Cooling rate influences in carbon fibre/PEEK composites. Part 1. Crystallinity and interface adhesion

The effect of cooling rate on the fibre–matrix interface adhesion for a carbon fibre/semicrystalline polyetheretherketone (PEEK) composite was characterised based on the fibre fragmentation, fibre pullout and short beam shear tests. The interface adhesion was correlated to the degree of crystallinity and the crystalline morphology, as well as the bulk mechanical properties of neat PEEK resin, all of which were in turn controlled by cooling rate. It was shown that the interface bond strength decreased with increasing cooling rate; the tensile strength and elastic modulus of PEEK resin decreased, while the ductility increased with increasing cooling rate through its dominant effect on crystallinity and spherullite size. The improvement of crystalline perfection and flattened lamella chains with high crystallinity at the interphase region were mainly responsible for the strong interface bond in composites processed at a low cooling rate. The interphase failure was characterised by brittle debonding in slow-cooled composites, whereas the amorphous PEEK-rich interphase introduced in fast cooled specimens failed in a ductile manner with extensive plastic yielding.     

Viscoelastic Model of Cross-Linked Polyethylene Including Effects of Temperature and Crystallinity

Characterization of the mechanical behavior of cross-linked polyethylene (XLPE) commonly used in high voltage cable insulation was performed by an extensive set of isothermal uniaxial tensile relaxation tests. Tensile relaxation experiments were complemented by pressure-volume-temperature experiments as well as density and crystallinity measurements. Based on the experimental results, a viscoelastic power law model with four parameters was formulated, incorporating temperature and crystallinity dependence. It was found that a master curve can be developed by both horizontal and vertical shifting of the relaxation curves. The model was evaluated by making comparisons of the predicted stress responses with the measured responses in relaxation tests with transient temperature histories.     

Experimental investigation of near crack tip creep deformation in alloy 800 at 650^C

Using a novel, noncontacting experimental method, the values for creep fracture parameters C(t) and C* have been experimentally quantified from full field, time-dependent surface deformation measurements in the crack-tip region for the first time. Near-tip experimental deformation results were obtained for a stationary crack in alloy IN800 at 650^C and compared to predictions based on both far-field measurement and finite element analyses. Results indicate that (a) for short times, the C(t)- integral is time and path dependent; (b) for long times, the C(t)-integral approached a constant for all paths which is agreement with C* values obtained from far-field experimental data. Comparisons of experimental results with FEM solutions were illustrated by means of radial and angular variations, full-field contour plots, as well as line integration form. Results from these comparisons indicate that (c) the experimental strain data portrays trends similar to those observed in the FEM solutions; (d) experimental, near-tip strain values were different in magnitude from the predicted values for all angles, and (e) the transition time obtained from the tests was about 10 times longer than that calculated from a formula proposed by Riedel and Rice (1980), indicating that primary creep cannot be neglected for this material. This revised version was published online in August 2006 with corrections to the Cover Date.     

Experimental Study of the Creep Lifetime of the 1.25Cr 0.5Mo Steel Pipes

Abstract: In this paper, physical parameters for the creep constitutive equations of the low alloy ferritic steel 1.25Cr0.5Mo have been determined using experimental data. This alloy is used mostly in power generation and petrochemical industries because of its high temperature creep resistance. Test samples have been obtained from a new super‐heater pipe wall of a steam‐generating boiler in Tabriz Petrochemical Plant according to the ASTM standards. By conducting creep rupture tests for 1.25Cr0.5Mo steel, creep behaviour and creep‐rupture properties were examined for this material. Creep rupture tests have been carried out at four temperatures of 700, 725, 750 and 800 °C, under applied uni‐axial stresses of 30, 35, 40 and 50 MPa. The experimental data have been used to obtain the constitutive parameters using numerical optimisation techniques. Also the temperature and stress dependency of the creep lifetime for this alloy has been investigated using Larson–Miller and Monkman–Grant parameters. The results show good agreement with other test data such as ASTM and API. Finally, these constitutive equations have been used to study the creep behaviour of the super‐heater pipe. The results show that the super‐heater tube has been over designed in terms of the creep lifetime and this is in accordance with the in‐plant observations.     

Experimental Study of the Creep Behaviour of the Cold‐Drawn 304L Stainless Steel

Abstract: In this article, the material and physical parameters for the creep constitutive equations of cold‐drawn 304L stainless steel have been determined using experimental data. Austenitic stainless steel 304L is used mostly in power generation and petrochemical industries because of its high‐temperature creep resistance even at above yield stresses. Test samples have been obtained from cold‐drawn bars, and the material conforms to ASTM A276‐05a specifications. The creep behaviour and properties have been examined for this material by conducting uni‐axial creep tests. Constant temperature and constant load uni‐axial creep tests have been carried out at three temperatures of 680,700 and 720 °C, subjected to constant loads which produce below and above yield initial stresses of 200, 250, 320, 340 and 360 MPa. The experimental data have been used to obtain the creep constitutive parameters using numerical optimisation techniques. In addition, the temperature and stress dependency of the creep properties for this alloy have been investigated using Larson–Miller and Monkman–Grant parameters.     

