Showing papers on "Stress relaxation published in 2020"

Cracking potential of alkali-activated slag and fly ash concrete subjected to restrained autogenous shrinkage

Abstract: This study aims to investigate the cracking potential of alkali-activated slag (AAS) and alkali-activated slag-fly ash (AASF) concrete subjected to restrained autogenous shrinkage Temperature Stress Testing Machine (TSTM) is utilized, for the first time, to monitor the stress evolution and to measure the cracking time of alkali-activated concrete (AAC) under restraint condition The stresses in AAS and AASF concrete are calculated based on the experimental results while taking into consideration the influence brought by creep and relaxation It is found that AAS and AASF concrete showed lower autogenous shrinkage-induced stress and later cracking compared to ordinary Portland cement (OPC) based concrete with similar compressive strength, despite the higher autogenous shrinkage of AAS and AASF concrete The low autogenous shrinkage-induced stress in the AAC is mainly attributed to the pronounced stress relaxation A good prediction of the stress evolution in AAC is obtained by taking into account the elastic part of the autogenous shrinkage and the stress relaxation In contrast, calculations ignoring the creep and relaxation would lead to a significant overestimation of the stress in AAC

Rheological, fatigue and relaxation properties of aged bitumen

Abstract: Bitumen ageing has a significant impact on the mechanical performance of asphalt concrete. This study utilises dynamic mechanical tests (frequency sweep, fatigue, and relaxation) to investigate the effect of ageing on the viscoelastic characteristics of bitumen. As ageing progressed, a shift of the Black diagrams curves towards lower phase angles was observed; at the same time the shape of the curves changed to a straight line and the curvature reduced. Interestingly, based on the standard fatigue analysis criteria, ageing appears to have a positive effect on bitumen fatigue life. Consequently, it occurs that studying only the stiffness and fatigue behaviour of bitumen cannot directly characterise ageing-induced degradation. The stress relaxation test was found to be more suitable to characterise ageing of bitumen. Aged bitumen had higher residual stresses and longer relaxation times. Hence, aged bitumen was found to be more susceptible to stress accumulation and thus cracking. A linear relationship was established between the crossover modulus and the stress relaxation ageing indices for the studied bitumen. This relationship suggests that crossover modulus has the potential to be used as a parameter to characterise the bitumen relaxation properties by means of routine tests such as the DSR frequency sweep tests.

Stress relaxation in symmetric ring-linear polymer blends at low ring fractions.

Abstract: We combine linear viscoelastic measurements and modeling in order to explore the dynamics of blends of the same-molecular-weight ring and linear polymers in the regime of the low volume fraction (0...

Alternative Form of Standard Linear Solid Model for Characterizing Stress Relaxation and Creep: Including a Novel Parameter for Quantifying the Ratio of Fluids to Solids of a Viscoelastic Solid

Abstract: The standard linear solid model (SLSM) is a typical and useful model for analyzing stress relaxation and creep behaviors of viscoelastic solids for obtaining the corresponding viscoelastic properties. However, the analysis results cannot be directly compared to the parameters commonly adopted for defining the mechanical properties of viscoelastic solids in the finite element simulation package such as the modulus of elasticity (Ee) and the two parameters in the dimensionless form of the relaxation modulus (g and τ1). The purpose of this paper is to introduce an alternative form of SLSM in terms of Ee, g and τ1 for characterizing stress relaxation and creep behaviors. A series of stress relaxation and creep curves with different Ee, g and τ1 was simulated by finite element simulation. The derived alternative form of SLSM was used to curve fit the simulated stress relaxation and creep curves to obtain the corresponding values of Ee, g and τ1. The results showed that the values of Ee, g and τ1 obtained from the simulation were approximately equal to the theoretical ones (i.e., those set in the simulation), showing that the alternative form of SLSM can accurately evaluate the corresponding Ee, g and τ1. In conclusion, the alternative form is formulated in terms of the parameters used to define the mechanical properties in the finite element simulation package, so that the parameters obtained by curve fitting can be directly compared to those set in the finite element simulation package. It was also found that the physical meaning of g is associated with the ratio of viscous fluids to solids of a viscoelastic solid.

