Stress corrosion cracking (SCC) in compressor impellers made of FV520B stainless steel frequently occurred, and hence was investigated by slow strain rate tests (SSRTs) and fracture toughness tests, combined with finite element analysis (FEA) based on ABAQUS and ZENCRACK software. SSRTs revealed that the FV520B stainless steel had slight susceptibility to aqueous brine solution, which increased with increasing temperature, and fracture surface exhibited ductile feature with dimples similar to that in nitrogen; however, this material showed significant susceptibility to NACE solution A at various temperatures, and fracture surface consisted of initial quasi-cleavage region and subsequent dimple region; both tempered martensite matrix and localized delta ferrites were susceptible to SCC. Fracture toughness K-IC (J(IC)) of this steel was 150MPam(1/2) (102KJ/m(2)) in air, but threshold stress intensity factor (K-ISCC) dramatically reduced to 48MPam(1/2) in NACE solution A, and Boltzman equation was modified to describe SCC propagation rate (da/dt-K) curves. Comparison with FEA results indicated that the fracture of SSRT specimen was initially affected by environmental factor and then mainly attributed to mechanical factor, whereas the whole fracture process of impeller was closely related to corrosive environment. This study was useful for SCC evaluation of materials and failure analysis of compressor impellers.

Experimental studies were conducted on the dynamic fracture behavior of AISI 5140 steel over a wide range of loading rates. True stress-strain relations of the steel were measured under different strain rates, and a dynamic constitutive model was then expressed with the strain-hardening term and the strain-rate hardening term. Fracture tests were investigated under quasi-static condition, instrumented Charpy impact condition and Hopkinson pressure bar impact condition. Fracture toughnesses corresponding to each test condition were also determined. Fracture characteristics were analyzed with observations of the fracture appearances. It was found that the fracture toughness decreased significantly with the increasing loading rate, and the fracture mechanisms of the steel at various loading rates were brittle fractures characterized by river markings and secondary cracks. Based on the fracture assessment method of the CEGB R6 procedure, the effects of the strain rate and the loading rate on the assessment curve were discussed.

This is Part II of mixed-mode fracture behaviors of AISI 1045 and AISI 5140 steels used for compressor crankshafts, to further investigate mixed-mode II-III fracture by using a modified loading device based on the mode I-III one. It was found that load-displacement curves began to deviate from linear relationships at lower load levels with increasing loading angle; at the same loading angle, stress-intensity-factor versus displacement curves were similar for specimens with different pre-crack lengths; fracture surfaces mainly showed shear feature (elongated dimples and frictional markings) except the fiber and sliding features under the mode II loading condition; crack propagated in the direction perpendicular to the maximum tangential stress under the mode II loading condition while cracks propagated in the pre-crack planes under other loading conditions. Finally, a criterion for mixed-mode II-III fracture was proposed and its validity for AISI 1045 and AISI 5140 steels has been verified.

In this work, an extended finite element method with partial Heaviside function enrichment (XFEM-P) in the crack tip element is proposed for fracture analysis for the first time. In this method, the crack tip element is divided into cracked and uncracked domains naturally by the crack geometry. In the cracked domain, Heaviside function enrichment is employed to simulate the crack discontinuity across the crack surface. In the uncracked domain, the displacement from standard finite element method is employed. Ramp function is used to ensure the displacement continuity in the method. Through this way, the crack tip can be arbitrarily located inside the crack tip element, which is a great advantage over traditional counterpart, i.e., XFEM with only Heaviside function enrichment (XFEM-H). Thus the mesh work for the XFEM-P is much easier than that of the XFEM-H, resulting in great conveniences for crack propagation. The new method is applied to several examples. From the computed results, it is observed that the XFEM-P offers more accurate results than the XFEM-H in terms of strain energy and SIFs. The XFEM-P performs better than XFEM-H in numerical properties such as convergence rate and condition number.

