Crack Closure Behavior Research Articles

Short fatigue crack growth under mean stress, uniaxial loading

The overload retardation phenomenon has been investigated in AISI 304 stainless steel under different baseline ΔKs, overload ratios and R ratios. Both the crack growth and crack closure behaviour were carefully monitored. It was found that the extent and degree of retardation increase with overload ratio, decrease with R ratio, but are not correlated in a simple manner with the baseline ΔK. For R ⩽0.6, momentary acceleration followed by well-defined delayed retardation were observed. The crack growth behaviour before maximum retardation can be correctly predicted by Paris' law taking into account the experimentally measured crack closure value. Such a correlation failed beyond the maximum retardation point because of the occurrence of a discontinuous closure mechanism. Artificial introduction of crack closure by vacuum-infiltrating epoxy resin into the crack flanks also produced a similar retardation phenomenon. On the other hand, for R ratio ⩾0.65, immediate retardation was observed. Mechanisms that have been proposed to explain the overload retardation phenomenon are reviewed and examined against the current experimental observations. It is concluded that the major mechanism causing overload retardation is plasticity-induced crack closure. Secondary mechanisms, such as crack-tip blunting and residual compressive stress ahead of the crack tip, are significant only at high R ratios when crack closure is suppressed.

Fatigue Crack Propagation in a Specimen with a Complicated Cross-Section; Characterization of the Stress Intensity Factor

Abstract In order to predict the fatigue life of machine equipment, the fatigue crack propagation law has up to now been investigated using specimens with a simple cross-section shape. In this study, a specimen with three grooves cut in the loading direction was used to clarify crack propagation and crack closure behavior at points of discontinuous change in thickness. It was found that the crack propagation curve showed a complicated variation in the vicinity of the grooves and that the variation of crack closure behavior correlated well with it. The stress intensity factor was evaluated using the calibration curve between the crack propagation rate and the effective stress intensity factor range obtained from a similar smooth specimen. Results showed that the stress intensity factor of the grooved specimen was higher in front of the first groove and lower behind it, compared to the smooth specimen. As the crack crossed the first groove, the crack propagation rate exhibited a local maximum as a function of the maximum value of stress intensity factor.

Crack growth and closure behavior of short fatigue cracks

Crack growth and closure behavior of short cracks are investigated for various stress ratios, using in-plane bending specimens of 2024-T351 aluminum alloy. Artificially prepared two-dimensional, short through-thickness cracks are used. Crack length and closure of short cracks are measured continuously during the test by employing an unloading elastic compliance technique and a personal computer system. For most of all the stress ratios tested, short cracks grow faster than long cracks in the low stress intensity factor range region, and the growth rates of short cracks merge with the long crack growth curve with increases in the value of stress intensity factor range. The variation in growth rates between short and long cracks is reduced as the value of stress ratio is increased, and finally disappears at a stress ratio of R = 0.5. The growth rates of short cracks are well described by the effective stress intensity factor range. The growth rates of short cracks in terms of the effective stress intensity factor range coincide well with the long crack data. However, the closure behavior of short cracks is significantly different from that of long cracks. The crack opening ratio of a short crack decreases from an initial high value corresponding to a fully open crack linearly with the value of effective stress intensity factor range, and merges with the long crack results. Based on the test results obtained, a relatively simple procedure is proposed to predict closure and growth behavior of short cracks. The validity of the proposed procedure is examined using the test data of other workers. In addition, some methods proposed by other workers are also discussed.

Plasticity induced fatigue crack closure in single and dual phase materials

The role of the presence of a non-deformable second phase in a soft matrix on the plasticity induced crack closure behavior of a growing fatigue crack is analyzed as a boundary value problem using elastic—plastic finite element techniques. Three cases of plasticity induced crack closure are simulated under plane-strain conditions: (a) response of the matrix; (b) response in the presence of the second phase; and (c) effects of interface failure between the matrix and the second phase as the fatigue crack crosses the second phase. Results indicate that the presence of the second phase considerably affects the stress and strain fields of a growing fatigue crack at both maximum and minimum load levels, and hence the crack opening and closure behavior. The interface bonding characteristics between the matrix and the second phase play an important role in this variation. Crack deflection and increased surface roughness are inevitable in two phase materials due to the localization of the plastic flow between the second phase. Therefore, it appears that the parameters of the second phase are important factors in the observed effects of plasticity induced crack closure.

