Long-term Creep Tests Research Articles

Rapid assessment of the creep rupture life of metals: A model enabling experimental design

Prediction of the creep rupture life of engineering metals is critical for qualification and design of new materials. The use of long-term creep tests and the need to quantify the performance variability in a priori similar systems hinder the rapid creep assessment of a given material. Therefore, it is essential to develop methods that can extrapolate the long-term performance of alloys and the associated variability from short-term experiments. To this end, this study introduces a new model which enables the estimation of the rupture life of a material for a given stress and temperature. This model relies on two components. First, a new relation for the minimum creep rate (MCR) of materials is introduced. It includes a stress dependent stress exponent allowing the model to capture the variation of MCR across a wide range of temperatures and stresses. Second, employing the Monkman-Grant (MG) law, we establish a relation between stress, temperature and creep rupture life. Together, these two elements yield a new closed-form mathematical expression for the Larson Miller parameter as a function of stress and temperature. This expression captures the creep rupture time for many metals (Gr91, Copper, Gr122 and 347H) and compares favorably with alternate empirical approaches. The model is then used to assess the minimum duration of creep rates necessary to qualify the material up to 100000h. It is found that depending on the material system, creep tests as few as five limited to 5000 h for steels (Gr91, Gr122, 347H) and 100 h for copper are sufficient to model creep lifetimes. Finally, using a Bayesian inference-based approach to calibrate the model, we demonstrate that variability in rupture life can be captured via the quantification of the uncertainty in the model parameters and extrapolated from a limited number of short to moderately short creep tests; thereby paving the way for accelerated creep testing.

Full-scale evaluation of creep coefficients and viscoelastic moduli in honeycomb sandwich pultruded GFRP composite cross-arms: Experimental and numerical study

The utilization of pultruded glass fibre-reinforced polymer composites (PGFRPC) to replace traditional wooden cross-arms in high transmission towers is a relatively recent development. While there have been numerous investigations into enhancing cross-arm structures, there remains a notable absence of research focused on the elastic characteristics of a full-scale PGFRPC cross-arm, particularly one enhanced with a honeycomb sandwich structure. To full-fill the gap, this paper presents an experimental and numerical study through cantilever beam flexural tests on assembled cross-arm condition to examine deflection behavior and the flexural creep response. For deflection behavior, the load was applied up to actual working load. For creep behavior, the hanging load was applied for 1000 h in open area condition followed ASTM D2990 standards. By using Findley's power law, confirming the ability of this empirical approach to simulate the viscoelastic response of the cross-arm. The results obtained prove that the addition of a honeycomb sandwich structure reduced deflection and improved resilience against bending forces, enhancing specific points' elastic modulus slightly. Long-term creep tests revealed Point Y3 had the highest strain, but the enhanced cross-arm displayed superior resistance and a shorter viscoelastic transition period, indicating increased stability. Besides that, the Findley's Power Law Model effectively represented creep behavior for both cross-arm types, with low errors. Over 50 years, both versions showed a significant reduction in average elastic modulus, with the enhanced variant 20 % stronger due to the honeycomb structure. In conclusion, this study validates the superior creep properties of the enhanced PGFRPC cross-arm and demonstrates the honeycomb sandwich structure's substantial role in increasing strength and extending the cross-arm's lifespan, making it a valuable enhancement for such applications.

