Showing papers on "Creep published in 2018"

Vitrimers Designed Both To Strongly Suppress Creep and To Recover Original Cross-Link Density after Reprocessing: Quantitative Theory and Experiments

Abstract: Vitrimers form a promising class of dynamic polymer networks, but they have an Achilles’ heel: elastomeric vitrimers exhibit significant creep under conditions where permanently cross-linked, elastomeric networks exhibit little or no creep. We demonstrate that vitrimers can be designed with strongly suppressed creep and excellent reprocessability by incorporating a substantial yet subcritical fraction of permanent cross-links. This critical fraction of permanent cross-links, which has little or no detrimental effect on reprocessability, is defined by the gelation point of only permanent cross-links leading to a percolated permanent network. Via a modification of classic Flory–Stockmayer theory, we have developed a simple theory that quantitatively predicts an approximate limiting fraction. To test our theory, we designed vitrimers with controlled fractions of permanent cross-links based on thiol–epoxy click chemistry. We characterized the rubbery plateau modulus before and after reprocessing as well as st...

Prediction of plasticity-controlled failure in polyamide 6:influence of temperature and relative humidity

Abstract: In this study, the influence of temperature and relative humidity on the plasticity controlled failure of polyamide 6 was investigated. Uniaxial tensile tests were performed at several temperatures, strain rates, and relative humidity; creep tests were performed at different relative humidity and applied load. In order to describe and predict the yield kinetics, the Ree–Eyring equation was employed and modified to include the effect of relative humidity. Subsequently, by the introduction of the concept of critical amount of accumulated plastic strain, the yield kinetics were successfully translated to predictions of time-to-failure. A good agreement between predictions and experimental results is obtained, showing that the model is a suitable and versatile tool to evaluate mechanical performance of a temperature and moisture sensitive material such as polyamide 6. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 135, 45942.

Physics Of Creep And Creep-Resistant Alloys

Abstract: Unique in its approach, this introduction to the physics of creep concentrates on the physical principles underlying observed phenomena. As such it provides a resource for graduate students in materials science, metallurgy, mechanical engineering, physics and chemistry as well as researchers in other fields. Following a brief mathematical treatment, the authors introduce creep phenomena together with some empirical laws and observations. The mechanisms of creep and diffusion under varying experimental conditions are subsequently analysed and developed. The second half of the text considers alloying in greater detail as well as exploring the structure and properties of superalloys and stress effects in these materials.

High-temperature mechanical properties of AlSi10Mg specimens fabricated by additive manufacturing using selective laser melting technologies (AM-SLM)

Abstract: Mechanical properties (tensile strength and creep) of AlSi10Mg specimens fabricated by selective laser melting (SLM) in the Z-direction were investigated in the 25–400 °C temperature range. Specimens were tested after stress relief treatment. The results revealed that yield stress (YS) significantly decreases and the elongation increases at temperatures higher than 200 °C. The ultimate tensile stress (UTS) continuously decreases with temperature. The creep parameters, namely stress exponent n and apparent activation energy Q, were found to be 25 ± 2 and 146 ± 20 kJ/mole, respectively. It was shown that plastic deformation during creep is governed by dislocation movements in primary aluminum grains. The tested material is actually an aluminum composite reinforced by sub-micron Si particles. The creep resistance of AlSi10Mg alloy fabricated by selective laser melting is close to that for aluminum matrix particles reinforced composites.

Integration of graphene in poly(lactic) acid by 3D printing to develop creep and wear-resistant hierarchical nanocomposites

Abstract: Polylactic acid (PLA) and graphene reinforced polylactic acid (PLA-graphene) composites have been fabricated by three-dimensional (3D) fused deposition modeling (FDM) printing. Indentation creep resistance was analyzed in terms of the strain-rate sensitivity index of PLA (0.11) and PLA-graphene (0.21). Enhanced creep resistance in PLA-graphene is attributed to the restriction of the polymeric chains by graphene, caused by low strain rates identified during secondary creep. The tribological properties of PLA and PLA-graphene composites were evaluated by ball-on-disk wear tests. Wear resistance was increased by a 14% in PLA-graphene as compared to PLA. A two-stage coefficient of friction (COF) behavior has been observed for PLA-graphene. Initially, PLA-graphene exhibits a 65% decrease in COF as compared to PLA. During the second stage, PLA-graphene approached similar COF behavior and value of PLA (∼0.58). PLA-graphene composites have shown significant improvement in creep and wear resistance demonstrating 3D printing to be a novel manufacturing route. POLYM. COMPOS., 2017. © 2017 Society of Plastics Engineers

