Crack Closure Behavior Research Articles

荷重制御下の弾塑性疲れき裂進展 線形破壊力学パラメータの有効範囲と塑性変形の影響

Load-controlled fatigue crack growth tests were carried out on several kinds of ductile materials over a wide region from the linear elastic to the post-yield region, and crack closure behavior was monitored through the tests. The effect of plasticity on crack growth behavior and the limit of validity of the linear fracture mechanics parameters were discussed along with ASTM E647 specimen size requirements. The bulk cyclic plastic strain, even if so small, had the effect to lower the crack opening point. ASTM size requirements were found rational for CT-specimens, while for center cracked specimens were not always sufficient to provide a stable relationship between crack growth rate, da/dn, and stress intensity range, ΔK.In addition, peculiar growth behavior of post-yield crack under variable loadings was also briefly noted in the present study.

A consideration of the unloading elastic compliance method.

In this paper unloading elastic method is investigated by the analytical results based on the D-B model and by the measured hysteresis curve of load-strain. The crack closure behavior is always accompanied with plastic deformation. When the sensitivity of the measurement is not so high as it detect plastic deformation, the hysteresis looks like to be straight in the range where the crack tip is open. In this case, it is difficult to measure the crack opening point from the hysteresis curve. The unloading curve under high accuracy is found to be a "S" shape, and the point of inflection of the unloading curve is considered to be the crack closure point. The crack closure point does not coincide with the opening point. The crack propagation rate under step loadings can be expressed unifyingly by the effective stress intensity range, ΔKcleff=Kmax-Kcl, independently of the loading history. Based on the "S" shaped unloading curve, it is shown that the tip of a non-propagating crack does not open even under the maximum tensile load during one loading cycle.

Near-threshold fatigue crack propagation rates of dual-phase steels

Specimen geometry (center notched CN versus single edge notched SEN) has been shown to affect the near-threshold fatigue crack propagation rate in dual-phase HY80 and type 1018 steels. The stress intensity range threshold ΔK th values of the SEN specimens were higher than those of the CN specimens. Closure stress, scanning electron microscopy fractography and surface roughness measurements were made to characterize the near-threshold crack closure behavior. In all cases, closure stress intensities K cl at a given stress intensity range ΔK were much higher in the SEN specimens. When ΔK th is corrected for crack closure to compute the effective stress intensity range threshold ΔK eff, th, the specimen geometry effect disappears within experimental error. Surface-roughness-induced crack closure is shown to be the origin of the specimen geometry effect.

高張力鋼の疲労き裂進展とき裂開閉口挙動に及ぼす真空効果

Environmental fatigue crack growth tests of a high-tensile strength steel HT 55 have been conducted in dry air and vacuum, and crack closure behavior as well as fracture surface morphologies were investigated. The crack growth rate da/dN was significantly reduced in vacuum due to fracture surface rewelding. da/dN in vacuum at R=0.1 was equivalent to the one at R=0.5 in case of ΔKeff greater than 15MPa·m1/2, whilst the former became higher than the latter at ΔKeff less than 15MPa·m1/2. Roughness-induced unconformity of fracture surfaces is considered to prevent the rewelding of newly exposed surface at low R. At high R, on the other hand, no crack closure and no prevention of the rewelding were observed, thereby reducing the growth rate particularly near the threshold region. The crack opening value Kop rised immediately and the growth rate decreased rapidly with the environmental change from dry air to vacuum. On the contrary, a rapid increase of da/dN as well as a rapid decrease of Kop was observed with the environmental change from vacuum to dry air. Fracture surface morphology also varied abruptly with the environmental change. It is concluded that an absorption of gas molecules to newly created surface affects directly the crack closure behavior. Clay-like products induced by a remarkable plastic flow and rewelding were observed on the fracture surface in vaccum.

爆接ステンレスクラッド鋼の疲労き裂進展挙動

The fatigue crack growth and crack closure behaviors of an explosively bonded stainless clad pressure vessel steel plate (SUS-SPV plate) were studied by using the through thickness crack specimens and the surface crack specimens. The observed behaviors of the fatigue crack growth and crack closure were complicated, mainly due to the influence of the residual stresses which had been introduced in the clad plate through the production process. The shape of the crack was also influenced strongly by the residual stress in the surface crack specimens. The crack was elongated in the thickness direction when the crack was propagated from SUS to SPV, while it was elongated in the surface direction when the crack was propagated from SPV to SUS. The exceptionally high and low values of the aspect ratio, a/c, of the surface crack, the maximum value of 2.2 and the minimum value of 0.35, were observed. The stress intensity factor range, ΔK, was not in good correlation with the crack growth rate for both types of specimens. The effective stress intensity factor range, ΔKeff, was successfully correlated with the crack growth rate for all the specimens tested in this study, including those of the parent materials of the clad plate.

