Crack Growth In Air Research Articles

Experimental investigation on corrosion fatigue crack initiation and growth of heat-treated U75V rail steel

Understanding the corrosion fatigue mechanism of rail materials is of great importance to guide the assessment of the structural integrity of modern railway components. To this purpose, the corrosion fatigue crack (CFC) initiation and growth behaviors of heat-treated U75V rail steel are investigated under 3.5 wt% NaCl solution in terms of high-cycle fatigue, fatigue crack growth (FCG) rate, together with surface morphology and microstructure. Experimental results reveal that the U75V rail steel is susceptible to corrosion fatigue failure. Examinations on fractured samples clearly show that the electric potential difference between ferrite and cementite triggers the preferential anodic dissolution of ferrite, leading to corrosion fatigue crack initiation. The crack initiation is not only solely determined by the size of the corrosion pit, but also strongly influenced by its local shape. In comparison with the fatigue crack growth in air, the corrosion FCG tends to grow in a relatively straight path, because of hydrogen embrittlement. Additionally, the CFC behaviors indicate a tendency to transverse through soft-oriented grains and deflect upon encountering hard-oriented grains.

Fatigue crack growth resistance of the austenitic stainless steel Alloy 709 at elevated temperatures

Fatigue crack growth resistance of an austenitic stainless steel Alloy 709 has been evaluated at temperatures of 550, 650 and 750 °C in air and vacuum. Tests were conducted at a frequency of 0.25 Hz and a stress ratio of 0.1. The linear elastic stress intensity factor range (ΔK) has been used to characterise fatigue crack growth resistance. A modest detrimental effect of air at elevated temperatures on fatigue crack growth is identified and discussed. Striated transgranular fatigue is found to be the failure mechanism for all test conditions. The formation of striations and the interaction of crack growth with slip traces are further investigated using in-situ testing (within a scanning electron microscope), together with transmission electron microscopy carried out on samples extracted by focused ion-beam milling perpendicular to fracture surfaces. Finally, an analytical model is proposed to predict fatigue crack growth in air for Alloy 709.

大理石におけるサブクリティカルき裂進展の周辺環境依存性

It is important to investigate time-dependent deformation and fracturing in rock for the purpose of ensuring the long-term stability of structures in a rock mass. Subcritical crack growth is one of the main causes of time-dependent fracturing in rock. It is known that subcritical crack growth is influenced by not only stress but also surrounding environment. Studies of subcritical crack growth have been widely conducted for silicate rocks such as igneous rocks and sandstones. On the other hand, information of subcritical crack growth in carbonate rocks is not enough. Especially, influence of surrounding environmental conditions on subcritical crack growth in carbonate rocks has not been clarified yet. In this study, we investigated subcritical crack growth in carbonate rocks. Specifically, we investigated the influence of temperature and relative humidity on subcritical crack growth in air and in water. Macedonian marble was used as a rock sample. To measure subcritical crack growth, we used the load relaxation method of the double-torsion test. It was shown that the crack velocity increased when the temperature was higher at a constant relative humidity in air and in water. In addition, the crack velocity increased with increasing the relative humidity at a constant temperature. It is concluded that the temperature and relative humidity influence subcritical crack growth in carbonate rock in air and in water. From the results obtained in this study, it can be suggested that the condition with low temperature and low humidity is desirable for the long-term integrity of a carbonate rock.

Characterisation of short fatigue cracks in titanium alloy IMI 834 using X-ray microtomography

A first attempt at the three-dimensional evaluation of naturally initiated surface connected and internal fatigue cracks is presented. Fatigue crack initiation and growth in air and vacuum environments have been investigated through X-ray microtomography in air and vacuum environments at elevated temperatures (350°C), accompanied by post-mortem electron microscopy of the fracture surfaces. In vacuum (<10−5mbar), multiple internal and surface-connected crack initiation was observed, but only the surface-connected cracks grew. In contrast, fewer cracks formed in air, these were mostly surface-connected and all were observed to grow. In all instances the initiation features were associated with globular primary α. An improved fatigue life was found in vacuum, which was mostly a consequence of delayed initiation, but was also due to slower fatigue crack propagation. The non-propagation of internal cracks was taken to imply that even the good laboratory vacuum obtained here was insufficient to simulate the conditions obtained for an internal crack in a component. The crack shape evolved towards a semi-circular shape a/c=1 in air during fatigue crack growth, whilst the vacuum cracks remained semi-elliptical (a/c≃1.4). This was taken to imply that oxide-induced crack closure played a role in fatigue crack growth in air.