Detection of Fatigue Crack Initiation under a Random Load.

A method of detecting fatigue crack initiation in some weak section of a machine element is examined using a holed specimen under random load conditions. Waveforms of two strain functions, that is h1 = εy1 + λεy2 and h2 = εy1 - λεy2, are successively observed. εy1 and εy2 are the strains in the vicinity of the hole and λ is the strain range ratio Δεy1/Δεy2. The function h1 can be used to substitute for the load parameter and the waveform of h2 changes its shape after the crack initiation due to the crack closure behavior. Therefore, we can detect the fatigue crack initiation at the hole by comparison of these two waveforms. Also, these waveforms are analyzed with FFT (Fast Fourier Transformation) to allow for a more effective detection of crack initiation. Power spectrum density of the waveform h2 varies immediately when the crack length measured from hole edge reaches about 0.5 mm. The figures showing the coherence of two waveforms h1 and h2 clearly vary at the same length 0.5 mm. Thus, the crack initiation of its length of 0.5 mm can be easily detected with the analysis of the strain waveform.

球状黒鉛鋳鉄の低サイクル疲労下巨視き裂伝ぱに及ぼす基地組織の影響

The effect of microstructure on the macroscopic low-cycle-fatigue crack growth rate was examined on ductile cast iron with the same chemical compositions but different microstructures, ie., ferritic (FDI), pearlitic (PDI) and austempered (ADI) structures. The crack growth rate was evaluated by measuring crack closure behavior. FDI showed the fastest crack growth rate, irrespective of the assessing fracture mechanics parameter of δK or δKeff. The crack opening ratio was constant in PDI and ADI, irrespective of the crack growth stage, while it suddenly increased in FDI when the value of Kmax exceeded a certain value. Each material microscopically indicated zigzag-like crack growth caused by the effect of spheroidal graphite. In particular, crack branching, which retarded the crack growth, was often observed in ADI. Each material showed no discernible difference in appearance and roughness of fracture surface at the main stage of crack growth.

A Detection Method of Fatigue Crack Initiation by Analyzing Strain Waveform

Abstract A detection method of fatigue crack initiation at some weak section of machine equipment during operation is examined using a partial-notched specimen. The waveform of the strain function composed of strains in the vicinity of the partial notch was observed successionally. It is found that such strain waveform changes its figure at the time when the crack length is 1 mm, due to the crack closure behavior. Therefore, in this experiment, the crack initiation could be detected when the crack length is about 1 mm. Since the engineering crack initiation size is frequently taken to be 1 mm, the present method of detecting crack initiation can be applied to real machine equipment. Also, this waveform was analyzed with FFT (Fast Fourier Transformation). Power spectrum density of the waveform varies immediately after the crack length reaches 1 mm. Furthermore, the figure of the coherence of two waveforms composed of the same strains clearly varies at the same crack length of 1 mm. Thus, it is thought that the engineering size crack initiation can be detected by the analysis of the strain waveform.

