正交玻璃布/环氧复合材料蠕变损伤实验研究

本文在实验基础上,研究了正交玻璃布/环氧复合材料的蠕变损伤行为。考虑到材料剪切屈服效应,对Kachanov-Rabotnov理论引入了附加项,得到了材料的损伤演化方程和蠕变表达式。实验结果表明:沿纤维方向材料的蠕变量很小,可略而不计。沿其它方向加载时,由于基体参予承载,蠕变十分明显。当剪切应力在其强度极限的45%～60%范围内时,实验数据较为稳定,和本文理论曲线吻合。     

Fatigue behaviour and lifing of two single crystal superalloys

A model has been developed to predict the high temperature cyclic life of single crystal superalloys RR2000 and CMSX-4 under conditions of creep and fatigue. A combined creep–fatigue model is used, although it is found that failure always occurs by creep or fatigue separately, and that creep–fatigue interaction has a minor influence. Microstructural investigation of a series of interrupted high- and low-frequency tests are presented, these are combined with the results of a series of interrupted creep tests to identify the separate and interactive mechanisms of creep and fatigue. When creep damage is present the material behaves homogeneously. Under these conditions crack growth is initiation controlled, the mechanism of failure is surface or casting pore-initiated planar crack growth followed by shear on crystallographic planes. As the temperature is lowered or the cyclic frequency increased, the material behaves less homogeneously and shear bands are formed during cycling. Crack growth under these conditions is again initiation controlled and failure is by rapid crystallographic crack growth along shear bands. Such a failure is a distinct fatigue failure and occurs when little creep damage is present. Under certain cyclic conditions, mainly those where the crystallographic failure mechanism is dominant, the material shows an anomalous increase in fatigue resistance with temperature up to approximately 950 °C. This behaviour has been quantified by relating it to the effect of strain rate and temperature on the yield strength of the material.     

Shear Creep Deformation of the Super Alloy Single Crystal CMSX‐4 at High Temperatures and Low Stresses

In the present paper we investigate the shear creep behavior of the single crystal super alloy CMSX‐4 at temperatures between 950 and 1100 °C and shear stresses ranging from 80 to 155 MPa. A double shear creep test technique is used to study the shear creep behavior of four specific macroscopic crystallographic shear systems defined by a specific crystallographic shear plane and a specific crystallographic shear direction (systems investigated: {001}<110>, {100}<010>, {011}<01‐1>, and {111}<01‐1>). The shear creep behavior is analyzed in terms of the shape of individual creep curves and in terms of the stress and the temperature dependence of the secondary shear creep rate. Individual creep curves are generally characterized by a pronounced primary creep range where creep rates decrease by up to three orders of magnitude. A sharp creep rate minimum is not observed. The secondary creep range starts at shear stresses of the order of 0.02 and is followed by a secondary creep range which extends over shear strain ranges of the order of 0.1. No pronounced increase of shear creep rate in the later stages of creep is observed. Norton plots yield power law stress exponents ranging from 5.5 to 9.7. The temperature dependence of the secondary creep rate is of an Arrhenius type and apparent activation energies between 549 and 690 kJ/mol were found. There is a clear influence of crystallography on shear creep rates, which vary between different macroscopic crystallographic shear systems; this effect decreases with increasing temperature. The shear creep results obtained in the present study are discussed in the light of results from uniaxial testing and in the light of underlying microscopic deformation processes.     

Creep behaviour of AISI 316H stainless steel under stress-varying creep loading conditions: primary creep regeneration

Primary creep regeneration (PCR) is an important reported observation from creep under stress-varying conditions for several alloys. For a specimen deforming in the secondary creep regime, a stress reversal leads to an enhanced creep rate upon reloading due to reactivation of the primary creep regime (i.e. PCR). This paper focuses on an investigation of the PCR phenomenon during stress-varying creep loading for AISI 316H stainless steel at 650°C. The experimental observations clarify the influence of different parameters (e.g. forward creep stress level, reverse stress magnitude and forward and reverse accumulated inelastic strain) on the extent of PCR activation. In addition, a correlation between the extent of PCR activation and inelastic strain accumulation during the reverse loading period was found, which was employed to develop an empirical–phenomenological model for prediction of the creep behaviour of the alloy after stress transients (e.g. stress reversals).     

