Term Creep Research Articles

Prediction of creep crack growth behaviour in 316H stainless steel for a range of specimen geometries

The specimen geometry and constraint effects on the creep crack growth behaviour of Type 316H stainless steel at 550 °C have been examined over a wide range of load levels using finite element simulations. Creep crack growth predictions are performed on a range of specimen geometries by employing stress dependent creep ductility and strain rate trends in creep damage calculations. The predicted creep crack growth rates are characterised using the C* fracture mechanics parameter and validated through comparison with the existing experimental data. Comparisons have been made between the predicted short term and long term creep crack growth behaviour in different specimens and the results are discussed in terms of the specimen constraint effects on the crack growth behaviour of the material. Two material states including as-received and pre-compressed conditions have been considered and their predicted creep crack growth results are compared in each of the specimen geometries examined.

Progressive shear-surface development in cohesive materials; implications for landslide behaviour

The aim of this study was to investigate mechanisms of progressive shear surface development using a series of specialised triaxial cell tests. Intact and remoulded samples of Gault Clay from the Ventnor Undercliff on the Isle of Wight in southern England were subjected to pore pressure reinflation (PPR) testing in a triaxial cell, in which failure is generated by increasing pore pressure under a constant total stress state. In addition, a novel very long term (>500days) creep test was undertaken, in which the sample eventually failed at a constant stress state below the failure envelope.The experiments showed that undisturbed samples of the Gault Clay failed in a brittle manner, generating a linear trend when plotted using the Saito technique. On the other hand, remoulded samples showed ductile behaviour, as indicated by a non-linear Saito trend. A number of otherwise identical PPR tests were conducted in which the rate of increase in pore water pressure was varied. These tests showed that strain rate generated at any point in the PPR tests depended on both the effective stress and the rate of change of effective stress. The latter is important because a change in stress generates a change in strain. Thus, whilst tests at different rates of change of effective stress are similar when plotted in q–p′ space and in strain–p′ space, they are markedly different in strain rate–p′ space.The long term creep test failed when the stress state had been constant for over 80days. This mechanism was reminiscent of creep rupture, occurring below the failure envelope defined in the conventional experiments.We conclude that first time failure in the Gault Clay is a progressive mechanism dominated by the development of micro-cracking, which leads to strain localisation and the development of one or more shear surfaces at failure. Whilst this mechanism may usually occur in response to a change in stress, the study indicates that failure can develop progressively. In the remoulded Gault Clay shear strains cannot localise along a singular shear surface.The results provide new insight into the mechanisms of landslide movement operating within the Ventnor landslide complex and indicate that present movements are likely to be occurring on a pre-existing shear surface. The lab tests suggest that this material is unlikely to undergo catastrophic failure.

Advanced Microstructures for Increased Creep Rupture Strength of MARBN Steels

Over the past three decades a lot of effort was made to optimize the chemical compositionof 9% Cr martensitic steels, aiming to increase the operating temperature up to 923K and thus im-proving the efficiency of thermal power plants. Under these service conditions (high temperature andstress exposure), the creep strength of such steels is closely related to the long term stability of theirmicrostructure. The time to rupture can also be understood as an equivalent to the time of microstruc-ture deterioration. Optimization of the initial microstructure and understanding of the microstructureevolution during creep exposure are therefore decisive to improve the creep behavior of 9% Cr steels.Selected chemical compositions of MarBN steels (Martensitic 9% Cr steels strengthened by Car-bides, Nitrides and Boron) were subjected to different heat treatments to produce an optimized mi-crostructure to improve the creep rupture time. The initial microstructure before creep exposure wasinvestigated using optical microscopy, SEM and EBSD. Short term creep rupture tests at 923K and150MPa were performed, followed by systematic microstructure investigations.Comparative EBSD investigations confirm an optimized microstructure for creep exposure, pro-duced by an appropriate heat treatment. From comparative creep test results, it can be concluded thatadvanced microstructures increase the time to rupture of the selected MarBN steels by more than 10percent, without reduction of the ductility.