EXAMINATION OF SHORT‐CRACK MEASUREMENT AND MODELLING UNDER CYCLIC INELASTIC CONDITIONS

AbstractThe opening and closure behaviour of short fatigue cracks is seen as one of the important phenomena which control fatigue life of components where a major part of life consists of the growth of short cracks. Therefore attempts are undertaken to experimentally assess and to model the behaviour of short cracks with respect to opening and closure. In this paper crack opening results obtained by Sunder et al. through SEM evaluation of striation patterns of 2000 series aluminium alloys are examined and compared to predictions using a model recently developed for fatigue life prediction based on fracture mechanics of short cracks. Sunder's technique for crack opening measurements involves particular load sequences with increasing and decreasing load ranges applied to notched specimens with naturally nucleated surface cracks where crack opening levels are identified by steady‐state striation widths for increasing load ranges. A detailed review of Sunder's results, however, indicates a number of inconsistencies and contradictions which are discussed.Opening and closure behaviour of short fatigue cracks, in particular for inelastic conditions, is compared to predictions obtained with the above‐mentioned model which incorporates a constant strain opening and closure assumption. For inelastic conditions that may develop at notches this assumption means that cracks would close at considerably lower stress levels as compared to the opening stress which becomes important when effective (local) stress‐strain ranges are to be determined for fatigue life prediction under spectrum loading. The constant strain assumption is supported by a number of experimental observations from the literature as discussed in the paper. The approximative nature of this assumption and further details of the model are pointed out which show a need for further developments.

Fatigue crack growth and closure behaviour of semicircular and semi-elliptical surface flaws

The growth behaviour of semi-elliptical and semicircular cracks subjected to crack extensions due to tensile constant amplitude loading ( R = 0.1) was investigated using a three-dimensional elastic-plastic finite-element analysis of fatigue crack growth and closure. Based on this analysis, for both semicircular and semi-elliptical meshes, the crack first closed on the exterior (free) surface ( θ = 0°) and then closed towards the interior region upon further unloading of the cyclic loading. The closure and opening behaviour of crack surfaces for both semicircular and semi-elliptical geometries are investigated. An experimental investigation of semi-elliptical flaws under fin spectrum loading was conducted to simulate realistic growth rate behaviour for the semi-elliptical geometry.

On the mechanisms and behaviour of overload retardation in AISI 304 stainless steel

The overload retardation phenomenon has been investigated in AISI 304 stainless steel under different baseline ΔKs, overload ratios and R ratios. Both the crack growth and crack closure behaviour were carefully monitored. It was found that the extent and degree of retardation increase with overload ratio, decrease with R ratio, but are not correlated in a simple manner with the baseline ΔK. For R ⩽0.6, momentary acceleration followed by well-defined delayed retardation were observed. The crack growth behaviour before maximum retardation can be correctly predicted by Paris' law taking into account the experimentally measured crack closure value. Such a correlation failed beyond the maximum retardation point because of the occurrence of a discontinuous closure mechanism. Artificial introduction of crack closure by vacuum-infiltrating epoxy resin into the crack flanks also produced a similar retardation phenomenon. On the other hand, for R ratio ⩾0.65, immediate retardation was observed. Mechanisms that have been proposed to explain the overload retardation phenomenon are reviewed and examined against the current experimental observations. It is concluded that the major mechanism causing overload retardation is plasticity-induced crack closure. Secondary mechanisms, such as crack-tip blunting and residual compressive stress ahead of the crack tip, are significant only at high R ratios when crack closure is suppressed.

Crack growth and closure behaviour of surface cracks under axial loading: Kim, J.-H. and Song, J.-H. Fatigue Fract Eng Mater Struct15 5 (May 1992) pp 477–489

Crack growth and closure behaviour of surface cracks under axial loading: Kim, J.-H. and Song, J.-H. Fatigue Fract Eng Mater Struct15 5 (May 1992) pp 477–489

Effects of Overload on Fatigue Crack Propagation of Fe-3% Si Alloy.

The effect of tensile overload on fatigue crack propagation behavior of Fe-3% Si alloy was investigated by the measurement of surface crack length and crack closure level. Crack propagation after the overloading was observed to be delayed under constant stress amplitude. The delayed propagation was predominated by the crack closure behavior, and when the crack propagation rate da/dN became lower, the crack closure level became higher. Also, the crack propagation law could be reasonably evaluated with the effective stress intensity factor range ΔKeff independent of the delayed propagation. It is expected for this reason that crack tip blunting hardly affected the crack propagation in the case of the present material.

