Increase In Crack Growth Rate Research Articles

Reliability-based assessment of deteriorating ship structures operating in multiple sea loading climates

A reliability-based model for assessment of deteriorating ships subjected to multiple environmental conditions is established. Deterioration due to combined crack growth and corrosion in structural components of a ship hull is accounted for based on a fracture mechanics formulation. The model enables to take into account the corrosion-induced increased crack growth rate in two ways: (1) the increased stress range produced by the plate thinning (wastage) effect and (2) corrosion fatigue itself. Sensitivity studies are carried out to evaluate the effect of inspection updating on a production ship subjected to two different climate conditions. The hazard rate concept is adopted as a measure of reliability and emphasized throughout the different case studies.

233 炭素量0.08mass%の低炭素鋼板の疲労き裂進展とストレッチゾーンに及ぼす水素の影響(OS8-1 水素環境が金属強度特性に及ぼす影響,OS8 水素環境が金属強度特性に及ぼす影響)

Fatigue crack growth tests were conducted on hydrogen charged and uncharged specimens of a 0.08 mass%C low carbon steel sheet at test frequency 0.001 -10Hz. After fatigue crack growth tests, a stretch zone was formed in order to investigate the mechanisms of fatigue crack growth in the hydrogen environment. From the observation of striation and stretch zone on the fracture surface and slip bands on the specimen surface, it was revealed that uncharged specimens have no dependency of crack growth rate on test frequency and on the other hand, the crack growth rate of hydrogen charged specimens at the lower test frequency is approximately 10 times faster than that of uncharged specimens. There exists an upper limit of the increase in crack growth rate of the hydrogen charged specimens at test frequency below 0.01 Hz. The upper limit is almost equal to the maximum of stretch zone width which is inclined. The experimental results show that the fatigue crack growth of hydrogen charged specimen is related to the hydrogen enhanced localized plasticity.

A viewpoint on environmental contributions to fatigue crack growth

Fatigue crack growth behaviour is examined using the Unified Damage Approach developed by the authors since 1994. The approach is based on the understanding that a fatigue damage requires two loading parameters; a requirement manifesting in terms of two fracture mechanics parameters: maximum stress intensity factor Kmax and a stress amplitude ΔK. Kmax governs the breakage of crack tip bonds allowing for crack extension and ΔK controls the extent of cycle damage needed ahead of the crack tip. The overall changes in crack growth depend on several factors like microstructure, deformation mode and its properties and environment. The Unified Damage Approach describes these closely related changes in the damage mechanisms due to synergistic effects of mechanical and environmental driving forces. Crack growth trajectory maps developed using the Unified Damage Approach summarises these changes in mechanisms that occur with increasing crack growth rate or stress intensity.

Fatigue crack propagation in microcapsule-toughened epoxy

The addition of liquid-filled urea-formaldehyde (UF) microcapsules to an epoxy matrix leads to significant reduction in fatigue crack growth rate and corresponding increase in fatigue life. Mode-I fatigue crack propagation is measured using a tapered double-cantilever beam (TDCB) specimen for a range of microcapsule concentrations and sizes: 0, 5, 10, and 20% by weight and 50, 180, and 460 μm diameter. Cyclic crack growth in both the neat epoxy and epoxy filled with microcapsules obeys the Paris power law. Above a transition value of the applied stress intensity factor ΔK T, which corresponds to loading conditions where the size of the plastic zone approaches the size of the embedded microcapsules, the Paris law exponent decreases with increasing content of microcapsules, ranging from 9.7 for neat epoxy to approximately 4.5 for concentrations above 10 wt% microcapsules. Improved resistance to fatigue crack propagation, indicated by both the decreased crack growth rates and increased cyclic stress intensity for the onset of unstable fatigue-crack growth, is attributed to toughening mechanisms induced by the embedded microcapsules as well as crack shielding due to the release of fluid as the capsules are ruptured. In addition to increasing the inherent fatigue life of epoxy, embedded microcapsules filled with an appropriate healing agent provide a potential mechanism for self-healing of fatigue damage.

Corrosion and fatigue characteristics of positively polarised Zeron 100 base & weld metal in synthetic seawater

It is envisaged that super duplex stainless steels, as currently used in the offshore oil and gas industries, will find application in the emergent renewable energy sector in areas such as offshore wind, wave and tidal electricity/hydrogen generation. Such applications typically involve engineering components experiencing fluctuating loads in a harsh environment such as seawater. In these circumstances, there is a real possibility of increased crack growth rates due to combined corrosion fatigue. In this way, the current paper investigates the corrosion and fatigue characteristics of Zeron 100 weld metal in synthetic seawater. High-imposed electrochemical potentials are specifically addressed, since it is well known that aerated and chlorinated seawater in contact with high alloy stainless steels give rise to high local electrochemical potentials. The corrosion resistance of the weld metal was similar to the base metal when positively polarised in synthetic seawater at ambient temperature. Crack propagation rates increased for the base and weld metal by over a factor of 2 compared to rates in laboratory air. High temperature solution significantly reduces the pitting potential and delays repassivation.