Influence of bonding on the tensile creep behavior of paper in a cyclic humidity environment

It has been shown, as paper structure is improved through increased bonding (by increasing relative bonded area or specific bond strength), a fully efficient loaded structure can be achieved. Once fully efficient, further improvements in bonding become redundant and have no effect on some paper deformation behaviors; deformation is dictated only by the characteristics of the fibers. Although previous work had shown this was true for elastic modulus and short time duration stress-strain behavior, it has only recently been shown to be true for constant humidity tensile creep behavior. In this study, the goal was to ascertain if cyclic humidity tensile creep behavior (known as accelerated creep) would follow the same trend. To accomplish this, sheets were made at differing levels of relative bonded area and specific bond strength. This was done by applying two different wet pressing levels (to alter relative bonded area) and using bonder and debonder (to change specific bond strength). It was found that accelerated creep behavior of paper sheets is no different than constant humidity creep behavior; changing bonding does not influence accelerated creep if the sheet has a fully efficient loaded structure. If the sheet structure is inefficiently loaded (there is no redundancy in bonding), accelerated creep will be affected by bonding. However, it is proposed that the only reason accelerated creep is influenced by bonding when inefficiently loaded is because constant humidity creep behavior determines the accelerated behavior and it is influenced, in this case, by bonding.     

Structural change of carbon-fibres at high temperatures under load

Creep tests were performed with carbon fibre‐bundles (Toho Tenax, HTA 5131) in a temperature range from 1500 to 1800 °C. The fibre‐bundles were electrically heated in vacuum (pressure 10^?4 mbar) and tensile load was applied in a hydraulic testing machine. The creep parameters were obtained from varying temperature and loading conditions. Accompanying structural investigations were performed with X‐rays (SAXS and WAXD). An increasing orientation of the graphene planes along the fibre (and thus the loading axis) could be observed with increasing temperature and load. No structural change and no creep, however, was observed for carbon fibres stabilized by an appropriate heat treatment.     

A new creep damage assessment method for metallic material under variable load conditions at elevated temperature

It is significant for structural design and maintenance to assess behaviour and life under varying load conditions. For structures operating in high‐temperature environments, creep is one mechanism responsible for material failures. In this paper, different damage accumulation rules were reviewed, and a new creep damage assessment method was proposed based on the creep damage tolerance parameter λ and load factor Φ. By introducing the creep damage tolerance parameter λ and the minimum creep rate, the loading process and creep behaviour of the material are taken into account in a damage assessment. The parameters in the model can be obtained by the simple variable load creep test, and the remaining life and strain can be predicted using uniaxial creep test data. To analyse the applicability and accuracy of this new model, the strain histories, the life of step load, and the constant load creep from experiments on a titanium alloy at 500°C were obtained, and the prediction results of the novel and previous methods were carefully investigated.     

Modeling basic creep of concrete since setting time

Modeling the early age evolution of concrete properties is necessary to predict the early age behavior of structures. In case of restrained shrinkage or application of prestress load [1], creep plays an important role in the determination of the effective stress. The difficulty lies in the fact that the modeling of creep must be based on experimental data at early age and this data must be obtained automatically because the hardening process of the concrete takes place rapidly during the first hours and also the first days. This paper presents a new methodology to model basic creep in compression since setting. Two kinds of tests are used: classical loadings and repeated minute-scale-duration loadings. The classical test is used to characterize the creep function for one age at loading and the repeated minute-scale-duration loadings test is used to define two ageing factors for the creep function. A new model based on the physical mechanisms and the two ageing factors is presented. A comparison with the Model Code 2010 is done and an advanced way to consider ageing with the Model Code 2010 is presented.     

沥青砂混合料粘弹塑力学特性研究

在0.1MPa、0.15MPa、0.2MPa、0.25MPa和0.3MPa下进行了沥青砂试样单轴压缩和蠕变实验,分析了其压缩和蠕变性质,根据变形机理提出了粘弹塑本构模型可由粘弹性和粘塑性的两个子模型串联构成,通过对粘塑性子模型中粘性系数进行改进,理论推导了模型蠕变本构方程,确定了模型参数,并求得模型参数与加载应力函数关系。进行模型预测与实验结果对比,结果表明:该模型能够描述沥青砂试样在不同应力下蠕变变形的3个阶段,反映了沥青砂混合料粘弹塑变形特点。     

