Relative Creep Research Articles

Creep Behaviors and Deformation Mechanisms at Intermediate Temperatures in a Novel γ‐γ′ Nickel‐Based Superalloy

The creep behaviors of a novel polycrystalline nickel‐based superalloy are investigated at a wide applied stress range of 500–950 MPa and temperature ranging from 650 to 750 °C. And the microstructure and deformation mechanisms are also analyzed. The results indicate that all creep curves of the novel nickel‐based superalloy are mainly consist of the primary and accelerated stages, whereas the steady‐state stage is not obviously observed. The minimum creep rate increases and the corresponding creep life decreases with the applied stress increasing at constant temperature. The relative longer creep life at 650 °C could be attributed to the coupling of stacking faults in the interior of γ′ and deformation twinning.

Masonry creep influence on the lateral displacement capacity of slender structures: a case-study of a tank-tower in Italy

Historical constructions were typically designed to be gravity-loads resistant. In the case of masonry towers, relevant structural weaknesses are related to the (axial and lateral) displacement capacity. In fact, the masonry is strongly affected by creep which generates both the axial shortening and stiffness softening. Therefore, the lateral displacement capacity may vary significantly during the time. Since towers are attended to be long-term preserved, it is reasonable to evaluate the seismic vulnerability by also considering the creep’s influence. To make preliminary evidence, a case-study is herein presented referring to a 1-century-old (masonry) tank-tower located in Italy. In fact, even if the conventional seismic capacity’s check, in term of maximum lateral displacement (according to the Italian Desing Guidelines), is satisfied at the present age, the same evaluation referring to future ages (<return period for the expected earthquake) may be drastically different when accounting the relative creep status. A series of four push-over analyses was performed with Finite Element Method (FEM). The first acted as reference being the conventional seismic analysis (age 0). The other three referred to future ages; for which the load-condition was kept unchanged; on the contrary, the lateral stiffness was varied according to the creep’s level estimation. The results were expressed in terms of lateral displacement capacity versus spectral acceleration. The findings highlighted the importance of considering the combined creep and seismic effects on the historical masonry tank-tower when the long-term preservation is the main aim.

Multiscale viscoelasticity response for bamboo after partial hemicellulose removal treatment

Hemicellulose degradation can decrease the hygroscopicity and result in an improvement of dimensional stability for nature materials. The mechanical properties has always been used as an indicator of hemicellulose degradation, but lack of an indicator that can be reflects the additive effects of treatment condition and the effects of time. This work aimed to use multiscale viscoelasticity response to evaluate the coupled effects of physical/chemical treatment and loading time. Different conditions (hot water, acidic and saturated steam) were tested on bamboo. Macroscopic creep of bamboo slices and microscopic creep behavior of cell wall were determined. The relationship between gradient structure and viscoelasticity has been obtained. The outer slice, i.e., bamboo green, had best strength and flexural toughness. Middle slice exhibits significant viscoelasticity response. Inner slice, also called bamboo yellow, exhibits little viscoelasticity response but bad strength. The relationship between treatment conditions and macroscopic/microscopic creep also has been obtained. At macro scale, the acid group exhibits significant viscoelasticity response, while the water group and steam group exhibit more elasticity response, especially for water groups. At micro scale, the steam-treated and acid-treated group are all showed significant viscoelasticity response, while the water-treated group had lowest relative creep. This paper could be a reasonable selection support of bamboo materials for stiffness or flexible products.

Creep behavior and prediction of fiber-reinforced polymer reinforced timbers under changing temperature and relative humidity

The creep behavior of fiber-reinforced polymer (FRP) reinforced larch laminar timber was studied with respect to temperature and humidity fluctuations. There was a control group of larch laminated with two small-diameter timbers with FRP (carbon FRP (CFRP), glass FRP (GFRP)) produced; the laminated lumber had the same cross-sectional area as the two laminated lumber specimens with reinforced tensile parts. A creep test with fluctuating temperature and humidity was conducted by applying a load of 25% of the bending strength of the control specimen. A total of 8 specimens, 2 for each type, were measured for creep deformation at 9:00 am and 6:00 pm daily for approximately 14,000 h. Temperature and humidity fluctuations were measured every hour. The equilibrium moisture content and humidity of the creep test space exhibited a proportional relationship, and the moisture content of each specimen did not show a noticeable correlation with humidity and equilibrium moisture content. The average relative creep was measured as approximately 0.67 for the control, 0.4 for Glulam, and 0.43 for both the CFRP- and GFRP-reinforced specimens. Thus, the creep deformation of all the reinforced pieces was confirmed to be lower than that of the control specimen.

