Behavior Of Surface Cracks Research Articles

A two-stage micromechanics model for short fatigue cracks

The influence of microstructure on the propagation behavior of short surface cracks is examined in a simulated HAZ microstructure in C Mn steel. The detection of crack initiation and crack growth measurement is carried out by the replication of hourglass specimens under torsional and push-pull loading. The length of ferrite at the prior austenite grain boundaries is characterized as the effective microstructural parameter. Propagation rate is affected by the microstructure; it decreases when the crack approaches the grain boundary and increases again when it crosses the grain boundary. The short crack growth models proposed by other researchers are critically examined and a modified model based on micromechanics is presented to incorporate the effect of microstructure on the rate of crack growth. The crack growth behavior predicted by the model matches closely with the experimental results.

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.

EFFECT OF CYCLE MEAN STRAIN ON SMALL CRACK GROWTH IN ALLOY 718 AT ELEVATED TEMPERATURES

AbstractThe influence of the cyclic compressive excursion on the fatigue crack growth behavior of small surface cracks in Alloy 718 at 650°C is experimentally studied. Test conditions were chosen to simulate the cyclic plasticity found at notch locations in high temperature structural components. During cycling, the crack lengths were continuously monitored using the direct current potential drop method while the near field crack mouth opening displacement and global cyclic stressstrain behavior were measured using a laser interferometry technique and mechanical extensometry, respectively. Two aspects related to cyclic compressive excursion have been studied; crack closure and crack tip plasticity. Attempts have been made to use several modified ΔK expressions as well as ΔJeff to account for the effects of closure and decrease crack tip plasticity. It was concluded that the compressive excursion is most prominent in this alloy in its effect on the global plasticity and the subsequent loss of constraint. Closure was not found to be significant in the consolidation of test data.

Microstructure and crack-shape effects on the growth behavior of small fatigue cracks in Ti24Al11Nb

The effects of four different microstructures in the titanium aluminide alloy Ti24Al11Nb (at.%) on the fatigue crack growth behavior of small surface cracks and large cracks have been investigated. The four microstructures were a Widmanstätten basketweave, a Widmanstätten aligned colony, an equiaxed primary α 2 in a Widmanstätten matrix and a completely equiaxed α 2 structure. Small cracks were found to develop arbitrary shapes owing to the effects of microstructure. The crack shapes (aspect ratios) were measured using a laser interferometric and photomicroscopic system, and these measurements allowed accurate calculation of crack growth rates at the surface position as well as at the depth position for the surface cracks. After accounting for the continuous variation in crack shape in crack growth rate calculations, the trends in small-crack growth rates agreed reasonably well with the corresponding large-crack growth rates. While the crack growth rates at depth positions for small cracks correlated well with large-crack data in the basketweave microstructure, crack growth rates at surface positions correlated well with the corresponding large-crack data in the other microstructures. The microstructural factors that may be responsible for this behavior are discussed.

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.

Behavior of surface crack in plates subjected to tensile loads: Analysis based on fully plastic solutions

The J- integral and the crack mouth opening displacement under fully plastic conditions are estimated by the finite element calculation for several semi-elliptical surface crack geometries in a plate subjected to uniform tension. The calculation is performed under the assumption of the power-law hardening stress-strain relationship and the deformation theory of plasticity. Some discussions are made on the distribution of the J- integral along the crack front line. Finally, the finite element solutions are interpolated and applied to the analysis of the crack growth experiment, which is performed by the multiple specimen method.

An Analysis of Surface Crack Growth in Circumferentially Grooved Components under Low-Cycle Fatigue

Circumferentially grooved specimens were low - cycle fatigued, and the local stress-strain relation was calculated at the notch root by F.E.M. (finite-element method). The fatigue lives for crack initiation and final failure were satisfactorily evaluated by both the axial and equivalent plastic strain ranges existing at the notch root, and Coffin-Manson law held for the notched specimens as well as for the smooth specimen. The growth behavior of surface cracks at the notch root was examined in association with the growth rates of the surface crack in the smooth specimen and of the long through-crack. The fatigue crack growth rates, da/dN, were successfully plotted against the cyclic J-integral range, ΔJ, and the strain intensity factor range, ΔKε, which were estimated using the stress-strain hysteresis loop analyzed at the notch roots by F.E.M: The plots of log da/dN versus log ΔJ and log da/dN versus log ΔKε for the notched specimens were expressed by linear lines which were nearly in accord with these relations for the surface crack in the smooth specimen.