Factors Affecting Properties of Polymer Grouted Sands

The aim of this research was to undertake laboratory testing to investigate the beneficial effects of epoxy resin grouts on the physical and mechanical properties of sands with a wide range of granulometric characteristics. Six sands of different particle size and uniformity coefficients were grouted using epoxy resin solutions with three ratios of epoxy resin to water (3.0, 2.0 and 1.5). A set of unconfined compressive strength tests were conducted on the grouted samples at different curing periods and a set of long-term unconfined compressive creep tests in dry and wet conditions after 180 days of curing were also carried out in order to evaluate the development of the mechanical properties of the sands, as well as the impact of water on them. The findings of the investigation showed that epoxy resin resulted in appreciable strength values in the specimens, especially those of fine sands or well graded sands, grouted with the different epoxy resin grouts. Whilst the higher compressive strength and elastic modulus values at the age of 180 days were obtained for the finer sand, which ranged from 2.6 to 5.6 MPa and 216 to 430 MPa, respectively, the lower compressive strength and elastic modulus values were attained for the coarser sand with low values of the coefficient of uniformity, which varied from 0.68 to 2.2 MPa and 75 to 185 MPa, respectively. Moreover, all grouted sands showed stable long-term creep behaviour, with high values of the creep limit ranging from 67.5 to 80% of compressive strength. The presence of water had a negative marginal effect in the majority of the grouted specimens. In terms of physical properties, the permeability and porosity were estimated. The permeability of fine sands or well graded sands was decreased by two to four orders of magnitude. Using laboratory results and regression analysis, three mathematical equations were developed that relate each of the dependent variables of compressive strength, elastic modulus and coefficient of permeability to particular explanatory variables.

Continuum Mechanics Models for Describing the Creep and Physical Aging Behaviors of Laminated Composites

Two types of carbon fiber-reinforced epoxy composite laminates are chosen for long-term tensile creep tests under different temperatures and load levels. Their time-dependent and non-monotonic deformations indicate clearly both temperature effect and physical aging effect. To characterize these viscoelastic behavior, two phenomenological constitutive models and one physical model are developed. The linear viscoelastic model based on the Boltzmann superposition principle is able to describe reasonably the deformations at relatively lower stress levels and temperatures. The nonlinear viscoelastic model of Schapery’s single-integral form, together with a usage of effective time theory, could describe nicely all the effects of temperature, stress, and physical aging. The physical model based on Ngai’s coupling mechanism concept is further combined with the framework of Schapery’s nonlinear viscoelastic theory, which may provide certain physical understanding about the effect of aging behavior on long-term creep deformation of the laminated composites. Numerical modelling by finite element method are implemented, and comparisons between the experimental and simulation results are demonstrated.

A multifunctional shear apparatus for rocks subjected to true triaxial stress and high temperature in real-time

Deep engineering disasters, such as rockbursts and collapses, are more related to the shear slip of rock joints. A novel multifunctional device was developed to study the shear failure mechanism in rocks. Using this device, the complete shear–deformation process and long-term shear creep tests could be performed on rocks under constant normal stiffness (CNS) or constant normal loading (CNL) conditions in real-time at high temperature and true-triaxial stress. During the research and development process, five key technologies were successfully broken through: (1) the ability to perform true-triaxial compression–shear loading tests on rock samples with high stiffness; (2) a shear box with ultra-low friction throughout the entire stress space of the rock sample during loading; (3) a control system capable of maintaining high stress for a long time and responding rapidly to the brittle fracture of a rock sample as well; (4) a refined ability to measure the volumetric deformation of rock samples subjected to true triaxial shearing; and (5) a heating system capable of maintaining uniform heating of the rock sample over a long time. By developing these technologies, loading under high true triaxial stress conditions was realized. The apparatus has a maximum normal stiffness of 1000 GPa/m and a maximum operating temperature of 300 °C. The differences in the surface temperature of the sample are constant to within ±5 °C. Five types of true triaxial shear tests were conducted on homogeneous sandstone to verify that the apparatus has good performance and reliability. The results show that temperature, lateral stress, normal stress and time influence the shear deformation, failure mode and strength of the sandstone. The novel apparatus can be reliably used to conduct true-triaxial shear tests on rocks subjected to high temperatures and stress.

Identification of load cells parameters under applying creep and dynamic force for dynamic force calibrations

Many mechanical systems in industry need dynamic force calibration for accurate measurements. This is due to the sensitivity of load cells changed greatly after their natural frequencies. Load cell parameters are used in predicting sensitivity and the frequency range where this cell can be used. These parameters values are constants depending on the load cell’s material and structural design. Consequently, they can be obtained under exerting static, or dynamic force. The aim of this paper is to demonstrate a new study for measuring the load cell parameters accurately under applying long-term creep test. Then, these parameters are compared to those measured while exerting dynamic force of sine mode. A clear difference has been observed between the two measurement methods. Rocking modes, and adapters in the dynamic measurement method affect the parameters accuracy. Generally, static measurement methods are well defined than dynamic in force application and deflection measurement, and less complex.

