Creep Of Materials Research Articles

Modelling thermal and irradiation creep with crystal plasticity theory and self-consistent method

In order to help comprehend the underlying deformation mechanisms related to irradiation creep of metallic polycrystalline materials, the crystal plasticity theory and viscoplastic self-consistent method are combined in this work. In the developed constitutive laws for single crystals, the influence of irradiation-induced defects on the creep strain rate has been effectively addressed by considering dislocation climb and glide. For the former, dislocation climb with irradiation effect can be enhanced through the absorption of point defects by dislocations, where the steady-state concentration of point defects is affected by the defect sinks including dislocations and defect clusters. As a comparison, dislocation glide may be retarded through the impediment of mobile dislocations by irradiation-induced defects. More importantly, the effect of dislocation static recovery has been incorporated into the dislocation evolution law, which is noticed to play a deterministic role for the annihilation of dislocations during the long-term creep process. In addition, the viscoplastic self-consistent method is taken as a cross-scale way to predict the irradiation creep properties of polycrystals. To validate the developed creep model, experimental data of both single crystalline and polycrystalline nickel has been considered under both thermal and irradiation creep. A good agreement between the theoretical results and experimental data is achieved, which offers a solid basis to further analyze the macroscopic irradiation creep deformation from the aspect of microstructure evolution.

Experimental study on the shear creep of rock-like materials with cracks

Experimental study on the shear creep of rock-like materials with cracks

A novel theoretical model for obtaining Norton's law of creep materials using different small specimens

The test method for evaluating the creep properties of a material using small specimens mostly relies on an empirical correlation equation derived through uniaxial tests, and it is difficult to obtain a unified theoretical model for creep specimens with different configurations. We propose a novel and unified theoretical model that describes the creep displacement–time curves based on the energy density equivalent principle, which can be expressed as h = h*B(P/A*)vt. A simple expression of the model parameters related to sizes was established for cantilever beam, ring compression, and flat small punch specimens using finite element analysis (FEA). Forward and reverse verification using FEA and experimental validation of the model using small specimens of a zinc alloy (ZA27) at 27°C were conducted. The results revealed that the Norton's law parameters predicted using the new creep model are in good agreement with the results obtained using the uniaxial creep test. The new model does not require an empirical correlation between the displacement and uniaxial reference equivalent strain and has a uniform characterization for four types of specimens with different configurations and sizes.

Creep and ageing of granular materials under isotropic pressure

Single- and multi-stage isotropic creep tests including bender element and acoustic emission measurements are carried out to investigate the relationship between ageing and creep in dense silica sand. In the considered pressure range, the experimental results show an inversely proportional relationship between ageing and creep: the small-strain shear modulus increases with decreasing isotropic pressure,, while the axial creep strain and the acoustic emissions show the opposite pressure dependence. In the multi-stage creep test, the small-strain shear modulus increases monotonically, while the rates of axial strain, number of acoustic emissions and small-strain shear modulus decrease with time according to a power law. In the single-stage creep tests, the ageing and creep indicators initially evolve as in the multi-stage creep test, but then the small-strain shear modulus reaches a peak value and decreases with time thereafter. At the same time, the rates of axial strain, number of acoustic emissions and small-strain shear modulus deviate from the power law. A conceptual model assuming a time-dependent behaviour of the normal and shear forces at the particle contacts in order to explain the experimental observations qualitatively is proposed. Accordingly, an increase in small-strain shear modulus results from homogenisation of the force chains, while a decrease in small-strain shear modulus results from a temporary formation followed by a time-delayed collapse of strong force chains.

