Local Crack Growth Research Articles

Prediction of effective mode II fatigue crack growth threshold for metallic materials

The fatigue crack growth threshold consists of the intrinsic component (effective threshold) and the extrinsic component (crack closure). Theoretical formulae for prediction of the mode II effective threshold in metallic materials are presented. The model is based on physical interpretation of experimental findings. For a given material the dominant local crack growth mode at the kinked crack front of a remote mode II loaded crack is identified and the local stress intensity factor is expressed, which represents a new concept for estimation of the mode II effective threshold. The local crack growth mode depends on microstructure and crystallographic structure of the material. Moreover, new experimental data for mode II effective thresholds of the Ti6Al4V alloy (ΔKIIeff,th=1.8MPam1/2) and pure zirconium (ΔKIIeff,th=1.3MPam1/2) are presented. These data extended the available database for metallic materials and confirmed a broad validity of the proposed theoretical relationships.

Simulation of crack propagation/deflection in ceramic matrix continuous fiber reinforced composites with weak interphase via the extended finite element method

Toughness in continuous ceramic fiber reinforced ceramic matrix composites (CMCs) with dense matrices depends on the properties of the fiber coating or interphase. Multiple criteria have been proposed to describe the mechanism of crack propagation/deflection at the filament scale in brittle matrix continuous fiber reinforced composites; however, most of these criteria fail to account for the presence of an interphase of finite thickness and/or employ unrealistic boundary conditions. Recent simulations employing the extended finite element method (XFEM) have shown that variations in interphase thickness and strength relative to the fibers and/or matrix can have a significant influence on the crack propagation/deflection mechanism. It is shown that primary crack deflection most often occurs when conditions favor secondary cracking in the interphase in front of an approaching matrix crack. Although this mechanism is similar to that argued by Cook and Gordon (1964), the simulations here indicate that the conditions for secondary crack initiation and deflection of the primary crack can be much different than that which was originally presented in their analytical model. Variations in the properties of the interphase are simulated to produce large deviations in the local crack growth behavior as a matrix crack grows into interphase. Results are discussed relative to what has been observed experimentally.

3D simulation of short fatigue crack propagation by finite element crystal plasticity and remeshing

An iterative method to simulate 3D fatigue crack propagation in crystalline materials is proposed and demonstrated on simple test cases. The method relies on the computation of a damage indicator based on plastic activity around the crack tip. By post-processing this quantity after a given loading sequence, local crack direction and growth rate are estimated along the crack path. Dedicated remeshing routines allow the crack to propagate in 3D and the iterative process can continue with the updated crack geometry. Two examples in a BCC single crystal featuring different slip systems demonstrate the crack propagation simulations under cyclic loading over distances comparable to the crystal size and with non-regular crack shapes such as zig-zag crack paths. The effect of transferring plastic fields between propagation steps is also analysed.

Analysis of Portland cement mortar under impact: A combined material characterization, micromechanics modeling, and dynamic testing approach

Toward the development of a constitutive model for describing high strain-rate behavior of cementitious materials, this study couples multiscale quantitative characterization of mortars with micromechanics modeling, validated against dynamic experiments. Blast or impact loads create a far-field compressive loading condition that drives local tensile crack growth from flaws. Two flaw types – round (e.g., entrained air voids) and slit (e.g., tandem effect of the interfacial transition zone and the aggregates) flaws – are considered in this flaw-driven, elastic model for predicting strength at high strain-rates. The model sheds light on the linkage between meso-scale fluctuations of the flaw populations (including size, shape, and spatial arrangement) and the material's overall dynamic response. Dynamic strength is interpreted in a statistical framework rather than a deterministic one. Micromechanics results agree with Kolsky bar tests on mortars with and without air entrainment. The model may be useful for engineering concrete for potentially at-risk elements.