Contact fatigue of veneer feldspathic porcelain on dental zirconia

ObjectivesTo understand the resistance to cyclic and static contact loading of feldspathic porcelain on dental zirconia (3Y-TZP), in order to understand the partial failure of porcelain (chipping or cracking). MethodsHertzian contact techniques were used to evaluate the appearance of damage as a ring crack in terms of applied load and number of cycles in air and simulated saliva. ResultsStatic contact loading showed the presence of stress corrosion cracking in the porcelain; the environmental crack growth in air was determined from the results of time to damage under static load. There was also a contribution of fatigue effects due to the interactions of the crack with the microstructure. From the obtained results, a time to failure was estimated depending on the material counterpart. Cracking can occur on porcelain coatings if the contact counterpart is teeth or porcelain in a time interval of a few years, consistent with clinical studies. SignificanceContact loading, particularly against teeth or other ceramic materials can be a significant cause of failure and chipping of feldspathic porcelain on zirconia, especially if the patient suffers from bruxism. Protection, by e.g. a guard, against repetitive contact against the porcelain can increase the lifetime of the veneer.

Underload‐induced crack growth behaviour of minor cycles of pipeline steel in near‐neutral pH environment

AbstractDespite decades of research, the mechanisms of near‐neutral pH stress corrosion cracking of pipeline steel are still not fully understood. This investigation was aimed to understand the effect of minor cycles with very high R ratios (minimum stress/maximum stress) on crack growth in air and in near‐neutral pH environments. It has been demonstrated that the minor cycle, even with an R ratio as high as 0.9, could significantly contribute to the crack growth of pipeline steels in the presence of a large underload cycle with a low R ratio of 0.5 in near‐neutral pH environment. Comparing with constant amplitude tests, an increase of crack growth rate by a factor of 3 and 5 was observed when some non‐propagating minor cycles were combined with an underload cycle for tests in air and in near‐neutral pH solution, respectively. Based on the test results, the crack growth mechanisms during minor cycle loading in near‐neutral pH environments were discussed and practical strategies aimed to minimise crack growth during pipeline operation were also proposed.

Vacuum crack growth behavior of austenitic stainless steel under fatigue loading

The fatigue crack growth behavior of an austenitic stainless steel in a vacuum environment has been studied. Fatigue crack growth tests were performed with the compact tension specimens in laboratory air and vacuum, and the environmental effect on the crack growth behavior was examined. The crack growth rate data were expressed in terms of the J-integral range during fatigue loading, while an elastic–plastic finite element analysis was employed to calculate the J-integral range. The fractographic examinations were also carried out to assess the crack growth mechanisms and correlate with fatigue characteristics. The results show an accelerated fatigue crack growth in air compared to that in vacuum and this environmental effect depends on the load ratio.

Hydrogen concentration behavior around a crack tip under the constant wind rate condition simulated to a moving crack with constant rate

Clarification of the hydrogen diffusion behavior in the metal is important for prevention of hydrogen embrittlement fracture. Previously, some numerical analysis has been performed about hydrogen diffusion behavior under local stress fields by using Finite Element Method or finite difference method. On the other hand, the fatigue crack growth in air, that is, in inert atmosphere, has original speed. However, there is few analysis of hydrogen diffusion behavior including original fatigue crack growth speed. In this research, the effect of crack growth rate on hydrogen diffusion behavior around a crack tip under fatigue condition was clarified based on the numerical analysis and these analyses were conducted under various yield stress. The crack growth speed was modeled by giving uniform flow velocity to the hydrogen distribution in the whole analytical area. The upwind difference method was used for uniform flow velocity. The analytic results were compared between upwind difference and central difference.