The essential work of plane-stress ductile fracture of poly(ether-ether ketone) thermoplastic

The total work of fracture in a ductile material is not a material constant and the linear elastic fracture mechanics is not appropriate. Only the work performed per unit area in the fracture process zone, called the specific essential work of fracture, is a material constant for a given specimen thickness. The results of an experimental investigation on the essential work of fracture of a crystalline and a non-crystalline poly(ether-ether ketone) (PEEK) films are reported. Single-edge notched specimens were used to determine the specific essential work by extrapolating the straight-line relationship between the specific work of fracture and ligament length to zero ligament length. In this way, the specific essential work of fracture for the crystalline PEEK film of thickness 0.1 mm was measured to be 65.02 kJ m^–2 and for the non-crystalline film of thickness 0.25 mm was measured as 62.71 kJ m^–2. Advancing crack opening displacements (COD) have also been analysed and the specific essential work of fracture calculated from the COD values compared extremely well with those results obtained from the intercept of specific work of fracture versus ligament length.     

Creep and Creep Recovery Response of Load Cells Tested According to U.S. and International Evaluation Procedures

The 60 min creep data from National Type Evaluation Procedure (NTEP) tests performed at the National Institute of Standards and Technology (NIST) on 65 load cells have been analyzed in order to compare their creep and creep recovery responses, and to compare the 60 min creep with creep over shorter time periods. To facilitate this comparison the data were fitted to a multiple-term exponential equation, which adequately describes the creep and creep recovery responses of load cells. The use of such a curve fit reduces the effect of the random error in the indicator readings on the calculated values of the load cell creep. Examination of the fitted curves show that the creep recovery responses, after inversion by a change in sign, are generally similar in shape to the creep response, but smaller in magnitude. The average ratio of the absolute value of the maximum creep recovery to the maximum creep is 0.86; however, no reliable correlation between creep and creep recovery can be drawn from the data. The fitted curves were also used to compare the 60 min creep of the NTEP analysis with the 30 min creep and other parameters calculated according to the Organization Internationale de Métrologie Légale (OIML) R 60 analysis. The average ratio of the 30 min creep value to the 60 min value is 0.84. The OIML class C creep tolerance is less than 0.5 of the NTEP tolerance for classes III and III L.     

Effects of Processing Variables on Fatigue in Molded PEEK and Its Short Fiber Composites

The effects of various material and processing parameters on the fatigue behavior of PEEK and its short glass and carbon fiber reinforced composites have been investigated. In particular, the significance of molecular weight, fiber type and increased matrix crystallinity, induced by post-forming annealing, are considered in this paper. Standard S-N (peak stress against log (cycles to failure(( fatigue data curves have been generated and crack propagation rates have been monitored as a function of the alternating stress intensity factor. Scanning electron microscopy (SEM), optical microscopy, X-ray diffraction and differential scanning calorimetry (DSC) have all been used to characterize the microstructural parameters and failure mechanisms. Results show that the increased crystallinity, induced by annealing, significantly improves fatigue crack resistance for both neat PEEK and its short fiber composites. Generally, superior fatigue behavior is associated with the higher molecular weight matrix and with carbon rather than glass fibers. However, it proved difficult to unravel the complex interdependence of various matrix factors such as molecular weight and crystallinity, and fiber type, length, aspect ratio and preferred orientation. Nevertheless, a combination of the tie molecule density concept and a reinforcing efficiency model, developed to take into account these various parameters, has helped to explain the complexity of the results.     

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.     

Activation theory for creep of matrix resin and carbon fibre-reinforced polymer composite

Activation theory used in metals and polymers has been used to model creep of unidirectional composite and resin matrix, using the concept of internal stress. The model fits the experimental creep curves very well for a range of materials. The results obtained for a brittle epoxy and its carbon fibre-reinforced composite with two fibre orientations are reported. The model parameters, such as internal stress, activation volume and activation energy, have been measured experimentally and compared with model-fit values, and their influence on creep is discussed. Finally, an approach to predict the creep rupture of unidirectional composites using internal stress is presented.     