Cyclic Load Impact Assessment of Long-Term Properties in Compression to Steel and Polyvinyl Alcohol Fibre-Reinforced Geopolymer Composites.

This study investigates the cyclic load application impact on fly-ash-based geopolymer composites that are reinforced with a low amount of fibre reinforcement. For reinforcement purposes, polyvinyl alcohol and steel fibres are used. For testing purposes, four geopolymer composite mixes were made, three of which had fibre reinforcement. Simultaneously, specimens were tested for shrinkage, static-load-induced creep, and cyclic-load-induced creep. For static and cyclic creep testing, specimens were loaded with 20% of their strength. For cyclic creep testing, load application and release cycles were seven days long. When each cycle was introduced, the load was applied in steps. Necessary load application or unloading lasted for 5 min and consisted of four steps, each 25% of the necessary load. From the long-term static and cyclic creep tests, it was seen that only the plain specimens showed that static creep strains are within cyclic creep strains. For all the other specimens, the static load was higher than the cyclic-load-induced creep amplitude. Also, 1% polyvinyl alcohol fibre-reinforced specimens showed the most elastic characteristics under cyclic load, and 1% steel fibre-reinforced specimens appeared to be the most resistant to the cyclic load introduction.

Lifetime prediction under dead-load long-term creep test and models analysis of wood-plastic composites for building materials

ABSTRACTOptimal formulation of wood-plastic composites (WPCs) from recycled polypropylene (rPP) and rubberwood flour (RWF) was determined. Mechanical properties and long-term creep behaviour of rPP and WPCs were investigated. This study revealed that an increasing of rubberwood flour content clearly increases the flexural, compressive, and tensile values. The modulus of composite samples linearly increased with an increasing amount of RWF. The creep behaviour test with load levels at 20, 30, and 40% of the ultimate flexural strength were conducted for 1000 h. The deflection of the composites at the same load levels of rPP gave the higher creep behaviour (2.45, 4.65, and 6.68 mm) than that of WPCs (1.53, 2.41, and 3.37 mm), respectively. Six-element for determining Burger model parameters was well fitted with the long-term creep of WPCs, this deflection result was compared to four-element Burger and Power law models. Finally, the master curve could predict lifetime of composite materials.

Tensile and long-term creep properties of structurally and functionally integrated carbon fiber reinforced plastic at room temperature

Carbon fiber reinforced plastic (CFRP) based on phenolic resin have been widely used in aerospace and pressure vessel fields because of their high stiffness to weight ratio and excellent mechanical properties. In this paper, carbon fiber reinforced plastic were prepared by molding method using phenolic resin as the matrix. The fiber distribution inside the material and its influence on tensile fracture mechanism and creep properties were investigated. The specimens were prepared along horizontal and vertical directions. The test results show that the fibers tend to be distributed horizontally within the material and are influenced by the mold. Horizontal specimens with a large number of fibers parallel to the tensile direction exhibit more excellent tensile properties. The vertical specimen is loaded by the resin and interface with weaker properties inside. The fracture mechanism of horizontal specimens is fiber breakage, fiber pullout and resin fracture, while the fracture mechanism of vertical specimens is interfacial debonding and resin fracture. The difference in tensile fracture mechanism of horizontal and vertical specimens affects the creep properties of CFRP. The creep resistance of fiber is better than that of resin. Therefore, horizontal specimens exhibit better creep resistance in long-term creep tests due to the loads borne by the fibers. Modified Time Hardening model can accurately fit all experimental data. Creep curves can be predicted based on the model parameters.