Fatigue Crack Extension in an Incrementally Formed Tube

Fatigue properties of a thermally assisted incrementally formed steel tube with a fine graded microstructure were investigated. The gradation is characterized by a recrystallized microstructure on the one side and a recovered one on the other side. It was found, that fatigue cracks are significantly deflected when reaching the recovered microstructure, but only a minor influence on the local crack growth rate was observed during the experiments.

Stress state factor evaluation based on a fractographic analysis for use in the crack growth FASTRAN retardation model of the AFGROW computing code

The FASTRAN crack growth retardation model uses stress state descriptors to consider the local stress state ahead of the crack tip. Using the FASTRAN model implemented in the AFGROW computing code, the descriptor is input into the retardation model as a constant tensile (compressive) constraint factor along with the corresponding crack growth rate.The paper presents an evaluation based on a fractographic analysis of fracture surfaces of middle tension M(T) specimens from Al-alloy, which were used for the standard crack growth rate measurement under constant amplitude loading. There are typically three zones on the fracture surface: the flat zone that is perpendicular to the loading force and represents the tension mode of specimen cracking, the slant zone that is approximately 45° deviated from the loading force direction and a transient zone between them. It is shown that a significant correlation between crack length, local crack growth rate and the sizes of the zones exist.Crack lengths relevant to the zones and corresponding crack growth rates were processed by statistical procedures. A continuous uniform distribution is a very good statistical model of the logarithm of the crack growth rates on both sides of the transient zone. In the retardation model, this distribution can be accounted for by using the means of the probability functions.A sub-scale structural model was tested for fatigue crack growth under a random loading process representing real operational conditions. Using the typical values of constraint factors for a “pure” plain stress and plain strain states and the crack growth rates evaluated by the fractographic analysis as input parameters, the crack growth curves calculated by the AFGROW code using the FASTRAN retardation model were compared with the crack growth data measured in the experiment.The AFGROW output crack growth curves fit to the experimental measured ones without any additional tuning of the retardation model parameters.

Effects of inclusions, grain boundaries and grain orientations on the fatigue crack initiation and propagation behavior of 2524-T3 Al alloy

Microstructural aspects have fundamental influences on the fatigue crack characteristics of materials. In this paper, effects of inclusions, grain boundaries (GBs) and grain orientations on the fatigue crack initiation and propagation behavior in a 2524-T3 aluminum alloy have been investigated using in-situ scanning electron microscope (SEM) fatigue testing and electron back scattering diffraction (EBSD). The results show that, potential fatigue cracks tend to nucleate along coarse and closely spaced inclusion particles or high-angle GBs. Coarse inclusion particles drastically accelerate local crack growth rates. A model of series crack growing stages is given based on the observation of initiation and growth of cracks at the inclusion region. GBs serve to impede the crack tip from propagation and cause large angle crack deflections, which greatly affects local crack propagation behaviors. In addition, fatigue crack shows a strong tendency to propagate transgranularly grains with high Schmid factors (SFs) and avoid grains with low SFs.

Local mechanical properties of a dissimilar metal welded joint in nuclear powersystems

In this paper, the local mechanical properties (yield/ultimate strength, ductility as well as stress–strain curves) of an Alloy52M dissimilar metal welded joint (DMWJ) in nuclear power systems were determined by using the mini-sized flat tensile specimens, and the local microstructures in the joint were characterized and analyzed. The results show that there are drastic variations in the local mechanical properties in the interface regions between materials. The heterogeneous mechanical properties are mainly related to the local microstructures of the DMWJ. The complex microstructures in the interface regions between materials were mainly caused by the heat flow and element migration during welding process. The local mechanical properties and their mismatches may have significant effect on crack-tip fracture mechanics parameter, plastic deformation behavior, local fracture resistance and crack growth behavior. Therefore, they need to be obtained and used in the integrity assessment of theDMWJs.

