Crack Closure Mechanisms Research Articles

Micromechanical Analysis of Crack Closure Mechanism for the Intelligent Material Containing TiNi Fibers. 1st Report, Modeling of Crack Closure Mechanism and Analysis of the Stress Intensity Factor.

Micromechanical modeling of an intelligent material containing TiNi fibers is performed by taking into consideration of the existence of crack bridging fibers with transformation strain which corresponds to the total amount of shrinkage of the fiber. In modeling, thermal expansion strain due to the mismatch of thermal expansion coefficients between TiNi fiber and matrix is also considered. The stress and strain fields in the matrix, fibers, and fictious eigenstrain in cracks can be found, hence the total potential energy of the model will be derived from these fields. By using the total potential energy, the stress intensity factor at the tip of crack in the material will be expressed successfully in terms of the transformation strain of fibers and the thermal expansion strain occurred in the material.

Micromechanical Analysis of Crack Closure Mechanism for the Intelligent Material Containing TiNi Fibers. 2nd Report, Numerical Calculation of the Stress Intensity Factor in the Process of Shape Memory Shrinkage of TiNi Fibers.

In the previous study, micromechanical modeling of the intelligent material containing TiNi fibers was performed and the stress intensity factor K1 could be expressed in terms of the magnitude of shape memory shrinkage of fibers and the thermal expansion strain occurred in the material. In this study, by using the analytical results, the value of K1 will be numerically calculated. As a result, we can see that the K1 value decreases with increasing in the shrink strain of fibers. This tendency agree with that of the experimental result obtained by Shimamoto et al. Moreover, it can be seen that there exists the optimal value of shrink strain of fibers to make the K1 value zero. The change in K1 with temperature during heating process to the inverse austenitic temperature of TiNi fiber is also consistent with the experimental result. The results obtained can be explained by the shrink strain, the thermal expansion strain, and the elastic moduli of TiNi fiber. These results may be useful in designing the intelligent material containing TiNi fibers from the viewpoint of crack closure.

An Experimental Investigation of Tubular T-Joints Under Cyclic Loads

This paper presents an experimental study of fatigue crack initiation and growth in tubular T-joints, simulating the extreme case of those used for offshore platforms. The crack locations and profiles were recorded and fractographic examinations were carried out to reveal the mechanisms of crack initiation, growth, and crack closure. The relationships between crack length/depth and the numbers of fatigue cycle were analyzed using the Paris equation. During testing, the static strain responses at selected gage locations were also recorded. The variations in the static strains caused by the formation and growth of cracks were correlated with fatigue cycles by a characteristic equation. These results and correlations are useful for understanding the fatigue damage process and for monitoring the development of such damage in tubular T-joints of offshore structures.

疲労 ＳＥＭ／ＡＦＭ破面観察による超長寿命疲労機構の考察

Since in superlong fatigue failure with Nf≥108, the average fatigue crack growth rate is much less than lattice spacing (-0.1A or 0.01nm or 10-11--12m/cycle), we cannot assume that crack growth occurs cycle by cycle in the early stage of fatigue process.In this paper, possible mechanisms for extremely high cycle fatigue are discussed. A special attention was paid to a newly found particular fatigue fracture morphology in the vicinity of fracture origin (nonmetallic inclusions) of a heat treated alloy steel, SCM435. The particular morphology looks a dark area inside fish-eye mark by optical microscopic observation. Specimens with short fatigue life of N=-105 do not have such dark area in fish-eye mark. SEM and AFM observations revealed that the dark area has a rough surface quite different from usual fatigue fracture surface in martensite lath structure. The predictions of fatigue limit by the √area parameter model are -10% unconservative for fatigue limit defined for N=107. Thus, the fatigue failure for N≥108 is presumed to be caused by a mechanism which induces breaking or releasing of fatigue crack closure phenomenon in small cracks. The breaking or releasing crack closure mechanism is presumed to be caused by environmental effects such as hydrogen embrittlement coupled with extremely high cycle fatigue. Some indirect evidences to support this hypothesis are shown.

Development of fatigue crack closure mechanism maps Part 1 – Basic concepts and boundary equations

The influence of various mechanical, metallurgical, and environmental parameters on fatigue crack growth behaviour has been broadly explained elsewhere using the crack closure concept. In this paper the concept of a crack closure mechanism map is proposed in order to predict crack closure mechanisms operating under a given set of loading conditions for any alloy/microstructure. The domains of plasticity induced, roughness induced, and oxide induced crack closure mechanisms are mapped in Kmax–∆K space using various boundary equations. In Part 1 of this paper, the boundary equations are derivedfrom conditions related to the loading conditions.