Study on Crack Growth Behaviors in Impact Fatigue : Part II, Crack Clusure Behavior under Simple Impact Load

Fatigue crack growth behaviors were investigated under a simple impact stress pattern by using center-notched 0.1%C aluminum killed carbon steel plate specimens, and results obtained were discussed in comparison with those obtained under non impact fatigue. At first, the fact that the crack growth rate in impact fatigue was higher than that in non impact fatigue and that there was a stress range dependence of impact fatigue crack growth rate demonstrated in the previous study done under somewhat complex impact stress pattern confirmed also in this study. Furthermore, the stress range dependence as mentioned above for impact fatigue was explained reasonably from the viewpoint of crack closure behavior, and crack tip plastic zone characteristics under impact fatigue loading were discussed tentatively based on the variation trend of effective stress range, U, or crack opening stress intensity factor, Kop, during crack growth process.

低ΔＫ領域におけるポリカーボネートの疲労き裂進展挙動に及ぼす応力比の影響

The effects of stress ratio R on fatigue crack propagation rate and fatigue fracture surface appearance of polycarbonate at low stress intensity factor range ΔK level were investigated. Fatigue crack closure behavior of these specimens was also examined. From these experiments, it was found that the dependence of fatigue crack propagation rate on R, which was observed in the relation between crack propagation rate and ΔK, was considerably masked by using the effective stress intensity factor range ΔKeff. But, the effect of R on crack propagation mechanism was observed, and so the data of crack propagation rate at each R were not perfectly well expressed even in terms of ΔKeff.Discontinuous growth bands (D. G. B.) were observed only when R was negative, and as R was decreased, D. G. B. seemed to be more easily formed. Fatigue crack closure also seemed to contribute to D. G. B. formation. These results suggested that the cyclic compressive stress at the crack tip played an important role for D. G. B. formation. This was considered to be due to the easy initiation of a single craze at the crack tip which was responsible for D. G. B. formation when cyclic tensile and compressive stress existed at the crack tip.

超音波法によるＳＣＭ４３５鋼の疲労き裂進展特性の評価

Fatigue crack growth tests in a wide range of fatigue crack propagation rates covering the threshold condition were conducted at various stress ratios using a quenched-tempered and high strength steel SCM435 with changing heat treatment. The crack closure behavior due to both the plastic deformation at the crack tip and the formation of oxide deposits on crack faces at an ultralow, near-threshold propagation rate were confirmed experimentally by the proposed Top-on ultrasonic method. The influences of stress ratio, strength level and microstructure on fatigue crack growth characteristics were discussed on the basis of the crack closure concept. It was shown that the fatigue crack propagation rate, da/dN, can be related uniquely to the effective stress intensity factor range, ΔKeff, and the threshold level defined at da/dN=10-8mm/cycle in terms of effective stress intensity factor range, ΔKeff, th, are nearly constant regardless of stress ratio and strength level. However, the relationship between da/dN and ΔKeff at such a high stress ratio as R=0.8 where the plasticity induced crack closure and the oxide induced crack closure don't exist, didn't agree with the above relationships at R=0.05, 0.15, 0.4 and 0.6, and ΔKeff, th was Large.

高張力低合金鋼における下限界近傍の疲労き裂進展挙動に及ぼす材料微視構造および応力比の影響

The effects of stress ratio and microstructure on the fatigue crack propagation near the threshold in a high-strength low alloy steel were analysed based on the crack closure measurement and the microscopic observation of fracture surfaces. The following results were obtained.(1) The effects of microstructure and stress ratio on the relation between the crack propagation rate and the stress intensity range were large in region A near the threshold. The threshold stress intensity range increased linearly with the square root of the mean free path. When compared at the same crack propagation rate, the relation between the stress intensity range and the stress ratio was dividable into two regions, i.e., Kmax controlled region with low R values, and ΔK controlled with high R values.(2) Region A was dividable into regions A1 and A2 in the relation between the rate and the effective stress intensity range. In regions A2 and B, the rate was expressed in a unique power function of the effective range without respect to the microstructure and the stress ratio. In region A1, very close to the threshold, the rate depended on the microstructure but not on the stress ratio. Thus, the effect of stress ratio on the crack growth rate can be ascribed to its effect on the crack closure.(3) The transition from region A to B can be caused by either the transition in crack closure behavior or the change in crack growth mechanism. The latter condition becomes predominant when the reverse plastic zone size is equal to the mean free path of dislocation motion. The transition in crack closure is dependent on the ratio of the reversed plastic zone size to the prior-austenite grain size as well as on the material itself.

