Value Of Stress Intensity Range Research Articles

Hydrogen-assisted fatigue crack propagation in a pure BCC iron. Part II: Accelerated regime manifested by quasi-cleavage fracture at relatively high stress intensity range values

Hydrogen effect on fatigue performance at relatively high values of stress intensity factor range, ΔK, of pure BCC iron has been studied with a combination of various electron microscopy techniques. Hydrogen-assisted fatigue crack growth rate is manifested by a change of fracture features at the fracture surface from ductile transgranular in air to quasi-cleavage in hydrogen gas. Grain reference orientation deviation (GROD) analysis has shown a dramatic suppression of plastic deformation around the crack wake in samples fatigued in hydrogen. These results were verified by preparing site-specific specimens from different fracture features by using Focused Ion Beam (FIB) technique and observing them with Transmission Electron Microscope (TEM). The FIB lamella taken from the sample fatigued in air was decorated with dislocation cell structure indicating high amount of plasticity, while the lamella taken from the quasi-cleavage surface of the sample fatigued in hydrogen revealed a distribution of dislocation tangles which corresponds to smaller plastic strain amplitude involved at the point of fracture. These results show that a combination of critical hydrogen concentration and critical stress during fatigue crack growth at high ΔK values triggers cleavage-like fracture due to reduction of cohesive force between matrix atoms.

Statistical fatigue properties and small fatigue crack propagation in bimodal harmonic structured Ti-6Al-4V alloy under four-point bending

Small fatigue crack propagation in bimodal harmonic structured titanium alloy (Ti-6Al-4V) with high strength and ductility was examined under four-point bending at a stress ratio of 0.1 in the ambient laboratory atmosphere. The crack profiles were observed using optical microscopy and scanning electron microscopy, and analyzed using an electron backscattered diffraction to examine the mechanism of small fatigue crack propagation. Fatigue crack paths were not influenced by the bimodal harmonic structure, and the crack growth rates, da/dN, in the harmonic structured Ti-6Al-4V were almost the same as those in a material with coarse acicular microstructure for comparable values of stress intensity range, ΔK. In contrast, the harmonic structured Ti-6Al-4V had a higher resistance of fatigue crack initiation due to the grain refinement induced by mechanical milling, which resulted in an increase of the fatigue life and fatigue limit. Furthermore, the statistical fatigue properties of Ti-6Al-4V alloy were analyzed using the stress dependence of Weibull parameters to quantitatively examine the effects of the bimodal harmonic structure on its fatigue life.

Experimental Investigation of Fatigue Crack Growth in the Welding Nugget of FSW Joints of a 6060 Aluminum Alloy

The work investigates the fatigue behavior of friction stir welded butt joints by means of fracture mechanics techniques. FSW joints of artificially aged AA6060 T6 aluminum alloy were studied. The welding was performed on 8 mm thick butt joined sheets by means of a CNC machine tool. Welding speed in the range between 117 and 683 mm/min and tool rotational speed between 838 and 1262 rpm were considered. Fatigue crack growth tests were performed according to ASTM E647 standard on CT specimens, under constant load amplitude conditions, at 0.1 minimum to maximum load ratio, with the notch placed in the dynamic recrystallization zone of weld nugget, oriented along the welding direction. The comparison of results demonstrates the crack growth rate is always equal or lower than the base material at low values of stress intensity range. At ΔK values above 12 MPa√m, crack growth rate was found to be higher than base material for high feed, low speed and high feed/speed ratio.

Fatigue Crack Propagation Behavior of an Inertia Friction Welded α/β Titanium Alloy

Abstract The inertia friction welding process is being extensively investigated for the joining of high strength titanium alloys for aerospace applications. Although it offers solid state joining, the thermal cycle and deformation involved results in microstructural inhomogeneity across the weld interface. In this paper, the fatigue crack propagation behavior in an inertia welded microstructure in a high strength, high temperature α/β titanium alloy is considered. The fatigue crack propagation behavior in corner notched weld specimens at varying stress ratios is studied at room and elevated temperatures and compared with that of the parent material. Fatigue crack growth rates at lower stress intensity ranges are comparable with those in the parent material. However, in weld specimens tested at room temperature, unstable crack growth occurs at lower stress intensity range values compared to that at high temperature. Fracture surface observations show that this difference is related to a change in fracture mode from transgranular to intergranular/mixed mode during room temperature tests. This change in fatigue crack growth mechanism is deduced to be due to low ductility intergranular failure of grain boundary α in the refined transformed beta microstructure across the weld interface.

