Development Of Internal Stress Research Articles

Unusual deformation mechanisms evoked by hetero-zone interaction in a heterostructured FCC high-entropy alloy

Understanding the synergistic mechanical effects of heterostructured materials remains challenging due to the complexities in the underlying deformation mechanisms, which are usually diverse, activated at different length scales and possibly interacting. Here, we unravel a deformation fundamental for heterostructures: in addition to the direct contribution on strength, hetero-zone interaction and the development of long-range internal stress could assist in evoking extra plastic mechanisms that are difficult to activate in their homogeneous counterparts. Specifically, the deformation of a heterostructure in Al0.1CoCrFeNi alloy, featuring nanostructured hard lamellae embedded in fine-grained soft matrix, is taken as an example for study. Drawing from experimental insights, the long-range internal stress buildup at elastoplastic transition stage due to intense dislocation pile-ups against hetero-zone boundary, which increases yield strength significantly, is theoretically analyzed. At the plastic stage, the high internal stress helps to activate phase transformation in the fine-grained zones, and the inter-zone constraint leads to form dispersed stable strain bands in the nanostructured zones. These extra mechanisms, together with enhanced deformation twinning, facilitate work hardening and coordinate the strain partitioning between zones, imparting improved ductility at high flow stress. These findings indicate a principle for heterostructural design: introducing strong hetero-zone interaction to enhance internal stress and thereby invoke new deformation mechanisms.

Mode I fracturing behavior of Malan loess and implications for the toppling failure of loess slopes

Loess-covered areas are susceptible to a variety of geohazards, among which toppling is one of the typical slope failures causing deadly and destructive threat on human life and properties. Loess toppling usually involves slab-like or columnar blocks being detached from the loess mass by vertical or sub-vertical cracks that develop in the manner of Mode I fracturing. Thus, understanding the static fracture behavior of loess is the key to deciphering the mechanism of such failures. This study carried out Mode I fracturing tests on pre-cracked specimens of undisturbed Malan loess following the semi-circular bend test method suggested by ISRM. Through digital image correlation and numerical simulation, the fracturing process (including development of surficial strain and internal stress) was examined, and it is found Malan loess experiences a four-stage fracturing, i.e., compaction, linear deformation, yield and post-failure. The fracturing exhibits fragile or ductile behavior depending on the physical properties and loading conditions. Low water content, high dry density, and/or fast loading rate favor the absorption of elastic strain energy, which tends to lead a fragile failure and a relatively high fracture toughness. The opposite conditions induce gradual consumption of plasto-damage strain energy, promoting a progressive propagation of crack and, therefore a ductile failure and a relatively low fracture toughness. Among these three factors, water content was found to have the first priority when influencing the fracture behavior of Malan loess. These findings depict the development of vertical cracks and toppling failure in loess area, and have practical implication in designing disaster risk management plan.

Improving mechanical properties of laminate heterogeneous GW103K/AZ31 alloys

The laminate heterogeneous GW103K (Mg-10Gd-3Y)/AZ31 (Mg-3Al-1Zn) alloys were obtained by diffusion bonding and hot rolling. The influence of the interfacial layer numbers on mechanical properties and microstructure evolution of laminate heterogeneous GW103K/AZ31 alloys were systematically studied. The 9-layer laminates exhibited an excellent combination of yield strength (210 MPa), ultimate tensile strength (269 MPa), and elongation (13.2%), which are much superior to the individual component materials. In addition, the 9-layer laminates also exhibited maximum extra strength and ductility that is higher than what is predicted by the rule of mixture. The strengthening and ductilization mechanisms of laminate heterogeneous GW103K/AZ31 alloys are discussed, which promotes the development of internal stress and can be correlated to the Bauschinger effect as measured by cyclic loading-unloading-reloading experiments. Detailed transmission electron microscopy observation was applied to characterize the interface structure of the laminate samples.

