Loss Of Material Strength Research Articles

An analysis method to assess in-Calandria corium retention time in PHWRs

In Pressurized Heavy Water Reactors (PHWRs), during a postulated severe accident involving the core collapse, the core internals would fall onto the Calandria bottom and form a debris bed. Under such conditions, the core debris bed would be contained by the Calandria, which also facilitates its cooling by the Calandria vault water. In absence of any accident management action, the Calandria temperatures would rise, causing loss of material strength that may also threaten the structural integrity of Calandria. The gross structural failure of Calandria would limit its ability to further retain the core debris and would jeopardize the in-vessel accident management strategies. The knowledge of structural degradation of Calandria with time till its ultimate failure is important for formulating and assessing the severe accident management guidelines. Assessment of Calandria failure under accident conditions involves several important aspects such as complex thermo-mechanical loadings, temperature dependent elastic-plastic and creep modeling, flexibility of different parts and boundary constraints etc. In view of this, a systematic analysis method based on the sequentially coupled thermo- mechanical analysis has been devised and used for the structural assessment of Calandria. The failure assessment has been carried out as per the specified failure criteria for prominent failure modes including large unbounded deformations and plastic instability, large inelastic strains/ ductility exhaustion and creep-stress rupture. The paper covers various considerations taken into account during the analyses along with few case studies on Indian PHWRs.

Seismic Modelling of Corroded Reinforced Concrete Columns

Deterioration of seismic resistance of columns due to existing corrosion damage is investigated using advanced non-linear finite element simulation and analysis of a large database of tests with the objective to develop performance criteria for seismic evaluation and assessment. Current guidelines for structural seismic evaluation do not address how to account for the condition of the reinforcement. Incorporating the degree of bar corrosion in the evaluation procedures would complicate significantly the problem of seismic assessment. However, bar section loss, embrittlement, loss of material strength and interfacial bond, all affect the member’s residual mechanical properties, the hierarchy of failure modes and the consequences on seismic performance of complete structural components. In this work, simple modifications of existing nonlinear assessment procedures are proposed, in order to account for the effects of corrosion on seismic performance. Reduction factors of reinforcement stress–strain properties and column stiffness, strength and deformation capacity were calibrated to an assembled database of experimentally tested corroded columns, and were verified through advanced nonlinear finite element analysis, after modeling the effect of mass loss on material properties.

Tornado-induced collapse analysis of a super-large reinforced concrete cooling tower

Wind loads are a predominant design factor for the gravity, thermal effects, and seismic actions of thin-walled super-large cooling towers. The current load code provisions relating to cooling towers in various countries are only suitable for straight-line boundary-layer wind fields under normal climates, rather than non-synoptic winds such as tornadoes, which have quite different wind field characteristics and are more vulnerable to structural safety. To further understand the tornado-resistant performances of super-large reinforced concrete cooling towers (RCCTs), wind tunnel tests with respect to a reduced-scale model of a prototype structure with a height of 215 m were conducted using a tornado vortex simulator. Wind loads acting on the tower shell due to tornadoes with different swirl ratios were studied, and then applied to a refined full-scale finite element model. The entire process of tornado-induced RCCT collapse was simulated and the critical failure wind velocities considering nonlinear collapse effects were compared with the results from the buckling stress state (BSS) approach, which is recommended in national codes and linear bifurcation buckling analysis. The results revealed that structural failure is more likely to occur when the RCCT is located at the tornado core radius because the structure experiences the most unfavorable tornadic wind loads within this region. The loss of material strength rather than the loss of stability is the key factor influencing tornado-induced RCCT failure. The structural collapse is initiated by the appearance and development of circumferential cracks in the windward region of the tangential wind flow owing to the tensile failure of the meridian reinforcement inside the tower shell. The cracks around the tower shell lead to the degradation of the natural frequencies of the structure and the change in vibration modes, and finally affect the tornado-resistant performance of the RCCT.

