Behavior Of Surface Cracks Research Articles

Simultaneous simulation of closure behavior and shape development of fatigue surface cracks

The shape development and crack closure behavior of a surface crack subjected to fatigue loadings are two important features studied by researchers in the fatigue community. In the current study, both features are considered in the finite element analysis, and closure behavior and shape development of surface cracks are obtained simultaneously. In the analysis, two types of finite element models are constructed, one developed for the closure analysis and the other for the stress intensity factor computation of surface cracks. In the closure analysis, a new node release scheme is developed and the crack front nodes are unnecessarily simultaneously released during a loading cycle. Hence, the closure analysis of a surface crack with an evolving front of arbitrary shape becomes possible. The calculated results show that the proposed two-finite element model approach can obtain reasonable opening stress levels and crack shapes.

Fracture of Ceramics with Near Surface Gradient Residual Stresses

The fracture toughness and strength of ceramics can be improved with respect to monolithic ceramics by developing graded materials as laminates composed of periodic alternating layers of one material separated by layers of a second material. The second layer must contain residual compressive stresses which are induced during densification because of differential thermal contraction of the layers. The overall residual stresses increase the apparent fracture toughness of the laminate. However, most deleterious natural flaws and most of the damage induced in service by the environment, contact loading, wear, etc, are small cracks on the surface of the outer layer, so that the effect of the laminate residual stresses on these cracks should be rationalised to understand their behaviour. This work presents an analysis of the influence of the gradient residual stresses on the behaviour of surface cracks under bending and indentation in materials with outer layers either with tensile or compressive residual stresses.

Predictions of Propagation Behavior of Semi-Elliptical Slant Surface Cracks in Residual Stress Fields Based on Simulations of the Partial Elastic Contact of Crack Surfaces

In residual stress fields, the so-called partial elastic contact of crack surfaces sometimes occurs, where a fatigue crack is closed at the crack mouth while it is open at the crack tip. The partial elastic contact has a significant effect on the lives of fatigue cracks. However there are few studies on the partial elastic contact of crack surfaces for three-dimensional cracks. In this paper, the propagation paths and lives of semi-elliptical slant surface fatigue cracks are predicted. The finite element method was employed for these simulations. The residual stress was introduced by applying an equivalent nodal force in correspondence to an initial strain to each finite element. The results of the propagation paths and lives of simulations considering the partial elastic contact of crack surfaces differed from those without the consideration of the elastic contact. Especially the estimated results of crack propagation rate sometimes brought one-order difference. These results demonstrate the necessity of simulation considering the partial elastic contact of crack surfaces.

Surface Crack Behavior in Socket Weld of Nuclear Piping under Fatigue Loading Condition

The ASME B & PV Code Sec. allows the socket weld for the nuclear piping in spite of the weakness on the weld integrity. Recently, the integrity of the socket weld is regarded as a safety concern in nuclear power plants because many failures and leaks have been reported in the socket weld. OPDE (OECD Piping Failure Data Exchange) database lists 108 socket weld failures among 2,399 nuclear piping failure cases during 1970 to 2001. Eleven failures in the socket weld were also reported in Korean NPPs. Many failure cases showed that the root cause of the failure is the fatigue and the gap requirement for the socket weld given in ASME Code was not satisfied. The purpose of this paper is to evaluate the fatigue crack behavior of a surface crack in the socket weld under fatigue loading condition considering the gap effect. Three-dimensional finite element analysis was performed to estimate the fatigue crack behavior of the surface crack. Three types of loading conditions such as the deflection due to vibration, the pressure transient ranging from P=0 to 15.51MPa, and the thermal transient ranging from T=25oC to 288oC were considered. The results are as follows; 1) The socket weld is susceptible to the vibration where the vibration levels exceed the requirement in the ASME Operation and Maintenance (OM) Code. 2) The effect of pressure or Temperature transient load on the socket weld integrity is not significant. 3) No-gap condition gives very high possibility of the crack initiation at the socket weld under vibration loading condition. 4) For the specific systems having the vibration condition to exceed the requirement in the ASME Code OM and/or the transient loading condition from P=0 and T=25oC to P=15.51MPa and T=288oC, radiographic examination to examine the gap during the construction stage is recommended.

