Creep Tendency Research Articles

Time-dependent soil–structure interaction analysis using a macro-element foundation model

The delayed settlement of foundations due to soil consolidation, creep or particle breakage can alter the internal load distribution and differential settlements in a superstructure through soil–structure interaction (SSI). The study introduces a novel methodology to simulate time-dependent SSI that overcomes the complexity of incorporating the time-dependent behaviour of foundations into routine SSI analysis. The proposed approach represents the superstructure as a condensed stiffness matrix, and replaces the foundations and underlying soils with macro-element foundation models that encapsulate the foundation–soil interaction into load–displacement relationships derived from constitutive models. To examine the performance of the proposed method, a macro-element model for time-dependent analysis of shallow foundations on sand was integrated with structural analysis to simulate two tests performed in a geotechnical centrifuge on a 3D-printed aluminium framed structure supported by footings on sand. The simulated responses of the superstructure and foundations were found to agree well with those observed in the centrifuge tests. Parametric analyses were conducted to investigate the effect of loading history, load level on the superstructure, and creep tendency of soil on post-construction load redistribution and differential settlements. The findings suggest that creep of foundations on sand facilitates load redistribution in the structure from heavily loaded sections to lightly loaded sections. Moreover, post-construction load redistribution depends on the differential creep between footings and should be considered for structures that are quickly constructed or at high levels of strength mobilisation (low factor of safety). Overall, the study highlights the potential of the proposed methodology in analysing the time-dependent SSI and its applicability in practical SSI analysis.

Modeling and analysis of creep in concrete containing supplementary cementitious materials based on machine learning

This paper used machine learning to model the prediction of creep and the analysis of characteristic factors for concrete containing supplementary cementitious materials (SCM). First, a creep database covering thirteen input parameters and one output parameter was developed. Then, based on this database, Linear Regression (LR), Support Vector Regression (SVR), Random Forest (RF) and Extreme Gradient Boosting (XGB) models were used to establish creep compliance models. Finally, the sensitivity analysis of each input parameter based on the shapley additive explanation (SHAP) model was performed to investigate the influence of each parameter on the prediction results of creep. The results show that the extreme gradient boosting (XGB) model can predict the creep compliance of SCM concrete very well. The creep compliance is the result of the combined effect of numerous positively and negatively correlated characteristic parameters. Among the many characteristic parameters, the stress-to-strength ratio is the most important factor affecting the prediction of creep. The creep compliance of concrete is positively correlated with cement content, while it is negatively correlated with silica fume (SF) content, and shows a negative and then positive change with slag and fly ash (FA) content. The creep tendency is smaller when the slag content is about 25% or the FA content is about 20%. Meanwhile, the interaction interval between different SCM admixtures and cement content differed in concrete creep prediction.

Assessment of the stepped isothermal method for accelerated creep testing of high-density polyethylene

Thermoplastic materials are increasingly used in demanding structural applications under, in some cases, long-term static loading over several decades. In this regard, the stepped isothermal method (SIM) with creep testing at stepwise increased temperature levels in combination with time-temperature superposition (TTSP) provides a very time efficient procedure for long-term creep characterization. In the present study, the creep behavior of an injection molded high-density polyethylene material (HDPE) was investigated by SIM in the thermally untreated state as well as after annealing.Due to experimental issues regarding the heating behavior of the specimens and non-linear viscoelastic behavior, particularly at elevated temperatures, bi-directional curve shifting was required in order to generate meaningful master curves for creep compliance. In a first step, an Arrhenius equation was used for the horizontal curve shifting, based on activation energies, determined in additional multi-frequency dynamic mechanical analysis (DMA). Continuous master curves were then obtained by empirical vertical shifting of the individual creep curve segments for the different temperature levels. In general, good agreement was observed between the resulting SIM master curves and the corresponding conventionally measured creep compliance curves at least for a time range up to 300 hours. Furthermore, significant differences in the creep tendency of the annealed material state compared to the thermally untreated condition revealed the distinct influence of the thermal history on the resulting creep behavior.

Failure analysis for gas storage salt cavern by thermo-mechanical modelling considering rock salt creep

Salt cavern is ideal vessel for underground gas storage due to the high deliverability. During cyclic gas operations of injection-and-withdrawal, the temperature change in salt cavern imposes thermal stress on cavern wall. The temperature change, together with the pressure change in salt cavern, causes the effective stress on cavern wall to alter, leading to variation of creep rate. In this study, a coupled thermo-mechanical model is proposed for failure analysis of salt cavern in rock salt prone to creep deformation. Six cases that consider rock salt with different creep tendency have been conceived for the coupled thermo-mechanical model to investigate the stability of the salt cavern in terms of cavern convergence and failure indices. The results indicate that the affected region by cyclic pressure and temperature is up to 10 m inside the rock salt from the cavern wall. It is also revealed from the results that largest displacement occurs on the top of the cavern, indicating that the cavern top is most liable to deformation damage. Although convergence of cavern fluctuates over the injection and withdrawal sessions, long-term convergence of cavern depends on the creep tendency of rock salt rather than the cyclic loading; rock salt with stronger creep tendency leads to larger cavern convergence. The results also demonstrate that there is stress concentration on the top and bottom of the cavern; the cavern wall is likely subject to shear failure according to Mohr‒Coulomb failure criteria, however, the cavern is unlikely to fail due to expansion according to Drucker‒Prager failure criteria. Both Mohr‒Coulomb and Drucker‒Prager failure indices are smaller in the cases where rock salt has stronger creep tendency. The proposed thermo-mechanical model provides an approach for evaluation of long-term stability of underground salt caverns.

