High-temperature Power Plant Research Articles

Influence characteristic and improvement mechanism of thermal storage performance of Packed-bed thermocline tank based on high-temperature molten salt

Chloride salts and carbonates thermal energy storage (TES) have great application potential in high-temperature concentrated solar power (CSP) plants. However, their thermal storage characteristics, performance improvement mechanism and economic feasibility of application in high-temperature conditions are not clear. In this study, a transient, two-dimensional and axisymmetric model is developed for packed-bed thermocline tanks. The effects of six commonly solid fillers on thermal storage performance of NaCl-KCl-MgCl2 salt and Na2CO3-K2CO3-Li2CO3 salt at an operation temperature of 450–800 °C are simulated. The suggestions for optimizing fillers are provided combined with thermal and economic performance. Results show that high volume heat capacity and low thermal conductivity of fillers play important roles in deferring migration and weakening diffusion of thermocline, thus promoting thermal storage capacity and efficiency of tanks. Concrete and cast iron are suitable fillers for chloride salts and carbonates TES. The concrete tank is suitable for systems with strict cost control and low TES requirements. The iron tank is suitable for systems with high TES and low cost control requirements, which significantly increases effective thermal storage capacity of chloride salts by 222.1 % and considerably reduces TES cost of carbonates by 41.1 %. The results can be beneficial for high-temperature TES system of CSP plants.

Modeling of Additively Manufactured Ceramic Heat Exchangers with Semi-Elliptical Cross-Section Flow Channels

An approximate and easily applied analytical model was developed for heat transfer calculations of heat exchangers consisting of multiple rows and columns of heat transfer fluid flow channels with semi-elliptical cross sections. Heat exchangers of this type are being developed by using ceramic material and additive manufacturing for high temperature and pressure-concentrating solar electric power plants. Calculations using the model require only the geometrical dimensions and flow conditions of the heat exchanger. Comparisons of modeling predictions, with both simulation results and experimental data, were conducted to verify the viability of the model. The results showed good agreement where almost all the modeling predictions were within 20% of the simulation results or the experimental data. The proposed modeling approach is more generally applicable to heat transfer analysis of heat exchangers with similar flow channel configurations to those considered in this study.

Analysis of ECCC cross-weld creep test data on grade 92 steel

ABSTRACT The UK High Temperature Power Plant Forum have completed a long term cross-weld creep test programme on a Grade 92 steel pipe. This paper describes an analysis of the creep test data. The weld strength factor (WSF), i.e. the ratio of cross-weld strength to base material strength for a specified rupture time and temperature, is shown to be a unique function of a dimensionless parameter S, the creep test stress divided by ultimate tensile strength at the test temperature. The key practical finding is that, since a single universal cross-weld creep life parametric is applicable to all temperatures, long-term behaviour at service temperatures should reliably be predictable from short-term tests at accelerated temperatures.

Effects of various fillers on the thermal shock and fouling resistance characteristics of zirconia-based composite coatings for high-temperature applications

Composite coating technology has been extensively developed to meet industrial demands for coatings with specifications that exceed the capabilities of conventional coating technologies and that can function in extreme environments and high temperatures. The composite coating in this study is focused on materials that have hydrophobicity. To achieve this hydrophobicity, the coating must have at least one restrictive property: antifouling. In this research, antifouling coating materials based on certain zirconia ceramic composites (ZrO2) and fillers, such as boron nitride, MoS2, and graphite, are chosen. The substrate used is a low-carbon steel, which is used in high-temperature power plants. The slurry spray coating method is chosen, which is a simple and low-cost method. Thermal shock and fouling resistance tests are conducted, and the samples are further observed with Scanning Electron Microscopy-Energy-Dispersive X-ray Spectroscopy (SEM-EDS). The results show that the higher the hBN filler content in the zirconia composite coating is, the better the thermal shock and antifouling resistance levels. This phenomenon is indicated by the lower delamination of the coating after the thermal shock treatment and the lower fouling of sodium sulfate deposits on the surface of the coating after the antifouling treatment than before. The same result is shown by the zirconia composite coating with hybrid filler (hBN-MoS2-Graphite).

