Fatigue Striation Spacing Research Articles

Strain controlled cyclic deformation behavior of an extruded magnesium alloy

Fatigue properties of an extruded AZ31B magnesium alloy were evaluated using strain-controlled push–pull cyclic tests at different total strain amplitudes at room temperature. The alloy exhibited an asymmetric sigmoidal-shaped hysteresis loop due to twinning in compression during the unloading phase and detwinning during the loading phase. As the total strain amplitude increased, the asymmetry of hysteresis loops, plastic strain amplitude, mean stress, and stress amplitude increased, while the ratcheting strain and pseudoelastic modulus decreased. As the cyclic deformation progressed at a given total strain amplitude (greater than 0.3%), an abrupt increase in the plastic strain amplitude was observed, representing the onset of fatigue crack initiation. The extent of this increase process, which decreased with increasing total strain amplitude, corresponded to the fatigue crack propagation prior to the final fast failure. Fatigue crack initiation was observed to occur at the specimen surface, and fatigue crack propagation was characterized by typical striations. The smaller spacing of fatigue striations and larger fatigue crack propagation zone at the lower total strain amplitude gave rise to a longer fatigue life. The Coffin–Manson and Basquin's relationships can be used to describe the fatigue lifetime of this alloy.

Laser and shot peening effects on fatigue crack growth in friction stir welded 7075-T7351 aluminum alloy joints

The influence of shot and laser peening on the fatigue crack growth behavior of friction stir welded (FSW) aluminum alloy (AA) 7075-T7351 sheets was investigated. The alterations resulting from this surface modification on the fatigue crack growth of FSW were characterized and evaluated for two different crack configurations. A systematic investigation of the various peening effects indicated a significant decrease in fatigue crack growth rates resulting from using laser peening compared with native welded and unwelded specimens. In contrast, shot peened specimens did not result in a significant reduction in fatigue crack growth. The fatigue striation spacings for the laser peened specimens were assessed and found to be small compared with the unpeened, and shot peened specimens. The reduction in striation spacing indicates a slower fatigue crack growth rate and is partially attributed to the deeper compressive residual stresses induced by the laser peening.

Reconstitution of fatigue crack growth in Al-alloy 2024-T3 open-hole specimens using microfractographic techniques

Fatigue is one of the main problems in the provision of service life and safety of aircraft structures. The menace of fatigue cracking is accentuated in areas of stress concentration, e.g., joints of structural components. An example is the fuselage where riveting is used. One of the techniques for improving the fatigue life of these connections is the cold expansion of the rivet hole. As part of a larger project on the fatigue behaviour of aeronautical structures, an experimental study of open-hole specimens in Al-alloy 2024-T3, with and without hole expansion, is presented. The residual stress field created by the cold expansion was experimentally assessed by using the X-ray technique and predicted by FEA. Fatigue tests were supplemented by SEM measurements of fatigue striation spacing along longitudinal and transverse directions in the crack surface of each specimen. Empirical models and fractographic techniques developed by Nedbal et al. are used for the analysis of the experimental data, and results of quantitative microfractography are presented. Crack tunnelling was quantified based on the reconstituted crack history and on the surface crack growth measurements.

Fatigue striation spacing and equivalent initial flaw size in Al 2024-T3 riveted specimens

One of the main concerns of aeronautical structures is structural damage due to fatigue. This phenomenon is accentuated in areas of stress concentration, namely the connection of components. The fuselage is one of the most important structural elements where the connection of the fuselage panels is often done by riveting. Specimens with several geometric configurations are used to study these structures. This work is concerned with the study of one of these specimens, i.e., the lap splice with three rows of rivets and one rivet column. It is part of the IDMEC-Porto contribution to the European project ADMIRE. Stress versus number of cycles results and measurements of fatigue striation spacing along two perpendicular directions using scanning electron microscopy (SEM) are presented. The fractographic reconstitution of fatigue crack growth was carried out. The value of equivalent initial flaw size (EIFS) is calculated using the back extrapolation technique and two models of fatigue crack growth. They consist of a model using the Paris law and another the strain energy density factor concept. The EIFS is used to predict the fatigue behaviour of this structural detail. It is intended to verify the applicability of empirical models based in experimental evidences.