Effect of Na2CO3 on the tensile creep of slag-fly ash systems activated with Na2SiO3

The use of Na2CO3 in alkali-activated system (AAS) has attracted an increased interest since it provides a suitable setting time, suitable mechanical properties and lower cost. However, the tensile creep behavior of Na2CO3-activated AAS remains unknown, which is crucial for controlling its early cracking. Therefore, this study aims to investigate the effect of Na2CO3 on the tensile creep of slag-fly ash systems activated with Na2SiO3 (ASF). The total Na2O equivalent (Na2O of hybrid activators/binder, by mass) was maintained at 6%. Various Na2CO3 contents (i.e. the ratio of Na2O equivalent of Na2CO3 to total Na2O equivalent) were used in this paper, namely, 0, 1/3, 1/2 and 2/3. The compressive strength, flexural strength, elastic modulus, internal relative humidity, autogenous shrinkage, tensile creep and microstructures (X-ray diffraction, thermogravimetry, mercury intrusion porosimetry, scanning electronic microscopy-energy dispersive spectroscopy and nanoindentation) were investigated for the ASF system. Results indicate that the tensile creep of ASF mortar (ASFm) firstly increases and then decreases as the Na2CO3 contents increases. ASFm with Na2CO3 content of 1/2 has the highest tensile creep, which is 236.3% higher than that of ASFm without Na2CO3. When the Na2CO3 content increases from 0 to 1/2, Na2CO3 promots C(N)-A-S-H gel formation, increases the autogenous shrinkage and decreases the contact creep modulus of gels, leading to the increases of the tensile creep of ASFm. However, excessive Na2CO3 (content＞1/2) reacts preferentially with Ca ion dissolved from slag to generate CaCO3, inhibiting the gel formation. Moreover, CaCO3 plays a role in limiting the gel deformation, which remarkably reduces autogenous shrinkage and increases the contact creep modulus of gels, and therefore the tensile creep of ASFm is reduced.

Creep in expanded clay concrete at different levels of stress under compression and tension

The paper analyzes the results of experimental studies of the creep of light concrete and the influence of various factors on it. A method for determining the boundary of linear creep of expanded clay concrete has been developed. Experimental studies have been carried out to determine the boundary of linear creep. Experimental creep data and the kinetics of the change in the creep measure of expanded clay concrete over time under axial compression at different levels of long–term load (0.2 - 0.7)Rb are obtained. The limiting values of the relative creep deformation and shrinkage of expanded clay concrete are determined. Empirical formulas for determining and describing the relative deformation of the nonlinear creep of expanded clay concrete, measures of the linear creep of expanded clay concrete under axial tension and compression (if there are no direct experimental data), and the nature of the change are proposed λ(t;τ1)═Cbt(t;τ1)/(t;τ1), depending on (t;τ1) according to the linear law at each site, the nature of the influence of previous loading on the strength and modulus of elasticity of concrete, the limit values of the relative creep deformation and shrinkage of expanded clay concrete.

The Long-Term Mechanical Properties of BS Perpendicular to the Grain

As a modern bamboo composite with good mechanical properties, bamboo scrimber (BS) has achieved prominence in the sustainable architecture field. When used as a structural material, it is inevitably under continual tension perpendicular to the grain, therefore its mechanical response under long-term loading is significant for structural design. In this study, tensile tests were conducted on BS under short-term and long-term loads perpendicular to the grain. The duration of load (DOL) effect on BS perpendicular to grain and its creep effect were analyzed. Compared with BS parallel to the grain, the DOL effect on BS perpendicular to the grain was less severe, and the capacity for creep resistance was weaker. The threshold stress ratio and relative creep strain of BS perpendicular to the grain were 0.40 and 0.87, respectively. It was found that the DOL models and the viscoelastic model accurately predicted the DOL factor and creep strain. This study provides a scientific reference for the safe lifetime service of BS in practical engineering.

Flexural creep in plain concrete: State of the Art

Concrete, being a viscoelastic material, creeps with time causing additional strains and deflections in flexural members. Most of the studies in creep of concrete are based on long-term tests on plain concrete standard cylinders under axial compression, very limited studies have been reported on creep of concrete in flexure and direct tension. Whether the mechanism of creep in tension is different from creep in compression is still being debated. Numerous studies have been reported in the literature showing varied results with regard to the effects of creep in direct compression, direct tension and flexure. This paper provides a brief review of various creep tests on plain concrete reported and attempts to understand the effects of various parameters on the creep behaviour of concrete. Some studies reported that the creep in tension is higher than that in compression, whereas a few others obtained contradictory results from creep tests. This may be attributed to the fact that the relative creep behaviour in tension and compression is highly sensitive to the material composition, exposure conditions and loading level. The studies by various researchers on flexural creep in plain concrete are also discussed.