Numerical simulations were performed on the damage behavior of carbon-fiber-wrapped composite cylinder subjected to impact by a flat-ended impactor. The simulation results were in agreement with the test in terms of both the damage morphology of the cylinder and the impact acceleration-time curve of the impactor. The relationship between the impact acceleration-time curve characteristics and the initiation and propagation process of the various damages was analyzed. The effects of the internal pressure on the damage morphology of the cylinder, the impact acceleration-time curve of the impactor, and the critical perforation energy of the cylinder wall were discussed. Change law of the residual burst pressure of the cylinder with different impact energies was obtained. The conclusions in this paper are helpful for the safety assessment of the composite cylinders subjected to impact by foreign objects.

针对离心压缩机叶轮出现的复合断裂问题,设计了Ⅰ-Ⅱ复合型断裂夹具和改进的紧凑拉伸试样,开展了叶轮典型材料FV520B不锈钢的试验研究.结果表明:FV520B钢Ⅰ-Ⅱ复合型断裂均属微孔聚集型韧性机制,只是在较大加载角度下,断口上的韧窝沿裂纹扩展方向被轻微拉长、深度变浅,主要由剪切分量占比增加所致;随加载角增加,材料总的断裂韧度增大,加载角为22.5°和45°时较纯Ⅰ型的分别增加了1.8倍和4.4倍,断裂韧度的Ⅰ型和Ⅱ型分量、及启裂角亦随之增大,采用复合型J积分断裂准则可以对试验结果进行较好地预测.本研究可为离心压缩机叶轮的复合断裂分析提供技术支撑.

A three-dimensional transient computational fluid dynamics (CFD) model has been established for the simulations of supercritical heat transfer of real liquefied natural gas (LNG) mixture in a single tube and a tube bundle of an intermediate fluid vaporizer (IFV). The influence of chemical composition of LNG on the thermal performance has been analyzed. The results have also been compared with those obtained from the one-dimensional steady-state calculations using the distributed parameter model (DPM). It is found that the current DPM approach can give reasonable prediction accuracy for the thermal performance in the tube bundle but un-satisfactory prediction accuracy for that in a single tube as compared with the corresponding CFD data. As benchmarked against pure methane, the vaporization of an LNG containing about 90% (mole fraction) of methane would lead to an absolute deviation of 5.5 K in the outlet NG temperature and a maximum relative deviation of 11.4% in the tube side HTC in a bundle of about 816 U tubes at the inlet pressure of 12 MPa and mass flux of 200 kgm(-2).s(-1). It is concluded that the influence of LNG composition on the thermal performance should be taken into consideration in order to obtain an economic and reliable design of an IFV.

Fracture behavior of a high-pressure vessel for food processing under monotonic and fatigue loadings was investigated by conducting both experiments and finite element analysis (FEA) based on abaqus and zencrack software. Finite element analysis results showed that cracks nucleated at the filets of pin-hole and propagated faster near the inner surface than near the outer surface of the pressure vessel, progressively deflected, and eventually coalesced with other cracks initiated from the counter pin hole under monotonic loading. Such crack growth behavior coincided with the experimental result of hydraulic pressurizing test. Based on fatigue crack growth test of the pressure vessel material, 17-4PH stainless steel, a new equation to express the da/dN−ΔK curves including threshold region, has been proposed and embedded into the zencrack software to simulate the fatigue behavior of the pressure vessel. The simulation results showed that fatigue lives could be accurately estimated including low pressure range. The present simulation methods would be the useful design tool for pressure vessel under monotonic and cyclic loadings.

针对离心压缩机叶轮叶片疲劳寿命研究的需要,考虑到服役环境对疲劳寿命的影响,开展叶轮典型材料FV520B钢在湿H2S+Cl-环境中的腐蚀疲劳试验研究,结合断口形貌观察分析其疲劳失效机理,建立疲劳寿命预测模型.结果表明,FV520B钢在湿H2S+C1-环境中的疲劳性能相较于大气环境大幅下降,并随着环境中H2S浓度升高而下降;随着应力幅值降低,介质中的H2S对疲劳性能的影响逐渐显现并随应力幅值降低而增大.疲劳断口形貌观察表明,试样表面存在点蚀坑,疲劳裂纹在点蚀坑处萌生并扩展.最后,在点蚀演化寿命预测模型的基础上,结合氢对疲劳演化过程的影响,建立考虑应力及腐蚀介质共同作用的疲劳寿命预测模型,预测结果与试验数据吻合较好.该模型可为湿H2S+Cl-环境下材料的疲劳寿命预测提供基础支持.

