Long-term Creep Behavior Research Articles

Construction and non-linear viscoelastic properties of nano-structure polymer bonded explosives filled with graphene

The 1,3,5-triamino-2,4,6-trinitrobenzene (TATB)-based polymer bonded explosives (PBXs) modified with different contents of graphene from 0.05 wt% to 0.5 wt% were prepared by the water suspension methods. The non-linear viscoelastic properties of the TATB-based PBXs were detailedly investigated. The experimental results indicated that with the incorporation of only a small amount of graphene, the storage modulus, the static mechanical properties, and creep resistance in the nanocomposites were effectively improved. A rigid filler effect and the strong sheet/polymer matrix interfacial interaction to restrict the mobility of polymer chains played an important role in the enhanced non-linear viscoelastic behaviors of nanostructured materials. The formation mechanisms were further interpreted based on the modeling of the creep behavior using a six-element mechanical model. The modeling results demonstrated that the introduction of the graphene into the TATB-based PBXs was an effective and fundamental method to enhance the elastic modulus of high elastic deformation and restrain the irrecoverable deformation of the materials. The long-term creep behaviors predicted using the time-temperature superposition principle revealed that PBXs with 0.15 wt% graphene exhibited a lowest long-term creep strain.

Dislocation-based modeling of long-term creep behaviors of Grade 91 steels

To meet the 30 years (∼263,000 h) design lifetime of a typical thermal power plant, understanding and modeling of the long-term creep behaviors of the power plant structural steels are critical to prevent premature structural failure due to creep. The extremely long exposure to high operation temperature results in evolving microstructure during service, which cannot be observed in standard short-term (<1 year) creep tests. In fact, creep rupture life predictions based on short-term creep testing data often overestimate the creep rupture times for long period of time, and are therefore insufficient to provide a reliable failure time prediction. In this article, a microstructure-sensitive, long-term creep model is developed and validated against existing long-term creep experimental data (∼80,000 h) for ferritic steel Grade 91 (Fe-8.7Cr-0.9Mo-0.22V-0.072Vb-0.28Ni in wt.%). The mechanistic creep model is based on fundamental dislocation creep mechanisms – the particle bypass model based on dislocation climb from Arzt and Rösler [1] - that describe the steady state creep strain rate as a function of stress, temperature, and microstructure. In particular, the model incorporates the evolution of the particle size via coarsening into the dislocation climb theory, dislocation detachment mechanism and back-stress generated by subgrain dislocation structures. Model inputs were obtained based on the microstructure information obtained from published literature from the National Institute of Material Science (NIMS, Japan) creep database. The model, which can be used on many types of alloys, shows excellent agreement with existing long-term creep experimental data (∼80,000 h) of Grade 91 ferritic steel.

Long-term creep behaviour of cast TiAl-Ta alloy

The long-term creep behaviour of the cast Ti-44.4Al-8.1Ta (at.%) alloy with an initial γ-TiAl(L10)+α2-Ti3Al(D019) microstructure was investigated at 700 °C. The alloy shows a remarkable creep resistance. The minimum creep rate is measured to be as low as 1.47 × 10−10 s−1 and total deformation is only 2.5% after an interrupted 30000 h creep test at 200 MPa. The creep exposure affects the stability of the initial microstructure. The discontinuous precipitation leads to the formation of γ + τ(Ti3Al2Ta) type of microstructure along the lamellar colony and grain boundaries. The decomposition of the α2 phase results in the formation of τ and γ + α2 + L12 segments within the lamellae and cellular γ + α2 + L12 network within the coarse particles. The intensive formation and coalescence of the cavities along the grain boundaries leads to predominantly intergranual type of creep fracture at the applied stresses of 250 and 300 MPa.

