Small Fatigue Crack Behavior Research Articles

Fatigue strength properties of precipitation strengthening stainless steel A286 focused attention on small fatigue crack behaviour

In order to safely use precipitation strengthening type A286 stainless steel in high cycles, the fundamental fatigue strength characteristics were investigated using fatigue tests at room temperature. During the fatigue test, the author paid attention to the behaviour of small fatigue cracks in order to clarify the fatigue strength properties in more detail than the usual fatigue test. As a result of the fatigue test, the fatigue strength characteristic of the A286 steel became clarified as shown below. The fatigue strength ratio of the A286 steel is lower than 0.5 which is the ratio value of most steels. The cracks propagate which result in a failure even after 107 cycles. It was found that there are two crack mechanisms propagation of the A286 steel; i.e., in a high load-stress, the cracks monotonously propagate and result in failure. Under a low load-stress, the cracks intermittently propagate even after 107 cycles. The non-propagating cracks coalesce with a new crack which initiates near the crack tip and it becomes longer, finally resulting in a failure. This is the reason why the fatigue failures occur even after 107 cycles.

Fatigue crack growth from small defects under out-of-phase combined loading

A modified linear elastic fracture mechanics analysis is presented for the evaluation of the crack growth and threshold behavior of small cracks initiated from small defects under combined loading fatigue. For the detailed evaluation of the behavior of small fatigue cracks, the Kitagawa effect, the elastic–plastic behavior of cracks in biaxial stress fields and crack closure effects are taken into account. In-phase and out-of-phase combined tension and torsion fatigue tests were conducted using annealed carbon steel specimens containing small holes. The direction of crack propagation, S– N curves and fatigue limits were found to be in good agreement with the theoretical predictions.

In-situ scanning electron microscopy studies of small fatigue crack growth in recrystallized layer of a directionally solidified superalloy

The small fatigue crack behavior in the surface recrystallized layer of a nickel-based directionally solidified superalloy DZ4 was studied by in-situ scanning electron microscopy. Transgranular cracking manner was identified at 350 °C, distinct from the intergranular fracture of recrystallized grains reported formerly. The transition of fracture mode is attributed to the effect of temperature, which should be particularly concerned in the safety of turbine components with recrystallization. The discontinuous crack growth in recrystallized layer is associated with the local microstructure. Carbide can act as crack source, and barrier to the crack growth as well. Effect of twin boundary on impeding the fatigue crack is evident.

Initiation and Growth Behavior of Small Surface Fatigue Cracks on SiC Particle-Reinforced Aluminum Composites

Reversed plane bending fatigue tests were conducted on SiC particle-reinforced aluminum composites. The initiation and growth behaviors of small surface fatigue cracks were continuously monitored by the replica technique and investigated in detail. The fatigue life of MMC is shorter than that of matrix because there exists interface debonding of SiC particles and matrix on the whole face of the notch part in the casting metal matrix composite (MMC). The coalescence of small cracks was observed in the surface after conducting fatigue tests at high stress levels. Due to the coalescence, a higher crack growth rate of small cracks rather than that of long cracks was recognized in da/dn – K max relationship.

An Analysis of the Behavior of Small Fatigue Cracks Emanating from Stress Concentrators

The plastic zone size (PZS) at the tip of a crack emanating from stress concentrators subjected to uniform tension was calculated based upon the Dugdale model. A method was proposed for the prediction of fatigue notch effects in terms of the McEvily method that has widely been used for the analysis of small fatigue crack growth. In modifying this method, the elastic-plastic effects due to stress concentration were taken into account using the values of PZS calculated in this study. The results predicted by this method accounted successfully for the behavior of small fatigue cracks near the threshold levels observed in the fatigue tests conducted using notched steel specimens.

Observation of the Behavior of Fatigue Cracks in Friction Stir Welded Aluminum Alloy Joints

Friction stir welding (FSW) is a new solid-state welding process that can produce low-cost and high-quality joints of especially aluminum and mgnesium alloys. The welding zone consists of different regions with characteristic microstructuralal details such as a weld nugget, a thermo-mechanically-affected zone (TMAZ) and a heat-affected zone (HAZ). Tension-compression fatigue tests were performed using FSW aluminum alloy AA5454 sheet specimens at a stress ratio of –1. To investigate the propagation behavior of small fatigue cracks in those regions, an artificial defect was introduced into different defined locations in the FSW specimens as well as into the parent material specimens. The crack propagation rates depended on the defined locations and were a function of the hardness; that is, the lower the hardness was, the higher the propagation rate was. The crack paths were mostly perpendicular to the applied stress axis, but some crack paths exhibited deviations by the influence of the local anisotropy of the microstructure.