The fatigue life and fatigue-crack-through-thickness behavior of a surface-cracked plate. (3rd report. In the case of combined tensile and bending stress).

The LBB (leak-before-break) design is one of the most important subjects for the evaluation and the assurance of safety in pressure vessels, piping systems, LNG carriers and various other ;structures. In the LBB design, it is necessary to evaluate precisely the lifetime of steel plate. 'Furthermore, the change in crack shape that occurs during the propagation after through thickness s of paramount importance. For this reason, in a previous report, the authors proposed a simplified evaluation model for the stress intensity factor after cracking through thickness. Using this model, he crack propagation behavior, crack-opening displacement and crack shape change of surface-cracked smooth specimens and surface-cracked specimens with a stress concentration were evaluated quantitatively. The present study was also done to investigate the fatigue crack propagation behavior of surface cracks subjected to combined tensile and bending stress. Estimation of fatigue :rack growth was done using the Newman-Raju formula before through thickness, and using formula (7) and (8) after through thickness. Crack length aT at just through thickness increases with increasing a bending stress. Calculated fatigue crack shape showed very good agreement with )experimental one. It was also found that particular crack growth behavior and change in crack shape after cracking through thickness can be explained quantitatively using the K value based on Eqs. (7) and (8).

Surface cracking of vitreous fused silica induced by MeV ion beam bombardment

The surface cracking behavior of vitreous fused silica (suprasil II) induced by MeV^(35)C1 and^(19)F ions was observed. The crack density increases with ion fluence, reaches a maximum (for 17 MeV^(35)C1 at 1 × 10^(13 35)Cl/cm^2 and for 17 MeV^(19)F at 5 × 10^(13 19)F/cm^2) and then declines. The cracks close up for subsequent higher ion fluences. The surface cracking occurs in different ion fluence ranges with ion mass, for 17 MeV^(35)C1 5xl0^(12)-lxl0^(14 35)Cl/cm^2; for 17 MeV^(19)F, 1 × 10^(13)-3 × 10^(14 19)F/cm^2. No surface cracks were observed for 2 MeV H^+ for the ion fluence range of 5 × 10,^4-10^(17) ‘H/cm^2. The measurements of the defects in silica samples with EPR indicate that the surface cracking occurs in the transition region where the ion damage tracks partly overlap. We tentatively suggest that the surface cracking may result from heavy ion tracks in the insulating materials, and cracks close up at higher ion fluence due to the compaction of the irradiated area.

機械構造用鋼における表面微小疲労き裂の発生及び進展速度の確率特性

Reversed plane bending fatigue tests were conducted on two kinds of structural steels, JIS S35C and SCM435. The behavior of small surface cracks were continuously sampled by replica technique, and their probabilistic properties were investigated. The distributions of fatigue crack initiation life and fracture life were found to be represented by log-normal distributions. The fatigue crack growth rate of S35C in the range of crack length from 0.1mm to 3mm could be expressed by the equation of da/dn=C·σa7.7·a1.3, where σa and a are the stress amplitude and a half of surface crack length, respectively. It was found that the distribution of coefficient C also obeyed a log-normal distribution and its variance was nearly constant irrespective of magnitude of stress amplitude and crack length. Some trials to estimate the distribution of fatigue crack growth life were made based upon the above mentioned experimental results of growth rate.

An analysis on growth of short surface crack at mild notch root under low cycle fatigue.

