Crack Growth In Air Research Articles

Short Surface Crack Growth of High-Strength Aluminum Alloy in Corrosion Fatigue.

The short surface fatigue crack growth behavior of a high-strength aluminum alloy, 7075-T651, was examined in deaerated aqueous 3.5% NaCl. The growth rate of long cracks in corrosion fatigue was almost the same as the rate in air when the stress intensity range was smaller than 7 MPa√(m). In this region, the growth rate of short surface cracks was higher than that of long cracks in the same environment. Since the da/dN-ΔK relation of short surface cracks in air was located between the da/dN-ΔK relation and da/dN-ΔKeff relation of long cracks, the difference in crack closure may be responsible for the acceleration of short crack growth in air. Since the da/dN-ΔK relation of short surface cracks in corrosion fatigue was located left of the da/dN-ΔKeff relation of long cracks, and the value of ΔK is always greater than ΔKeff, the acceleration of short surface crack growth in corrosion fatigue cannot be explained solely by the difference in crack closure. The observations of fracture surface by scanning electron microscopy showed that the difference in the crack growth mode was also responsible for the acceleration.

FATIGUE CRACK PROPAGATION OF SINTERED SILICON NITRIDE IN VACUUM AND AIR

Abstract— Static and cyclic fatigue in vacuum and air have been investigated for pressureless sintered silicon nitride. Cyclic crack propagation occurred even in vacuum where environmental effects are negligible. Hence such damage is attributed to pure mechanical fatigue unaffected by environment. Crack growth occurs in air even below KISCC and the growth rate is higher than in vacuum. Therefore, it is believed that cyclic crack growth in air is accelerated by stress corrosion cracking for a wide range of Kmax values.

Environmentally assisted crack growth in a low alloy boiler steel in high temperature water containing oxygen

The crack growth behaviour of the higher strength steel 17 MnMoV 6 4 in deionized high temperature water containing oxygen was investigated with respect to static loading. The tests were performed in an autoclave testing facility at an oxygen content of 8 ppm, a pressure of 70 bar and a temperature of 240°C under quasi-stagnant flow conditions. The stress intensities at the beginning of the tests were 17, 20, 27, 34, 40 and 58 MPa√m. In contrast to the higher loaded specimens no evidence of environmentally assisted cracking was found at stress intensities of 17 and 20 MPa√m. The maximum crack growth rate in the range where only environmetally assisted cracking occured amounted to about 4 x 10 -5 mm/s. The critical stress intensity K IJ for the onset of stable ductile crack growth in air is 100 MPa√m. On the basis of fractographic studies the crack development found can be ascribed to the “Strain Induced Corrosion Cracking (SICC)” mechanism. This mechanism, used as a working hypothesis, gives a satisfactory explanation for the occurence of stress corrosion cracking of unalloyed and low alloyed steels in high temperature water. SICC is particularly characterized by aggravated corrosive attack occuring as soon as the magnetite/haematite protective layer has been locally disturbed. The stress concentration then just becomes so great that in the region of the resulting crack tips, yield/creep deformation within the critical range of strain rates occurs.

On the influence of the ambient environment on the fatigue crack growth process in steels

A comparison of the crack opening and fatigue crack growth characteristics of three tempered martensitic steels; a modified 4135, 2 1/4Cr-1Mo and a modified 9Cr-1Mo; in ambient air and in vacuum (3 × 10 −5 torr) has been carried out. In vacuum, roughness-induced closure is responsible for closure in the near-threshold region and the level of closure is independent of ΔK. The presence of oxygen in the ambient environment (50% relative humidity) can increase the overall closure level by as much as 50% in the case of 4135 and 2 1/4Cr-1Mo steels, but does not contribute significantly to closure in the case of the 9Cr-1Mo steel. Roughness-induced closure increases with increase in tempering temperature, but the extent of the environmental effect on closure in a given alloy is much less dependent on the tempering temperature. Rewelding is not responsible for the high threshold levels found in vacuum. Oxide film rupture as well as hydrogen embrittlement increase the rate of crack growth in air over that observed in vacuum.