Small fatigue crack growth in metallic materials: A model and its application to engineering alloys

Characterization of the growth behavior of small fatigue cracks is important for materials used in structurally demanding applications such as aircraft turbine discs and some automotive engine components. Here, we present a general, dislocation-based fracture mechanics approach to predict the growth rate of small fatigue cracks in metallic materials. The applicability of the model to the small fatigue crack growth behavior of four engineering alloys was examined. Small fatigue cracks were initiated and propagated, in a controlled manner, from micronotches fabricated by femtosecond pulsed laser micromachining. The results suggest that a methodology consisting of crack-tip damage accumulation and fracture provides a common framework to estimate the fatigue crack propagation lifetime of structural materials.

The transition between small and long fatigue crack behavior and its relation to microstructure

The transition point from small to long crack behavior is experimentally studied in a single phase aluminum alloy. It is shown that scatter decreases until reaching a steady state value for long crack growth. This point is defined as the transition from micro-structurally small to long crack growth and is shown to correspond to the point when the growing crack front intersects approximately 15 grains. This transition point is experimentally validated from fatigue crack growth data both from single, corner micro-cracks and multi-site micro-cracks on a smooth surface.

P-19 Fatigue crack initiation and small fatigue crack propagation behavior in high vacuum on Ti- and Al alloys

In this study, we investigated initiation and propagation behavior of small fatigue crack on A7075 and Ti-6Al-4V in vacuum environment. As well as the ambient air environment, so-called small fatigue crack behavior were observed in vacuum. In both materials, the fatigue crack initiation life and failure life in vacuum environment are always longer than that of ambient air environment. In case of A7075, effect of the air on small fatigue crack propagation rate is less than that of obtained on Ti-6Al-4V.

Prediction of the behavior of small fatigue cracks

A modified linear-elastic fracture mechanics approach has been proposed and developed by McEvily and co-workers, with which the fatigue crack growth rates of both small cracks and large cracks can be quantitatively evaluated. This paper reviews the recent results of applications of this approach to the following fatigue problems involving the prediction of the behavior of small fatigue cracks: (1) in an examination of the use of Miner's rule in fatigue life estimation under variable amplitude loading, (2) in the verification of the a r e a parameter model, and (3) in the prediction of the fatigue life and strength of defect-containing components subjected to multiaxial stress.

Initiation and Growth Behavior of Small Fatigue Cracks in Ultra-fine Grained P/M Aluminum Alloy at Elevated Temperatures

Small fatigue crack initiation and growth behavior at elevated temperatures were investigated using ultra-fine grained P/M (Powder Metallurgy) aluminum alloy of which grain size was from 200 to 500nm. Reversed plane bending fatigue tests were conducted at three different temperatures of room temperature (R.T.), 200°C (473K) and 250°C (523K), and crack initiation and small crack growth were studied in detail by means of replication technique. The fatigue strength decreased as test temperature increased. The cracks initiated at the boundary between powders regardless of test temperatures, and total fatigue life was dominated by crack initiation life. The fatigue crack initiation and growth behavior was almost irrelevant to the test temperatures. Fatigue crack growth rates, da/dn, were slightly accelerated at a fixed Kmax with increasing test temperature, while da/dn evaluated in terms of Kmax/E (E : Elastic modulus) agreed well. The dependence of fatigue strength and crack growth rate on test temperature was much smaller than that of conventional aluminum alloys, indicating the high heat resistance of ultra-fine grained P/M aluminum alloy.

In-situ observations of high cycle fatigue mechanisms in cast AM60B magnesium in vacuum and water vapor environments

We present in situ scanning electron microscopy (SEM) observations regarding the formation and propagation of small fatigue cracks in cast AM60B magnesium. Using an environmental SEM, observations were made in vacuum and in the presence of water vapor at 20 Torr. In the vacuum environment, fatigue cracks in the magnesium formed preferentially at pores, sometimes precluded by observable cyclic slip accumulation. At higher cycle numbers in the vacuum environment, additional cracks were discovered to initiate at persistent slip bands within relatively large magnesium dendrite cells. The propagation behavior of small fatigue cracks (a < 6–10 dendrite cells) was found to depend strongly on both environment and microstructure. Small fatigue cracks in the magnesium cycled under vacuum were discovered to propagate along interdendritic regions, along crystallographic planes, and through the dendrite cells. The preference to choose a given path is driven by the presence of microporosity, persistent slip bands, and slip incompatibilities between adjacent dendrite cells. Fatigue cracks formed more rapidly at certain locations in the water vapor environment compared to the vacuum environment, leading to a smaller total number of cracks in the water vapor environment. The majority of small cracks in magnesium cycled in the water vapor environment propagated straight through the dendrite cells, at a faster rate than the cracks in the vacuum. In the water vapor environment, cracks were observed to grow less frequently through interdendritic regions, even in the presence of microporosity, and cracks did not grow via persistent slip bands. The propagation behavior of slightly larger fatigue cracks (a > 6–10 dendrite cells) was found to be Mode I-dominated in both environments.

