Term Creep Research Articles

Short term creep properties of WPC at different temperatures—An experimental and numerical investigation at different stress levels

AbstractUsing a two‐step molding process, wood plastic composite (WPC) were prepared under temperatures ranging from 160 to 180°C. Uniaxial compression tests of WPC were conducted at temperatures of −20°C, 0°C, 20°C, and 40°C, determining stress levels of 30%, 45%, and 60% based on the test results. Subsequently, 24‐hour compression creep tests were performed for WPC under 12 different conditions. The Findley power law model was employed to fit the experimental results, indicating that creep deformation increased with both stress level and environmental temperature. Notably, specimens at 0°C failed after sustaining a load for 13.6 hours at 60% stress, while those at 20°C failed after only 0.72 hours under the same conditions. Overall, the Findley model effectively predicted the creep behavior of WPC under all temperature conditions and stress levels.Highlights The influence of temperature variation on the strength of WPC. Short term creep of WPC under different stress levels. Creep of WPC at different temperatures. Prediction of creep performance under different operating conditions.

Foundations in Permafrost of Northern Canada: Review of Geotechnical Considerations in Current Practice and Design Examples

In northern Canada where permafrost is prevalent, a persistent shortage of accessible, affordable, and high-quality housing has been ongoing for decades. The design of foundations in permafrost presents unique engineering challenges due to permafrost soil mechanics and the effects of climate change. There is no specific design code for pile or shallow foundations in northern Canada. Consequently, the design process heavily relies on the experience of Arctic engineers. To clearly document the current practice and provide guidance to engineers or professionals, a comprehensive review of the practice in foundation design in the Arctic would be necessary. The main objective of this paper is to provide an overview of the common foundations in permafrost and the geotechnical considerations adopted for building on frozen soils. This study conducted a review of current practices in deep and shallow foundations used in northern Canada. The review summarized the current methods for estimating key factors, including the adfreeze strength, creep settlement, and frost heave, used in foundation design in permafrost. To understand the geotechnical considerations in foundation design, this study carried out interviews with several engineers or professionals experienced in designing foundations in permafrost; the findings and the interviewees’ opinions were summarized. Lastly, in order to demonstrate the design methods obtained from the interviews and review, the paper presents two design examples where screw piles and steel pipe piles were designed to support a residential building in northern Canada, according to the current principles for adfreeze strength, long term creep settlement, and frost heave. The permafrost was assumed to be at −1.5 °C, and the design life span was assumed to be 50 years. The design examples suggested that for an axial load of 75 kN, a 12-m-long steel pipe pile or a 7-m-long screw pile would be needed.

The effect of salinity on the rigidity and settling behavior of reconstituted water-saturated kaolinite sediments

In sandy sediments, the salinity of the pore water only affects the sound speed of the mixture by way of affecting the sound speed of the pore water. With clay mineral and saltwater mixtures, the intergranular forces that depend on the salinity of the pore water affect the sediment acoustic properties beyond simply affecting the sound speed of the pore water. These dependencies were investigated via image analyses of prepared samples of two kaolinite clay types (Flat DS and RSA) via analyses of their settling dynamics, porosity, and rigidity as a function of pore water salinity. There exists an observable dependence on salinity to mixture porosity and rate of settling. In addition, three regimes of slip-stick dynamics are shown to exist at different salinity ranges: (a) an immediate transition to liquid-like behavior; (b) a viscoelastic transition to liquid-like behavior with long term creep; and (c) a delayed transition to viscoelastic behavior with long term creep. [Work supported by ONR.]

3D phase-field simulations to machine-learn 3D information from 2D micrographs

A novel approach is developed to support retrieval of 3D information from 2D experimental micrographs. The approach utilizes 3D phase-field simulations to train an artificial intelligence machine. In a first step, the phase-field simulations have to be validated to reproduce microstructural features which characterize elementary processes which govern processing and high temperature service exposure. The qualified 3D simulation setup is then applied to produce a high number of 2D simulated micrographs by automated sectioning. These simulated micrographs are then used to train a gradient boosting regression model together with the 3D information from the simulations. In the final step, the model is applied to 2D experimental micrographs to retrieve the hidden 3D features. The approach is generally applicable to all kinds of metallic materials, minerals or ceramics which can be treated quantitatively by phase-field simulations. In this paper we concentrate on the process of directional coarsening, referred to as ‘rafting’, in the field of creep of single crystal Ni-base superalloys. The experimental and modeling aspects of the evolution of the volume fraction of the phase during long term creep are discussed.

