Lower Crack Propagation Rate Research Articles

Critical fracture toughness, JC and δ5C, of unidirectional fibre–metal laminates

Fibre–metal laminates (FMLs) are structural composites designed aiming to produce a damage-tolerant and high strength material. Their main characteristic is their very low fatigue crack propagation rates when compared to traditional aeronautical Al alloys. Their application in aeronautical structures demands a deep knowledge of a wide set of mechanical properties and technological values, including both fracture toughness and residual strength. The objectives of the present work were to present critical toughness values ( J C and δ 5C) of unidirectional FMLs obtained following a recently proposed methodology and to use them for critical crack length and residual strength predictions. Residual strengths of middle centre-cracked panels of Arall ® 2 and 3 were predicted and compared to experimental values from the literature. The results showed that all FMLs evaluated presented higher fracture toughness and crack tolerance than their constituent alloys and that the measured fracture toughness was useful for an accurate prediction of residual strength in centre-cracked plates of Arall.

Fatigue-crack-propagation thresholds in a nickel-base superalloy at high frequencies and temperatures

Fatigue-crack-propagation (FCP) tests were conducted on the powder metallurgy nickel-base superalloy KM4 at temperatures of 20 °C, 550 °C, and 650 °C. Two different heat treatments were investigated, one yielding a relatively coarse grain size of 55 µm and another yielding a fine grain size of 6 µm. Tests were conducted at 100 Hz and 1000 Hz and at load ratios between 0.3 and 0.7. In the Paris regime, trends observed at high frequencies for KM4 were identical to those observed by earlier investigators at lower frequencies: coarse grains, low load ratios, low temperatures, and higher frequencies generally resulted in lower crack-propagation rates. However, in contrast to the Paris-regime behavior, thresholds were a complicated function of microstructure, load ratio, temperature, and frequency, and the only variable that resulted in a consistent trend in threshold was the load ratio. For example, thresholds increased from 100 to 1000 Hz for the fine-grained material at 550 °C, but decreased with the same frequency variation at 650 °C. One reason for this complexity was a change to intergranular fracture in the fine-grained microstructure at 650 °C, which was beneficial for high-frequency thresholds. Higher load ratios and lower frequencies promoted intergranular fracture. However, not all of the complexity could be explained by changing fracture mechanisms. Scanning electron microscope (SEM) stereofractography was utilized to determine quantitative measures of fracture-surface roughness. The most useful quantitative measure was found to be the standard deviation of the fracture-surface height, which is a physically meaningful length parameter and which corresponded to about half the grain size during room-temperature fatigue at near-threshold ΔK levels. The roughness of the fracture surface was found to increase as the load ratio was increased for both microstructures. For the coarse-grained microstructure, there was a direct correlation between fracture-surface roughness and FCP threshold over the entire range of temperatures, frequencies, and load ratios. However, measurements of closure loads indicated that roughness-induced closure was not the sole reason for the varying FCP thresholds.

［フェライト/ベイナイト］ハイブリッド材における疲労き裂進展挙動

We have examined fatigue crack propagation behavior in carbon steels employing a steel containing mixed ferrite and pearlite phases and a Hybrid steel prepared by diffusion bonding between ferrite and bainite materials. No special steel-microstractural effect was observed in the ferrite-pearlite mixed steel. In the Hybrid steel, crack propagation in the ferrite phase decelerated with reaching the [ferrite/bainite]-interface, probably due to the supression of plastic deformation at the crack tip. Low crack propagation rate was also observed in the bainite phase. This can be interpreted in terms of cyclic softening at the crack tip. It is concluded that the fatigue crack propagation rate is strongly affected by phase and interphase interface in steels.

The Influence of Microstructural Morphology and Prestrain on Fatigue Crack Propagation of Dual-phase Steels in the Near-threshold Region.

The influence of microstructure and prestrain on fatigue crack propagation in the near threshold region was studied for dual-phase steels with different microstructural morphologies. The material with martensite phases dispersed in ferrite matrix exhibited a higher nominal stress intensity range and a lower crack propagation rate than those of material with a continuous martensite phase around ferrite grains. Prestrain prior to fatigue testing resulted in a decrease of nominal stress intensity range in both materials. However there was a slight difference among all the materials in the effective stress intensity range. The microstructural morphology and prestrain exerted a great influence on crack path and that caused a large difference in crack closure. A quantitative correlation existed between crack closure level and surface roughness of fatigue fracture.