Effects of plastic strain and reloading stress on the magneto-mechanical behavior of electrical steels: Experiments and modeling

The properties of electro-mechanical systems are intimately linked to the mechanical state of the materials used for their design. Notably, the processing stages can lead to significant mechanical stresses, with a strong impact on the final magnetic behavior, through plasticity and residual stress effects. This work presents a thorough magneto-mechanical characterization, both hysteretic and anhysteretic, of an electrical steel (DC04) at different levels of plastic deformation and applied tension stress. The mechanical characterization of the material leads to the identification of two hardening stages: a first stage attributed to the development of long-range internal back stress, and a second stage dominated by intragranular stresses connected to the formation of dislocation structures. The magnetic characterization shows that, under no applied stress, plastic strain involves a significant degradation of the magnetic behavior. Mechanical reloading allows recovering part of the magnetic properties of the virgin material. A simplified multiscale modeling tool is proposed for the magneto-mechanical behavior, including the effects of internal stress and dislocation density. The model is notably used to predict the effect of a reloading stress on the magnetic behavior of a plasticized material, with a very satisfactory agreement. For the first time, a three-dimensional modeling approach is proposed for the magneto-mechanical behavior of materials including levels of plasticity up to necking. The very low computation cost of the modeling approach makes it suitable for the numerical study of magnetic devices under various mechanical states. In addition, this formulation opens a route for estimating the mechanical state of a plastically deformed material through the analysis of its magnetic behavior.

Relaxation of Flexure‐Induced Strengthening of Ice

AbstractIncreasing fetch as sea ice retreats with global warming is increasing the amplitude of ocean waves, motivating the need for a better understanding of the impact of episodic flexing on the strength of ice. Unexpectedly, recent studies showed that the flexural strength of ice increases by as much as a factor of two or more upon cyclic loading (unlike earlier results where ice had a thermo‐mechanical history that could account for the difference), possibly owing to the development of internal back stress originating from dislocation pileups. New systematic experiments reveal that the cyclically‐induced increase in flexural strength of columnar‐grained S2 freshwater and saline ice is fully relaxed upon annealing at high homologous temperatures (Th = 0.91 and 0.96). Moreover, the ice can be repeatedly strengthened to the same level by cyclic loading if allowed to anneal after each episode of strengthening. The relaxation of the original strength is attributed to the relaxation of the cyclically‐induced internal back stress.

Microstructure-Based Random Finite Element Method Model for Freezing Effects in Soils and Cold Region Retaining Walls

This paper describes the development of a random finite element model that allows holistic simulation of the phase transition and consequent development of internal stress and volume changes in frozen soils. The simulation capabilities of the model are first validated with laboratory scale experiments. The validated model is then implemented to study the soil lateral stress on the retaining wall subjected to frost action. The results show that the frost action leads to an increase of lateral stresses along the retaining wall. Strategies to mitigate the frost loads on the retaining wall are analyzed with this model; drainage of water in the backfill and use of thermal insulation layer both help to mitigate the lateral frost loads. Overall, by accounting for the spatial nonhomogeneity and coupled thermo-mechanical responses in frozen soils, the model provides holistic simulation of the responses of retaining walls subjected to freezing conditions.

Model analogies between pattern formation in deforming engineering materials & morphogenesis in ageing human brains

AbstractMathematical models developed within the material mechanics and material physics communities have been routinely adapted to interpret and further understand physiological and biological processes. The field of biomechanics, in particular, has emerged from a direct application of elasticity and fluid mechanics theories to model cell and tissue behavior, as well as bone fracture and blood flow. On the other hand, Turing’s reaction-diffusion model of morphogenesis for biochemical systems has been adapted to interpret pattern formation in deforming materials. An important aspect, however, that has not been sufficiently examined is to investigate the role of an externally applied or internally developed stress. Another, equally interesting issue that has not been adequately explored, concerns the development of a common effective methodology to analyze signals and images for both humanmade and naturemade systems, especially when differential equations are not available to use for this purpose. The article is an initial modest effort to discuss such common features between nonliving and living materials. It focuses, in particular, to modeling analogies between pattern formation of defects in deforming engineering materials under application of external stress and morphogenesis of cellular structures in ageing brain tissue under development of internal stress.