The Influence of Water Saturation on the Short- and Long-Term Mechanical Behavior of Red Sandstone

Abstract: The presence of water greatly influences time-dependent rock deformation. An understanding of how water can affect the time-dependent mechanical behavior of rock is important when assessing the long-term stability of geotechnical projects. While the previous studies have performed brittle creep experiments on oven-dry or fully-saturated rocks, we report here on a study designed to better understand brittle creep at different levels of saturation. We performed brittle creep experiments on oven-dry samples of red sandstone (Hunan province, China) and samples of the sandstone pre-immersed in water for different durations (from 2 to 8 days). These samples were deformed at a constant stress in one of either two conditions: dry or submerged in water. Before performing creep experiments, we first performed a series of water absorption and constant stress rate experiments to guide the stresses required for our creep tests and to assist with their interpretation. Our creep experiments show that immersion in water greatly increased the minimum creep strain rate and greatly shortened the time-to-failure when compared to the dry state. In detail, the minimum creep strain rate and time-to-failure increased and decreased, respectively, as pre-immersion duration increased from 4 to 6 days, but did not change as the duration was further increased to 8 days. We attribute this to the saturation of microcracks between 4 and 6 days (i.e., water imbibition was complete, or close to completion, following 6 days). We also show that oven-dry samples deformed at a constant stress underwater fail at stresses much lower than oven-dry samples deformed under dry conditions, due to the imbibition of water during deformation. Samples pre-immersed in water, but deformed in the dry condition were characterized by lower strain rates and longer time-to-failure than those pre-immersed in water and deformed underwater. Our explanation for this is that, due to the availability of water, crack tips can remain hydrated when the sample is deformed underwater, thus increasing the efficacy of stress corrosion cracking. The relationships and data provided herein inform on the long-term stability of engineering structures.

Creep and Long-Term Permeability of a Red Sandstone Subjected to Cyclic Loading After Thermal Treatments

Abstract: Long-term experiments were performed on red sandstones after different thermal treatments (25, 300, 700 and 1000 °C) under multi-step loading and unloading cycles and a confining pressure of 25 MPa. Furthermore, to quantitatively analyse the temperature influence on the deformation behaviours of the specimens, the concept of the temperature–strain rate was proposed to describe the relationship between strain and temperature, and the experimental results were corrected to identical temperatures (i.e., 20 °C), to overcome the influence of periodic fluctuations in ambient temperature. The results show that the axial mean temperature–strain rate first increased as temperature increased from 25 to 300 °C and then decreased with increasing temperature, whereas the lateral mean temperature–strain rate decreased with increasing temperature. The total strain was divided into the instantaneous elastic strain, the instantaneous plastic strain, the visco-elastic strain and the visco-plastic strain. The total axial strain increased with increasing deviatoric stress, and the irrecoverable strain increased with increasing loading and unloading history. Furthermore, the total axial strain increased with increasing temperature; specifically, at 1000 °C, it was approximately two times that at 700 °C and three times those at 25 and 300 °C. The instantaneous elastic strain and the instantaneous plastic strain increased approximately linearly with increasing deviatoric stress, whereas the creep strain varied with deviatoric stress in complicated ways at different temperatures. However, under identical deviatoric stress, the instantaneous elastic strain and the instantaneous plastic strain increased slightly as temperature increased from 25 to 700 °C and then increased substantially as temperature reached 1000 °C, whereas the variations in the creep strain, the visco-elastic strain and the visco-plastic strain were dependent on temperature and stress level. Finally, the permeability first decreased slightly as temperature increased from 25 to 300 °C and then increased with increasing temperature.

