Creep Behavior Of Materials Research Articles

Determination of uniaxial creep properties using equivalent factors by sharp indentation

The uniaxial creep properties, including the creep coefficient and stress exponent, are crucial to characterize the creep behavior of materials. Instrumented sharp indentation test (ISIT) is a non-destructive technique to characterize the uniaxial creep properties using the stress equivalent factor and strain rate equivalent factor. However, the strain hardening exponent of tested material is ignored in the contact model, leading to inaccurate estimation of uniaxial creep coefficient. Based on finite element simulations, the effects of the strain hardening exponent on equivalent factors were investigated. The relationships between the equivalent factors, the strain hardening exponent, and the yield strain were established. Indentation and uniaxial tests on Pb95Sn5 and Sn99Cu1 were conducted to validate the established equivalent factors. Then those factors were used to determine the uniaxial creep coefficient of Pb95Sn5 and Sn99Cu1 by ISIT. This work demonstrates the feasibility of measuring uniaxial creep properties of work hardened material by ISIT.

Research on Polyurethane Creep Prediction Model Based on BWO-BP

Abstract Polyurethane foam materials can creep under the influence of temperature, humidity, and pressure during use, which seriously affects their safety and state of use. Therefore, studying the creep behavior of polyurethane foam materials and developing corresponding creep mechanism models have significant scientific value for understanding their damage patterns. As a type of machine learning, Backpropagation (BP) neural network can be used for data analysis. However, there are many issues when determining the network structure. Whale Optimization (BWO) algorithm, as a type of swarm intelligence algorithm, can efficiently determine the framework of the BP model. BP which has been optimized through the BWO algorithm, has been utilized to construct a model for the prediction of polyurethane’s creep failure. The optimization process has significantly lowered the mean squared error from 0.30 in the non-optimized BP model to 0.07 in the enhanced BWO-BP model, which validates the effectiveness of the proposed algorithm.

Dynamic creep of Al2O3 whisker-reinforced ZrO2 ceramic composite

Previous studies on the creep behavior of ceramic materials were primarily conducted under static loads. However, the majority of applications for ceramic materials are subject to dynamic loads, and there is a paucity of research conducted in this area. In this work, we report for the first time the creep behavior of Al2O3 whisker-reinforced ZrO2 composites under dynamic pressure. The results showed that the sample crept under dynamic pressure produced higher creep rate and lower activation energy compared to static pressure. By analyzing the creep data and microstructures, we showed that the dynamic pressure can enhance dislocation motion and transform the creep mechanism from diffusion-controlled grain-boundary sliding to dislocation-controlled grain-boundary sliding. This work provides a foundation for subsequent detailed studies of the creep behavior of ceramic materials under dynamic pressure.

The reduced order model for creep using dynamic mode decomposition

Given the computational challenges associated with finite element methods in analyzing creep behavior in high-temperature components, this work presents an advanced method for modeling thermal creep in complex structures through a reduced order model (ROM) developed via dynamic mode decomposition (DMD). The results indicate that the ROM using DMD can predict long-term structural responses related to creep based on short-term data with high accuracy, enhancing significantly computational efficiency. The application of ROM allows for the rapid estimation of creep effects in complex structures, like monoliths in the MegaPower reactor, thereby improving the design efficiency. The eigenvalues of the creep related ROM all fall in the unit circle on the complex plane, indicating that creep is a stable development process. The DMD with improved time mode coefficient can reconstruct strain-rate related plastic deformation and cyclic visco-plastic deformation process. Additionally, we derive the necessary stress updating algorithm and consistent tangent operator matrix for the 316H unified visco-plastic constitutive equation in 2023 edition ASME code, ensuring accurate modeling of material creep behavior under high temperatures in this paper. This work provides a valuable tool for the safe design and operation of critical components in nuclear engineering and other high-temperature applications.