Residual stress effect on fatigue striation spacing in a cold-worked rivet hole

The residual stress effect due to cold-working is studied in relation to fatigue striation spacing. Cold-working introduces a compressive stress field around the hole reducing the tendency for fatigue cracks to initiate and grow under cyclic mechanical loading. It is known that fatigue lifetime assessment requires a detailed knowledge of the residual stress profile. X-ray diffraction and 3D finite element analysis (FEA) can be used to determine the residual stress profile. Scanning electron microscopy (SEM) measurements were performed for measuring the striation spacing.

Fatigue behavior of 20% cold-worked 316 stainless steel under in situ irradiation with 17 MeV protons at 60 °C

Load-controlled fatigue tests in the mode of tension–tension were performed for the side-notched 20% cold-worked 316 stainless steel under in situ irradiation and following irradiation with 17 MeV protons at 60 °C. In comparison with the unirradiation tests, fatigue life was substantially prolonged for in situ irradiation tests, while a slight increase of fatigue life was detected in post-irradiation condition. The SEM measurements of spacing of fatigue striations on fracture surface suggested higher resistance to fatigue fracture in the in situ irradiation specimens. Some essential differences in the irradiation effects between in situ and post-irradiation conditions were summarized in this paper.

A remaining life assessment of a cracked attemperator steam line

The present paper attempts to describe a series of detailed remaining life assessments of a cracked reducer weld location in a attemperator steam line in a 120 MW fossil-fired unit. Crack growth occurred by either pure fatigue or by creep-fatigue depending upon the combinations of stress and crack depth, i.e. maximum stress intensity level, K MAX. From consideration of certain fatigue and creep threshold details and measured fatigue striation spacings on the failure surface a stress of 80 MPa was estimated to be driving the cracking process. Also high temperature fatigue crack growth data was treated in terms of a series of constant probability crack growth curves which allowed real probability values to be assigned to any particular remaining life assessment. It was shown that for probability levels of 10 −4 and an estimated active stress of around 80 MPa the remaining life of initial 4 mm and 7 mm deep cracks was 231 and 87 starts respectively.

Fatigue fracture of AMg6N alloy under loading at sound frequencies

Cyclic crack growth resistance tests of AMg6N alloy under loading at frequencies of 20 to 10 kHz have demonstrated that the rate of fatigue crack propagation decreases with increasing frequency and the threshold stress intensity factors increase exponentially with the frequency of strain cycling. Fractographic observations of fracture surfaces of the specimens have revealed that an increase in the loading frequency is accompanied by a decrease in the fatigue striation spacing and in the size of the striation microzones by intnsifying the processes of secondary cracking and the formation of fretting products. This leads to a decrease in the rate of fatigue crack propagation and an increase in the threshold values of the stress intensity factors. An increase in the asymmetry of the loading cycles reduces the contribution of delamination and the formation of fretting products to the process of fracture of the alloy and results in a smaller fraction of the striation relief and in an earlier occurrence of the elements of quasistatic fracture by dimples, which is the cause of the reduction in characteristics of the cyclic crack growth resistance under asymmetrical loading.

QUANTITATIVE FRACTOGRAPHIC ANALYSES OF FATIGUE CRACK GROWTH IN LONGERONS OF IN‐SERVICE HELICOPTER ROTOR‐BLADES

Abstract— An analysis is made of striation spacing measurements as observed on fatigue crack surfaces of specimens and longerons of helicopter rotor‐blades made from the aluminium alloy AVT. It is shown that, for the combined cyclic tensile stresses at various R‐ratios and static rotation angles, α, striation spacings were controlled by an equivalent intensity factor Ke=K1F(α, R). Values of the F(α) for Ri= const were computed.The crack growth life in terms of the number of flights and stress equivalents were computed for various longeron sections of the helicopter rotor‐blades which had failed in service.SUMMARY AND CONCLUSIONS(1) A fatigue striation spacing analysis was made from measurements on AVT Al‐alloy specimens tested in air under combined cyclic tension and static‐rotation loads at various a‐angles of rotation, and stress ratios, R. Fractographic analyses have shown similarities in terms of striation spacing formation in the investigated range of α‐angles and R ratios.(2) The similarity in the fatigue fracture mechanisms attested to the possibility of characterizing the striation spacing formation by a unified kinetic diagram via a Ke= K1F(α, R) approach for different α‐angles and R ratios. The function F(α, R) was determined and it can be used to calculate the tensile stress equivalent according to values of σi measurements taken from components broken in tests or in service.(3) It was a longeron of the rotor‐blade of helicopters manufactured from AVT Al‐alloy broken in service and tested cyclically that were investigated fractographically. An analysis of the formation of striations and their spacing has shown similarities with the investigated longerons fatigued in tests and in service. The crack growth life versus number of flights and stress equivalent was calculated according to values of the σi measurements.(4) It was shown that a fatigue crack will not nucleate in longerons in service if the depth of defects on the external surface have a size less than 0.3 mm. The stress for tested longerons is near to the calculated equivalent for longerons that failed in service.