분자동력학을 이용한 결정립 제어 레오로지 소재의 나노 변형거동 전산모사

In this study, the nano-deformation behavior of semi-solid Al-Si alloy was investigated using a molecular dynamics simulation as a part of the research on the surface crack behavior in thixoformed automobile parts. The microstructure of the grain-size controlled Al-Si alloy consists of primary and eutectic regions. In eutectic regions the crack initiation begins with initial fracture of the eutectic silicon particles and inside other intermetallic phases. Nano-deformation characteristics in the eutectic and primary phase of the grain-size controlled Al-Si alloy were investigated through the molecular dynamics simulation. The primary phase was assumed to be single crystal aluminum. It was shown that the vacancy occurred at the zone where silicon molecules were.

Propagation of surface fatigue cracks in human cortical bone

An understanding of how fatigue cracks grow in bone is of importance as fatigue is thought to be the main cause of clinical stress fractures. This study presents new results on the fatigue-crack growth behavior of small surface cracks (∼75–1000 μm in size) in human cortical bone, and compares their growth rates with data from other published studies on the behavior of both surface cracks and many millimeter, through-thickness large cracks. Results are obtained with a cyclically loaded cantilever-beam geometry using optical microscopy to examine for crack growth after every 100–500 cycles. Based on the current and previous results, small fatigue cracks appear to become more resistant to fatigue-crack growth with crack extension, analogous to the way the fracture resistance of cortical bone increases with crack growth. Mechanistically, a theory attributing such behavior to the development of bridges in the wake of the crack with crack growth is presented. The existence of such bridges is directly confirmed using optical microscopy.

A study on nano-deformation behavior of rheo-formed Al–Si alloy based on depth-sensing indentation with three-dimensional surface analysis

A nanoindentation study of semi-solid processed Al–Si–Mg alloy was performed as part of the research on surface crack behavior in automotive applications. The microstructure of semi-solid processed Al–Si alloy usually consists of an Al region (with a α-Al globular structure) and a eutectic region (with Al/Si eutectic phases). The region of interest in the failure process is the eutectic region in which crack initiation begins with initial fracture of the eutectic silicon particles and inside other intermetallic phases. In this study, the deformation behaviors (surface morphology after indentation and pile-up) and mechanical properties (indentation hardness and elastic modulus) in the primary Al region, the eutectic region, and the primary Al–eutectic boundary were investigated through nanoindentation experiments and atomic force microscopy (AFM) observation. The pile-up was observed around indents in both the primary Al region and the eutectic region. The indent in the primary Al region showed a clean surface and apexes. Contrarily, distorted valleys were mostly observed in the indent in the eutectic region. For the primary Al region, almost constant values for the elastic modulus ( E) and the hardness ( H) were obtained under the different test conditions. The hardness and elastic modulus of the eutectic region were more irregular than that of the primary Al region due to the inhomogenity of the eutectic structure. It was also found that the measured mechanical properties of some regions were strongly affected by those of the substrate region. In addition, the primary Al region showed higher pile-up and less elastic recovery than the eutectic region because it had higher values of E/ H.

Effect of Atmospheric Humidity on the Fatigue Crack Propagation Behavior of Short Cracks in Silicon Nitride

The effect of the environment on crack-growth processes in silicon nitride was studied by investigating the static and fatigue crack-growth behavior of small surface cracks, as influenced by testing (i) in the ambient environment, (ii) in distilled water, (iii) under vacuum, and (iv) in toluene. A principal finding was that testing under cyclic conditions led to crack-growth rates that were much higher in air than in toluene, whereas testing under static conditions in air or toluene led to minor differences in the rate of static fatigue crack growth. This difference in sensitivity to the environment under static and cyclic loading conditions was attributed, in part, to a much-greater extent of microcracking at the surface ahead of the main crack in air under cyclic conditions, in comparison to that in other environments. This propensity for microcracking at the surface in air under cyclic conditions also was reflected in the aspect ratios of the crack shapes that developed.

