DD6单晶合金蠕变特性及断裂机理

在500MPa、900℃和300MPa、1000℃两种环境条件下,通过实验研究了[001]、[011]、[111]三种取向的镍基单晶合金DD6的蠕变特性及断裂机理。结果表明:DD6单晶高温合金蠕变性能具有明显的晶体取向相关性,在相同的温度和应力条件下,三种取向的单晶合金寿命差别很大,温度是影响[001]取向单晶合金蠕变寿命的主要因素,而应力则是影响[011]和[111]取向单晶合金蠕变寿命的主要原因。同时,晶体取向和实验条件的不同,都会使最小蠕变率发生变化,随着温度的升高,不同取向晶体的各向异性减弱,相同条件下,[111]取向最大蠕变量最大,[001]取向次之,[011]取向最小。而晶体取向对断裂机理有直接的影响,[001]、[111]取向DD6单晶合金的断裂是由微孔引起的断裂,[011]取向的DD6单晶合金在900℃、500MPa条件下的蠕变断裂为滑移断裂,1000℃、300MPa条件下的断裂为滑移面断裂和韧窝断裂二者兼有的混合型断裂。     

Ein neues Zeitstandgerät für Kriechversuche bei wechselnden Belastungen und Temperaturen

A New Instrument for Measuring Creep under Alternating Loading and Temperature Conditions. It is an established fact that plastics are particularly subject to creep, i. e. deformation over a period of time under load. Under practical conditions unchanged load and constant temperature are seldom realized. The deformations associated with changing mechanical loads cannot generally be derived from the results of ordinary creep tests. In order to be able to study creep behaviour of plastics under intermittent load conditions (i. e. a sequence of loading and unloading), a special instrument has been developed at the Austrian Plastics Institute. The period of time as well as the ratio of loading and unloading phase can be programmed within wide ranges. Creep tests at varying temperatures can also be carried out with this instrument. Under intermittent load very significant differentiation of deformation and fracturing behaviour depending on the polymeric structure has been observed. Comparison of results of ordinary creep with those under intermittent loading at constant temperature shows, that in the case of amorphous thermoplastics destruction by stress crazing and fracture is markedly accelerated. On the other hand semicrystalline thermoplastics have excellent resistance against intermittent load.     

LIFE PREDICTION OF 316 STAINLESS STEEL UNDER CREEP-FATIGUE LOADING

Life prediction for creep-fatigue loading conditions should be related to creep damage mechanisms. In order to examine the effect of the creep damage mode on rupture life under creep-fatigue loading, a “combined creep-fatigue loading test” was carried out on 316 stainless steel. In this method, creep loading and fatigue loading are repeated alternately. The fracture criteria under combined loading closely depend on the creep fracture modes of the static creep test. A new life prediction method which uses this new fracture criterion is proposed. The criteria are changed when the creep damage mode varies. In order to verify the adequacy of this method, fatigue tests with a tensile strain-hold wave form were carried out. It is clear that rupture life in such fatigue tests is dependent on the chosen fracture criteria.     

Parameteranalyse zum Zeitstandinnendruckversuch und Schädigungsmechanismus bei chemikalienbelasteten Thermoplastrohren (System: LDPE/H2SO4 (98%))

Parametric Analysis of Creep Tests under Pressur and Kind of Failure in Thermoplastic Tubes under Chemical Load Creep tests under pressure are a method to evaluate the long term behaviour of tubes under mechanical, thermic and chemical loading. However, this method does not enclude essential criteria of failure. In form of a parametric analysis this paper proves that “weeping” takes place much earlier than total failure by rupture. Investigations on the different kinds of damage within the tube material using light microscopy and electron scanning microscopy reveal- that it is not correct to transform results of creep tests with small and thin tubes to such of practically used dimensions. This is on the one hand based on the existance of different microstructures which are specific to the semicristalline material and on the macrostructure determined by production process on the other hand. In order to determine failure of tubes sufficiently new test methods are proposed.     

Kriech- und Bruchverhalten warmfester Stähle in kennzeichnenden Fällen veränderlicher Zeitstandbeanspruchung

Creep and Creep Rupture Behaviour of Heat Resistant Steels under Variable Loading Conditions The creep and creep rupture behaviour under rectangular cyclical conditions was measured on 8 typical heat resistant steels up to 35 000 h test duration. The results are described by the modified life fraction rule. The main dependencies of the characteristic of that rule, the relative life, are investigated. Concepts for the evaluation of the relative life are presented in regard to design and supervision of components.     