Numerical analysis of retained residual stresses in C(T) specimen extracted from a multi-pass austenitic weld and their effect on crack growth

Small scale fracture mechanics test specimens of austenitic stainless steel weld and heat affected zone material are often extracted from non-heat-treated weldments, which contain significant weld residual stresses. Although these stresses are substantially relaxed by the process of specimen extraction, they may still reach levels that can affect subsequent testing if the applied loads are low and deformation is elastic. Long-term creep crack growth testing is one such case, where failure to take account of retained residual stresses could result in unrealistically high measurements of creep crack growth at applied load levels equivalent to those in operating plant. This paper describes a research programme to predict the start-of-creep-test levels of retained residual stress and residual stress intensity factor in compact tension C(T) specimen blanks extracted from non-post heat-treated AISI 316 weldments. The simulations were validated using neutron diffraction and slitting residual stress measurements and stress intensity factor measurement. A pass-by-pass finite element simulation of the original weldment is performed first, and followed by extraction of the C(T) specimen blank. The predicted retained residual stresses in the specimen are compared with residual stress measurements made on similar blank using neutron diffraction, and slitting techniques. The elastic stress intensity factor due to residual stress is then evaluated on the crack plane of the C(T) specimen and compared with experimental measurements made using the slitting method. Good agreement is achieved between measurement and simulation, providing validated basis for future modelling of long term creep crack growth tests.

Long term response of glass–PVB double-lap joints

The paper presents the results of an experimental research on the viscoelastic behavior of laminated glasses with polyvinyl-butyral interlayers. The problem is investigated with double lap joints under compressive loadings over a temperature range between −20°C and 50°C. The viscoelastic mechanical parameters of polyvinyl-butyral play a fundamental role in determining the response of laminated glass and consequently, their knowledge is required for a reliable design, especially in structural application, such as beams, plates, fins, columns, walls, façade and roofs. Tests were performed on specimens made of three glass plies under long-term loads in different conditions of temperature and relative humidity. Shear stresses and relative displacements among the glass-interlayer interfaces were measured. The response is presented and discussed. To obtain a long term creep model using time–temperature superposition, an eight terms Prony series and the shift factors obtained from the Williams, Landel and Ferry equation were considered.

Viscoelastic properties of kenaf reinforced unsaturated polyester composites

In order to quantify the effect of temperature on the mechanical and dynamic properties of kenaf fiber unsaturated polyester composites, formulations containing 10 wt.% to 40 wt.% kenaf fiber were produced and tested at two representative temperatures of 30°C and 50°C. Dynamic mechanical analysis was performed, to obtain the strain and creep compliance for kenaf composites at various styrene concentrations. It is possible to obtain creep curves at different temperature levels which can be shifted along the time axis to generate a single curve known as a master curve. This technique is known as the time–temperature superposition principle. Shift factors conformed to a William–Landel–Ferry (WLF) equation. However, more long term creep data was needed in order to further validate the applicability of time-temperature superposition principle (TTSP) to this material. The primary creep strain model was fitted to 60 min creep data. The resulting equation was then extrapolated to 5.5 days; the creep strain model of power-law was successfully used to predict the long-term creep behavior of natural fiber/thermoset composites.

Assessment of the stepped isostress method in the prediction of long term creep of thermoplastics

To predict long term creep of thermoplastics, methods based on the time–temperature superposition principle (TTSP) or on the time–stress superposition principle (TSSP) are commonly used. These methods enable the construction of a creep master curve without a lengthy experimental program. Recently, a new accelerated creep testing method, termed the stepped isostress method (SSM), was proposed and used to predict long term creep of technical yarns. This paper focuses on the processing aspects of the SSM test data and its validity in the creep prediction of thick thermoplastic specimens. Excellent correlation is obtained between the master curves constructed by the classical TSSP method and those constructed by the SSM method. The variation of the SSM testing parameters has no significant effect on the obtained master curves, which constitutes proof of the SSM robustness. Further, the trend of the SSM shift factors in terms of the creep stress obeys the Eyring equation.

Modeling and Experimental Study of Long Term Creep Damage in Austenitic Stainless Steels