Prediction of stable crack growth geometry in residually stressed glass

Processing procedures have been developed that produce stress distributions in glass with a maximum compressive stress below the surface. These glasses can exhibit rising apparent fracture toughness behavior and reduced strength variability associated with the stable growth of surface cracks under applied tensile stress. A weight function approach was used to determine stress intensity factors as a function of crack geometry for surface cracks under the effects of stress distributions similar to those found in these glasses. These calculations were then used to predict the growth behavior of surface cracks as a function of the applied and residual stress fields.

나노인덴터와 원자력간 현미경을 이용한 결정립 제어 레오로지 소재의 변형거동에 관한 연구

In this study, the deformation behavior of semi-solid Al-Si alloy was investigated by nanoindenter as a part of the research on the surface crack behavior in thixoformed automobile component. The microstructure of semi-solid Al-Si alloy consists of primary and eutectic regions. In eutectic regions the crack initiation begins with initial fracture of the eutectic silicon particles and inside other intermetallic phases. Nano-deformation characteristics in the eutectic and primary phase of semi-solid aluminium alloy were investigated through the nano-indentation experiments and the AFM observation. In addition, mechanical properties of each region were investigated and compared with each other.

Modeling of surface and subsurface crack behavior under contact load in the presence of lubricant

A new mathematical model for lubricated elastic solids weakened by cracks is proposed. Surface and subsurface cracks are taken into account, and the interaction of lubricant with elastic solids within cavities of surface cracks is regarded as the most interesting aspect of the problem. The boundary conditions characterizing the behavior of lubricant within crack cavities such as pressure rise in crack cavities fully filled with lubricant as well as other boundary and additional conditions are derived. The problem is reduced to a system of integro-differential equations with nonlinear boundary conditions in the form of alternating equations and inequalities. A new iterative numerical method is developed for solution of the proposed problem. The method guarantees conservation of lubricant volumes trapped within closed crack cavities and allows for all three functions (normal and tangential displacement jumps and normal stress applied to crack faces) characterizing the problem solution to be determined simultaneously. Examples of numerical results for surface and subsurface cracks are presented and numerical and asymptotic results for small subsurface cracks are compared to each other. The numerical analysis indicates that depending on a surface crack orientation its normal stress intensity factor may be two or more orders of magnitude higher than the one for a similar subsurface one.

Crack growth and closure behaviour of surface cracks

This paper investigated the crack growth and closure behaviour of surface cracks in AISI 4130 alloy steel under the constant amplitude loading and single-overload conditions. The compliance measurements of depth- and surface-wise crack closure were performed using the backface strain gauge and the near-tip gauge, respectively. Under the constant amplitude loading, the surface growth was faster than the depth growth, whereas at each Δ K level, the surface growth was slower because the surface-wise crack closure was stronger. Each single overload condition increased the amount of crack closure at the surface over that at the depth, thereby, lowering the growth rate at the surface. The measured crack shape developments under the constant amplitude loading and the single-overload conditions correlated well with the crack-closure inferred counterparts.

Analysis of Shape Variation of a Fatigue Surface Crack in a Pipeline

The authors have elaborated a model of nonthrough surface flaws in a pipeline under variable internal pressure. The model involves a proposed scheme allowing for variation of local fracture stresses along a curvilinear crack front. Verification of the theoretical model is carried out by comparing it with the available experimental data reported elsewhere. The authors have performed a comprehensive parametric study of the influence of initial flaw geometry, pipeline dimensions and variation of properties of steels due to heat treatment and test temperature on the surface crack behavior.