Creep properties of Sn-3.5Ag-0.5Cu lead-free solder under step-loading

The purpose of this study is to investigate the creep behavior and develop a suitable life assessment model for a lead-free Sn-3.5Ag-0.5Cu solder subjected to variable-step creep loading. Creep tests were conducted under two-step loading with various combinations of stress and temperature. Experimental results showed existence of sequence effects on cumulative creep damage for the given ternary alloy under two-step loading or temperature. A high-low sequence is more damaging than a low-high sequence for all given two-step loading/temperature conditions. Furthermore, the creep rate in the second step of a high-low sequence was greater than that under single-step loading while no significant difference in creep rate was observed in the second step of a low-high sequence compared with the single-step loading one. A nonlinear cumulative creep damage model was proposed and made reasonably good predictions of creep lifetime for all the given two-step creep testing conditions. This model also predicts the sequence effects that a low-high step loading would produce a larger sum of lifetime fraction than a high-low one.     

Creep properties of a U‐type notch plate in Ni‐based superalloy

In the present investigation, the effect of notch on creep rupture behavior and creep rupture life of a Ni‐based superalloy has been assessed by performing creep tests on smooth and U‐notched plate specimen under 0°C. The finite element analysis coupled with continuum damage mechanics are carried out to understand the stress distribution across the notch throat and the creep damage evolution under multi‐axial stress state. The creep rupture life of U‐notched specimen is much larger than that of plane plate specimen under the same stress condition, indicating that there is a strengthening effect on notch specimen. Creep rupture life increases with increasing the notch radius, the smaller notch radius can induce the creep rupture easier. The effect of notch on the creep damage is also studied. It is found that the location of the maximum creep damage and the maximum equivalent creep strain initiates first at the notch root and gradually moves to the inside as the notch radius increases.     

Langzeitschwingfestigkeitsverhalten bei Temperaturen im Kriechgebiet

Long-term High Cycle Fatigue Behaviour at Temperatures in the Creep Range – Investigation of the Correlation with Creep Rupture Properties and Effect of Superimposed Low Cycle Fatigue Loading The long-term high cycle fatigue behaviour of the martensitic steel X 22 CrMoV 12 1 and the Ni-base superalloy IN 792 has been investigated. For that purpose, the influence of the ratio of static mean stress to HCF stress amplitude was examined. In addition, the influence of notches on the long-term HCF endurance has been determined. The HCF data can be correlated with creep rupture data by a combination of the method of Keil and Maier and of the Moore-Kommers-Jasper-diagram, additionally, interpolations for any ratio of static mean stress to HCF stress amplitude can be performed. That means, that it will be possible in certain cases to replace costly long-term HCF tests by short-term tests at higher temperature. The superposition of HCF and LCF loading leads to lifetime reductions, which can be explained by simple damage accumulation rules only for small HCF stress amplitudes.     

Fatigue behaviour of synthetic fibres,yarns, and ropes

S-N fatigue and creep-rupture data have been obtained for nylon 6,6 single fibres, interlaced yarns, and small ropes under a variety of loading conditions. The results show a similar degradation rate at each level of structure, with no apparent influence of inter-fibre effects. Cyclic lifetimes of single fibres of nylon 6,6 as well as polyester and aramid can be predicted from a creep rupture model. Consistent with this model, the time to failure is insensitive to frequency over a broad range. For each level of structure the strain at failure is the same whether tested in simple tension or under cyclic or creep loading. Failure modes were generally similar in creep rupture and cyclic fatigue tests; no effect of a slack load on each cycle was evident either in the failure mode or specimen lifetime.     