Deformations of elastic aftereffect: the effect of long – term strength loadings and concrete work diagram

The article presents data on the deformation of the elastic aftereffect of concretes of various compositions, at different stress levels of prolonged loading, the effect of prolonged loading on the strength and the diagram of concrete work. Development of relative creep deformations and elastic aftereffect of steamed concrete. The development of relative creep deformations and elastic aftereffects of natural hardening concrete of the B series under stress. Data on the effect of prolonged loading on the strength of the studied concretes. Development of relative creep deformations e_p(t) including deformations of elastic aftereffect after unloading of samples. Data on the effect of prolonged loading on the strength of the studied concretes.

Effect of nickel slag on the basic tensile creep of alkali-activated slag/nickel slag mortars

Alkali-activated slag (AAS) is used as a green cementitious material with high strength and good durability, while the cracking damage due to high shrinkage limits the application of AAS in engineering project. The tensile stresses due to shrinkage can easily lead to the cracking of structures, while basic tensile creep can relax part of the tensile stresses and reduce the cracking risk. This study aims to investigate the effect of nickel slag (NS) on the basic tensile creep of alkali-activated slag/nickel slag mortars (ASNms). NS content refers to the weight ratio of NS to the summation of slag and NS. The properties of ASNms, including microstructure, hydration heat, internal relative humidity, mechanical properties, autogenous shrinkage and basic tensile creep, are studied. Results show that the mechanical properties of ASNms decrease with the increase in NS content. By reducing the hydration degree, the incorporation of NS decreases the amount of calcium aluminosilicate hydrate (C-(A-)SH) of ASNms, and the internal relative humidity and nominal crystal-binder ratio of ASNms are increased. As a result, the autogenous shrinkage of ASNms was significantly mitigated by incorporating NS. Autogenous shrinkage is the driving force of tensile creep. With the increase in NS content, the Na+ content absorbed by C-(A-)SH decrease, and therefore reduces the basic tensile creep of ASNms.

The strain characteristics of cracked anchored soft rock under short-term creep

Abstract The short-term creep properties of cracked soft rock are the basis for analyzing and predicting the long-term deformation characteristics of engineering rock mass. Based on orthogonal test design and by means of physical testing and theoretical analysis, this paper systematically studies the short-term creep strain properties of soft rock mass with a vertical anchor form under different combinations of crack number, crack discontinuity degree and crack inclination. The results show the following: (i) as the stress increases, the growth rate of the instantaneous strain of the anchored soft rock slows down, the growth rate of the creep strain increases and the growth rate of the total strain decreases at first and before it then increases. Based on this, the idea is proposed that the stress corresponding to the turning stress of the total strain change rate, from small to large, is the turning stress of the strain rate. This stress value is approximately 50% of its uniaxial compressive strength (UCS). (ii) Statistical analysis shows that for the same cracked rock mass, the three characteristics of cracks have different effects on the instantaneous strain, creep strain and total strain of anchored soft rock, and a certain number of cracks under specific geometric conditions are beneficial to the long-term deformation control of anchored soft rock. (iii) A relative creep model based on mutation is proposed. Considering the effects of the test instrument, the model can be divided into the power function model, the logarithmic function model and the exponential function model, according to goodness of fit. All three models can describe the initial and stable creep stage of anchored soft rock with cracks.

Long-term mechanical properties of bamboo scrimber

Bamboo scrimber (BS), a main bamboo composite, has wide application prospects in green buildings. As a viscoelastic biomass material, long-term mechanical properties are the key indicators for its safe lifetime service. In this study, both short-term and long-term tension tests parallel to grain of BS were conducted. The long-term mechanical properties of BS including the duration of load (DOL) and creep effects were investigated from the macro and micro levels. Results indicated that the DOL effect of BS was more severe than other structural wooden materials, but its relative creep strain was smaller. From microlevel characterization, the DOL effect was strongly correlated with its microfracture morphology and bamboo fiber damage, while the creep effect was related to the decomposition of hemicellulose. Finally, time-dependent constitutive models were developed and provided good prediction for the DOL factor and creep strain of BS under different load durations. This study was beneficial for the long-term safe application of BS as a new structural material.

