Crack Opening Point Research Articles

Small Fatigue Crack Initiation and Growth behavior of Low Carbon Steel with Simulated HAZ Heat Treatment

Naturally initiated small fatigue crack and its growth behavior of several simulated HAZ microstructures were successively observed using automatic microscope system. Simulated HAZ microstructure specimen were prepared by heat treatment with controlling maximum temperature and cooling rate. In addition, crack initiation and early stage crack opening-closing behaviors were evaluated by Digital Image Correlation (DIC) technique. As a results, fatigue life of coarse grained microstructure was shorter than that of finer grained one due to the difference from not crack initiation life but crack growth life. Since initiated crack size, depending on microstructural unit size, of the coarse grained specimen was larger than the other, remained fatigue crack growth life became shorter. In addition, early stage crack opening stress of naturally initiated small fatigue crack were successfully measured by combination of automatic microscope system and DIC technique. This result showed that small fatigue crack just after initiation opens near the minimum load, under the compressive stress, and it was increased depending on crack length. However, crack opening stress of coarse grained specimen increased more slowly than that of finer grained specimen. This behavior also affected to fatigue crack growth life. Furthermore, early stage small crack growth life became comparable to long crack data by using effective J-integral range evaluated from measured crack opening point. Therefore, it was indicated that initial crack size and early stage crack closure model depending on microstructure are needed to calculate fatigue life.

Comparative assessment of cyclic J-R curve determination by different methods in a pressure vessel steel

Cyclic J-R behaviour of a reactor pressure vessel steel using different methods available in literature has been examined to identify the best suitable method for cyclic fracture problems. Crack opening point was determined by moving average method. The η factor was experimentally determined for cyclic loading conditions and found to be similar to that of ASTM value. Analyses showed that adopting a procedure analogous to the ASTM standard for monotonic fracture is reasonable for cyclic fracture problems, and makes the comparison to monotonic fracture results straightforward.

Effect of Mean Stress on Small Fatigue Crack Growth Rate on Low Carbon Steel with Several Simulated HAZ Heat Treatment

In this study, to clarify the effect of microstructure of Heat Affected Zone (HAZ) and residual stress of weld on small fatigue crack growth behavior, uniaxial fatigue testing with surface small fatigue crack growth observation were carried out for low carbon steel with three simulated HAZ heat treatment under several mean stress conditions. As a results, εna type small crack growth law was also effective to HAZ microstructure under zero mean stress. Under tensile mean stress condition, fatigue fracture life and small fatigue crack growth life were decreased. In addition, it is clarified that small fatigue crack growth rate acceleration under tensile mean stress was able to be evaluated by using Smith-Watson-Topper equivalent strain εeq which is obtained from mean stress dependency of fatigue fracture life for smooth specimen and εeqna type modified small crack growth law was proposed. Furthermore, it is clarified that εeq was proportional to εeff which was evaluated by crack opening point measured with Digital Image Correlation technique. It is considered that equivalent strain εeq implicitly includes small crack growth life and effect of crack closure.

Behavior of Short Fatigue Crack at Notch Root

It has been recognized that the threshold stress intensity factor range Kth of a short crack is lower than that of a long crack. The short crack behavior in plain specimen has been studied by many researchers. However, the behavior of a short crack at the root of a long notch is not yet clear. The crack closure behavior is considered to be affected by the constraint at notch root and it is dependent on the length and the root radius of notch. In this study, fatigue tests on specimens with short pre-crack at long notch were done and the difference in crack closure behavior was studied. As a result, short crack effect appeared in any notch root radius. In a sharp notch, the crack opening point easily reached its stable condition after a small amount of crack extension. On the contrary in a dull notch, the opening point was lower than the stable condition and consequently short crack effect lasted in relatively wide range of crack extension. The small crack effect of notched specimen was discussed based on crack closure behavior.

Noise reduction for fatigue crack growth test data under random loading

The noise included in electrical signal significantly influences on the estimating results of crack length and crack opening point. The noise can be effectively reduced by averaging some consecutive loading cycles without distortion of the signal for constant amplitude loading tests, whereas the averaging technique cannot be applied to random loading tests due to the variation of load. In this study, a noise reduction technique was developed by using a load-based averaging technique, which can be applied to fatigue crack growth test data under random loading. Additionally, the developed noise reduction method was applied to random loading test data to verify the effectiveness of the method.