Different batches of austenitic stainless steels (316LN) are subjected to numerous creep tests carried out at various stresses and temperatures between 525°C to 750°C up to nearly 50•103h. Interrupted creep tests show an acceleration of the creep deformation only during the last 15% of creep lifetime which corresponds to macroscopic necking. The modeling of necking using the Norton flow law allows lifetime predictions in fair agreement with experimental data up to a few thousand hours only. In fact, the experimental results show that, the extrapolation of the ‘stress – lifetime’ curves obtained at high stress leads to large overestimations of lifetimes at low stress. After FEG–SEM observations, these overestimates are mainly due to additional intergranular cavitation along grain boundaries as often observed in many metallic materials. The modeling of cavity growth by vacancy diffusion along grain boundaries coupled with continuous nucleation proposed by Riedel is carried out. For each specimen, ten FEG–SEM images (about 250 observed grains) are analyzed to determine the rate of cavity nucleation assumed to be constant during each creep test in agreement with many literature results. This constant rate is the only measured parameter which is used as input of the Riedel model. Lifetimes for long term creep are rather fairly well predicted by either the necking model or the Riedel model with respect to experimental lifetimes up to 200000hours for temperatures between 525°C and 700°C. A transition time as well as a transition stress is defined by the intersection of the lifetime curves based on the necking and Riedel modellings. This is due to a change in damage mechanism. The scatter in lifetimes predicted by the Riedel model induced by the uncertainly of some parameter values is around 50%. This model is also validated for martensitic steels (Lim et al., 2011.) and for other austenitic SSs 304H, 316H, 321H (creep rupture data provided by Dr. F. Abe, NIMS). A transition from power-law to viscous creep behavior is reported in the literature at 650°C–750°C. It allows us to predict even better lifetimes up to 200000hours at very high temperature.

Effects of varying temperature on the performance of reinforced concrete

To obtain a more robust understanding of the effects of temperature on crack widths, stiffness, strength and short term creep behaviour of reinforced concrete elements to improve structural assessment techniques. Crack widths are monitored in four small scale reinforced concrete tension specimens, while exposed to temperature cycles. Four large scale beams, two at room temperature and two at −20 °C, are loaded under four point bending for 48 h at service load before being tested to failure. Crack width behaviour and mid-span deflections are monitored. A comparison of the room temperature and low temperature test results show that the crack widths tend to decrease at lower temperatures in members that are free to expand and contract. The beams also saw an increase in strength that was dependent on the failure mode, but saw no noticeable increase in stiffness. Lastly, short-term creep behaviour was reduced in the low temperature beams. The observed cracking behaviour and beam test results indicate the potential for an increase in shear capacity for beams at lower temperatures.

Modeling and Experimental Study of Long Term Creep Damage in Austenitic Stainless Steels

Austenitic Stainless Steels (SSs) are presently being investigated as appropriate candidates for structural components for the future Generation IV nuclear reactors. Austenitic SSs of different grades will operate at high temperature and suffer low stress loading for decades. At the laboratory, austenitic SSs have been subjected to creep tests at various stresses and temperatures between 500°C to 700°C, up to nearly 50·103h. Interrupted creep tests show an acceleration of the reduction in cross-section only during the last 15% of creep lifetime which may be called macroscopic necking. The modeling of necking using a modified Norton power-law allows lifetime predictions in agreement with experimental data up to a few thousand hours only. And the experimental results show that, the extrapolation of the 'stress lifetime curves obtained at high stress leads to large overestimations of lifetimes at low stress. After FEGSEM observations, these overestimates are mainly due to additional intergranular cavitation along grain boundaries. The modeling of cavity growth by vacancy diffusion along grain boundaries coupled with continuous nucleation proposed by Riedel has been carried out. Lifetimes for long term creep are rather correctly predicted with respect to experimental lifetimes. The lifetime curves predicted by either the necking model or the creep cavity one cross each other, defining transition times of five to ten thousand hours for temperatures between 600°C and 700°C, in agreement with experimental curves.

Creep fracture mechanism in welded joints of P91 steel

The creep rupture strength of the welded joint of P91 steel (9Cr–1Mo–V–Nb) is limited by that of the fine grained region of its heat affected zone (HAZ). The fine grained HAZ has the lowest creep rupture strength, and the creep voids are mostly concentrated in this region in comparison with the coarse grained HAZ, the weld metal and the base metal of the welded joint. This is mainly due to the higher rate of recovery of the subgrain structure (coarsening of subgrains and recovery of dislocations within the subgrains) in the fine grained HAZ. Type IV fracture occurred in the fine grained HAZ of the cross-weld specimens after the long term creep tests at low stresses at 600 and 650°C. It occurred by the formation of creep voids. There are two types of voids. The first is the voids that form wedge type cracks at the triple points between the prior austenite grain boundaries at intermediate stresses at 600 and 650°C. The second is the voids that form round type cracks on the prior austenite grain boundaries and the subgrain boundaries at low stresses in long term creep tests at 600 and 650°C. These voids form at the interfaces between the matrix and the coarse intergranular particles of Laves, Z and Cr rich carbide phases.