Creep Behavior of Fiber Reinforced Mortars and Its Effect to Reduce the Differential Shrinkage Stress

This research aims to develop durable repair materials that can resist shrinkage cracking by exploring the role of creep in reducing shrinkage stress. In this regard, the creep effect can only be quantified if an accurate creep prediction model and theoretical analysis of the shrinkage stress in the patch repair system exist. For this purpose, the research was carried out in the following sequences: first, the research investigated the short-term creep of the patch repair materials containing accelerator and micro-synthetic fibers in the 0.00–0.12% volume fraction range. This short-term creep was measured on five-cylinder specimens (having a diameter of 75 mm and a height of 275 mm). Three specimens were used to determine the deformation of the repair material under unloading conditions, while those remaining were used to determine the total deformation under loading conditions. The amount of creep deformation was determined by taking away the unloaded (shrinkage) and instantaneous (elastic) deformations from the total deformation of the loaded specimens. Secondly, a modified prediction model of ACI 209R-08 is introduced to accurately capture the rate and magnitude of the observed creep of the repair materials. Finally, a formulated theoretical analysis of shrinkage stress in the patch repair system was proposed to examine how creep potentially reduces the repair material's cracking tendency. The results show that the asymptotic value of the creep curve is attained at an earlier age and that its magnitude is greater than that of most concrete. The modified ACI 209R-08 prediction model can closely estimate the repair materials' creep behavior. The best-fit line, residual values, and coefficient of error analyses confirm the modified model's prediction accuracy. The analysis of tensile stress development in the repair layer suggests that creep can reduce stress by up to 50%. With such a reduction, the repair material is expected to be durable in resisting shrinkage and cracking tendency. Doi: 10.28991/CEJ-2023-09-08-014 Full Text: PDF

Creep and microstructure stability of different purity aluminium and its alloy processed by ECAP method

Severe plastic deformation (SPD) is a perspective method for preparation of ultrafine-grained materials. A coarse-grained aluminium with different purity and Al-0.2Sc alloy were processed by equal-channel angular pressing (ECAP) to evaluate the effect of severe plastic deformation on the thermal stability of resulting ultrafine-grained (UFG) microstructure and on creep behaviour of the SPD processed samples. These materials were subjected to ECAP up to 8 passes by route BC at room temperature with a die that had an internal angle of 90° between the two parts of the channel. The microstructural investigations were performed using transmission electron microscopy (TEM) and scanning electron microscopy (SEM) equipped with an electron back-scatter diffraction (EBSD) unit. Tension creep tests in constant load regime were conducted at temperature of 473 K on a both coarse-grained and UFG states of materials. It was found that creep behaviour of aluminium and its alloy is influences by their grain size, stability during creep test, material purity, solid solution and precipitate strengthening and fraction of high-angle grain boundaries development during ECAP process. While creep resistance of pure aluminium at elevated temperatures is improved already after 1 ECAP pass, further ECAP passes lead to degradation of creep life in case of low purity Al and Al-0.2Sc alloy.

Applying Anand versus Garofalo creep constitutive models for simulating sintered silver die attachments in power electronics

This paper aims to examine the thermomechanical response of sintered silver die attachments in power electronics using finite element analysis (FEA). In this work, several material parameters of the sintered silver bonds are investigated. Additionally, two common solder creep constitutive laws including Anand and Garofalo models are also studied. To ensure the fidelity of the simulation procedures, the finite element (FE) models are first correlated with digital image correlation data. Afterward, the FE models are utilized to examine the influence of the material and creep models on the die attach stresses, strains, and plastic works. The expected fatigue and lifetime predictions of the sintered silver layer are thoroughly discussed, accordingly. The results proved that the die attach layer mechanical response is highly driven by the material parameters and creep modeling procedures considered throughout the simulations. Thus, the resulting fatigue life is evaluated. Finally, a general modeling guideline for simulating thermomechanical response of sintered silver die attachments in power electronics are provided in great detail.