Thermal stability of film forming amines‐based corrosion inhibitors in high temperature power plant water solutions

AbstractFilm forming amines (FFA) are corrosion inhibitors added to power plant water. The major concern associated with their application is the thermal stability in the high temperature power plant water medium, along with the risk of decomposition into low molecular weight organic acids that can cause corrosive damages in the water/steam cycle. However, there is still a lack of sufficient data on the thermal stability of FFA corrosion inhibitors. This paper presents a comprehensive critical review and state‐of‐the‐art assessment of the results obtained from studying the thermolysis of FFA corrosion inhibitors in power plant water/steam cycle conditions, highlighting the relevance for practical application and research needs. Temperature, exposure time, initial concentration, and alkalizing agents were identified as key factors influencing the thermal stability of FFA in high temperature power plant water. Organic acids are found in concentrations harmless to metal tubes. Advanced scientific background information and additional research are required on this topic.

Recent progress in the microstructurally based creep modelling of Ni-based alloy 617

ABSTRACT Solid solution strengthened (SSS) Ni-based superalloys, such as A617, show superior creep resistance at 700°C. Established for many years in land- and aero-based gas turbines, these materials are increasingly being considered for use in high-temperature thermal power plants. Apart from SSS, the creep strength in A617 stems from γ’ and carbide precipitates. In this work, a microstructurally based creep model for A617 is presented. Mobile dislocations in the model interact with fine grain-interior precipitates, and grain boundaries act as dislocation sources/sinks. The model is capable of simulating creep curves and time-to-rupture (TTR) diagrams based on the evolution of mobile dislocations. At lower stresses, the accuracy of modelled TTR can be improved by adding diffusion creep to dislocation creep. The simulated evolution of dislocation densities is realistic compared to the literature data. The reduction of area of ruptured samples was included in a damage factor, enabling the consideration of creep ductility.

Solving the issue of reliability data for FOAK equipment in an innovative nuclear energy system

There is a demand for the improved safety and economics of advanced nuclear energy systems. The world's first high-temperature gas-cooled reactor pebble-bed module nuclear power plant (HTR-PM Demo Project) is about to be completed in China. This power plant achieved initial full power at the end of 2022. The success of the HTR-PM Demo Project shows the importance of risk-informed initiatives throughout the project with solid support from comprehensive risk assessment. This paper focuses on a technical issue that needs to be solved when conducting risk-informed initiatives for HTR-PM, namely the issue of reliability data for first-of-a-kind (FOAK) equipment. In short, no data for the FOAK equipment are available in existing reliability databases. On the basis of the HTR-PM pilot experience, this paper proposes a process of estimating the required data for the reliability parameters of FOAK equipment. The process has five steps: (1) identification of key parts and assemblies, (2) preliminary determination of the reliability parameters and the associated reference values for the key parts and assemblies, (3) analysis of maintenance management tactics, (4) estimation of reliability parameters for the FOAK equipment using a Bayesian network, and (5) Bayesian updating with operational data. The helium circulator of HTR-PM is taken as an example to demonstrate the feasibility of the proposed process. A lack of reliability data is a common issue for innovative nuclear energy systems. The proposed process is supposed to be universal to all types of innovative design.

Multi-scale modelling of creep cavity nucleation and growth in polycrystalline Type 316 stainless steel

ABSTRACT A common creep damage mode in Type 316 stainless steel under high-temperature power plant conditions is intergranular cavitation. A review of the literature has confirmed that cavitation in Type 316 is controlled by nucleation, which is not fully understood. In order to provide further insights into the physics of this process, existing strain-based empirical and stress-based (classical nucleation theory) nucleation models are modified in this study by considering experimentally-observed features of cavity nucleation in Type 316. The models are employed locally within a newly-developed crystal plasticity finite element (CPFE)-interface element framework. Modelling results suggest that the strain-based model as a function of local inelastic strain rate does not explain the physical nature of the nucleation process as observed experimentally. By contrast, the modified classical nucleation theory is able to capture features of the observed macroscopic failure response and the distribution of cavities in the microstructure. A number of missing features are identified in the mechanistic model, which need to be incorporated in future unified cavity nucleation theories. These findings highlight key aspects of the nucleation process, which need to be examined experimentally.