Micro-, Meso- and Macro-Texture and Fatigue Crack Roughness in Al-Li 2090 T8E41

ABSTRACTThe use of synchrotron polychromatic x-ray microbeams in the transmission geometry is described for mapping grain orientation as a function of position and for relating this microtexture to the formation of large asperities on fatigue crack surfaces in Al-Li 2090 T8E41. In common with the centers of rolled plates of many aluminum alloys, Al-Li 2090 T8E41 has a sharp average texture or macrotexture different from that in the outer portions of the plate. The geometry of large asperities in Al-Li 2090 has been related to this macrotexture, and the resulting roughness-induced crack closure is recognized to be responsible for the very low crack propagation rates in certain plate orientations. This report focuses on why asperities form at certain positions and why the crack remains relatively planar elsewhere. The microtexture (i.e., the grain-to-grain orientation variation) seems to be organized into a specific type of mesotexture: multiple adjacent grains have nearly identical orientations and form substantial volumes of near-single-crystal material. Transitions between differently oriented near-singlecrystal volumes or between a near-single- crystal region and more randomly oriented grains appear to bound asperities.

Studies on Relation between Low Cycle Fatigue Strength and Fatigue Crack Propagation Rate in Structural Materials.

The estimation of fatigue crack propagation rate in structural materials and the clarification of crack propagation mechanism are extremely important. In this study, a low cycle fatigue test and a fatigue crack propagation test were carried out. Then the relation between the low cycle fatigue strength and the fatigue crack propagation rate was investigated, and the factors affecting the fatigue crack propagation rate were clarified on the basis of a damage accumulation model. Consequently it was found that Young's modulus E, the fracture ductility ef and the yield strain eN considerably affected the fatigue crack propagation rate da/dN, and the following equation involving the effective stress intensity factor range ΔKeff was proposed in order to consider the effect of material properties on the fatigue crack propagation rate. da/dN=506(ΔKeff/E)2.79/(ef√(ey))

The influence of SiC particulates on fatigue crack propagation in a rapidly solidified Al-Fe-V-Si alloy

The fatigue crack propagation properties of a rapidly solidified aluminum alloy are compared with those of a metal matrix composite (MMC) made of the same base alloy with the addition of 11.5 vol pct SiC particulate. The high-temperature base material, alloy 8009 produced by Allied-Signal, Inc. (Morristown, NJ), is solidified and processed using powder metallurgy techniques; these techniques yield a fine-grained, nonequilibrium microstructure. A direct comparison between the fatigue crack propagation properties of the reinforced and unreinforced materials is possible, because alloy 8009 requires no postprocessing heat treatment. As a consequence, this comparison reflects the influence of the SiC particulate and not differences in microstructure that could arise during processing and aging. The experimental data demonstrate that the SiC-reinforced material exhibits modestly superior fatigue crack propagation properties: slower crack growth rates for a given ΔK, at near-threshold crack growth rates. Even when the data are corrected for crack closure using an effective stress intensity factor, ΔKeff, the composite exhibits lower crack propagation rates than the unreinforced matrix alloy. Microscopic evidence shows a rougher fracture surface and a more tortuous crack path in the composite than in the base alloy. It is argued that the lower crack growth rates and higher intrinsic threshold stress intensity factor observed in the composite are associated with crack deflection around SiC particles.

Fatigue fracture behaviour of carbon-fiber-reinforced modified bismaleimide composites

The fracture toughness and fatigue fracture behaviour of carbon-fiber-reinforced modified bismaleimide (BMI) composites have been studied. These composites were found to have higher fracture toughnes, better damage tolerance and longer fatigue life than carbon-fiber composites with epoxy matrices. Delamination is the major mode of failure in fatigue and it is controlled by the properties of the matrix and interface. The improved performance is due to the presence of thermoplastic particles in the modified BMI matrix which gives rise to enhanced fiber/matrix adhesion and more extensive plastic deformation. The fatigue behaviour also depends on the stacking sequence, with the multidirectional [45/90/-45/0] fiber-reinforced modified BMI composite having a lower crack propagation rate and longer fatigue life than the unidirectional laminate. This arises because of the constraint on the damage processes due to the different fiber orientation in the plies.

