Influence of stress ratio on the threshold level for fatigue crack propagation in high strength steels

Centrally cracked specimens of JIS SM58Q and HT80 steels were fatigued. The fatigue crack growth rates, da/dn, and the stress intensity threshold levels, ΔK_th were measured over the range of stress ratio, R, from ?1 to 0.8 by the use of an automatic method of continuously decreasing stress intensity factor with crack extension. The measured ΔK_th was well represented as |ΔK_th/2|_R=(1?R)γ|ΔK_th/2|_R=0; and the propagation rate, as da/dn = A(1?R)^γm[(ΔK/2)^m ? {(1 ? R)^γ|ΔK_th/2|R=0}^m] for ?1≦R≦0.33 or da/dn = A(1 ? 0.33)^?γm [(ΔK/2)^m {(1 ? R)^γ |ΔK_th/2|R=0}^m] for 0.33 < ≦ 0.8.     

The cyclic strain resistance, fatigue life and final fracture behavior of magnesium alloys

This paper summarizes the results of a comprehensive study on the cyclic strain resistance, low-cycle fatigue life and fracture behavior of three rapidly solidification processed magnesium alloys. Test specimens of the magnesium alloy were cyclically deformed under fully-reversed total strain amplitude control straining, over a range of strain amplitudes, giving less than 10⁴ cycles to failure. The cyclic stress response characteristics, strain resistance and low-cycle fatigue life of the alloys are discussed in light of alloy composition. All three alloys follow the Basquin and Coffin-Manson strain relationships, and exhibit a single slope for the variation of cyclic elastic and cyclic plastic strain amplitude with reversals-to-fatigue failure. The cyclic stress response characteristics, fatigue life and final fracture behavior of the alloy are discussed in light of competing and synergistic influences of cyclic total strain amplitude, response stress, intrinsic microstructural effects and dislocation-microstructural feature interactions during fully-reserved strain cycling.     

Influence of nano-sized carbon nanotube reinforcements on tensile deformation,cyclic fatigue,and final fracture behavior of a magnesium alloy

Magnesium alloy (AZ31) based metal matrix composite reinforced with carbon nanotubes (CNTs) was fabricated using the technique of disintegrated melt deposition followed by hot extrusion. In this research paper, the microstructure, hardness, tensile properties, tensile fracture, high cycle fatigue characteristics, and final fracture behavior of CNTs-reinforced magnesium alloy composite (denoted as AZ31/1.0 vol.% CNT or AZ31/CNT) is presented, discussed, and compared with the unreinforced counterpart (AZ31). The elastic modulus, yield strength, tensile strength of the reinforced magnesium alloy was noticeably higher compared to the unreinforced counterpart. The ductility, quantified both by elongation-to-failure and reduction in cross-section area of the composite was higher than the monolithic counterpart. A comparison of the CNT-reinforced magnesium alloy with the unreinforced counterpart revealed a noticeable improvement in cyclic fatigue life at the load ratios tested. At all values of maximum stress, both the reinforced and unreinforced magnesium alloy was found to degrade the cyclic fatigue life at a lower ratio, i.e., under conditions of fully reversed loading. The viable mechanisms responsible for the enhanced cyclic fatigue life and tensile behavior of the composite are rationalized in light of macroscopic fracture mode and intrinsic microscopic mechanisms governing fracture.     

On the embrittlement of fracture toughness specimens of two high strength steels

Hydrogen embrittlement of two commercial materials, a 1$\text{1}\text{2}$ Ni-Cr-Mo steel and a 5 Cr-V-Mo steel both quenched and tempered to a nominal ilrength of 1.60 × 10³MN/m² has been studied. Frecture toughness testing was carried out on each steel in three conditions, (a) heat-treated and tested in air, (b) thermally charged with hydrogen and tested in air, and (c) uncharged and tested a an atmosphere of dry hydrogen. The effects of hydrogen pressure and loading rate on embrittlement were examined. The experimental results show that thermal charging had a greater embrittling effect on the 1$\text{1}\text{2}$ Ni-Cr-Mo steel than on the 5 Cr-V-Mo steel although the latter had a higher hydrogen concentration. In a hydrogen environment the 1$\text{1}\text{2}$ Ni-Cr-Mo steel was much more sensitive to embrittlement than the 5 Cr-V-Mo steel.     