Study on Cure Deformation of Metal/CFRP Laminated Composites

During the curing process of composite materials, the differences of the heat transfer ability and mechanical properties of material in various directions result in residual stresses and curing deformation in the composite after curing. Curing deformation has adverse effect on the mechanical performance of composites. The development of internal stress during curing may cause matrix cracking, delamination, warpage and rebound deformation of composites. In this paper, numerical simulation is used to predict the curing deformation of components, thereby reducing production costs and improving production efficiency as well. According to the curing kinetic equation, the HETVAL subroutine was firstly developed to calculate the temperature field and the curing degree field of the curing process. UMAT and UEXPAN subroutines were then applied to solve the mechanical constants and thermal strain and curing shrinkage strain during the curing process. After the proposed model was verified, the deformation of the metal composite co-cured components was studied. It was found that when the thickness ratio of metal to CFRP is one to one the composite gains the greatest geometrical deviation after curing.

Study on the stress relaxation properties of self-compacting concrete under restrained autogenous shrinkage

The development of internal stress and stress relaxation induced by restrained autogenous shrinkage of self-compacting concrete (SCC) with different water to binder ratios and different amount of cementitious materials was investigated. Test results at more than 100 days showed that the free autogenous deformation expands within 1 day and then shrinks rapidly from 3 days to 7 days, after that, the autogenous shrinkage grows slowly and steadily. The induced stress in the restrained autogenous shrinkage also had the similar law; the stress relaxation of restrained autogenous shrinkage mainly developed in the early ages within 7 days, and then growth of the stress relaxation was almost stagnant. Stress relaxation rate of self-compacting concrete increased with the increase of cementitious content, and it increased significantly with the increase of water to binder ratio.

Stress Relaxation in Epoxy Thermosets via a Ferrocene-Based Amine Curing Agent

Physical stress relaxation in rubbery, thermoset polymers is limited by cross-links, which impede segmental motion and restrict relaxation to network defects, such as chain ends. In parallel, the cure shrinkage associated with thermoset polymerizations leads to the development of internal residual stress that cannot be effectively relaxed. Recent strategies have reduced or eliminated such cure stress in thermoset polymers largely by exploiting chemical relaxation processes, wherein temporary cross-links or otherwise transient bonds are incorporated into the polymer network. Here, we explore an alternative approach, wherein physical relaxation is enhanced by the incorporation of organometallic sandwich moieties into the backbone of the polymer network. A standard epoxy resin is cured with a diamine derivative of ferrocene and compared to conventional diamine curing agents. The ferrocene-based thermoset is clearly distinguished from the conventional materials by reduced cure stress with increasing cure temp...

Swelling and acoustic emission behaviour of unconfined and confined coal during sorption of CO2

This paper presents the results of simultaneous measurements of strains, stress and acoustic emission (AE) induced in cylindrical coal samples by CO2 sorption. The experiment comprised two sorption tests. In the first one, an unconfined sample was used, while in the second, before sorption, the sample was axially compressed under high vacuum up to 25–30% of its compressive strength (previously determined on a parallel sample in a uniaxial compression test under vacuum). The experimental set-up consisted of a pressure vessel mounted in a loading frame and the associated pressurisation and monitoring systems. Measurements were made keeping the gas pressure inside the test vessel constant.The experiments have shown that an applied axial stress results in the development of internal stress within the coal and also in the reduction of both swelling and acoustic emission by about 60% and 80%, respectively. This may indicate that the effect of self-stressing as a result of CO2 sorption by coal under constrained conditions should be taken into account in ECBM reservoir modelling.