Flexural creep behaviors of bamboo subjected to different gradient variation directions and relative humidity

As more and more bamboo materials have been used for buildings, the bamboo creep property deserves more explorations because it is important to structural design and plays a critical role in serviceability of structure. In this study, the creep mechanism of bamboo over a wide range of scales from microstructure and bending flexural property to creep behavior were investigated. Firstly, the gradient functions of different vascular bundle distribution directions were obtained. The average value of gradient index n was respectively 2.16 and 0.29 when x axis ranged from different directions. Secondly, the flexural property test under different loading directions were analyzed. There were two groups, one group (OT) was the fiber-rich outer culm wall in tension side, the other group (Oc) was the fiber-rich outer culm wall in compression side. It was concluded that Group OT showed larger MOR and smaller MOE than Group OC. In addition, the total energy consumed of Group OT was larger than Group OC, while the elastic energy was smaller. Thirdly, the creep property under different loading directions and humidity were examined, which is unique because no research about the bamboo creep performance under different humidity conditions has been completed according to the literature review. The creep deflection of Group OT was larger than Group OC whatever constant humidity or variable humidity was used. The relative creep of Group OT was smaller than Group OC under high humidity (90% RH) and variable humidity (30% RH-90% RH-30% RH), while larger under low humidity (30% RH). Compared to bamboo-based products, the stress level was more sensitive on the creep property of bamboo, and the anti-creep property of bamboo was lower than bamboo-based products.

Experimental Investigation of the Creep Behaviour of Glulam Compression-Bending Members

Creep test results of glulam members under compression and bending were studied in this paper. The creep tests were conducted to investigate the influence of the stress level and relative eccentricity on the creep deformation of glulam members. The test results showed that the creep deformation trends of glulam members under long-term compression and bending loading were similar; the creep deformation increased with increases in both the stress level and relative eccentricity. However, the relative creep deformation decreased with the increase in both the stress level and relative eccentricity under long-term loading, and a five-parameter creep model was proposed to analyse the creep mechanism of glulam members under compression and bending. The good fitness of the test results indicated that the five-parameter model was able to accurately simulate the creep deformation of glulam compression-bending members. Moreover, a numerical model was developed using the creep equation, which was related to the parameter and . The simulation results were in good agreement with the test results, and the parameters and were correlated to the relative eccentricity and stress level, respectively. Based on the corrected parameters and , the relative creep deformation of glulam compression-bending members was predicted over 50 years.

Long-term bending properties of cross-laminated timber made from Japanese larch under constant environment

Cross-laminated timber (CLT) has been used extensively in timber construction. CLT panels are typically used in roofs and floors that carry a continuous load, and it is important to examine the long-term loading capacity of CLT. However, studies that focus on the long-term loading capacity of CLT are limited. To this end, we conducted long-term out-of-plane bending tests on seven-layer CLT made from Japanese larch (Larix kaempferi) under constant environmental conditions, investigated creep performance and duration of load, and experimentally analyzed creep rupture behavior. The mean estimated relative creep after 50 years was 1.49. The sample showed a satisfactory resistance to creep as a building material. The duration of load of most of the specimens in this study was shorter than the conventional value of small clear wood specimens. Specimens had a lower duration of load capacity than solid lumber. According to the results of survival analysis, a loading level of 70% or more caused the initial failure of specimens. Creep rupture of most of the specimens occurred at less deflection than displacement at failure in the short-term loading test. Additional studies focusing on the effects of finger joints, transverse layers, and width of a specimen on creep rupture behavior are suggested.

Experimental investigation on the rheological behavior of timber in longitudinal and transverse compression

Understanding the rheological behavior of timber is vital for evaluating the long-term performance of timber structures. In this study, short-term experiments were first conducted to obtain the short-term mechanical properties of timber including yield strength and elastic modulus in both longitudinal and transverse compression. Subsequently, creep tests were carried out to capture the rheological behavior of timber over 360 days. Different loading types of compression and stress levels were chosen in the creep tests. Environmental temperature and relative humidity, moisture content, environmental strain and creep strain were monitored. The coefficients of environmental deformation and irrecoverable mechano-sorptive creep were obtained in longitudinal and transverse directions. Various creep models of timber were calibrated against the experimental data. Weighted nonlinear regression was adopted to better simulate the secondary creep behavior. The creep coefficients over 50 years were estimated using the calibrated models, which were compared with values from existing national standards and research. The experimental results demonstrate that the limit of linear creep in longitudinal compression was bigger than that in transverse compression. Also, the transverse timber creep was much bigger than the longitudinal creep.