基于广义Kelvin链钢管混凝土徐变研究

混凝土徐变是混凝土材料本身固有的一个时变特性,是结构响应中一个重要组成部分,其计算方法通常是建立在单轴试验和理论基础上。为探讨钢管混凝土徐变特性,该文采用自制的压力自平衡混凝土徐变试验装置对混凝土圆柱和圆钢管混凝土柱进行了徐变试验,结果表明:钢管混凝土柱徐变变形要比普通混凝土柱的徐变变形小,在该文试验中两者相差接近50%,这可能是密闭钢管内核心混凝土无法与外界发生水分交换而不发生干燥收缩和干燥徐变以及钢管围压所致。根据粘弹性理论,引入多参数Kelvin链粘弹性元件模型,建立了求解单轴应力状态下混凝土徐变的Volterra型积分方程,模型参数近似表示为连续粘滞谱。通过离散时间变量t和分步积分,进一步得到了单轴应力状态下混凝土徐变应力-应变增量本构模型。依据徐变叠加原理,考虑Poisson效应,进一步将单轴应力状态下混凝土徐变应力-应变增量本构模型拓展到三轴应力状态,用于钢管混凝土徐变分析。对有限元商用软件Ansys进行二次开发,将反映三轴应力状态下混凝土徐变性能的本构方程引入Ansys提供的用户子程序Usermat中,并采用Fortran语言编程,从而实现了钢管混凝土徐变长期性能的有限元分析计算。将有限元数值解与试验结果进行对比分析,发现该文提出的模型是科学的和有效的。该文提出的方法将为混凝土徐变计算提供了另一条有效途径。     

Constitutive description of creep behavior of M-4Al-1Ca alloy

Creep behavior of an advanced magnesium alloy AX41 (4 wt.% Al, 1 wt.% Ca, Mg balanced) was investigated in temperature interval from 343 to 673 K and stresses from 2 to 200 MPa. Compressive creep experiments with stepwise loading were used in order to obtain stress dependence of the creep rate in interval from 10⁻⁹ to 10⁻³ s⁻¹ for a given temperature. All stress dependences can be well described by the Garofalo sinh relationship with natural exponent n = 5. An analysis of the parameters of this relationship has shown that lattice diffusion controls creep at all experimental conditions. While climb-controlled creep mechanism is decisive at lower stresses and higher temperatures, glide-controlled mechanisms act at higher stresses and lower temperatures. A typical power-law breakdown is observed at intermediate stresses and temperatures. __________ Translated from Problemy Prochnosti, No. 1, pp. 36–39, January–February, 2008.     

Nonlinearly Viscoelastic Behavior of Polycarbonate. I. Response under Pure Shear

The nonlinearly thermo-mechanical creep behavior of (bisphenol A)polycarbonate was investigated at different temperatures (0 to 140°C) under pure shear loading. The shear creep in the linearlyviscoelastic range was measured with a torsiometer for referencepurposes and a master curve, along with a shift factor curve, wasconstructed. While the master curve is well defined with no detectabledeviation, the shift factor can be represented by essentially twostraight lines and offset at the transition temperature ofpolycarbonate. The shear creep tests in the nonlinearly viscoelasticrange were conducted on an Arcan specimen geometry at differenttemperatures and under different stress levels, utilizing digital imagecorrelation for recording the creep strains. The difference between thenominal stress and the actual stress distribution in the Arcan specimenwas explored via numerical simulations (ABAQUS) by assuming linearlyquasi-elastic and quasi-plastic analyses in place of the as yetuncertain material characterization. Isochronal plots were created fromthe creep data. Nonlinearly viscoelastic behavior starts to take effectnear 1% strain at the temperatures considered. The applicability of thestress-clock representation for material characterization has beenexplored and is found, at best, to be dubious for this material. The`yield-like' behavior of polycarbonate has been examined in terms of theisochronal stress-strain response and a corresponding `yield-like shearstress' has been determined to be a monotonically decreasing function ofthe temperature, but again with an interruption or `jog' at the transition temperature. Time-temperature trade-off as practiced for`time-temperature shifting' at small strains does not apply in thenonlinear domain. The results are generally in agreement with thosefound for Poly(Methyl Methacrylate), thus fostering the idea that thepresent results can be generalized – with additional work – to other amorphous polymers.     

Influence of humidity on creep response of sandwich beam with a viscoelastic soft core

Polymer foam cored sandwich beams are widely used in load-bearing components due to their high strength to weight ratio. To improve the reliability in using sandwich beams, it is essential to understand their long-term creep response in terms of variation of stresses and deformations with time under external mechanical and environmental stimuli. This paper presents an analytical model for investigating the creep response of sandwich beams made with a viscoelastic soft core, including the effect of the variable ambient humidity under the sustained load and its influence on the creep behavior. The model is based on a high-order viscoelastic structural modeling. The soft core is modeled as a viscoelastic material using differential-type constitutive relations that are based on the linear Boltzman’s principle of superposition and accounting for the deformability of the core in shear and through its thickness. Several numerical examples are presented in order to show the capability of the model and to investigate the effect of moisture on the creep behavior of sandwich beams. Finite element simulations of the creep response of sandwich beams are also performed using ABAQUS software to validate the proposed theoretical model. The results show the concentrations of shear and transverse normal stresses near the edges and their variation in time and with the change of humidity.     