研究金属材料在不同应力比下的疲劳裂纹扩展速率描述公式,包括Ⅰ型、Ⅲ型和Ⅰ-Ⅲ型三种模式.基于现有疲劳裂纹扩展速率公式和对试验数据进行归纳,建立了以,J积分变化幅△,J为控制参量的疲劳裂纹扩展速率改进公式.在公式中,通过引入应力比修正系数M将不同应力比R下的数据ΔJ等效到R=0时对应的ΔJeqo,并给出了M～R的经验关系式.最后,通过对文献中Ⅰ型、Ⅲ型和Ⅰ-Ⅲ型的疲劳裂纹扩展速率试验数据进行分析发现,本文建立的改进公式可以对不同应力比下的试验数据进行较好的描述.

This study focused on the impact behavior of carbon-fiber-wrapped composite cylinders subjected to impact from flat-ended, hemispherical-nosed and conical-nosed impactors. Damage morphologies of the cylinders and mechanisms of the damage were analyzed. Change laws of the maximum impact forces, durations of impact processes and energies absorbed by the cylinders after impact with different impactors and impact energies were obtained. A finite element model was developed and the simulation results were in reasonable agreement with the tests. Finally, taking the flat-ended impactor as an example, stress distributions of the cylinders under pressurization and impact were discussed.

Stress corrosion cracking (SCC) susceptibility of FV520B martensitic precipitated hardening stainless steel exposed to aqueous hydrogen sulfide solution,sodium chloride solution and aqueous hydrogen sulfide containing with 5% sodium chloride solution were investigated by slow strain rate tensile test.The influence of hydrogen sulfide concentration was also taken into account.FV520B stainless steel in aqueous hydrogen sulfide containing with 5% sodium chloride solution revealed a higher susceptibility to SCC than that exposed to other two mediums;the susceptibility increases with increasing of the concentration of hydrogen sulfide.Scanning electron microscopy analyses showed that the fracture of SSRT was transited from cavity fracture to brittle fracture when the concentration of H2S increased.The constant displacement stress corrosion tests of FV520B steel exposed to aqueous hydrogen sulfide with chloride displayed that the fracture toughness of FV520B significantly decreased due to the influence of corrosive medium and threshold stress intensity factor (KISCC) of FV520B decreases with the concentration of hydrogen sulfide increasing.Fracture morphology analyses showed that the fracture was characterized by quasi-cleavage and the SCC mechanism of FV520B stainless steel exposed to aqueous hydrogen sulfide with chloride is hydrogen embrittlement (HE) type of SCC.

The corrosion fatigue behavior of FV520B steel in water and salt-spray environments at different temperatures is the focus of this study. Temperature has a significant effect on the fatigue behavior of FV520B steel in water and salt-spray environments, but has only a negligible effect in an air environment. Fatigue life decreases as temperature increases in water and salt-spray environments, and this trend is much more remarkable at a low stress amplitude level. As compared to air and water-spray, it was found that the chloride ion in salt-spray accelerated the fatigue damage of FV520B steel. Micro observation revealed that the fatigue cracks were initiated on the surfaces of the specimens in all experimental environments, and significant evidence of corrosion pits and micro cracks was found on the surfaces of the specimens in the salt-spray environment. Based on the pitting corrosion mechanism, a modified fatigue life prediction model that considered the combined effects of stress, temperature and corrosion was proposed. The study results show that the model predictions are in good agreement with the test data.