Welding, mechanical properties and microstructure of different grades of austenitic stainless steels: A review

Nowadays the power of the industry standard materials has gained a lot with the advancement in technology. The requirement for materials with high creep rupture strength, high thermal conductivity, and strong corrosion resistance rose dramatically when the power plant was operated at greater temperatures and pressures. Creep strength improved steels, nickel based alloys, and austenitic stainless steels are the most often utilised materials in high temperature and high pressure power plants. To meet the strength criteria, reduced cost investment, and service standards must be met, dissimilar welding between Stainless steels is required. This necessitates the combining of different metals. An effective weld among different metals is one which is as strong as that of the weakest of the two different metals being connected, that is, it has enough ductility and tensile strength to prevent the junction from failing during the weld. Such joins may be made from a number of metals and using a range of welding methods. Before selecting the best way to join the metals the evaluation of both the base metals and the filler material is to be done. Here the welding process done between steels is TIG welding. Challenges in the current research are to join the different materials without disturbing the properties of parent metals. Some of the frequently occurring problems are: wider HAZ (heat affected zone), Difference in coefficient of thermal expansions, Difference in chemical composition, Multi pass welds to weld high thickness materials. Current research deals with the Optimization the TIG Welding Process and To Understand the effect of Process Parameters on Mechanical Characteristics of the dissimilar joints and to carry out Numerical Investigation on the dissimilar welding of Austenitic Stainless Steel.

Inflation-Deflation Episodes in the Hengill and Hrómundartindur Volcanic Complexes, SW Iceland

Non-eruptive uplift and subsidence episodes remain a challenge for monitoring and hazard assessments in active volcanic systems worldwide. Sources of such deformation may relate to processes such as magma inflow and outflow, motion and phase changes of hydrothermal fluids or magma volatiles, heat transfer from magmatic bodies and heat-mining from geothermal extraction. The Hengill area, in southwest Iceland, hosts two active volcanic systems, Hengill and Hrómundartindur, and two high-temperature geothermal power plants, Hellisheiði and Nesjavellir. Using a combination of geodetic data sets (GNSS and InSAR; Global Navigation Satellite Systems and Interferometry Synthetic Aperture Radar, respectively) and a non-linear inversion scheme to estimate the optimal analytical model parameters, we investigate the ground deformation between 2017–2018. Due to other ongoing deformation processes in the area, such as plate motion, subsidence in the two geothermal production fields, and deep-seated source of contraction since 2006, we estimate 2017–2018 difference velocities by subtracting background deformation, determined from data spanning 2015–2017 (InSAR) or 2009–2017 (GNSS). This method highlights changes in ground deformation observed in 2017–2018 compared to prior years: uplift signal of ∼10 km diameter located in the eastern part of the Hengill area, and geothermal production-related temporal changes in deformation near Húsmúli, in the western part of the Hengill area. We find an inflation source located between the Hengill and Hrómundartindur volcanic complexes, lasting for ∼5 months, with a maximum uplift of ∼12 mm. Our model inversions give a source at depth of ∼6–7 km, located approximately in the same crustal volume as an inferred contracting source in 2006–2017, within the local brittle-ductile transition zone. No significant changes were observed in local seismicity, borehole temperatures and pressures during the uplift episode. These transient inflation and deflation sources are located ∼3 km NW from a source of non-eruptive uplift in the area (1993–1999). We consider possible magmatic and hydrothermal processes as the causes for these inflation-deflation episodes and conclude that further geophysical and geological studies are needed to better understand such episodes.

Creep-fatigue interactions in type 316H under typical high-temperature power plant operating conditions

A micromechanical crystal plasticity self-consistent model (SCM) is used to analyse loading histories in 316H stainless steel. SCM predictions on changes in cyclic and creep properties are compared to analyses via the R5 assessment procedure. Plant-relevant cyclic-creep histories at 550 °C are examined with focus on the estimation of accumulated creep strain. The effect of cyclic plasticity on creep is quantified. The levels of creep strain accumulated during different dwells, following loading from different macroscopic stress states, is also evaluated and explained through experimental observations. Assessing the impact of displacement- and load-controlled dwells on the hysteresis loop evolution revealed how different assumptions for creep strain accumulation could employ SCM results to reduce uncertainty. Guidance is provided in the development of extrapolation frameworks for structural parameters based on the SCM results which could be used directly in structural assessment to estimate the inelastic strain accumulation with a lower degree of conservatism.

