Why is creep accelerated at higher temperatures

As the stress increases, the creep deformation increases, the slope of the creep curve increases, and the creep rate is prone to stabilise, which is characterised by two stages of primary creep and steady-state creep. Under the last stress level, the creep strain response increases exponentially, resulting in the failure of the specimen, which can be divided into three stages including primary creep, steady-state creep, and acceleration creep [ 33 ].

Furthermore, the influence of exposure to different temperatures on the concrete creep duration varies. Indeed, the time to failure is reduced from It is evident that the rising temperature promotes crack propagation and increases the internal damage, as well as accelerating the concrete failure process. The relationships between the transient strain and stress level are demonstrated in Figure 7. As shown, a good correlation exists between the transient strain and stress level, which expresses that the transient strain increases rapidly with the increase of stress level.

Indeed, the increasing temperature weakens the resistance to transient deformation. Relationship between the stress level and the transient strain of GHBC, exposed to different temperatures. Figure 8 presents the creep strain under the stress level between 0. When the stress level is 0. When the stress level increases to 0. The above comparison shows that the effect of stress level on the creep behaviour of GHBC is related to the temperature.

The influence of temperature on creep deformation under high stress is more significant, given that the high stress aggravates the deterioration of GHBC. Under long-term stress, the cracks and pores are supposed to have sufficient time to expand.

Besides, the slip distance between the particles increases, and the edges of coarse particles are crushed, hence the weak particles break, which eventually results in a reduction in creep resistance [ 34 ].

The ratio of each stress and its corresponding axial transient strain is defined as the transient deformation modulus E 0 , which can be expressed, as follows:. The transient deformation moduli E 0 of GHBC exposed to various temperatures under different stress levels are presented in Figure 9. It can be observed the E 0 of the GHBC specimen fluctuates within a certain range, under various stress levels.

With reference to the research by Ma et al. The total deterioration degree and phase deterioration degree of GHBC exposed to different temperatures. This can be attributed to the dense internal structure, which enhances the resistance to transient deformation to some extent.

As the temperature increases, the transient deformation modulus of GHBC decreases, while the total deterioration gradually increases. As the temperature increase, the total deterioration degree increases to Moreover, the phase deterioration degree increases, indicating that the deterioration of GHBC by high temperature is a cumulative process.

The creep rate reaches a relatively high value at the moment of loading Figure 11 a. The latter corresponds to the considerable transient deformation of the specimen and gradual increases of the transient strain rate as the stress level increases. Once loading is finished, the creep rate decreases sharply. After the decay of the transient rate, the creep rate of the specimen at room temperature remains relatively low, with small fluctuations, when the loading ratio is lower than 0.

When the stress level fluctuates between 0. Unlike the creep, the strain is practically constant under a lower stress level during the steady-state creep stage, and the creep strain at the high stress levels continues to grow. At the last stress level, the creep keeps stable for a short period of time after the decay of transient strain, followed by a dramatic increase until the specimen fails. Furthermore, the fluctuation range of the steady-state creep rate increases significantly as the temperature increases.

The variation range of the steady-state creep rate of the GHBC specimen at room temperature is between 0. This can be explained by the exposure to the high temperature, accompanied by the formation of cracks and voids. Creep stress accelerates the development of the cracks and the expansion of the internal weakening zone, presenting obvious inhomogeneous characteristics [ 36 ].

All of the specimens are destroyed at the last stress level. It can be seen that the creep failures of GHBC specimens, exposed to different target temperatures, follow a similar trend under the last stress level. The specimens develop from the primary creep through the steady-state creep, and finally, the failure occurs at the accelerated creep stage, in which the primary creep and accelerated creep stage show shorter durations, while the accelerated creep stage presents longer durations.

The durations of creep under failure stress are presented in Table 5. It can be seen that the threshold stress of creep failure of the specimen at room temperature is 0. Comparing the specimens which failed under the same loading ratio, it can be seen that the higher the temperature, the larger the creep failure strain and the shorter the duration.

Hence, the proportions of primary creep and accelerated creep duration to the total creep duration gradually increase, reaching Similar results are reported by Li et al. When the stress increase to 0.

As presented in Figure 13 , each curve has a critical point, dividing the curve into the decreasing and increasing parts. The critical point is 0. The critical point moves forward with the increase of temperature, indicating that the GHBC is more prone to instability after exposure to high temperatures. Based on Yu et al. However, the linear creep under the stress below the critical stress level is negligible, while the nonlinear creep occurs when the stress level exceeds the critical point, leading to the creep failure [ 39 ].

Results, demonstrated in Figure 13 suggest that this relationship can be served as a design criterion for determining the critical stress level of creep failure via a multistage creep test. Once the stress exceeds the point, the GHBC exhibits nonlinear creep behaviour, and the creep rate is determined by the stress level and temperature condition.

The creep curves in Figure 6 demonstrate that the GHBC specimens experience a transient and steady-state creep stage at each stress level. Furthermore, the creep strain rate increases at first, followed by a decrease, until reaching a constant value.

Furthermore, the creep behaviour of concrete is usually described by the Burgers model, consisting of the Kelvin model and Maxwell model in series, as shown in Figure The creep equation of the model is as follows:. The first term in the model represents the transient or the elastic strain, which is independent of time. The second term represents the creep strain, which is time related.