331 スペクトラム波形の単純化(材料の動的特性)

A spectrum wave pattern for a plane is a load form based on the data in which many external forces including uncertain elements were collected from many flights. After overloading in a spectrum wave pattern, the delay of the fatigue crack growth may occur. In this study, the model spectrum wave was made by eliminating amplitude of little crack growth from the original spectrum wave pattern, in consideration of the crack opening point after overload. The crack growth behavior obtained from both waves were similar trend and were grouped to three crack growth rate regions such as 1) crack growth acceleration just after the maximum overload in overloads 2) crack growth by overloads after the maximum overload and retardation 3) retardation after the maximum overload. From three crack growth rates, the crack growth curve was able to be predicted.

A procedure for determining crack opening load from differential displacement data

In this study, a numerical differentiation method is proposed to determine the crack opening load from differential displacement data. The derivatives were calculated using a linear least squares fit. The crack opening point is determined from the third derivative of the differential displacement data. Calculating the derivative by using a straight fit line effectively reduces the scatter of the measured crack opening results. The proposed method was applied in practice to constant amplitude loading tests and the measured crack opening results were compared with those of the ASTM method. The proposed method gives stable and consistent results and the measured crack opening values are relatively reasonable.

Fatigue Crack Growth Behavior of Ti-6Al-4V Alloy with Bimodal Microstructure under Repeated Two-step Load Sequences

The effect of microstructure on fatigue crack growth behavior under repeated two-step load sequences was investigated using compact tension specimens of Ti-6A1-4V alloy with bimodal microstructure. Fatigue crack growth rate and macroscopic crack closure were measured by means of an unloading elastic compliance method. The da/dn-ΔKeff relationship exhibited a trilinear form in so-called Paris region corresponding to the change of growth mechanism from the intergranular fracture in the low ΔKeff region to the transgranular one in the high ΔKeff region. The crack opening point, Kop, was constant during one block of varying loading in the case that the crack extension during one block loading was relatively short. On the other hand, in the case that the crack grew beyond the cyclic plastic zone introduced by ΔKH, Kop under low level loading decreased from the initial high value to the stationary low one as the crack grew. Depending on the test conditions, the fatigue crack growth rate under varying loading was found to be higher or lower than that estimated by da/dn-ΔKeff relation under constant amplitude loading, which indicates that the load variation resulted in both crack grown acceleration and deceleration in terms of ΔKeff. The crack growth acceleration was attributed to the change of growth mechanism due to the load variation, while the crack arrest during one block loading brought about the crack growth deceleration.

Corrosion fatigue characteristics in the weld of multi-pass welded A106 Gr B steel pipe

In order to investigate the corrosion fatigue characteristics in the weld of multi-pass welded A106 Gr B steel pipe, corrosion fatigue tests were performed under the various stress ratios and 3.5 wt% NaCl solution at room temperature. The corrosion fatigue characteristic curves were represented using crack closure concept. The obtained results are as follows ; when the load frequency is 1.0 Hz, the crack opening point is transited in the region of Kmax=20-32 MPa-msu1/2. In the low stress intensity factor range, the crack opening point is higher than that in air. However, in the high stress intensity factor range, it is lower than that in air. In the cases of 0.1 Hz and 0.01 Hz, the crack opening point gradually decreases to K(min) with Kmax increase.

되풀이 소성영역 크기를 이용한 피로 균열 닫힘 거동의 유한요소해석

An elastic-plastic finite element analysis is performed to investigate detailed closure behaviour of<br/> fatigue cracks in residual stress fields and the numerical results are compared with experimental results. The finite element analysis performed under plane stress using contact elements can predict fatigue crack closure behaviour. The mesh of constant element size along crack surface can not predict the opening level of fatigue crack. Specially, the mesh of element sizes depending upon the reversed plastic zone size included the effect of crack opening point can precisely predict the opening level. By using the concept of the mesh of element sizes depending upon the reversed plastic zone size included the effect of crack opening point, the opening level of fatigue crack can be determined very well.<br/>