Experimental and Numerical Investigation on Compression Creep Behavior of Wood

Abstract This article provides an efficient and intuitive way to predict the long-term (12-h) compression creep behavior of wood by a finite element method (FEM). In this study, oriental beech (Fagus orientalis L.) was used as an example. First, a short-term (3-h) compression creep experiment was conducted to obtain the original creep data of beech, and then a Kelvin model (KM) with two elements in series was applied to fit the curve of creep strain and time to calculate parameters used in FEM. Second, long-term compression creep behavior was numerically analyzed by the FEM based on the short-term creep data. Finally, the results of the FEM and the KM were compared with that of the long-term creep experiment. The results showed that the creep deformations increased in the order of longitudinal (L), radial (R), and tangential (T) directions under the same loading level, and the creep behavior of beech in the L direction stopped after a short time and stayed constant. Additionally, in the same orientation, the creep deformation increased with the growth of loading levels below the ultimate creep strength. In addition, the KM did not work well for predicting the long-term creep behavior of beech. By contrast, the results of FEM were highly consistent with those of the experiment, and errors in L, R, and T directions were all less than the standard deviation. As a result, the numerical method provided acceptable accuracy to predict the long-term creep behavior of wood construction and wood products.

The long-term creep and shrinkage behaviors of green concrete designed for bridge girder using a densified mixture design algorithm

Creep and shrinkage behaviors are critical factors in the precast/prestressed concrete industry because these factors allow engineers to assess the long-term performance of concrete and to develop life-cycle estimates for concrete structures. The current study presents the results of an experimental work that addresses creep and shrinkage behaviors as well as the development of compressive strength in ordinary Portland cement concrete (OPC), high-performance concrete (HPC), and self-consolidating concrete (SCC). The concrete mixtures created for the present study were used to fabricate prestressed bridge girders. A conventional method (ACI) was used to design the mixture proportion for OPC and a densified mixture design algorithm (DMDA) was used to design the mixture proportions for HPC and SCC. All concrete mixtures had the same target strength of 69 MPa (10000 psi) at 56 days. Additionally, a comparative performance in terms of strength development and creep and shrinkage behaviors of ACI and DMDA concrete is performed in the present study. Test results show that all of the samples attained the target strength after 28 days of curing and that the strengths of each continued to increase afterward. Importantly, the incorporation of pozzolanic materials into concrete mixtures affected the propagation of creep strain and shrinkage positively. Furthermore, the DMDA concrete sample delivered better long-term performance than ACI concrete in terms of compressive strength, creep strain, and shrinkage.

Creep of Posidonia Shale at Elevated Pressure and Temperature

The economic production of gas and oil from shales requires repeated hydraulic fracturing operations to stimulate these tight reservoir rocks. Besides simple depletion, the often observed decay of production rate with time may arise from creep-induced fracture closure. We examined experimentally the creep behavior of an immature carbonate-rich Posidonia shale, subjected to constant stress conditions at temperatures between 50 and 200 °C and confining pressures of 50–200 MPa, simulating elevated in situ depth conditions. Samples showed transient creep in the semibrittle regime with high deformation rates at high differential stress, high temperature and low confinement. Strain was mainly accommodated by deformation of the weak organic matter and phyllosilicates and by pore space reduction. The primary decelerating creep phase observed at relatively low stress can be described by an empirical power law relation between strain and time, where the fitted parameters vary with temperature, pressure and stress. Our results suggest that healing of hydraulic fractures at low stresses by creep-induced proppant embedment is unlikely within a creep period of several years. At higher differential stress, as may be expected in situ at contact areas due to stress concentrations, the shale showed secondary creep, followed by tertiary creep until failure. In this regime, microcrack propagation and coalescence may be assisted by stress corrosion. Secondary creep rates were also described by a power law, predicting faster fracture closure rates than for primary creep, likely contributing to production rate decline. Comparison of our data with published primary creep data on other shales suggests that the long-term creep behavior of shales can be correlated with their brittleness estimated from composition. Low creep strain is supported by a high fraction of strong minerals that can build up a load-bearing framework.

Strain-rate sensitivity of cement paste by microindentation continuous stiffness measurement: Implication to isotache approach for creep modeling

Strain rate (ε̇) of concrete is crucial for structures due to its significant effect on the mechanical properties of concrete. Understanding strain rate effect can ensure safe design of concrete structures with enhanced performance. In this study, strain rate effect on time-dependent deformation and mechanical properties of hardened cement pastes is investigated by performing continuous stiffness measurement (CSM). The w/c ratios of hardened cement pastes are 0.3, 0.4, and 0.5, the applied strain rates are 0.01s−1, 0.05s−1, 0.1s−1, and 0.5s−1. Experimental results show that the contact hardness (H) increases with increasing strain rate, and their relationship can be well captured by an empirical power law equation. However, strain rate has a negligible effect on the elastic modulus (E). The viscoplastic depth (hvp, shown in Fig. 2d)-force (P) curve, which is controlled by the micro creep behavior of cement pastes, is also affected by the strain rate. Similar to the long-term creep behavior of soils, a unique isotache-type hvp-P-ε̇ relationship is found for hardened cement pastes at micro level. The results preliminarily confirm the applicability of isotache approach to characterize the time-dependent behavior of cement pastes at micro level.

