Intrinsic Crack Growth Resistance Research Articles

Local crack arrestability and deformation microstructure evolution of hydrogen-related fracture in martensitic steel

Intergranular cracks in the uncharged specimen were arrested at low-angle prior austenite grain boundary (PAGB) segments of several micrometers. In contrast, even small, low-angle PAGB segments with sub-micrometer sizes impeded the propagation of hydrogen-related intergranular crack. At the hydrogen-related quasi-cleavage crack tip, the crystallographic orientation changed abruptly, and deformation microstructures developed, including the formation of low-energy dislocation structures. A certain degree of crack growth resistance (intrinsic crack growth resistance) in the hydrogen-related fractures could be attributed to the intense localized plastic works involved in the arrest of intergranular cracks and the propagation of quasi-cleavage cracks.

Effects of microstructure on fretting fatigue behavior of IN100

Fretting fatigue behavior of a nickel-base superalloy, IN100, was investigated at room temperature. Two microstructures of IN100 were tested which varied primarily by the gamma grain size (3 μm versus 7 μm). Fretting fatigue tests were conducted at various stress levels using cylinder-on-flat contact configuration. An increase in the grain size was associated with decrease in the fretting fatigue strength/life of IN100. Microscopic analysis showed that the 3 μm grain microstructure provided a higher microstructural barrier to the fretting fatigue crack nucleation and initiation. On the other hand, the 7 μm grain microstructure had a higher intrinsic crack growth resistance due to the tortuous crack path requiring more energy. These features were in agreement with the plain fatigue where fine microstructures generally provide higher resistance to crack initiation but reduce crack propagation resistance while coarse microstructures have the opposite behavior.

Microstructure variation effects on room temperature fatigue threshold and crack propagation in Udimet 720Li Ni‐base superalloy

ABSTRACTAn assessment of the effects of microstructure on room temperature fatigue threshold and crack propagation behaviour has been carried out on microstructural variants of U720Li, i.e. as‐received U720Li, U720Li‐LG (large grain variant) and U720Li‐LP (large intragranular coherent γ′ variant). Fatigue tests were carried out at room temperature using a 20 Hz sinusoidal cycling waveform at an R‐ratio = 0.1 on 12.5 mm × 12.5 mm square cross‐section SENB specimens with a 60° starter notch. U720Li‐LG showed the highest threshold ΔK (ΔKth), whilst U720Li‐LP showed the lowest ΔKth value. U720Li‐LP also showed higher crack growth rates in the near‐threshold regime and at high ΔK (although at higher ΔK levels the difference was less marked). Crack growth rates of U720Li and U720Li‐LG were relatively similar both in the near‐threshold regime and at high ΔK. The materials showed crystallographic stage I type crack growth in the near‐threshold regime, with U720Li showing distinct crystallographic facets on the fracture surface, while U720Li‐LG and U720Li‐LP showed mostly microfacets and a lower proportion of large facets. At high ΔK, crack growth in the materials becomes flat and featureless indicative of stage II type crack growth. The observed fatigue behaviour, which is an effect of the combined contributions of intrinsic and extrinsic crack growth resistances, is rationalized in terms of the microstructural characteristics of the materials. Enhanced room temperature fatigue threshold and near‐threshold long crack growth resistance are seen for materials with larger grain size and higher degree of planar slip which may be related to increased extrinsic crack growth resistance contributions from crack tip shielding and roughness‐induced crack closure. Differences in the deformation behaviour, either homogeneous or heterogeneous due to microstructural variation in this set of materials may provide approximately equivalent intrinsic crack growth resistance contributions at room temperature.

Effect of temperature on high cycle fatigue behaviour in 18Cr–2Mo ferritic stainless steel

Fully reversed axial fatigue tests have been performed using smooth specimens of 18Cr–2Mo ferritic stainless steel (type 444) at ambient temperature, 673 K and 773 K in laboratory air, in order to understand the effect of temperature on high cycle fatigue behaviour. Fatigue strength significantly decreased with increasing temperature. When characterized in terms of fatigue ratio, fatigue strength still decreased at elevated temperatures compared with at ambient temperature. At all temperatures studied, cracks were generated at the specimen surface due to cyclic slip deformation, but crack initiation occurred much earlier at elevated temperatures than at ambient temperature. Subsequent small crack growth was considerably faster at elevated temperatures even though difference in elastic modulus was taken into account, indicating the decrease in the intrinsic crack growth resistance. Fractographic analysis revealed some brittle features in fracture surface near the crack initiation site at elevated temperatures, which was more pronounced and extensive at 773 K, indicating that embrittlement has occurred at the temperature range studied. It was believed that such embrittlement was responsible for the observed decrease in both crack initiation resistance and small crack growth resistance.