FRACTOGRAPHIC ANALYSIS OF FATIGUE CRACK GROWTH IN ENGINE COMPRESSOR DISKS OF Ti‐6Al‐3Mo‐2Cr TITANIUM ALLOY

Abstract—Fractographic features related to fatigue crack growth in a Ti‐6Al‐3Mo alloy are studied using compressor disks tested on a hydraulic test bed and which simulate operational multiaxial cyclic loading conditions. The hold‐time of a cycle results in the formation of a fracture relief which reflects mainly the two‐phase (α+β) lamellar structure of the titanium alloy and a fragmentary fatigue striation formation. Correlation between the number of fatigue striations on the fracture surface and the number of applied blocks of loading (imitating the service conditions of compressor disks) has been obtained. The hold‐time duration of the cycle does not affect the crack growth rate and the formation of the fracture relief in this material. An analytic expression is suggested to describe the relationship between fatigue striation spacing, δ, and the stress intensity factor KcI as applied to quarter‐ellipse‐shaped cracks; it is of the form δ= C[f(τ, FCi)KcI]4, where f(τ, FCi) accounts for the hold‐time, τ, and the programmed loading together with their influence on the fatigue crack growth behaviour. The particular threshold value of stress intensity factor (KcI) is established at 20 MPa m. The work indicates that the role of τ manifests itself via a considerable acceleration of crack growth.

A thermal-transient test of an FBR piping-bellows model: Saito, T., Umeda, H., Kanazawa, S. Watashi, K. and Imazu, A. Int. J. Pressure Vess. Piping 1990 44, (1), 117–135

Four-point-bend fatigue experiments are conducted on rectangular beam samples of a Zr50Cu37Al10Pd3 (in atomic percent) bulk metallic glass (BMG). In general, a fatigue crack initiates from the corner of the beam sample at the tensile stress surface and subsequently grows inside during cyclic loading. As a result, many shear bands form around the crack growth path due to the plastic deformation at the crack tip. Moreover, the size of the shear-band zone shows a good relationship with the stress intensity factor range, which is similar to the plastic zone in the crystalline alloys. Moreover, the shear-band spacing is consistent with the coarse fatigue striation spacing, which implies that the fatigue-crack growth in BMGs is related to the formation of shear bands. A reasonable mechanism is proposed to understand the fatigue-crack-growth behavior in BMGs.

PbSnSbIn solder alloy: Kawai, K., Fukuma, N. and Matsui, A. Patent: US4869871, USA Mar. 1989, Off. Gaz. Sept. 1989

Fatigue is one of the main problems in the provision of service life and safety of aircraft structures. The menace of fatigue cracking is accentuated in areas of stress concentration, e.g., joints of structural components. An example is the fuselage where riveting is used. One of the techniques for improving the fatigue life of these connections is the cold expansion of the rivet hole. As part of a larger project on the fatigue behaviour of aeronautical structures, an experimental study of open-hole specimens in Al-alloy 2024-T3, with and without hole expansion, is presented. The residual stress field created by the cold expansion was experimentally assessed by using the X-ray technique and predicted by FEA. Fatigue tests were supplemented by SEM measurements of fatigue striation spacing along longitudinal and transverse directions in the crack surface of each specimen. Empirical models and fractographic techniques developed by Nedbal et al. are used for the analysis of the experimental data, and results of quantitative microfractography are presented. Crack tunnelling was quantified based on the reconstituted crack history and on the surface crack growth measurements.