Material characterisation of additively manufactured elastomers at different strain rates and build orientations

Abstract: Material jetting, particularly PolyJet technology, is an additive manufacturing (AM) process which has introduced novel flexible elastomers used in bio-inspired soft robots, compliant structures and dampers. Finite Element Analysis (FEA) is a key tool for the development of such applications, which requires comprehensive material characterisation utilising advanced material models. However, in contrast to conventional rubbers, PolyJet elastomers have been less explored leading to a few material models with various limitations in fidelity. Therefore, one aim of this study was to characterise the mechanical response of the latest PolyJet elastomers, Agilus30 (A30) and Tango+ (T+), under large strain tension-compression and time-dependent high-frequency/relaxation loadings. Another aim was to calibrate a visco-hyperelastic material model to accurately predict these responses. Tensile, compressive, cyclic, dynamic mechanical analysis (DMA) and stress relaxation tests were carried out on pristine A30 and T+ samples. Quasi-static tension-compression tests were used to calibrate a 3-term Ogden hyperelastic model. Stress relaxation and DMA results were combined to determine the constants of a 5-term Prony series across a large window of relaxation time (10 μs–100 s). A numerical time-stepping scheme was employed to predict the visco-hyperelastic response of the 3D-printed elastomers at large strains and different strain rates. In addition, the anisotropy in the elastomers, which stemmed from build orientation, was explored. Highly nonlinear stress-strain relationships were observed in both elastomers, with a strong dependency on strain rate. Relaxation tests revealed that A30 and T+ elastomers relax to 50 % and 70 % of their peak stress values respectively in less than 20 s. The effect of orientation on the loading response was most pronounced with prints along the Z-direction, particularly at large strains and lower strain rates. Moreover, the visco-hyperelastic material model accurately predicted the large strain and time-dependent behaviour of both elastomers. Our findings will allow for the development of more accurate computational models of 3D-printed elastomers, which can be utilised for computer-aided design in novel applications requiring flexible or rate-sensitive AM materials.

Thermal crack formation in TiCN/α-Al2O3 bilayer coatings grown by thermal CVD on WC-Co substrates with varied Co content

Abstract: Within this work, the thermal stress build-up of chemically vapor deposited TiCN/α-Al2O3 bilayer coatings was controlled by tuning the coefficient of thermal expansion (CTE) of the substrate material. This was implemented through a Co content variation from 6 to 15 wt.% in WC-Co substrates, which exhibit higher CTEs with increasing Co contents and thereby approach the CTE values of TiCN and α-Al2O3. High temperature X-ray diffraction was employed to determine thermal expansion of an α-Al2O3 powder. Crystallographic texture of the α-Al2O3 coating layer was evaluated by electron backscatter diffraction and taken into consideration in order to assign the appropriate in-plane CTE. This consideration indicated a lower CTE mismatch of α-Al2O3 with WC-Co, compared to TiCN with WC-Co. X-ray diffraction was further utilized for the determination of residual stress in TiCN and α-Al2O3, demonstrating a decrease in both layers for Co contents below 12.5 wt.%. Decreasing stress signaled the formation of thermal crack networks confirmed by scanning electron microscopy surface images. Lower residual stresses were determined in TiCN compared to α-Al2O3 layers of bilayer coatings, contradicting finite element simulations of thermo-elastic stress, that were carried out to illustrate the stress relaxation effects caused by thermal cracks. Monolayer TiCN coatings were annealed at 1000 °C, to replicate stress relaxation taking place during α-Al2O3 deposition, exhibiting a similar residual stress state to TiCN base layers of bilayer coatings. Thermal crack formation was found to be the dominating stress relaxation mechanism in α-Al2O3, while TiCN undergoes further relaxation through secondary mechanisms.

Effects of Glass Transition and Structural Relaxation on Crystal Nucleation: Theoretical Description and Model Analysis.