Effect of stress ratio on the mechanism of environmentally-assisted fatigue crack growth in high strength steels

The environmentally-assisted fatigue crack growth (FCG) behavior of high strength steels, the effect of cathodic potential on the FCG rates, and fracture surfaces were examined in some aqueous solutions. The effect of stress ratio on the critical values of maximum stress intensity, as well as cyclic stress intensity range and da/dN, which define quantitatively the role of hydrogen-induced cracking and local anodic dissolution in crack propagation, were also examined.Published inProblemy Prochnosti, Nos. 1–2, pp. 94–102, January–February, 1995.     

Multiphase microstructure formation and its effect on fracture behavior of medium carbon high silicon high strength steel

This work investigated the evolution of multiphase microstructure and impact fracture behavior of medium carbon high silicon high strength steel subjected to the austempering treatment at 240,360,and 400 ℃.The results show that martensite,bainite,and retained austenite (RA) are the main microstructural phases.The austempering treatments at 360 and 400 ℃ caused the formation of carbon-poor ferrite in the matrix,and the transformation of ultrafine bainite into coarse lath bainite and granular bainite,respectively.Thick filmy RA was distributed between bainite laths.The polygonal martensiteaustenite islands and blocky RA formed along the grain boundaries.The average carbon concentration in the matrix decreased with the temperature increase,while the impact toughness initially increased and then dropped with temperature.The quasi-cleavage brittle fracture dominated the impact fracture mechanism of the sample austempered at 240 ℃ by forming tearing surfaces and tearing steps.The microcracks disappeared in the RA on the prior austenite grain boundaries.On the other side,the fracture surface of the sample austempered at 360 ℃ exhibited ductile fracture with deep dimples and brittle fracture with cleavage river patterns.The polygonal martensite-austenite islands or blocky RA constrained the microcracks.After austempered at 400 ℃,the brittle fracture was dominant,showing river patterns,and the microcracks propagated through the granular bainite without any resistance.     

Influence of the mode of thermal treatment and load ratio on the cyclic crack-growth resistance of wheel steels

We study the microstructure, mechanical properties, and micromechanism of fatigue fracture of high-strength railway-wheel steel (0.63% C and 0.1% V). The necessity of elevation of the cyclic fracture toughness of this type of steel by isothermal hardening and tempering at 500°C is substantiated. It is shown that the cyclic crack-growth resistance of steel decreases for high loading amplitudes as the load ratio (simulating the influence of residual thermomechanical stresses) increases.     

Near-threshold fatigue crack propagation of several high strength steels

In support of Douglas Aircraft Co. structural damage tolerance analysis, crack propagation rates were measured in humid air from 10^?8 to 10^?3 mm/cycle of several high strength martensitic steels at stress ratios of $\text{R = 0.1}$ and $\text{R = 0.5}$. Increasing the stress ratio was found to decrease the threshold stress intensity. Increasing the tensile yield material strength in these quench and tempered steels resulted in a nearly linear inverse relationship with the threshold stress intensity. The threshold stress intensity appeared to be independent of the fracture toughness of the alloy steel.     

Influence of directionality on strength and impact behaviour of high strength steels

Abstract

The directionality of the strength and impact behaviour of control rolled 110 grade high strength steel has been examined. Tensile specimens tested along the rolling direction were found to give strengths of ～60 MPa lower than when tested transversely in the rolling direction. Charpy V notch impact samples, in contrast, gave lower impact transition temperatures when taken in the rolling direction (35°C lower). The difference in strength between the two directions is believed to be mainly as a result of the strong texture produced on control rolling. The difference in impact behaviour is due to the more severe fissuring on the fracture surface of the longitudinally tested specimens. Fissuring is more marked on the longitudinally tested sample because of the greater grain elongation in that direction, the initial fracture path being along the grain boundaries.     

Microstructural features affecting fracture toughness of high strength steels

The fracture toughness of quenched and tempered steels, such as AISI 4340, AISI 4130 and 300M, can be increased by 50–100% by minor changes in heat treating procedures. Certain microstructural features, particularly blocky ferrite, upper bahnte and twinned martensite plates, are deleterious to fracture toughness. Similarly, the presence of undissolved carbides and sulfide inclusions, which act as crack nuclei, can lower fracture toughness by 25–50%. Other microstructural constituents, such as lower bainte, autotempered martensite, and retained austenite can enhance fracture toughness. By controlling the amounts and distributions of the microstructural constituents, the fracture toughness values of AISI 4340, AISI 4130 and 300M can be raised to the fracture toughness level of 18Ni maraging steel at equivalent values of yield strength.     