Numerical study on ratcheting performance of heavy haul rail flash-butt welds in curved tracks

Ratcheting at the rail heads due to cyclic rolling contact stresses is often observed in heavy haul railways, especially when there are large steering forces developed in low-radius curves. Additionally, ratcheting behaviour can be exacerbated when associated with material strength loss in the heat affected zones of rail welds. However, the numerical evaluation of this failure mode requires a sophisticated representation of elastic–plastic contact pressures and traction distributions, which, to the best of the authors’ knowledge, cannot be directly measured by any existing tools. In this paper, in order to accurately evaluate the ratcheting performance at rail welds in curved tracks, attempts were conducted to modify the FaStrip algorithm to analytically estimate the traction distributions based on contact pressures and creepages obtained from static finite element analysis and multi-body dynamic simulations, respectively. Cyclic rolling contact was then simulated by repeatedly applying contact pressure and traction on the rail surface consisting of both the rail weld and the parent rail regions. The weld region was divided into 23 sub-zones in the rolling direction to represent the material inhomogeneity within the heat affected zones. Cases on a 2000 m radius curved track and a tangent track were studied for comparison. It was found that the ratcheting strain rate in curved tracks can be significantly elevated due to the higher magnitude of traction, and the location of the elements with the highest ratcheting strain rate was dependent on the traction distribution patterns. The proposed method provides a general tool for predicting rolling contact fatigue initiation life and location, which will benefit the maintenance efficiency of heavy haul rail welds in curved track.

ПРИДАНИЕ ОГНЕЗАЩИТНЫХ СВОЙСТВ ЦЕЛЛЮЛОЗНЫМ ТЕКСТИЛЬНЫМ МАТЕРИАЛАМ С ПРИМЕНЕНИЕМ ЗОЛЬ - ГЕЛЬ ТЕХНОЛОГИИ

The article presents studies on the use of a new composition based on sodium silicate, urea and sodium hydrogen phosphate to impart fire-retardant properties to cellulosic textile materials. The influence of the concentration of the starting components, temperature, and heat treatment time on the flame retardant properties was studied. The change in the fire retardant properties of cotton fabric is given for three heat treatment modes: at 80, 90 and 100 °C. Compared to the initial fabric, the samples treated with a flame retardant have indicators of flame retardant properties. Untreated fabric with a size of 220×170 mm when tested for flammability at an ignition time of 15 s completely burns out in 60 s. In samples treated with a flame retardant, at an ignition time of 15 s, the smoldering time is practically reduced to zero. With an increase in the concentration of the flame retardant, and the temperature of the heat treatment, the loss of material strength, breaking load, and the appearance of the fabric change slightly. Using electron scanning microscopy and energy dispersive microanalysis, it was shown that pure cotton fabric contains 68.77% carbon and 31.22% oxygen; after modification, particles of sodium – 0.02%, phosphorus – 0.04% and potassium – 0.05% are formed on the surface of the treated fabric. distributed fairly unevenly. It has been shown that in cellulosic materials modified with compositions based on sodium silicate and urea, sodium hydrogen phosphate, flame retardant properties increase. The proposed composition provides the achievement of higher fire resistance. Processing can be carried out on standard equipment of finishing enterprises without the stage of high-temperature fixation of the drug.

Experimental study on the behavior of mismatched butt welded joints of high strength steel

An experimental study on the load carrying capacity and deformation capacity of 108 mismatched butt welded joints of high strength steel (HSS) under tension load is performed, to investigate the strain distribution development over the specimen along the whole loading process by using digital image correlation technique (DIC). Three simplified hardness distribution patterns were identified based on the existence of hardened HAZ zone (HHZ) and softened HAZ zone (SHZ). For butt joints with Pattern I or Pattern II hardness distribution (92 specimens), the maximum load increases almost linearly with increasing values of mismatch ratio until mismatch ratio reaches 1.0, while for the specimens (42 specimens) with Pattern III hardness distribution, the strength are significantly lower than the estimated value from EC3, which shows the unsafety of EC3 with wide SHZ. In almost all the cases, mismatch (undermatch or overmatch) leads to a decrease in joint ductility because of strain localization inside the weaker zone, which varies depending on the failure location and the hardening properties in these failure zone. It is noted that the loss of material strength in SHZ can only be mitigated with thin SHZ due to the constraint provided by the unyielded adjacent material.