Einfluß des Reinheitsgrades,der Kaltverformung und des Schweißens auf das Zeitstandverhalten von X20 CrMoV 12 1-Rohrstahl bei 550 °C

Influence of purity, cold work and welding on creep rupture behaviour of X20 CrMoV 12 1 tubes at 550 °C In the water steam circuit of the German Thorium High Temperature Reactor (THTR 300) a high tempered martensitic X20 CrMoV 12 1 steel with low P and S contents and improved impact toughness was used at the first time for live steam and reheater pipes. There were similar and dissimilar weldings (with austenitic steel alloy 800). Small pipes were cold bended. Therefore ABB started a material testing programm supported by BMFT. Creep rupture tests at 550 °C til 60 000 h led to following results:

• Creep rupture strength and 1 % creep strain limit of the tougher grade lie in the lower half of the commercial steel scatterband.
• Under service creep loadings tertiary creep begins after a life-time of about 60% resp. a plastic elongation of about 1 %.
• The creep damage can be described by microstructural classes.
• Only for small stress ranges there are nearly constant creep stress exponents which are independent from the melts.
• Low loading speeds reduce static elastic moduls.
• Cold work reduces creep rupture strength and deformation.
• Under stress transverse to welding joints the design according to creep rupture strength of the base material must include a reduction factor. The fine grained part of the HAZ is the weakest area.

With the results of long-time examinations the design of future HTR-plants, fossil-fired power stations, and chemical plants may furtherly be optimized.     

Creep Crack Growth in Polyethylene Under Combined Mode I and Mode II Loading

Creep crack growth characteristics under various combined mode I and mode II loadings were studied using the compact tension shear (CTS) specimens of polyethylene. Creep crack growth rates da/dtunder combined mode I and mode II loading can be correlated with a single effective stress intensity factor K _Ieffderived from the combined — mode fracture toughness envelope. The steady state or constant crack growth rates which occupy the significant part of creep failure life increase with the increasing initial effective stress intensity factor.     

Creep and stress rupture behaviour of Be/Al composite material Albemet 162

Abstract

Creep and stress rupture tests were conducted on Albemet 162, a Be particulate reinforced Al matrix composite with a 70% volume fraction of reinforcement. A power law relationship was observed between steady state creep rate, stress rupture life, and applied stress. Considerable differences were seen between transverse and longitudinal specimens in terms of creep and rupture behaviour. Fractography of the specimens revealed that particle agglomeration of the Be phase is highly susceptible to creep damage and the primary cause of creep failure, which was observed in specimens of both orientations.     

Effect of strain waveform on creep-fatigue life for Sn–8Zn–3Bi solder

This paper describes the creep‐fatigue life of Sn–8Zn–3Bi under push–pull loading. Creep‐fatigue tests were carried out using Sn–8Zn–3Bi specimens in fast–fast, fast–slow, slow–fast, slow–slow and hold–time strain waveforms. Creep‐fatigue lives in the slow–slow and hold‐time waveforms showed a small reduction from the fast–fast lives but those in the slow–fast and fast–slow waveforms showed a significant reduction from the fast–fast lives. Conventional creep‐fatigue life prediction methods were applied to the experimental data and the applicability of the methods was discussed. Creep‐fatigue characteristics of Sn–8Zn–3Bi were compared with those of Sn–3.5Ag and Sn–37Pb.     

Effect of cyclic loading on subsequent creep behaviour and its implications in creep–fatigue life assessment

Abstract

Evaluation of creep–fatigue failure is essential in design and fitness evaluation of high-temperature components in power generation plants. Cyclic deformation may alter the creep properties of the material and taking cyclic effects into account may improve the accuracy of creep–fatigue failure life prediction. To evaluate such a possibility, creep tests were conducted on 316FR and modified 9Cr–1Mo steel specimens subjected to prior cyclic loading; their creep deformation and rupture behaviours were compared with those of as-received materials. It was found that creep rupture life and elongation generally decreased following cyclic loading in both materials. In particular, the rupture elongation of 316FR in long-term creep conditions drastically decreases as a result of being cyclically deformed at a large strain range. Use of creep rupture properties after cyclic deformation, instead of those of as-received material, in strain-based and energy-based life estimation approaches brought about a clear improvement of creep–fatigue life prediction.     

High-temperature fatigue behaviour of a second generation near-γ titanium aluminide sheet material under isothermal and thermomechanical conditions

The high-temperature deformation behaviour of a second generation γ-TiAl sheet material with near-γ microstructure was characterised under tensile, creep, isothermal and thermomechanical fatigue (TMF) loading conditions. Test temperature ranged from 500 to 750 °C in isothermal tests and these temperatures were also used as minimum and maximum temperature of in-phase (IP) and out-of-phase (OP) thermomechanical fatigue tests. Under tensile loading, a ductile-to-brittle transition temperature (DBTT) of about 650 °C was observed. At this temperature the material experiences a temperature dependent change in the fracture morphology. Creep tests carried out in the temperature range from 650 to 800 °C under true constant stress conditions revealed a temperature and stress dependence of the Norton stress exponent n and the apparent activation energy for creep Q_app. With increasing temperature, isothermal fatigue life at constant strain amplitude decreased in vacuum, but increased in air indicating an abnormal (inverse) environmental effect. Under IP loading, fatigue is characterised by cyclic softening due to dynamic recrystallisation. OP loading drastically reduces fatigue life and turned out to be an extremely critical loading situation for γ-TiAl alloys.     