Epitaxial stress and texture in thin oxide layers grown on Fe–Al alloys

The development of internal stress and texture in the oxide scales grown on Fe–15at.% Al (100) and (111) single crystals at 700°C was determined in situ using synchrotron X-ray diffraction. At this low oxidation temperature oxide scale thickness as revealed by X-ray photoelectron spectroscopy and transmission electron microscopy was situated between 100 and 150nm after 650min oxidation. Trigonal α-Fe2O3 was found to act as a crystallographic template for the nucleation of isostructural α-Al2O3. Strain behavior during oxidation was found to be influenced by the epitaxial relationship between α-Al2O3 and α-Fe2O3.

Coking theory: Internal stress in the coal batch

The development of local internal stress in the coal batch is analyzed on the basis of recent theoretical and experimental data. Its influence on coke quality is demonstrated. The influence of mineralized rock particles and large hard-to-crush particles of highly metamorphic coal (including petrographically uniform coal) is considered. The results of appropriate preparation of coal batch for coking are described.

Tensor‐resolved stress analysis in silicon MEMS device by polarized Raman spectroscopy

AbstractA polarized Raman method is described to measure the tensor elements of the residual stress state (induced upon fabrication) in a thin silicon plate, a suspended structure belonging to the test element group (TEG) chip of a microelectromechanical system (MEMS). The residual stress data experimentally retrieved by the Raman methodology are compared to those calculated according to a finite element model (FEM) of the MEMS. While FEM required the input of the thermal history experienced by the suspended bridge silicon structure, experimental Raman data directly revealed the stress tensor components without any assumption needed. An improved analytical procedure is shown that enables retrieving individual stress tensor components from polarized Raman light. Tensor‐resolved Raman analysis, capable of de‐coupling the hydrostatic and the deviatoric parts of the stress tensor, represents a key‐point in rationalizing the role of thermal history as well as of other processing parameters on the development of internal stress in MEMS devices made of silicon.

Effect of Nb additions on the microstructure, thermal stability and mechanical behavior of high pressure Zr phases under ambient conditions

This paper analyzes the influence of Nb on the shear-induced α→ω→β transformation taking place when processing Zr by high pressure torsion (HPT) under suitable conditions of pressure and shear. With that purpose, pure Zr and Zr–2.5%Nb were processed by HPT at room temperature and at pressures ranging from 0.25 to 6GPa using 5 anvil turns. Nb causes a further reduction of the transition pressures, which are already lower when applying shear besides pressure. Thus, the transition pressure to the β phase is reduced at least 100 times in the Zr–Nb alloy. Alloying with Nb decreases the grain size of the transformed phases, significantly enhances their thermal stability and increases their UTS and elongation to failure. Selected post-HPT annealing treatments lead to the development of very tough, multiphase Zr and Zr–Nb with bimodal grain size distributions. The retention of the high pressure phases under ambient conditions is explained by the development of a high internal stress during processing. This stress is measured by synchrotron radiation diffraction at HZB-BESSY II. It is proposed that the presence of Nb reduces the internal stress level required for the retention of the high pressure phases.

The strain induced martensite transformation in austenitic stainless steels: Part 2 – Effect of internal stresses on mechanical response

In Part 1 of the present study attention was given to the influence of the deformation temperature on the structural aspects of the strain induced martensite transformation in austenitic stainless steels. In this part the mechanical response is examined, both during and after the martensite transformation, with particular attention to the development of internal stress. The influence of internal stresses on the bulk mechanical response concerns both the nature of the elastoplastic transition and the extent of kinematic hardening. The approach taken to assess the development of internal stresses is to use strain reversal (Bauschinger) tests and tensile experiments performed on a neutron diffractometer. In this way the evolution of stresses in partially transformed samples can be measured in the absence of any further martensite transformation.

Comparison using neutron diffraction of martensitic transformation in Fe–Mn–Si shape memory alloys with and without VN precipitates

Neutron diffraction studies have been performed in situ during thermomechanical testing to quantify the effect of VN precipitates on the development of martensitic transformation, internal stress and texture in Fe–Mn–Si shape memory alloy. To this end, the behaviour of the alloy with and without the addition of VN precipitates has been compared and contrasted. The precipitates promote increased martensitic transformation, while pronouncing differences in extent of transformation between differently oriented austenite grains, leading to increased intergranular stresses. The role of back stress due to Orowan loop formation on the behaviour of the VN bearing material is examined, supported by additional evidence from cyclic tensile and strain recovery tests.