Predicting the behavior of commercially available building foundation isolation materials using a four-parameter fractional Zener model

In this paper we apply a four-parameter fractional Zener model to describe the short and long term creep behavior of a viscoelastic building foundation isolating material currently in use globally in noise and vibration mitigation applications. Building from the classical rheological models composed of springs and dashpots suitably combined (Maxwell, Kelvin-Voight, Zener), we include a fractional derivative component in the form of a Scott-Blair element composed of a Riemann-Liouville derivative of non-integer order. The parameters are identified empirically through measured stress-strain results and hysteresis curves for the material in compression. The model is then compared to the results of a 168h constant load creep test of three commercially available materials.

Effect of Axial Shortening of Column in Building with and without Soft Storey

The construction of tall concrete building became popular. As a result, we can see the problem of shortening of column due to huge loads carry by column .During construction of a tall building, the vertical elements would be subjected to a number of load increments. Each load increment would cause elastic shortening of columns. It will lead to long term creep and shrinkage shortening of such elements. Shortening of column cause effect on horizontal elements such as beams and slabs seriously. The slabs may not be truly horizontal after some time. Beams could be subjected to higher bending moments when connected to outer columns and inner shear walls. These could have certain structural effects and also could affect the nonstructural elements like damage to floor finishes, services etc.so it became important to study column shortening. In this paper we are going to study the influence of column shortening in case of building with and without soft storey once without considering construction stage analysis and once with considering it and also creep and shrinkage effect include in construction stage by using ETABS Software and comparing the effect.

Engineering properties of Norwegian peat for calculation of settlements

Despite some 9% of Norway's area being underlain by peat the geotechnical characteristics of the material have not been well documented. As peatlands form an excellent carbon sink there is much pressure on planning authorities to avoid the excavation of peat for infrastructure development. In engineering projects in peat often the resulting settlements both in the short and long term are of greatest concern. Because of the high variability of peat, several, largely empirically based, methods exist for predicting settlement on peat. These were often developed based on specific cases. These techniques have shown to be inadequate to generically predict long term creep settlements and the development of settlement with time. Here the engineering characteristics of peat from several sites in the Trondheim area of mid-Norway are studied using a series of field and laboratory tests including three types of oedometer test. The laboratory and field data, together with empirical correlations found in the literature, were used to provide input into the commercially available constitutive model Soft Soil Creep (SSC) in the computer code PLAXIS. The model was initially calibrated using the laboratory test results and then applied to the back-analysis of two full scale field trials in the Trondheim area, where the peat properties were significantly different. The modelling showed that SSC captured well the vertical settlement versus time behaviour of the peat. Guidance is provided for selecting the critical input parameters for SSC such as stiffness, yield stress and permeability. This work contributes towards efforts being made to bridge the gap between the numerical modelling community and practicing engineers.

Short and long term creep behaviour of polyamide ropes for mooring applications

Polyamide 6 fibres are of interest for mooring lines of floating wind turbines and these are continuously loaded throughout their service life. Such applications require the long term response of polyamide 6 ropes in water to be characterized. This paper presents results describing the long term creep behaviour of polyamide 6 sub-ropes with tests that lasted 2 years. A specially designed experimental set-up for long term creep test in water is presented first. Then, a kinetic study comparing creep and recovery is performed using a logarithmic identification of the strain rate. The need for performing long term creep tests is evaluated by comparing the long term creep results to those from short term creep tests lasting 3 h. The results show that a 3 h long creep test provides a reasonable prediction of long term creep strain using a single logarithmic linear law. Finally, a latch-based Weibull model is compared to a single linear logarithmic law to describe and predict creep and recovery response. It is shown that the Weibull model allows a better description of the recovery behaviour of polyamide 6 but is less well adapted for the description of creep.

Evaluation of Creep Life Prediction Models for Low Alloy Creep Resistance Steel Grade 22 (2.25Cr-1Mo) using Short-term Creep Test

Traditional Power Law equations and modern creep equations were evaluated to estimate long term creep life of Grade 22 material. Evaluation of models made on a short-term database for predicting its capacity of precise long-term creep life. Linear trend line curve fitting method used for extrapolation of data for long-term creep life. Open SourceNRIM creep rupture data for Grade 22tube in annealed/tempered condition and plates in quenched/tempered plates used in this evaluation. This evaluation is helpful for power plant industries for selecting an economically viable and precise model.