A numerical study of the behaviour of surface cracks under dry-sliding conditions

The shape and amount of the surface roughness of a machine part is based on the manufacturing processes. The irregularities cause serious stress concentrations due to the loads acting on the surface. When the loads effect on the surface repeatedly, the irregularities penetrate to turn into a crack. The load may act as normal and/or fractional forces. The latter is caused in general due to friction under rolling–sliding contact conditions. In this study, the behaviour of surface cracks under the normal and tangential loads were analysed. Three angular location for the crack is considered (i.e. −π/4, π/2 and π/4) The problem is considered under condition of linear elastic fracture mechanics and the finite element method was used for numerical solution. The intensity factors at crack tip, K I and K II , were calculated using displacement correlation method. Maximum principal stress theory was used to determine the direction of crack propagation. Effect of the friction coefficient on the stress intensity factors K I and K II and crack growth direction was also investigated.

Crack growth and closure behaviour of surface cracks under pure bending loading

Fatigue crack growth and closure behaviour of surface cracks as well as through-thickness cracks in aluminium alloy 7075-T651 are investigated under pure bending loading. We have placed a special focus on the comparison of the surface crack closure behaviour under two loading cases, axial and bending, and also with the through-thickness cracks. The ratio of crack opening in the crack length direction to that in the crack depth direction is close to 0.9 regardless of the applied loading types and the stress ratios. Growth rates of surface cracks can be successfully explained in terms of the effective stress intensity factor range, independently of the loading type, the crack shape and the stress ratio.

疲労 ＳＳ４００炭素鋼のモードＩＩ，モードＩＩＩ疲労き裂の進展挙動

Fatigue characteristics are of fundamental importance when choosing a structural materials, especially when deciding upon the materials for reactor structure where long-term safety is primary importance. In this field, recently, the propagation behavior of mode II and mode III fatigue cracks has attracted special interest. The mode II and mode III fatigue testings involve greater difficulties compared with the mode I testing and the testing procedure has not been established. SS400 carbon steel was fatigue tested by a torsion fatigue testing machine. In the torsion test, fatigue cracks can propagate in all modes. To identify their behavior, surfaces of the specimen were observed by CCD camera and the behavior of surface cracks were recorded in a time-lapse video recorder which can record up to for 40 days. To reveal the morphology of the mode II and mode III crack propagation, some of test interrupted specimens were sliced and observed by a two dimension measuring microscope.To investigate the crack morphology three dimension crack demonstrate computer program was developed. In the torsion test of plain surface specimen the mode II crack initiates first, but it transits to mode I in the early stage. However, according to the 3D view a large mode III crack propagates into inside in this stage. Persistent slip band like patterns were observed on the surface and the cross section of the fatigue cracked specimen and all mode III cracks propagate in the band. Some of them grew across the center of the specimen. The da/dN-ΔK diagram of the surface mode II fatigue crack propagation was compared with that of mode I by CT type specimen of the same material. mode II propagation rate agreed with mode I propagation rate where the stress intensity factor range was converted as ΔKI=2ΔKII. It means that to design the structure by mode I crack propagation data alone may be acceptable under above conversion. To establish this relationship and to elucidate the behavior of mode III fatigue cracking, more investigations are required.

Martensite transformation and surface cracking of hydrogen charged and outgassed type 304 stainless steel

The effects of cathodic hydrogen charging and outgassing on martensite transformation, surface cracking and anodic dissolution behavior of type 304 stainless steel have been investigated. Cathodic charging and aging results in the formation of a considerable amount of ε and α′ martensites. Optical observation combined with X-ray diffraction analysis shows that there are critical charging conditions for the martensite transformation. The minimum charging current density to induce the phase transformation is 0.2 mA cm −2. Surface cracks are observed on the specimens, which are hydrogen charged for a longer time than the critical charging time for the matensite transformation. The surface cracks mainly initiate during aging instead of during cathodic charging and propagate along martensite/austenite interfaces, grain boundaries and twin boundaries. The anodic dissolution tests in 0.5 M H 2SO 4+0.1 M HCl solution reveal that an obvious increase in the weight loss is due to the preferential dissolution of martensite phases and the weight loss decreases with the outgassing time.

