Mass Change Method Research Articles

Research on the capillary absorption capacity of concrete of different particle sizes after high temperature based on nuclear magnetic resonance technology

Concrete is subjected to thermal stress and thermal loading in a high temperature environment, resulting in the expansion and contraction of internal pores, which adversely affects the stability and durability of concrete structures. To explore the effects of temperature and aggregate particle size on the capillary absorption capacity of concrete, the capillary absorption tests were carried out on the specimens with aggregate particle sizes of 1.25–2.5 mm, 2.5–5.0 mm and 5.0–10 mm after high temperature treatment at 300 °C, 400 °C and 500 °C.In this paper, The capillary absorption coefficient was measured by mass change method, and the migration change law and spatial distribution of water during capillary absorption were monitored by NMR technology, the changes in water content in different pores during water absorption were calculated by nuclear magnetic resonance T2 spectroscopy, and a method for calculating the capillary absorption coefficient based on T2 spectra was proposed. The results showed that the capillary absorption coefficient of the samples without high-temperature treatment decreased with increasing of particle size. The capillary absorption coefficient increases at 300 °C and decreases slightly at 400 °C. At 500 °C, the capillary absorption coefficient increases dramatically. This shows that at 500 °C, the durability of concrete is drastically affected. The capillary absorption coefficient calculated based on the T2 spectrum is linearly related to the results of the traditional weighing method. In this paper, the fractal geometry theory and regarding methods are used to deduce the relationship between the average capillary rise height and t12DT, and the rise rate is related to the pore structure. And the correctness of the model was verified by MRI. This study can provide theoretical guidance for the safety assessment of concrete structures after high temperature exposure.

Assessment method of carbonation degree and activity of a wide range of calcium/magnesium containing solids

This work provides a systematic analysis and comparison of the assessment methods for determining the carbonation degree of materials containing calcium or magnesium. The analysis is conducted from three key perspectives, namely the consumption of carbonation reactive components, the production of carbonation product and the dry mass change resulted from carbonation reaction. The carbonation degree of steel slag, tricalcium silicate and carbide slag were assessed at liquid-to-solid ratios of 0 and 0.15 using quantitative X-ray diffraction, thermogravimetry and weighing at temperature of 25 °C, relative humidity of 60% and carbon dioxide concentration of 40%. The utilization of scanning electron microscope was employed to reveal the significant impact of liquid water on the carbonation degree. The results indicate that the carbonation degrees obtained by X-ray diffraction and thermogravimetry are quite similar, whereas the values obtained through weighing method drift away a little, particularly when liquid water is present. Nevertheless, all the methods investigated in this study exhibit great consistency and reliability, including the dry mass change method given its acceptable deviation from others and its convenience in testing. Furthermore, a novel method on assessing the carbonation activity is proposed based on the 24-h carbonation degree at the liquid-to-solid ratio of 0.15, which enables to efficiently distinguish the carbonation potential of different materials. In the end, carbonation activity inventory is established in which a wide range of calcium/magnesium contained materials are categorized into five groups, suggesting its suitability for evaluating the carbonation activity of various solid wastes and cement-based materials.

Dynamics of bridges during proof load tests and determination of mass-normalized mode shapes from OMA

Modal shapes derived from Operational Modal Analysis (OMA) lack normalization with respect to masses, hindering direct estimation of effective mass or modal participation factors. The mass-change method is suitable to solve this issue since it proposes an experimental derivation of scaling factors for modes based on OMA results in different load conditions. However, in the case of bridges, the method application is challenging, as significant static masses are required on the deck for OMA in loaded conditions. To address this challenge, trucks can be used as added masses during standard static proof load tests for bridges, making the mass-change method feasible. However, the presence of trucks introduces dynamic coupling effects among bridge and vehicles, potentially complicating the dynamic identification. This paper proposes a methodology to exploit data obtained from OMA on bridges loaded with stationed trucks within the framework of the mass-change method in order to get the modal scaling factors for the mass normalisation of mode shapes and taking into account possible effects of the dynamic coupling between the two systems. To this purpose, the bridge dynamics with added stationed trucks is studied by introducing a finite element model to address the bridge-vehicles dynamic coupling. Then, a simple but effective 2-degree of freedom system is introduced to develop a parametric investigation in a non-dimensional manner in order to provide a practical tool for engineers to account for the bridge-truck dynamic coupling effects. Finally, numerical applications are proposed to derive some conclusions about the effectiveness of the proposed procedure and its suitability in real applications.