Effects of carbon nanotube-polydopamine hybridization on the mechanical properties of short carbon fiber/polyetherimide composites

Recently, it is shown that hybrid sizing agents of nanomaterials and polymers are more effective than their single components in enhancing the mechanical properties of fiber reinforced polymer composites. However, the effects of the hybrid sizing agents are scarcely examined from both experimental and theoretical aspects. In this study, short carbon fiber (SCF) surfaces were modified by applying the hybrid sizing agents of polydopamine (PDA) and carbon nanotubes (CNTs). Then the polyetherimide (PEI) composites reinforced with modified SCFs were manufactured by extrusion compounding and injection molding techniques. A single fiber fragmentation test was experimentally carried out and the results were numerically analyzed to elucidate the effects of CNT-PDA sizing layer on the interfacial properties. The effects of hybrid sizing agent on the mechanical behaviors of PEI composites were investigated from both experiment work and theoretical predictions. The PEI composites with PDA-CNT treated SCFs exhibit the improved tensile strength at 25 °C and −60 °C due to the enhanced interfacial properties between SCFs and PEI after PDA-CNT treatment. The Fu-Lauke model predictions were conducted to interpret the reinforcing mechanisms of coated SCFs by hybrid sizing agent. Moreover, the temperature-dependent tensile behaviors of the composites were studied to examine the suitability of hybrid PDA-CNT sizing in modifying SCFs at various temperatures. Finally, the short term creep behavior of PEI composites was investigated by using Burger's model and the results indicate that the creep resistance of the PEI composites was obviously enhanced by the CNT-PDA sizing treatment at 25 °C.

Creep, chloride, carbonation and sulphate attack on concrete

Deterioration of concrete structures with age and environmental factors like creep, chloride, carbonation and sulphate attack in the presence of a corrosive environment leads to a decrease in their performance in terms of load-bearing capacity and serviceability. The present work attempts to develop a model for the impairment in the strength of RC beams because of the combined effects of creep and sulphate attack. It also aims at studying the individual effects of chloride carbonation attacks with the help of established deterioration models. The equations of instantaneous elastic modulus and long term creep described in Indian standard codes (IRC 112) enable modelling the deterioration of modulus of elasticity of concrete as a function of time. The influence of sulphate attack on concrete strength against time is obtained from an empirical research study. Standard numerical models are employed to understand the effects of chloride ingress and carbonation phenomena. The consequences of the aforementioned attacks on an M30 grade concrete girder beam are presented in this paper. The results showed that the creep and sulphate attacks tend to dwindle over the course of time, while the effect of the former is more dominant than that of the latter. In addition, the initiation times for carbonation and chloride attacks were estimated, and it was observed that for a pristine RC beam chloride attack is highly imminent than carbonation.

Heterogeneous Microstructure-Induced Creep Failure Responses in Various Sub-Zones of Modified 310S Welded Joints

In order to reveal the creep failure behavior of novel modified 310S austenite steel welded joints, the creep life and microstructure evolution of the 310S austenite steel welded joints were investigated in this study. The rupture life was assessed to estimate the damage of the welded joint based on creep rupture tests performed at 600 °C in the stress range of 170–238 MPa. Compared with WM, HAZ facilitated the occurrence of creep failure in long term creep due to the combination of a smaller hardness value, a more heterogenous microstructure accompanied by coarsened M23C6, a larger grain size, higher KAM and Schmid factor. Discontinuous Laves phases appeared near the boundaries between the δ-ferrite and γ-austenite grains in the WM, and dislocation strengthening and precipitation strengthening were observed near the boundary in the BM. Furthermore, segregation elements were detected by APT and EDS adjacent to the boundary. Cr and C segregation near grain boundaries weaken the creep resistance in long term creep service.