Development of fatigue crack closure mechanism maps Part 1 – Basic concepts and boundary equations

The influence of various mechanical, metallurgical, and environmental parameters on fatigue crack growth behaviour has been broadly explained elsewhere using the crack closure concept. In this paper the concept of a crack closure mechanism map is proposed in order to predict crack closure mechanisms operating under a given set of loading conditions for any alloy/microstructure. The domains of plasticity induced, roughness induced, and oxide induced crack closure mechanisms are mapped in Kmax–∆K space using various boundary equations. In Part 1 of this paper, the boundary equations are derivedfrom conditions related to the loading conditions.

Development of fatigue crack closure mechanism maps Part 2 – Construction of maps for various materials

Crack closure mechanism maps are constructed for a variety of ferrous and non-ferrous alloys using the boundary equations derived in Part 1. Extensive fatigue crack growth studies involving crack closure measurements and fractography were performed to validate the predictions of the crack closure mechanism maps developed in this way. It is demonstrated that the dominance of a particular mechanism under a given set of loading conditions is dependent on the deformation behaviour, controlling microstructure unit, yield strength, texture, and component thickness. Finally, some possible engineering applications of crack closure mechanism maps are suggested.

Development of fatigue crack closure mechanism maps Part 2 – Construction of maps for various materials

Crack closure mechanism maps are constructed for a variety of ferrous and non-ferrous alloys using the boundary equations derived in Part 1. Extensive fatigue crack growth studies involving crack closure measurements and fractography were performed to validate the predictions of the crack closure mechanism maps developed in this way. It is demonstrated that the dominance of a particular mechanism under a given set of loading conditions is dependent on the deformation behaviour, controlling microstructure unit, yield strength, texture, and component thickness. Finally, some possible engineering applications of crack closure mechanism maps are suggested.

Environment-sensitive closure and fatigue crack propagation behavior of Al 2090

Fatigue crack propagation (FCP) behavior of Al-Li alloys is largely influenced by extrinsic crack closure. The studies on closure have been confined to the relatively benign moist air environment. The synergistic interaction between the aggressive environment and closure on FCP, particularly that in the near-threshold ΔK regime, has often been neglected due to the difficulties in closure measurement in aqueous environments. In the present study, FCP behavior of Al 2090 was examined in moist air, vacuum, and aqueous 3.5 pct NaCl at different R ratios with an emphasis on the understanding of environment-sensitive crack closure mechanisms. The closure in Al 2090 was strongly affected by applied ΔK, R ratio, and environment. The increase in near-threshold da/dN in aqueous 3.5 pct NaCl over that in moist air mostly disappeared after crack closure correction, indicating that environment-sensitive closure was a major determining factor for the FCP rates of high-closure materials. Interestingly, Al 2090 showed a lower closure level in vacuum than in moist air, despite the enhanced fracture surface roughness and reduced FCP rates in low and intermediate ΔK regimes. The increasing R ratio tends to reduce closure contribution and eventually increase the FCP rates of Al 2090 for each environment. The trends observed in the present study show that closure is determined by complex interactions between the environment and R ratio, particularly in a low stress intensity regime.

The merging of fatigue and fracture mechanics concepts: a historical perspective

In this review, some of the technical developments that have occurred during the past 40 years are presented which have led to the merger of fatigue and fracture mechanics concepts. This review is made from the viewpoint of “crack propagation”. As methods to observe the “fatigue” process have improved, the formation of fatigue micro-cracks have been observed earlier in life and the measured crack sizes have become smaller. These observations suggest that fatigue damage can now be characterized by “crack size”. In parallel, the crack-growth analysis methods, using stress-intensity factors, have also improved. But the effects of material inhomogeneities, crack-fracture mechanisms, and nonlinear behavior must now be included in these analyses. The discovery of crack-closure mechanisms, such as plasticity, roughness, and oxide/corrosion/fretting product debris, and the use of the effective stress-intensity factor range, has provided an engineering tool to predict small- and large-crack-growth rate behavior under service loading conditions. These mechanisms have also provided a rationale for developing new, damage-tolerant materials. This review suggests that small-crack growth behavior should be viewed as typical behavior, whereas large-crack threshold behavior should be viewed as the anomaly. Small-crack theory has unified “fatigue” and “fracture mechanics” concepts; and has bridged the gap between safe-life and durability/damage-tolerance design concepts.

Fatigue crack propagation in β Ti–15Mo–5Zr–3Al alloy

This paper describes the fatigue crack propagation (FCP) for five materials having different microstructures of β Ti–15Mo–5Zr–3Al alloy. With one exception, the microstructure was found to have very little or no influence on FCP behavior. The one exception was the material having the largest β grain size, which showed higher FCP resistance than all other materials due to higher crack closure levels. The effects of stress ratio on the FCP behavior could not be realized solely in terms of the crack closure mechanisms, because static fracture mechanisms operated during FCP depending on the magnitude of the maximum stress intensity factor, K max. Furthermore, it was found that the FCP resistance of all the materials was considerably lower than that of α/ β Ti–6Al–4V alloy, which was primarily attributed to lower crack closure levels of the β alloy.