Creep analysis of the flax fiber-reinforced polymer composites based on the time–temperature superposition principle

Abstract Natural plant fiber-reinforced polymer composites (PFRP) have emerged as an environmental-friendly material in the construction industry, but their creep behavior is a critical concern for load-bearing structures. This study investigates the creep behavior of flax fiber-reinforced polymer composites (FFRP) using the time–temperature superposition principle (TTSP). Due to the application of TTSP on the tensile creep behavior of FFRP is not fully understood, three potential methods for calculating the critical parameters during TTSP are compared to obtain an efficient application method to build the creep master curve. A 2,000-h long-term creep test is conducted parallelly on the same sample to validate the accuracy of the creep analysis results. The study proposes an ideal method to determine the key parameters in TTSP, providing valuable insights for the practical application of PFRP in the construction industry. Meanwhile, the research results in this study would be helpful in better understanding the creep behavior of FFRP via short-term accelerated tests.

Experimental and analytical study of flexural creep of impregnated woven fabric-reinforced concrete

Textile-reinforced concrete (TRC) is a promising composite material that has growing demands in the construction industry. The time-dependent behavior of TRC is a less-studied characteristic of these materials. Since the wide use of TRC in constructing thin-walled concrete structures, the present study performed an experimental and analytical study on the flexural creep of concrete reinforced with resin-impregnated woven fabrics. An analytical viscoelastic model is developed based on multi-scale micromechanics, classical lamination theory, and Alfrey’s correspondence principle to predict the upper and lower limits of creep deformation of TRC. Due to uncertainty observed in the cracking force of TRC, this parameter is considered a random variable with normal distribution. Probabilistic intervals are calculated for TRC midspan creep deflections. Short-term and long-term creep tests are conducted to evaluate the proposed model. The results showed that a probabilistic model is more appropriate to predict the creep behavior of TRC rather than a deterministic model.

Low strain rate yield characteristics and failure modes of solid composite propellant

Solid motor propellant grain is in low strain rate creep state under long-term storage condition. To analyze the yield characteristics and failure modes of HTPB (hydroxyl-terminated polybutadiene) propellant under different strain rate conditions, the constant speed tensile test of 2.38 × 10−5 s−1～2.38 × 10−2 s−1, 12-week long-term relaxation test and 4-week long-term creep test were carried out. The dynamic yield function of elastic-viscoplastic model was proposed, and the function parameters were fitted and verified by experimental data. The results show that the mechanical properties of the propellant show obvious rate dependence under lower strain rate, and the breaking strength and maximum elongation decrease significantly with the decrease of strain rate. The propellant starts to yield after strain reaching 15.0%, and the meso-structure enters an unstable state. The matrix and particle interface dewetting gradually becomes serious, developing into large particle shedding and matrix tearing, and finally causing macroscopic failure. The dynamic yield function can well describe the relationship between the dynamic yield strength, viscoplastic strain rate and static yield strength. The static yield strength of the propellant was analyzed and obtained as 0.06 MPa. The obtained yield strain and static yield strength can support the establishment of structural integrity failure criteria for long-term storage solid rocket motor.

Effect of Mortise and Tenon Structure on the Properties of Wood Flour Polyvinyl Chloride-Laminated Veneer Lumber Co-Extruded Composites.

Core-shell composites with strong weather resistance, mechanical strength and creep resistance can be prepared using co-extrusion technology. Considering the weak bonding strength between core-shell interfaces, this study started from the concept of a mortise and tenon combination; three types of conical, rectangular and trapezoidal mortise and tenon joints were prepared, and their bending properties, long-term creep properties, interfacial bonding properties, and dimensional stability properties were tested. Results showed that the mortise and tenon structure could form a mechanical interlock between the outer-shell-layer polyvinyl chloride (PVC) wood-plastic composite (WPVC) and the inner-core-layer laminated veneer lumber (LVL), which could effectively improve the interface bonding property between the two layers. Among them, the trapezoidal mortise and tenon structure had the largest interface bonding force compared with the tapered and rectangular mortise and tenon structure, where the interface bonding strength reached 1.01 MPa. Excellent interface bonding can effectively transfer and disperse stress, so the trapezoidal mortise and tenon structure had the best bending properties and creep resistance, with a bending strength of 59.54 MPa and a bending modulus of 5.56 GPa. In the long-term creep test, the deformation was also the smallest at about 0.2%, and its bending properties, creep resistance and interface bonding performance were also the best. The bending strength was 59.54 MPa and the bending modulus was 5.56 GPa; in the long-term creep test, the strain curve was the lowest, about 0.2%. In addition, the mortise and tenon structure could disperse the stress of the inner shell LVL after water absorption and expansion, thus significantly improving the dimensional stability of the co-extruded composite after water absorption.