The influence of pre-compression on the creep deformation and failure behaviour of Type 316H stainless steel

The influence of pre-compression to 8% plastic strain at room temperature has been examined on the tensile properties, uniaxial creep deformation, creep crack initiation and growth behaviour of Type 316H stainless steel at 550°C. Uniaxial creep and crack growth tests have been performed on the pre-compressed (PC) material and the results compared to existing long term (>15,000h) and short term test data on as-received (AR) (i.e. uncompressed) material. Pre-compression has been found to increase the materials subsequent yield stress in tension. Therefore the extent of non-linearity observed on the load–displacement curves of uniaxial creep rupture and crack growth tests on PC material is limited compared to AR material. In addition pre-compression causes a significant reduction in creep ductility and rupture time, although similar average and minimum creep strain rates are found in PC and AR materials. The creep crack growth (CCG) data on PC and AR materials have been characterised using the steady state creep C* parameter employing appropriate validity criteria and geometry dependent fracture mechanics parameter solutions. The CCG results are compared to the creep crack growth prediction models. Based on the creep properties, creep ductility and metallurgical observations of the fracture behaviour of the AR and PC materials, it has been shown that short term creep crack growth tests on PC material may be used to predict long term creep crack initiation (CCI) and CCG behaviour of AR material at 550°C.

Difficulties in interpreting data from creep crack growth tests on type 316H weldments

Experimental creep crack growth data are generally obtained by following standard methods such as ASTM E1457-07 and subsequently characterised using the C* parameter. These data are then used in assessment procedures, such as R5, together with reference stress estimates of C* in the component, to predict creep crack growth behaviour. Some modifications to the ASTM E1457 creep crack growth testing and analysis methods have already been proposed following a previous analysis of data from long term creep crack growth tests on type 316H parent material. This paper reports the results of creep crack growth tests on type 316H heat affected zone material at 550°C using compact tension (CT) specimens manufactured from non-stress relieved thick section butt welds. It is shown that interpretation of the data from these weldment tests is complicated by both the discontinuous nature of the cracking process and the presence of significant residual stresses in the CT specimens. Further modifications to creep crack growth testing and analysis methods are proposed to address difficulties arising from the discontinuous nature of the cracking process, and further work is identified to investigate the influence of the residual stresses present in the specimens on the observed crack growth behaviour.

Modeling of Long Term Effects on Plasma Facing Components for a Fusion Power Reactor

In a fusion power reactor the Plasma Facing Components (PFC) will experience a thermal and neutron irradiation induced creep together with tensile properties degradation and swelling due to neutron irradiation. So the investigation of the long term creep effects on the materials used for the PFC’s in a fusion power plant are of vital importance for the design and safe operation of the device. On the other hand the creep behavior study for a given material requires long and expensive test campaigns, repeated on specimens at different levels of neutron irradiation, because of the material parameters variation due to the cumulated irradiation.In this work we want to investigate if the numerical mechanical simulations employment, according to a proper methodology, could reduce the number of needed creep tests, because this would be a valuable help in defining suitable materials and valid conceptual designs for PFC’s. For this reason a method based on the systematic variation of the parameters of the empirical law, e.g. the Norton-Bailey, is outlined. To exemplify it, the behavior of a simplified model is analyzed under thermal and mechanical cyclic loading in a time transient elasto-plastic simulation, including the creep behavior, varying the parameters in the empirical creep law for the material.

Exploring the applicability of the LICON methodology for the creep assessment of a dissimilar metal weld

The LICON methodology is an approach for predicting the lifetime of materials under creep loading conditions. The LICON method predicts long-time uniaxial creep strength using the results from several short duration creep crack incubation (CCI) tests in conjunction with the outcome of a mechanical analysis for the adopted multiaxial specimen geometry. The applicability of the methodology for long term creep strength prediction of martensitic 9%Cr and bainitic 1%Cr steels has already been demonstrated. This study has examined the applicability of the procedure for predicting long term uniaxial creep strengths for a dissimilar metal weld (DMW). It has required new developments to the original formulation. Application of the developed formulation for predicting uniaxial creep rupture behaviour of the investigated DMW shows an acceptable agreement with experimental observations which was not previously achievable.

