Bending Creep Research Articles

STRUCTURAL CHANGES IN A POWDER METALLURGY 66FE-14MN-6SI-9CR-5NI (MASS. %) SHAPE MEMORY ALLOY SUBJECTED TO BENDING CREEP

Fe-14Mn-6Si-9Cr-5Ni (mass. %) shape memory alloy (SMA) has been developed as a commercial grade of Fe-Mn-Si based SMAs with excellent formability and corrosion resistance. Producing FeMnSiCrNi SMAs by powder metallurgy enables a better control of chemical composition and grain size. This paper discusses the variation of bending creep deformation, as a function of time, temperature and force, in the case of a powder metallurgy (PM) Fe-14Mn-6Si-9Cr-5Ni SMA with 50 % mechanically alloyed powder volume. Creep test temperatures were selected based on phase transitions determined by dynamic mechanical analysis (DMA), during heating. The results suggested the tendency of martensite plate variants to reorient along a common direction, after creep.

Aging Characteristics of Plant Hot-Mix Recycled Asphalt and Its Induced Aging on New Asphalt

In order to explore the aging process and mechanism of new and old asphalt in plant hot-mix recycled asphalt mixture during the mixing and use process, the liquid viscosity test and low-temperature bending creep test are carried out to test the dynamic viscosity, stiffness modulus, creep rate, and low-temperature flexibility of the matrix asphalt, recycled asphalt, and old and new asphalt after rolling thin film oven test (RTFOT) aging and pressurized aging vessel (PAV) aging. The macroscopic performance attenuation law of new and old asphalt during the aging process in thermal regeneration is compared and analyzed. After that, the aging process and mechanism of new and old asphalt are explored by infrared spectroscopy and differential calorimetric analysis scanning. The results show that RTFOT aging and PAV aging make the viscosity of recycled asphalt rise significantly and the low temperature performance decline rapidly. After RTFOT aging and PAV aging of new and old asphalt, the dynamic viscosity and low-temperature performance change range is much higher than that of the matrix asphalt. Some technical indicators are even closer to the recycled asphalt after aging, which proves that its aging speed is faster than that of the matrix asphalt. Meanwhile, the results of infrared spectroscopy and differential scanning calorimetry analysis show that in addition to the independent aging of new asphalt and recycled asphalt, there is also a chemical effect between them–that is, some active groups in recycled asphalt have a more obvious promotion effect on the aging process of new asphalt, here called “induced aging”. This induced aging changes the aging mechanism of the matrix asphalt by changing the aging process of it, which greatly limits the popularization and application of thermal regeneration technology.

High temperature creep behaviour of glass-ceramic joined Nextel 610™/Al2O3-ZrO2 composites

Ox/ox CMC based on Nextel 610™ fibers with 75 wt% alumina – 25 wt% zirconia matrix have been joined by a glass-ceramic and tested in creep in four-point bending (DIN EN 820–4) up to 50 h at 850 °C and 1000 °C, between 32 and 41 MPa. As a comparison, the creep limit of the non-joined ox/ox CMC was investigated in this range and also at higher temperatures and stresses. The creep experiments at 850 °C did not show any significant difference between joined and ox/ox CMC after 50 h, while a certain one was detected at 1000 °C on joined ox/ox CMC, suggesting that its creep mechanism is dominated by visco-elastic flow of the glass-ceramic.

Flexural Creep Behaviour of Pultruded GFRP Composites Cross-Arm: A Comparative Study on the Effects of Stacking Sequence.

Pultruded glass fibre reinforced polymer (pGFRP) composites provide outstanding properties for composite polymer cross arms in power transmission line applications. This study has investigated the effects of various stacking sequences of fibres directions of pGFRP on flexural strength and creep behaviour. The use of static four-point bending flexural tests revealed that Stacking Sequence 2 (±45/0/90/0/90/0) had a significant flexural strength of 399.9 MPa while Stacking Sequence 1 (±45/90/0/±45) had a flexural strength of 242.5 MPa. Furthermore, the four-point bending creep experiments were performed at three distinct stress levels, notably 12%, 24%, and 37% of the ultimate flexural strength, to characterise the creep behaviour of distinct stacking sequences. Moreover, Findley’s power law equation for bending creep behaviour has revealed that the time-dependent reduction factor of Stacking Sequence 1 and Stacking Sequence 2 estimates a drop in flexural modulus of 23% and 10% respectively.