The Crack Growth Evaluation of A335 P22 Steels by Acoustic Emission

Acoustic emission (AE) technique was used to investigate fatigue crack growth on compact tension specimens of aging materials at room temperature. Test materials have been sampled steam pipe serviced the actual operation conditions for a long time in fossil power plant. The compact tension test specimens were subjected to load stress ratios of 0.33, 0.5, and 0.66. All the fatigue tests were performed at a frequency of 1Hz. The test results indicate that acoustic emission counts show reasonable correlation with crack propagation rates for applied stress ratios. When the crack growth rates increase, AE’s counts and energies show increment. Also, the higher stress ratios, the faster crack propagation rates. Based on these relationships it may be possible to predict the remaining service life of fatigue-damaged steam pipes.

Theoretical-Experimental Model for Predicting Crack Growth Rate in Structural Alloys of Aircraft Gas Turbine Engine Disks with Regard to Fatigue and Creep

Hold times of 300 and 3600 s under trapezoidal cyclic loading are shown to result in an increase in crack growth rates by many factors of ten for EP962 alloy and several times for EP742 alloy at a temperature 973 K. It has been found that in case the creep crack growth diagram has a first segment where the crack growth rate decreases, the crack growth kinetics under trapezoidal cyclic loading can be predicted by means of the hypothesis of linear summation of fatigue and creep crack growth rates considering special features of the first segment of the creep crack growth diagram. The authors put forward empirical approaches to determining the mean rate on the first segment of the creep crack growth diagram.

Fatigue crack growth behavior of titanium alloys

Fatigue crack growth behavior of titanium alloys is examined using the Unified Approach developed by the authors. The approach is based on the realization that fatigue requires two load parameters for unambiguous description. This requirement manifests as two fracture mechanics parameters: maximum stress intensity factor, K max, and stress intensity amplitude, Δ K. The former essentially governs the breaking of the bonds contributing to crack growth, while the latter controls the extent of cyclic damage required ahead of the crack tip. The former can also be associated with the monotonic plastic zone, while the latter with the cyclic plastic zone. The crack growth behavior in Ti-alloys is complicated by several factors such as microstructure and its dependent flow properties, planarity and heterogeneity of slip, environmental sensitivity of the alloy, and changes in the flow behavior due to segregation of internal hydrogen contributing to localized creep and embrittlement. In the past, several anomalies in the behavior have been noted. The Unified Approach demonstrates that these observed anomalies are closely related to changes in the mechanisms of crack growth due to synergistic effects of mechanical and chemical driving forces. Crack growth trajectory maps developed under the Unified Approach document these changes in the mechanisms that occur with increasing crack growth rate or stress intensity.

Fatigue behavior of nanocrystalline metals and alloys

In this work, the stress-life fatigue behavior and fatigue crack growth characteristics of pure Ni were studied as a function of grain size spanning a range of tens of nanometer to tens of micrometer. The fatigue response of electrodeposited, fully dense, nanocrystalline pure Ni, with average and total range of grain sizes well below 100 nm, was compared and contrasted with that of electrodeposited ultrafine-crystalline pure Ni with an average grain size of about 300 nm and conventional microcrystalline Ni with an average grain size in excess of 10 μm. It was found that grain refinement to the nanocrystalline regime generally leads to an increase in total life under stress-controlled fatigue whereas a deleterious effect was seen on the resistance to fatigue crack growth at low and high tensile load ratio levels. To explore the generality of the above trends, systematic experiments were also performed in ultrafine-crystalline pure Ti produced by equal-channel angular pressing where a reduction in grain size was found to cause an increase in fatigue crack growth rates at different tensile load ratios. Grain refinement from the microcrystalline to the ultrafine-crystalline regime by cryomilling of Al alloys also showed a similar response. Possible mechanistic origins of such trends are explored, and some general conclusions are extracted on strategies for improvements in the fatigue resistance of engineering structures by recourse to grain refinement down to the nanocrystalline regime.

Crack Growth Anomalies in Base Steel P91 and in HAZ

The growth of cracks in base steel P91 of 9Cr-1Mo class and in intercritical layer of HAZ is measured under creep coditions. For long term tests, a material degradation was detected consisting in an increase of crack growth rate and in a decrease of crack initiation time. An attempt is made to connect these effects with drop in ductility during thermomechanicalexposition.