Assessing 3D Seismic Plastic Damages of Arch Dams Considering Optimum Mesh Spaces Based on FOS Analyses and 2023 Kahramanmaraş, Hatay, Malatya, and Gaziantep Earthquakes in Turkey

In this paper, the three-dimensional (3D) seismic plastic damage performance of the Ermenek Arch Dam (220 m), which was built in Karaman, Turkey, in 2009, is investigated by including different gallery spaces. 3D modeling of the dam is performed using the finite-difference method, and four various gallery spaces are added to the dam model considering their original oval geometries. WIPP-Drucker (WD) material model is utilized for the dam’s concrete material in creep and seismic damage analyses. Moreover, the Mohr–Coulomb material model is utilized for the foundation. Quiet nonreflecting and free-field boundary conditions are taken into account in the earthquake analyses, and reflecting (fix) boundary condition is used in the factor of safety (FOS) analyses in order to minimize the reflection of earthquake waves at the boundaries. First, the FOS analyses of the Ermenek Dam are performed considering the WD material model, and the optimum mesh space is determined according to FOS analyses. Then, 3D earthquake analyses are performed for 10 important strong ground motions that occurred in Kahramanmaraş, Hatay, Malatya, and Gaziantep in 2023. As a result of the FOS analyses, it is suggested that the mesh length of arch dams should not be chosen randomly while performing the earthquake analyses, and the FOS analyses of arch dams should be carried out using the WD material model before choosing the mesh space. Besides, it is concluded that selected ground motions for seismic analyses have created significant plastic damage around the galleries of the Ermenek Arch Dam, and gallery spaces are of great importance for the seismic plastic damage behavior of arch dams.

STABILITY OF RODS WITH INITIAL IMPERFECTIONS IN THE FORM OF ECCENTRICITY OF LOAD APPLICATION UNDER LINEAR AND NON-LINEAR CREEP CONDITIONS

Stability of a compressed rod having initial imperfections in the form of eccentricity of applied load under conditions of linear and nonlinear creep is considered. It is noted that all real elements have some initial imperfections in the form of technological deflections, eccentricities of applied loads, etc., so they begin to bulge from the very beginning of loading. Another important factor in stability theory is the consideration of material creep. In this regard, the loading process is divided into two phases: the instantaneous loading process and the creep phase under constant external load. Moreover, creep can be time-limited or unrestricted. In the paper formulas for determination of critical forces of stability loss of the rod having initial imperfections, under short-term and long-term action of load are obtained. The equation allowing to determine time of the first crack appearance is derived. Derived are equations the roots of which are loads at action of which the first cracks appear at initial moment of time and at arbitrarily long period of load action. Analysis of acting force determining the character of rod deformation is executed. From the constructed stability equation it is possible to determine the critical force corresponding to the critical length of the section with cracks. For similar problems in nonlinear formulation formulas for determining critical force and critical displacement corresponding to maximum load are obtained. For the case of long duration load the equation which establishes relationship between load and displacement is obtained. Equation for determination of critical force under prolonged action of load has been derived. It has been established that critical displacement is the same under short- and continuous action of load. It is shown that at any intermediate moment critical displacement can be achieved under load lying in certain interval. Keywords: stability, rod, initial imperfection, eccentricity, linear creep, non-linear creep, critical force, crack, critical displacement.

How to define the parameters of the kernel of the transverse creep

The article is devoted to the methodology for determining the parameters of the kernels of the transverse creep of the linear-viscosity materials for the minds of a folding stressed frame. Issues: One of the main tasks in the theory of viscosity is the task of choosing the structure of the kernel of decay and the assignment of parameters of the kernels. The task is actual, shards of the correct designation of unknown parameters in the present to lay down the terms of operation of structural elements.Purpose: The method of work is the development of the method of assigning the parameters of the kernels of transverse creep and the approbation of the results on the problems of analysis of the deformation of transverse creep. Implementation methodology: If the given methodology is only valid for linear materials, then at the first stage, linearity in viscous powers is entrenched. The parameters of the kernels are significant on the basis of the hypothesis of a single function of creep, the basis of which is presented in a confidence interval of 10%. In the capacity of the core of the decay of the vicorist, there is a shot-exponential kernel. The task is to reduce to the minimization of the functionality of the quadratic adjustment of the experimental data in the species of rozrachunks. On the basis of the results, it is possible to create a conclusions about the practicality of the induced technique, both for "Plaxiglass ST-1" and for other linear materials. The maximum allowance for calculation data from experimental warehouses is 5%.