Correlation between fatigue crack growth rate and striation spacing in XZM

Much information can be learned from the fracture surface of a metallic component. At a microscopic level, the relationship between a fatigue fracture surface and the rate at which the crack propagated through the material is of particular interest. At certain growth rates, a fatigue fracture surface is comprised of striations which are the results of a cyclical loading being applied to a component. When evaluating a fatigue fracture surface to determine crack growth rates, a Transmission Electron Microscope (TEM) is employed to observe the striations. However, the following question arises, “Does one fatigue striation equal one load cycle”? In previously published literature, the relationship of one striation being equal to one load cycle has been confirmed for various aluminum alloys. The test data presented herein is for steel, i.e., a High Hot Hardness (HHH) gear material utilized in helicopter transmissions. The correlation of one striation being equal to one load cycle was obtained and documented during a fatigue crack growth test of X2M gear steel. A prime use of this data would be in the evaluation of a fatigue fracture surface to determine the initiation time and propagation rate for a service related failure.

The plane problem of thermal conductivity and thermal elasticity for a finite piecewise uniform body with cracks

ture of polycrystalline minerals," Author's Abstract of Doctoral Thesis, Technical Sciences, Kiev (1979). 3. A. A. Shanyavskii and V. M. Grigor'ev, "The kinetic fatigue failure curve of structural elements in the Paris stage under various forms and conditions of loading," in: The Crack Resistance of Materials and Structural Elements: Summaries of Papers for the Second All-Union Symposium on "Fracture Mechanics" [in Russian], Vol. III, Kiev (1985), p. 89. 4. A. A. Shanyavskii, A Method of Qualitative Evaluation of the Parameters of Fatigue Failure of Aircraft Parts from the Results of Fractographic Analysis [in Russian], Otd. Nauch. Tekh. Inst. Gos. Nauch.-Issled. Inst. Grazh. Aviatsii, Moscow (1985). 5. J. J. Au and J. S. Ke, "Correlation between fatigue crack growth rate and fatigue striation spacing in AISI 9310 (AMS)," in: Fractography and Material Science. ASTM STP 733, ASTM, Philadephia (1981), pp. 202-221. 6. A. A. Shanyavskii, "The curve of discrete fatigue crack growth under conditions of selfmodeling," Fiz. Khim. Mekh. Mater., No. i, 45-52 (1983). 7. A. A. Shanyavskii and A. Yu. Sasov, "Metrological aspects of quantitative fractographic investigations of the reasons for failure of aircraft parts in service," in: Increasing the Reliability of the Parts of Aircraft Assemblies and Evaluation of their Technical Condition during Servicing and Repair [in Russian], Kiev. Inst. Inzh. Grazh. Aviatsii, Kiev (1985), pp. 75-81. 8. A. A. Shanyavskii and M. Zo Koronov, "Fractographic evaluations of the parameters of service fatigue failures," in: Proceedings of the State Scientific-Research Institute for Civil Aviation [in Russian], No. 260 (1987), pp. 18-24. 9. A. A. Shanyavskii, Ko V. Vantsovich, and A. V. Karasev, "The rules of development of blind fatigue cracks in the biaxial stressed state of elements of aircraft structures," ibid., No. 238 (1985), pp. 57-61.

FRACTURE TOUGHNESS OF NODULAR GRAPHITE CAST IRONS UNDER STATIC, IMPACT AND CYCLIC LOADING

Abstract— The paper deals with the fracture toughness of nodular cast irons of differing microstructure and manganese content. Fracture toughness was determined for static and impact loading in the temperature range from 77 to 523 K. The dependences of fracture toughness on manganese content and microstructure were obtained.Fatigue crack growth rate curves were obtained at room temperature and a correlation between the parameters characterizing fatigue crack growth rate was found.Fractographic studies revealed an important role of graphite inclusions in the process of fracture. On the one hand, as stress raisers they favour the nucleation of microcracks while on the other hand, they cause local retardation of fatigue crack growth probably due to the residual compressive stress surrounding them. In the mid‐range of the fatigue crack growth rate curve a correlation was found between fatigue striation (or blocks of striations) spacings and the crack growth rate.

Spacing of fatigue striations and crack growth rate

Spacing of fatigue striations and crack growth rate

腐食損傷面に対する２次元画像処理技術の応用

Two-dimensional quantitative analysis of SEM (Scanning Electron Microscope) images of fracture and corrosion damaged surfaces was made by using the computer image processing and pattern recognition technique. This SEM image analysis was found to be successful with enough accuracy for automatic measurement of fatigue striation spacing, identification of corrosion pits and their statistical distribution, and classification of intergranular and transgranular cracking portions. The quantitative and statistical analysis of SEM images, which have required specialists's experience, a great deal of labor and much time, can be carried out now quickly and accurately by using a digital computer with fast-processing capability.