Long-term creep behavior of self-reinforced PET composites

Long-term creep behavior of self-reinforced PET composites

Micro-mechanism during long-term creep of a precipitation-strengthened Ni-based superalloy

The long-term creep behavior of a Ni-based superalloy Haynes 282 at 700 and 750 °C was investigated. The creep curves exhibit the traditional shape with three creep stages. The coarsening of the γ′ phase during creep at 700 and 750 °C can be detected. The applied stress plays an important role in the coarsening of γ′ particles because of the lattice misfit and the difference of elastic modulus between the matrix and γ′ phase. Dislocation shearing into the γ′ phase and the Orowan process are the dominant creep deformation mechanisms at 700 °C/322 MPa. Dislocations tend to shearing into γ′ phase at first; nevertheless, the Orowan bowing mechanism replaces the process of shear as the coarsening of γ′ phase. The dominant deformation mechanism at 750 °C/187 MPa and 750 °C/215 MPa is dislocation gliding combined with dislocation climbing. Dislocation networks distributed in the interface of γ/γ′ phase may change the direction of dislocations and promote them to climb over the γ′ phase. The fracture surfaces were observed by scanning electron microscopy. Intergranular fracture is the dominant failure mode of the three samples because of the softening of grain boundary and stress concentration. Quasi-cleavage fracture, which are attributed to the stress concentration at the carbides/matrix interface, can be observed on the fracture surface of the specimen crept at 700 °C/322 MPa, whereas dimples with small precipitates inside can be detected on the fracture surface of the samples crept at 750 °C/187 MPa and 750 °C/215 MPa.

Creep Properties of Walikukun (Schouthenia ovata) Timber Beams

This study presents an evaluation of creep constants of Walikukun (Schoutheniaovata) timber beams when rheological model of four solid elements, which is obtained byassembling Kelvin and Maxwell bodies in parallel configuration, was adopted. Creep behaviorobtained by this method was further discussed and compared with creep behavior developedusing phenomenological model of the previous study. Creep data of previous study was deformationmeasurement of Walikukun beams having cross-section of 15 mm by 20 mm with a clearspan of 550 mm loaded for three weeks period under two different room conditions: with andwithout Air Conditioner. Creep behavior given by both four solid elements model and phenomenological(in this case are power functions) had good agreement during the period of creepmeasurement, but they give different prediction of creep factor beyond this period. The powerfunction of phenomenological model could give a reasonable creep prediction, while for the foursolid elements model a necessary modification is required to adjust its long-term creep behavior.

Creep deformation mechanisms in a γ titanium aluminide

Titanium aluminides (TiAl) are considered as potential alternatives to replace nickel-based alloys of greater density for selected components within future gas turbine aero-engines. This is attributed to the high specific strength as well as the good oxidation resistance at elevated temperatures. The gamma (γ) titanium aluminide system Ti-45Al-2Mn-2Nb has previously demonstrated promising performance in terms of its physical and mechanical properties. The main aim of the current study, which is a continuation of a previously published paper, aims at evaluating the performance of this titanium aluminide system under high temperature creep conditions. Of particular interest, the paper is strongly demonstrating the precise capability of the Wilshire Equations technique in predicting the long-term creep behaviour of this alloy. Moreover, it presents a physically meaningful understanding of the various creep mechanisms expected under various testing conditions. To achieve this, two creep specimens, tested under distinctly different stress levels at 700°C have been extensively examined. Detailed microstructural investigations and supporting transmission electron microscopy (TEM) have explored the differences in creep mechanisms active under the two stress regimes, with the deformation mechanisms correlated to Wilshire creep life prediction curves.