Heterogeneous creep deformation behavior of functionally graded transition joints (GTJs)

Dissimilar metal welds (DMWs) made between ferritic steels and austenitic alloys are used extensively in high-temperature power plant components. These DMWs experience premature creep failure in the ferritic steel’s heat affected zone (HAZ), close to the weld interface. Steep variations in microstructure and chemical composition across the dissimilar weld interface have been the contributing factors for the failure. Graded transition joints (GTJs), developed by functionally grading the chemical composition in layers, have been proposed as potential candidates to replace DMWs. In this research, GTJ coupons were fabricated between 2.25Cr-1Mo steel and Alloy 800H base material using two filler materials: (i) Inconel 82 and (ii) P87. These samples were aged at 600°C for 2000h to accelerate high-temperature microstructural evolution seen in service conditions, before subjecting them to short-term (~1 month) creep tests. Surface strains were measured using digital image correlation (DIC) technique to extract creep strain rates at different locations within GTJs. Both the GTJs exhibited heterogeneous creep strain distribution. Creep strain accumulated in the FGHAZ of 2.25Cr-1Mo steel, similar to type IV failure associated with Cr-Mo steel weldments. Microstructure based creep model framework was developed to describe the discrete creep strain rates observed in various 2.25Cr-1Mo steel regions of GTJs.

Phase change materials integrated solar thermal energy systems: Global trends and current practices in experimental approaches

Abstract Solar thermal systems (STSs) are gaining boom in the globalized market since last two decades to combat the menace of global warming. Considerable research has been carried out in the field of solar thermal system for efficiency enhancement. Improvement in terms of efficiency and performance would make solar thermal systems a better option for replacing the conventional energy systems. Phase change Materials (PCMs) have emerged as an alternative to enhance the performance of the solar heating systems by acting as thermal storage batteries. In this review article an attempt has been made to consolidate the global trends and practices that has been underwent incorporating Phase change materials (PCMs) in solar thermal systems. Research on PCM based solar cooker has found to be extinct. PCM based PCM based high temperature power plant applications are on current trends of research. Application of phase change materials for low, medium and high temperature solar thermal systems are comprehensively reviewed and discussed in this article. As well the environmental benefits in terms decline in CO2 emission, due to the use of STS in day-today life and the economic analysis of PCM based STS in terms of cost and payback period is presented.

SMAW ve GTAW Metotları ile Kaynaklanmış Yüksek Sıcaklık ve Basınç Dirençli Nükleer Enerji Santrali Çeliğinin Mekanik Özelliklerine Kaynak Sonrası Isıl İşleminin Etkisi.

This research aims at effect of Post Welding Heat Treatment (PWHT) on mechanical properties of 2.25Cr1Mo (P22) high temperature and pressure resistant nuclear power plant steel welded with GTAW (gas tungsten arc welding) and SMAW (shielded metal arc welding). Pre-heating was applied to the materials to be welded at 200°C before welding processes. Welding processes of materials were performed at room temperature. After welding processes, post weld heat treatment (PWHT) was applied at 750⁰C for 2 hours. Before and after the PWHT, welded materials were prepared in accordance with EN standards for tensile, bending, impact, hardness tests and macrostructure examinations for the investigation of mechanical properties.

Convective heat transfer characteristics of molten salts flowing in complex flow structures: Experimental studies and progress