溝を有する試験片の疲労き裂伝ぱと応力拡大係数の評価について

In order to predict the fatigue life of machine equipment, the fatigue crack propagation law is usually investigated by using smooth specimens. However, few investigations of the crack propagation have been carried out on the specimens having the cross-section of complicated shape. In this study, the specimen with three grooves which were cut in the loading direction was used to clarify the crack propagation and the crack closure behavior at the suddenly changed site of the thickness. The crack propagation rate was observed to be once accelerated and then decelerated in the vicinity of the first groove. The variation of crack closure point was different between the grooved specimen and the smooth specimen. The stress intensity factor was evaluated using the calibration curve between the crack propagation rate and the effective stress intensity factor range obtained by the smooth specimen. The results showed that the stress intensity factor of the grooved specimen was higher in front of the first groove and lower behind it, compared to the smooth specimen. The crack propagation law evaluated with Kmax becomes complicated in the vicinity of the first groove due to the change of thickness. Thus, the crack propagation rate once increases and then decreases with the maximum value of stress intensity factor Kmax when the crack crosses the groove.

有効応力拡大係数範囲ΔＫｅｆｆによる疲労き裂伝ぱ挙動の評価に関して

Fatigue crack propagation law in the following two cases was investigated under the negative stress ratio R(=σmin/σmax) condition. The one is the crack propagation in the field of welding-residual stress, and the other is that after the overloading. In the former case, the crack propagation rate da/dN depended on the stress ratio R. Due to the tensile residual stress, the crack propagation rate increased as the stress ratio decreased in the range of 0≤R≤-2. In the latter case, the crack propagation rate da/dN after the overloading was higher at first and then became lower than that of the constant stress amplitude. In the present experimental cases, the crack propagation rate was affected by the residual stress and the overloading. However, the crack propagation behavior had a good correlation with the crack closure behavior. Therefore, in the present cases, the crack propagation law was reasonably evaluated with the effective stress intensity factor range ΔKeff.

Propagation and Closure Behavior of Small Fatigue Cracks.

An interferometric strain/displacement gauge (ISDG) system with a laser diode was developed to dynamically measure the crack closure behavior of small surface cracks. The timing of the laser radiation was controlled by a computer and was synchronized with the fatigue load. Smooth low-carbon steel specimens were fatigued. The crack mouth opening displacement of small fatigue cracks was measured dynamically as well as statically with the ISDG system. The crack opening stress was determined using the unloading elastic compliance method. The crack opening stress obtained at dynamic loading was smaller than that obtained at stepwise loading. The relationship between the crack propagation rate and the effective stress intensity range obtained at dynamic loading agreed very well with that for long cracks.

Hydroelastic response and fatigue analysis of a multi-module very large floating structure: Wang, W., Du, S. and Ertekin, R.C. Conf. Fatigue and Fracture in Steel and Concrete Structures, ISFF '91, Madras, India, Dec 1991 Vol 2 (Oxford & IBH Publishing Co. Pvt. Ltd, New Delhi, India, 1991) 1277–1291

The fatigue crack propagation and the crack closure behaviour of 15Mn mild steel in 3.5% sodium chloride solution are studied. An equation to calculate the crack closure effect induced by corrosion products is proposed. The results reveal that the crack closure effect has a remarkable influence on corrosion fatigue. The crack closure effect decreases and disappears with crack propagation. The effect of crack closure on the crack growth rate and on the corrosion fatigue mechanisms is discussed.

The influence of air humidity on near-threshold fatigue crack growth of 2024-T3 aluminum alloy: Stanzl, S.F., Mayer, H.R., and Tschegg, E.K. Mater. Sci. Eng. A Oct. 1991 A147 (1) 45–54