Creep damage characteristics and evolution of HR3C austenitic steel during long term creep

HR3C austenitic heat resistant steel has been gradually applied as tubes to manufacture components of superheaters and reheaters in ultra-supercritical (USC) power plants. Creep tests were conducted at 650 °C under the applied stresses in the range of 150 MPa–250 MPa. Scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and X-ray diffraction (XRD) were employed to investigate the creep damage characteristics and evolution. The results show that intergranular microcracks and dense creep cavities along grain boundaries are the major features of creep damage. Different states of creep damage characteristic distribution are related to the applied stress and creep duration at high temperature. The wedge-cracking is prone to take place at the triple junctions of grain boundaries, and creep cavities are nucleated preferentially at M23C6 precipitates along grain boundaries. The nucleation and growth of creep cavities and wedge-cracks have a crystallographic orientation relationship with the neighboring austenite grains. The adjacent grains have a large difference of Schmid factor, and one of them is close to <1 1 1>//the stress axis orientation, which has greater resistance to deformation. The Orientation Deviation (KAM) obtained from EBSD analysis implies that the creep damage is obviously affected by the stress level. Microcracks under high stress are mainly attributed to the local deformation, and dense creep cavities under low stress are dominated by time-dependent diffusion. Under the intermediate stress level, local deformation and time-dependent diffusion act together, leading to the most serious damage.

Creep strain modeling for alloy 690 SG tube material based on modified theta projection method

During a severe accident, steam generator (SG) tubes undergo rapid changes in the pressure and temperature. Therefore, an appropriate creep model to predict a short term creep damage is essential. In this paper, a novel creep model for Alloy 690 SG tube material was proposed. It is based on the theta (θ) projection method that can represent all three stages of the creep process. The original θ projection method poses a limitation owing to its inability to represent experimental creep curves for SG tube materials for a large strain rate in the tertiary creep region. Therefore, a new modified θ projection method is proposed; subsequently, a master curve for Alloy 690 SG material is also proposed to optimize the creep model parameters, θi (i = 1–5). To adapt the implicit creep scheme to the finite element code, a partial derivative of incremental creep with respect to the stress is necessary. Accordingly, creep model parameters with a strictly linear relationship with the stress and temperature were proposed. The effectiveness of the model was validated using a commercial finite element analysis software. The creep model can be applied to evaluate the creep rupture behavior of SG tubes in nuclear power plants.

TEM replica analysis of particle phases in a tempered martensite ferritic Cr steel after long term creep

Tempered martensite ferritic steels (TMFSs) have been and are being used for critical components in high temperature plant operating in the 600 °C range. They are exposed to creep conditions for long time periods, exceeding 100,000 h. In the present study we investigate a 12% Cr TMFS, after creep-testing at 550 °C at 120 MPa for 139,000 h. We had previously investigated this material in the TEM using thin foils. We now use an extraction replica technique to analyze four particle families: M23C6, MX, Laves-phase and Z-phase, considering statistically relevant numbers of particles (between 120 and 720). We show how EELS mapping can help in identifying Z-phase particles and use Cr-V-maps to differentiate between the four particle families. The chemical evolution of particles is investigated. The experimental results are discussed in the light of previous thin foil data and with respect to predictions from computational thermodynamics. The strength and weakness of thin foil and replica procedures are compared. Improvements for thermodynamic databases are suggested.

Characterization of viscoelastic bending stiffness of uncured carbon-epoxy prepreg slit tape

In support of computational modeling of tow placement at elevated temperature, the creep response of IM7/8552 uncured prepreg slit tape material during 3-point bend loading at different temperatures is measured using an RSA III dynamic mechanical analyzer. Short term creep experiments were conducted for a duration of 1000 s at ten different temperatures ranging from −5°C (below Tg of 0 °C) to 40 °C, with the latter being the nominal processing temperature for the tow. Results show that the tow material obeys the time-temperature superposition principle (TTSP) in the temperature range selected for the creep experiments. Using the TTSP, creep compliance master curve spanning more than eight logarithmic decades is obtained for the nominal processing temperature of 40 °C. The time–temperature shift factor is observed to follow closely the William-Landel-Ferry (WLF) model, with the WLF parameters obtained by least square fitting of the experimentally determined shift factors utilizing closed form shifting algorithm (CFS). The WLF model for the material is used to extend the master curve to a range of temperatures above glass transition temperature (Tg) of the material. The creep compliance data is further employed to obtain the retardation spectra of the material using an algorithm based on the work of a co-author and applied recently to a different material system by the authors. Details for extraction of the retardation spectra from the creep compliance results are provided. Results from these studies demonstrate that the creep compliance reconstructed from the retardation spectra accurately represent the experimental results while providing baseline data to quantify the importance of viscoelastic behavior in wrinkle formation during advanced manufacturing of aerospace components.