Exact analytical solutions for bending creep behavior of viscoelastic laminated arches with interlayers

In this work, an exact analytical solution is proposed in order to investigate the bending creep behavior of viscoelastic laminated arches with interlayers under a distributed load. In the analytical model, both the lamina layers and bonding interlayers are considered as viscoelastic materials, which are described by Burgers model for transient and steady-state creep stages. The stress and displacement fields in each viscoelastic lamina layer are obtained on the basis of the exact elasticity theory as well as Boltzman superposition principle. The general solution of stresses and displacements with unknown coefficients is derived by utilizing the Fourier series expansion approach. The interlayer slip effect in the laminated arches is considered in the continuity conditions. By employing analytical Laplace transformation, the coefficients are further determined according to the continuity and load conditions. The present solution is compared to the finite element solution and they show a good agreement. The influences of various parameters are investigated in detail on the bending creep behavior of the arch.

Sample Size-Effect on Creep in Bending: An Interplay between Strain Gradient and Surface Proximity Effects

Sample Size-Effect on Creep in Bending: An Interplay between Strain Gradient and Surface Proximity Effects

Laboratory investigation on performance and mechanism of polyphosphoric acid modified bio-asphalt

As a green and clean road material, the bio-asphalt is a research hotspot in road engineering. It is known, the poor high-temperature performance of bio-asphalt limits its application in practical engineering. In order to solve this problem, the polyphosphoric acid (PPA) was used to modify the bio-asphalt. According to the principle of polymer compatibility, the liquid polymer can be well compatible with the asphalt. The matrix asphalt was first heated to a fluid state, and different contents of PPA and bio-oil were then added and stirred. The high-speed shear equipment was finally used to prepared the polyphosphoric acid modified bio-asphalt (PPAMB). The viscosity, dynamic shear rheology, and low-temperature bending creep tests of PPAMB were carried out, and its mechanism was revealed by the Fourier transform infrared spectroscopy (FTIR), fluorescence microscope (FM), scanning electron microscope (SEM), and atomic force microscope (AFM). The results showed that the PPAMB had excellent rheological performance at high and low temperatures. The optimum content of PPA and bio-oil in PPAMB for the composite modification of asphalt were 2% and 7.5% respectively. The PPA reacted with the asphalt to form new functional groups, but the bio-oil had no chemical reaction. Through the FM and SEM analysis, it could be seen that the PPAMB had outstanding uniformity, and smooth surface without aggregation phenomenon; compared with that of bio-asphalt, the size of bee-like structure of PPAMB increased, but its number decreased; compared with that of PPA modified asphalt, the number of bee-like structure of PPAMB increased instead.

Viscoelastic Mechanical Model of Asphalt Concrete Considering the Influence of Characteristic Parameter of Fiber Content

This paper carries out bending creep tests on polyester fiber-reinforced asphalt concrete beams, and investigates how the volume ratio and aspect ratio of the fibers influence the parameters of viscoelastic mechanical model and the viscoelastic performance of the asphalt concrete. The results show that: with the growing volume ratio and aspect ratio of the fibers, the midspan deflection and bottom flexural-tensile strain of asphalt concrete beams first dropped and then rose over time; the characteristic parameter of fiber content (FCCP) could reflect the overall effects of the volume ratio and aspect ratio of the fibers. On this basis, a viscoelastic mechanical model was established for the asphalt concrete in the light of the influence of FCCP. The test and theoretical results show that, in our tests, the optimal volume ratio of fibers, optimal aspect ratio of fibers, and optimal FCCP are 0.348, 324, and 1.128 for polyester fiber-reinforced asphalt concrete.

Laboratory Characterization of Geosynthetics-Reinforced Asphalt Mixture.

In order to improve the mechanical properties of asphalt pavement, geosynthetics can be employed in asphalt mixture. This research designed 12 reinforced schemes based on the types of geosynthetics, bonding layers and reinforced position. For the relative tests carried out, reinforced specimens were prepared according to each individual scheme. Moreover, rutting tests, bending creep tests and split fatigue tests were carried out on reinforced specimens in the laboratory. The results obtained in this investigation showed that the dynamic stability, bending creep rate and fatigue life of geocell-reinforced specimens are better than those of geogrid-reinforced specimens. The bonding layer of Styrene-Butadiene-Styrene (SBS) modified asphalt is better than epoxy modified asphalt. The dynamic stability and fatigue life of middle reinforcement are better than those of the lower reinforcement, while the bending creep rate of the lower reinforcement is better than middle reinforcement. In addition, reinforced scheme (9) has the largest increase in dynamic stability and fatigue life by 103 and 137%, respectively, and reinforced scheme (12) has the largest reduction in bending creep rate by 46%. However, scheme (9) improved dynamic stability and fatigue life by 43 and 29% higher than scheme (12), while the reduction of flexural creep rate of scheme (12) is only 7% higher than that of scheme (9).