Simulation of Internal Stress in Injection Molded Parts

The development of internal stress in injection molded parts is analyzed. Different from other researches, this study uses a new modified Maxwell model to calculate the internal stress. On the basis of the creep experiments of injection molded parts, a non-linear constitutive equation is proposed. Non-linear finite element equation to calculate the internal stress is derived. By means of this model, the internal stress of an injection molded polystyrene plate is simulated. The effects of mold wall temperature, cooling time and packing pressure on the development of internal stress are investigated. The predicting results are in good agreement with experimental data.

The role of internal stresses on the plastic deformation of the Al–Mg–Si–Cu alloy AA6111

In this work, we have investigated the internal stress contribution to the flow stress for a commercial 6xxx aluminium alloy (AA6111). In contrast to stresses from forest and precipitation hardening, the internal stress cannot be assessed properly with a uniaxial tensile test. Instead, tension–compression tests have been used to measure the Bauschinger stress and produce a comprehensive study which examines its evolution with (i) the precipitation structure, and (ii) a wide range of applied strain. A large set of ageing conditions was investigated to explore the effect of the precipitation state on the development of internal stress within the material. It is shown that the Bauschinger stress generally increases with the applied strain and critically depends on the average radius of the precipitate and is thus linked to the shearable/non-shearable transition. Further work in the case of non-shearable particles shows that higher strain eventually leads to particle fracture and the Bauschinger stress then rapidly decreases. Following the seminal work of Brown et al. a physically based approach including plastic relaxation and particle fracture is developed to predict the evolution of the internal stress as a function of the applied strain. Knowing the main characteristics of the precipitation structure–such as the average precipitate radius, length and volume fraction–allows one to estimate accurately the internal stress contribution to the flow stress with this model.

Reduction in vegetative growth of the water mold Saprolegnia parasitica (Coker) by humic substance of different qualities

Humic substances (HS) account for 50–80% of the dissolved organic matter in non-eutrophicated freshwater ecosystems. HS are not inert, but are taken up by and may interact with aquatic organisms. However, at present no information is available on the interaction of HS with fungi, for instance, the fish-pathogenous species Saprolegnia parasitica. To fill this gap, we tested effects of HS on S. parasitica growth in-vitro using 25–500 mg L −1 carbon of HS on GY-agar. We investigated 20 HS including natural organic matter (NOM) samples, two lignite derived HS, and one synthetic HS. The aim was (1) to find out, if there are inhibiting effects and (2) if potential effects can be explained by humic matter properties by structure activity relationships. The growth of S. parasitica was related to the growth on HS-free agar controls. Characterization of HS and NOMs included elemental analysis, high-pressure size exclusion chromatography (HPSEC), UV/VIS, FTIR-, and EPR-spectroscopy in order to obtain information on elemental and structural composition including various metals, molecular weights of the HS fraction, aromaticity, free organic radicals, and functional groups. NOMs with high moieties of high-molecular carbohydrates supported the growth of S. parasitica, all other HS and NOMs reduced it. However, no inhibition of the development of the sporangia and primary zoospores was found. Therefore, the impact of the HS on S. parasitica has to be classified as fungistatic, rather than fungicidal. Synthetic and lignite-derived HS were among the most efficient HS sources. Growth inhibition was correlated ( p < 0.05) with the molecular weights of the HS-fraction, sUVa, COOH groups, C and H. Our results suggest that especially HS with higher molecular weights and aromaticity which contain a high number of organic radicals are the most efficient in reducing fungal growth. Furthermore, highly functionalized HS seem to be important for the observed effect. The development of internal oxidative stress could be a mechanism explaining the observed growth inhibition of S. parasitica.