Experimental characterization and phenomenological modeling of nonlinear viscoelasticity, plasticity and damage of continuous carbon fiber-reinforced thermoplastics

The presented work addresses the experimental characterization and phenomenological modeling of the complex rate dependent mechanical response of continuous fiber-reinforced thermoplastics (cFRTP). The theoretical basis is given by the concept of an equilibrium curve of the material which provides a comprehensive framework for separating the mechanical response into a rate dependent overstress and a rate independent equilibrium stress. In long-term creep and relaxation tests, it is shown that relaxation tests provide several advantages over creep tests when determining the equilibrium response. A stepwise loading–unloading relaxation (SR) test is proposed which makes it possible to extract nonlinear viscoelasticity, damage and plasticity present in the material from a single test. The application to cFRTP is exemplified by using the proposed method for the experimental characterization of a continuous carbon fiber-reinforced polycarbonate (CF-PC). Based on the obtained experimental data, a phenomenological material model is developed which describes the mechanical response of a unidirectional ply. The model consists of an overstress branch addressing the nonlinear viscoelastic material response and an equilibrium branch that utilizes a coupled damage-plasticity approach. After calibrating the model using the experimental data, it is shown that the proposed model is able to accurately reproduce the mechanical response of cFRTP.

Investigation on very long-term brittle creep test and creep-damage constitutive model for granite

Investigation on very long-term brittle creep test and creep-damage constitutive model for granite

Rock salt creep: a cross-check test to cover the full relevant range of deviatoric stresses

The time-dependent response of rock salt has been mainly investigated using confined creep tests covering a differential stress range between 5 and 20 MPa. In recent years, efforts have been made to investigate the range 0.1–4.5 MPa, using dead-end drifts in underground mines to take advantage of the very stable ambient conditions (temperature, relative humidity). Up to now, the combination of experimental data in the two ranges is difficult because of the use of different salt facies, sample preparation methods, test temperatures and experimental conditions (e.g., confined vs. unconfined tests, scale of measurements). In this work, we conduct two long-term creep tests on two natural salt samples from the same origin and prepared using the same protocol. The thermo-mechanical loading path is the same for the two tests, with only a small difference in the lateral load. One test is performed in a remote drift in a salt mine and the other test is performed in a climatic chamber in the laboratory. The comparison of the results is consistent, allowing to investigate a large range of deviatoric stresses by combining results in the two facilities. The final goal of this approach is to reduce stress extrapolation by investigating the whole deviatoric range that is relevant for underground operations. Next steps include investigating in more detail the effect of intergranular fluids, testing different temperatures and performing confined tests in the mine.

Development and performance of INCONEL® alloy 740H® seam-welded piping

INCONEL® alloy 740H® is an age-hardenable nickel-based alloy approved for pressure vessels and piping within ASME Section I, VIII, and B31.1. Currently, the code applies a weld strength reduction factor (WSRF) of 0.7 to the allowable stresses for longitudinal seam welds in the time dependent creep regime. In this work, a full-scale seam weld was successfully produced using typical industrial practices. The component was solution heat-treated and aged after fabrication to improve the performance of the weldment. Tensile, bend, impact, and low-cycle fatigue tests showed the component met all the specification requirements and elevated temperature properties were within base metal expectations with failures predominately in the weld metal. Long-term creep tests, including large samples more representative of the entire weldment, were fabricated and tested to times in excess of 10,000 h at multiple temperatures. Analysis of the creep data supports a WSRF of 0.9 for the solution annealed + aged weldments in contrast to the WSRF of 0.7 currently applied to welded + aged weldments. Metallurgical analysis shows that the solution annealing causes recrystallization of the weld metal and reduces the chemical compositional and microstructural gradients in the weldments resulting in acceptable short-term performance. In long-term creep, evidence for coarsened zones in the weldments due to discontinuous coarsening reactions were identified as the mechanism leading to accelerated creep damage formation in the weld metal and sample failure.