Effects of shrinkage variation within beam depth and bending creep on flexural behavior of RC members

The effects of shrinkage variation within the beam depth and bending creep on the time-dependent flexural behavior of reinforced concrete (RC) beams were examined via two sets of experiments and numerical analyses. Each test set included four types of experiments: axial creep, beam shrinkage, bending creep, and time-dependent RC beam bending tests. The variations in beam shrinkage and bending creep strain were determined using the American Concrete Institute’s shrinkage and creep prediction models, respectively. Time-dependent finite-element beam equilibrium equations were presented using an incremental form of the age-dependent constitutive equation. The mathematical creep model of the constitutive equation accounted for the aging effects of the elastic modulus and the creep development under a multi-sequential loading condition with a single creep curve. The resulting finite-element formulation captured the mechanical and non-mechanical phenomena of the aging effect of concrete at an early age, shrinkage variations within the beam depth, and bending creep. Comparisons between the age-dependent responses of the measured and predicted results of the RC beam test specimens illustrated the significant effects of shrinkage variation and bending creep on the time-dependent behaviors of RC flexural members.

A subcritical microcrack informed creep-damage model for quasi-brittle materials

A subcritical microcrack informed creep-damage model is proposed to describe the creep-failure behavior of quasi-brittle materials where the subcritical microcracks propagation is characterized by the subcritical damage. Total free energy is decomposed into the time-dependent component and time-independent component where the time-dependent Helmholtz free energy is associated with time-dependent behavior. Driven by the time-dependent free energy the subcritical damage growth is originated from the nano-scale fracture kinetics. Within the continuum damage framework, a modified stochastic damage model is introduced to account for instantaneous behavior and subcritical damage evolution. The viscoelastic behavior is characterized by the Burgers model. The proposed model is validated through several numerical simulations including uniaxial loading tests, uniaxial creep tests and bending creep test. The results demonstrate that the present model is promising for the long-term analysis of underground structures.

Experimental study and comparative numerical modeling of creep behavior of white oak wood with various distributions of earlywood vessel belt

Many researches have been conducted to investigate creep behavior of wood; however, the effects of structure on wood creep behavior remain unclear. Therefore, the effects of existence and distribution of earlywood vessel belt on creep behavior of white oak (Quercus alba L.) wood were investigated by dynamic thermal mechanical analyzer (DMA) with double cantilever bending in this study. Besides, a comparative numerical modeling simulation on strain curves of white oak specimens was completed using Burger and Five-parameter model. Results revealed that instantaneous strain and 45-min strain of specimens decreased with increase in the distance between earlywood vessel belt and stress acting surface obviously. Additionally, instantaneous strain and 45-min strain of specimens remarkably increased with increase in temperature from 20 to 80 °C. An obvious bending creep behavior was observed with increase in temperature from 20 to 80 °C. Both Burger and Five-parameter model can effectively simulate the creep behavior of white oak specimens with R2 values greater than 0.90. Furthermore, Five-parameter model illustrated a better fitting effect than Burger model in the final creep stage due to the introduction of a non-linear creep strain growth expression. It concluded that creep behavior of white oak wood strongly depends on the existence and distribution of earlywood vessel belt.

Response of round-hole tubes submitted to pure bending creep and pure bending relaxation

This paper presents experimental study on the response of 6061-T6 aluminum alloy round-hole tubes with five different hole diameters of 2, 4, 6, 8, and 10 mm and four different diameter-to-thickness ratios of 30, 40, 50, and 60 submitted to pure bending creep and pure bending relaxation. Pure bending creep or relaxation is defined as bending the tube to the required moment or curvature and maintaining that moment or curvature for a period of time. The experimental results of pure bending creep show that the curvature increases with time. In addition, larger holding moment, diameter-to-thickness ratio, or hole diameter results in larger creep curvature. As the curvature continues to increase, the round-hole tube eventually breaks. The experimental results of pure bending relaxation show that the relaxation moment decreases sharply with time and tends to a stable value. In addition, larger holding curvature, diameter-to-thickness ratio, or hole diameter results in larger drop of the relaxation moment. Due to fixed curvature, the round-hole tube does not break. Finally, formulas proposed by the research team of Pan et al. were respectively improved to simulate the creep curvature-time relationship for pure bending creep in the initial and the secondary stages and the relaxation moment-time for pure bending relaxation. After comparing with the experimental results, it is found that theoretical analysis can reproduce the experimental results reasonably.