Evolution of Long-Term Load Reduction Using Borrowed Soil

AbstractThe effectiveness of load-reduction techniques often diminishes due to creep behavior observed in geomaterials, as loess backfill is used, the load reduction rate of high-filled cut-and-cover tunnels (HFCCTs) after creep will decrease by 10.83%, posing a threat to the long-term stability of deeply buried structures such as HFCCTs. Therefore, a geotechnical solution is crucial to ensuring sustained effectiveness in load-reduction strategies over time. This study utilizes a finite-difference method to examine three promising measures for mitigating creep effects. Our analysis focuses on the time-dependent changes in earth pressure atop the cut-and-cover tunnel (CCT) and the internal distribution of cross-sectional forces, including bending moment, shear force, axial force, and displacement. Results indicate that the creep behavior of load-reduction materials significantly influences the internal force distribution. Furthermore, sustained load reduction is achieved when utilizing low-creep materials like dry sandy gravel as backfill soil, which needs to be borrowed from other sites. Additionally, integrating concrete wedges with load-reduction techniques facilitates a more uniform stress distribution atop CCTs.

Development and validation of fractional constitutive models for viscoelastic-plastic creep in time-dependent materials: Rapid experimental data fitting

This study aims to address the core challenges in the field of fractional models: developing fractional viscoelastic models with clear physical meanings and efficient, precise methods for solving fractional differential equations. Through the analysis of experimental data, this study proposes several concise and accurate models to predict creep phenomena, characterized by a minimal number of parameters, each with a clear physical interpretation. These models demonstrate high accuracy in describing the nonlinear behavior during the accelerated creep stage and successfully overcome the difficulty of the fractional order exceeding 1. Additionally, the research introduces an improved memoryless technique for the numerical solution of fractional viscoelastic models, enhancing the efficiency of parameter identification. The methodology of this study is practical and feasible, offering valuable references for understanding and predicting the creep behavior of viscoelastic materials.

Climatic Ageing of Low Density Polyethylene in Agricultural Greenhouses

Abstract Low-Density Polyethylene (LDPE) is a commonly employed plastic in the production of greenhouse covers due to its advantageous attributes of flexibility, durability, and transparency. Notably, LDPE’s ability to resist UV radiation plays a crucial role in protecting plants from harmful solar rays. However, the extended lifespan initially anticipated for this material is frequently undermined by the intricate process of LDPE aging. This study examines how the mechanical properties and creep behavior of monolayer and tri-layer PE materials change over nine months of natural aging in the Tiaret region of northwest of Algeria. Tri-layer polyethylene (PE) film outperforms mono-layer PE in strength (2.37x), elasticity (35%), and creep resistance due to its enhanced barrier against environmental factors. It maintains 50% elongation even after 9 months, while mono-layer PE degrades 50% in six months with a tensile strength of 14 MPa.

Comprehensive Understanding of the Effect of TGO Growth Modes on Thermal Barrier Coating Failure Based on a Simulation.

The growth stress induced by thermally grown oxide (TGO) is one of the main reasons for the failure of thermal barrier coatings (TBCs). In this study, the failure behavior of TBCs was examined based on different growth modes of TGO. A TBC thermo-mechanical model with a simplified sinusoidal interface morphology was established by the secondary development of a numerical simulation. The plasticity and creep behavior of materials were considered. Based on the subroutine development, the non-uniform growth of the TGO layer was realized. Cohesive elements were also applied to the TC/TGO interface. The stress distribution and evolution at the TC/TGO interface were investigated. Then, the cracking behavior near the interface was studied. The results show that lateral growth causes the off-valley site to replace the previous off-peak site as a vulnerable site. The non-uniform growth accelerates damage in the off-valley site, which leads to a change in the failure behavior. These results will provide significant guidance for understanding the TBC failure and the development of advanced TBCs.

Analysis of contact creep behaviour of nanofilled composites

An effective modelling method for analysing the contact creep behaviour of composite materials with different nanofillers was proposed. Considering both the contact and creep material nonlinearities, an augmented Lagrangian algorithm was used to treat the unilateral contact constraints, and a time-hardening creep constitutive equation was adopted to describe the deformation characteristics of the nanocomposites. The constitutive parameters were extracted through a series of linear fittings based on the multiaxial creep theory and creep test curves, and the accuracy of the constitutive equation was validated. The applications of the proposed modelling method were demonstrated using a rope-wheel contact system (RWCS), in which the wheels were made of polystyrene (PS) with carbon black (CB), multi-walled carbon nanotubes (MWCNT), or chemically reduced graphene oxide (CRGO). The results indicated that the addition of nanofillers significantly reduced the creep deformation of the contact zone. The CRGO exhibited a better anti-creep performance than that of CB and MWCNT. Compared with pure PS, the maximum deformation on the contact path of the nanocomposite wheel with 5.0 wt% CRGO sheets at 1500 s and 104 s was reduced by approximately 22 % and 52 %, respectively. The coefficient of friction μ had a strong influence on the contact creep deformation when μ < 0.3. Additionally, the anti-creep capability of the added nanofillers became more evident as the loading time increased. The contact creep deformation of the CRGO composite was less than one-tenth that of the matrix material after 30 d.