Effect of niobium on high-temperature properties for ferritic stainless steel : Fujita, N., Ohmura, K., Kikuchi, M., Suzuki, T., Funaki, S. and Hiroshige, I. Scripta Materialia (1996) 35, 705–710

Fully reversed axial fatigue tests have been performed using smooth specimens of 18Cr–2Mo ferritic stainless steel (type 444) at ambient temperature, 673 K and 773 K in laboratory air, in order to understand the effect of temperature on high cycle fatigue behaviour. Fatigue strength significantly decreased with increasing temperature. When characterized in terms of fatigue ratio, fatigue strength still decreased at elevated temperatures compared with at ambient temperature. At all temperatures studied, cracks were generated at the specimen surface due to cyclic slip deformation, but crack initiation occurred much earlier at elevated temperatures than at ambient temperature. Subsequent small crack growth was considerably faster at elevated temperatures even though difference in elastic modulus was taken into account, indicating the decrease in the intrinsic crack growth resistance. Fractographic analysis revealed some brittle features in fracture surface near the crack initiation site at elevated temperatures, which was more pronounced and extensive at 773 K, indicating that embrittlement has occurred at the temperature range studied. It was believed that such embrittlement was responsible for the observed decrease in both crack initiation resistance and small crack growth resistance.

Fatigue Crack Growth in Aluminum Laminate Composites

AbstractFatigue crack growth has been measured in a laminated metal composite (LMC) consisting of alternating layers of AA6090/SiC/25p metal matrix composite (MMC) and AA5182 alloy. This material was tested in both as-pressed (F temper) and aged (T6 temper) conditions. Corresponding crack growth measurements were made in self-laminates of both the MMC and AA5182 materials to examine the role of the interfaces.The LMC-T6 material has a significantly higher fracture toughness than its MMC constituent but exhibits only modest improvements in nominal fatigue crack growth resistance and a lower nominal threshold. The LMC-T6 shows high levels of crack closure which reveal poor intrinsic crack growth resistance. Self-laminated AA6090/SiC/25p MMC in the F temper has lower microhardness, lower strength and superior intrinsic crack growth resistance compared to this material in the T6 condition. The fatigue fracture surface of T6 temper MMC remains macroscopically smooth despite the development of a large amount of crack closure. The selflaminated AA5182 exhibits a significant improvement in fatigue crack growth resistance after heat treatment and a corresponding change in crack morphology from smooth to very rough. This change leads to a large increase in roughness-induced closure. The effects of heat treatment are attributed to differences in interfacial strength.

Ｔｉ‐６Ａｌ‐４Ｖ合金の疲労き裂進展特性に及ぼす組織の影響

The characteristics of fatigue crack growth were investigated in Ti-6Al-4V alloy with three kinds of microstructure prepared with different heat treatments. The effect of microstructure on the crack growth behavior was attributed to the development of crack tip shielding, primarily resulting from the role of the crack path morphology in inducing crack closure and crack deflection. Roughness induced crack closure played an important role on the near-threshold growth behavior at the load ratio of 0.05, but the growth rate data plotted in terms of effective stress intensity factor range ΔKeff (after allowing for closure) still exhibited the effect of microstructure. Fractographic observations were performed using a scanning electron microscope (SEM) with the aid of image processing, which enables the three dimensional observation using a stereo pair of SEM micrographs. The roughness was evaluated quantitatively by the ratio of the total area of the three dimensional fracture surface to its projected area. As the fracture surface roughness was taken into account in re-evaluating the crack growth characteristics, the effect of microstructure disappeared, indicating that the intrinsic crack growth resistance was the same for all microstructure. Thus, fracture surface roughness is a controlling parameter of the fatigue crack growth in Ti-6Al-4V alloy. Furthermore, the crack growth rate were compared with those of other structural materials such as steels, nodular cast iron and an aluminum alloy.

Mechanisms of cleavage during fatigue crack growth in Ti6Al4V alloy

Beta processed microstructures of titanium alloys consisting of Widmanstatten structures offer attractive combinations of strength, fracture toughness and resistance to fatigue crack growth (FCG) for damage-tolerance applications [1-3]. This is generally attributed to large prior beta grain sizes incorporating coarse lamellar colonies across the boundaries of which cracks are deflected or deviated [4]. FCG resistance of these microstructures is far superior that of other microstructures owing to increased levels of crack deflection or deviation causing an increase in the intrinsic crack growth resistance, $\Delta K_e_f_f_,_t_h$, as well as crack closure [5]. However, the fatigue crack initiation resistance is lowfor these structures due to the formation of large intense planar slip bands. Growth of fatigue cracks along these bands normal to $\alpha/\beta$ interfaces is easy without interruption at the interfaces and this has been largely attributed [6] to the poor fatigue crack initiation characteristics, tn the present study, some important and unusual fracture features pertaining to the effect of $\beta$ phase in retarding cleavage cracks produced during fatigue crack growth are presented. The factors responsible for these effects are highlighted.