Fracture mechanism of a dissimilar metal welded joint in nuclear power plant

In this paper, fracture tests and microscopic observations were conducted on an Alloy52M dissimilar metal welded joint (DMWJ) between A508 ferritic steel and 316L stainless steel in nuclear power plant. The fracture mechanism of different regions within the joint was investigated, and the relationship between fracture mechanism in local microstructures and local fracture resistance of the DMWJ was analyzed. The results show the fracture mechanism of A508 and 316L base metals and heat-affected-zone (HAZ) of 316L is typical ductile fracture of nucleation, growth and coalescence of voids, and different crack growth resistance is mainly related to microstructures of them. The fracture mode in A508 HAZ and A508/52Mb interface region with predominant martensite microstructure is mixed brittle and ductile fracture, which leads to lower crack growth resistance. The columnar austenite crystal orientation relative to main crack growth direction apparently affects fracture mechanism and crack growth resistance of buttering Alloy52Mb and weld Alloy52Mw. The cracks in Alloy52Mb propagate across columnar crystal boundaries in a ductile mode, and higher fracture resistance is produced. While the cracks in weld Alloy52Mw propagate along the weak columnar crystal boundaries in a brittle mode, which leads to lower crack growth resistance. The cracks in interface regions and HAZs generally grow towards the material sides with lower strength, which is caused by local strength mismatch and affects local crack growth resistance.

AEROSPACE STIFFENED PANEL INITIAL SIZING WITH NOVEL SKIN SUB-STIFFENING FEATURES

The introduction of skin sub-stiffening features has the potential to modify the local stability and fatigue crack growth performance of stiffened panels. Proposed herein is a method to enable initial static strength sizing of panels with such skin sub-stiffening features. The method uses bespoke skin buckling coefficients, automatically generated by Finite Element analysis and thus limits the modification to the conventional aerospace panel initial sizing process. The approach is demonstrated herein and validated for prismatic sub-stiffening features. Moreover, examination of the generated buckling coefficient data illustrates the influence of skin sub-stiffening on buckling behavior, with static strength increases typically corresponding to a reduction in the number of initial skin longitudinal buckle half-waves.

An experimental investigation of local fracture resistance and crack growth paths in a dissimilar metal welded joint

An experimental investigation on the local fracture resistance and crack growth behavior in a Alloy52M dissimilar metal welded joint (DMWJ) between A508 ferritic steel and 316L stainless steel has been carried out by using the single-edge notched bend (SENB) specimens. The local J-resistance curves and crack growth paths of 13 cracks located at various positions in the DMWJ were determined, and the effects of the local strength mismatch on local fracture resistance, crack growth paths and integrity assessment for the DMWJ were analyzed. The results show that the cracks always deviate to the materials with lower strength, and the crack path deviations are mainly controlled by the strength mismatch, rather than toughness mismatch. The J-resistance curve with larger crack path deviation only reflect the apparent fracture resistance along the crack growth region, rather than the intrinsic fracture resistance of the material at the initial crack-tip region. Without considering the local fracture resistance properties of heat affected zone (HAZ), interface and near interface zone, the use of the J-resistance curves of base metals or weld metals following present codes will unavoidably produce non-conservative (unsafe) or excessive conservative assessment results. In most cases, the assessment results will be potentially unsafe. Therefore, it is recommended to obtain and use local mechanical and fracture resistance properties of all regions of the DMWJ if the complex local mismatch situation is a concern. And new integrity assessment methods based on local damage and fracture models also need to be developed for the DMWJs.

Local Mechanical Properties and Microstructures of Alloy52M Dissimilar Metal Welded Joint between A508 Ferritic Steel and 316L Stainless Steel

The local mechanical properties and microstructures in a real dissimilar metal welded joint (DMWJ) of nuclear power plant were investigated. Results show that the distributions of mechanical properties in the DMWJ are very inhomogeneous. Especially in the three interface regions of A508/Alloy52Mb, Alloy52Mb/Alloy52Mw and Alloy52Mw/316L, the inhomogeneous characteristics are most prominent, and some local softening and hardening zones are formed. This non-uniform distribution on mechanical properties is mainly caused by the different microstructures in the DMWJ. The local dramatic variations in mechanical properties and local mismatch effect in the softening and hardening zones have significant effects on macroscopic fracture mechanics parameter, local crack initiation and growth behavior. But they are not considered in present structure integrity assessment codes and Leak-Before-Break (LBB) analysis. For assessing integrity and conducting LBB analysis on the DMWJ accurately, it is necessary to determine the local mechanical properties and their local mismatch ratios. In addition, the new integrity assessment and LBB analysis methods based on local damage and fracture need to be developed.

