Effects of hydrogen on the mixed mode I/III toughness of a high-purity rotor steel

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Effects of hydrogen on the mixed mode I/III toughness of a high-purity rotor steel

J integral tests, both pure mode I and mixed mode I/III in character, were performed on a high-purity Ni-Cr-Mo-V rotor steel. The steel was tested in the uncharged condition and after gaseous charging with hydrogen to a level of 2 at. ppm. Hydrogen degraded the J integral toughness incrementally by approximately 30 pct with the degree of degradation increasing mode III loading component. The results are discussed in terms of a model in which, augmenting the effect of the mode III loading, hydrogen further increases the tendency for plastic instability in the form of shear bands. An erratum to this article is available at .     

Mixed mode I/III fracture toughness of an experimental rotor steel

Mixed mode I/III toughness tests were performed on a Ni-Cr-Mo-V rotor steel that exhibited very high mode I toughness. The minimum toughness occurred under mixed mode loading conditions. The influence of mode III loading on total toughness is analyzed in terms of incompatibility effects at particles ahead of the crack tip.     

Combined mode I-mode III fracture toughness of a spherodized 1090 steel

The aim of this investigation was to determine the fracture behavior of a spherodized 1090 steel under combined mode I-mode III loading conditions. Suitably defined formulations of the J integral denoted J_ic and J_iiic were used to characterize the elastic-plastic fracture of this steel. As the mode III component in the system is increased, the resolved mode I J integral at initiation decreases, its mode III counterpart increases and the total J value remains nearly a constant. This implies a constant energy requirement for fracture initiation under mixed mode loading. As the crack plane becomes less inclined to the load line, the slopes of the mode I and total J resistance curves increase from their pure mode I values until a crack inclination angle of about 65° is reached. Somewhere in the region of 65-55°, a maximum in these values is reached and they fall off rapidly for larger mode III components. This drop is accompanied by the breakup of the crack front into mode I and mode III steps, which is shown to be an energetically more favorable process for this steel.     

Mixed mode I/III fracture toughness of 2034 aluminum alloys

A study of the mixed mode I and III fracture resistance of four 2034 aluminum alloys with varying manganese content is presented in this paper. The mixed mode fracture tests are carried out using modified compact tension specimens. As there is no standard test method for mixed mode fracture, special formulations of the J integral are used to characterize the mixed mode fracture resistance. The results indicate that the overall effect of mode III shear component is to lower the critical J-integral energy by enhancing the tendency for shear instability and early void formation. Manganese present in small amounts forms intermediate size dispersoids which increase the strength and work hardening ability without the loss of fracture toughness. In larger amounts manganese forms large particles which lower the fracture toughness significantly. These micromechanisms and those of mixed mode fracture are discussed.     

Effect of loading mode on the fracture toughness of a ferritic/martensitic stainless steel

The critical J integrals of mode I (J_IC), mixed-mode I/III (J_TC), and mode III (J_IIIC) were examined for a ferritic stainless steel (F-82H) at ambient temperature. A determination of J_TC was made using modified compact-tension specimens. Different ratios of tension/shear stress were achieved by varying the principal axis of the crack plane between 0 and 55 deg from the load line. The value for J_IC was determined by means of specially designed specimens. The results showed that F-82H steel had high fracture toughness. Both J_IC and J_IIIC were about 500 kJ/m₂, and the mode I tearing modulus J_Ida) was about 360 (kJ/m²)/mm. However, J_TC and mixed-mode tearing modulus (dJ_T/da) values varied with the crack angles and were lower than their mode I and mode III counterparts. Both the minimum J_TC and dJ_T/da values occurred at a crack angle between 40 and 50 deg, at which the load ratio of σ_iii/σ, was 0.84 to 1.2. The J_min was 240 kJ/m², and ratios of J_lC/J_min and J_IIICJ_min were 2.1 and 1.9, respectively. The morphology of the fracture surfaces was consistent with the change in J_TC and dJ_T/da values. While the upper shelf-fracture toughness of F-82H depended on loading mode, the J_min value remained high. Other important considerations include the effect of mixed-mode loading on the ductile-brittle-transition temperature and effects of hydrogen and irradiation on J^.     