Fatigue crack growth behavior of D6AC space shuttle steel

Fatigue crack propagation rate properties of a D6AC steel were developed, covering a wide range of crack growth rates from approximately 10 −3 m/cycle (4 × 10 −2 in./cycle) to 10 −10m/cycle (4 × 10 −9 in./cycle). The low crack propagation rates were obtained using a decreasing ΔK test method, while the fast crack growth rates were generated by the J- controlled test technique. At high ΔK levels [> 122 MPa √ m (111 ksi√ in.)], dimpled fracture was the dominant fracture mode, while at ΔK levels between 122 MPa √m (111ksi √in.) and 176.6 MPa√m (16 ksi√in.), the transgranular fracture mode was found. Below ΔK levels of 17.6 MPa√m (16 ksi√in.), some intergranular fracture mixed with transgranular fracture was observed. At near-threshold levels, oxide deposits were present on fracture surfaces. Oxide thickness was measured, and was found to be much less than crack tip opening displacement. Thus, oxide-induced crack closure was negligible during near-threshold crack growth testing.

ELEVATED TEMPERATURE FATIGUE CRACK PROPAGATION IN HOT WORK TOOL STEEL

Cyclic tests have been carried out on a hot work tool steel (H13) over a range of temperatures using different waveforms. At a temperature of 125°C, a slow-fast wave resulted in lower crack propagation rates than those with a balanced (or symmetrical) waveform. However, at 500°C normal fatigue crack propagation behaviour was observed in which the slow-fast wave was more damaging than a balanced waveform at the same frequency. Dynamic strain aging is attributed to the reversed crack propagation effect at 125°C.

The microstructures of mild steels and corrosion fatigue: Xing, Z., Huang, S., Song, Y. and Tu, M. Acta Metall. Sin. (China) Dec. 1988 24, (6), B398–B403 (in Chinese)

Fibre–metal laminates (FMLs) are structural composites designed aiming to produce a damage-tolerant and high strength material. Their main characteristic is their very low fatigue crack propagation rates when compared to traditional aeronautical Al alloys. Their application in aeronautical structures demands a deep knowledge of a wide set of mechanical properties and technological values, including both fracture toughness and residual strength. The objectives of the present work were to present critical toughness values (JC and δ5C) of unidirectional FMLs obtained following a recently proposed methodology and to use them for critical crack length and residual strength predictions. Residual strengths of middle centre-cracked panels of Arall® 2 and 3 were predicted and compared to experimental values from the literature. The results showed that all FMLs evaluated presented higher fracture toughness and crack tolerance than their constituent alloys and that the measured fracture toughness was useful for an accurate prediction of residual strength in centre-cracked plates of Arall.

Stress Corrosion Cracking of Conventional and Super-Clean 3.5-NiCrMoV Rotor Steels in Simulated Condensates

Abstract The stress corrosion cracking (SCC) behavior of two bainitic 3.5-NiCrMoV low-pressure (LP) rotor steels has been studied at 95 C in environments containing species that are possible contaminants in steam condensates. The steels were a conventional modern rotor material of regular chemical composition and a super-clean steel with low levels of Mn, Si, S, and P. The environments were 3.5 M NaOH, 1 M Na2CO3 + 1 M NaHCO3 and a saturated CO2/H2O solution. A combination of slow strain rate tensile testing methods and fracture mechanics techniques were used to produce SCC. A unique specimen configuration, based on a modification of the notched double cantilever beam geometry, was developed to improve the ease of testing by fracture mechanics methods. Cracking was observed only in the NaOH and carbonate/bicarbonate environments and the SCC behavior of the two steels was reasonably similar, with the super-clean steel having slightly lower crack propagation rates in the NaOH solution. Based on 500-h exposu...

Numerical simulation of microstructually short crack propagation in creep.

A numerical simulation model of the propagation of a microstructually short crack in monotonic creep was proposed. In the model, the distribution of the crack propagation rate caused by the microstructual inhomogeneity was expressed by means of some random variables showing the grain boundary length and the crack propagation rates at the grain boundary triple points and their midway. The cumulative probability of the crack length was calculated using the Monte Carlo Method and the effects of some factors such as the length was between the grain boundary triple points, the initial crack length, the upper bound and the lower bound of crack propagation rates, and the waveform of crack propagation rate between two adjacent triple points were discussed. The lower crack propagation rate and the initial crack length had greater effects on the probability than did the other factors. The cumulative probability of the crack length in the case of continuous crack initiation during creep was also simulated on the basis of the model, and it coincided very well with the experimental results.