Effects of morphology and volume fraction of α 2 phase on the fatigue crack propagation of a Ti-24Al-11Nb alloy

The main objective of the present investigation is to study the individual effect of α 2 morphology and volume fraction on the fatigue crack propagation behavior of a two-phase (α 2+β) Ti-24Al-11Nb alloy. With systematic heat treatments, three distinct morphologies have been obtained: fine colony, fine equiaxed, and fine basket-weave structures. Volume fraction of α 2 has been varied from 40 to 68 pct by changing the two-phase annealing temperature. It has been shown that the volume fraction and morphology of α 2 phase have a large influence on the fatigue crack propagation behavior. Fine colony structure shows no variation in the fatigue crack propagation rate with the α 2 volume fraction. On the other hand, equiaxed and basket-weave structures show the increased resistance to fatigue crack propagation as α 2 volume fraction decreases. Such increased resistance to crack growth is attributed primarily to tortuous crack paths, which result in a reduction in the crack driving force from crack deflection and roughness-induced crack closure mechanism. Quantitative metallography and experimental crack closure measurements are presented to substantiate such interpretations.

Crack growth and closure mechanisms of shear cracks under constant amplitude biaxial straining and periodic compressive overstraining in 1045 steel

Crack growth and roughness induced closure mechanisms of shear cracks under 180° out-of-phase strain histories having constant amplitude straining and periodic compressive overstrains (PCOs) were investigated. The results revealed that shear cracks initially nucleated on a slip band at 45° to the axis of the specimen which coincides with the plane of maximum shear strain. Growth on the shear planes (microcracks) into the specimen surface occupied up to 90–95% of fatigue life during which time the surface length of the microcracks remained nearly constant. Failure then occurred by a rapid linking of microcracks at the end of a test. The opening stress of microcracks was taken to be the applied static stress level at which the crack depth stopped increasing with increasing stress. Observations indicated that as the number of cycles increased the crack depth on the maximum shear plane increased but the crack opening stress did not change appreciably. The crack growth rate in the depth direction on shear planes increased significantly and fatigue strength was reduced by a factor of 1.4 and 1.7 at short and long lives, respectively, when PCOs of near yield point magnitude were applied. Confocal scanning laser microscopy and scanning electron microscopy examinations of fracture surfaces revealed that these PCOs flattened mismatch asperities near the crack tip. The reduction in the height of these irregularities on the fracture surface was accompanied by a reduced crack closure stress and a higher crack growth rate.

Mechanisms of ambient temperature fatigue crack growth in Ti−46.5Al−3Nb−2Cr−0.2W

Fatigue crack growth studies have been conducted on a two-phase alloy with a nominal composition of Ti−46.5Al−3Nb−2Cr−0.2W (at. pct), heat treated to produce duplex and lamellar microstructures. Fatigue crack growth tests were conducted at 23°C using computer-controlled servohydraulic loading at a cyclic frequency of 20 Hz. Several test methods were used to obtain fatigue crack growth rate data, including decreasing-load-range-threshold, constant-load-range, and constant-K max increasing-load-ratio crack growth control. The lamellar microstructure showed substantial improvement in crack growth resistance and an increase in the threshold stress intensity factor range, ΔK th, when compared with the behavior of the duplex microstructure. The stress ratio had a significant influence on crack growth behavior in both microstructures, which appeared to be a result of roughness-induced crack closure mechanisms. Fractographic characterization of fatigue crack propagation modes indicated a highly tortuous crack path in the fully lamellar microstructure, compared to the duplex microstructure. In addition, limited shear ligament bridging and secondary cracking parallel to the lamellar interfaces were observed in the fully lamellar microstructure during fatigue crack propagation. These observations were incorporated into a model that analyzes the contribution of intrinsic vs extrinsic mechanisms, such as shear ligament bridging and roughness-induced crack closure, to the increased fatigue crack growth resistance observed for the fully lamellar microstructure.

Fatigue crack propagation and closure behavior of modified 1070 steel: Experimental results

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.

Analysing the fatigue crack growth in structural details

A unified procedure is proposed to compute both the stress intensity factor (SIF) and the fatigue crack growth for part-through cracks in complex structural details. This method is an extension of the approximate three-dimensional (3D) weight function (WF) method in solving both SIF and fatigue crack growth problems for complex structural details using only the stress distribution in perspective crack sites. The fatigue crack growth analysis is based on the plastic deformation induced crack closure mechanism which is determined according to the strip yielding model. Cycle-by-cycle fatigue crack growth predictions have been made based on Elber's crack growth relation. The fatigue crack growth analysis is closely related to the cyclic stress experience, the configuration of structural detail, the applied load spectrum and the material property. The advantage and limitation of the method have been discussed. Some examples have been provided for the fatigue crack growth analyses for structural details of a Swedish fighter/attacker aircraft to demonstrate the methodology. This paper shows that an efficient, solid, unified theoretical frame can be established for reliable analyses of the fatigue crack growth in complex structural details for general engineering applications.

