Fracture mechanical properties in controlled rolled CMn thermomechanically treated steels with splittings

Fracture mechanical properties of a thermomechanically treated CMn steel were investigated in both longitudinal and transverse directions relative to the rolling direction. The CTOD fracture toughness testing was performed at three deformation rates in the temperature range from −60 to +40°C. Fracture initiation was investigated at room temperature. The CTOD fracture toughness depended very much on the specimen orientation with respect to sulphide inclusions. In the transverse specimens, maximum load was reached just after yielding, hence the recorded CTOD_m values were nearly independent of the rate of deformation and testing temperature.     

钙处理对AH36船体钢耐点蚀性能的影响

为提高AH36钢的耐海水腐蚀性能,炼钢时引入了钢水钙处理工艺.本文通过室内间浸挂片试验、交流阻抗试验、动电位极化试验及极化试验后的SEM分析,研究了钙处理对AH36钢中的夹杂物及耐点蚀性能的影响.结果表明:变性夹杂物中的CaS对钙处理钢的耐点蚀性能有明显的影响.通过钙处理把钢中氧化铝夹杂、硅铝酸盐夹杂等球化变性成无CaS或低CaS的球状CaO-Al2O3复合夹杂,可有效提高钢的耐点蚀性能;变性球状复合夹杂中含较多的CaS会促进诱发夹杂物周围基体腐蚀,提高点蚀诱发敏感性;含硫量较高的AH36钢在钙处理不充分时,形成CaS夹杂或以CaS为主的复合夹杂,会使AH36钢的耐点蚀性能恶化.     

样品取向对轧制钢板拉伸性能的影响

分别对不同厚度、不同取向(α=0°,45°,90°)的SPCC冷轧钢板、SPHC热轧钢板、SUS304冷轧不锈钢板和SUS304热轧不锈钢板进行了拉伸试验,研究了取向因素对轧制钢板拉伸性能的影响,并分别探讨了钢板厚度、轧制状态和钢板材料等因素引起的拉伸性能各向异性程度的差异。结果表明:样品取向对轧制钢板的拉伸性能有明显影响,轧制方向的抗拉强度和断后伸长率较高,而屈服强度的变化规律则较为复杂;钢板厚度、轧制状态、钢板材料等因素对轧制钢板拉伸性能的各向异性都有不同程度的影响。     

The crack growth resistance of thin steel sheets under eccentric tension

The stable crack growth in thin steel sheets is the topic of this paper. The crack opening was observed using a videoextensometry system, allowing the crack extension determination. J_R-curve and δ_R-curve were established from obtained data. The ductile tearing properties of different thin sheets of steel were determined, including the impact of the specimen orientation, from test performed on compact tension specimens loaded under two conditions. The effect of the material, the rolling direction, and loading rate on the crack growth resistance of thin steel sheets was analyzed. In addition to the crack growth resistance, J-integral values for crack initiation were also estimated. The relation between J _i and J_0.2 was assessed using the basic mathematical and statistical methods. This relation was described by a linear regression model.     

Hydrogen-enhanced cracking of 2205 duplex stainless steel

Tensile and fatigue crack growth tests of 2205 duplex stainless steel (DSS) were performed in laboratory air, gaseous hydrogen at 0.2 MPa and saturated H₂S solution. The longitudinal specimen showed a lesser degradation of tensile properties than the transverse ones in saturated H₂S solution. The orientation of specimens with respect to rolling direction had little influence on the fatigue crack growth rate (FCGR) of the alloy in air. Furthermore, 2205 duplex stainless steel was susceptible to hydrogen‐enhanced fatigue crack growth. Transmission electron micrographs, in addition to X‐ray diffraction, revealed that the strain‐induced austenite to martensite transformation occurred near the crack surface within a rather narrow depth. Fatigue fractography of the specimens tested in air showed mainly transgranular fatigue fracture with a small amount of flat facet fracture. Furthermore, extensive quasi‐cleavage fracture of 2205 duplex stainless steel was associated with the hydrogen‐enhanced crack growth.     