The influence of test frequency on fatigue crack growth in air, and crack surface oxide formation

The oxide layers often observed on fatigue crack surfaces raise a number of problems and data concerning the kinetics of their formation and their influence on fatigue crack growth rate (FCGR) is scarce. The research work conducted in this field shows that a close relationship exists between the fatigue crack growth rate and the thickness of the oxide layer formed on the crack surfaces. The present study clearly shows the influence of testing conditions, especially the load cycle frequency, on oxide formation and FCGR.

鋭敏化した304ステンレス鋼の腐食疲労過程におけるAE発生におよぼす環境，力学，材料条件の破面解析による検討

Corrosion fatigue (CF) tests were conducted for sensitized 304 austenitic stainless steel to investigate effects of environments, mechanical conditions, and sensitization on crack growth and AE (Acoustic Emission) behavior. The same experimental method as in the previous work conducted for solution treated specimens was employed. The crack growth rates of sensitized specimens were found to be accelerated in air as compared with those in the solution treated case. Those obtained in transpassive region (+1.1 V) and at corrosion potential (Ecorr) for sensitized specimens were more accelerated, due to superposition of the acceleration by sensitization and environmental effects such as intergranular corrosion, metal dissolution, or hydrogen evolved from cathodic reaction. The AE activity during crack growth of sensitized specimens was much higher than that of solution treated specimens through all the ΔK values in this study. This indicates that AE techniques are very useful to monitor crack growth processes under sensitized condition. SEM observations of the sensitized fracture surfaces revealed that many microcracks were nucleated at grain boundaries perpendicularly to the main crack surfaces at higher ΔK than ∼34 MPa·m1⁄2. These microcracks (separations) were concluded to be the main AE source during the crack growth in air under sensitized condition, since the numbers of the separations were proportional to the detected AE event counts. In addition to separations, intergranular-like cracks were observed on the sensitized fracture surfaces tested in transpassive region. The high AE activity (event count rate) at relatively low ΔK level was ascribed to the nucleation of these intergranular-like cracks which arose from intergranular corrosion and triaxial stress in the crack tip plastic zone. Intergranular-like cracks due to hydrogen and triaxial stress as well as the separations were observed on the sensitized fracture surfaces tested at Ecorr. The AE sources which provided the high AE activity during CF crack growth under sensitized condition were shown to be such microcracking processes as separations and intergranular-like cracks caused by the co-operation of environments (intergranular corrosion or hydrogen), triaxial stress (mechanical conditions), and sensitization.

Creep crack growth behaviour in air and sodium for an unstabilized austenitic stainless steel and assessment of evaluation concepts

The austenitic stainless steels used for permanent structures in the heat transfer systems of liquid metal fast breeder reactors (LMFBR's) are subjected to flowing sodium at elevated temperatures. During this exposure to sodium environment the structural materials can undergo compositional and microstructural changes which could alter the mechanical behaviour as compared to air. An investigation of creep crack growth in air and flowing liquid sodium was performed on the austenitic stainless steel X6 CrNi 18 11 (similar to AISI 304 ss) under constant load at 823 K using compact tension specimens. The sodium conditions are characterized by an upstream position in a non-isothermal loop with a temperature gradient of 170 K. The experimental data are evaluated correlating the crack growth rate with the net section stress σ net , the stress intensity factor K Q , and the energy rate integral C ∗ , respectively.

Shear lips on fatigue fractures in aluminium alloy sheet material

An analysis is made of shear lip width measurements and the transition of tensile mode fatigue cracks to shear mode fatigue cracks, as observed on fatigue crack surfaces of aluminium alloy sheet material. It could be shown that these phenomena were controlled by ΔK eff , rather than K max or ΔK. For crack growth in air the shear lip width was approximately proportional to (ΔK eff) 2 , but it was significantly larger than the estimated size of the reversed plastic zone. The initiation of shear lips, the transition from plane stress to plane strain along the crack front and the environmental effect on shear lips are briefly considered in the discussion.

