Effect of distance of loading points on long brittle crack propagation/arrest behaviour

Abstract

The effect of the distance between loading points on a long brittle crack arrest behaviour was investigated in the ultrawide duplex ESSO tests. The effect of loading conditions was also analysed. Arrest of a long brittle crack was more facilitated in the tests with a distance between loading points of 5 m than in those with 10 m. A load drop was larger in the tests with a loading point distance of 5 m than in those with 10 m. If the specimen length is 6·8 m or more, or the distance between loading points are 10 m or more in the ultrawide duplex ESSO test, the effect of the unloading stress wave on the dynamic stress intensity factor of a crack penetrating into the test plate becomes negligible, within the range of brittle crack propagation rate between 500 and 800 m s^?1.     

Effect of unwelded length on behaviour of brittle crack arrest in T-joint structure

In the large heat input weld joint of heavy gauge steel plate used for large container carriers, a brittle crack possibly propagates straight along a weld joint without diverging to the base metal. This phenomenon is discussed for its application to the large heat input weld of heavy gauge plate to ship structures. In this study, to arrest a brittle crack at the T-joint embedding unwelded face, the effect of the unwelded face on behaviour of brittle crack propagation/arrest in the T-joint structure was investigated and analysed. The ESSO test of the T-joint structure was carried out, supposing that a brittle crack which propagates along a weld joint of hatch coaming rush into T-joint of hatch coaming and strength deck. The test results showed that the brittle crack arrested at the T-joint embedding the unwelded face, and the brittle crack arrested easier, the longer the unwelded face. The results of static FEM analysis showed that the stress intensity factor of the brittle crack was increased by the unwelded face, until the brittle crack reached a flange. However, when the brittle crack propagated into the flange, the stress intensity factor of the brittle crack was decreased by the unwelded face. The crack-arrest effect of the unwelded face appears after the brittle crack propagates into the flange.     

激光表面熔凝处理对共晶铝硅合金疲劳裂纹扩展的影响

研究了共晶铝硅合金在激光表面熔凝处理后的显微组织变化及激光表面熔凝处理对合金疲劳裂纹扩展的影响.结果表明,激光表面熔凝处理使试验合金的表层微观组织发生显著变化,熔凝层中的初生α-Al枝晶相和共晶Si相均有明显细化,熔凝层的硬度显著提高;在应力比R为0.1和0.5的试验条件下,激光表面熔凝处理能显著提高共晶铝硅合金的疲劳裂纹扩展抗力.     

回火温度对Si-Mn低碳贝氏体钢塑韧性的影响

采用扫描电镜、X射线衍射仪、透射电镜研究了不同回火温度对Si-Mn低碳贝氏体钢的力学性能、微观组织、残留奥氏体、冲击断裂裂纹扩展的影响。结果表明,提高低碳贝氏体钢回火温度,降低残留奥氏体量,增加残留奥氏体的稳定性,有利于塑韧性的改善。但回火温度达到500℃以上,残留奥氏体量都发生转变或分解,塑韧性会变差。稳定的残留奥氏体会增大裂纹扩展能量,从而改善塑韧性。     

In situ investigation of the effect of Hydrogen on the plastic deformation ahead of the crack trip and the crack propagation for 0.15C-1.5Mn-0.17V-0.012N steel

The influence of hydrogen on the deformation ahead of the crack tip and the crack propagation were observed and studied in situ under transmission electron microscopy with dynamic tensile deformation for bridge steel. The results show that hydrogen can promote local plastic deformation ahead of the crack tip and change the mode of crack propagation so that the crack will propagate in a zigzag path.     