含Ce2O3氧化物对改善双相不锈钢亚稳态点蚀的机理研究

利用扫描电镜、原子力显微镜、恒电位脉冲等研究了2205双相不锈钢在中性含Cl-环境下氧化物引起点蚀萌生的机理。实验发现MgO-Al_2O_3系夹杂物中MgO偏聚处以及MgO-Al_2O_3-CaO系夹杂物中CaO富集处会引起夹杂物处基体同周围基体接触电势差增加。此外,CaO富集处易使夹杂物表面出现显微缝隙并使基体裸露,产生亚稳态蚀坑。经Ce处理后发现夹杂物成分变为含Ce_2O_3·11Al_2O_3或Ce_2O_3·Al_2O_3为主的复合夹杂,夹杂物与基体接触电势差减小并且在含Ce_2O_3复合夹杂物处未发现点蚀萌生现象,最后阐述了非金属氧化物引起点蚀的机理以及Ce与氧化物反应的机制。     

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.     

Induction time approach to surface induced crystallization in polyethylene/poly (ɛ-caprolactone) melt

The surface induced crystallization of poly (ɛ-caprolactone) (PCL) on an ultra-high modulus polyethylene (PE) fibre was investigated using a new approach based on the induction timet _i. This approach allows estimation of the free energy difference function Δσ as it appears in the theory of heterogeneous nucleation. The classical approach based on the rate of heterogeneous nucleationl is not applicable to transcrystallization because the nucleation density at the fibre surface cannot be measured. A relationship betweenl andt _i is proposed and a theoretical justification is presented. Good agreement between the two approaches is obtained for a verification case where bothl andt _i can be determined. A transcrystalline growth rate study yields an estimate of the parameter σσ_e for PCL of 680 erg²cm⁻⁴. The maximum growth rateg ^* is also obtained. The results obtained indicate the influence of certain parameters on the appearance of transcrystallinity. It is also shown how the interfacial morphology can be controlled by the knowledge of the variations of the induction time with temperature.     

Estimation of high‐temperature fracture parameters for small punch specimen with a surface crack

An outline of a newly proposed methodology for evaluating creep crack growth (CCG) parameters using cracked small‐punch (SP) specimens is explained. Three‐dimensional finite element analyses were performed to calculate the stress intensity factor along the crack front for a surface crack formed at the centre of a SP specimen. Effects of crack ratio, (a/t); crack aspect ratio, (a/c); and thickness of the specimen, (t), on the fracture parameters were studied. It was observed that the minimum variation of K‐value along the crack front can be achieved when a/c was 0.50 except the location very near the intersection of the crack and free surface. This condition is similar to the case of constant K‐values along the crack front of the conventional compact tension specimen. Thus, it can be argued that the SP specimen with a surface crack is a suitable specimen geometry for CCG testing. The proposed CCG test method was found to be practically applicable for the crack geometry of 0.10 to 0.30 of a/t with constant aspect ratio of 0.50. An estimation of the K and C_t‐parameter under the small scale creep condition was derived. Future work for further development of the suggested CCG testing is discussed.     

Pits with coloured halos formed on 1Cr18Ni9Ti stainless steel surface after ennoblement in seawater

Micropits surrounded by coloured halos were observed under incident-light microscope on 1Cr18Ni9Ti stainless steel surface after “ennoblement” in seawater. Ennoblement has been attributed to biofilm formation on stainless steel surface in seawater. In this study, the environment in biofilm for ennoblement was simulated by adding H₂O₂ into seawater at concentrations that were reported to detect in marine biofilm [N. Washizu, Y. Katada, T. Kodama, Corros. Sci.46 (2004)1291.]. H₂O₂ increased the passivity of stainless steel in seawater, but this passivation was not uniform. The probability of pitting corrosion was increased after ennoblement. Equal thickness interference on the deposition film around the pits was believed to be the reason for the coloured fringes. It is conceivable that haloed pits on the stainless steel surface are the characteristic morphological indications for pitting corrosion formed under ennoblement condition in seawater. Examinations on the microbial and structural effects of the biofilm were not included in this study.     