熱間鍛造用型鋼の高温破面解析

In order to investigate the high temperature fracture behavior of hot forging die steel, low cycle fatigue life and crack propagation tests of SKD62 were carried out. The effects of testing temperature, hardness of base metal and surface hardening treatments such as ion nitriding, tuftriding and sul-surf treatment on low cycle fatigue strength and crack propagation rate were investigated.Decrease of low cycle fatigue strength and increase of crack propagation rate were comparatively prominent at 450°C. Increase of hardness of base metal was most effective to improve the low cycle fatigue strength of SKD62 due to delay of crack initiation. Among various surface treatments, ion nitriding was most effective to improve the low cycle fatigue strength of SKD62.Striation was predominant in the fracture surfaces of all low cycle fatigue tested specimens in this experiments.Thermal fatigue crack initiation and propagation tests were also conducted by heating specimens up to 1000°C by use of a laboratory made thermal fatigue testing machine. By lowering the temperature difference between heating and cooling, crack initiation life was increased and crack propagation rate was decelerated. Striation like pattern appeared remarkably on the thermal fatigue fracture surface. The same type of fracture surface was observed on the thermal fatigue failed fracture surface of hot forging die.

Kinetics and mechanisms of creep crack growth in a creep-resisting steel

1. In EI415 steel, rapid subcritical creep crack extension Δa=0.5–2 mm at 103 h at a temperature of 560°C is detected at KI=0.15KIc, at T=510°C it is detected at KI=0.25KIc, and at T=460°C at KI=0.3KIc. 2. At comparatively high values of SIF, creep crack propagation takes place by means of the development of intergranular microcracks propagating in the opposite direction from the region positioned at a distance of 2–6 grain diameters from the crack tip, and is of jumplike nature. 3. At comparatively low values of SIF, the creep crack propagates by means of disintegration of the material of the crack tip. Voids appear both at the grain boundaries and in the body of the grain. It is evident that crack growth is almost continuous. 4. In intergranular failure, the incubation period of crack propagation, i.e., the time to the first jump, represents a large part of the total life, whereas at low crack propagation rates, the incubation period has only conventional meaning and amounts only to a small part of the total life. 5. Running for 137, 000 h at 520°C causes no marked reduction of the resistance of EI415 steel to the propagation of creep cracks.

Influence of dislocation–precipitate interaction on low cycle fatigue resistance of Alloy 800 at 600°C

AbstractThe high temperature, low cycle fatigue behaviour of Alloy 800 has been investigated at 600°C. The results showed that a prior aging treatment of 1000 h in the temperature range 550–600°C improved the fatigue resistance as compared to solution treated material. The strengthening effect was particularly evident in material aged at 550°C. A microstructural investigation revealed that theγ′ precipitate dispersion, which formed intragranularly during aging, interacted very strongly with mobile dislocations. The relatively large γ′ particles formed in material aged at 600°C were bypassed according to the Orowan mechanism, whereas the small γ′ particles in material aged at 550°C were bypassed by particle cutting. Measurements of striation spacings showed that the high fatigue strength of material aged at 550°C was accompanied by a low crack propagation rate. The results can be rationalized in terms of reversibility of plastic flow at the crack tip, which can be expected to be more pronounced when partic...