Semi-analytical creep model to obtain Norton's law of materials under flat indentation and its applications

In traditional indentation test methods for obtaining Norton's law of creep materials, an empirical correlation formula represents the relationship between the indentation strain rate and the uniaxial reference strain rate or indentation displacement rate. This study proposes an equivalent method for representative volume elements (RVE) of creep materials from a complex stress state to a simple stress state. Considering Norton's law as a constitutive relationship based on the equivalent method of stress state, a semi-analytical model describing the creep displacement vs. time relationship of flat cylindrical indentation (flat indentation) and a method for determining the parameters of the model were established under constant load conditions in this investigation. The finite element analysis (FEA), creep test results of zinc alloy ZA27, and results in the current literature were used to verify the semi-analytical creep model. The results showed that Norton's law and the displacement rate predicted by different indentation creep models are relatively close, and the partial results predicted by the new model are slightly better than those obtained by methods reported in the literature.

Повзучість малопластичних жароміцних матеріалів при вигині…

Express method of testing the initial creep stagiess of low-plastic heat-resistant materials that work in extreme conditions using the bending scheme proposed and developed. The features of stress and deformation calculation are analyzed. The limitations of using the elastic approximation are outlined: the degree of plastic deformation of the sample should not exceed 1—1,5%. The deflection should not exceed 10% of the distance between the supports, the height should not be greater than 1/10 of the distance between the supports. Under these conditions, the first and second stages are well distinguished on the creep curves. This makes it possible to analyze the influence of phase and structural changes on the mechanisms of creep at each stage, and the conditions for the transition to stationary creep. The results of model experiments performed on TNM titanium aluminide alloys and Fe3Al powder alloy samples indicate the expediency of using the three-point bending scheme for researching the initial stagiess of creep of heat-resistant materials. Experimentally determined values of the deformation rate vary in the range έ ~ 10-5—10-8, which is the most characteristic for the creep of heat-resistant materials. For low-plastic intermetallics, the influence of temperature and loading force on creep curves was studied. The dependences deformation rate vs time on the first and second stages of creep were obtained from bending tests. Thermal activation parameters are defined for the stage of permanent creep. The proposed method allows to study the speed sensitivity and to determine the thermal activation parameters of creep. In extreme conditions of operation of low-plastic heat-resistant materials, the proposed method allows to take into account and analyze the contribution to the creep of cracking and slow destruction processes. Keywords: three-point bending test, rate of deformation, initial stages of creep,high temperature materials, thermal activation parameters.

P35: Characterization and Comparison of Thermoplastic Material Performance Using Pressure-Volume Relationships

Purpose: Characterizing mechanical performance of finished materials can be challenging due to geometric constraints and the effects of downstream processing on raw material behavior. The following describes a more sensitive test method to accurately characterize the performance of a thin polymeric sheet material. Methods: Two different polymers, Thermoplastic Polyurethane, TPU, and Thermoplastic Elastomer, TPE, were selected for characterization. Each was molded and assembled into a manifold. Both polymers were under a constant pressure load over time, as a control and measurement technique. Fluid was injected incrementally into the assembly and the resulting injectate volume was measured by mass difference up to a limit volume to understand material behavior post processing. Results: The plotted pressure-volume relationships were distinct between TPU and TPE at two time intervals, as shown in Figure 1. A reduced pressure response to the same volume at test repetition demonstrated that both materials showed permanent set over time. Volume offset between sample runs illustrated reduced mechanical performance over time due to material creep. Conclusion: The pressure-volume testing method successfully characterized performance of the materials over time to discriminate the material set properties between two thin polymeric sheet material, which could not be detected for a finished component using traditional tensile test methods. This repeated method can be used during material selection in various applications for materials with a manufactured finished component, especially for the medical device industry where polymers are carefully chosen and designed for a specific function.