定常変動荷重下の疲労き裂進展とき裂開閉口挙動 （アルミニウム合金の場合）

Fatigue crack propagation tests were carried out on an aluminum alloy, ZK141-T7, under stationary varying loading conditions including random loading at relatively high stress intensity region and crack closure behaviors were investigated.It was found that the crack opening point under varying loadings was determined by the maximum range-pair of K and its R ratio and equal to that of the constant amplitude test of the same ΔK and R. Moreover, the estimation method of fatigue crack propagation rate based on the modified Miner's rule of crack propagation in terms of the effective stress intensity range-pair, which was proposed previously for the tests on steels, was confirmed to be applicable in this case, too.

STRESS‐RATIO EFFECT OF FATIGUE CRACK PROPAGATION IN A BIAXIAL STRESS FIELD

AbstractFatigue crack propagation tests were conducted under conditions of equibiaxial, uniaxial and shear loading by using a cruciform specimen in a servo hydraulic testing machine. The effect of non‐singular stress cycling on the fatigue crack propagation rate was examined based on the observation of crack opening behavior. The crack propagation rate was significantly influenced by the non‐singular stress parallel to the crack when it was correlated to the stress intensity range. The crack closure behavior was greatly affected by the non‐singular stress. The crack propagation rate was uniquely correlated to the effective range of the stress intensity factor except for the case of completely reversed shear loading where significant plasticity was detected. The crack opening displacement range was concluded to be a parameter controlling the crack propagation rate for all the stress conditions examined in the present experiments. Some discussion is made on the effect of material anisotropy on fatigue crack propagation in a biaxial stress field.

Low Cycle Fatigue Crack Growth under Fully Reversed Strain Control

Low cycle fatigue crack growth behavior of 2 1/4 Cr-1 Mo Steel is investigated under fully reversed strain control at room temperature for center cracked specimen. Application of the cyclic J-integral (ΔJ) approach is attempted.Experimental results for center cracked specimen under fully reversed strain control are also compared with those for center cracked specimen and CT specimen under slop line control, and those for CT specimen under load control.It is known that ΔJ is a successful parameter for low cycle fatigue crack growth and the estimation of crack opening behavior rather than crack closure behavior is important for the improvement of accuracy under fully reversed strain control.It is important to verify the crack growth estimation by the experimental data in application of ΔJ to structural components, therefore some investigation is made for the prediction of low cycle fatigue crack growth in center cracked specimen.

変動荷重下の疲労き裂進展とき裂開閉口挙動

変動荷重下の疲労き裂進展とき裂開閉口挙動

The effects of stress ratio and grain size on near-threshold fatigue crack propagation in low-carbon steel

The effects of the stress ratio and the grain size on the fatigue crack growth near the threshold in a low carbon steel were analysed based on the crack-closure measurement and the microscopic observations of cracktip slip deformation and the fracture surface. The low-rate region A was divided into regions A1 and A2 in the relation of the rate against the effective stress intensity range. In regions A2 and B, the rate was expressed in a unique power function of the effective range without respect to the stress ratio and the grain size. In region A1 very close to the threshold, the rate was slower for larger grain sized material, and the effective threshold stress intensity factor increased linearly with the square root of the grain size. The slip-band zone in this region was rather independent of the stress intensity and was sized by the grain dimension. A model of the crack-tip slip bands blocked by the grain boundary was confirmed to be useful for analysing very slow growth as well as the threshold condition. The shear-mode fracture surface observed on the surface in region A1suggests the repeated nucleation mechanism for crack growth. The effects of the stress ratio and the grain size on the crack closure behavior near the threshold was quantified.

爆接ならびにロール圧接した黄銅クラッド鋼の疲労き裂伝ぱ挙動とき裂開閉口

The fatigue crack growth and crack closure behaviors of an explosively-bonded and a roll-bonded brass-steel clad plate were examined. An emphasis was put on the comparison of the behaviors between the two kinds of clad plates manufactured through the different bonding processes.It was found that poor correlations were observed between the crack growth rate and the stress intensity range both for the explosively-bonded and the roll-bonded clad plates. The crack growth and closure behaviors were affected by the residual stresses which had been introduced in the production processes both for the two clad plates, although the reduced influence was observed in the roll-bonded clad plate. A good correlation was found between the crack growth rate and the effective stress intensity range, ΔKeff, which was calculated on the basis of the crack closure behavior. However, the adjustment was not fully successful for the crack growth behavior very near the material interface. Further studies are needed for more detail of the fatigue crack growth behavior of clad materials.