Analysis of the effects of exfoliation corrosion on the fatigue behaviour of the 2024-T351 aluminium alloy using the fatigue damage map

The concept of the Fatigue Damage Map (FDM) is implemented to quantify the effects of exfoliation corrosion damage on the fatigue behaviour of the 2024-T351 aluminium alloy. This is achieved by using extensive experimental data involving tensile, fracture toughness, fatigue as well as fatigue crack growth tests on pre-exfoliated 2024-T351 specimens. The analysis suggests that exfoliation exposure leads to an increase in accelerated crack growth stage (Stage III) at the expense of Stage II (long crack growth). It also suggests that the transition from Stage I crack growth (short crack) to Stage II for the same stress level occurs at smaller crack lengths for the exfoliated material. Furthermore, stress intensity range values for onset of crack growth rate acceleration (Stage III) obtained from the experimental results for the pre-corroded specimens are in accordance with the results of the FDM analysis. The outcome of the analysis, demonstrates that the a priori negligence of the corrosion effect in structural integrity analyses of aged aircraft components may lead to significant overestimation of the damage tolerance ability of the structure.

A study on fatigue crack growth in dual phase martensitic steel in air environment

Dual phase (DP) steel was intercritically annealed at different temperatures from fully martensitic state to achieve martensite plus ferrite, microstructures with martensite contents in the range of 32 to 76%. Fatigue crack growth (FCG) and fracture toughness tests were carried out as per ASTM standards E 647 and E 399, respectively to evaluate the potential of DP steels. The crack growth rates (da/dN) at different stress intensity ranges (ΔK) were determined to obtain the threshold value of stress intensity range (ΔKth). Crack path morphology was studied to determine the influence of microstructure on crack growth characteristics. After the examination of crack tortuosity, the compact tension (CT) specimens were pulled in static mode to determine fracture toughness values. FCG rates decreased and threshold values increased with increase in vol.% martensite in the DP steel. This is attributed to the lower carbon content in the martensite formed at higher intercritical annealing (ICA) temperatures, causing retardation of crack growth rate by crack tip blunting and/or deflection. Roughness induced crack closure was also found to contribute to the improved crack growth resistance at higher levels of martensite content. Scanning electron fractography of DP steel in the near threshold region revealed transgranular cleavage fracture with secondary cracking. Results indicate the possibility that the DP steels may be treated to obtain an excellent combination of strength and fatigue properties.

反応焼結ＴｉＡｌ金属間化合物の長い疲労き裂および切欠き底からの微小疲労き裂伝ぱ挙動

Smooth and notched specimens of reactive-sintered Ti-45at%Al-1.6at%Mn intermetallic compound with lamellar and equiaxed γ microstructures were fatigued under cyclic axial tension compression at room temperature. The effect of microstructures on the crack propagation behavior was investigated in comparison with cast Ti-47.2at%Al with larger grain size.Fatigue crack paths were tortuous in the lamellar microstructure in contrast to the straight crack paths in the equiaxed γ microstructure. In comparison with cast TiAl, reactive-sintered TiAl exhibited higher crack propagation rates when compared at the same stress intensity range. The values of stress intensity range at threshold were greater in cast TiAl. Those threshold values had a tendency to increase with the roughness of the fracture surface. The rate of increase in crack opening stress intensity factor at threshold with non-propagating crack length was smaller than that of cast TiAl. Predicted fatigue limits for crack initiation and propagation based on the fatigue-crackgrowth resistance-curve method were in agreement with the experimental results. The fatigue limits of notched specimens were smaller than those of cast TiAl in spite of the superior mechanical properties of reactive-sintered TiAl.

Fatigue threshold in steels—Mean stress and microstructure influences

The purpose of the present paper was (i) to closely investigate the effect of R-ratio on a large variety of microstructures commonly found in low C steels and (ii) to assess the predictability of a selection of approaches or models which deal specifically with R-ratio effects on threshold fatigue stress intensity range values, ΔK th . Essentially the extent of the R-ratio response on the ΔK th was found to be significantly microstructure dependent but the trends were by no means simple. Generally the predictions of the various models dealing with R-ratio-ΔK th trends exhibited poor commonality with the experimentally determined ΔK th values. Marked complex yield strength σ y effects on ΔK th were observed at low R-ratio levels ( R ⋍ 0 ), viz. (a) ΔK th levels initially increased with σ y , (b) attained a maximum ΔK th value of 9 MPa✓m which remained constant with increasing σ y level over the range 300–500 MPa and (c) above σ y ⋍ 600 MPa ΔK th decreased with σ y .