The large-scale utilization of traditional fossil fuels causes global warming and environment pollution, which is constraining the sustainable development of human society. Recently, new energy utilization technologies, including solar power and nuclear power, have witnessed rapid developments because they can relieve these problems. For the new energy utilization, molten salts are widely used. Therefore, having a deep understanding on the convective heat transfer characteristics of molten salts flowing in complex heat transfer structures is a critical issue to realize the high heat-transfer performance for new energy utilization technologies, especially the next-generation advanced nuclear power plant and high-temperature concentrated solar power (CSP) plant. In this paper, experimental studies on the convective heat transfer characteristics of the molten salt are firstly reviewed. It can be concluded that these studies experienced a development process, which is from smooth tubes to enhanced tubes, from tube side to shell side. Meanwhile, it is pointed out that studying on the convective heat transfer characteristics of molten salt flowing in complex structures is of great importance in the future. Then, previous experimental studies by our team on the convective heat transfer characteristics of the molten salt flowing in 5 heat exchangers with complex structures are introduced systematically, including shell-and-tube heat exchangers (STHE) without baffle plate, STHE with large and small hole baffles, STHE with segmental baffles, STHE with rod baffles, and airfoil printed circuit heat exchanger (PCHE). Based on the above experimental studies, corresponding convective heat transfer correlations for the molten salt flowing in the five complex heat transfer structures were obtained; and the applicable temperature and Reynolds number ranges for these correlations were also provided. The results show that the obtained correlations have simple forms and high precision, which can meet the engineering requirement. Moreover, heat transfer performances of the above five heat exchangers are also compared with each other. The results show that Nusselt numbers of the five heat exchangers have the same changing trend with Reynolds number; and the order of the heat transfer performances of the STHEs is: STHE with large and small hole baffles > STHE with rod baffles > STHE with segmental baffles > STHE without baffle plate. In addition, the heat transfer rate per unit volume of the airfoil PCHE is 70 times that of the STHE. To conclude the experimental research status of the friction and heat transfer characteristics of molten salt flowing in typical heat exchangers with complex structures, it can be found that most of the investigations focused on the convective heat transfer performance, lacking the analysis of friction characteristics. Therefore, the friction performances of the heat exchangers with complex structures should be further studied to obtain the comprehensive performance. Moreover, most of the experimental studies on the convective heat transfer performance of molten salts are related with the traditional heat exchangers. Thus, novel heat exchangers, which can achieve high heat transfer performance and low pressure drop, should be investigated and developed to meet the need of high efficiency, high temperature, and high pressure in the next-generation advanced nuclear power plant and high-temperature CSP plant.

Component Testing and Numerical Calculation of a Bolted High Temperature Power Plant Pipe Flange Under Complex, Near-Service Loads

Abstract To ensure a reliable power supply with a minimum environmental impact in the future, further increases in efficiency and flexibility of fossil-fired power plants are major challenges to address in recent R&D activities. In order to counterbalance substantial fluctuations in the electricity grid due to the rising share of renewable resources, frequent start-ups and shut-downs of turbomachinery will be claimed by the market. Hence, along with the development of advanced materials for elevated steam parameters, contemplations with respect to altered boundary conditions of established materials, e.g., for bolted pipe flange connections, are required. In this paper, a selection of results from a recently finished research work on stress relaxation will be presented. Together with European power plant component manufacturers, a newly developed test rig comprising a scaled intermediate pressure (IP) steam turbine pipe flange now allows investigations under near-service conditions. Before, throughout, and after performance of the experiments, an extensive measurement setup enables the examination of effects that cannot be studied in standardized relaxation tests. Within this work, a loss in bolt pretension of up to almost 50% over a comparatively short period of experimental time was observed. By means of numerical calculations, creep deformation in the transition areas between pipe and flange plate sections was identified to be a major originator of these pretension drops. Particularly, good correlations between experiments and finite element method (FEM) simulations could be achieved through the implementation of enhancements concerning the fitting strategy of the Graham–Walles-type creep model which was used.

Microstructure and Creep Behavior Property of Dissimilar Joints Between Incoloy 800HT and P91 Steel

Dissimilar joints are commonly encountered in the fabrication of high-temperature power plant components. In this current work, dissimilar joints between P91 and Incoloy 800HT have been produced using two different types of filler materials like ER505 and ERNiCr-3 with the GTAW welding process. Microstructure characterization of the weld metal reveals the presence of columnar dendritic structures. The microhardness value is higher in ER 505 filler-based weldment due to the formation of δ ferrite, M23C6 and MX precipitates. Impression creep test of dissimilar weldments has been conducted with the help of flat bottom cylindrical punch. The impression creep test has been carried out at the temperature of 873.15 K and 923.15 K with a constant load of 135 Mpa with 7200 s of holding time by using a vacuum of the order of 1 × 10−6 mbar in the heating chamber. The impression creep test result has revealed that the depth of penetration of weld with ER505 filler metal has a higher value compared to other weldments. The creep strength of the specimen is compared by the calculated activation energy of each specimen. The creep strength of the weld with ER505 filler metal (Exp No 4) is higher than that of all the other weld’s specimens. Hence, the dissimilar weld with ER505 as a filler material by GTAW process will produce better results.