Effect of environment on crack growth behavior in austenitic stainless steels under creep and fatigue conditions

Crack growth behavior at high temperatures under cyclic, static, and combined loads was studied in annealed and 20 pct cold-worked Type 316 and 20 pct cold-worked Type 304 austenitic stainless steels in air and vacuum. Under cyclic load, crack growth rates in annealed Type 316 steel are slightly lower in vacuum than in air, but this difference decreases with increase in crack growth rate. Most importantly, the effect of temperature on crack growth is present even in vacuum and arises mostly from the variation of elastic modulus with temperature. In the cold-worked Type 316 steel, the pronounced hold-time effects on fatigue crack growth in air reported in the literature persists even in vacuum. This implies that at high crack growth rates these hold-time effects arise mostly from creep-fatigue interaction rather than environment fatigue interaction. Environment has a negligible effect also on crack growth under static load. Thus, time dependent crack growth in these steels is due to creep processes. Crack growth behavior in annealed and cold-worked materials are compared and reasons for the enhanced time dependent crack growth in cold-worked material are discussed in detail.

Étude du phénomène de rupture différée dans l'air de l'alliage TA6V

We studied the influence of the hydrogen content of the alloy Ti6A14V on slow crack growth in air and on the fracture toughness. Three hydrogen concentrations were used: 10, 45 and 105 ppm. Tests were performed at room temperature on single-edge-notched precracked specimens. The results showed that the fracture toughness decreased as the hydrogen content increased whereas the sustained-load cracking was improved. The alloy containing 105 ppm hydrogen was immune to slow crack growth in air. Metallographic examinations enabled us to establish that sustained-load cracking is due to the cleavage of the α phase along a plane inclined at about 14° to the basal plane.

AN INVESTIGATION OF THE CREEP‐FATIGUE‐ENVIRONMENT INTERACTION IN A Ni‐BASE SUPERALLOY

Abstract— This work was conducted to study the significant increases in fatigue crack growth rates at low frequencies commonly observed at high temperature. Creep tests and fatigue tests at 0·01, 0·1 and 1·0 Hz were conducted at 650°C in air and in helium on samples of INCONEL alloy 718 in two different heat treatments. Creep crack growth rates were 50–100 times greater in air than in helium. Fatigue tests in helium showed little sensitivity to frequency, but tests in air showed considerable increases in crack growth rates at lower frequencies. The results indicate the air environment played a predominant role in both the creep and the fatigue tests. Oxygen diffusion into grain boundaries appeared responsible for the accelerated crack growth in air. An overaging heat treatment reduced the crack growth rates.

Review of Oxidation Effects on Cyclic Life at Elevated Temperature

Abstract The literature reporting on comparative studies of fatigue life and fatigue crack growth rates in oxidizing environment and vacuum has been reviewed. The emphasis has been on the properties of steel, nickel and super alloys at elevated temperature. A number of effects emerge: (i) The crack growth in air is an order of magnitude faster than in vacuum; (ii) There is a stepwise or gradual transition from vacuum behaviour to air behaviour between approx. 10−6 and 1 torr; (iii) The apparent activation energy for the oxidation effect on crack growth rate is small, less than 10 kcal mole−1; (iv) Oxidation promotes intergranular cracking but also accelerates transgranular cracking; (v) The frequency dependent is stronger in oxidation environment than in vacuum, but is also present in vacuum; (vi) The oxidation effect disappears at high strain amplitudes NF < 102...

Fatigue crack growth rates and thresholds in steels under oxidising conditions

The effect of R (minimum load/maximum load) on fatigue crack growth in tests on CrMoV steels at 550°C was investigated under oxidising conditions. Most results were described adequately by linear elastic fracture mechanics using just the tensile loading to calculate stress intensity amplitude ΔK. Crack growth rates in air and under vacuum were decreased by loading at R = 0 compared with tests at R = −1 or R > 0 especially near the threshold ΔK 0 · R = −1 tests produced upper bound crack growth rates for any conditions. Crack growth in air was faster than under vacuum except at low amplitudes, so that for tests in air at 10 −2 Hz and R = 0 the threshold amplitude was higher than under vacuum. Higher frequencies ( e.g. 1 Hz), an inert environment and compressive or raised mean tensile stresses all lowered ΔK 0. These effects are interpreted in terms of the cyclic crack tip displacement being reduced by shear lips at the crack surface, and oxide filling. Limited creep ratcheting per cycle in these creep-ductile steels was found not to increase crack growth rates.