Fracture behavior of quaternary Al-Li-Cu-Mg alloy under mixed mode I/III loading

The fracture toughness under mixed-mode I/III loading conditions was evaluated for a quaternary 8090 Al-Li-Cu-Mg alloy in underaged and peak-aged conditions. The mixed-mode fracture behavior was found to be significantly different for the two aging conditions. Super-imposed mode III component lowered the fracture resistance of the alloy in underaged condition, whereas it had no significant effect in peak-aged condition. The results obtained have been discussed in the light of the prevalent fracture processes, namely, transgranular shear and ductile intergranular fracture mechanisms. Further, these results are analyzed in terms of different frac-ture criteria and they were found to deviate significantly from those predicted by the energy release rate criterion. On leave from the Defense Metallurgical Research Laboratory, Hyderbad     

Ductile fracture in HY100 steel under mixed mode I/Mode II loading

A number of criteria have been proposed which predict the direction of cracking under mixed Mode I/Mode II loading. All have been evaluated for brittle materials, in which a crack subjected to tension and shear propagates normal to the maximum tensile stress (i.e. fracture is of the Mode I type). In a ductile material, however, a notch subjected to mixed Mode I/Mode II loading may initiate a crack in the direction of maximum shear. This paper shows that the profile of the notch tip changes with increasing mixed mode load in such a way that one side of the tip blunts while the other sharpens. Various specimens, subjected to the same mixed mode ratio, were unloaded from different points on the load-displacement curves to study the change in notch-tip profile. Studies under the Scanning Electron Microscope (SEM) have shown that cracks initiate at the sharpened end, along a microscopic shear band. Using a dislocation pile-up model for decohesion of the carbide-matrix interface, a micromechanical model has been proposed for crack initiation in the shear band. It is shown that a theoretical prediction of the shear strain required for decohesion gives a result that is, of magnitude similar to that of the shear strain at crack initiation measured in the experiments.     

Effects of microstructure on fracture toughness of a high-strength low-alloy steel

Fracture toughness and ductile-brittle transition behavior were measured for a copper-bearing HSLA steel. The value ofK _lc for cleavage failure was independent of heat treatment, whileJ _lc for ductile failure decreased monotonically with increasing strength level. With both failure modes, fracture appears to be controlled by cracking of sulfide inclusions. The decrease in ductile-failureJ _lc is caused by decreased work-hardening rates that suppress cleavage and facilitate void coalescence. Both higher austenitizing temperature and quenching rate after austenitization influence the ductile/brittle transition temperature, either through grain-size and precipitate refinement or through an increase in the resistance of the steel to shear failure. Formerly Graduate Student, The Ohio State University     

晶粒尺寸对车轮钢解理断裂韧性的影响

通过紧凑拉伸试验研究了碳的质量分数约为0.5%的C50车轮钢解理断裂韧性K_IC(即条件断裂韧性K_Q)与晶粒尺寸的关系.结果表明,晶粒尺寸对试样的断裂韧性有明显的影响,但决定车轮钢解理断裂韧性的是组织中最大的晶粒尺寸,而不是平均晶粒尺寸,最大晶粒尺寸越大,断裂韧性越低.对于C50车轮钢,当前5%的最大晶粒平均尺寸为30~73μm时,车轮钢的条件断裂韧性K_Q与晶粒平均尺寸的对数呈线性关系.     

The equilibrium concentration of hydrogen atoms ahead of a mixed mode I-Mode III crack tip in single crystal iron

Calculations of the equilibrium hydrogen concentration profiles about a mixed mode I-mode III crack in single crystal iron were performed. Both material anisotropy and the tetragonal nature of the distortion induced in the iron crystal structure by interstitial hydrogen were incorporated. Results show that, unlike the case of a spherical distortion, a strong coupling exists between the strain field of the interstitial hydrogen and the stress field of the crack for orientations of the crack plane that are not coincident with the cube axes of the lattice. As a result, the predicated enhancement of hydrogen in the crack tip region increases with increasing levels of mode III loading for those orientations. The results may help reconcile conflicting observations concerning the potential role of shear stresses in hydrogen embrittlemet and preferential cracking of grains ahead of loaded crack tips in sustained load cracking experiments.     