Circumferentially grooved specimens were low cycle fatigued, and the local stress-strain relation was calculated at the notch root by FEM. The fatigue lives for crack initiation and final failure were satisfactorily evaluated by both the axial and equivalent plastic strain ranges produced at the notch root, and the Coffin-Manson law held for the notched specimen as well as for the smoothed specimen. The growth behavior of short surface crack at the notch root was examined in association with growth rates of the short surface crack in the smoothed specimen and of the long through-crack. The fatigue crack growth rates, da/dN, were successfully plotted against the cyclic J-integral range, ΔJ, and the strain intensity factor range ΔKe. The log da/dN vs. log ΔJ plots in the notched specimens were situated between those for the small surface crack in the smoothed specimen and for the long through-crack in the lower crack growth rate region, where the differences between each two were quite small to be allowable in a pratical sense, while these three da/dN vs. ΔJ relatives were nearly in accord with one another in the higher crack growth rate region.

Suspension of work on fatigue standards proposed by ISO

Twelve wide plate tests have been performed in recent years, as part of the European Fast Reactor development programme, to examine the influence of residual stresses on the fracture behaviour of Type 316 stainless steel plates and weldments. The first ten tests consisted of a study of through-thickness cracks and the last two tests addressed the behaviour of surface cracks. Results obtained from this series of tests are summarised and the ability of the R6 fracture assessment procedure to predict the results is reviewed. Significant amounts of ductile tearing under increasing applied load was shown to have occurred in both the plain plate and weldment through-thickness crack tests, prior to instability conditions being attained. R6 calculations were shown to conservatively underpredict instability applied loads for all through-thickness cracks tested. The through-thickness crack weldment tests indicated that residual stresses should be included in predicting initiation and the earliest stages of crack growth, but that they may be excluded in predicting the later stages of crack growth leading to instability. There was some evidence to suggest however that having to include residual stresses or not may depend on whether the section containing the crack is in contained yield or not. A wide plate test of a semi-elliptical crack in parent material, loaded in tension, indicated that the global limit load, although being slightly overpredictive in terms of applied load, was significantly more accurate than the local limit load approach.

複数表面欠陥からの疲労亀裂伝播挙動評価法 ＩＩＩ 複数表面欠陥付曲管試験体の疲労試験結果と亀裂伝播評価結果との比較

The evaluation of multiple surface crack growth is of practical importance in assessing the fatigue life of structural components. Authors have been conducting the experimental investigation on the growth behavior of multiple surface cracks in order to provide an appropriate fatigue life evaluation procedure.The evaluation procedure, in which the influence of the front free surface and crack interaction are considered, was already proposed in the previous paper and the fatigue behaviors of straight pipes with multiple surface cracks in the inner surface were also compared with the analytically predicted behavior.Subsequently to the straight pipe test, bend pipes with surface cracks in the inner surface were tested. In this test, the growth behavior of multiple surface cracks was investigated under cyclic in-plane bending.In this paper, the result of bend pipe test is presented and the applicability of the proposed evaluation procedure is discussed.

複数表面欠陥からの疲労亀裂伝ぱ挙動評価法 ＩＩ 複数表面欠陥付直管試験体の疲労試験結果と亀裂伝ぱ評価結果との比較

It is one of the important subjects to evaluate appropriately the growth of surface cracks to ensure the integrity of a structural component with surface flaws, since the flaws are most frequently detected at the surface of a structural component by non-destructive examinations. Authors have been investigating the growth of surface cracks and the crack growth evaluation method, in which interaction and surface effect in multiple crack growth are considered, was proposed previously using empirical expressions. In the meanwhile, verification tests using structural component models were performed to confirm the validity of the evaluation method. In this paper is described the growth behavior of multiple surface cracks obtained from fatigue tests of straight pipes with multiple surface cracks in the inner surface, and is discussed the applicability of the growth evaluation method proposed in this study and that specified by the ASME Code Sec. E. Reasonable fatigue behavior evaluations were obtained by the proposed method for the straight pipes, whereas the method of the ASME Code Sec. XI gave overly conservative fatigue lives.