Structural response of a steel-frame building to horizontal and vertical travelling fires in multiple floors

During previous fire events such as the World Trade Centre Towers (WTC) 1, 2 & 7 in New York (2001), the Windsor Tower in Madrid (2005), and the Plasco building in Iran (2017), flames were observed to travel horizontally across the floor plate and vertically to different floors. Such fires are not considered as part of the traditional prescriptive structural design for fire. Recently, the Travelling Fires Methodology (TFM) has been developed to account for such horizontally travelling nature of fires. A dozen of studies have investigated the structural response of steel, concrete, and composite structures to a single-floor travelling fire. 5 out of 6 of the vertically travelling fire studies have been limited to the structures with a long span composite truss system as in the WTC Towers. The aim of this work is to investigate the response of a substantially different structural system, i.e. a generic multi-storey steel frame, subjected to travelling fires in multiple floors, and varying the number of fire floors, including horizontal and vertical fire spread. A two-dimensional 10-storey 5-bay steel frame is modelled in the finite element software LS-DYNA. The number of multiple fire floors is varied between 1 and 10, and for each of these scenarios, 5 different fire types are investigated. They include four travelling fire scenarios and the standard fire. In total, 51 fire simulations are considered. The development of deflections, axial forces, bending moments and frame utilization are analysed. Results show that the largest stresses develop in the fire floors adjacent to cool floors, and their behaviour is independent of the number of fire floors. Results indicate that both the fire type and the number of fire floors have a significant effect on the failure time (i.e. exceeded element load carrying capacity) and the type of collapse mechanism. In the cases with a low number of fire floors (1–3) failure is dominated by the loss of material strength, while in the cases with larger number of fire floors (5–10) failure is dominated by thermal expansion. Collapse is mainly initiated by the pull-in of external columns (1–3-floor fires; 1–9-floor fires for 2.5% TFM) or swaying of the frame to the side of fire origin (5–10-floor fires). This study has assessed a different structural form compared to previous literature under an extensive range of multiple floor travelling fire scenarios. We find that although vertically travelling fires result in larger beam axial forces and initial deflections, simultaneous travelling fires result in shorter failure times and represent a more onerous scenario for the steel frame investigated.

Fire response of exterior reinforced concrete beam-column subassemblages

Investigating the structural response of reinforced concrete beam-column sub-assemblies at elevated temperatures is the purpose of this paper. This goal was achieved by conducting the ISO-834 standard fire test on two identical one-third scaled reinforced concrete beam-column subassemblage test specimens. The test specimens, which each consisted of one reinforced concrete cantilever beam anchored at the mid-height of a reinforced concrete column, were installed together in a full scale furnace and subjected to downward and upward service loads, respectively. The fire compartment fully engulfed the cantilever beams (except the beams’ top face and the loading points), the beam-column connections and the lower columns. The fire test terminated after 74min as soon as the tensile longitudinal steel bars of the upward-loaded cantilever beam attained the predefined critical temperature 530°C. The lower columns exhibited partial concrete spalling and typical diagonal cracks appeared at the beam-column connections. Based on the recorded internal temperature distributions at the joint cores it was found that the material strength loss in the fire had insignificant impact on the load bearing mechanism of the joints. On the other hand, the gradual decrease in rotation capacity of the beam ends during the fire course considerably influenced the load-deflection relationship. A detailed numerical work has been carried out to simulate the response of the test specimens and will be published elsewhere.

Reducing machining distortion in AA 6061 alloy through re-heating technique

Solution-treated AA 6061 alloy contains residual stresses which cause unwanted deformation during the machining operation rendering the parts unacceptable for use. Usually for AA 6061 alloy, stress relieving is performed by re-heating the parts at 343°C for 1 h. This stress relieving is however accompanied by a considerable loss of material strength which subsequently reduces the functionality of the parts. This paper is based on an effort to evaluate the effectiveness of lower re-heating temperatures for stress relieving without significant loss of strength. Temperatures within the range of 200–343°C were used and treated samples were tested for both the strength and machining distortion. The experimental results indicate 60% reduction in machining distortions with 21% decrease in the strength.