Effect of structural state on the fatigue fracture of magnesium alloy MA12 in air and in a vacuum. Report No. 2. Microscopic and macroscopic features of fatigue crack growth in air

Effect of structural state on the fatigue fracture of magnesium alloy MA12 in air and in a vacuum. Report No. 2. Microscopic and macroscopic features of fatigue crack growth in air

Effects of hydrogen in titanium alloys on subcritical crack growth under sustained load

The susceptibility of three titanium alloys to subcritical crack growth in air under sustained load was determined as a function of hydrogen content. The alloys examined contained from 7 to 76 p.p.m. and included Ti6Al4V, Ti8Al1Mo1V and Ti4Al3Mo1V. Tests were conducted with 0.25-inch-thick, fatigue-precracked compact tension specimens. Crack-growth behavior was quite sensitive to hydrogen content; time to failure increased as hydrogen content was decreased, while the threshold stress intensity — defined as the highest initial stress intensity leading to failure within 1 month — appeared to attain a minimum value at an intermediate hydrogen content. Hydrogen content also was shown to have a significant effect on fracture toughness; dramatic improvements in toughness accompanied reductions in hydrogen content to low levels. The effects of hydrogen on toughness were independent of loading rates varying between 60 and 4600 k.s.i.-inch 1 2 per minute.

Internal fatigue origins in two alpha-beta titanium alloys

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 (<10−5 mbar), 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.

Subcritical crack growth in two titanium alloys

Measurement of subcritical crack growth during static loading of precracked titanium alloys in salt water using samples too thin for plane strain loading to predominate was examined as a method for determiningKIscc, the critical stress intensity for crack propagation in salt water. Significant internal crack growth followed by arrest was found at quite low stress intensities, but crack growth rates were relatively low. Assuming these techniques provided a reliable measurement ofKIscc, the value for annealed Ti-4Al-1.5Mo-0.5V alloy was apparently about 35 ksi-in.1/2, while that for annealed Ti-4Al-3Mo-lV was below 45 ksi-in.1/2. Crack growth was also observed in tests conducted in both alloys in an air environment. At 65 ksi-in.1/2, the extent of crack growth was greater in air than in salt water. Ti-4Al-3Mo-lV showed arrested crack growth in air at a stress intensity of 45 ksi-in.1/2.

Delayed Crack Growth in Polycarbonate

The delayed crack growth rate dc/dt is generally expressed as a function of stress intensity factor K as follows:dc/dt=a1Kmwhere a1 and m are constants that depend on several external factors. From the nucleation theory, Yokobori suggested that these constants were functions of temperature, surface energy, etc. However, there have been few experimental studies about the dependence of these constants upon external factors, although such a study is important to clarify the delayed crack growth mechanism.The present experiment was carried out to investigate the effect of environment on a1 and m by using a high polymeric solid, namely polycarbonate. It is hoped that the results are applicable to the study of metals because of the similarity in delayed crack growth rate between high polymers and metals. The main results obtained are summarized as follows:(1) There are two types of delayed crack growth; one is characteristic of the crack growth in air and another is that in the environment.(2) The environmental delayed crack growth process consists of three stages.(3) At the stage of stable crack growth, the growth rate is expressed as the power function of stress intensity factor. The power varies depending upon the kind of environment.

Some Chemical Effects in Fatigue Cracking of Vulcanized Rubbers

Abstract An apparatus is described for studying the fatigue of rubber strips under repeated simple extensions in different atmospheres. The fatigue life is shown to be greatly increased in vacuo for vulcanizates of natural rubber and SBR. The effect of oxidation inhibitors and radical acceptors has been examined; they are found to prolong the life in air, but have little effect in vacuo. The results suggest that a substantial proportion of crack growth in air is due to oxidative deterioration of the rubber at the crack tip, probably initiated by mechanical rupture.

Some chemical effects in fatigue cracking of vulcanized rubbers

AbstractAn apparatus is described for studying the fatigue of rubber strips under repeated simple extensions in different atmospheres. The fatigue life is shown to be greatly increased in vacuo for vulcanizates of natural rubber and SBR. The effect of oxidation inhibitors and radical acceptors has been examined; they are found to prolong the life in air, but have little effect in vacuo. The results suggest that a substantial proportion of crack growth in air is due to oxidative deterioration of the rubber at the crack tip, probably initiated by mechanical rupture.