Thermal activation of fatigue crack growth: Analysing the mechanisms of fatigue crack propagation in superalloys

A method for deriving apparent activation energies, E app, for fatigue crack growth (FCG) is presented. It is shown how these E app values can be used to analyse the main underlying mechanism causing the progression of the crack. The E app values for fatigue crack growth in air and vacuum, and with and without a dwell are obtained for a range of Ni-based superalloys for turbine disc applications. The apparent activation energies range from about 0 to about 300 kJ/mol. The analysis allows identification of alloys that are prone to FCG through diffusion-controlled mechanisms, notably grain boundary oxidation or grain boundary creep, as well as alloys that are nearly immune to FCG by grain boundary oxidation or creep.

A fatigue-to-creep correlation in air for application to environmental stress cracking of polyethylene

The present study was undertaken to determine whether the correlation between fatigue and creep established for polyethylene in air could be extended to environmental liquids. Fatigue and creep tests under various conditions of stress, R-ratio (defined as the ratio of minimum to maximum load in the fatigue loading cycle), and frequency were performed in air and in Igepal solutions. The load–displacement curves indicated that stepwise fatigue crack growth in air was preserved in Igepal solutions at 50 °C, the temperature specified for the ASTM standard. In air, systematically decreasing the dynamic component of fatigue loading by increasing the R-ratio to R = 1 (creep) steadily increased the lifetime. In contrast, the lifetime in Igepal was affected to a much smaller extent. The fatigue to creep correlation in air was previously established primarily for tests at 21 °C. Before testing the correlation in Igepal, it was necessary to establish the correlation in air at 50 °C. Microscopic methods were used to verify stepwise crack growth by the sequential formation and breakdown of a craze zone, and to confirm the fatigue to creep correlation. The crack growth rate under various loading conditions was related to the maximum stress and R-ratio by a power law relationship. Alternatively, a strain rate approach, which considered a creep contribution and a fatigue acceleration factor that depended only on strain rate, reliably correlated fatigue and creep in air at 50 °C under most loading conditions of stress, R-ratio and frequency. The exceptions were fatigue loading under conditions of R = 0.1 and frequency less than 1 Hz. It was speculated that compression and bending of highly extended craze fibrils were responsible for unexpectedly high crack speeds.

Fatigue as a process of cyclic brittle microfracture

ABSTRACTWhile fatigue crack growth in vacuum may occur by slip alone, environmental fatigue including crack growth in air is strongly influenced by crack‐tip surface chemistry that adversely affects ductility. Cumulative diffusion, combined with adsorption and chemisorption in the loading half‐cycle may promote instantaneous crack extension by brittle microfracture (BMF). Unlike slip, the BMF component will be sensitive to parameters that affect near‐tip stresses, such as load history and constraint. BMF dominates near‐threshold environmental fatigue. Being a surface phenomenon, it loses its significance with increasing growth rate, as slip‐driven crack extension gains momentum and growth becomes less sensitive to environment. The BMF model provides for the first time, a scientific rationale for the residual stress effect as well as the related connection between stress–strain hysteresis and load‐sequence sensitivity of metal fatigue including notch response. Experimental evidence obtained on a variety of materials under different loading conditions in air and vacuum appears to support the proposed model and its implications.

Creep–fatigue crack growth of intermetallic compound Ni3 Al(B) at elevated temperatures

The creep–fatigue crack growth of Ni3 Al(B) alloy was investigated at elevated temperatures in air under four different loading waveforms. Two types of time dependent damage mechanisms have been identified: oxidation and creep effects. As compared with fatigue crack growth in air at room temperature, the effect of oxidation at the crack tip on the crack growth acceleration is significant. Creep effects, on the other hand, are dominant for tensile holding and slow–fast loading waveforms. The complicated interaction between creep–fatigue, oxidation induced embrittlement, and oxide induced crack closure determined the different fatigue crack growth behaviours for different loading waveforms at elevated temperature. The relationship between the constants C and m in the Paris formula and loading waveform were examined, and the influence of loading waveform on the crack propagation were also discussed.