Evaluation of the Durability of the Fiber-Reinforced Concrete Beam Under Long-Term Pure Bending and Local Creep

Evaluation of the Durability of the Fiber-Reinforced Concrete Beam Under Long-Term Pure Bending and Local Creep

Experimental investigation of the long-term behavior of reconstituted bamboo beams with various loading levels

To study the creep behavior of bamboo beams, the long-term bending creep tests of twelve bamboo beams with different loading levels were carried out in an indoor environment. The following results were obtained. The creep of reconstituted bamboo beams under different loading levels exhibits the three-stage characteristics of short rapid creep, sustained moderate-speed creep, and stable low-speed creep; the growth ratios of creep deflection are between 30.9% and 39.1% for the reconstituted bamboo beams at the 10–50% loading level. The creep and the creep rate increase with an increase in the loading level. Meanwhile, the long-term creep prediction model of the reconstituted bamboo beam is derived based on the Burgers model; the proposed model may reflect the significant effects of the loading level and is able to accurately evaluate the deflection-time behavior of the recombinant bamboo by taking the loading level into consideration. Based on the prediction, the creep value of reconstituted bamboo beams is large and develops rapidly, and the creep deflection of the reconstituted bamboo beams at high loading level (30–50%) will exceed the maximum deflection corresponding to the ultimate bending capacity within 5 years.

Bending Behaviour of Polymeric Materials Used on Biomechanics Orthodontic Appliances.

This paper discusses the issues of strength and creep of polymeric materials used in orthodontic appliances. Orthodontic biomechanics is focused on the movement of individual teeth or dental groups as a result of the force applied by orthodontic appliances. Stresses in the construction of functional and biomechanical appliances is generated when using the apparatus in the oral cavity. The orthodontic appliance must maintain its shape and not be damaged during treatment so strength and creep resistance are fundamental properties. It was assumed that the clinical success of orthodontic appliances can be determined by these performance properties. The aim of the work was the experimental assessment of comparative bending strength and creep resistance of selected popular polymer materials used in the production of biomechanical orthodontic appliances. Four commercial materials manufactured by the world class producers were tested: NextDent Ortho Rigid (Vertex-Dental B.V., Soesterberg, The Netherlands) marked as “1A”; Erkocryl (ERKODENT Erich Kopp GmbH, Pfalzgrafenweiler, Germany)-“2A”; Vertex Orthoplast (Vertex Dental B.V.), blue, marked as “3A” and material with the same name as “3A” but orange, marked in the article as “4A”. All the tests were carried out after aging in artificial saliva for 48 h at a temperature of 37 °C. Flexular strength and flexular modulus were made using the three point bending method according to the ISO 178 technical standard. Creep tests were carried out according to the method contained in ISO 899-2. The creep test was carried out in an artificial saliva bath at 37 °C. The creep tests showed significant differences in the strength, modulus and deformability of the tested materials. The strength reliability of the tested materials also varied. The research shows that the 2A material can be used for orthodontic applications in which long-term stresses should be lower than 20 MPa.

Customized High-Temperature Bending with DIC for High-Throughput Determination of Creep Parameters: Technique, Instrumentation, and Optimization

Bending is emerging as an alternative to uniaxial testing for high-throughput determination of primary-cum-secondary creep parameters. This is feasible because of augmentation of cantilever bending tests with digital image correlation (DIC). Here, we provide guidelines for selecting the patterning medium, spray conditions, and other important parameters for laying high-resolution, high contrast ratio DIC speckle-patterns that are optimal for long-term creep tests performed up to 800°C. Furthermore, an experimental setup for efficiently performing creep experiments in bending at high temperatures is designed and developed. Proper choices of paint, spraying technique, high-resolution image capturing device, and the appropriate heat management system in the mechanical testing unit yield unambiguous correlation in the “DIC augmented bending creep” tests. The developed methodology and the equipment are validated by evaluating the creep behavior of a stainless steel at 750°C by obtaining multiple stress–strain rate pairs from a single test. An excellent match between uniaxial and bending data is observed, thereby paving the way for using DIC augmented bending creep for studying the creep response of materials at high temperatures in a high-throughput fashion.