疲労き裂伝ぱを律するパラメータに関する考察

It is generally believed that ΔKeff is the most appropriate parameter which controls the fatigue crack propagation rate. On the other hand, the difference of the crack closing load from the crack opening load or the difference of the re-tensile plastic zone's generated load (RPG Load) from the crack opening load was also pointed out. However, the accuracy of usual measurements seems to be not sufficient to discuss quantitatively the above mentioned difference, in other words, the superiority of one of the fatigue crack propagation parameters ΔKop, ΔKcl, and ΔKRP.Under this background, successful measurement of the change in strain near fatigue crack tip with high accuracy was made in our previous report, and in this paper, matured consideration on the fatigue crack propagation parameter on the basis of the experimental results was carried out. From dividing the loading and unloading processes into several characteristic areas on the basis of the tendency which the compliance and the change rate of compliance show, the following observations have been obtained as conclusions.(1) In the unloading process, it seems to be impossible to isolate the area corresponding to the creation of the re-compressive plastic zone and there is also doubt about using the stress intensity factor because of the influence of crack closing. So it seems to be rational to adopt the area in the loading process, corresponding to the creation of the re-tensile plastic zone, for the fatigue crack propagation parameter.(2) According to the experimental results, it is clear that the re-tensile plastic zone's generated point disagrees with the crack opening point.(3) From the facts described above (1) & (2), it may be concluded that ΔKRP, not ΔKop nor ΔKcl, is considered the most suitable parameter which controls the fatigue crack propagation.

疲労き裂先端ひずみ変化の詳細観察

It is well known that the crack opening/closure phenomenon found by Elber plays an important role in the fatigue crack propagation behavior under any kind of loading condition. On the other hand, the retensile plastic zone's generated load (RPG Load) was newly proposed by Toyosada as for a fatigue crack propagation parameter in place of the crack opening/closure load. However, the accuracy of usual measurements seems to be not sufficient to discuss the fatigue crack opening/closure phenomenon quantitatively.In this paper, for the purpose of proper understanding of the fatigue crack opening/closure phenomenon, a detailed observation of change in strain near the fatigue crack tip was carried out. And successful measurement of the hysteresis curve of load vs. strain (ε') with high accuracy was made. By introducing the curve of compliance (dε'/dp) and the change rate of compliance (d2ε'/dp2) during one cycle, the following conclusions have been obtained.1) The unloading part of the hysteresis curve is found to be “S” shape, and the point of inflection of the unloading part is considered to be the crack closure point as Nisitani pointed out. So if the accuracy of measurement is high enough to be able to obtain a smooth curve of compliance, the crack closure point can be measured automatically as the maximum point of the unloading part of this curve.2) The loading part of the curve of compliance is found to be “S” shape, and the point of inflection of the loading part is considered to be the RPG point. So if the accuracy of measurement is high enough to be able to obtain a smooth curve of the change rate of compliance, the RPG point can be measured automatically as the minimum point of the loading part of this curve.3) Even though the accuracy of measurement is high, it seems to be difficult to measure automatically the crack opening point, because it shows no distinguished feature.

An automated procedure for determining crack opening level from differential displacement signal data

The crack opening point is commonly measured from load-displacement records or load-differential displacement records. In this study, an automated method is developed to determine the crack opening point from differential displacement signal curves. To improve measurement precision, a noise reduction method using a digital low-pass filter was employed. In order to assess the measurement accuracy and precision of the proposed method, computer simulation was extensively performed, generating differential displacement signals of various S/N ratios. The measured results of crack opening points for these simulated signals show good accuracy and precision. The proposed method was applied in practice to constant-amplitude loading, and two kinds of random loading: narrow- and wide-band random loading. As a result, the method is found to be very effective even for random loadings.

Analysis of Crack Growth under Cyclic Loading with Mean Stress by Means of Elastic-Plastic Finite-Element Method.

The stress-strain field was assessed by means of the elastic-plastic finite-element method in the vicinity of the crack tip of the through-thickness-center cracked specimen subjected to cyclic loading under test conditions where ratchet deformation occurs. Then the cyclic J-integral range was calculated and its validity was examined. Both the maximum and minimum axial total strains at the crack tip during one stress cycle were greatly increased when ratchet deformation was generated. The path independence of ΔJop failed with the beginning of ratchet deformation, where ΔJop is cyclic J-integral range corresponding to load ranges above a crack opening point. Cyclic J-integral range ΔJ calculated through Dowling's conventional method, which was plotted against the crack tip opening displacement range ΔΦt calculated at 200μm behind crack tip, was situated on the smaller ΔJ side under test condition accompanied with ratchet deformation than under that without ratchet deformation. Meanwhile, the modified parameterΔJ/(1-Φminc/Φmaxc)0.2 showed a good linear correlation with ΔΦt under all the test conditions with and without ratchet deformation. The earlier work in which one of the authors showed that the elastic-plastic fatigue crack growth rates were controlled quite well by means of the parameter ΔJ/(1-Φminc/Φmaxc)0.2 might be realized in this way.