Pitting susceptibility of UNS NO. 8904 stainless steel in bromide-sulphide solution

The effect of sulphide ion on the pitting corrosion behaviour of UNS* no. 8904 (904L) stainless steel (SS) in 0.6 M NaBr at 25 °C has been investigated by using potentiodynamic anodic polarization and electrochemical impedance spectroscopy techniques. The pitting potential, E _pit, in 0.6 M NaBr + 10^–2 M Na₂S was more negative than that obtained in 0.6 M NaBr, and decreased with increasing temperature in the range 25–60 °C. Scanning electron microscopy observation showed that elemental sulphur formed on the steel surface before E _pit was reached in 0.6 M NaBr + 10^–2 M Na₂S at 60°C. E _pit of 904L SS pH independent in the pH range from 3–10 of 0.6 M NaBr + 10^–2 M Na₂S at 25 °C, while at high pH values the pitting was suppressed. The impedance measurements showed that the charge transfer resistance, R _t, decreased with time, when the controlled potential became higher than E _pit.     

Influence of the rheological properties on the steel fibre distribution and orientation in self-compacting concrete

The interest in potential applications produced with self-compacting fibre reinforced concrete continues to grow, but in practice, problems associated with an uneven distribution and orientation of fibres in the concrete structure occur. It is not clear what exactly influences uneven distribution of fibres in self-compacting concrete (SCC) mixtures, especially during the casting and how different factors influence fibre orientation. The objective of this work was to investigate how rheological properties influence the steel fibre distribution in self-compacting concrete. This work also focuses on the investigation of steel fibre spatial orientation dependence on rheological properties of SCC, while keeping other casting parameters and the proportions of mixture components constant. Mixtures with three different rheological properties were chosen based on slump flow, slump flow time t₅₀₀ and static segregation values. The steel fibre orientation, volumetric concentration and spatial distribution values were determined in separate beam sections using three different non-destructive testing methods: electromagnetic induction, image analysis and computed tomography (CT scan). The comparison of the results is presented. The results show how different rheological properties of SCC affect the steel fibre orientation and distribution for the case of beams produced with the flow-induced casting method.     

Prediction of ductile fracture initiation using micromechanical analysis

In the paper ductile fracture initiation analysis of low-alloyed ferritic steel has been made by application of two micromechanical models: the Rice–Tracey void growth model and the Gurson–Tvergaard–Needleman (GTN) model. The aim of the study was to analyse transferability of micromechanical parameters determined on specimens without initial crack to pre-cracked specimens. A significant part of the research has been carried out through participation in the round robin project organised by the European Structural Integrity Society (ESIS). Tensile tests have been performed on cylindrical smooth specimens and CT specimens. Critical values of micromechanical parameters determined on smooth specimen for both applied models, have been used for prediction of the crack growth initiation in CT specimen. Modelling of the first phase of ductile fracture––void nucleation––has been carried out using quantitative metallographic analysis of non-metallic inclusion content in tested steel. For determination of critical values of model parameters corresponding to ductile fracture initiation a simple procedure has been applied based on a combination of experimental and numerical results. Evaluated J-integral values corresponding to onset of crack growth, J_i, are in good agreement with experimental result and both models have proved to be suitable for determination of the ductile fracture initiation in tested steel. The effect of FE size at a crack tip on J_i-value has been particularly analysed: it has been established that the calculation with FE size corresponding to the mean free path λ between inclusions in steel gives results that are in accordance with the experimental ones.     

The effect of sulfur on the fracture of CrNiMoS steel at low temperatures

Sulfur, whose solubility in ferrite at room and subzero temperatures is low, is present in steel in the form of nonmetallic inclusions (sulfides and oxysulfides), the type and nature of which are determined by the final deoxidizing treatment. The nature and quantity of nonmetallic inclusions have a marked effect on such mechanical properties of steel as, , anda _i [1–3].     