热处理工艺对新型R5系泊链钢组织和断裂韧性的影响

采用裂纹尖端张开位移(CTOD)实验方法对研究了不同热处理工艺对新型R5系泊链钢断裂韧性的影响,使用偏光显微镜、光学显微镜和扫描电镜等对CTOD试样裂纹扩展区组织及断口形貌进行分析,并用高低温材料试验机测试了R5系泊链钢低温-20℃下的力学性能。结果表明,回火温度对R5系泊链钢CTOD断裂韧性有显著影响。在570~600℃范围内回火,组织保留马氏体位向,碳化物呈片状及长条状不均匀分布在原奥氏体晶界、马氏体板条界及板条内部,应力作用下容易导致局部应力集中,裂纹扩展速率大,CTOD特征值接近零。随回火温度升高,在600~690℃范围内回火,碳化物逐渐聚集、球化并在基体中弥散均匀分布,应力作用下不易引起局部应力集中,并且在裂纹扩展时可使裂纹扩展方向发生偏转,裂纹扩展路径延长,裂纹扩展受到抑制,CTOD特征值随回火温度升高不断增大。经950℃淬火+630~660℃区间回火,R5系泊链钢同时具有高强度和良好的CTOD断裂韧性。     

Effect of electron beam surface hardening on fatigue crack growth rate in AISI 4340 steel

This paper studies the effect of electron beam (EB) surface hardening on the fatigue crack growth rate in AISI 4340 steel. The heat treatment conditions were varied to consider the influence of microstructure and residual stress. The results show that increasing the EB heat input increases the compressive residual stress in the hardened layer. Thus EB surface-hardening treatment improves the fatigue crack growth resistance. This effect increases with increasing EB heat input but disappears as the ΔK value increases. The fracture mechanism of the hardened layer is intergranular fracture, while that of the base material is transgranular quasi-cleavage.     

Effect of selective dissolution on fatigue crack initiation in 2205 duplex stainless steel

The effect of selective dissolution on fatigue crack initiation of 2205 duplex stainless steel (DSS) was investigated in this study. In mixed sulfuric and hydrochloric acid aqueous solution, there existed two distinctly separated anodic peaks in the active-to-passive transition region of the polarization curve. Either ferritic or austenitic phase was selectively dissolved at each characteristic anodic peak potential. Under sinusoidal cyclic loading condition, however, selective dissolution did not assist fatigue crack initiation instead resulting in the elimination of stress concentration site in the selectively dissolving phase. As a consequence, under selective dissolution condition, fatigue crack initiated in the phase while its dissolution rate was lower with respect to the other constituent phase in the duplex stainless steel. The microstructural evolution of the corrosion fatigue crack initiation in 2205 DSS in the mixed sulfuric and hydrochloric acid solution is highlighted in this investigation.     

Effect of metallurgical variables on the stress corrosion crack growth behaviour of AISI type 316LN stainless steel

Mill-annealed AISI type 316LN stainless steels, received from two different sources (one indigenous (SS-2) and the other foreign (SS-1)), were tested for stress corrosion cracking (SCC) resistance in a boiling acidified environment of NaCl. SCC results indicated a remarkably lower value of plateau crack growth rate (PCGR) and higher values of K_ISCC and J_ISCC for SS-2, which was attributed to the lower effective grain boundary energy resulting from a higher amount of copper in it. Cold working reduced K_ISCC and PCGR; while thermal aging and welding decreased K_ISCC and increased PCGR vis-à-vis the annealed material.     

Effect of hydrogen peroxide on crack growth rate in X70 pipeline steel in weak acid solution

An investigation of the effect of hydrogen peroxide (H₂O₂) on corrosion crack growth can be considered as a necessary stage in a study of the SCC of pipeline steels in the presence of dissolved oxygen and other oxidants. It was found that the presence of hydrogen peroxide at a low concentration (5?mM) results in a deceleration of the crack growth. With an increase in the concentration of H₂O₂, the crack growth rate increases. The change in the steel corrosion rate at various H₂O₂ concentrations agrees with the dependence of the crack growth rate on the oxidant concentration. The conclusion has been made that the crack growth in a weakly acidic electrolyte (pH 5.5) is determined by the metal dissolution process. Hydrogen charging of the metal indirectly affects the crack growth by increasing the surface coverage with hydrogen, which decreases the steel dissolution rate.     