Study on Coalescence Probability of Cracks Originating from Randomly Spaced Surface Defects and Associated Fatigue Life Distribution

Rotating bending fatigue tests were carried out on S50C steel specimens having four drilled holes with random circumferential distances in the critical section to investigate statistically the propagation behavior of surface cracks and their coalescence leading to a failure of the specimen. The important findings in the present study are summarized as follows: It was observed that the first crack coalescence occludes between adjacent holes with the minimum or second minimum distance. There was little difference between the number of cycles to crack coalescence and that to a failure. The distribution of the numbers of cycles to crack coalescence has been theoretically derived on the basis of crack coalescence probability assuming that the first crack coalescence occurs between adjacent two holes of the minimum distance or the second minimum distance. The theoretical distribution thus derived shows good agreement with the experimental result.

Estimation of J-integral resistance using fully plastic solutions of surface cracks.

For the Leak Before Break (LBB) assessment of the nuclear piping system, analysis of the surface crack behavior in the ductile materials is indispensable. In this paper, the surface crack problems are treated with the finite element method based on the power-law hardening constitutive relation and the deformation theory of plasticity. The normalized solutions for the global J-integral and the displacement are obtained under the fully plastic condition. The models analyzed are plates with a semielliptical surface crack subjected to uniform tension. The solutions are applied to estimate the J-integral resistance for stainless steel type 304 from the experimental relation between load and crack opening describes well the constraint around the tip of the surface crack. The general J-integral characteristics of the surface crack penetration should be clarified by performing also experimental studies.

Elastic wave scattering by a surface-breaking or subsurface planar crack. II. Three-dimensional geometry

The theory of the dynamical behavior of surface cracks which has previously been presented is here applied to the computation of the scattering of elastic waves in 3D geometry from planar cracks of a variety of shapes perpendicular to the surface of an elastic half-space. Effects of crack size, shape, burial depth, and incoming wave frequency are illustrated. Appendices include discussions of reciprocity, simplifications in the long-wavelength limit, the determination of model parameters from surface-breaking criteria, results for in-plane scattering from surface cracks in a 2D geometry, and scattering from buried and from partly closed 2D cracks.

Analysis of surface cracking in lubricated rolling/ sliding spherical contact.

Growth behavior of three-dimensional surface cracks formed on lubricated rolling-sliding spherical contact surfaces is studied by calculating stress intensity factors. It is shown from the viewpoint of the fatigue crack propagation that a tiny crack extends along the original crack plane by the shear mode crack growth. As the crack extends, the growth mode is altered to the tensile mode. This is due to the influence of the fluid pressure by the lubricant oil penetrated into the crack interior. The surface traction is the controlling factor for booth modes.

A Statistical Study on Fatigue Life Distribution Based on the Coalescence of Cracks from Surface Defects

Rotating bending fatigue tests were carried out on SUS304 stainless steel specimens having three drilled holes with equal distance at the critical section to investigate statistically the propagation behavior of surface cracks and their coalescence leading to a failure of the specimen. Based on the distribution characteristics of crack length at an arbitrary number of cycles, a statistical evaluation was made on the coalescence of cracks as a function of the number of cycles, and the fatigue life distribution was theoretically derived from the above evaluation assuming that the failure occurs immediately after the coalescence of cracks. The analytical results obtained are in good agreement with the experimental results of fatigue life distribution.

Study on coalescence probability of cracks from randomly spaced surface defects and associated fatigue life distribution.

Rotating bending fatigue tests were carried out on S50C steel specimens having four drilled holes with random distance at the critical section to investigate statistically the propagation behavior of surface cracks and their coalescence leading to the failure of specimen. The important findings in the present study are summarized as follows. It was observed that the first crack coalescence was encountered between adjacent holes with the minimum distance or second small distance. There was little difference between the number of cycles to crack coalescence and that to failure. The distribution of the number of cycles to crack coalescence has been theoretically derived on the basis of crack coalescence probability as a result of theoretical analysis with the assumption of the first crack coalescence occurring between adjacent two holes of the minimum distance or second small distance. The theoretical distribution thus derived shows a good agreement with that of experimental trends.