Effects of tensile prestrain on the notch toughness of low-alloy steel

Tensile prestrains of various levels were applied to blank steel specimens. Four-point bend tests of notched specimens at various temperatures revealed an appreciable drop in the notch toughness of the specimens, which experienced 3 pct tensile prestrain. Further prestrains of up to 20 pct had a negligible effect on the notch toughness despite additional increases in the yield strength. Microscopic analyses combined with finite element method (FEM) calculations revealed that the decrease in toughness resulted from a change of the critical event controlling the cleavage fracture. The increase in yield strength provided by prestraining allowed the normal tensile stress at the notch tip to exceed the local fracture stress σ _f for propagating a just-nucleated microcrack. As a result, for the coarsegrained steel with low σ _f tested presently, the critical event was changed from tensile stress-controlled propagation of a nucleated microcrack to plastic strain-controlled nucleation of the microcrack at the notch tip. A reduction of toughness was induced as a result of this. The increase in yield strength provided by decreasing the test temperature acted in the same way.     

Combined mode I-mode III fracture of a high strength low-alloy steel

Special formulations of theJ integral were used to quantify the combined mode I-mode III fracture behavior of ASTM A710 Grade A steel, a tough material which displays elastic-plastic behavior at room temperature. Experimental fracture initiation loci for two heat-treated conditions were linear inJ _i-J_iii space, agreeing with analytical predictions based on linear elasticity. As commonly observed in mode I fracture behavior, large tearing modulus values accompanied largeJ values for mode I and mode III components, individually as well as for totalJ values. There was a pronounced tendency toward antiplane shear flow in the modified compact tension specimens tested. This may result from the concentration of mode III shear strains in the plane of the propagating crack as predicted by slip line theory. Shear fractures with the crack propagating at inclined angles to both load line and plate free surfaces are favored energetically over pure mode I cracks even though the total surface area formed by the latter is much smaller.     

Combined mode I-mode III fracture of a high strength low-alloy steel

Special formulations of theJ integral were used to quantify the combined mode I-mode III fracture behavior of ASTM A710 Grade A steel, a tough material which displays elastic-plastic behavior at room temperature. Experimental fracture initiation loci for two heat-treated conditions were linear inJ _i-J_iii space, agreeing with analytical predictions based on linear elasticity. As commonly observed in mode I fracture behavior, large tearing modulus values accompanied largeJ values for mode I and mode III components, individually as well as for totalJ values. There was a pronounced tendency toward antiplane shear flow in the modified compact tension specimens tested. This may result from the concentration of mode III shear strains in the plane of the propagating crack as predicted by slip line theory. Shear fractures with the crack propagating at inclined angles to both load line and plate free surfaces are favored energetically over pure mode I cracks even though the total surface area formed by the latter is much smaller. formerly Graduate Research Assistant, The Ohio State University     

Ce对00Cr17高纯铁素体不锈钢再结晶及晶粒长大的影响

利用显微硬度计、金相显微镜、差热分析仪和XRD研究了稀土元素Ce对00Cr17高纯铁素体不锈钢再结晶及晶粒长大的影响.结果表明,00Cr17钢中加入Ce降低了再结晶温度,促进了再结晶的发生.主要是由于微量Ce固溶到00Cr17钢中引起晶格畸变,增加了冷变形储存能,从而增大了00Cr17钢的再结晶驱动力.00Cr17钢中加入Ce增加了晶粒长大激活能,减小了晶粒生长指数,阻碍了00Cr17钢的晶粒长大,这是由于Ce偏聚到晶界,增大晶界运动的阻力.     

Effects of hydrogen on the properties of iron and steel

The effects of hydrogen on the physical and mechanical properties of iron and steel are reviewed. A new mechanism for the cold work peak for hydrogen in iron is considered. Together, internal friction and mechanical properties indicate that hydrogen softens iron by enhancing screw dislocation mobility at room temperature but hardens iron by core interactions at low temperatures. No single mechanism exists for the degradation of the properties of steel by hydrogen. Instead a complex process involving many of the proposed mechanisms as contributing factors is shown to account for most degradation phenomena.     

Hardenability of a high-purity 0.3 % Mo eutectoid steel

Determination of the hardenability in 0.3 % Mo and 0 % Mo steels of eutectoid compositions using quantitative standard methods. Slight effect on the Ae₁ temperature, but increase of the incubation period and the completion of austenite decomposition to pearlite by addition of 0.3 % Mo. Evidence by Jominy end quench tests that the addition of molybdenum increases the hardenability of steel.