Nonlinear creep-damage constitutive model of surrounding rock in salt cavern reservoir

Salt cavern energy storage is a key technology to promote the optimization of China's energy reserve structure, but the surrounding rock of the reservoir cavity is prone to creep and damage during the long-term injection production process, which seriously endangers the safety of the reservoir operation. Therefore, it is urgent to carry out salt rock creep test research to reveal the creep mechanical properties and damage evolution failure mechanism of the surrounding rock of the reservoir cavity, so as to provide a theoretical basis for the safe construction and operation of the salt cavern reservoir. In order to deeply study the creep mechanical properties of salt rock, a long-term graded loading creep test was carried out on salt rock specimens. The loading time of each stage was about 30 days, and the total creep time exceeded 3 months. The long-term strength of salt rock is determined to be about 8 MPa by the improved isochronous stress-strain method and steady-state creep rate method. A nonlinear creep-damage constitutive model of salt rock based on elastic element and dashpot element is established and compared with the fitting results of several models for the sake of describing the creep behavior of salt rock more accurately. The local and global sensitivity analysis methods are applied to the model to verify its robustness and the rationality of parameter setting. The results showed that the fitting results of the model were in good agreement with the test data, and could well describe the whole creep process of salt rock, especially the nonlinear accelerated creep stage. In addition, the constitutive relation of the established model is simple, and the meaning of the model parameters is clear, which provides a theoretical basis for the study of the creep law of salt rock and the safe operation of the salt cavern storage.

Modified Z-phase formation in a 12% Cr tempered martensite ferritic steel during long-term creep

The formation of modified Z-phase in a 12Cr1MoV (German grade: X20) tempered martensite ferritic (TMF) steel subjected to interrupted long-term creep testing at 550°C and 120 MPa was investigated. Quantitative volumetric measurements collected from thin-foil and extraction replica samples showed that modified Z-phase precipitated in both the uniformly-elongated gauge (fv: 0.23 ± 0.02 %) and thread regions (fv: 0.06 ± 0.01 %) of the sample that ruptured after 139 kh. The formation of modified Z-phase was accompanied by a progressive dissolution of MX precipitates, which decreased from (fv: 0.16 ± 0.02%) for the initial state to (fv: 0.03 ± 0.01%) in the uniformly-elongated gauge section of the sample tested to failure. The interparticle spacing of the creep strengthening MX particles increased from (λ3D: 0.55 ± 0.05 μm) in the initial state to (λ3D: 1.01 ± 0.10 μm) for the uniformly-elongated gauge section of the ruptured sample, while the thread region had an interparticle spacing of (λ3D: 0.60 ± 0.05 μm). The locally deformed fracture region had an increased phase fraction of modified Z-phase (fv: 0.40 ± 0.20%), which implies that localised creep-strain strongly promotes the formation of modified Z-phase.

Comparison of concrete creep test methods as per Russian and foreign normative documents. Part 2. Testing and processing of results

Introduction. The main approaches to long-term creep tests of concrete regulated by GOST 24544-2020, valid in the Russian Federation, as well as foreign ISO, ASTM, and EN standards, are analyzed.Aim. In this work, fundamental differences in conducting tests following Russian and foreign standards are determined. These efforts are essential and urgent, since, in order to use test results obtained abroad in Russia, it is necessary to harmonize test methods and initial test parameters (samples, measuring and test equipment, etc.).Materials and methods. The article reviews testing procedures, controlled parameters, and processing of result; part 1 of the article covered the application, testing and measuring equipment, manufacturing process, sample parameters and fittings, and test preparation.Results. ISO, ASTM, and EN are comparable; however, they differ greatly from GOST 24544 in some respects, in particular, regarding the sample parameters and the methods of their installation in the testing equipment.Conclusion. Further harmonization of GOST 24544 with the aforementioned foreign standards is necessary.