Abstract: In the application of classical nucleation theory (CNT) and all other theoretical models of crystallization of liquids and glasses it is always assumed that nucleation proceeds only after the supercooled liquid or the glass have completed structural relaxation processes towards the metastable equilibrium state. Only employing such an assumption, the thermodynamic driving force of crystallization and the surface tension can be determined in the way it is commonly performed. The present paper is devoted to the theoretical treatment of a different situation, when nucleation proceeds concomitantly with structural relaxation. To treat the nucleation kinetics theoretically for such cases, we need adequate expressions for the thermodynamic driving force and the surface tension accounting for the contributions caused by the deviation of the supercooled liquid from metastable equilibrium. In the present paper, such relations are derived. They are expressed via deviations of structural order parameters from their equilibrium values. Relaxation processes result in changes of the structural order parameters with time. As a consequence, the thermodynamic driving force and surface tension, and basic characteristics of crystal nucleation, such as the work of critical cluster formation and the steady-state nucleation rate, also become time-dependent. We show that this scenario may be realized in the vicinity and below the glass transition temperature, and it may occur only if diffusion (controlling nucleation) and viscosity (controlling the alpha-relaxation process) in the liquid decouple. Analytical estimates are illustrated and confirmed by numerical computations for a model system. The theory is successfully applied to the interpretation of experimental data. Several further consequences of this newly developed theoretical treatment are discussed in detail. In line with our previous investigations, we reconfirm that only when the characteristic times of structural relaxation are of similar order of magnitude or longer than the characteristic times of crystal nucleation, elastic stresses evolving in nucleation may significantly affect this process. Advancing the methods of theoretical analysis of elastic stress effects on nucleation, for the first time expressions are derived for the dependence of the surface tension of critical crystallites on elastic stresses. As the result, a comprehensive theoretical description of crystal nucleation accounting appropriately for the effects of deviations of the liquid from the metastable states and of relaxation on crystal nucleation of glass-forming liquids, including the effect of simultaneous stress evolution and stress relaxation on nucleation, is now available. As one of its applications, this theoretical treatment provides a new tool for the explanation of the low-temperature anomaly in nucleation in silicate and polymer glasses (the so-called “breakdown” of CNT at temperatures below the temperature of the maximum steady-state nucleation rate). We show that this anomaly results from much more complex features of crystal nucleation in glasses caused by deviations from metastable equilibrium (resulting in changes of the thermodynamic driving force, the surface tension, and the work of critical cluster formation, in the necessity to account of structural relaxation and stress effects) than assumed so far. If these effects are properly accounted for, then CNT appropriately describes both the initial, the intermediate, and the final states of crystal nucleation.

Epoxy-Anhydride Vitrimers from Aminoglycidyl Resins with High Glass Transition Temperature and Efficient Stress Relaxation.

Abstract: Epoxy-anhydride vitrimers are covalent adaptable networks, which undergo associative bond exchange reactions at elevated temperature. Their service temperature is influenced by the glass transition temperature (Tg) as well as the topology freezing transition temperature (Tv), at which the covalent bond exchange reactions become significantly fast. The present work highlights the design of high-Tg epoxy-anhydride vitrimers that comprise an efficient stress relaxation at elevated temperature. Networks are prepared by thermally curing aminoglycidyl monomers with glutaric anhydride in different stoichiometric ratios. The tertiary amine groups present in the structure of the aminoglycidyl derivatives not only accelerate the curing reaction but also catalyse the transesterification reaction above Tv, as shown in stress relaxation measurements. The topology rearrangements render the networks recyclable, which is demonstrated by reprocessing a grinded powder of the cured materials in a hot press. The epoxy-anhydride vitrimers are characterised by a high Tg (up to 140 °C) and an adequate storage modulus at 25 °C (~2.5 GPa), which makes them interesting candidates for structural applications operating at high service temperature.