Meso-scale analysis on the effect of coarse aggregate properties on the creep behaviors of concrete based on the 3D particle-based method

Coarse aggregate, as the main component of concrete, has a significant influence on its behaviors. In this work, a 3D particle-based method was utilized to investigate the effect of coarse aggregate properties on the creep behaviors of concrete at a meso-scale level, in which concrete was composed of mortar, coarse aggregate, and interfacial transition zone (ITZ). Firstly, cluster-based modules for aggregate were generated based on the real aggregate geometry obtained by 3D scanning, and then, the developed aggregates were randomly distributed in the model according to the specified aggregate gradation and content. Then, the Burger model, a linear viscoelastic-plastic model, was employed to define the contact mechanical characteristics of mortar and ITZ. Meanwhile, the linear parallel bond model, a linear elastic bonding interface with dimensions that can transmit forces and moments, was utilized to describe the contact bonds between balls in the aggregate. On this basis, the effects of coarse aggregate properties (i.e., surface topography, shape, gradation, content, and elastic modulus) on the concrete creep were studied quantitatively. The results show that the coarser the surface topography of aggregate, the smaller creep. Also, with the increase in the content or elastic modulus of coarse aggregate, concrete creep showed a decreasing trend. In addition, the constraint action of the aggregate can be illustrated by the displacement fields. Overall, the 3D particle-based proposed in this work can provide an effective tool to explain the creep behavior of concrete materials from the meso-scale.

Contribution of high-temperature 3D-Digital Image Correlation to investigate the creep behaviour of refractory materials

Potential contributions of 3D-Digital Image Correlation are investigated to understand the dilatant behaviour of a refractory concrete during high-temperature creep experiments. An experimental analysis of high-temperature image quality is performed. The measurement uncertainty is assessed and compared to the sensitivity of computed kinematic fields to the parameters of the Drucker-Prager model that control the dilatancy. Promising results are found i) concerning deflection and neutral axis location measurement in bending creep tests and ii) contactless measurement of axial strain in compression creep tests, iii) to support the assumption of dilatant behaviour from bending and diametral compression tests and iv) to provide data for a rough parameter identification from compression creep tests.

A discrete dislocation dynamics model of creep in polycrystals

The dislocation creep represents high temperature plastic deformation of metallic materials. However, the subject is usually approached with constitutive and phenomenological formulations. The notable exceptions are the recent use of numerical modeling. For example, single-crystal discrete dislocation dynamics (DDD) has been adopted to involve dislocation glide and climb together with diffusion of vacancies. This naturally provides a physics-based framework for dislocation creep. However, the existing model(s) are limited to single crystals and hence are limited in scope. This study extended a single crystal dislocation creep model into polycrystalline ensemble. This was then used to simulate steady state creep rate (ε˙) with respect to creep stress (σ), temperature (T) and grain size (d) in polycrystalline aluminum. Our virtual experiments showed an Arrhenius relationship between ε˙ and T, and a power law scaling with σ. Further, our simulations also revealed a power law increase in ε˙ with d. This is striking, as available experimental data indicate both grain size dependence as well as independence. The answer to this apparent contradiction also emerged from our numerical simulations. It was shown that ε˙ is controlled by interactions of dislocations with static obstacles and grain boundaries. Dominance of static obstacles, in particular, significantly diminished the grain size effect. The role of dislocation mean free path on the pinning and dislocation creep behavior of polycrystalline metallic material was thus mechanistically established.