纯铝中间层厚度对铝/钢点焊接头性能的影响

采用纯铝作为中间层对铝合金与低碳钢进行了电阻点焊,分析中间夹层厚度对界面反应层厚度和接头抗拉强度的影响.在钢/中间夹层界面观察到有界面反应层生成,其主要由靠近钢侧的Fe2Al5和靠近中间夹层铝侧的FeAl3两种金属间化合物组成.与不加中间夹层相比,利用纯铝作为中间夹层点焊的铝合金与低碳钢的接头具有较薄的界面反应层和较高的接合强度.随着中间夹层厚度的增加,界面反应层厚度逐渐减小,而接头抗拉强度则呈增大趋势.结果表明,采用纯铝作为中间夹层点焊铝合金与钢具有一定的有效性.     

Effect of cavitation on the corrosion behaviour of welded and non-welded duplex stainless steel in aqueous LiBr solutions

The influence of cavitation on the corrosion resistance of EN 1.4462, its filler metal, and the welded metal obtained by GTAW was investigated by potentiodynamic curves in LiBr heavy brine solution under static and dynamic conditions at 25 °C. The corrosion behaviour of the tested alloys is modified by the presence of chromates (inhibitor) in the medium. The corrosion potential in the solution with chromates shifted towards the active direction when cavitation was present; on the contrary, in the solution without inhibitor, cavitation led to a noble shift in the corrosion potential favouring the cathodic reaction.     

Effect of tempering time on the oxidation behavior of 21/4Cr-1Mo steel

21/4Cr-1Mo ferritic steel is generally used in the normalized and tempered condition for steam-generator tubes in fast breeder reactors. The microstructure of the alloy changes on tempering, depending on the type of carbides formed, as a function of tempering time. At the very early stage of tempering (up to 30 min), most of the cementite (Fe₃C) dissolves, causing trapping of Cr and Mo to form their carbides. With an increase in the tempering time more Cr and Mo are used up for carbide formation. This leads to a decrease in the available Cr content, which is generally responsible for the formation of a protective oxide scale. It is thus expected that a decrease in the oxidation resistance will occur with an increase in tempering time. Oxidation tests were carried out on specimens prepared after five tempering treatments; viz. 10 min, 30 min, 60min, 120min, and 180min at 998 K. Oxidation was carried out in air both for long duration (1000 hr at 773 K) and for short duration (6hr at 873, 973, and 1073 K). Results indicate a decrease in the oxidation resistance with increasing tempering time. At 1073 K, however, no significant change in oxidation was noticed. This could be perhaps due to the vanishing of the tempering effect by the dissolution of carbides at this temperature.     

Effect of niobium on the corrosion behaviour of low alloy steel in sulfuric acid solution

This study examines the effect of niobium (Nb) addition on the electrochemical properties of low alloy steel using electrochemical techniques in a 10 wt.% sulfuric acid solution as well as surface analysis techniques. The potentiodynamic test reveals the passive behaviour of all specimens and a decrease in the passive currents with increasing Nb content. Electrochemical impedance spectroscopy (EIS) shows that the Nb-containing steels have higher passive and charge transfer resistance than the control samples. These results suggest that the interaction of elements improves the corrosion resistance of low alloy steel due to the formation of Nb, C, S, P, and Fe products on the surface.     

Wet/dry accelerated laboratory test to simulate the formation of multilayered rust on carbon steel in marine atmospheres

The prolonged exposure of carbon steel in marine atmospheres with high chloride deposition rates and long times of wetness of the metallic surface leads to the formation of thick multilayered rust. The present work proposes an accelerated cyclic laboratory test based on immersion (4.2 min) in a 3.5% NaCl solution followed by drying (12 min) under infra-red lamps in the laboratory atmosphere. The carbon steel corrosion process is thus accelerated, giving rise to the generation of thick rust layers in relatively short times. The rust phases and the structure of the rust layers formed offer a good approximation to the multilayered rust formed in marine atmospheres. The study includes a gravimetric evaluation of the magnitude of corrosive attack and a characterisation of the rust phases and corrosion layers formed using XRD, optical microscopy and SEM.     