Resistance of PVC-U and PVC-M to cyclic fatigue

PVC pipes may be used in applications where they are subjected to cyclic pressures generated by the operation of pumps and valves. It is therefore important that the material's response to cyclic loading is quantified and incorporated into pipeline design when appropriate. Using rotational bending, PVC-U and PVC-M have been shown to exhibit very similar resistance to crack growth. Further, both materials exhibited a similar increase in fatigue crack growth rate as the test temperature increased. The rotational bending method was also used to investigate the presence of a fatigue threshold for PVC-M and PVC-U. It is concluded that the same cyclic fatigue design principles should be applied to both PVC-U and PVC-M pipes. Moreover, the existence of a threshold value should be taken into account so that derating for fatigue is not required if the amplitude of the stress intensity factor is below the threshold.

Effects of mixed mode loading on fatigue and creep–fatigue in SRR-99 single crystals

Fatigue tests have been carried out at 20, 650 and 850 °C over a range of frequencies on SRR-99 single crystals under both pure mode I and mixed mode I/II loading conditions in vacuum at a stress-ratio of 0.5. At low temperatures, predominantly slip band crack growth (which remained effectively in-plane) was observed, although the scale of the facets increased under mixed mode loading. At high temperatures mixed mode loading resulted in deflected co-operative slip crack growth along a direction that experienced the maximum opening mode (analogous to Stage II crack growth). Some pore cavitation and interlinking was observed at the highest temperatures and lowest frequencies. The effect of increased dwell time at 850 °C was to increase crack growth rates compared with the highest frequency test, although differences in the magnitude of these effects were seen for the two different loading conditions. The increased constraint experienced by the deflected crack offers a partial explanation for this.

Stress intensity variations in bone microcracks during the repair process

Microscopic cracks form and grow in compact bone in vivo due to cyclic loading. Their growth can cause stress fractures and has been implicated in the processes of remodelling and adaptation. These cracks are repaired by the actions of BMUs which are mobile resorption cavities. In this work, we studied the interaction between cracks and BMUs by making finite element models representing different stages in the repair process. The tendency of the crack to grow was measured by its stress intensity factor, K. We found that K changes in a complex manner during the repair process, both decreasing and increasing depending on the size of the crack and the type of loading applied. For loading conditions similar to those that exist in vivo, the presence of the BMU can cause K to rise significantly, in some cases by more than 20%, implying a substantial increase in crack growth rate. This information is important for our general understanding of the complexities of the repair process, and especially for the development of theoretical models to simulate damage and repair in bone.

Liquid-Metal Embrittlement of Type 316L Stainless Steel by Gallium as Measured by Elastic-Plastic Fracture Mechanics

Abstract In order to bound failure of austenitic stainless steel storage containers housing Ga-containing compounds, the liquid-metal embrittlement of Type 316L (UNS S31603) stainless steel (SS) by Ga was investigated. Type 316L SS compact tension specimens were exposed to liquid Ga using a depassivation technique to wet the specimen. Linear elastic and elastic-plastic fracture mechanics methods were used to compare the fatigue and fracture behavior. Mild liquid-metal embrittlement was observed, as indicated by increased fatigue crack growth rate, decreased number of fatigue cycles to failure, decreased crack initiation resistance, and increased crack growth rate. Stable cracking was observed for all test conditions. A small amount of intergranular cracking was observed following Ga exposure. No effect of test temperature on embrittlement was observed over the small temperature range examined (35°C to 75°C). Decreasing crosshead displacement rate promoted Ga embrittlement. Based on fractography, profilometry, and mechanics, it appears that both adsorption-induced decohesion and adsorption-enhanced plasticity mechanisms are operative in the Type 316L SS-Ga system.

Effects of loading waveform on fatigue crack growth of Udimet 720 Li superalloy

The influence of loading waveform, including constant load tests, on fatigue crack growth rate (FCGR) of the Ni-base superalloy Udimet 720 Li has been investigated in air at the temperature of 700 °C, using corner crack specimens. The fatigue crack growth is controlled by oxidation mechanisms and the mode of fracture is completely intergranular. The loading ramp has the same effect on crack growth rate as the unloading ramp. The time spent at maximum load produces higher crack growth rate than the same time consumed during loading or unloading ramp. High strain rate due to high loading ramp of about 300 kN/s followed by slow unloading leads to considerable increase in crack growth rate, which is attributed to interaction between crack tip plasticity, creep and oxidation.