Numerical and experimental investigation on creep response of 3D printed Polylactic acid (PLA) samples. Part I: The effect of building direction and unidirectional raster orientation

The main aim of this research work is to investigate the effects of building directions and raster orientations on the creep behavior of 3D-printed plastic material and to develop rheological constitutive models to estimate the creep behavior of components. These components have been manufactured through the Fused Deposition Modeling (FDM) technique in which materials are heated and extruded through a nozzle to create 3D Polylactic acid (PLA) specimens. Since 3D-printed specimens exhibit anisotropic behavior, studying their building condition is necessary. Both building direction and raster orientation are among the fabrication conditions that play a major role in the mechanical behavior of the specimens. The tensile behavior of 3D-produced PLA specimens and their creep behavior were evaluated. To model the creep behavior of 3D printed PLA, three different types of rheological constitutive models, Zener, Burgers, and modified Burgers were used analytically and numerically. The finite element (FE) model of the 3D printed unnotched samples was developed to predict the creep behavior of notched samples. The results show that 3D FE models can predict the creep behavior of AM-notched specimens with high accuracy.

Creep in Nanostructured Materials

The creep behaviour and properties of nanostructured materials are attributed to their operating deformation mechanisms, which could be different from those in their coarse-grained counterparts. Accordingly, in this review, recent progress on the creep behaviour of nanostructured materials will be described. The results of large sets of tensile creep tests on selected more complex metallic materials are analysed for evaluating the effect of different SPD processing methods on creep resistance at high temperatures. The resultant creep characteristics are compared with those attained in unprocessed conditions of the same materials. By contrast to the creep behaviour of UFG pure metals SPD processing of more complex materials mostly exhibits no essentially improved creep resistance. Evaluated stress dependences of the creep rate and the creep life suggest that creep deformation mechanisms in UFG materials are similar to those operating in coarse-grained materials. However, creep mechanisms in SPD processed materials are not clearly resolved and this is due to complexity of phenomenon and very small number of studies that have been carried out before now.

Microstructural Effects on Irradiation Creep of Reactor Core Materials

The processes that control irradiation creep are dependent on the temperature and the rate of production of freely migrating point defects, affecting both the microstructure and the mechanisms of mass transport. Because of the experimental difficulties in studying irradiation creep, many different hypothetical models have been developed that either favour a dislocation slip or a mass transport mechanism. Irradiation creep mechanisms and models that are dependent on the microstructure, which are either fully or partially mechanistic in nature, are described and discussed in terms of their ability to account for the in-reactor creep behaviour of various nuclear reactor core materials. A rate theory model for creep of Zr-2.5Nb pressure tubing in CANDU reactors incorporating the as-fabricated microstructure has been developed that gives good agreement with measurements for tubes manufactured by different fabrication routes having very different microstructures. One can therefore conclude that for Zr-alloys at temperatures < 300 °C and stresses < 150 MPa, diffusional mass transport is the dominant creep mechanism. The most important microstructural parameter controlling irradiation creep for these conditions is the grain structure. Austenitic alloys follow similar microstructural dependencies as Zr-alloys, but up to higher temperature and stress ranges. The exception is that dislocation slip is dominant in austenitic alloys at temperatures < 100 °C because there are few barriers to dislocation slip at these low temperatures, which is linked to the enhanced recombination of irradiation-induced point defects.

Effects of Microwave‐Assisted Cross‐Linking on the Creep Resistance and Sensing Accuracy of a Coaxial‐Structured Fiber Strain Sensor

AbstractNanocomposites have been widely studied for sensor applications because of their high design flexibility. However, the creep behavior of polymeric materials is a crucial issue of nanocomposite‐based strain sensors in terms of sensing accuracy in high‐temperature environments, which acts as a bottleneck for the use of nanocomposites in embedded type sensors for monitoring the curing reactions and structural health of composite. Herein, we suggest a microwave‐assisted cross‐linking method for enhancing the reliability of polymer‐based coaxial‐structured fiber strain sensors in high temperatures. A dry‐jet wet‐spun ultra‐high molecular weight polyethylene core fiber with a microwave post‐treatment in a peroxide bath provided superior strength (2.1 GPa), modulus (69 GPa), and enhanced creep resistance (&gt;94%). By using a creep resistance‐enhanced core fiber, coaxial‐structured fiber strain sensors, which have multi‐layer structured shell parts, could be fabricated via the simple dip‐coating method. The fabricated coaxial‐structured fiber strain sensor showed a high gauge factor (4.12 with strain range of 0%–1%), a low‐temperature sensitivity coefficient (−6.5 × 10−4), and an improved sensing accuracy (&lt;3% error at 80 °C) in high‐temperature environments. From these results, it can be confirmed that the microwave‐assisted cross‐linking method is very promising for use in nanocomposite‐based strain sensors for various application fields.