A new rock creep model based on variable-order fractional derivatives and continuum damage mechanics

Abstract: The creep model is the main form of rock model used to describe the rheological behavior of rocks. A large number of creep models have been proposed, but many are complicated and/or are not able to fully simulate the three stages of rock creep. Hence, an important focus of research on rock creep has been to develop a model with few parameters and better simulation performance. To achieve this, in this study, we propose a new four-element creep model, based on variable-order fractional derivatives and continuum damage mechanics. The newly proposed creep model agrees well with experimental data for Changshan rock salt. The results show that the segmentation treatment is an effective approach for simulating the creep behavior of rocks.

The Effect of Post-Processes on the Microstructure and Creep Properties of Alloy718 Built Up by Selective Laser Melting

Abstract: The selective laser melting (SLM) process was used to fabricate an Alloy718 specimen. The microstructure and creep properties were characterized in both the as-built and post-processed SLM materials. Post-processing involved several heat treatments and a combination of hot isostatic pressing (HIP) and solution treatment and aging (STA) to homogenize the microstructure. The experimental results showed that the originally recommended heat treatment process, STA-980 °C, for cast and wrought materials was not effective for SLM-processed specimens. Obvious grain growth structures were obtained in the STA-1180 °C/1 h and STA-1180 °C/4 h specimens. However, the grain size was uneven since heavy distortion or high-density dislocation formed during the SLM process, which would be harmful for the mechanical properties of SLM-fabricated materials. The HIP+ direct aging process was the most effective method among the post-processes to improve the creep behavior at 650 °C. The creep rupture life of the HIP+ direct aging condition approached 800 h since the HIP process had the benefit of being free of pores, thus preventing microcrack nucleation and the formation of a serrated grain boundary.

Mechanical Properties of High-Strength Q690 Steel at Elevated Temperature

Abstract: High-strength steels are finding wide applications in steel-framed buildings. Therefore, fire resistance design of high-strength steel structures has gained more attention in recent years. ...

Homogenization of P91 weldments using varying normalizing and tempering treatment

Abstract: Creep strength enhanced ferritic/martensitic P91 steel is considered as a candidate material for the reactor pressure vessels and reactor internals of Very High Temperature Reactor (VHTR). Heterogeneous microstructure formation across the P91 weldments lead to premature Type IV cracking and makes the weldability of P91 steel a serious issue. The present research work describes the effect of normalizing and tempering (N&T) treatment on microstructure evolution in various zones of gas tungsten arc welded (GTAW) P91 pipe weldments. For N&T treatment, P91 pipe weldments were subjected to various normalizing (950–1150 °C) and tempering (730–800 °C) temperature. The effect of varying heat treatment on tensile properties and hardness of P91 pipe weldments were studied for V-groove and narrow-groove weld designs. The effect of increase in normalizing temperature (fixed tempering temperature) resulted in increase in strength and hardness, while increase in tempering temperature (fixed normalizing temperature) resulted in the decrease in strength and hardness of P91 steel weldments. The better combination of strength, ductility and microstructure were obtained for the maximum normalizing temperature of 1050 °C and tempering temperature of 760 °C.

Comparison of Short-Term and Long-Term Creep Experiments in Shales and Carbonates from Unconventional Gas Reservoirs

Abstract: We carried out a series of long-term creep experiments on clay- and carbonate-rich shale samples from unconventional gas reservoirs to investigate creep over both relatively short-term (4-h) and long-term (4-week) periods. Results from each set of experiments were compared to evaluate the ability to predict the long-term behavior of reservoir rocks using relatively short-term creep experiments. The triaxial deformation experiments were performed in a time-cycling pattern, which included a series of four stages of loading, creep, unloading and recovery experiments conducted over different time spans. The loading conditions (tens of MPa) reflect current reservoir conditions and were far below the strength of the samples. Experiments were conducted on both horizontal and vertical shale samples to address anisotropy introduced by the bedding. A power-law model was fitted to the creep data to predict the long-term behavior of shale samples. Regardless of the applied loading history, results of the experiments show that the shale samples follow a single trend representing their creep behavior through time. We show that the simple power-law model is capable of describing creep over multiple time periods. Additionally, the value of the creep compliance factor is consistent over different creep testing periods and it is possible to characterize the behavior of these samples from relatively short-term (1 day) creep experiments.