Numerical Investigation on Laser-Assisted Flow Forming Process

Recent years laser-assisted manufacturing techniques are increasingly the focus of the automotive and aerospace industry due to cost price, low energy efficiency and safety. Among these techniques is laser-assisted flow forming process that combines laser beam technology with conventional flow forming process. During forming the material undergoes a rapidly heating and cooling cycles with the irradiative action of the laser beam, which is quite different from the traditional process. A transient three-dimensional model was developed to investigate the influence of laser irradiation on the forming process, in which a user-defined function was created to overcome the problem of a moving Gaussian heating source. The result indicates that a ring-shaped temperature field is acquired when the heating source scans workpiece along a thread; the deformation area on the workpiece during forming is extensively and continuously heated by intense laser radiation and the thermally induced loss of material strength significantly increases the formability concerning the achievable material reduction rate and tool service life; the possibility of accuracy improvement by laser assistance has been investigated and an accuracy error decreases 10%-28.5%.

Thermodynamic admissibility of continuum damage representations

Representations of continuum damage mechanics introduce internal damage parameters into the constitutive stress–strain relations to account for the growth of microscopic flaws and the loss of material strength. Nominally elastic materials are characterized by their elasticity parameters, such as Young’s modulus and Poisson’s ratio, which become dependent on the internal damage parameters. An increase of internal damage involves an energy release similar to the well-known energy release rate in fracture mechanics. This energy release must satisfy the second law of thermodynamics. The implications for the representations of the elasticity parameters in terms of the internal damage parameters are investigated. Necessary conditions for the thermodynamic admissibility of the continuum damage representations are derived based on theoretical models. Several possibilities are given for materials with isotropic or with orthotropic elastic behaviour in the damaged state.

Method for fire protection of wool-containing cloths for special clothing

An effective method for fire protection of wool-containing textiles by phosphorus-containing flame retardants under the influence of CO2-laser radiation was proposed. It was demonstrated that the activating effect of the laser-radiation photons facilitated sorption processes, which helped to increase the flame-retardant content and to form a more thermally resistant fiber structure. Such modification provided reduced combustibility without a loss of material strength.

Identification of concrete deteriorating minerals by polarizing and scanning electron microscopy

The deterioration of concrete represents one of the most serious problems of civil engineering worldwide. Besides other processes, deterioration of concrete consists of sulfate attack and carbonation. Sulfate attack results in the formation of gypsum, ettringite and thaumasite in hardened concrete. Products of sulfate attack may cause a loss of material strength and a risk of collapse of the concrete constructions. The authors focused especially on the microscopical research of sulfate attack. Concrete samples were taken from the Charles Bridge in Prague, Czech Republic. A succession of degrading mineral formation was suggested. Microscope methods represent a new approach to solving the deterioration problems. They enable evaluation of the state of concrete constructions and in cooperation with hydro-geochemistry, mathematics and statistics permit prediction of the durability of a structure. Considering the number of concrete constructions and their age, research of concrete deterioration has an increasing importance. The results obtained can also be useful for future construction, because they identify the risk factors associated with formation of minerals known to degrade structures.

Modelling deterioration of structural behaviour of reinforced concrete beams under saline environment corrosion

In the assessment of existing structures the likely future deterioration of strength and stiffness is of interest. For reinforced concrete beams one of the most important influencing factors is the loss of reinforcement cross-sectional area at critical locations. Because corrosion is a long-term process for well-designed structures, there is only limited field experience available on which to draw and with which to make empirical rules. The considerable research effort in corrosion of reinforcement in concrete structures has focused mainly on the rate of migration of chloride ions from the environment through the concrete to the corroding steel surface. This will estimate the time before reinforcement corrosion becomes significant but does not provide the rate of structural deterioration. In the present paper a method is proposed for estimating structural deterioration, based on evidence that (a) reinforcement corrosion causes internal damage to and loss of bond with the surrounding concrete and (b) chloride diffusion is influenced by stresses set up in the concrete by external loading. Both these factors affect the rate of chloride migration locally and hence the amount of corrosion and material strength loss at any point in time. The proposed method is outlined and an example application given. Reasonably good agreement between the results of the proposed procedure and the few experimental results was obtained.