Prediction of long-term viscoelastic behavior of amorphous resin based on the time-temperature superposition principle

This paper deals with the prediction of long-term viscoelastic behavior of amorphous resin at a temperature below the glass transition temperature T _g from measuring the short-term viscoelastic behavior at elevated temperatures based on the time-temperature superposition principle (TTSP) with vertical shift as well as horizontal shift. The long-term creep compliance as well as short-term and medium-term creep compliances were measured at elevated temperatures. The master curves of creep compliance can be constructed from measured data by shifting vertically as well as horizontally. The master curves of creep compliance constructed from measured data by short-term and medium-term creep tests agree well with those measured by long-term creep tests. Furthermore, the horizontal and vertical shift factors obtained from constructing the master curve are independent of the time period of creep tests. Therefore, the long-term viscoelastic behavior at a temperature below T _g can be predicted accurately from measuring the short-term viscoelastic behavior at elevated temperatures based on the TTSP with vertical shift as well as horizontal shift.     

Characterization of the global deformation behaviour of engineering plastics rolls

In real service application of polymer rolls as rolling elements, the time-dependent deformation behaviour leads to a negative influence on the running smoothness after a long-lasting static load. By means of two polymeric materials (POM and PEEK) with different viscoelastic properties over a wide temperature range the influence of material properties on the running smoothness was investigated. A combination of static and dynamic tests as well as finite element simulations were performed. Observed differences in the behaviour of both materials in the service could be reflected well by this work. POM revealed creep flattening and a running performance which could be modelled well by viscoelastic material laws. By contrast, PEEK showed a different performance and its behaviour could not be represented adequately by viscoelastic material models.     

Beschreibung des Zeitdehnverhaltens warmfester Stähle Teil 2: Kriechgleichungen für die Stahlsorten 10 CrMo 9 10 und X 20(22) CrMoV 12 1

Creep Behaviour of Heat Resistant Steels Part II: Creep Equations for Steels 2.25 Cr-1 Mo and 12 Cr-1 Mo-0.3 V Creep data scatter bands of steels 2.25 Cr-1 Mo and 12 Cr-1 Mo-0.3 V were evaluated with the aid of model functions based on time temperature parameters. From the times to reach given strain values, mean isostrain curves in the stress time diagramme were calculated and therefrom, mean creep curves were derived. On this basis, creep equations were established, which include primary-, secondary- and tertiary-creep and are valid in the main range of application of each steel. Further, mean stress strain curves from hot tensile tests were used to describe the initial plastic strain in the creep equations. The values calculated with the established creep equations agreed relatively well with the correspondent original scatter band values from the creep tests.     

Some experimental findings in compression-after-impact (CAI) tests of CF/PEEK (APC-2) and conventional CF/epoxy flat plates

Compression-after-impact (CAI) tests have been conducted for quasi-isotropic thick plates with 48 plies by using the NASA method and on plates with 32 plies by using the SACMA method. Specimens are made of CF/PEEK (APC-2) and conventional CF/epoxy for the NASA plates and CF/epoxy for the SACMA plates. In the NASA CAI tests, the sequence of delamination buckling and its propagation is clearly revealed through various experimental techniques. One major technique is moiré topography, and the other is thermo-mechanical stress analysis with a high-accuracy infrared sensor. The arrest of delamination propagation just before catastrophic failure due to high fracture toughness is clearly captured by the moiré camera. This behavior provides good CAI values of CF/PEEK. The initial buckling properties of the delaminated region by the impact are then extensively discussed. Numerical predictions of initial buckling stress have been obtained by modelled geometry of the delaminated region simplified from its precise structure clarified by ultrasonic C-scanning. They agree fairly well with the experimental results. The in-plane stress distribution in the delaminated region before initial buckling is measured by an infrared stress graphic system. This compared favorably with finite element predictions. Two types of symmetric buckling modes with respect to the central plate surface, twin and single peak ones, are experimentally captured.     

The creep behaviour of poly(vinylidene fluoride)/“bud-branched” nanotubes nanocomposites

The creep behaviour of poly(vinylidene fluoride) (PVDF)/multiwall carbon nanotubes nanocomposites has been studied at different stress levels and temperatures. To fine-tune the ability to transfer stress from matrix to carbon nanotubes, bud-branched nanotubes, were fabricated. The PVDF showed improved creep resistance with the addition of carbon nanotubes. However, bud-branched nanotubes showed a modified stress–temperature-dependent creep resistance compared with carbon nanotubes. At low stress levels and low temperatures, bud-branched nanotubes showed better improvement of the creep resistance than that of virgin carbon nanotubes, while at high stress levels and high temperatures, the virgin carbon nanotubes presented better creep resistance than that of bud-branched nanotubes. DSC, WAXD, and FTIR were employed to characterise the crystalline structures and dynamic mechanical properties were characterised by DMA testing. The Burgers’ model and the Findley power law were employed to model the creep behaviour, and both were found well describe the creep behaviour of PVDF and its nanocomposites. The relationship between the structures and properties was analysed based on the parameters of the modelling. The improved creep resistance for PVDF by the addition of nanotubes would benefit its application in thermoset composite welding technology.     