Role of reinforcement/matrix interfacial strength in fatigue crack propagation in particulate SiC reinforced aluminium alloy 8090

A study has been made of the influence of the reinforcement/matrix interfacial strength on fatigue crack propagation in a powder metallurgy aluminium alloy 8090–SiC particulate composite. The interfacial region has been altered by two separate routes, the first involving aging of the 8090 matrix, with the subsequent formation of precipitate free zones at the boundaries, and the second consisting of oxidising the surface of the SiC particles before their incorporation into the composite. In the naturally aged condition, oxidation of the SiC leads to a reduction in fatigue crack growth resistance at higher values of stress intensity range ∆K. This is due to a proportion of the crack growth occurring through voids formed in association with many of the weak SiC interfaces which have retained a layer of thick surface oxide after processing. On overaging no difference in crack growth rate is discernible between the oxidised and unoxidised SiC composites. It is proposed that this is due to similar levels of interfacial weakening having occurred in both composites, indicating that this is an important factor in the reduction of the high ∆K crack growth resistance of the unoxidised SiC composite on aging.MST/1605

The influence of mean stress or [formula omitted] on the fatigue crack threshold characteristics of steels—A review

The present review attempts to describe an assessment of the effects of the R- Ratio on ΔK th values in steels and compares the predictions of the various approaches or models dealing with R- Ratio response, with experimentally determined threshold stress intensity range values. Essentially significant R- Ratio effects on ΔK th have been observed and that the extent of such effects are highly dependent on microstructural considerations. The two classical approaches dealing with R- Ratio effects, viz., the Barsom and the Klesnil and Lukas models, can adequately describe the R- Ratio response when the data are considered in terms of microstructural characteristics. However, the other approaches of Mazumdar and Conrad and McEvily and Groeger fall short in predictive terms while the Musuva and Radon approach needs more detailed high R- Ratio ΔK th data before it can be realistically assessed.

Crack deflection: Implications for the growth of long and short fatigue cracks

The influences of crack deflection on the growth rates ofnominally Mode I fatigue cracks are examined. Previous theoretical analyses of stress intensity solutions for kinked elastic cracks are reviewed. Simple elastic deflection models are developed to estimate the growth rates of nonlinear fatigue cracks subjected to various degrees of deflection, by incorporating changes in the effective driving force and in the apparent propagation rates. Experimental data are presented for intermediate-quenched and step-quenched conditions of Fe/2Si/0.1C ferrite-martensite dual phase steel, where variations in crack morphology alone influence considerably the fatigue crack propagation rates and threshold stress intensity range values. Such results are found to be in good quantitative agreement with the deflection model predictions of propagation rates for nonlinear cracks. Experimental information on crack deflection, induced by variable amplitude loading, is also provided for 2020-T651 aluminum alloy. It is demonstrated with the aid of elastic analyses and experiments that crack deflection models offer a physically-appealing rationale for the apparently slower growth rates of long fatigue cracks subjected to constant and variable amplitude loading and for the apparent deceleration and/or arrest of short cracks. The changes in the propagation rates of deflected fatigue cracks are discussed in terms of thelocal mode of crack advance, microstructure, effective driving force, growth mechanisms, mean stress, slip characteristics, and crack closure.

50-Fold Difference in Region-II Fatigue Crack Propagation Resistance of Titanium Alloys: A Grain-Size Effect

Fatigue crack growth rates (da/dN) in ambient laboratory air have been determined for a wide variety of materials from four basic α + β titanium alloy systems. Each material was cyclically loaded with a haversine waveform and a load ratio, R = 0.10. The results indicate that, at a constant value of stress-intensity range (ΔK), the width of the da/dN data band exceeds an order of magnitude. For example, at ΔK = 21 MPa·m1/2, a 50-fold difference in fatigue crack propagation rates is observed. Analysis of the crack growth rate data at this point indicates a systematic dependence on grain size (l), viz. that da/dN decreases with increasing l. An interpretation of this effect is offered in terms of reversed (cyclic) plastic zone size considerations.

Investigation of the Effect of Vacuum Environment on the Fatigue and Fracture Behavior of 7075–T6

Axial-load fatigue-life, fatigue-crack-propagation, and fracture-toughness experiments were conducted on sheet specimens made of 7075-T6 aluminum alloy. These experiments were conducted at air pressures ranging from 101kN/m2 to 7 μN/m2 to determine the effect of air pressure on fatigue behavior. Analysis of the results from the fatigue-life experiments indicated that for a given stress level, the lower the air pressure was the longer the fatigue life. At a pressure of 7 μN/m2, fatigue lives were 15–30 times longer than at 101 kN/m2. Analysis of the results from the fatigue-crack-growth experiments indicates that at low values of stress-intensity range, the fatigue-crack-growth rates were approximately twice as high at atmospheric pressure as in vacuum. However, at higher values of stress-intensity range, the fatigue-crack-growth rates were nominally the same in vacuum and at atmospheric pressure. An empirical equation developed by Forman, Kearney, and Engle was fitted to the crack-propagation data using least-squares techniques. This equation gave an excellent fit to the data generated both in vacuum and at atmospheric pressure. Analysis of the fracture-toughness data showed there was essentially no difference between the fracture toughness of 7075-T6 in vacuum and at atmospheric pressure

Estimation Of Local Cyclic Stress Ratio Due To Confined Notch Plasticity

Estimation Of Local Cyclic Stress Ratio Due To Confined Notch Plasticity