Effects of Na3PW12O40 on the corrosion behavior of carbon steel in 55 %LiBr solution

Electrochemical technologies, chemical immersion tests and surface detection technologies were adopted to study the inhibition performance of Na₃PW₁₂O₄₀ inhibitor on carbon steel in 55 % LiBr + 0.07 mol/L LiOH solution, and the inhibition mechanism was discussed. Results indicated that Na₃PW₁₂O₄₀ inhibitor caused the decrease of both anodic and cathodic polarization current density and a widening of the passive potential region. It behaved as a mixed inhibitor. When the concentration of Na₃PW₁₂O₄₀ in 55 % LiBr + 0.07 mol/L LiOH solution was 300 mg/L, excellent inhibition performance on carbon steel was obtained. When the solution temperature were 145°C, 160°C, 180°C and 200°C, respectively, the corrosion rates of carbon steel were 13.07 μm/y, 17.74 μm/y, 24.55 μm/y and 73.52 μm/y accordingly. Na₃PW₁₂O₄₀ inhibitor exhibited an excellent inhibition performance at high temperature. Na₃PW₁₂O₄₀ is a strong oxidant. Fe was oxidized to Fe₂O₃ and itself was deoxidized to heteropoly blue. By this mechanism, an integrate compact passive film mainly comprising Fe₂O₃ may be formed on the carbon steel surface, and the corrosion of carbon steel in 55 % LiBr solution may be retarded by this passive film.     

Effect of Mo and Mn additions on the corrosion behaviour of AISI 304 and 316 stainless steels in H2SO4

The work addresses the influence of Mn and Mo additions on corrosion resistance of AISI 304 and 316 stainless steels in 30 wt.% H₂SO₄ at 25 and 50 °C. Corrosion mechanism was determined by gravimetric tests, DC polarization measurements and electrochemical impedance spectroscopy (EIS). The morphology and nature of the reaction products formed on the material surface were analysed by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). Reduction of temperature from 50 to 25 °C drastically decreased the corrosion rate of AISI 304 and 316 stainless steels in sulphuric acid solution. Mn additions did not affect significantly the general corrosion resistance due to its low ability to form insoluble compounds in acid medium. Meanwhile, the formation of molybdenum insoluble oxides enhanced the corrosion performance.     

Effect of electrolyte composition on the active-to-passive transition behavior of 2205 duplex stainless steel in H2SO4/HCl solutions

Selective dissolution could occur in duplex stainless steels (DSSs) due to the difference in chemical composition between the two constituent phases. In this study, the effect of H₂SO₄/HCl composition on the selective dissolution behavior was investigated. The results indicated that there were two distinct peaks appeared in the active-to-passive transition region in the polarization curve. The peak appeared at a lower potential region was associated with the preferential dissolution of ferrite phase while that for austenite at a higher potential. In the concentration ranges of 0.25-2 M of H₂SO₄ and 0.25-2 M of HCl, the magnitude of the peak anodic current density and the resolution between these two peaks greatly depended on the composition of H₂SO₄/HCl. However, the anodic peaks corresponding to the respective dissolutions of ferrite and austenite became less distinguishable when the concentrations of HCl exceeded 1.2 M. Image analysis using scanning electron microscopy (SEM) was performed to confirm the selective dissolution of each constituent phase after potentiostatic polarization at the respective anodic peak potential.     

Effect of electrochemically reactive rust layers on the corrosion of steel in a Ca(OH)2 solution

Electrochemical and gravimetric measurements are used to study the effect of rust layers on the corrosion of rusted steel in a saturated Ca(OH)₂ solution imitating the liquid phase in concrete pores. The results indicate that the reduction of rust is the main cathodic reaction in the first phase of the corrosion process of rusted steel. Subsequently the oxygen reduction reaction becomes the cathodic predominant reaction, in which the rust seems to act as a porous electrode. In this research rust clearly favours the corrosion of rusted steel, and an approximately direct correlation is seen between the rust mass and the mass loss due to steel corrosion. In the electrochemical measurements, attention is paid to the overestimation of the corrosion rate deduced from the polarisation resistance in the case of specimens with the highest rust contents, a problem that is attributed to the inappropriateness of the conventional measuring procedure in the presence of high capacitance values and redox processes.