Obtaining mass normalized mode shapes of motorway bridges based on the effect of traffic movement

Ambient vibration testing has become a promising alternative for the forced vibration method due to its well-established practical advantages on bridge structures. However, because of the unavailability of the input data in output-only vibration tests, the extracted mode shapes are not mass normalized. Mass normalized mode shapes are essential in various modal analysis applications such as flexibility-based damage detection techniques. Mass change method was proposed to resolve this issue for Operational Modal Analysis (OMA); however, there are certain practical issues that limit the applicability of this method to real-world structures. In this paper, we propose to utilize the dynamic effects of the traffic movement over bridges as the required perturbation to the structure's modal properties. Numerous traffic conditions are considered, and uncertainty analyses are performed to evaluate the suitability of the loads in different traffic scenarios. Furthermore, a numerical case study is conducted on Hafez Bridge in the city of Tehran, Iran. Actual traffic conditions are simulated using the traffic micro-simulation method, and vehicle forces and masses are applied to the finite element model of the bridge using the equivalent nodal forces method. OMA is conducted in the time domain, and modal parameters are extracted using the Stochastic Subspace Identification method. Finally, it is shown that using the proposed framework, not only the modal scaling factors are estimated with satisfying accuracy, but also the practical difficulties of the conventional mass change method such as bridge shutdowns are eliminated.

Evaluating tool point dynamics using output-only modal analysis with mass-change methods

This paper discusses the evaluation of tool point dynamics using output-only modal analysis methods. As compared to the traditional experimental modal analysis procedures that rely on the input and the output, both being known, output-only methods discussed herein can evaluate modal parameters using only the measured response of the tool subjected to unknown impact-based excitations. Since the input is not known, the estimated shapes are unscaled. To correctly scale shapes, we use the mass-change method. We present analytical models to guide the optimal selection of the mass to be added as well as to guide its placement on the tool. Analytical models systematically characterize errors in scaling the eigenvector and errors due to uncertainties in the estimation of the natural frequencies. The model suggests placement of the mass to be added at the anti-node(s) for the mode(s) of interest. These models guide experiments on two tools – a slender boring bar, and an end mill. Tool point frequency response functions (FRFs) reconstructed with the natural frequencies, damping ratios, and scaled mode shapes evaluated with the output-only mass-change method are compared with and found to be in good agreement with FRFs obtained from the traditional experimental modal analysis procedures. Since output-only methods require one less transducer, and since these methods are robust to uncertainties and inconsistencies in the input, output-only methods promise advantages from the industrial point of view.

Modal analysis of machine tools using visual vibrometry and output-only methods

This paper presents visual vibrometry as a new video camera-based vibration measuring technique for machine tools. Pixels within images from recordings of the vibrating machine are treated as motion sensors to detect and track vibrating edges. Modal parameters are extracted from the measured pixel-displacement time series data using output-only mass-change methods. Methods are experimentally demonstrated on a small milling machine, on a slender boring bar, and on a regular end mill. Modal parameters extracted using visual vibrometry agree with those extracted using experimental modal analysis procedures. Moreover, visual vibrometry aids shape visualization and maintains advantages of other non-contact measurement techniques.

An accurate estimation of frequency response functions in output-only measurements

Frequency response functions (FRFs) can be estimated only from responses of structures without knowledge of input forces. Mode shapes identified from operational modal analysis (OMA) methods are unscaled, and the unscaled mode shapes provide unscaled FRFs. In this paper, mass change and mass–stiffness methods are employed to construct FRFs from the response-only measurements. A numerical case study of a cantilever beam is investigated using the finite element method. It is shown that the mass–stiffness change method provides more accurate results compared to the mass change method in the low-frequency range. A laboratory-scale steel beam is also tested using an OMA method and conventional hammer test. The experimental results show better accuracy of the identified FRFs in the low-frequency range when the mass–stiffness method is used.

Influence of Dopant Type and Orientation of Silicon Anodes on Performance, Efficiency and Corrosion of Silicon–Air Cells with EMIm(HF)2.3F Electrolyte

Intermediate term discharge experiments were performed for Si–air full cells using As-, Sb- and B-doped Si-wafer anodes, with 〈100〉 and 〈111〉 orientations for each type. Discharge characteristics were analyzed in the range of 0.05 to 0.5 mA/cm2 during 20 h runs, corrosion rates were determined via the mass-change method and surface morphologies after discharge were observed by laser scanning microscopy and atomic force microscopy. Corresponding to these experiments, potentiodynamic polarization curves were recorded and analyzed with respect to current-potential characteristics and corrosion rates. Both, discharge and potentiodynamic experiments, confirmed that the most pronounced influence of potentials – and thus on performance – results from the dopant type. Most important, the corrosion rates calculated from the potentiodynamic experiments severely underestimate the fraction of anode material consumed in reactions that do not contribute to the conversion of anode mass to electrical energy. With respect to materials selection, the estimates of performance from intermediate term discharge and polarization experiments lead to the same conclusions, favoring 〈100〉 and 〈111〉 As-doped Si-wafer anodes. However, the losses in the 〈111〉 As-doped Si-anodes are by 20% lower, so considering the mass conversion efficiency this type of anode is most suitable for application in non-aqueous Si–air batteries.