The effects of solvent sorption on the properties of poly(ether ether ketone)

A study has been carried out on the sorption of ortho-dichlorobenzene, N,N-dimethylformamide and water by poly(ether ether ketone) (PEEK). Two types of PEEK samples have been analysed; the first in the amorphous state and the second with a high crystallinity level. The sorption and desorption curves have been determined and the effect of the solvent presence on the mechanical properties has been analysed by means of the tensile test. PEEK is affected to a different extent by the solvents studied and also by the crystallinity of the polymer. Sorption takes place only in the case of amorphous PEEK. Its effect on the mechanical properties of PEEK is explained on the basis of the two concomitant processes that are the consequence of sorption, these are: plasticization and induced crystallization.     

应力水平和纤维角度对CGF/PP复合材料蠕变行为的影响及其Burgers模型参数的数值预测

采用DMA的Creep模式分别测试了短时间内（15 min）聚丙烯（PP）在不同应力水平和温度下的单向拉伸蠕变行为,长时间内（10 h）连续玻璃纤维增强聚丙烯（CGF/PP）复合材料单层板在不同应力水平和不同纤维角度上的拉伸蠕变行为。利用Burgers黏弹性模型拟合了蠕变测试数据,构建了相关参数与应力水平和纤维角度的依赖性。结果表明:PP和CGF/PP单层板的蠕变柔量均随应力增大而显著增加,稳态蠕变速率也随之增加,蠕变模量保留率明显下降,PP基体的黏弹性主要决定了CGF/PP单层板在低应力水平下的蠕变行为; 30%应力水平下,偏轴拉伸的纤维角度在0°~90°范围内存在拉-剪耦合效应,在45°时最为显著,此时稳态蠕变速率和蠕变变形量最大;利用四元件Burgers黏弹性模型拟合各条件下蠕变曲线得到的数值模型与实验数据具有较好的相关性,相关系数达到0.99,从得到的数值模型可知相关模型参数存在明显的应力和角度依赖关系;利用模型参数的数值拟合公式分别预测10 MPa应力下0°纤维方向的蠕变曲线及45°纤维方向上30%应力水平的偏轴蠕变曲线均与实验曲线一致,表明本文得到的数值模型的可靠性。      

Thin film and bulk deformation behaviour of poly(ether ether ketone)/poly(ether imide) blends

The deformation behaviour of amorphous thin films of poly(ether ether ketone) (PEEK)/poly(ether imide) (PEI) blends was investigated over a wide temperature range by optical and transmission electron microscopy. All the materials showed localized shear deformation at temperatures well below Tg. In pure PEI and in blends with up to 60 wt% PEEK content, a transition from shear deformation to disentanglement crazing occurred as the temperature was raised. However, this transition was absent in PEEK, which deformed by shear over the whole temperature range, and similar behaviour was found for PEI/80 wt% PEEK. It is argued that at high PEEK content disentanglement crazing is suppressed by strain-induced crystallization and some evidence for crystalline order in deformed regions of initially amorphous PEEK thin films was obtained by electron diffraction. The thin film deformation behaviour of the blends was also shown to be consistent with their bulk deformation behaviour, a high temperature ductile–brittle transition being observed at low PEEK content in tensile tests. This revised version was published online in November 2006 with corrections to the Cover Date.     

Thermomechanical fatigue behaviour of nickel base superalloy IN738LC Part 1 – Comparison of two unified constitutive models for description of mechanical behaviour

Abstract

The capabilities of two unified constitutive models to predict the mechanical behaviour of nickel base superalloy IN738LC under uniaxial loading conditions have been investigated over the temperature range 450–850°C. The material parameters of each model have been identified from an experimental investigation and complemented by available data from the literature. Mechanical responses from isothermal tests performed at 450 and 850°C (such as creep, monotonic, and fully reversed isothermal cyclic tests) were used for the identification of model parameters, and model capabilities were examined by comparison with in phase and out of phase thermomechanical fatigue and stress relaxation tests. Both models were found to capture all features of the material responses under uniaxial loading, although with varying degrees of accuracy.     

A small cantilever beam test for determination of creep properties of materials

The application of small specimen creep test techniques in the evaluation of creep properties of materials in‐service has been increasing. To obtain the creep data accurately and conveniently, a new creep test method with small cantilever beam specimens is proposed. Analytical equations are derived that can convert the load to equivalent uniaxial stress and the displacement rate to equivalent uniaxial strain rate. Three types of the cantilever beam specimens are designed. The optimal configuration of the cantilever beam specimens is recommended with the aid of finite element method, which is further validated by the cantilever beam and uniaxial specimen tests. The results show that parameters obtained from the cantilever beam tests correspond reasonably well with those from uniaxial tests at low stress levels. With a relatively large equivalent gauge length, the cantilever beam specimen allows the small creep strain rate data obtained with a high accuracy.