Frequency varying rheology-based fluid–structure-interactions waves in liquid-filled visco-elastic pipes

This contribution proposes a new rheology-based model for water-hammer wave propagation in visco-elastic pipes. Using a long wavelength analysis and a generalized frequency-dependent Hooke-law for the stress/strain relation, the pressure/longitudinal stress coupled wave system is derived. In this general framework, a visco-elastic Fluid–Structure Interaction (FSI) four equations model is derived by having four visco-elastic kernels associated with the non-local time response of the visco-elastic solid. The explicit dependence of these kernels with the material creep function and the pipe dimension is found. Considering a general linear visco-elastic rheology, the four visco-elastic kernels, and the corresponding creep function are explicitly derived in frequency and time-domain versus four visco-elastic parameters. For a given set of boundary conditions, a general analytical solution for the pressure/stress water hammer wave is obtained in frequency domain. The model’s predictions are successfully compared with experimental measurements as well as with other models adjusted to the same experimental data set by calibrating the model’s parameter. The proposed model can be used in many other contexts with the specific ability to distinguish the intrinsic visco-elastic rheology from the considered pipe geometry and boundary conditions.

Effect of UV-C Radiation on 3D Printed ABS-PC Polymers.

During the initial stages of the COVID-19 pandemic, healthcare facilities experienced severe shortages of personal protective equipment (PPE) and other medical supplies. Employing 3D printing to rapidly fabricate functional parts and equipment was one of the emergency solutions used to tackle these shortages. Using ultraviolet light in the UV-C band (wavelengths of 200 nm to 280 nm) might prove useful in sterilizing 3D printed parts, enabling their reusability. Most polymers, however, degrade under UV-C radiation, so it becomes necessary to determine what 3D printing materials can withstand the conditions found during medical equipment sterilization with UV-C. This paper analyzes the effect of accelerated aging through prolonged exposure to UV-C on the mechanical properties of parts 3D printed from a polycarbonate and acrylonitrile butadiene styrene polymer (ABS-PC). Samples 3D printed using a material extrusion process (MEX) went through a 24-h UV-C exposure aging cycle and then were tested versus a control group for changes in tensile strength, compressive strength and some selected material creep characteristics. Testing showed minimal mechanical property degradation following the irradiation procedure, with tensile strength being statistically the same for irradiated parts as those in the control group. Irradiated parts showed small losses in stiffness (5.2%) and compressive strength (6.5%). Scanning electron microscopy (SEM) was employed in order to assess if any changes occurred in the material structure.

History effects in the creep of a disordered brittle material

We study the creep behavior of a disordered brittle material (concrete) under successive loading steps, using acoustic emission and ultrasonic sensing to track internal damage. The primary creep rate is observed to follow a (Omori-type) power-law decay in the strain rate, the number of acoustic emission events, as well as the amplitudes of the ultrasonic beams, supporting a brittle-creep mechanism. The distribution of acoustic emission event energies is observed to have a scale-free power-law distribution instead of a truncated one expected for a system approaching a critical point at failure. The main outcome is, however, the discovery of unexpected history effects that make the material less prone to creep when it has been previously deformed and damaged under primary creep. With the help of a progressive damage model implementing thermal activation, we interpret this as an aging-under-stress phenomenon: During an initial creep step at relatively low applied stress, the easy-to-damage sites are exhausted first, depleting the excitation spectrum at low stress gap values. Consequently on reloading, although previously damaged, the primary creep restarts but the material creeps (and damages) less than it would under the same stress but without precreeping. Besides shedding a new light on the fundamental physics of creep of disordered brittle materials, this has important practical consequences in the interpretation of some experimental procedures, such as stress-stepping experiments.