Consideration of the mechanical behaviour of small fatigue cracks

The mechanical behaviour of small fatigue cracks is investigated for a low, medium and high strength material. At first an elastic consideration is performed which give a good impression how the stress fields change with crack size. In part 2 a full elastic-plastic analysis of short cracks is performed using a new numerical scheme to simulate the growth of shear bands emanating from the crack tip. The influence of material and loading paramters as well as of the crack size on the plastic crack tip opening displacement is discussed. It is also investigated how it is possible to get a conservative estimate of the crack tip deformation at small cracks.

Analysis of Plasticity-induced Crack Closure of Fatigue Crack Interacting with Microstructure.

The propagation behavior of small fatigue cracks was numerically simulated to evaluate the development of crack closure with crack growth. The effect of the interaction between crack-tip slip band and grain boundaries on plasticity-induced crack closure was analyzed under plane stress condition. The crack and slip bands were assumed to be coplanar. When the slip band crosses the grain boundary, the larger resistance of slip motion in the next grain induces a reduction in the crack opening displacement range and an increase in the crack opening stress. The distribution of the residual stretch was geometrically similar to the change of the maximum crack tip opening displacement with crack extension. The relation between the crack opening displacement range and the effective stress intensity range was nearly unique. When the slip band extends over many grains, the irregularity of crack opening displacement range and crack opening stress diminishes. The crack propagation life can be calculated as a function of the crack length and the stress level on the basis of the crack-tip opening displacement range.

The effect of solidification rate on the growth of small fatigue cracks in a cast 319-type aluminum alloy

A study was conducted to investigate the effect of solidification rate on the growth behavior of small fatigue cracks in a 319-type aluminum alloy, a common Al-Si-Cu alloy used in automotive castings. Fatigue specimens were taken from cast material that underwent a hot isostatic pressing (HIP) process in order to eliminate shrinkage pores and to facilitate the observation of surface-initiated cracks by replication. Naturally initiated surface cracks ranging in length from 17 µm to 2 mm were measured using a replication technique. Growth rates of the small cracks were calculated as a function of the elastic stress-intensity-factor range (ΔK). Long-crack growth-rate data (10 mm≤length≤25 mm) were obtained from compact-tension (CT) specimens, and comparison to the small-crack data indicates the existence of a significant small-crack effect in this alloy. The solidification rate is shown to have a significant influence on small-crack growth behavior, with faster solidification rates resulting in slower growth rates at equivalent ΔK levels. A stress-level effect is also observed for both solidification rates, with faster growth rates occurring at higher applied-stress amplitudes at a given ΔK. A crack-growth relation proposed by Nisitani and others is modified to give reasonable correlation of small-crack growth data to different solidification rates and stress levels.

Statistical evolution of small fatigue crack in 1Cr18Ni9Ti weld metal

Statistical evolution of small fatigue crack behavior is analyzed. Three considerations are given. They consist of the total fit, the consistency of statistical parameters with test data, and the practice of commonly used distributions, namely Weibull (two- and three-parameters), normal, lognormal, extreme minimum, extreme maximum, and exponential. The crack growth rate data of 1Cr18Ni9Ti stainless steel-weld metal reveal that fatigue damage is a gradual process involving an initial non-ordered (chaotic) or load history-independent random state to an ordered load history-dependent random state. The seven distributions are examined. Different from existent conclusion, appropriate assumed distribution for characterizing the evolution data is the normal or extreme minimum value model rather than the Weibull two- or three-parameter. The other distributions do not fit the data as they violated two or all three of the mentioned considerations.