Design of 10%Cr martensitic steels for improved creep resistance in power plant applications

The drive to increase the efficiency of fossil fired power generation to reduce CO2 emissions and to conserve energy resources has led Tata Steel to design new 10%Cr martensitic compositions for use at temperatures of 620°C and above with creep properties superior to steel 92 (9%Cr, 0·5%Mo, and 2%W), which is the best currently available steel of this type. In the new alloys, the chromium content was set at 10% to ensure the required oxidation resistance. The long term creep performance of steels with this level of chromium has been limited by the precipitation of Z phase nitrogen rich particles. These form at the expense of vanadium rich MN precipitates that are vital for long term creep strength. Compositions have been designed with the aim of suppressing or delaying Z phase formation. The principles underlying the new steel chemistries are discussed, and a detailed explanation is given of the role of heat treatment in optimising creep rupture strength at temperatures in excess of 600°C. The new steel compositions are based on published work supplemented by thermodynamic calculations to optimise the heat treatment cycle, to produce a fully martensitic microstructure and to maximise the stable nanoprecipitates that are essential to maximise creep rupture life.

Creep fracture of the centrifugally-cast superaustenitic steels, HK40 and HP40

Practical experience of creep failure of components in service is discussed in relation to the creep rupture properties of HK40 (25Cr–20Ni–0.4C) and HP40 (25Cr–35Ni), which are considered using traditional and new data analysis procedures. The new approach is shown to offer not only a sensible mechanistic interpretation of the observed behaviour patterns but also a reasonable method for predicting long-term fracture strengths by extrapolation of short term creep life values.

적층 천연고무 면진장치의 장기성능과 크리프에 대한 예측

Seismic isolation has been considered and utilized in various industries as a way to prevent huge damage on to structures by large earthquakes in various industries. The laminated Laminated rubber bearings is are most frequently used in seismic isolation systems. The structural Structural safety could not be assured unless the performance of the rubber bearing is not guaranteed for the life time of the structure under the consideration that the bearing is a critical structural member to sustain vertical loads in the seismically isolated structure. However, there are few studies on the deterioration problems of rubber bearings during their service life. The long term performance of the rubber bearings was not considered in past designs of seismically isolated structures. This study evaluates the long term performance and creep characteristics of laminated natural rubber bearings that are used in seismically isolated buildings. For the this study, a set of accelerated thermal aging tests and creep tests are were performed on real specimens. The experimental results show that the natural rubber bearings would have a stable change rate of change for durability under severe environmental conditions for a long time.

Recent Advances in Creep Modelling of the Nickel Base Superalloy, Alloy 720Li.

Recent work in the creep field has indicated that the traditional methodologies involving power law equations are not sufficient to describe wide ranging creep behaviour. More recent approaches such as the Wilshire equations however, have shown promise in a wide range of materials, particularly in extrapolation of short term results to long term predictions. In the aerospace industry however, long term creep behaviour is not critical and more focus is required on the prediction of times to specific creep strains. The current paper illustrates the capability of the Wilshire equations to recreate full creep curves in a modern nickel superalloy. Furthermore, a finite-element model based on this method has been shown to accurately predict stress relaxation behaviour allowing more accurate component lifing.

Application of a Smooth Approximation of the Schmid's Law to a Single Crystal Gas Turbine Blade

In industrial practice the choice of the most suitable material model does not solely rely on the ability of the model in describing the intended phenomena. Most of the choice is often based on a trade-off between a great variety of factors. Robustness, cost, and time for the minimum testing campaign necessary to identify the model and preexisting standard practices are only a few of them. This is particularly true in the case of nonlinear structural analyses because of their intrinsic difficulties and the higher level of skills needed to carefully exploit their full potential. So, despite the great progress in this field, in certain cases it is desirable to use plasticity models that are rate independent and possess very simple hardening terms. This is for example the case in which long term creep can be an issue or when the designer may want to treat separately different phenomena contributing to inelastic deformation. If the material to be modeled is isotropic, commercial finite element (FE) packages are able to deal with such problems in almost every case. On the contrary for anisotropic materials like Ni-based superalloys cast as single crystals, the choice of the designer is more limited and despite the large amount of research literature on the subject, single crystal constitutive models remain quite difficult to handle, to implement into FE codes, to calibrate, and to validate. Such difficulties, coupled with the unavoidable approximations introduced by any model, often force the practice of using oversimplifications of the material behavior. In what follows this problem is addressed by showing how single crystal plasticity modeling can be reduced to the adoption of an anisotropic elastic behavior with a sort of von Mises yield surface.