Investigating the relaxation of small craters emplaced on solidified melt sheets on the Moon

Larger impact craters and basins on the Moon display a persistent discrepancy in crater size frequency distributions between their interior melt surfaces and exterior ejecta blankets. The population of smaller craters <1 km across used to date the melt and ejecta surfaces often indicate younger surface ages for the melt surface, even though the two should be the same age having formed at the same time from the initial impact. This work investigates the possibility of removal or deformation of these small craters by topographic relaxation via the solid-state creep of lunar mare and highlands materials that have formed on a warm but recently solidified melt sheet on the Moon. We first determine the thermal state of a solidified volume of melt as it cools from the surface to a melt interface at depth that is thick enough to accommodate the emplacement of craters 200 m or 500 m in diameter. We then examine the extent of topographic relaxation under a favorable steady state temperature profile using material rheologies for the applicable lunar materials under lunar gravity. Ultimately, our simulations show that despite elevated thermal conditions that are favorable to relaxation, the low stress state and stiffness of the material rheologies inhibit any significant crater deformation, with displacements <1 m over 10 kyr. This finding suggests that other mechanisms such as self-secondary cratering or target material contrasts, among others and possibly in combination, are likely responsible for the discrepancy in CSFD ages between the melt and ejecta surfaces.

Measured changes in the bridge mobility and radiativity of violins due to material creep following tensioning of strings

Violins have been observed to undergo a small, but measurable, deformation of the corpus after being brought into tune following a prolonged period without string tension. This behavior, which is analagous to the flexing of a bow, can be attributed to the viscoelastic properties of wood. Because of the relatively long time scale of deformation, the process is sometimes referred to as material “creep.” To assess the impact of creep on the sound of a violin, we measured the vibrational and acoustic response of multiple violins during this period of deformation. The measurement process involves tapping the bridge with a small modal impact hammer and recording the velocity response of the top plate using a laser Doppler vibrometer, as well as the far-field acoustic response in an anechoic chamber. Results show modest changes in the amplitude of the main body modes, most notably the CBR and B1- modes, and a consistent shift in the peak frequency of higher modes, over a two-day period after the strings were tuned.

Восстановление деформированных судовых валов правкой

The article focuses on studying the crankshaft as an essential part of the marine internal combustion engine. Typical defects of crankshafts of marine diesel engines are considered. The loads causing the curvature of a crankshaft axis under operating conditions are indicated. The causes and nature of violation of the shaft axis geometry are determined. The statistical assessment in the cases of axle warping during diesel operation is found necessary. The recommendations of the Russian Maritime Register of Shipping on eliminating the curvature of the shaft axis are considered. A long-term practical experience of straightening crankshafts at the ship repair plants is taken into account. Methods for eliminating shaft deflections under repair conditions are described. Methods of mechanical straightening without heating are considered. The advantages and disadvantages of mechanical methods of shaft straightening are clarified. The conditions and necessary requirements for thermomechanical shafts straightening are described. The conditions for applying the thermal method of straightening ship shafts are substantiated. High labor intensity and instability of a shaft shape after straightening are pointed out. There has been proposed method of straightening crankshafts of marine internal combustion engines by stress relaxation based on the creep of the shaft material. Theoretical foundations of the processes of creep and stress relaxation in the shaft material are considered. There has been inferred a necessity of further research of operational reliability of crankshafts of marine internal combustion engines after their restoration by the stress relaxation method.

Maximum steady stress of various cross-section beams under combined loadings during creep

In the simplified method based on stress classification for estimating the stress at the steady stage during creep, an empirical correction factor is needed to account for the relaxation of primary bending stress during creep. An analysis of a rectangular beam subjected to tensile loading and bending moment during creep is carried out to demonstrate the applicability of the Kt, which is the correction factor proposed by the ASME code, in this work. In view of the effects of material creep property and the primary membrane stress on the relaxation of the primary bending stress, a method for accessing the parameter Kt is proposed and validated. The results reveal that the parameter Kt will increase with the tensile loading due to the relaxation of the primary bending stress accelerated by the primary membrane stress. Good agreement between the results of inelastic analysis and the modified method is observed for the beams with different material properties, combined loadings, and cross-section shapes.