Verminderung des Verformungsvermögens bei mehrachsigen Spannungszuständen unter Kriechbeanspruchung

Degradation of the Deformation Capacity under Creep due to Multiaxial States of Stress A method is proposed, which describes the degradation of the uniform true (rupture) strain φ due to the multiaxial stress states in creep. The method is based on the formulas of Siebel which describe the ratios of the uniform true strains in the 3 principle stress directions for any given states of stress: σ_m is the hydrostatic stress. These values for different given stress states are normalized by the principle stress σ_IZV for uniaxial tension with the same amount as for the multiaxial stress states. From such ratios it is possible to find out the real uniform true (rupture) strains φ for the different stress states in taking the uniform true (rupture) strain for uniaxial tension, φ_IBZV, as a basis: The comparison of predicted and tested φ-values proves that the formulas describe the toughness-degradations with the right tendency. For multiaxial states of stress it has to be kept in mind that we have two counterbalancing influences which are on the one hand the reduction of the creep rate due to lower v. Mises equivalent stresses and on the other hand the reduction of the deformation capacity, described above. A model ist proposed to estimate these influences of the states of stress on the creep rupture times.     

Effects of temperature on the creep behaviour of woven and stitched composites

This paper presents an investigation of the tensile creep behaviour of woven fibre composite stitched, through the thickness, with cotton or carbon threads along the loading direction. Creep tests were conducted at various temperatures. It was found that the through-thickness stitching significantly improved the creep deformation and creep rupture resistance of these composites. The creep data were analysed using the ‘timetemperature-stress superposition principle’ theory (TTSSP). The long-term behaviour of the material could then be predicted by means of a master curve. Finite-element analyses of the composites was also carried out and the stitching was found to considerably reduce the interlaminar stresses.     

蠕变/疲劳共同作用下寿命估算方法

通过载荷谱转换，将带保载时间的蠕变／疲劳循环用不带保载时间的纯疲劳循环代替，提出蠕变／疲劳共同作用时的寿命估算方法。对１２Ｃｒ１ＭｏＶ钢母材和焊材的蠕变／疲劳交互作用试验数据的分析结果表明，本方法方便、实用。提出一个表征蠕变／疲劳连续加载时交互作用行为的参数，蠕变／疲劳寿命比。分析认为，材料的蠕变疲劳交互作用行为与该比值的大小腾。材料在蠕变／疲劳共同作用下呈这是呈负交经作用，并非材料固有的特性，还取决于载荷条件。     

Influence of joint thickness on Type IV cracking behaviour of modified 9Cr-1Mo steel weld joint

Effect of joint thickness on Type IV cracking behaviour of modified 9Cr-1Mo steel weld joint has been investigated. Creep tests on multi-pass double-V cross weld joint flat specimens of the steel having thicknesses in the range 1–17 mm have been carried out at 923 K (650°C) and 50 MPa stress. Creep rupture life of the weld joint was found to increase with thickness and reached a maximum value around 10 mm of thickness followed by decrease with further increase in thickness. Failure in the weld joints occurred in the soft intercritical region of the heat-affected zone (HAZ). Creep strain localisation was observed at the fractured location and was more in the thinner weld joints than in the thicker weld joint. Creep cavitation in the intercritical region of HAZ close to the unaffected base metal was more extensive at the mid-location of the weld pass, where the HAZ width was relatively larger and hardness was lowest. The type IV cavitation in intercritical HAZ was more extensive in thicker joint, whereas creep strain concentration in the intercritical HAZ was more in thinner weld joint. Creep cavitation in the joint was more pronounced at near mid-thickness locations than those beneath the specimen surface. Joints of intermediate thickness possessed higher creep rupture life because of relatively less accumulation of creep deformation coupled with lower creep cavitation in the intercritical region of HAZ.