Nonlinear Shear Behavior of Poly(vinyl acetate) Material

Because of the strong environmental sensitivity of poly(vinyl acetate), PVAc, especially with respect to moisture, and the fact that shear deformation is essentially equivoluminal up to moderate strain levels, little has been reported in the literature on the nonlinear mechanical creep behavior of this polymeric material loaded in shear. This paper presents the results of torsional tests which establish the shear response through the linear zone and well into the nonlinear region. Test specimens were thin-walled cylinders giving an approximately uniform deformation field. Because of carefully chosen wall thickness to length ratio, it is considered that these measurements represent some of the most accurate nonlinear shear results to date in the strain range above 1%. Measurements of stress, strain and creep compliance were made at temperatures near the glass transition temperature and somewhat below it. Isochronal shear stress-strain dependence into the nonlinear range was used to establish limits of viscoelastic linearity during creep. As temperature is increased toward the glass transition, the limit shows a greater dependence on stress than on strain. The stored distortional strain energy at the limit of linearity was not a constant but varied with temperature and load. Thus, these results appear not to support the concept of stored energy as a material property defining the threshold for nonlinear viscoelastic behavior. Strain during the short-time load-up period gives evidence that PVAc is also subject to nonlinear elasticity in the glassy response region.     

Effect of a viscoelastic interphase on the creep and stress/strain behavior of fiber-reinforced polymer matrix composites

The influence of a viscoelastic interphase on the overall creep compliances and stress/strain relationships of fiber-reinforced polymer-matrix composites under a constant stress and a constant strain-rate loading are examined. The fibers are taken to be elastic but the matrix is also viscoelastic. Evaluation of the overall property is based upon the composite cylinder assemblage and the generalized self-consistent scheme. It is found that, except for the axial tensile behavior, which is fiber-dominated, the creep and stress/strain responses under transverse tension, transverse shear, axial shear, and plane-strain biaxial tension, are all significantly influenced by the interphase. A detailed examination of these effects in the light of the interphase property and volume concentration is carried out, and the results reveal that, when the interphase is viscoelastically softer than the matrix, its presence will cause a very pronounced influence on the creep strength and load-carrying capacity of the three-phase system.     

Physical aging in the creep behavior of thermosetting and thermoplastic composites

The creep behavior of a series of fiber-reinforced plastics (FRP) and corresponding resins has been studied, with emphasis on elucidating the role of physical aging effects on FRP viscoelastic behavior. Thermosetting and thermoplastic composites were studied, representing semicrystalline, amorphous, and highly filled amorphous polymer matrix FRPs. It was found that physical aging effects are operative for all FRPs, including the semicrystalline systems. Time/ aging-time and time/temperature superposition are found to be valid procedures for short-term creep behaviour; they cannot be applied to long-term creep behavior. However, long-term creep can be satisfactorily predicted from momentary creep by using an effective time theory. Evidence of a universal, temperature shift factor temperature dependence is presented.     

Advances in numerical analysis of creep effect in time-dependent deflection of light-weight concrete slabs

Abstract

Using a wide range of creep models, the experimental results of long-term deflection of lightweight concrete slabs subjected to two levels of early-age loading are investigated. Different creep models give considerably different estimation of the experimental deflection of slabs. The included factors in each creep model to simulate the experimental creep behavior of the concrete, and loading level on the slabs are the main causes of different results. Among the investigated models, the BP1 and FIBMC-2010 models including the aggregate type and concrete density is shown to be in good agreement with the experimental data in both loading levels.     

Viscoplastic analysis of structural polymer composites using stress relaxation and creep data

A structural carbon based composite material has been investigated for its high temperature viscoplastic properties using a model based on an overbearing stress concept and using the data obtained from load relaxation and creep. The time dependent viscoplastic properties were obtained at several load and temperature levels. An elastic–viscoplastic constitutive model (proposed by Gates) was used for the modeling efforts. The model is based on an overstress concept appropriate to inelastic properties of composites. The materials parameters for the model are obtained from a set of load relaxation experiments. The model predictions have been compared to the results of creep tests. The results show that the model is capable of predicting the creep behavior at shorter time periods and lower temperatures. As the temperature is increased or as the creep is prolonged the model predictions deviate from the experimental results.