き裂開閉口によるき裂先端付近の局所応力とひずみの変動およびき裂開閉口点の測定について

The local stress and strain in the vicinity of crack tip were calculated by means of a finite element method to examine the measuring method of crack closure points. The fatigue crack closure behavior affected the local stress σx of cracking direction much more than the local stress σy, of loading direction in the vicinity of crack tip. The diagram between the nominal stress σ representing cyclic loading and σx had some inflection points. One of them well coincided with the crack opening point. However, some undesirable inflection points due to plastic constraint or crack blunting etc. were also found on the σ-σx diagram. On the other hand, the diagram between σ and the local shear stress τ in the vicinity of crack tip was found to have clear inflection points related to the crack closure behavior. It is expected that the crack closure points can be measured easily with an adequate strain function in the vicinity of crack tip.

薄板材における疲労き裂開閉口点の測定とその問題点について

The measurement of fatigue crack tip opening and closing points in a machine element with complicated shape is expected to involve difficulties. In the present study, the measurement of crack opening and closing points in a center notched thin plate was carried out by the unloading elastic compliance method and the strain interference method, and the application of these methods to an actual machine equipment was discussed. The strain measurements were carried out at several positions around the notch and crack for the purpose of determining crack opening and closing points. When the bending of the specimen arises during a push-pull fatigue process, the measurement of crack opening and closing points becomes more difficult unless the proper measuring position is chosen. However, when the strain function consisting of strains measured at two diffent positions on the same longitudinal axis is used in the case of strain interference method, the crack opening point can easily be measured with sufficient accuracy.

負の応力比下でのＳＭ５０Ａ鋼溶接材の疲労き裂伝ぱ挙動

The crack propagation behavior of welded material SM50A under a negative stress ratio condition was evaluated with the maximum stress intensity factor Kmax and the effective stress intensity factor range ΔKeff. The crack opening stress σop was measured by means of the strain interference method. The crack opening stress σop in the welded metal was lower than that in the base metal due to the tensile residual stress. Also, the negative value of the crack opening stress σop was measured in the case of the welded metal. Therefore, the relation between Kmax and the crack propagation rate da/dN was found to be different between the welded metal and the base metal. However, the relationship between ΔKeff and da/dN could be evaluated with the same equation in all the present experimental cases. Thus, it is confirmed that ΔKeff is one of the reasonable parameters for the evaluation of fatigue crack propagation and the strain interference method is useful for the measurement of crack opening point.

Crack closure behavior of small fatigue cracks measured with an interferometric strain/displacement gage.

An interferometric strain/displacement gage system was developed to measure the crack closure behavior of small fatigue cracks nucleated in smooth specimens by fatigue. Interference takes place between laser beams reflected by two Vickers indentations. Linear image sensors were used to detect the movement of interferometric fringes due to strain. The resolution in displacement of our system was ± 1nm for the gage length of 100μm. The system was first applied to a through-thickness fatigue crack at the center of a low-carbon steel plate. The profile of crack opening displacement agreed with the prediction based on the Dugdale model. The crack opening point of small surface cracks in fatigue specimens of aluminum alloy 2024-T3 was determined from the record of load versus displacement. Surface cracks smaller than about 100μm in length mostly showed faster growth rates than those given by the da/dN versus ΔKeff relation for long cracks.

Observations on evaluating fatigue crack closure from compliance traces

Fatigue crack closure generally has a large effect on crack growth rates, particularly in the threshold regime. Knowledge of closure values and mechanisms is important not only from a predictive viewpoint, but also in developing more fatigue-resistant materials through utilization of crack tip shielding. Closure measurement remains, however, an area of difficulty. This report considers two areas where problems can arise in compliance-based closure measurements: non-linearity of compliance traces due to plasticity and situations where the measured crack opening point does not appear to correspond with the effective crack opening point.

残留応力場における２段繰返し変動荷重下の下限界近傍疲労き裂進展挙動

In order to make clear the effect of residual stress on fatigue crack growth behavior, fatigue crack growth tests under constant amplitude and repeated two-step loadings were carried out on SM50A steel in the near threshold region by using the specimens with or without longitudinal residual stress. The crack length and crack opening point were measured by the unloading elastic compliance method.The results obtained are as follows.(1) Under varying loadings, a fatigue crack was found to grow in the residual stress field even when the low level effective stress intensity range (ΔKeff)L was less than (ΔKeff)th.(2) However, the fatigue crack growth rate under low level load (da/dn)*L was influenced by the number of cycles of low level load NL. When NL was rather large, the crack growth increment ΔaL became saturated and the averaged growth rate decreased.(3) The modified Miner type linear accumulation law of crack growth in terms of ΔKeff was confirmed to give a conservative estimation of near threshold fatigue crack growth rate under varying loadings even in the residual stress field.