クラッド材の接合界面を横切る疲労き裂の伝ぱとき裂開閉口挙動

The fatigue crack growth and the crack closure behaviors of a clad material were examined by using an explosively-bonded brass clad steel plate, in the case when the crack was propagated across the interface of the clad material. The apparent fatigue crack growth behavior of this material was complicated. Both acceleration and retardation were observed near the material interface. Corresponding to these crack growth behaviors, the extremely high and low values of the crack opening ratio, U, were observed. It was found that there was a poor correlation between the crack growth rate and the stress intensity factor range, ΔK. It was concluded that this was mainly due to the residual stresses which had been introduced in the material production process. A good correlation was found between the crack growth rate and the effective range of the stress intensity factor, ΔΔKeff. The result of the finite element analysis on the relation between the residual stress and the crack closure supported the conclusion that the observed complicated crack growth behavior of the clad material was caused by the residual stress.

Kinetics of environmental fatigue crack growth in nickel-copper alloy: Part I. In vacuum and oxygen

Fatigue crack growth rates have been determined in a nickel-copper alloy in the mild test environments of vacuum and oxygen. It is found that the fatigue crack growth mode in both vacuum and oxygen is ductile and the growth rate is sensibly independent of the maximum stress intensity levels and stress intensity ratio. The growth rate is found to be lower in vacuum than in oxygen, and the growth rate dependency on the cyclic stress intensity range is more pronounced in vacuum than in oxygen. These differences in behavior may be consistent with differences in crack closure behavior for the two environments.

Ｋ?Sｔｈ?Tレベルをはさむ繰返し２段変動荷重下の疲労き裂進展

Fatigue crack propagation rate and crack closure behavior under repeated two-step tests composed of loads above and below the threshold condition were studied on a medium carbon steel and an aluminum alloy.By means of electron fractography, striations corresponding to high and low level loadings were found on the fractured surface of both materials, which confirmed that crack propagation occurred even below the threshold level under varying loading conditions. And such behavior of crack propagation was observed even under the pulsating loading test as well as the reversed loading test.The acceleration of crack propagation was found to be contributed by both reasons of the acceleration under high level loading due to the increase in the effective stress intensity range and the above mentioned crack propagation below the threshold level. Furthermore, the effect of the cycle ratio of high level load to low level one on crack propagation rate was investigated.

疲労き裂進展過程の走査電子顕微鏡による連続観察とき裂進展機構

Fatigue crack propagation tests were carried out on a pure iron and a grain oriented silicon iron by using a specially designed testing apparatus in a scanning electron microscope. It was found from the tests on the poly-crystalline pure iron that surface cracks showed different behaviors depending on the grains. So it seems difficult to estimate the fatigue crack propagation mechanism under plane strain condition from observations of the surface crack propagation behaviors.On the other hand, in the case of the grain oriented silicon iron nearly plane strain condition could be realized up to the specimen surface by selection of loading axis to the specific orientation of the material. From the sequential observations of crack opening and closure behavior on this material, the mechanism of Mode I type fatigue crack propagation was confirmed to be an alternating slipping-off process at the crack tip apex.Quantitative evaluation of a relation between crack tip opening displacement (CTOD) and crack growth length was given by considering the slip direction, the crack tip angle and the amount of rewelding.

A tentative explanation for two parameters, C and m, in Paris equation of fatigue crack growth

The two parameters, C and m, which characterize the Paris equation for fatigue crack growth are explained in relation to the crack closure concept suggested by Elber. It is proposed that the range of effective incremental change in stress intensity factor (ΔK) needed for crack growth should have a second power correlation with the growth rate. The crack growth is essentially determined by cumulative damage to the material in cycled plastic zone near the crack tip, and is relatively insensitive to the applied ΔK-values and the mechanical properties of material. However, the crack closure behavior is expected to depend on both the stress range and the material properties. Thus it is concluded that the exponent parameter m reflects mainly the dependency of crack closure behavior on ΔK. For example, in the case of m=4 the crack opening level increases linearly with increase in ΔK, while in the case of m=2 it remains constant. It is suggested that the cyclic straining at the crack tip possibly varies with ΔK, thus changing primarily the crack closure behavior rather than the damage accumulation process in the plastic zone.