Experimental analysis of high-temperature creep in FV566 steel based on digital image correlation.

Digital image correlation (DIC) is an optical measurement method of material strain/displacement based on visible light illumination, which can be used for the measurement of long-term mechanical behavior. In this paper, an experimental method for analyzing high-temperature creep in FV566 steel material based on DIC was independently designed. Aiming at the problems of glass observation window medium refraction and thermal airflow disturbance in high-temperature testing, the corresponding correction methods were proposed to improve the measurement accuracy. Based on the above methods, high-temperature creep tests were carried out on three specimens with different shapes, and the strain concentration area at 600°C was calculated. Then, the influences of shape and other properties on material creep failure, stress distribution, and actual strain were investigated. Finally, the DIC calculation results were analyzed and compared with results of finite element analysis and the final fracture position of the specimen. The three results had a high degree of consistency, which verified that the proposed method can accurately measure and analyze the creep behavior of materials.

Creep behaviour of eco-friendly sandwich composite materials under hygrothermal conditions

This work describes a new class of sandwich panels made of hemp fibre composites and eco-friendly core materials (balsa, paper honeycomb and recycled polyethylene terephthalate foam). The biobased panels are designed to replace analogous glass fibre mat solutions widespread in the automotive sector. Compared to their fossil benchmarks, these panels show a significant reduction in the global warming potential (from 29 to 40%). Moreover, creep/recovery tests performed under two hygrothermal environments (23 °C-50% RH and 70 °C-65% RH) suggest that the time-delayed behaviour must be considered in the design of biobased sandwich panels, particularly for severe environments.

Creep properties of siltstone-like materials with different unloading confining pressures under seepage

Deep enclosing rocks exhibit non-homogeneous characteristics of dense joints and fissure development. Soft rocks subjected to multiple factors, such as high stress and hydrostatic pressure, are prone to damage and significant deformation, which can lead to instability of the surrounding rocks, failure of the supporting structure, and other accidents. In this study, we investigated the creep behaviour of deep soft rocks and siltstone-like materials subjected to different unloading confining pressures coupled with a high stress field and seepage. Subsequently, the laws and behaviours of parameters, such as the transient strain, creep deformation, and creep rate were derived and analysed under various conditions. The results indicate that the radial creep curve exhibits a variation pattern similar to that of the axial creep. However, the extent of radial creep exceeds that of axial creep in soft siltstone-like rocks under unloading confining pressure conditions. We derived expressions for the constitutive relation of siltstone-like specimens under various unloading confining pressure conditions in the presence of seepage using an improved viscoelasticmodel that considered the coupling effect of fissure and seepage flow. The correlation coefficients of the calculated model values with the experimental values, as obtained by the non-linear least-squares fitting, were all above 0.9178, indicating that the proposed model can accurately characterise the creep process in fissured siltstone.

Study of Material Properties and Creep Behavior of a Large Block of AISI 316L Steel Produced by SLM Technology

The additive manufacturing (3D printing) of metallic materials is a relatively new technology and its use is quickly increasing. Although it is of interest to many researchers, there are still areas which are not fully explored. One of those areas is the behavior of large components and/or semi-products processed by 3D printing. This work is focused on the study of material properties of additive manufactured large block made of AISI 316L steel in two heat treatment conditions (as-printed and solution annealed) and their comparison with the properties of hot-rolled plate performed by tensile tests, Charpy V-notch tests, small punch tests and stress rupture tests. Mechanical tests were complemented by microstructural investigation and the fractographic analysis of fracture surfaces. We found out that mechanical and long-term properties of large 3D printed blocks of this steel are excellent and comparable with other published results obtained on small-sized and intentionally produced test pieces. The observed lower ductility is the result of printing imperfections in microstructure. The results of small punch tests confirmed the possibility of exploiting the existing database and using the correlation between small punch tests and tensile tests results even for 3D-printed AISI 316L steel.