A combined experimental and finite element study of fatigue crack propagation and crack closure behavior, in a modified 1070 steel, has been conducted. In this paper, the experimental aspects of this study are presented and discussed. A comparison of crack closure measurement techniques using crack mouth opening displacement, back-face strain gage and a new surface strain gage method was performed. For two thicknesses of compact tension specimens, a series of constant maximum stress intensity and constant load ratio, constant load and constant load ratio, constant maximum stress intensity and increasing load ratio, single tensile overload, and conventional fatigue crack propagation tests were conducted. Implications of the influence of specimen thickness, crack length and test conditions on closure and crack growth behavior are detailed. Electron microscopy observations of the fatigue fracture surfaces were performed to assess the relative importance of crack path meandering, oxide-induced ind surface roughness-induced crack closure mechanisms. In the domain of stable crack growth, closure is dictated by the mechanism of plasticity-induced crack closure; while near-threshold behavior was found to be dominated by the conjoint and mutually interactive mechanisms of oxide-induced and surface roughness-induced crack closure. Test results reveal a significant influence of specimen thickness on closure and overall growth rate behavior. The salient advantages of the new surface strain gage method are elucidated and the conformance of experimentally measured and calculated growth rates is highlighted.

FATIGUE CRACK GROWTH OF SILICON NITRIDE WITH CRACK WEDGING BY FINE FRAGMENTS

Abstract— Fatigue crack growth tests of silicon nitride Si3N4 were carried out under four‐point bending using square bar specimens at room temperature. A pre‐crack was introduced by a bridge indentation method. Decreasing K‐type tests at stress ratios of R= 0.1 and 0.6 and also under static load were first carried out, and after observing the cessation of crack growth, K‐increasing tests were performed except for the case of a static load. Crack closure was observed on most specimens by the elastic compliance method. Furthermore, SEM observations of the crack paths were made to see what was happening during crack growth. The threshold and the region of steady crack growth were observed more clearly under cyclic loading, and an effect of load cycling certainly existed which became more evident when the maximum stress intensity factor Kmax approached the threshold. A wedge effect, caused by fine fragments on the crack surface, played an important role in crack closure behaviour of each specimen, and it is suggested that the crack growth rate is controlled by both the effective stress intensity range ΔKeff and the effective mean stress intensity factor Km,eff at least as a first approximation.

CRACK GROWTH AND CLOSURE BEHAVIOUR OF SURFACE CRACKS UNDER AXIAL LOADING

Abstract— Crack growth and closure behaviour of surface cracks in 7075‐T6 aluminium alloy are investigated under axial loading, noting the difference in fatigue growth behaviour at the maximum crack depth point and at the surface intersection point and also with through‐thickness crack growth behaviour. The plane strain closure response at the point of maximum depth of a surface crack is monitored using an extensometer spanning the surface crack at the midpoint of its length. The plane stress closure at the surface intersection point is observed by multiple strain gauges placed at appropriate intervals ahead of the crack tip and continuously monitored without interrupting the fatigue test. The crack opening ratio is found to be about 10% greater at the maximum depth point than at the surface intersection point. Under axial loading, the difference in plane strain crack closure behaviour between the surface crack and the through‐thickness crack is relatively small. Growth rates of surface cracks can be well described by the effective stress intensity factor range based on the closure measurements made in this study. The growth rates in terms of the effective stress intensity factor range seem to be slightly slower in surface cracks than in through‐thickness cracks.

Fatigue crack propagation in weathering weldable-steel KT 315 silicon: Benke, A., Schadlich, S. and Zouhar, G. Steel Res. July 1991 62, (7), 320–324

A study has been made of the role of aging treatment in influencing fatigue crack propagation and crack closure behavior in a high purity ingot metallurgy aluminum alloy 7150, with specific reference to crack growth at low and high load ratios in the near-threshold regime. A trend of increasing growth rates and decreasing threshold stress intensity ΔKth values with increased aging was seen to be consistent with lower measured levels of crack closure and a decreasing tortuosity in crack path. On the basis of crack growth measurements in moist room air, where closure due to corrosion product formation was found to be negligible in this alloy, the superior fatigue resistance of underaged microstructured (compared with overaged structures of similar strength and peak-aged structures of higher strength) was attributed to greater slip reversibility and to enhanced roughness-induced crack closure and deflection from the more tortuous crack paths. Such factors are promoted in alloy systems hardened by coherent shearable precipitates where the mode of deformation is one of non-homogeneous planar slip.