The Effects of Hold Period on the Fatigue Crack Growth Rate of 2 1/4 Cr-1Mo Steel at Elevated Temperatures

The effects of various loading wave forms on the fatigue crack growth rate of 2 1/4 Cr-1Mo steel at 482 and 538°C tested in air have been determined. Scanning electron fractography was used to measure the fatigue striation spacing on the fracture surface of strain controlled push-pull fatigue tests. The fatigue striation spacing was then used as a monitor of the fatigue crack growth rate in these specimens once a fatigue crack had been initiated. Constant strain hold periods were introduced during each cycle at various points of the hysteresis loop. Both increasing temperature and strain range increased the fatigue crack growth rate for continuously cycled tests as expected. However, when a hold period was introduced, the crack growth rate was dominated by the time that preceded each strain reversal; temperature and strain range effects became secondary to hold time. The effect of hold periods for the conditions examined is primarily crack tip oxidation rather than creep crack growth during the hold period.

ＳＵＳ３０４鋼およびＡ５３３Ｂ鋼の溶接継手における疲労き裂伝ぱ挙動

Fatigue crack growth characteristics in type SUS 304 stainless steel shielded-metal-arc weldments and A533 Grade B Class 1 steel submerged-arc weldments were studied using CT specimens at room temperature. The growth rates in SUS 304 stainless steel weldments were also examined at-100°C to evaluate the effect of low temperature on the growth rate of weldments. The low temperature was achieved by changing the flow quantity of liquid nitrogen. Crack closure of the weldments during fatigue crack growth was measured by a compliance method. An electron fractographic analysis was employed to determine the mechanism of crack growth and to measure striation spacings.The resistance to crack growth in the SUS 304 stainless steel weldments was considerably higher than that of the base metal due to the presence of high compressive residual stress at the crack tip in the CT specimen. The growth rates in the stress relieved weldments in A533B steel and the base metal fell within a relatively narrow band of scatter. Therefore, the growth rates of stress relieved weldments were found not to be sensitive to the changes of microstructure or strength of materials. The fatigue crack growth rates in the SUS 304 stainless steel weldments at -100°C were slightly reduced as compared with those of the weldments at room temperature. The crack growth mechanism in the weldments of both steels was controlled by striation formation, even at -100°C. The fatigue crack growth rates and striation spacings in the base metal and the weldments with residual stress at different temperatures were closely correlated with ΔKeff based on the crack closure measurement.

高強度鋼の衝撃疲労および衝撃でない疲労のき裂進展速度の比較について 衝撃負荷と応力比および強度レベルの関係

The impact fatigue tests were carried out by using a rotating disk type impact fatigue testing machine. The influence of impact loading, stress ratio and yield strength on fatigue crack growth rate was investigated by means of fracture mechanics and fractography in quenched and tempered 0.50% carbon steel. Then, the results of impact fatigue tests were compared with those of nonimpact fatigue tests. The results obtained are summarized as follows.(1) For the 423K tempered material, the dominant fracture appearance at impact fatigue was the dimple mode and the cleavage facet while that at non-impact fatigue was the intergranular facet. Such a fracture appearance was called the monotonic one. It was apparent that the acceleration of crack growth rate resulted from the rapid increase in the area percentage of monotonic fracture appearance regardless of the impact fatigue and the non-impact fatigue.(2) For the other tempered materials except the 423K tempered one, the striation was the dominant fracture appearance. However, the fracture appearance of the 573 and 873K tempered ones contained some inclusions and precipitation particles among the striation.(3) The effect of impact loading, Eac, defined by the ratio of impact fatigue striation spacing, Simp, to non-impact fatigue one, Snon, approached to about 1 as the yield strength, σys increased. The effect of impact loading could also be replaced by the equivalent stress ratio, Req. Then, the Req has related to σys as follows, Req=1-α·Eac-0.5=1-1.88×10-3·α·σys0.84.The Req decreased with increasing σys. This tendency was similar to the effect of stress ratio in non-impact fatigue.(4) The impact fatigue crack growth rate at a given ΔK level was larger than that in non-impact fatigue. This difference in crack growth rate increased with increasing tempering temperature. This fact could be well explained by the concept of equivalent stress ratio.