracture mechanics and linear notch mechanics. σ_<w1> and fatigue limit of plain specimen σ_<w0> were evaluated from ΔK_<th>. The σ_<w1> and σ_<w0> of annealed steel can be estimated from regarding the average ferrite grain size as the initial crack.

Effects of Laser Peening Treatment on High Cycle Fatigue and Crack Propagation Behaviors in Austenitic Stainless Steel

Laser peening without protective coating (LPwC) treatment is one of surface enhancement techniques using an impact wave of high pressure plasma induced by laser pulse irradiation. High compressive residual stress was induced by the LPwC treatment on the surface of low-carbon type austenitic stainless steel SUS316L. The affected depth reached about 1mm from the surface. High cycle fatigue tests with four-points rotating bending loading were carried out to confirm the effects of the LPwC treatment on fatigue strength and surface fatigue crack propagation behaviors. The fatigue strength was remarkably improved by the LPwC treatment over the whole regime of fatigue life up to 108 cycles. Specimens with a pre-crack from a small artificial hole due to fatigue loading were used for the quantitative study on the effect of the LPwC treatment. The fracture mechanics investigation on the pre-cracked specimens showed that the LPwC treatment restrained the further propagation of the pre-crack if the stress intensity factor range ΔK on the crack tip was less than 7.6 MPa√m. Surface cracks preferentially propagated into the depth direction as predicted through ΔK analysis on the crack by taking account of the compressive residual stresses due to the LPwC treatment.

A review of the volume-based strain energy density approach applied to V-notches and welded structures

A large bulk of experimental data from static tests of sharp and blunt V-notches and from fatigue tests of welded joints are presented in an unified way by using the mean value of the Strain Energy Density (SED) over a given finite-size volume surrounding the highly stressed regions. When the notch is blunt, the control area assumes a crescent shape and R 0 is its width as measured along the notch bisector line. In plane problems, when cracks or pointed V-notches are considered, the volume becomes a circle or a circular sector, respectively. The radius R 0 depends on material fracture toughness, ultimate tensile strength and Poisson’s ratio in the case of static loads; it depends on the fatigue strength Δ σ A of the butt ground welded joints and the Notch Stress Intensity Factor (NSIF) range ΔK 1 in the case of welded joints under high cycle fatigue loading (with Δ σ A and ΔK 1 valid for 5 × 10 6 cycles). Dealing with welded joints characterised by a plate thickness greater than 6 mm, the final synthesis based on SED summarises nine hundred data taken from the literature while a new synthesis from spot-welded joints under tension and shear loading, characterised by a limited thickness of the main plate, is presented here for the first time (more than two hundred data). Dealing with static tests, about one thousand experimental data as taken from the recent literature are involved in the synthesis. The strong variability of the non-dimensional radius R/ R 0, ranging from about zero to about 1000, makes the check of the approach based on the mean value of the SED severe.

The Behavior of Shallow Cracks in a Pipeline Steel Operating in a Sour Environment

Setting conditions for the avoidance of in-service crack growth in aggressive corroding environments has long been a major challenge due to the number of variables that have a significant effect on material behavior. One area where both experimental data and a validated assessment methodology are lacking is the behavior of shallow cracks. This paper describes the early results of an ongoing research program aimed at addressing the shortfall in experimental data to characterize material behavior in the shallow-crack regime, with the long-term aim of improving the understanding and assessment of the early stages of environment assisted cracking. There is an industry need for a better understanding of material behavior under these conditions and for the development of a more robust assessment methodology. API 5L X65 pipeline steel parent material was tested in a sour environment with initial flaw sizes in the range 1–2 mm. Fatigue crack growth rate tests have been performed to investigate the influence of crack depth on crack growth rate (da/dN). Initial results suggest that crack growth rates for deep flaws can increase by a factor of 5–100 compared with air depending on the applied stress intensity factor range (ΔK). Shallow cracks have been shown to grow up to 130 times faster in a sour environment than in air and up to an order of magnitude faster than deep cracks in a sour environment at the same value of ΔK. Constant load tests have also been performed to investigate the influence of crack depth on the threshold stress intensity factor for stress corrosion cracking (KISCC). Preliminary results suggest that in this case there is no crack depth dependence in the range of flaw sizes tested. While further experimental work is required, the results obtained to date highlight the potential nonconservatism associated with extrapolating deep-crack data. Guidance is therefore provided on how to generate appropriate experimental data to ensure that subsequent fitness for service assessments are conservative.