Aspects on viscoelasticity modeling of HDPE using fractional derivatives: Interpolation procedures and efficient numerical scheme

Among the wide range of structural polymers currently available, this work deals with high-density polyethylene (HDPE). The typical viscoelastic behavior of this material is not trivial to model and has already been investigated by many authors. We employ the fractional Zener model to fit our experimental creep results of HDPE evaluated at different stress levels. This model produces fractional constitutive equations with excellent curve-fitting properties and fewer parameters to be identified in relation to traditional models. The results are compared with those ones provided by the application of the Prony series method. The first novelty of this paper is the application of the time-stress equivalence principle (TSEP), coupled to the fractional model, to estimate creep at intermediate stress levels, that in turn, were not measured experimentally but lie within the stress range used to calibrate the model. We compare the results provided by this method with those based on linear interpolation of the parameters. Although there is clear benefits requiring fewer parameters, fractional derivatives render costly computations due to their history memory. To cope with this, we propose a new algorithm, called GPE, which shows a compromise between enhanced efficiency and accuracy when compared with other proposals of the literature. These features are verified with simulations for simple functions, and a long term creep test with the fractional Zener model. The combined application of fractional derivatives, TSEP and the new GPE algorithm results in a novel efficient and effective alternative to account for the creep modeling of HDPE.

Long term creep and shrinkage of nano silica modified high volume fly ash concrete

The long-term creep and shrinkage behaviour of two High-Volume Fly Ash (HVFA) concretes incorporating nano silica with 65% and 80% replacement of cement has been investigated. This comprised a detailed analysis of the microstructure, pore structure and chemistry of the two HVFA systems up to a period of 450 days. The compressive strength and modulus of elasticity of HVFA-65 concrete increased from 32 to 73 MPa and 30.3 to 40.5 GPa, respectively between 7 and 450 days. The HVFA-80 concrete achieved compressive strength values of 22 and 71 MPa and elastic modulus values of 28.9 and 37 GPa. After a total loading period of 450 days, HVFA-65 and HVFA-80 concretes displayed creep parameters, which were significantly below the values predicted by AS 3600, ACI 209 and CEB-FIP standard model equations. After a total drying period of 450 days 28-day cured specimens showed significantly reduced shrinkage compared to 7-day cured specimens. On the other hand, HVFA-80 concrete displayed higher shrinkage compared to the HVFA-65 specimens throughout the period. All specimens except for 7-day cured HVFA-80 concrete were within the maximum permissible shrinkage of 800 microns recommended for Australian construction practices. HVFA-65 concrete showed a denser microstructure and a stronger, better packed interfacial transition zone (ITZ) compared to HVFA-80 at all ages. The XRD and FTIR analysis data identified the formation of hydration products including C-S-H and C-A-S-H which contributed towards both the strength gain as well as the creep and shrinkage properties displayed by the HVFA concrete by minimizing the total porosity and pore size.

Isothermal and anisothermal creep behavior of the nickel base Alloy 602 CA

AbstractComponents in industrial furnaces and apparatus construction are usually exposed to temperatures up to 1,200 °C as well as mechanical loading and corrosive attack of the environmental atmosphere. Furthermore, they undergo numerous temperature cycles in service. To describe the characteristics of the nickel base Alloy 602 CA, which is often used in this industrial sector, the long term creep behavior of this alloy was studied at temperatures from 800 °C to 1,200 °C with and without thermal cycling. During these tests unexpected effects could be noticed. At load levels representing typical component loads and isothermal temperature control an anomalous decrease in creep rate in the third creep stage was observed. Experiments under thermal cycling showed a considerably higher creep rate in the whole range than in the isothermal case. Within the present article, these findings are outlined in detail together with a brief discussion of causes and consequences based on extensive microstructural examinations.

The test on the inhomogeneous strain field and creep rupture behavior of the weld joint at elevated temperature by digital image correlation method

The digital image correlation (DIC) method was applied for monitoring the short and long term inhomogeneous deformation and rupture behavior of the weld joint at elevated temperature. In order to prevent the surface oxidation of the metal during the long time exposure at elevated temperature and offer a series of stable surface patterns for the DIC sensing, the thin metallic coating was introduced. Then the strain field and rupture of the weld joint were tested successfully not only during the short term tensile test but also the long term creep test. The results showed a good performance on distinguishing the deformation differences at local zone of the weld joint. Especially, the three-stage creep behavior of the local zones could be measured simultaneously, even though the weakest zone controlling the creep rupture, which is helpful to study the creep rupture behavior of the weld joint and develop the creep life evaluation method or the creep failure monitoring technology. Also the limitations of this method were discussed briefly.