Isothermal fatigue of an aluminide-coated single-crystal superalloy: Part II. effects of brittle precracking

The effect of brittle coating precracking on the fatigue behavior of a high-activity aluminide-coated single-crystal nickel-base superalloy has been studied using hollow cylindrical specimens at test temperatures of 600 °, 800 °, and 1000 °. Three types of precrack were studied: narrow precracks formed at room temperature, wide precracks formed at room temperature, and narrow precracks formed at elevated temperature. The effect of precracking on fatigue life at 600 ° was found to depend strongly on the type of precrack. No failure was observed for specimens with narrow room-temperature precracks because of crack arrestvia an oxidation-induced crack closure mechanism, while the behavior of wide precracks and precracks formed at elevated temperature mirrored the non-precracked behavior. Crack retardation also occurred for narrow room-temperature precracks tested at 800 °—in this case, fatigue cracks leading to failure initiated in a layer of recrystallized grains on the inside surface of the specimen. A significant reduction in fatigue life at 800 ° relative to non-precracked specimens was observed for wide precracks and elevated temperature precracks. The presence of precracks bypassed the initiation and growth of coating fatigue cracks necessary for failure in non-precracked material. No effect of precracking was observed at 1000 °.

In situ SEM studies on the effects of particulate reinforcement on fatigue crack growth mechanism of aluminium-based metal-matrix composite

The effects of particulate reinforcement on the fatigue behaviour and fatigue mechanisms of two 6061 aluminium-based metal-matrix composites (MMCs) in three different heattreatment conditions were studied in situ with a scanning electron microscope and compared to the unreinforced alloy in the as-received condition. It was observed that the fatigue properties of the MMCs were influenced by the ceramic particles in two ways: firstly the particles increased the fatigue stress intensity threshold mainly by crack-deflection and crack-closure mechanisms, and secondly, the particles raised the fatigue crack growth rates in the Paris region by providing an easy crack path. The effect of ageing was small on the fatigue stress intensity threshold of MMCs, but for the peak-aged MMCs the fatigue crack growth rates in the Paris region were faster. The mechanism of fatigue crack growth was largely associated with the matrix/particle interface and the linkage with subcracks initiated ahead of the main crack at high applied stress intensity factors.

Application of the shadow optical method to fatigue and crack closure studies

AbstractIn this paper it will be shown that crack closure and related crack shielding mechanisms can be studied successfully by applying the shadow optical method, also known as the method of caustics. It is shown that at least one mechanism ‐ plasticity induced crack closure ‐ can be identified and determined from measurements of the transverse diameter of the caustic in the crack tip region. For given fatigue conditions at which crack closure occurs, several experiments have been performed to investigate the effectiveness of the crack opening in a typical fatigue cycle.Two different methods of closure determination have been used. A comparison is made between one of the most currently used methods of back face strain (BFS) compliance measurement and the shadow optical method (SOM). The underlying features of these two experimental techniques provide the key to finding the extent of the load in the fatigue cycles over which the crack is actually open. This redefines the value of Kmin and the meaning of ΔK. The driving force, ΔKeff is shown to be reduced in or near the fatigue threshold of the alloy, because crack closure is most effective in this part of fatigue loading. The introduction of SOM to fatigue crack closure provides a suitable alternative for finding the effective part of the stress intensity range between the minimum and maximum loads. Conclusions drawn from this work were that in addition to determining that part of the fatigue cycle over which the crack is actually open, the shadow optical method allowed an accurate interpretation of the entire fatigue cycle between Kmax and Kmin

β型Ti-15Mo-5Zr-3Al合金の疲労き裂進展特性

Fatigue crack propagation (FCP) in beta Ti-15Mo-5Zr-3Al alloy has been studied in laboratory air for five different microstructures which were solution-treated at 700°C, 735°C, 765°C, 850°C, and 1000°C followed by aged at 500°C. With one exception, microstructure was found to have very little or no influence on FCP behaviour. The one exception was the microstructure having the largest beta grain size whose FCP resistance was higher than that of the other microstructures due to higher crack closure levels. The effect of stress ratio on FCP could not be realized only in terms of crack closure mechanism, because static fracture mechanisms operated in FCP behaviour depending on the magnitude of the maximum stress intensity factor. Furthermore, the FCP resistance of all the microstructures was considerably lower than that of alpha-beta Ti-6Al-4V alloy, which was primarily attributed to lower crack closure levels of the present alloy.