Three-dimensional in situ observations of short fatigue crack growth in magnesium

Three-dimensional (3D) short fatigue crack growth behaviour in a cast magnesium alloy, Elektron 21, was studied using a combination of X-ray diffraction contrast tomography (DCT) and microtomography at the European Synchrotron Radiation Facility. A 3D map of grain shapes and crystallographic orientations was produced in a miniature fatigue specimen using DCT. A focused ion beam instrument was used to introduce small notches in selected grains. Synchrotron microtomography was used to study the evolution of fatigue cracks from these notches during interrupted in situ fatigue cycling. Stage I crack growth occurred preferentially on the basal plane of the magnesium hexagonal close packed crystal, with local crack growth rates between 4 and 40 nm cycle −1. Retardations in the local crack growth rate were observed in certain grains and at certain grain boundaries. The observed interactions between the crack and the polycrystalline microstructure can be explained using Schmid factors and a development of the tilt–twist model of Zhai and Wilkinson (Zhai T, Wilkinson AJ, Martin JW. Acta Mater 2000;48;4917).

A method of global-local analyses of structures involving local heterogeneities and propagating cracks

This paper presents the global-local finite cover method (GL-FCM) that is capable of analyzing structures involving local heterogeneities and propagating cracks. The suggested method is composed of two techniques. One of them is the FCM, which is one of the PU-based generalized finite element methods, for the analysis of local cohesive crack growth. The mechanical behavior evaluated in local heterogeneous structures by the FCM is transferred to the overall (global) structure by the so-called mortar method. The other is a method of mesh superposition for hierarchical modeling, which enables us to evaluate the average stiffness by the analysis of local heterogeneous structures not subjected to crack propagation. Several numerical experiments are conducted to validate the accuracy of the proposed method. The capability and applicability of the proposed method is demonstrated in an illustrative numerical example, in which we predict the mechanical deterioration of a reinforced concrete (RC) structure, whose local regions are subjected to propagating cracks induced by reinforcement corrosion.

Localized deformation during fracture of high-strength aluminum alloy

X-ray microtomography has been utilized for the observation of crack initiation and growth in a high-strength aluminum alloy. Procedures for evaluating local crack-driving forces such as J and CTOD, which were developed by the present authors, have been applied to analyze localized crack initiation and growth behaviors. The former is realized by tracking microstructural features that are located in tomographic imaged in high density. The latter is directly measured in 3D images. In order to raise the accuracy of the microstructural tracking technique, a trajectory prediction technique has been introduced, thereby substantial improvement in the ratio of tracked markers has been realized. Extensive formation of micro cracks ahead of a crack tip has been observed, being followed by premature crack initiation and growth. It has been clarified that the micro cracking occurs far beyond (more than 10 times) a large strain region that has been predicted in the conventional fracture mechanics. The current methods could offer a highly effective way of assessing such 3D local fracture behavior quantitatively.

3-D growth of a short fatigue crack within a polycrystalline microstructure studied using combined diffraction and phase-contrast X-ray tomography

X-ray diffraction contrast tomography is a recently developed, non-destructive synchrotron imaging technique which characterizes microstructure and grain orientation in polycrystalline materials in three dimensions. By combining it with propagation-based phase-contrast tomography it is possible to get a full picture description for the analysis of local crack growth rate of short fatigue cracks in three dimensions: the three-dimensional crack morphology at different propagation stages, and the shape and orientation of the grains around the crack. An approach has been developed on the metastable beta titanium alloy Ti 21S that allows for visualization and analysis of the growth rate and crystallographic orientation of the fracture surface.