Evaluating long-term performance of Glass Fiber Reinforced Plastic pipes subjected to internal pressure

A computational modeling procedure is developed for simulating long-term creep behavior of Glass Fiber Reinforced Plastic (GFRP) mortar pipes subjected to internal pressure. The modeling procedure includes creep evaluation, stress analysis, failure evaluation and degradation of mechanical properties. Different levels of internal pressure are examined to obtain sufficient data point on failure-pressure versus time-to-failure graph up to 100,000min. At each specific pressure level, the modeling procedure continues till the functional failure is distinguished implying on weepage phenomenon. Obtained data are extended to 50years using extrapolation technique for predicting the remaining strength of investigated GFRP pipe after 50years.

Special zone in multi-layer and multi-pass welded metal and its role in the creep behavior of 9Cr[sbnd]1Mo welded joint

The creep properties of 9Cr1Mo welded joint are determined by special zone (SZ) in equiaxed grain zone of welded metal (WM) suffered from multiple thermal cycles during multi-layer and multi-pass welding process. The SZ in WM was characterized in detail and its role in creep behavior was discussed in this paper. The creep test of 9Cr1Mo welded joint was carried out in 566°C under 270MPa and 250MPa, and great difference of fracture morphology was observed as “cup and cone” and “wave” shapes, respectively. The cup and cone fracture consists of many deep and cone-shaped dimples. For wave-shaped fracture, some honeycomb-like structure is observed at bottom and top of wave, which is shallow and inclined to coalesce. Banded structure along prior austenite grain boundaries inside SZ is responsible for wave fracture. Some small grains with size of about 2μm are observed in banded structure, which increases the real length of the grain boundary and might deteriorate the creep performance. For long-term creep behavior, creep voids and cracks firstly form along boundaries of small grains in banded structure of SZ, and then creep crack propagates along the fusion line direction of weld beads. Grain boundary failure induced by banded structure is considered as the main rupture mechanism in this condition.

Creep behavior and modeling of neat, talc-filled, and short glass fiber reinforced thermoplastics

Abstract Creep behavior of neat, talc-filled, and short glass fiber reinforced injection molded thermoplastic composites were investigated and modeled at room and elevated temperatures. Creep strength decreased and both creep strain and creep rate increased with increasing temperature. The temperature effect was more significant for samples with glass fibers in the transverse to the load direction, as compared with the longitudinal direction. The Larson–Miller parameter was able to correlate the creep rupture data of all materials. The Monkman–Grant relation and its modification were successfully used to correlate minimum creep rate, time to rupture, and strain at rupture data. The Findley power law and time–stress superposition principle (TSS) were used to represent nonlinear viscoelastic creep curves. Long-term creep behavior was also satisfactory predicted based on short-term test data using the TSS principle.

Bio-Based Resin Reinforced with Flax Fiber as Thermorheologically Complex Materials.

With the increase in structural applications of bio-based composites, the study of long-term creep behavior of these materials turns into a significant issue. Because of their bond type and structure, natural fibers and thermoset resins exhibit nonlinear viscoelastic behavior. Time-temperature superposition (TTS) provides a useful tool to overcome the challenge of the long time required to perform the tests. The TTS principle assumes that the effect of temperature and time are equivalent when considering the creep behavior, therefore creep tests performed at elevated temperatures may be converted to tests performed at longer times. In this study, flax fiber composites were processed with a novel liquid molding methacrylated epoxidized sucrose soyate (MESS) resin. Frequency scans of flax/MESS composites were obtained at different temperatures and storage modulus and loss modulus were recorded and the application of horizontal and vertical shift factors to these viscoelastic functions were studied. In addition, short-term strain creep at different temperatures was measured and curves were shifted with solely horizontal, and with both horizontal and vertical shift factors. The resulting master curves were compared with a 24-h creep test and two extrapolated creep models. The findings revealed that use of both horizontal and vertical shift factors will result in a smoother master curves for loss modulus and storage modulus, while use of only horizontal shift factors for creep data provides acceptable creep strain master curves. Based on the findings of this study, flax/MESS composites can be considered as thermorheologically complex materials.

Prediction of Long-Term Creep Behavior of High Performance Composite Material for Spacecraft

Even though carbon/epoxy composite materials have been widely used in aircraft for many years, different requirements are often required for composite materials in spacecraft application due to the space environment. Some of the considerations include modulus and thermal properties for spacecraft while as strength is important for aircraft. For heat pipe panels, space radiator, antennas or optical benches, thermal stability is required for the life of the satellite. In this study, long term creep behavior of high performance composite material is investigated with time-temperature shift factor. The fiber is pitch-based carbon which shows high thermal conductivity and high modulus. It was possible to predict creep compliance after 5 years from dynamic mechanical analysis data. This information is useful to understand the property change during the intended service life of the spacecraft.