Fatigue crack propagation in Ti3 Al based alloys

Effects of microstructure, stress ratio, and environment on the fatigue crack growth resistance of Ti–23Al–9Nb–2Mo–1Zr–1·2Si and Ti–23Al–11Nb–0·9Si (at.-%) Ti3 Al based alloys have been studied at room and elevated temperatures. Only modest effects of microstructure on fatigue crack growth resistance have been obtained at room temperature, and these tend to reduce further at the elevated temperatures of 600 and 700°C both in air and in vacuum. At room temperature the fatigue crack growth resistance of Ti3 Al based alloys is controlled primarily by the thickness of the retained βphase rather than by its volume fraction and the microstructure with a larger average thickness of retained β laths shows improved fatigue crack growth resistance. However, in some microstructures, the spatial distribution of the β phase can also be deduced to be important. A marked difference on crack growth resistance is obtained for stress ratios of 0·1 and 0·5 both at room temperature and at a temperature of 600°C. The mechanisms of fatigue crack growth in air and vacuum are discussed.

INCONEL® Alloy 783: An Oxidation-Resistant, Low Expansion Superalloy for Gas Turbine Applications

INCONEL® alloy 783 is an oxidation-resistant low coefficient of thermal expansion (low CTE) superalloy developed for gas turbine applications. Turbine efficiency can be increased through the use of low-CTE shrouds and case components that maintain tight blade tip clearances at different turbine operating temperatures. To achieve low CTE, alloys based on Ni–Fe–Co compositions require Cr content be maintained at low levels. Added Cr lowers the Curie temperature and thereby increases thermal expansion rate over a wider temperature range. The necessary lack of Cr minimizes resistance to both general oxidation and stress-accelerated grain boundary oxygen enhanced cracking (SAGBO). Increased amounts of Al in alloys strengthened by γ′ alone also promotes SAGBO. Alloy 783 is the culmination in the development of an alloy system with very high aluminum content that, in addition to forming γ′, causes β aluminide phase precipitation in the austenitic matrix. It was discovered that this type of structure can be processed to resist both SAGBO and general oxidation, while providing low thermal expansion and useful mechanical properties up to 700°C. The high Al content also reduces density to 5 percent below that of superalloys such as INCONEL alloy 718. Key aspects of the alloy development are presented, including the assessment of SAGBO resistance by evaluating elevated temperature crack growth in air. The alloy, now commercially available, has been successfully fabricated and welded into gas turbine engine components.

The effect of temperature on fatigue crack growth in liquid metal environments

Previous studies of fatigue crack growth in liquid metal environments have shown that at Δ K > Δ K th crack growth rates are approximately one order of magnitude faster than those observed in air. At low Δ K values (low fatigue crack growth rates) and/or where the crack tip is exposed to the liquid metal for long periods of time, crack tip blunting by dissolution occurs. This decreases the rate of crack growth at low Δ K values and results in significantly higher apparent Δ K th values. In this paper the effect of temperature on fatigue crack growth in air and liquid metal environments is investigated using standard LEFM testing procedures. It is found that as the temperature increases the apparent Δ K th value increases significantly. These results are explained in terms of the crack tip blunting hypothesis proposed previously.

NEAR-THRESHOLD PROPAGATION OF DELAMINATION FATIGUE CRACKS IN UNIDIRECTIONAL CF/PEEK LAMINATES IN AIR AND IN WATER

The effects of stress ratio and a water environment on the near-threshold growth of delamination fatigue cracks were investigated with unidirectional laminates made from ICI APC-2 thermoplastic prepregs (AS4/PEEK). Tests were carried out under mode I opening loading by using double cantilever beam specimens. The fatigue crack growth rate near the threshold region was mainly controlled by the stress intensity range rather than the maximum energy release rate and the energy release rate range. The fatigue crack growth resistance of AS4/PEEK laminates was much higher than that of conventional CF/epoxy laminates even near the threshold region. However, the increase in fatigue resistance was smaller than that in fracture toughness. Tests in water did not show large detrimental effects. The crack propagation rate in water was expressed as two power functions of the energy release rate range. The exponent of the power function in the low propagation rate region was smaller than that in the high propagation rate region, and the growth threshold was not observed. The mechanism of the existence of the two power functions was discussed by conducting tests in water at lower frequency of stress cycling. Mesoscopic mechanism of delamination fatigue crack growth in air and in water was discussed on the basis of the fractographic observation.