Theoretical modelling of the effects of grain size on the threshold for fatigue crack growth

The dependence of the threshold for fatigue crack growth, ΔK th grain size has been modelled theoretically using fundamental micromechanistic processes of crack extension in polycrystalline materials. Near threshold, fatigue cracks generally propagate along slip bands piled up at the grain boundary. The effect of grain size on ΔK th principally comes through the increase in the intrinsic fatigue threshold, ΔK eff,th due to larger degree of crack deflection in coarse grained structures and the accompanying high levels of crack closure as a consequence of zig-zag crack growth. Using Minakawa and McEvily's description of roughness induced crack closure, the magnitude of closure stress intensity at threshold, K cl,th is quantified. A strong pile-up is formed at the grain boundary before the crack attempts to propagate inside a grain. It is assumed that dislocations pile-up in the forward loading and are largely irreversible during unloading. By assuming a continuously distributed dislocation configuration, the number of dislocations which are in equilibrium with the resolved shear shear long the slip plane can be calculated. The grain boundary sustains the pile-up until the shear stress in the slip band reaches a critical value required for nucleating slip in the adjacent grain. Assuming complete slip irreversibility during unloading, the magnitude of crack flank disregistry due to the interference of asperities just behind the crack tip can be obtained in terms of the number of dislocations and the Burger's vector of the operative slip system. The magnitude of K cl,th can then be expressed in terms of slip length or grain size, macroscopic yield stress, critical resolved shear stress and the angle between slip plane and crack plane. Theoretically predicted closure levels as a function of grain size have been shown to be in good agreement with the experimentally obtained data. The magnitude of ΔK eff,th as a function of grain size can be derived by considering the piled-up slip band as a kinked crack with the length of the kink being equal to the slip band length. The crack propagation is assumed to occur incrementally along the slip band. The intrinsic crack growth resistance is then formed by two contributions, one is the stress intensity necessary to maintain a critically stressed pile-up and the other is that required to incrementally cleave the slip band in each cycle. By assuming K cl,th and ΔK eff,th, ΔK th can be expressed as a function of grain size. A comparison of the theoretically predicted trend with the experimental data for a variety of alloy systems indicate good agreement. The factors responsible for the deviations are discussed.

Near threshold fatigue crack growth behavior of a titanium alloy: Ti-6A1-4V

Near threshold fatigue crack growth behavior of Ti-6A1-4V alloy was investigated as function of Widmanstatten microstructures. In particular, the effect of colony size on ΔK th, ΔK eff,th and K cl,th has been studied. It has been found that crack growth rates are strongly affected by the size of microstructural features such as colonies and α laths. However, the microstructural units controlling crack growth are colonies in fast cooled microstructures consisting of fine Widmanstatten colonies while they are α laths in relatively slow cooled ones with coarse colonies. As a result, the projection in literature of increased fatigue crack growth resistance at large colony sizes in titanium alloys can not be generalized. This distinction appears to be brought about by the thick continuous interplatelet β phase present in slow cooled structures. In fast cooled structures, thin discontinuous β phase is seen to be ineffective in arresting slip or crack. However, in slow cooled ones thick β phase appears to effectively retard slip/crack in fatigue. The thickness, composition, intrinsic properties such as modulus, ductility of β phase and an added environmental effect have been suggested to be important in this respect. The crack growth rates and the magnitudes of ΔK th and ΔK eff,th can be uniquely ordered when compared in terms of the controlling microstructural units in respective microstructures. However, crack closure levels at threshold appear to be dependent on colony size. In addition, an increase in the intrinsic crack growth resistance ΔK eff,th appears to exist when the cyclic plastic zone size approaches the thickness of α laths in the microstructure.

The effect of microstructure on fatigue crack growth behaviors of a low carbon steel and a spheroidal graphite cast iron.

It has been a serious problem whether the microstructure dependence of crack growth arises due to the change in crack closure level or intrinsic crack growth resistance. In order to investigate the effect of microstructure on fatigue crack growth behaviors at the threshold region, three kinds of ferrite base materials were fatigued at room temperature and slightly elevated temperatures. The fatigue results on the low carbon steel can be explained well by the ΔKeff conception, that is, the crack growth resistance is not affected by the structural factors, such as cyclic strain aging and precipitation hardening by low temperature quench aging. On the other hand, the results on spheroidal graphite cast iron cannot be explained only by ΔKeff conception. It may be considered that the graphite particles act as promoters at higher ΔK level and as obstacles at lower ΔK level to crack growth.

The use of non-linear fracture mechanics to study the fracture properties of soils

When forces are applied to soil by natural processes or by mechanical cultivation, it may fracture. This paper shows how methods of elastic-plastic fracture mechanics which, are normally used to characterize the fracture properties of metals, have been used to quantify some of the fracture properties of soils. Experiments have involved the verification and use of an electrical potential method for the measurement of crack length and the development of a method of propagating cracks through very weak materials in a controlled manner. The soils tested ranged from clays in a plastic state, to granular clays stabilized by the addition of ordinary Portland cement. Like tough metals, the soils tested showed a pronounced increase in their intrinsic crack growth resistance ( J R ), as the cracks grew. This increase ceased after extensive crack propagation, a tendency which is also seen in metals.