On deformation of metals along flat curvilinear trajectories of variable curvature. Part 1. Vector and scalar properties

We report and discuss the results of experimental investigation of complex loading and unloading of 45 steel along “Archimedean spiral” type trajectories in the planet ₁−t ₃ in the Il'yushin's deviator strain subspacet _i (i=1,2,3), which was carried out using an SN-éVM computerized calculation and testing system. We consider the regularities of variation of scalar and vector properties of the material along these trajectories and compare them with experimental results obtained for flat trajectories of constant curvature. Tver' State Technical University, Tver', Russia. Translated from Problemy Prochnosti, No. 4, pp. 5–11, July–August, 1999.     

Corrosion fatigue behaviour and microstructural characterisation of G20Mn5QT cast steel in 3.5‐wt% NaCl solution

Corrosion fatigue behaviour and microstructural characterisation of G20Mn5QT cast steel are investigated in simulated seawater. Fractography is performed by using scanning electron microscopy (SEM). The macroscale fracture surface and microstructure of the failed specimen are acquired including the crack initiation, crack propagation, and pitting evolution. The maximum cyclic stress (S) versus number of cycles to failure (N) curves is derived by three‐parameter fatigue curve method. Fatigue life is predominantly controlled by the corrosion pitting‐induced crack initiation when tested in simulated seawater at lower stress levels. As the maximum cyclic stress is less than 185 MPa, the chloride ion erosion is the main influence factor, which affects the fatigue failure of the G20Mn5QT cast steel in simulated seawater.     

Effect of microstructure on the thermo-oxidation of solid isotactic polypropylene-based polyolefins

Abstract

In the present work we aim to clarify the role of the microstructure and the crystalline distribution from the thermo-oxidation of solid isotactic PP (iPP) and ethylene-propylene (EP) copolymers. The effects of the content and quality of the isotacticity interruptions, together with the associated average isotactic length, on the induction time (t_i) as well as on the activation energy (E_act) of the thermo-oxidation are analysed. Both parameters have been found to change markedly at an average isotactic length (n₁) of 30 propylene units. While t_i reaches a minimum when n₁ is approximately 30 units, E_act increases quasi-exponentially as the number of units decreases from 30. This variation can be explained in terms of changes induced in the crystalline interphase, i.e. local molecular dynamics, which are closely linked to the initiation of the thermo-oxidation of isotactic PP-based polyolefins.     

A new approach to detect and size closed cracks by ultrasonics

The effect of a crack on time-of-flight of shear waves (4.5 MHz) polarized in perpendicular (t ) and parallel (t ) directions to the crack surface, propagating parallel to the direction of crack growth is investigated. The first and second back-wall echoes are used instead of the weak crack-tip echo for the measurement of time-of-flight. The measurement is made for fatigue cracks grown by different loading histories in ferritic steel (pressure vessel steel A533B-1) under the condition of no loading. The normalized time-of-flight (t –t )/t at the crack position is found to change proportionally as the ratio of crack depth to specimen width increases. The change is mainly due to the effect of plastic deformation occurring around the crack ont . It is shown that the depth of tightly closed fatigue crack in austenitic stainless steel (AISI 304) also can be evaluated under the condition of no loading by using this relationship.     

The effect of specimen thickness and temperature on plastic yielding and fracture properties of mild steel

The effect of thickness and temperature on deformation and fracture behavior is investigated in notched bend specimens of mild steel in low temperature region using a photo-elastic coating technique. The plastic constraint factor is determined and compared with the theoretical value obtained by slip line field theory. The difference of general yielding properties between surface and interior of specimen is examined by the observation of yield propagation. From these results, a qualitative picture of distribution of σ_z (the stress of thickness direction) is deduced. The effect of thickness on the plastic constraint factor and the plastic stress concentration factor R at T_GY (the critical temperature at which general yielding and fracture coincide) is discussed and so-called “thick specimen” is defined in cleavage fracture region. Then, the thickness effect on twin formation is studied. It becomes clear that the critical temperature T_t above which no twins are formed can be determined by the constant value of the plastic constraint factor. Finally, the relation between the plastic constraint factor, specimen thickness and temperature is embodied by the three dimensional diagram and the condition of plane stress is pointed out.