Experimental method for creep characterization of polymeric foam materials in media immersion

Polymeric flexible foam materials are widely used as damping materials in structural applications primarily to reduce unwanted system vibrations and related noise generation. Due to the viscoelastic nature of polymers and high compressibility of soft polymeric foams, their damping quality is strongly dependent on the overall loading situation, which occasionally means complex mechanical loading scenarios combined with specific ambient service conditions. In the case of superimposed constant compressive loading the deformation of the damping components is basically dependent on the fundamental creep tendency of certain material type and is also strongly influenced by service temperature and the surrounding contact media. Thus the chosen test methodology for proper creep characterization has to reflect these major influencing parameters.In this regard, a specific creep testing device was built up for the performance of small load compression creep experiments on soft foam specimens immersed in liquid media, which was mineral oil in the present study. Moreover, the thermo-mechanical behavior of the foam materials was investigated by dynamic-mechanical analysis (DMA). The resulting temperature-dependent modulus and damping characteristics showed a good correlation with the corresponding creep behavior, enabling a rough estimation of the creep tendency within corresponding temperature ranges.

Experimental and Numerical Investigations on the Creep Behaviour of Heat-Resisting Chromium Steel X10CrMoVnb9-1 by Means of Small Punch Test

The mechanical characterisation of materials at operating temperatures is of great importance for many industrial branches. Miniaturised specimens are required if the available amount of material is not sufficient for standard sized samples. For that reason, a material testing device is presented, which allows the mechanical investigation of round plate small scale specimens at temperatures up to 1273 K. Modern high temperature materials for nuclear power plant components of the next generation are subjected to temperatures above 873 K. A constant long-term exposure at this temperature level results in creep of the material. The reduction of the creep tendency of heat-resisting chromium steel X10CrMoVNb9-1 (P91) is achieved by temperature stable carbides and carbonitrides at the grain and sub-grain boundaries. However, this ensures no absolute inhibition of creep. In the present investigation the high temperature small punch test (HT-SPT) is used to experimentally and numerically characterise the creep behaviour of P91. The experimental part includes the performance and evaluation of HT-SPT creep tests at constant force (creep rupture tests) at different temperature and load levels without clamping of the samples. The numerical part covers the identification of Norton creep parameters by numerical optimisation based on a calibration of finite element simulated and experimentally obtained creep curve results for the material P91. For the microstructure evaluation of the steel P91, a pre and post mortem investigation of the HT-SPT creep specimens is performed by light optical microscopy (LOM), scanning electron microscopy (SEM), energy-dispersive X-ray microanalysis (EDX), and an oxalic acid test.

Improving Belleville Washer Spring Characteristics Using Elastomer Filling

Abstract Belleville washer steel springs are characterized by a long fatigue life, better space utilization, low creep tendency, and high force bearing capacity with a small spring deflection. In the case of thicker springs, a greater force bearing and greater stiffness are obtained, but the deflection of the spring is reduced. In such a case, the fatigue life is reduced and there is a very high probability that a Belleville washer spring may fail in a brittle manner, causing additional damage to machinery. In order to prevent such a fracture of a Belleville washer, an elastomeric filling was used on both free surfaces of the spring. Experimental testing and numerical analyses show that enhanced loading characteristics were obtained when the elastomer filling was increasingly involved in the force bearing process. When the elastomer filling is compressed, the stresses in the Belleville washer steel are reduced, because the majority of the deflection stress is shared by the elastomer instead of the steel.

SOME CONSIDERATIONS ON THE MECHANICAL TESTING OF ALUMINUM-STEEL CONDUCTORS AND THEIR CORES

Safe electrical energy transport on high-voltage overhead lines requires the physical-mechanical characteristics of aluminum-steel conductors and steel cores to be experimentally determined and compared to standard values. The basic characteristics of aluminum-steel conductors can be obtained by tensile testing to failure. Such tensile tests are regulated by the standards: CEI-IEC 1089, CEI-IEC 61089, SR-CEI 1089 and BS EN 50182. Because of a wide rigidity range, aluminum-steel conductors require adapted testing conditions and experimental data analysis, ensuring that by tensile testing the most important mechanical characteristics can be put into perspective, for the whole conductor and for its core. Therefore, a complete conductor testing procedure has been developed in this paper, according to the European standards in effect and applied for two conductors with a rigidity ratio of 4.42. The experimental tests were carried out on a horizontal tensile machine, designed and built in the Laboratory for Conductors and Cables of the “Politehnica” University of Timisoara, Romania, having a length of 13 m and developing a force up to 2500 kN. The tensile tests have firstly shown that a (sometimes significant) difference can be observed between the stress-strain curves obtained according to the standards in effect and the ones based on the strains measured at the beginning of each hold period. This shows the creep tendency of conductors over a short period of time, when exposed to overloads. Secondly, it has been found that in case of high rigidity conductor steel cores, current standard testing is limited only to the elastic region. DOI: http://dx.doi.org/10.5755/j01.test.10.1.5105