Influence of closure on the 3D propagation of fatigue cracks in a nodular cast iron investigated by X-ray tomography and 3D volume correlation

Synchrotron X-ray tomography was performed during in situ fatigue crack propagation in two small-size specimens made of nodular graphite cast iron. While direct image analysis allows us to retrieve the successive positions of the crack front, and to detect local crack retardation, volume correlation allows for the measurement of displacement fields in the bulk of the specimen. The stress intensity factors (SIFs), which are extracted from the measured displacement fields and the corresponding local crack growth rate all along the front, are in good agreement with published results. In particular, it is possible to link the non-propagation of a crack with crack closure in the crack opening displacement maps or with a local value of the measured SIF range. It is shown that a non-uniform closure process along the crack front induces an asymmetric arrest/growth of the crack.

Three-Dimensional Characterization of Fatigue Crack Propagation Behavior in an Aluminum Alloy Using High Resolution X-Ray Microtomography

Synchrotron X-ray microtomography was used to investigate the three-dimensional fatigue crack propagation behavior of an aluminum alloy. Local crack growth rates along specimen thickness were calculated. Crack observation revealed that crack propagation was retarded in three regions where there exist overlapping crack segments. Crack growth retardation was found to occur in these crack overlapping regions due to stress shielding. Observation of crack propagation process indicated that one of the overlapping crack segments initiated from the twisted crack. Crack tip opening displacement was measured and it was found that crack opening was larger in single planar crack region than that in crack overlapping region.

Fatigue creep damage at the cement–bone interface: An experimental and a micro-mechanical finite element study

The goal of this study was to quantify the micromechanics of the cement–bone interface under tensile fatigue loading using finite element analysis (FEA) and to understand the underlying mechanisms that play a role in the fatigue behavior of this interface. Laboratory cement–bone specimens were subjected to a tensile fatigue load, while local displacements and crack growth on the specimen's surface were monitored. FEA models were created from these specimens based upon micro-computed tomography data. To accurately model interfacial gaps at the interface between the bone and cement, a custom-written erosion algorithm was applied to the bone model. A fatigue load was simulated in the FEA models while monitoring the local displacements and crack propagation. The results showed the FEA models were able to capture the general experimental creep damage behavior and creep stages of the interface. Consistent with the experiments, the majority of the deformation took place at the contact interface. Additionally, the FEA models predicted fatigue crack patterns similar to experimental findings. Experimental surface cracks correlated moderately with FEA surface cracks ( r 2=0.43), but did not correlate with the simulated crack volume fraction ( r 2=0.06). Although there was no relationship between experimental surface cracks and experimental creep damage displacement ( r 2=0.07), there was a strong relationship between the FEA crack volume fraction and the FEA creep damage displacement ( r 2=0.76). This study shows the additional value of FEA of the cement–bone interface relative to experimental studies and can therefore be used to optimize its mechanical properties.

A New Correction Procedure to Correct the Predicted Crack Extension Direction of a Mixed Mode Crack Path

In an incremental crack extension analysis each crack increment is in general modelled with a straight extension. In order to avoid introduction of an error when the local crack growth criterion is used with an incremental formulation, each straight crack extension would have to be infinitesimal as the crack growth direction changes when the crack grows. A correction procedure to correct the extension direction of the increment can however be applied to ensure that a unique crack path is achieved with different analyses of the same problem performed with different size of the crack-extension increments. A proposed correction procedure and an reference correction procedure are demonstrated by solving a computational crack growth example. The demonstration shows that analyses of the crack path performed with big crack extensions and the proposed crack correction procedure are in excellent agreement with analyses of the crack path performed with very small crack extensions. Furthermore it is shown that the reference correction procedure has a tendency to overcorrect the crack growth direction if the stop criterion for the iterative correction procedure is not specified for each new crack growth analysis.