Dynamic fracture behaviour of AISI 1045 steel for compressor crankshaft was studied by experimental and numerical methods. True stress-strain relations of the material under different strain rates were measured, and dynamic constitutive model with consideration to strain-hardening and strain-rate hardening was proposed. Dynamic fracture tests loaded by Hopkinson pressure bar were carried out, and fracture toughness was determined using a finite element method with the combination of ABAQUS and Zencrack software. Loading states of the specimen and determination methods of the dynamic fracture toughness were discussed. By comparing the fracture behaviours under quasi-static and dynamic conditions, it was found that the fracture modes exhibited a transition from ductile to brittle fracture with the increasing loading rate, and the dynamic fracture toughness value was less than the quasi-static one.

The scatter behavior of fatigue strength for 17-4 PH stainless steel (JIS SUS630, equivalent to ASTM-A563 Grade 630), which exhibited a duplex S–N curve, was investigated. It was found that the fatigue strength scatter of the SUS630 at 106 and 107 cycles, which were corresponding to the fatigue strengths for the surface-induced and the subsurface-inclusion-induced crack nucleation modes, respectively, were significantly small and comparable to each other. The scatter of inclusion size in the present material was small and resulted in the small scatter of fatigue strength comparable to that for the surface-induced crack nucleation mode. The variation in size of non-metallic inclusions is known as one of the main factors for the fatigue strength scatter (Murakami, 2002) [1]. It was also found that the scatter of fatigue strength was in the same order of tensile strength.

Microstructure evolution due to carburising of a 25Cr35NiNb+MA ethylene pyrolysis furnace tube was investigated after service for approximately 4 years. The microstructure of 25Cr35NiNb+MA alloy was examined by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) equipped with energy-dispersive X-ray spectroscopy and high-resolution transmission electron microscopy (TEM). The results revealed that M23C6 carbides of the 25Cr35NiNb+MA alloy had coarsened and transformed to M7C3 carbides with a heavily faulted structure during carburising. The content of M7C3 carbide increased closer to the inner wall. A large number of dislocations had been observed surrounding the carbides and some formed dislocation walls. The carbide transformation mechanism and the effect of the dislocation walls are discussed.

The dynamic fracture assessment method applied to a cracked centrifugal impeller subjected to dynamic loading is the focus of this study. J-integral histories and loading rates at crack tips were evaluated by numerical simulation through ABAQUS and Zencrack software. True stress-strain relations, fracture mechanism and fracture toughness of the impeller material FV520B steel were investigated over a wide range of loading rates by experimental and theoretical methods. Fracture assessment with consideration to rate effects was then conducted based on the CEGB R6 procedure. The present method demonstrated advantages over existing methods evaluated by an equivalent static load. (C) 2015 Elsevier Ltd. All rights reserved.

对S30408奥氏体不锈钢进行室温应变强化,在深冷温区(77K～4.2K)进一步时效处理,利用金相显微镜、X射线衍射仪(XRD)和透射电子显微镜(TEM)研究了应变诱发和热诱发马氏体相变的微观特征,探讨了马氏体相变的微观机理.结果表明:马氏体相变的含量、微观形貌、形核特征和位错组态随着预应变量的增加和温度的降低而改变,且应变强化比温度对相变的影响要大.诱发马氏体与母相奥氏体的位相关系符合K-S关系,其相变的微观机理为γ(fcc)→ε(hcp)、γ(fcc)→α'(bcc)、γ(fcc)→ε(hcp)→α'(bcc)、γ(fcc)→形变孪晶→α'(bcc).

Stress and damage analyses are performed on a composite pressure vessel with aluminum liner under internal pressure through numerical simulations. The variational thickness and wound angle of the head is reflected in the finite element model in accordance with the actual structure. A progressive damage model considering four failure modes are adopted for the composite, i.e., the fiber tension and compression, the matrix tension and compression. The regularities of the initiation and distribution of the various damages are studied, and the effects of the damage evolution on the determinations of the burst pressure and the autofrettage pressure are analyzed. The effects of the autofrettage pressure on the mean stress and the stress amplitude of the liner under zero and service pressures are also discussed. The conclusions in this paper are helpful to the design and use of composite pressure vessels.