Buckling experiments on polyethylene liners with egg-shaped cross-sections

In Germany, egg-shaped concrete and masonry sewers can be renovated with short thermoplastic liners of corresponding cross-sections. The decisive variable for dimensioning of the wall thickness is the groundwater pressure on the liner surface. The necessary calculations are, as a rule, executed by data processing procedures. Buckling experiments are important for the development of the theoretical approaches. The specific material behaviour of thermoplastics, and in particular their creep tendency, is considered in the experimental planning. In total, the results of 11 buckling experiments with different imperfections (none, annular gap or local imperfection) concurred with the buckling pressures determined by calculation. Ten small-scale experiments and one full-scale test were carried out at constant external pressure as creep tests of over 2–3 h in duration. The evaluation of the 11 experiments shows that the application of the German design code ATV-M 127-2 for the calculation of the stability of egg-shaped profiles subjected to water pressure (host pipe states I and II) is suitable. The small-scale experiments showed that less extensive set-ups still supply useful information when examining the load-carrying capacity of non-circular liners. Such experiments could be carried out within the framework of first examinations and for quality assurance.

Diminishing of work hardening in electroformed polycrystalline copper with nano-sized and uf-sized twins

The diminishing of strain hardening effect has been an important issue in understanding the special deformation mechanism in the nano- and ultrafine- (uf-)grained materials. The deformation of an electroformed polycrystalline pure copper with both nano- and uf-sized twins was performed by uniaxial tension. Strain hardening rate was studied using the Kocks–Mecking plot approach. Microstructure evolutions of the two-scale sized twins at different strain conditions were observed and compared by means of transmission electron microscopy. A link between the abnormal hardening rate and the deformation mode inside nano- and uf-sized twins was established, and the diminishing of the hardening rate is attributed to the enhanced creep tendency and premature recovery of the special microstructures.

Morphology – Property Correlations of PP Materials by Means of Microhardness

AbstractThe present work deals with the influence of crystallization temperature, cooling rate and annealing conditions on microhardness, indentation modulus and creep behaviour of ethylene/propylene (E/P) random copolymers with 4, 6 and 8 mol% ethylene as well as α‐ and β‐spherulites in a homopolymer and an E/P random copolymer. The materials are unnucleated, the formation of β‐spherulites occurs sporadically. Additionally the indentation creep behaviour of α‐ and β‐nucleated PP is investigated.A nearly linear correlation between hardness as well as indentation modulus and crystallintity of the E/P copolymers can be proved. An increasing cooling rate leads to decreasing hardness and modulus values due to the hindered crystallization.For the investigation of the α‐ and β‐phases different crystallization and annealing temperatures are used. Independent of these conditions, microhardness and modulus determined by indentation testing are lower for the β‐phase in both materials. Increasing crystallization temperature and annealing lead to an increasing hardness and modulus in both phases. However, an effective annealing effect takes place only at short times and elevated temperatures above 100 °C. The increasing of microhardness and modulus is correlated with an increasing in lamellae thicknesses. Additionally, indentation creep experiments were carried out on nucleated materials that show the stronger creep tendency of the β‐phase PP and the stronger influence of annealing on this phase.

Temperature dependence of the hardness of iron-molybdenum and nickel-molybdenum alloys

1. The effect of alloying iron and nickel with molybdenum has been studied, and it has been shown that the Mo concentration converting the alloys from one-phase to two-phase structures, hardens them over the whole temperature range studied. 2. Up to 700°C (1290°F) there is precipitation hardening in the two-phase region, which greatly increases hardness. 3. The long-time hardness of the nickel alloy with 25% molybdenum is higher at 600 and 700 than at 5.00°C (1110, 1290 and 930°F); the differences in the 30 and 3000-second hardnesses characterizing the creep tendency of the alloy, are approximately the same at 500. 600 and 700°C. 4. Increase of the test temperature to 800 and 900°C (1470 and 1650°F) shows that the 25% molybdenum alloy (Ni-base) softens more readily than the alloy with 7% Mo. 5. At 1000°C (1830°F) nickel-base alloys suffer identical softening irrespective of their Mo contents, and the hardness/time curve is similar to that obtained at 500–600°C. 6. Increase of the Mo content within the range studied only slightly affects the elastic modulus, which continuously decreases with increase of test temperature. 7. The logarithmic decrement of elastic vibrations does not change up to 500°C (930°F) in any of the alloys studied, but with further increase of test temperature, there is a sharp increase in internal friction; this process starts in one-phase alloys at a lower temperature than in two-phase alloys.