Dynamic simulation and thermoeconomic analysis of a novel solar cooling system for a triple-pressure combined cycle power plant

This paper presents the design of a novel high-temperature solar assisted triple-pressure level combined cycle power plant. The system includes innovative high-temperature flat plate evacuated solar thermal collectors, a double stage lithium bromide/water absorption chiller, pumps, heat exchangers, storage tanks, mixers, diverters, controllers and a triple-pressure combined cycle power plant. This novel layout uses high-vacuum non-concentrating flat plate solar thermal collectors driving a double effect absorption chiller, leading to an overall high coefficient of performance for the solar cooling section. The provided cooling energy is used to cool gas turbine inlet air to enhance system efficiency and electrical capacity. This effect is mainly performed during central hours of the day when the conventional gas-fired combined cycles dramatically suffer for efficiency and capacity reduction, as a consequence of the corresponding increase of external air temperature. Such increase is related to the higher availability of solar radiation. This technology may be a viable solution, especially for hot and dry areas, in terms of primary energy savings and increased revenues. This prototypal system was numerically analysed for a combined cycle with a rated electrical power of 99 MWe and an electrical efficiency of 56%, developing a dynamic simulation model and detailed thermo-economic optimizations. Special attention is also paid to the design of novel control strategies aiming at maximizing the solar cooling effect. The results of the dynamic simulations show a very high average thermal efficiency of the solar collectors, equal to 34%. Implications of the performed work allowed an increase of the power output up to 5.5% and a satisfactory payback period equal to 10.7 years.

The problems of using a high-temperature sodium coolant in nuclear power plants for the production of hydrogen and other innovative applications

The intensity of the hydrogen sources arriving from the third contour of installation in second in comparison with the hydrogen sources on NPP BN-600 increases by two – three order at using of high-temperature nuclear power plants with the sodium coolant (HT-NPP) for drawing of hydrogen and other innovative applications (gasification and a liquefaction of coal, profound oil refining, transformation of biomass to liquid fuel, in the chemical industry, metallurgy, the food-processing industry etc.). For these conditions basic new technological solutions are offered. The main condition of their implementation is raise of hydrogen concentration in the sodium coolant on two – three order in comparison with the modern NPP, in a combination to hydrogen removal from sodium and its pumping out through membranes from vanadium or niobium. The researches with use diffusive model have shown possibility to expel a casium inflow in sodium through a leakproof shell of fuel rods if vary such parameters as a material of fuel rods shell, its thickness and maintenance time at design of fuel rods for high-temperature NPP. However maintenance of high-temperature NPP in the presence of casium in sodium is inevitable at loss of leakproof of a fuel rods shell. In these conditions for minimisation of casium diffusion in structural materials it is necessary to provide deep clearing of sodium from cesium.

Testing and assessment of cracking in P91 steels under creep-fatigue loading conditions

Abstract Future green energy options dictate that renewable energy source must be utilized when available. This poses a challenge to conventional power plant, which has generally been designed for base-load conditions, as they now need to operate in a ‘flexible’ manner. This flexible operation, in high temperature power plant components, could lead to a combination of creep and fatigue crack growth failure. Thus a better characterization of interactive creep-fatigue crack growth behaviour is required especially for assessing long term failure in plant. The industrial codes, such as R5 or BS7910, treat this interaction using linear accumulation of damage. However, this does not consider the degradation of properties and reduction in creep ductility in long term operations and their effect on the subsequent creep/fatigue behaviour. In this work creep-fatigue crack growth (CFCG) tests were performed on compact tension specimen of P91 steel in the as received and ex-service conditions at temperatures ranging between 600 °C to 625 °C, with the hold-times ranging from static to 600 s. The experimental results in addition to appropriate data from the literature have been analysed using stress intensity factor range, Δ K appropriate under fatigue control and the creep fracture mechanics parameter C* relevant under creep control. Scanning electron macroscopy (SEM) analysis confirms the influence of frequency on the mode of cracking. Within the scatter of experimental data for the present short term accelerated tests a linear cumulative damage rule can still predict the creep/fatigue interaction. However the effects due to low frequency cyclic loading as well as degraded the steel under ex-service conditions tending to reductions in creep ductility show factors of two or more faster cracking rates compared to as received static testing. Unavailability of long term tests at low stresses may pose additional problems under creep since creep is stress state controlled but fatigue in not. However using plane strain predictions of crack initiation and growth data using the NSW multiaxial ductility creep crack growth model suggest that conservative predictions of long term cracking can still be made under creep/fatigue and when there is a marked reduction creep failure ductility.