On the fatigue behavior of γ-based titanium aluminides: role of small cracks

Gamma-TiAl based alloys have recently received attention for potential elevated temperature applications in gas-turbine engines. However, although expected critical crack sizes for some targeted applications (e.g. gas-turbine engine blades) may be less than ∼500 μm, most fatigue-crack growth studies to date have focused on the behavior of large (on the order of a few millimeters) through-thickness cracks. Since successful implementation of damage-tolerant life-prediction methodologies requires that the fatigue properties be understood for crack sizes representative of those seen in service conditions, the present work is focused on characterizing the initiation and growth behavior of small ( a∼25–300 μm) fatigue cracks in a γ-TiAl based alloy, of composition Ti–47Al–2Nb–2Cr–0.2B (at.%), with both duplex (average grain size of ∼17 μm) and refined lamellar (average colony size of ∼145 μm) microstructures. Results are compared to the behavior of large ( a>5 mm), through-thickness cracks from a previous study. Superior crack initiation resistance is observed in the duplex microstructure, with no cracks nucleating after up to 500 000 cycles at maximum stress levels ( R=0.1) in excess of the monotonic yield stress, σ y. Comparatively, in the lamellar microstructure cracks nucleated readily at applied maximum stresses below the yield stress (85% σ y) after as few as 500 cycles. In terms of crack growth, measurements for small fatigue cracks in the duplex and lamellar microstructures showed that both microstructures have comparable intrinsic fatigue-crack growth resistance in the presence of small flaws. This observation contrasts previous comparisons of large-crack data, where the lamellar structure showed far superior fatigue-crack growth resistance than the duplex structure. Such “small-crack effects” are examined both in terms of similitude (i.e. crack tip shielding) and continuum (i.e. biased microstructural sampling) limitations of traditional linear elastic fracture mechanics.

ＳｉＣｐ／Ａｌ２０２４複合材料の疲労き裂発生および伝ぱ挙動に及ぼす予ひずみの影響

The effect of prestrain on the fatigue crack initiation and propagation behavior of an aluminum alloy (2024-T6) reinforced with 20 volume percent of SiC particles was investigated. Cracks were mainly nucleated in the matrix by prestrain of about 0.32% strain. The X-ray diffraction method was used to measure the change of residual stress in the SiC particles and in the matrix before and after prestrain. The residual stress changed from -134 to -107MPa in the SiC phase and from 0.6 to -12MPa in the matrix phase. Fatigue tests were conducted using a specially designed servo-hydraulic fatigue testing machine in a scanning electron microscope. The initiation and propagation behavior of small fatigue cracks in a smooth specimen was observed. Fatigue cracks were mainly nucleated from the matrix cracking and propagated in the matrix near the particles. When compared at the same maximum stress intensity factor, Kmax, the crack propagation rate, dc/dN, of the prestrained material was higher than that of the non-prestrained material because of the damage near the interface.

Small Fatigue Crack Initiation and Growth of SiC Whisker Reinforced Aluminum Composite.

Reversed plane bending fatigue tests were conducted on SiC whisker reinforced aluminum composite specimens with shallow surface notch. Initation and growth behavior of small surface fatigue cracks were investigated by means of a plastic replica technique. Fatigue strength of composite was higher than that of unreinforced alloy because SiC whiskers restricted cyclic slip deformation of matrix. which resulted in the delay of fatigue crack initiation. the initiation of small fatigue crack in composite was found characterized by the interface cracking between matrix and SiC whiskers, where the lump of SiC whiskers was composed in a manufacturing process. The many small cracks initiated as the lumps of SiC whiskers distributed around specimen surface irrespective of the stress revel, while several cracks grew simultaneously in the tests with high stress amplitude and one main crack propagated in those with low stress one. Microscopically, the coalescence of main crack and micro cracks was observed frequently. The difference of small fatigue crack growth rate between composite and unreinforced alloy was small, which implied SiC whisker was not resistant to crack growth but to crack initiation. The crack growth rate of small cracks was found faster than that of long cracks.

Bone maintenance and remodeling: a control system based on fatigue damage.

This paper explores the relationship between damage, repair, and remodeling in compact bone. A model of microcrack growth is developed that takes account of recent findings on the behaviour of small fatigue cracks in other materials. This is combined with a simple model of a repair process, envisaged as a constant rate of decrease in crack length. The system that results is capable of achieving a stable and precise control of crack length without the need to measure it. This is very useful because it implies that bone does not require the complexities of crack-measuring transducers or active decision-making processes. A simple explanation is suggested for the presence of a "lazy zone" of remodeling equilibrium strains, and the limits of this zone are quantified. The model is developed through a necessarily simplified geometry and loading scheme but can be extended to provide a general solution applicable to in vivo conditions.