Dynamic Mechanical Analysis on a PolyMethyl Methacrylate (PMMA) Polymer Optical Fiber

Abstract: Curvature sensors based on polymer optical fibers (POFs) present some advantages over the conventional technologies for joint angle assessment such as compactness, electromagnetic field immunity, and multiplexing capabilities. However, the polymer is a viscoelastic material, which does not have a constant response with stress or strain. In order to understand and model this effect, this paper presents the dynamic characterization of a POF. The effects of temperature, frequency, and loads on the fiber are analyzed for obtaining the influence of these parameters on the polymer dynamic Young modulus and time constant. Results show that a temperature on the range between 24 °C and 45 °C does not lead to considerable variations on the sensor output. Moreover, it is possible to estimate the storage modulus and loss factor from the frequency and temperature. The polymer time constant is defined on creep recovery experiments. Since the viscoelastic parameters are evaluated in different conditions of temperature, frequency, and load, a model for the stress behavior of the fiber is proposed. Such model leads to a root mean squared error between the modeled and measured results over 15 times lower than the one obtained with the model for bending stress without account the POF viscoelastic behavior.

Creep properties, creep deformation behavior, and microstructural evolution of 9Cr-3W-3Co-1CuVNbB martensite ferritic steel

Abstract: Creep deformation behavior and microstructure evolution of G115 steel were systematically investigated for temperatures of 625–675 °C under uniaxial tensile stress of 120–220 MPa. The relationship between minimum creep rate and applied stress followed the Bird–Mukherjee–Dorn (BMD) equation. The modified BMD equation was proposed using threshold stress to elucidate the actual creep deformation mechanism. The values of the threshold stress were determined to be 177.8, 91.4 and 87.6 MPa at 625, 650, and 675 °C, respectively. The true creep activation energy and the true stress exponent were 275.76 kJ/mol and 6, respectively. Thus, the dominant creep deformation mechanism was identified as dislocation climb. Three types of precipitates can be revealed after creep deformation: W-rich Laves, Nb-rich MX, and Cu-rich phases. The creep damage of G115 steel after creep deformation was characterized by martensite cracks and martensite fractures owing to the hardness and brittleness of the lath martensite structure. Further, a dense array of deep and equiaxed dimples appeared in the central region of fracture surfaces under the tested creep conditions. Ductile fracturing was the main fracture mechanism during creep deformation.

The Modeling of Time-Dependent Deformation and Fracturing of Brittle Rocks Under Varying Confining and Pore Pressures

Abstract: A numerical hydro-mechanical model for brittle creep is proposed to describe the time-dependent deformation of heterogeneous brittle rock under constant confining and pore pressures. Material heterogeneity and a local material degradation law are incorporated into the model at the mesoscale which affects the mechanical behavior of rocks to capture the co-operative interaction between microcracks in the transition from distributed to localized damage. The model also describes the spatiotemporal acoustic emissions in the rock during the progressive damage process. The approach presented in this contribution differs from macroscopic approaches based on constitutive laws and microscopic approaches focused on fracture propagation. The model is first validated using experimental data for porous sandstone and is then used to simulate brittle creep tests under varying constant confining and pore pressures and applied differential stresses. We further explore the influence of sample homogeneity on brittle creep. The model accurately replicates the classic creep behavior observed in laboratory brittle creep experiments. In agreement with experimental observations, our model shows that decreasing effective pressure, increasing the applied differential stress, and decreasing sample homogeneity increase the creep strain rate and decrease the time-to-failure, respectively. The model shows that complex macroscopic time-dependent behavior can be explained by the microscale interaction of elements. The fact that the simulations are able to capture a similar hydro-mechanical time-dependent response to that of laboratory experiments implies that the model is an appropriate tool to investigate the complex time-dependent behavior of heterogeneous brittle rocks under coupled hydro-mechanical loading.