Using multiaxial testing to determine the effects of temperature on high density polyethylene geomembrane liners

Geomembranes (GMs) are increasingly being used to line containment facilities for both hazardous and municipal wastes. The most common GM in the USA is high density polyethylene (HDPE), a tough polymer showing good chemical resistance, thereby helping to protect the environment from leachate emissions from landfills. The work presented here uses a form of multiaxial testing, called the comprehensive test system (CTS), for geomembranes to perform testing in a manner reflective of the field application. The key mechanical testing distinction of the CTS is that of applying load to the sample with granular media. Liners are exposed to a wide range of temperatures from pre- to postconstruction, and under a wide range of seasonal and latitudinal conditions. This work evaluated the effect of temperature on GMs using the CTS approach. The test time required to elicit changes in the material properties as a function of elevated temperature was considerably reduced by comparison with the US Environmental Protection Agency standard tests. The data showed that the delta modulus (a measure of the material strength) decreased with increasing temperature, indicating a loss of material strength. Most importantly, an increase in temperature weakened the GM in a linear fashion throughout the range studied, with no indication of a threshold temperature.

The “Chunnel” Fire. I: Chemoplastic Softening in Rapidly Heated Concrete

This paper and its companion paper present the main results of an assessment of the fire in the Channel Tunel (the “Chunnel”), which destroyed a part of the concrete tunnel rings by thermal spalling. The study seeks (1) to evaluate the effect of thermal damage (loss of elastic stiffness) and thermal decohesion (loss of material strength) upon the stress state and cracking at a structural level; and (2) to check whether restrained thermal dilatation can explain the thermal spalling observed during the fire. In the present paper, a macroscopic material model for rapidly heated concrete is developed. It accounts explicitly for the dehydration of concrete and its cross-effects with deformation (chemomechanical couplings) and temperature (chemothermal couplings). The thermal decohesion is considered as chemoplastic softening within the theoretical framework of chemoplasticity. Furthermore, kinetics of dehydration, dimensional analysis, and thermodynamic equilibrium considerations show that a unique thermal dehydration function exists that relates the hydration degree to the temperature rise, provided that the characteristic time of dehydration is much inferior to the characteristic time of structural heat conduction. The experimental determination of the thermal dehydration function from in-situ measurements of the elastic modulus versus furnace temperature rise is shown from experimental data available from the chunnel concrete. Finally, by way of an example, the proposed constitutive model for rapidly heated concrete is combined with the three-parameter William-Warnke criterion extended to isotropic chemoplastic softening.

Modelling of interfacial effects on the mechanical properties of fibre-reinforced composites

Abstract The implementation of an imperfect bond between the fibre and the matrix in the micromechanical model of cell assembly is achieved by two approaches. In the first one, a displacement jump at the fibre–matrix interface is introduced. The second approach replaces solely the fibre and the interphase by an equivalent medium. Our results show that the two approaches give the same estimation of effective elastic properties which describe the behaviour of the composite in its transverse plane. The combination of the tangential and normal interphase stiffness effect has been observed by loading a composite at a 45° off-axis. When the displacement discontinuity approach is used, the fibre–matrix interface has an important effect on failure modes generated in the composite. We have noticed that the loss of material strength is much greater as the fibre–matrix bond becomes poor.

Advanced Ceramic Materials for Use in High-Temperature Particulate Removal Systems

First-generation monolithic porous ceramic filter materials have experienced thermal fatigue, high-temperature creep, and a loss of material strength when operated for extended periods of time in advanced coal-fired combustion and gasification systems. Fiber-reinforced and advanced ceramic composites potentially provide a means to mitigate the degradation mechanisms encountered by monolithic filter matrices and ultimately extend operating life. In this paper we will review advancements which have recently been made during the development of the second-generation porous ceramic filter materials and provide insight into the performance of the filter elements during bench-scale qualification testing and operation in demonstration plant test facilities.

Application of the Žurkov‐Abasov theory for describing the strength properties of polyethylene terephthalate fibres (in Russian)

AbstractThe modification of Žurkov equation extending the range of its application to materials characterized by strong flow during stretching is described. For such materials the structural coefficient γ depends on stretching rate and temperature. This results in loss of material strength with increasing stretching rate, when the stretching rate reaches high values, and the temperature dependence is considerably higher than assumed by Žurkov. Moreover, the effect of intermolecular bonds on the stress transfer in material, responsible for linear loss in activation energy of destruction with increase in temperature is considered. The analysis of the Žurkov equation is confirmed by data of measurements carried out on polyester fibres of small stretching ratio.