To the Issue of Low-Alloyed Copper Alloys for Contact Wires

This paper is about a comparative analysis of the requirements of the Russian standard, the standard of the European Union and the PRC for overhead contact wires for electrified high-speed railways. New generation contact wires must have a high level of mechanical, electrical and operational properties. The Russian standard does not regulate the chemical composition of alloys used for the contact wires manufacture. Unlike the standards of the European Union and China, the Russian standard regulates the maximum value of the relative creep. Compositions, mechanical properties, electrical resistivity and recrystallization temperature for the contact wires with a cross section of 120 mm2 are given. The requirements for the material for the contact wire are presented. The need to use more durable conductive materials instead of copper is noted since mechanical loads and operating temperatures are constantly increasing. The values of mechanical properties, recrystallization temperature and creep of contact wires made of copper alloys are given. Based on the analysis results of the literature data and the experiments, alloys of the Cu-Mg system are recommended as a material for the contact wire manufacture for an overhead network system for high-speed railways.

Bending Strength of Wood Treated with Propolis Extract and Silicon Compounds.

The modification of wood and its treatment with various preservatives may affect its mechanical properties, hence the knowledge of the character changes in wood caused by impregnation is of great importance. Therefore, the aim of the research was to determine the effect of impregnation, with the propolis-silane preparation (EEP-MPTMOS/TEOS) consisting of the propolis extract (EEP) and silicon compounds: 3-(trimethoxysilyl)propyl methacrylate (MPTMOS) and tetraethoxysilane (TEOS), on the bending strength of treated wood. Moreover, in the study wood treated with components of the propolis-silane formulation was used, namely 70% ethanol, the propolis extract, and silanes (MPTMOS/TEOS). In order to determine whether the impregnation of wood affects its long-term bending, creep tests were performed depending on the humidity. The impregnation of wood with the propolis extract and the propolis-silane preparation (EEP-MPTMOS/TEOS) contributed to the increase in modulus of rapture and work to maximum load values compared to the untreated wood. In dry wood condition, the wood treated with EEP and EEP-MPTMOS/TEOS was characterized by lower modulus of elasticity values than the control samples. In turn, in wet wood condition, wood treated with the propolis-silane preparation showed an increase in the MOE value. Moreover, the impregnation of wood had an influence on the wood creep process under bending loads. The treated wood was characterized by higher relative creep compliance than the untreated wood. The exception was the wood impregnated with EEP-MPTMOS/TEOS, which showed comparable relative creep compliance to the control samples. The presented results indicate that wood treated with a bio-friendly preparation based on propolis and silicon compounds can be used in various application and also in variable humidity conditions.

Bending Creep Properties of Cross-Laminated Wood Panels Made with Tropical Hardwood and Domestic Temperate Wood

For efficient use and expansion of domestic small- and medium-diameter woods, cross-laminated wood panels composed of tropical hardwoods and domestic temperate woods were fabricated, and the bending creep behavior under long-term loading was investigated. The bending creep curve of the cross-laminated wood panels showed an exponential function graph with a sharp increase at the top right side. The wood panel composed of a teak top layer and larch core and bottom layers recorded the highest initial deformation, and that composed of a merbau top layer and tulip core and bottom layers showed the lowest initial deformation. Creep deformation of the cross-laminated wood panels showed the highest value in that composed of a teak top layer and larch core and bottom layers and showed the lowest value in that composed of a merbau top layer and tulip core and bottom layers. The obtained creep deformation is 3.1–4.3 times that of merbau, however, it is remarkably lower than that of tulip and larch. The highest relative creep was recorded by the wood panel composed of merbau top layer and larch core and bottom layers, whereas that composed of the teak top layer and tulip core and bottom layers showed the lowest relative creep.

Bending Creep of Glulam and Bolted Glulam under Changing Relative Humidity

This study was carried out in order to evaluate the bending creep deflection of glulams and bolted glulams beam-to-beam connection with steel-gusset plates and bolts under changing relative humidity. The two types of glulam beams (130 mm in width, 175 mm in thickness, and 3000 mm in length) used in this study were made from domestic larch and composed of seven layers. The gussets were made of 8-mm-thick steel plates. Creep testing was conducted under constant loads in an uncontrolled environment. The test was carried out in a room that was well ventilated through a window. The creep test specimens were loaded for 33,000 hours. A bending creep test for the glulams was conducted through four-point loading. The applied stresses were 20% and 30% of the MOR in the static bending test for the glulam and bolted glulam, respectively. After 33,000 hours, the creep deflection of the glulam at a 20% stress level increased by 39% to 99%, while the creep deflection of the glulam at a 30% stress level increased by 27% to 67%, as compared with instantaneous elastic deflection. The relative creep increased during autumn and winter, and recovered during spring and summer. The relative creep of the bolted glulams was changed abruptly by loading up to 5,000 hours, but stabilized after 5,000 hours, and then gradually increased until 33,000 hours. The relative creep of the bolted glulam increased 2.11 times on average after 33,000 hours.