High Temperature Fatigue Crack Growth Behavior of Ti-6Al-4V

Experimental evaluation of high temperature, Fatigue Crack Growth Rate (FCGR) data for Ti-6Al-4V, a titanium alloy, is presented. The FCGR data were measured at room temperature, 175, 230, 290 and 345°C using the Direct Current Potential Difference (DCPD) technique. Compact Tension (CT) specimens were used in the program and crack growth rates (da/dN) vs. Mode I stress intensity factor ranges (AK) were plotted as a function of temperature. A temperature rise from 175 to 345°C did not cause a substantial increase in crack growth rates within the Stage II region where a linear relationship describes the behavior. Formation of secondary cracks, observed at higher temperatures, may have slowed the crack propagation as observed in the fractography.

The influence of microstructure and environment on the crack growth behavior of powder metallurgy nickel superalloy RR1000

A study of crack growth in vacuum and air at 725 °C (T/T m=0.6) highlights the relative importance of creep and environmental crack-tip damage mechanisms in Powder Metallurgy (P/M) disc alloy RR1000. Both of these mechanisms are associated with a transition to intergranular fracture during fatigue crack growth at 0.25 Hz. Crack growth under sustained loads reveals the precise nature of these mechanisms in RR1000. The severity of creep and environmental mechanisms is controlled by the grain-boundary microstructure and the crack-tip stress. Near-tip cavitation leads to fracture in vacuum. Sigma-phase precipitation causes an increase in crack growth rate through increased crack-tip cavity nucleation. Rapid near-tip stress relaxation induced by γ′ coarsening has a beneficial effect on the severity of this type of damage. In air, increases in crack growth rates are associated with near-tip intergranular oxidation. It is proposed that the extent of this damage and subsequent growth rates are increased by sigma-phase precipitation through enhanced oxidation due to chromium depletion and subsequent decreased passivation. Again, a beneficial effect of rapid near-tip stress relaxation due to selective γ′ coarsening is apparent and environmental damage is reduced under these conditions.

Fatigue fracture of fully saturated bonded joints

The objective of this study was to compare the fatigue crack growth characteristics of adhesively bonded joints at several temperatures in air and salt water. Motivated by current trends in automotive structures, the specimens consisted of urethane adhesive bonding composite adherends with swirled glass fibers in an isocyanurate matrix. A modified cracked lap shear configuration under four-point bending was used to load the specimens, which were reinforced in order to ensure that cracking at or near the interface could be examined under all conditions. Potentially ambiguous diffusion transients were removed by fully saturating the specimens that were to be tested in salt water. Paris Law parameters and threshold values of energy release rate were extracted for each environmental condition that was studied. In air, both high and low temperatures increased crack growth rates and reduced threshold values relative to room temperature. Increasing temperature from 10°C consistently lowered the bond durability in salt water. The effects of load level, temperature, and environment on the fracture parameters were related to fracture mechanisms via recorded side views and fractographic evidence.

A study on fatigue crack propagation mechanism of GFRP in synthetic sea water

This paper evaluates the fatigue fracture behavior of a chopped strand glass mat/polyester composite both in air and sea water. Bending fatigue (R=−1) was performed on dry and wet specimens, that is respectively in air and sea water. Where the pH concentration of sea water was controlled to 6.0, 8.2, 10.0 and the wet specimens were immersed in the sea waters for 4 months. Throughout the tests, fatigue cracks both in the dry and wet specimens, tested in the air or sea water, occurred at the beginning of the cycle, followed by either of two regions: one decreasing and the other increasing as the crack growth rate increases.

Influence of stress state on high temperature fatigue crack growth in Inconel 718

The influence of stress state on fatigue crack growth in nickel‐base superalloys at high temperature is considered, based on studies in corner crack specimens of Inconel 718 at 600 °C. At high frequency and low R, cycle‐dependent trans‐granular crack growth occurs along the whole crack front, and growth rates are similar at the surface and within the interior of specimens, maintaining the original quarter‐circular shape. For conditions of low frequency and high R, increased crack growth rate per cycle is observed with the crack tunnelling ahead at the centre. A time‐dependent intergranular crack propagation mode occurs in the plane strain interior, attributed to an oxidation mechanism, whereas near the surfaces under plane stress, a trans‐granular cyclic plasticity mechanism is observed. It is proposed that in addition to frequency and R, that stress state influences the competition between the mechanisms controlling crack growth and the transition between them: plane strain in the interior favouring an oxidation‐controlled intergranular cracking mechanism as compared with the plane stress surfaces where cyclic plasticity dominates. An FEM study suggests that this influence of stress state is not associated with variation of ΔK along the crack front.