SHEAR LIPS ON FATIGUE FRACTURES OF ALUMINIUM ALLOY SHEETS SUBJECTED TO BIAXIAL CYCLIC LOADS AT VARIOUS R‐RATIOS

AbstractAn analysis is made of shear lip measurements as observed on fatigue crack surfaces of an aluminium alloy sheet material. It is shown that, for biaxial cyclic loads at stress ratios 1 and various R‐ratios, shear lips were controlled by an equivalent intensity factor Ke=K1F(λ,R).For crack growth in air the shear lip width ts was approximately proportional to K2e at ts &lt; t*s= 2.5 × 10−4 m and K4E, at ts &gt; t*s where t*s is the critical value of the shear lip dimension. The initiation of shear lips and the orientation of crack growth for different parameters of the loading cycle are briefly considered in the discussion.

FATIGUE CRACK GROWTH IN NICKEL‐BASED SUPERALLOYS AT 500‐700°C. I: WASPALOY

Abstract— The effects of cyclic frequency, hold time, and stress‐intensity‐factor range (δK) on rates of fatigue crack growth in air at 500‐700°C have been studied for Waspaloy—a nickel‐based superalloy used for gas‐turbine engine discs. The main effects observed were: (i) higher rates of crack growth for lower cyclic frequencies at high δK at 600 and 700°C. and (ii) lower rates of crack growth at low δK (and higher δK thresholds) for longer hold times at 700°C, compared with those at a baseline frequency of 2 Hz. Metallographic and fractographic observations suggested that the effects of cyclic frequency and hold time could be rationalised in terms of the competing effects of enhancement of cracking due to creep and inhibition of cracking caused by oxide‐induced crack closure, fracture‐surface‐roughness induced crack closure, and crack‐branching/deflection. Possible mechanisms for promoting intergranular and transgranular cracking at low cyclic frequencies or long hold times are discussed.

FATIGUE CRACK GROWTH IN NICKEL‐BASED SUPERALLOYS AT 500‐700°C. II: DIRECT‐AGED ALLOY 718

Abstract— The effects of cyclic frequency, hold time, and stress‐intensity factor range (ΔK) on rates of fatigue crack growth in air at 500°C and 700°C have been studied for the direct‐aged version of the nickel‐based superalloy 718. The main effects were similar to those observed for Waspaloy (Part I), namely: (i) small effects of cyclic frequency and hold time at 500°C, (ii) higher rates of crack growth at lower cyclic frequencies at high ΔK at 700°C, and (iii) lower rates of crack growth at low ΔK (and higher ΔK thresholds) for longer hold times at 700°C. For DA 718, there was no increase in crack growth rates with increasing hold times (0–60 s) at high ΔK at 700°C (unlike the large increases reported for standard processed Alloy 718). Metallographic and fractographic observations showed that crack growth was predominantly transgranular at 500°C, and predominantly intergranular at 700°C except at 2 Hz at high ΔK. The precise intergranular crack path, extent of branching, and fracture‐surface appearance depended on ΔK, wave‐form and cycle‐period. The mechanisms responsible for the observed effects, and possible explanations for the different behaviour of DA 718 and standard processed Alloy 718, are discussed.

Effects of Stress Ratio and Oxygen Concentration on Short Surface Crack Growth of High-Strength Aluminum Alloy in Corrosion Fatigue.

The fatigue crack growth behavior of a high-strength aluminum alloy, 7075-T651, was examined in deaerated and aerated 3.5% NaCl solution, in addition to in air. The relation between stress intensity range and crack growth rate in the same environment depended on stress ratio. For long cracks, the crack growth rate was always higher at higher stress ratio, while it was sometimes lower at higher stress ratio for short surface cracks. The effect of oxygen concentration was not observed for either long cracks or short surface cracks. The growth rate of long cracks in corrosion fatigue was almost the same as that in air when the stress intensity range was smaller than 7 MPam1/2. In this region, however, the growth rate of short surface cracks in the solution was higher than that in air, and the growth rate of short surface cracks was higher than that of long cracks in the same environment. Although the difference in crack closure was considered to be responsible for the acceleration of short surface crack growth in air, the acceleration of short surface crack growth rate in corrosion fatigue could not be explained solely by the difference in crack closure. The difference in chemical environment within the crack should be considered.