Mechanism of corrosion fatigue crack propagation in high strength steels

The crack propagation velocity in corrosion fatigue $\text{(d a/d N)}{}_{\mathrm{c}}$ were measured on the Ni-Cr-Mo steel quenched and tempered at 473 or 773 K.The steel with high sensitivity to delayed failure reveals the largest $\text{(d a/d N)}{}_{\mathrm{c}}$ under square load and the smaller $\text{(d a/d N)}{}_{\mathrm{c}}$ under positive saw tooth load. The frequency dependency of crack propagation characteristics indicates that the interaction between hydrogen atoms and the cyclic moving of triaxial position at crack tip acts an important role in the crack propagation mechanism, i.e. hydrogen concentration process controls the crack propagation of the steel.The steel with low susceptibility to delayed failure reveals, on the other hand, the largest $\text{(d a/d N)}{}_{\mathrm{c}}$ under the positive saw tooth load but the smallest $\text{(d a/d N)}{}_{\mathrm{c}}$ under the square load, i.e. the stress increasing time is important and the hydrogen invasion process is the controlling factor for the crack propagation.     

On effect of hydrogen on very high cycle fatigue behaviours of high strength steels

Abstract

In order to analyse the effect of hydrogen on very high cycle fatigue properties, hydrogen was precharged into two high strength steels. The applied stress intensity factor range at the periphery of inclusions before and after being precharged is approximately proportional to the cubic root of inclusion size. In addition, the applied stress intensity factor range at the periphery of inclusions after being precharged was lower compared with uncharged specimens. The additional stress intensity factor range generated by hydrogen ΔK_H is raised after the hydrogen was precharged. A simple prediction equation of S–N curve was proposed by introducing the hydrogen influence factor. The proposed prediction equation can reasonably describe the S–N curves for precharged specimens.     

Effect of slid-roll ratio on the contact fatigue behavior of sintered and hardened steels

Powder metal processed gears and bearings find increasing application in automotive and other engineering industries. Failure of these machine elements is mostly due to the presence of sliding action in addition to the prevailing rolling action during transmission of motion and power. This rolling-sliding contact fatigue behavior of sintered and hardened steel rollers was investigated using a test machine designed and developed in the laboratory. Sintered and hardened steel rollers were mated against the hardened wrought tool steel roller at different slide-roll ratios and contact stresses. Contact stress versus number of cycles-to-failure data was generated under drop lubrication conditions. An increase in slideroll ratio significantly affected the life of the rollers at all contact stress levels investigated. Failed surfaces were analyzed using the optical microscope. Significant peeling-type failure was observed in the specimens tested, but no marked difference in the failure appearance was noted due to the introduction of traction forces.     

Early detection and monitoring of fatigue in high strength steels with MWM-Arrays

Fatigue monitoring of cyclically loaded shot peened high-strength steel components can be accomplished via magnetic permeability measurements during laboratory tests or in service. These measurements can be performed either continuously using permanently mounted Meandering Winding Magnetometer Arrays (MWM^®-Arrays) or intermittently with scanning MWM-Arrays. The results obtained to date suggest that MWM-Array permeability measurements can provide early detection of fatigue damage in steels before conventional methods can detect any changes. This has been demonstrated to be particularly significant in the presence of high compressive stresses introduced by shot peening. One of the fatigue tests was suspended when accelerating changes in local permeability were detected. Examination of the fatigue specimen in a scanning electron microscope detected only a few relatively small cracks, e.g. 50–200 μm long at the surface. Fractography, however, revealed significantly longer cracks. For the same specimen, conventional eddy current and ultrasonic testing failed to provide any indications of cracks, and fluorescent liquid penetrant detected only an inconclusive spot indication. This paper provides a comparison of the permeability changes and fractography data with a fatigue crack growth curve based on a FASTRAN analysis accounting for residual stresses from shot peening. A comparison of the experimental data and crack growth analysis results suggested that MWM-Array magnetic permeability measurements may detect cracks in the compressive stress field when they are about 50 μm deep.     

Comparison of mode I and mode II elastic-plastic fracture toughness for two low alloyed high strength steels

In the present work, mode I and mode II tests were carried out on two low alloyed high strength steels. An asymmetrical four point bend specimen and J _II-integral vs. crack growth resistance curve technique were used for determining the mode II elastic-plastic fracture toughness, J _IIc · J _II-integral expression of the specimen was calibrated by finite element method. The results indicate that the present procedure for determining the J _IIc values is easy to use. Moreover, the mode I fracture toughness J _Ic is very sensitive to the rolling direction of the test steels, but the mode II fracture toughness J _IIc is completely insensitive to the rolling direction of the steels, and the J _IIc /J _Ic ratio is not a constant for the two steels, including the same steel with different orientations. Finally, the difference of the fracture toughness between the mode I and mode II is discussed with consideration of the different fracture mechanisms.