A new method for predicting cracking at the interface between solid and lattice support during laser powder bed fusion additive manufacturing

Abstract: For laser powder bed fusion (L-PBF) additive manufactured (AM) metals, residual stress-induced cracking often occurs at the interface between the solid and lattice support, and hence it is important to characterize the as-built critical J-integral of the interface to prevent cracking to occur. However, the standard testing method for the critical J-integral of the interface (ASTM E1820-01) does not work well in this situation for four reasons: 1) standard test blocks consisting of half solid and half lattice support crack during the printing process; 2) even after reinforcing the block with side walls to prevent cracking, post-stress relief causes the yield strength to change significantly, which would affect J-integral significantly; 3) post-build machining processes to obtain the required standard specimen geometry release a significant amount of residual stress, which also gives incorrect J-integral value; 4) the interface is so brittle that it is very difficult to machine it to the required standard configuration. Hence a more effective method that combines printing experiments and residual stress simulation is proposed to determine the as-built critical J-integral of the interface. First, a number of rectangular block specimens with lattice supports of identical height overlaid by solids of different heights are built by L-PBF in Inconel 718 in order to determine the critical height that the block would crack. Next, the experimentally-validated modified inherent strain method is utilized to simulate residual stress and compute the critical J-integral at where the interfacial cracking occurs. The proposed method is subsequently validated using the obtained critical J-integral to predict cracking in different geometries. This method eliminates the uncertainties associated with stress relaxation by heat treatment and machining on mechanical properties, as well as sheds light on crack prediction for as-built L-PBF components.

Crystal plasticity modeling and neutron diffraction measurements of a magnesium AZ31B plate: Effects of plastic anisotropy and surrounding grains

Abstract: This study used in-situ neutron diffraction measurements and Elastic ViscoPlastic Self-Consistent polycrystal plasticity model, which incorporates a Twinning and DeTwinning scheme (denoted by EVPSC-TDT), to examine the macro-and micro-mechanical behaviors of a rolled AZ31B plate subjected to uniaxial tension. Three specimens were specifically designed for minimum, maximum and intermediate twinning: (1) loading along the rolling direction, (2) loading along the plate normal, and (3) loading along the direction 45° with respect to the plate normal. Apart from the macroscopic stress strain response, the measured diffraction intensities and internal elastic strains were obtained to examine the activities of the deformation modes at the grain level. The diffraction intensity evolution signaled the volume fraction change of twinning, while the internal elastic strain evolution designated the stress partitioning among the grain orientations. The effect of the surrounding grains on the development of the internal elastic strain was investigated by identifying the corresponding deformation mechanisms. Notably, the corresponding modeling work revealed that the EVPSC-TDT model permitted the prediction of the strain hardening and anisotropic behavior along the directions with minimum, maximum and intermediate twinning at the macroscale, and the evolution of the diffraction intensities and internal strains at the microscale. The results provide a physical understanding of the effects of the load direction, texture and surrounding grains on the role of the deformation modes in hexagonal close-packed polycrystalline materials.

Strain-induced precipitation in Ti microalloyed steel by two-stage controlled rolling process

Abstract: Strain-induced precipitation kinetics and precipitates' characteristics in a Ti microalloyed steel subjected to a two-stage controlled rolling process, simulating rough and finish rolling process of industrial production, were quantitatively investigated by stress relaxation experiments and transmission electron microscopy (TEM), as well as inductively coupled plasma-atomic emission spectroscopy (ICP-AES). In the present work, precipitation during the cooling stage after rough rolling was taken into consideration for the first time. It was found that a small amount of Ti-bearing precipitates, consuming about 10% of the elemental Ti, were formed on dislocations after rough rolling and cooling to 900 °C. The obtained precipitation-time-temperature (PTT) curves exhibited a classic C shape with a nose temperature of 900 °C and the shortest incubation time of 60 s. The PTT curves moved to the lower left by the introduction of 20% deformation at 1050 °C, which was attributed to more nucleation sites for strain-induced precipitation and the decrease of Ti concentration in deformed austenite. During the stress relaxation stage, strain-induced precipitates preferentially nucleated on dislocations and sub-boundaries and were identified as TiC particles. The mean size of precipitated TiC particles increased from 10.2 ± 2.1 to 25.2 ± 2.8 nm, as the holding time was increased from 100 to 1800 s at 900 °C. When the holding time exceeded 600 s the migration of austenite grain boundaries could not be completely inhibited due to the coarsening of precipitates. Isothermal treatment at 900 °C with a holding time of 60–100 s is suggested here as a viable combination for the processing of the 0.05C-0.21Si-1.05Mn-0.13Ti (wt. %) steel due to the shortest incubation time period and effective pinning of grain boundaries by strain-induced precipitates.