Modeling and analysis of the creep behavior of jute/green epoxy composites incorporated with chemically treated pulverized nano/micro jute fibers

This paper reports the creep behavior of alkali treated jute/green epoxy composites incorporated with various loadings (1, 5 and 10wt%) of chemically treated pulverized jute fibers (PJF) at different environment temperatures. Composites were prepared by hand layup method and compression molding technique. The creep and dynamic mechanical tests were performed in three-point bending mode by dynamic mechanical analyzer (DMA). The incorporation of PJF is found to significantly improve the creep resistance and strain rate of composites. Three creep models i.e. Burger’s model, Findley’s power law model and a simpler two-parameter power law model were used to model the creep behavior in this study. The time temperature superposition principle (TTSP) was applied to predict the long-term creep performance. The Findley’s power law model was found to be satisfactory in predicting the long-term creep behavior. Dynamic mechanical thermal analysis (DMTA) results revealed the increase in storage modulus, glass transition temperature and reduction in the tangent delta peak height of composites with higher loading of PJF.

An Accelerated Method for Creep Prediction From Short Term Stress Relaxation Tests

With the development of ultrasupercritical power generation technology, creep strength of high-temperature materials should be considered for safety evaluation and engineering design. However, long-time creep testing should be conducted by traditional creep assessment methods. This paper established a high-efficient prediction method for steady creep strain rate and creep strength based on short-term relaxation tests. Equivalent stress relaxation time and equivalent stress relaxation rate were defined according to stress relaxation characteristics and the Maxwell equation. An accelerated creep prediction approach from short-term stress relaxation tests was proposed by defining the equivalent relaxation rate as the creep rate during the steady stage. Stress relaxation and creep tests using high-temperature material 1Cr10NiMoW2VNbN steel were performed to validate the proposed model. Results showed that the experimental data are in good agreement with those predicted solutions. This indicates that short-term stress relaxation tests can be used to predict long-term creep behavior conveniently and reliably, and the proposed method is suitable for creep strength design and creep life prediction of 9–12%Cr steel used in ultrasupercritical unit at 600 °C.

Effect of fiber types on creep behavior of concrete

In this paper, the effect of fiber types such as steel fiber, polyvinyl alcohol (PVA) fiber, polypropylene (PP) fiber and basalt fiber, on the creep of concrete after one-year-loading was studied and the principle of fiber’s effect on concrete creep was analyzed. The elastic modulus of fibers is shown to be the most significant factor influencing concrete creep. Fibers with elastic modulus much higher than plain concrete can clearly restrict creep, while fibers with lower elastic modulus have the opposite effect. For example, 2% volumetric blending of steel fiber reduces specific creep by 25.1%, compared with plain concrete, while 0.9kg/m3 mass blending of PVA fibers increases it by 19.9%. The internal defects introduced by fiber addition, i.e., the fiber-concrete interfacial zone and non-uniform fiber distribution, weakens its creep resistance. There is a clear correlation between the 28days elastic modulus of fiber reinforced concrete (FRC) and its long-term creep behavior, indicating that they are influenced by similar factors. Larger elastic modulus at 28days tends to yield less specific creep at 1year.

Experimental research on creep behaviors of sandstone under uniaxial compressive and tensile stresses

The consideration of time dependence is essential for the study of deformation and fracturing processes of rock materials, especially for those subjected to strong compressive and tensile stresses. In this paper, the self-developed direct tension device and creep testing machine RLW-2000M are used to conduct the creep tests on red sandstone under uniaxial compressive and tensile stresses. The short-term and long-term creep behaviors of rocks under compressive and tensile stresses are investigated, as well as the long-term strength of rocks. It is shown that, under low-stress levels, the creep curve of sandstone consists of decay and steady creep stages; while under high-stress levels, it presents the accelerated creep stage and creep fracture presents characteristics of brittle materials. The relationship between tensile stress and time under uniaxial tension is also put forward. Finally, a nonlinear viscoelastoplastic creep model is used to describe the creep behaviors of rocks under uniaxial compressive and tensile stresses.