Finally, the third term represents the primary creep with a decreasing creep rate [ 40 ]. The creep test data and theoretical curve are plotted and presented in Figure It can be observed that the predicted data agrees well with the experimental data. Table 6 represents the parameters of the Burgers creep equation after exposure to various target temperatures. The values of coefficient of determination R 2 are higher than 0. The results, provided in Table 6 , reveal the transient elastic strain increases and the E M decreases to some extent as the temperature increases.

As mentioned above, the parameters of the Burgers model i. It is essential to determine the parameters of the model under different temperatures to validate its applicability [ 43 , 44 ].

Therefore, data regression analyses are undertaken on the parameters, presented in Table 6. The fitting formulas are as follows:. Once the target temperature and the stress of the GHBC are determined, the creep behaviour under high temperature, predicted by the parameters of Burgers model, can provide the reference for the fire resistance design.

In this study, a series of uniaxial compression tests and multistage creep tests were carried out to study the influences of different target temperatures on the GHBC. The mechanisms of high temperature on the GHBC were analysed from both perspectives of the failure mode and the internal defects. The main findings of our study are as follows:. The weight and compressive strength of the GHBC decrease as the temperature increases. The failure mode shows the transformation from tensile failure to shear failure as the temperature increases.

The creep strain and creep rate increase with the increase in the temperature and the stress level, while the creep failure threshold stress and creep duration are reduced significantly. The higher the temperature, the more sensitive the stress is to the creep. The transient deformation modulus of the GHBC decreases as a power function with increasing temperature, while the deterioration degree increases.

The ratio of creep strain to total strain decreases at first, followed by an increase. The inflection point can be considered as the critical stress level of creep failure, which decreases with the increase of temperature. Finally, the parameters of the Burgers model are identified based on the experimental results. The theoretical curve of the model shows a satisfactory agreement with the creep test data at the primary creep and steady-state creep stages, which could potentially be applied in the fire resistance designs of the GHBC structures.

The research described in this paper was financially supported by the Funding Project of Anhui University of Science and Technology No. Conceptualization, Y. All authors have read and agreed to the published version of the manuscript. The authors declare that there are no conflict of interest regarding the publication of this paper. National Center for Biotechnology Information , U. Journal List Materials Basel v. Materials Basel. Published online Aug Find articles by Yu-shan Liu.

Author information Article notes Copyright and License information Disclaimer. Received Jul 1; Accepted Aug Abstract Glazed hollow bead insulation concrete GHBC presents a promising application prospect in terms of its light weight and superior fire resistance. Keywords: glazed hollow bead insulation concrete, high temperature, creep behaviour, failure analysis.

Introduction Owing to its relatively lighter weight, lower thermal conductivity, and better fire resistance compared to other types of concrete, glazed hollow bead insulation concrete GHBC has attracted increasing attention in recent years [ 1 ]. Materials and Methods 2. Purpose and Scope The main purpose of this study is to investigate the mechanical properties and creep behaviour of GHBC after exposure to high temperature, which will be useful in the fire safety designs of GHBC structures.

Raw Materials The binder material used in the experiment consists of the Chinese standard Portland cement and fly ash. Table 1 The chemical composition of the binder material. Open in a separate window. The third stage, tertiary creep, occurs when the creep life is almost exhausted, voids have formed in the material and the effective cross sectional area has been reduced. The creep rate accelerates as the stress per unit area increases until the specimen finally fails.

A typical failed specimen is illustrated in Fig. The creep test has the objective of precisely measuring the rate at which secondary or steady state creep occurs. Increasing the stress or temperature has the effect of increasing the slope of the line ie the amount of deformation in a given time increases. This enables the designer to calculate how the component will change in shape during service and hence to specify its design creep life.

This is of particular importance where dimensional control is crucial, in a gas turbine for instance, but of less importance where changes in shape do not significantly affect the operation of the component, perhaps a pressure vessel suspended from the top and which can expand downwards without being compromised.

There are therefore two additional variations on the creep test that use the same equipment and test specimen as the standard creep test and that are used to provide data for use by the designer in the latter case. These are the creep rupture test and the stress rupture test. As the names suggest both of these tests are continued until the specimen fails. In the creep rupture test the amount of creep that has occurred at the point of failure is recorded.

This data, if properly interpreted, is useful in specifying the design life of components when dimensional changes due to creep are not important since they give a measure of the load carrying capacity of a material as a function of time. Support for SMEs. Software Products. Go to Technical knowledge Search. The factors that affect creep of concrete are similar to the factors affecting shrinkage, which are as following:.

Creep is dependent on temperature and stress. Increase in temperature for a constant stress can decrease the time of the second stage and hence accelerate failure. Why is creep accelerated by heat? Creep is a type of metal deformation that occurs at stresses below the yield strength of a metal, generally at elevated temperatures.

Creep is generally related to elevated temperatures. The rate of failure increases at a comparatively higher temperature. In general, the creepy characteristics tapped into three core factors: They make us fearful or anxious; creepiness is seen as part of the personality of the individual rather than just their behavior; and we think they may have a sexual interest in us.

Begin typing your search term above and press enter to search. Press ESC to cancel. Skip to content Home Essay What is creep of concrete? Ben Davis May 1, What is creep of concrete? What is shrinkage and creep of concrete? Why does concrete creep occur? What is creep effect? What are the 3 stages of creep? What is creep failure? How do you stop creep failure? What is creep fatigue?

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