Characteristics of Mode I Fatigue Crack Growth of Aged Prestrained Carbon Steel

The fatigue crack growth property of prestrained carbon steels was investigated to predict the fatigue life of the structures and mechanical parts, the materials of which were prestrained by metal working such as machining, pressing, and forging. In this study, the relationship between the ultimate tensile strengthσ B of prestrained specimens and the crack growth rate dl/dN is the focus. The following results were obtained. Under the large-scale yielding (LSY) condition, there is a correlation between dl/dN and σB, where the dl/dN versus σnal characteristics are determined by σB regardless of the degree of prestrain. Under the small-scale yielding (SSY) condition, dl/dN appears to be determined by the stress intensity factor range ΔK rather than σB, in comparison with the case under the LSY condition. In addition, the relationship between σB and the cyclic yield strength σyc is discussed in terms of the cyclic stress-strain characteristics of prestrained 0.1% carbon steel and other nonstrained carbon steels. It is considered that σB represents the material resistance against the plastic deformation by which fatigue cracks propagate regardless of the existence of prestrain.

Energy-Based Micromechanics Analysis on Fatigue Crack Propagation Behavior in Sn-Ag Eutectic Solder

Sn-Ag-eutectic-based solders are replacing Sn-Pb eutectic solders in the electronics industry. The current paper extends the recently developed approach based on phase transformation theory, micromechanics, and fracture mechanics to treat fatigue crack nucleation and propagation for steels and alloys to predict fatigue crack propagation in solder alloys. To verify the proposed method, fatigue experiments were conducted on Sn-3.5Ag solder alloys. Finite element analysis is performed to predict the stress intensity factor range ΔK and the required energy U to increase the crack by a unit area. Unified creep-plasticity theory and a cohesive zone model are incorporated to predict the creep and hysteresis effects on fatigue crack propagation in solder and the interfacial behavior between the solder alloy and the intermetallic layer, respectively. With U determined numerically, the predicted fatigue crack propagation rate using phase transformation theory is compared with experimental data for Sn-3.5Ag and Sn-37Pb eutectic solders. Reasonable agreement between theoretical predictions and experimental results is obtained.

Factors Affecting the Crack Growth Rates of Reactor Pressure Vessel Steels under Simulated Boiling Water Reactor Conditions

Abstract Crack growth rate (CGR) measurement is an important tool to understand the behaviors of environmentally assisted cracking (EAC) of various materials used in light water reactor (LWR) environments and to produce quantitative data for structural integrity analyses, as well as for dispositioning existing or postulated defects. In this paper, the results of corrosion fatigue (CF) and stress corrosion cracking (SCC) experiments with two different reactor pressure vessel (RPV) steels at 288°C (˜561 K) under simulated boiling water reactor (BWR) conditions are presented. CGR was found to be dependent upon various mechanical factors such as the stress intensity factor range (ΔK), loading frequency, and hold time (ΔtH) at maximum load. In CF tests, it was observed that whether the loading frequency was high or low, the fatigue crack growth rate (FCGR) increased as ΔK increased under the same water quality and loading conditions. In SCC tests, the longer the ΔtH, the slower the CGR. On the other hand, it w...

3333 ナノクリスタルAl合金製CT試験片における疲労き裂進展挙動(S18-3 微細粒組織と強度,S18 材料の強度と組織)

The present study has investigated crack growth behavior in the nano-crystal powder alloys of GIGAS 20, 30 and 50. Crack propagation tests were made on CT specimens of these alloys at 297K in air under tension-to-tension fatigue loads at three levels of stress ratios, R=0.05 to 0.2. Curved crack propagation was observed in CT specimens of GIGAS 20 and 50, whereas cracks always propagated straight in those of GIGAS 30. Crack closure effects were measured by strain gages mounted near crack tips and on the back surfaces. Crack propagation paths were tracked by advanced CCD camera mounted on a tri-axial X-Y-Z positioning table. The values of the mode I and II stress intensity factors K_I and K_II for curved cracks so observed were calculated by FEM. Crack propagation rates da/dN were plotted against the effective stress intensity factor range ΔK_<eff> and compared for three types of the present nano-crystal alloys tested. The relations between instantaneous crack propagation angle θ and K_II/K_I ratio were also calculated and found to take a sigmoidal curve.

3903 デジタル画像相関法を用いたき裂開口変位分布計測と疲労き裂伝ぱ速度の評価(S17-1 デジタル画像相関法,S17 実験力学における最近の展開)

A digital image correlation method was used for measurement of COD distribution along the crack using slip pattern around fatigue cracks. In this measurement, selecting a subset in high contrast region was important for correct search. In addition, location of subsets near the crack and distance between subsets crossing the crack were found to have less effect on the COD distributions. Based on the COD distributions, stress intensity factor range ΔK_<est> was evaluated and used for estimating fatigue crack propagation rate.