Plastic deformation at short edge cracks under fatigue loading

The mechanical behaviour of short surface cracks is investigated and analysed with respect to its importance for fatigue crack growth. The elastic stress field and the plastic deformation are computed by means of the Boundary Element Method. The influence of material parameters, crack length and loading conditions on the crack tip deformation are investigated. The results are compared to the usual long crack equations based on Linear Elastic Fracture Mechanics (LEFM) and conditions are identified when LEFM becomes non-conservative. A yield stress reduction method is proposed to modify the LEFM equations for such cases in order to obtain reasonable and always conservative predictions for the crack tip deformation.

Estimates of Crack-Driving Force in Surface-Cracked Elbows

The objective of this effort was to assess whether a simple relationship could be developed between the behavior of surface cracks in straight pipe and in elbows. If such a geometric relationship could be developed, then a simple multiplier could be applied to the current straight-pipe solutions that are already used in codes and standards such as the ASME or other codes. In order to accomplish this objective, solutions from elbow and straight-pipe elastic-plastic fracture mechanics (EPFM) analyses were used along with experimental data. The elbow EPFM solution came from a J-estimation scheme developed during the IPIRG-2 program. These solutions were for an elbow with a pressure at the design stress limits of Section III of the ASME Code for typical nuclear piping steels. Significant efforts were undertaken in that program to develop J-estimation schemes for axial (along the side of the elbow) and circumferential surface cracks (centered on the extrados) in elbows under constant pressure and in-plane bending. These analyses were developed using the GE/EPRI methodology of determining an elastic and plastic contribution to J, and developing the appropriate functions through a matrix of EPFM finite element analyses. Even with this large matrix of FEM analyses, only one circumferential crack length and one axial crack length were investigated. Hence, it was desirable to develop a method to extend the analysis capabilities to other crack geometry, as well as developing a simplified procedure. A comparison of the elbow to straight-pipe moment versus crack-driving force curves showed that there is a simple multiplier linearly related to the ASME B2 stress index for elbows of different R/t ratios. Hence, a simplified procedure was determined where the straight-pipe solution could be multiplied by a function of the elbow stress indices to give the maximum load prediction of the surface-cracked elbow. Comparisons were made to circumferential surface-cracked elbow data from the IPIRG-2 program, and an axial surface-cracked elbow test conducted by EDF. The comparisons showed the simplified methods to be quite promising.

Behaviour of longitudinal surface cracks in the hot rolling of steel slabs

The behaviour of a longitudinal V-shaped crack, on the surface of a continuously cast steel slab, is studied during hot rolling. The analysis is carried out by means of the commercial FE-code LS-DYNA3D. Process parameters obtained from industry are used as a reference. The slab of initial width 1000 mm and thickness 220 mm is rolled down to 30 mm. It is assumed that the material can be treated as rigid-perfectly plastic and that the cracks do not propagate. The latter assumption is in agreement with industrial observations for a steel grade similar to that analysed here. The aim of the study is to investigate the possibility of controlling the plastic deformation so that the cracks disappear or so that their deteriorating effects are minimised. The analysis is focused upon the influence of friction, roll radius and rolling schedule on the change in the shape of a crack of initial depth 20 mm and a crack angle of 6°. The reliability of the simulations is checked by pilot-plant experiments using aluminium as the model material for steel. The results indicate that it is not possible to prevent the bottom side surfaces of the crack from coming into contact, especially not for small reductions/pass and small roll radii. The influence of friction was found to be marginal. Contact between the crack surfaces is found already at the beginning of the rolling, as the V-shaped crack is being changed to Y-shape. Considering the upper part of the crack, this remained open during the majority of the schedules studied. However for heavy reduction/pass and a large roll radius, this part of the crack is closed also, but not before the final passes. If the bottom side surfaces of the crack are in complete contact, they are prevented from further oxidisation. It is assumed that for such conditions the deteriorating influence of the bottom part of the defect decreases during subsequent rolling. During the elongation of the workpiece, the oxide flake of the Y-crack bottom is broken into splinters with oxide free material in between, making the creation of a high performance weld possible. Provided that this supposition is correct, the best results should be obtained for light reductions/pass at the beginning of the rolling, resulting in an early closure of the crack bottom, followed by heavy reductions/pass enabling also the closure and oxide splintering of the upper part of the folded crack.