小穴を有する軟鋼板の疲労き裂進展特性と微視的破壊機構（面外曲げ疲労の場合）

The present investigation was undertaken to examine the relation between fatigue crack propagation characteristics and microscopic fracture mechanisms at low crack propagation rates in a mild steel. In particular fractographic aspects on the fracture surfaces were examined and discussed in connection with the knee of the crack propagation rate-stress intensity factor range curve (log(dl⁄dN)−log(ΔK) curve) and with the transition from Stage 2a to Stage 2b in crack propagation. Fatigue tests were performed under pulsating ex-plane bending using notched specimens with a small circular hole. The fracture surfaces of specimens were examined by transmission and scanning electron microscopies. The resuls are summarized as follows:(1) The value of crack propagetion rate, dl⁄dN, changes in a pulsating manner at an early stage of crack propagation in maximum stress-constant tests. In this range three types of microscopic features, that is, intergranular facets, a striation-like pattern and a shear mode pattern superimposed by the striation-like pattern are observed.(2) In the range of dl⁄dN above the knee of the log(dl⁄dN)−log(ΔK) curve, Paris’ power law is valid. In this range the plastic zone size developed at the crack tip increases with crack propagation, and the shear mode pattern disappears.(3) At the transiton point from Stage 2a to Stage 2b, the area fraction of intergranular facets takes a maximum value.(4) The striation spacing is larger than the value of dl⁄dN and takes a constant value at Stage 2a, whereas it is proportional to dl⁄dN at Stage 2b.(5) In the range of dl⁄dN below the knee, the area of the shear mode pattern becomes larger, as dl⁄dN becomes lower.

Fracture toughness of vitreous enamel coatings including reinforced coatings

1. It is possible to estimate the cohesive strength of vitreous enamel coatings from their fracture toughness G1C, determined in OHB-specimens with a glued layer by the applied linear fracture mechanics method. In particular, this method of estimation makes it possible to find the optimum firing schedule providing the maximum value of G1C for a given coating chemical composition. 2. Reinforcing vitreous enamel coatings with single-crystal ZrO2 filaments promotes an increase in their fracture toughness. 3. The relatively low crack propagation rate in the coating materials studied is apparently caused by their marked porosity.

FATIGUE MACROCRACK GROWTH IN TEMPERED HY80, HY130, AND 4140 STEELS:THRESHOLD AND MID-?K RANGE

— –The rate of propagation of macrofatigue cracks down to near threshold was measured in air in three tempered martensitic steels; HY80, HY130 and 4140 (650°C temper). The value of ΔKth was determined by the load-shedding technique in center notched panel specimens. Of the three steels, 4140 tempered at 650°C had the lowest ΔKth, 3–5 MN/m3/2, while HY80 had the highest, 4.2 MN/m3/2. The 4140 (650°C temper) is intermediate in strength between HY80 and HY130. The results are discussed in terms of a recent theory of one of the authors. The fatigue crack propagation rates in the mid-ΔKrange in HY80 and HY130 in argon were also studied by measuring, with foil strain gages, the cyclic plastic work to propagate a fatigue crack by a unit area, U.HY80 has a lower crack propagation rate and correspondingly higher U.This was attributed in part to the higher yield strength of HY130 but the dislocation structure and carbide composition and morphology also play roles. Microstructural changes due to cyclic plastic deformation inside the plastic zone in HY80 and HY130 were observed by TEM of thin foils. SEM studies of the fracture surfaces at ΔK= 20 MN/m3/2 indicate a more ductile fracture mode for HY80 than for HY130. The fatigue crack propagation rate of HY130 is substantially higher in laboratory air (47% relative humidity) than in dry argon. This is not the case for HY80.

Effect of overaging on mechanical properties and stress corrosion cracking of HY-180M steel

The tensile properties, fracture toughness and stress corrosion cracking (SCC) behavior of HY-180 M steel at 22 °C were studied after final 5 h overaging treatments >510 ≤650 °C. SCC tests were conducted for 1000 h with compact tension specimens in aqueous 3.5 pct NaCl solutions at a noble (anodic) potential of −0.28 VSHE ( −0.48 VAg/AgC1) and a cathodic protection potential of −0.80 VSHE (−1.0 VAg/AgC1). The SCC resistance improved at aging temperatures >565 °C, the most significant improvement being at −0.80 VShe, especially after 650 ° aging whereK ISCC was raised to at least 110 MPa · m1/2. However, this was at the expense of mechanical properties. Provided low crack propagation rates of ∼3 X 10−11 m/s at −0.80V SHEmay be tolerated, the best compromise between strength, toughness, and SCC resistance was obtained after 594 °C aging. Under these conditions, stress intensities as high as ∼ 110 MPa · m1/2 can be used, with a yield strength of ∼ 1150 MPa and fracture toughness of ∼ 170 MPa · m1/2. The retained austenite content after aging increased with aging temperature up to 25 pct by vol at 650 °C. It appeared to correlate with improved SCC resistance, but other microstructural effects associated with aging may be involved.