Static bending creep properties of furfurylated poplar wood

Moisture content (MC) and its variation have a significant influence on creep in wood. Therefore, reducing moisture absorptive/desorptive effect is one of the best means to minimize creep deformation in wood. Moreover, furfuration treatment of wood is one of the most economical methods to achieve this purpose. In this project, samples of Poplar wood (Populuseuramevicana) were treated with 30% furfuryl alcohol solution (furfurylated wood) and then tested for normal creep (NC) and mechanical sorptive creep (MSC) at different stress levels in several relative humidity (RH) conditions. Results from the experiment and analysis indicated that the normal creep performance of furfurylated wood was similar to that of classical untreated wood material. Nevertheless, furfurylated wood at different stress levels (less than 35%) or in different relative humidity environments did not follow MSC performance (i.e. creep deflection increases during moisture desorption while decreases during moisture absorption) of untreated wood and only exhibited wood NC characteristics (i.e. creep deflection increases continuously with time). Moreover, the maximum MSC deflection of furfurylated wood was 72.3% and 72.2% lower than that of untreated wood at the same 10% stress level and the same range of 65–98% relative humidity variation, respectively. The stress level was positively related to the deformations of instantaneous elastic, viscoelastic, viscous and bending creep, but negatively related to deformation of furfurylated wood creep recovery. Furthermore, it was also found that variation in RH did not have a significant impact on MSC deflection of furfurylated wood compared with untreated wood. The 4-element Burgers model and the 2-parameter Weibull distribution could well simulate the normal creep and creep-recovery responses respectively for furfurylated wood.

Performance evaluation of BDM/unsaturated polyester resin-modified asphalt mixture for application in bridge deck pavement

Although epoxy resin-modified asphalt mixture (ERMAM) shows excellent performances as used in bridge deck pavement, its cost is high and low-temperature anti-cracking resistance is poor, so unsaturated polyester resin-modified asphalt mixture (UPRMAM) is proposed to replace ERMAM. However, there is still a great gap in strength between UPRMAM and ERMAM. In this paper, 4,4′-bismaleimidodi-phenylmethane (BDM)/unsaturated polyester resin-modified asphalt mixture (BDM/UPRMAM) is presented for bridge deck pavement. Marshall, splitting, three-point bending, rutting, bending creep, immersed Marshall, freeze–thaw splitting, four-point bending fatigue and oil corrosion tests are performed to compare the performances of BDM/UPRMAM, pure unsaturated polyester resin-modified asphalt mixture (PUPRMAM) and ERMAM. Results show that the addition of BDM into PUPRMAM makes the splitting tensile strength, flexural-tensile strength and dynamic stability increase by 17.6%, 29.2% and 80.94%, respectively. It also improves the resistance to water damage and fatigue disease significantly while it decreases low-temperature performance. Compared with ERMAM, BDM/UPRMAM has similar splitting tensile strength, worse flexural-tensile strength, dynamic stability and fatigue resistance, better low-temperature crack resistance and lower cost. As a result, BDM/UPRMAM can be used as the bridge deck surfacing material.

Study on Creep Behavior Of Asphalt Mixture Based on Discrete Element Method

A three-dimensional (3D) microstructure-based discrete element (DE) model was developed to study the creep behaviour of high viscoelastic asphalt sand (HVAS) with the uniaxial compression creep tests. The three-point bending creep tests of asphalt mortar were carried out in order to obtain the parameters of the Burger model, to determine the transformation method of macroscopic parameters and microscopic parameters of the model in theory, to obtain the parameters used in the discrete element model, and then establish the discrete element analysis model for the asphalt mixture. A 3D-DE digital specimen was composed of coarse aggregates, asphalt mortar and air voids, which could also take gradation, irregular shape, random distribution of aggregate and air voids into consideration, and the boundary conditions of the model were set through the simulation of the uniaxial compression creep tests. An accurate and extensive mapping model of HVAS was built by 3D-PFC (Particle Flow Code), which can provide a simple alternative to the laboratory tests. This method can simulate a series of numerical examples based on different stress levels, coarse aggregate homogenizations, mortar homogenizations and temperatures in a single factor method. Comparison of results of laboratory and numerical tests shows that the 3D-PFC-viscoelastic model can reflect the creep mechanical behaviour of asphalt mixture accurately. It provides the theoretical basis and auxiliary means for analysing the mechanical properties of asphalt mixtures using PFC software. The research on creep behaviour of the asphalt mixture by numerical simulation opens up a new way for the research on creep behaviour of the asphalt mixture, it is of considerable theoretical value and has broad application prospects.