質量変更法による正規化固有モードの推定手法に関する研究

Experimental modal analysis is widely used as a basic technology obtaining dynamic characteristic for mechanical systems design. In experimental modal analysis, input and output signals are measured. However, it is difficult to measure exiting force of mechanical structures because of a given boundary condition. Moreover, Operational Modal Analysis has been used on a field test, and the test structure is excited by the ambient force. When the input signals are unknown, the scaled mode shapes cannot be obtained. In this study, an estimation procedure for the scaling factors of normalized modal shapes is proposed. Furthermore, the real natural frequencies which excluded the effect of additional mass are identified by the mass change method.

705 質量変更法を用いた正規化固有モードと固有振動数の推定(機械力学・計測制御VII)

705 質量変更法を用いた正規化固有モードと固有振動数の推定(機械力学・計測制御VII)

A method to predict position-dependent structural natural frequencies of machine tool

Machine tool structure has a strong influence on the dynamic properties of the tool. The change of a machine tool's structure will cause variations in the dynamic parameters of the entire tool, such as its natural frequency, which will result in changes to the stability of the tool and poor machining quality. Thus, a study on the variations of machine tool dynamics is essential for high performance cutting. In this paper, using the mass change method, a basic mathematical model for predicting the natural frequency change resulting from structural change was presented followed by an experimental validation of the model. The mathematical model indicates that structural change will lead to the outward variation of the natural frequency, which is essentially related to the change of the squared mode shape values between the original position and the modified position of the moving component. With this natural frequency change rate prediction model, the natural frequency in the case of structural change can be easily predicted. The predicted results indicate that the positional change of different moving components has differing influences on the natural frequency of the machine tool.

405 質量変更法を用いた正規化固有モードの推定

405 質量変更法を用いた正規化固有モードの推定

Modal scaling in operational modal analysis using a finite element model

Operational modal analysis (OMA) is a technique that has been widely used on civil and mechanical structures in the last 10 years. As the force is unknown, mode shapes cannot be mass normalized from the responses used for modal identification. In the past few years, several formulations have been proposed to scale mode shapes using the mass-change method, which consists of repeating modal testing after changing the mass at different points of the structure where the mode shapes are known. This technique is easy to use in small systems but it has important drawbacks in medium and large structures due to the difficulties of applying masses of sufficient magnitude. In this paper a more simple methodology is proposed based on scaling the experimental mode shapes of a structure using the mass matrix of a finite element model. Two approaches are compared; one approach where the mass matrix is reduced to the set of measurement points using SEREP and a second approach where the experimental mode shapes are expanded to all DOFs in the model using a newly published principle called the local correspondence (LC) principle. The two approaches are compared in two case studies: a numerical example and a real experimental case. In the numerical example a finite element model was assembled in MATLAB, which was considered the experimental model, and then one thousand finite element models were simulated changing the material and the section properties of each element of the model, in order to study the accuracy provided by the different techniques. As regarding the experimental case, the scaling factors of a cantilever beam were estimated by the mass change method and with the equations that consider the mass matrix of the finite element model. The effect of discretization of the finite element model was studied by assembling several finite element models with different numbers of degree of freedoms.

Effect of biofuel‐derived contaminants on coated industrial gas turbines blade materials

Combined cycle power systems offer increased efficiency of electricity generation and lower environmental emissions of CO2, SOx and NO2, as well as being adaptable to most fossil/biofuels. Industrial gas turbines are at the heart of such power stations and are being developed to perform at higher firing temperatures and pressures to achieve even greater efficiencies, with lower emissions. Fuel gases derived from renewable fuels, such as biogases from digestors or syngases from solid fuel gasification, may contain contaminants that are extremely corrosive to the gas turbine components (e.g. blades and vanes) located in the hot combusted gas path. Such damage can result in a gradual loss of turbine efficiency and reliability. Therefore, it is of paramount importance that the materials used for gas turbine components that operate in these environments provide acceptable and predictable in‐service life times. Single crystal superalloys (e.g. CMSX‐4) were developed to have improved mechanical properties (creep and fatigue) at increasing component operating temperatures, especially in relatively clean aero‐engine operating environments. This paper describes work carried out to investigate the development of hot corrosion processes on CMSX‐4 (uncoated and Pt‐Al coated) in a range of potential environments for blade materials in industrial gas turbines fired on biomass derived fuel gases. A series of laboratory tests has been carried out using the ‘deposit recoat’ technique, with exposure conditions covering: deposits of 80/20 and 50/50 (Na/K)2SO4, with additions of lead, a gas composition of 100 vpm SOx, 100 vpm HCl in simulated combustion gases, deposition flux of 15 µg/cm2/h, temperature of 700 °C, for periods up to 1000 h. During their exposure the materials were monitored using traditional mass change methods. However, quantitative damage data in terms of metal loss was obtained using dimensional metrology, pre‐exposure contact measurements combined with post‐exposure measurements of damage observed by optical microscopy on polished cross‐sections. These measurement methods allowed the distribution of damage to be determined and the material sensitivity to such hot corrosion processes to be quantified.

Trends in fireside corrosion damage to superheaters in air and oxy-firing of coal/biomass

This paper compares the laboratory-based fireside corrosion tests on superheater/reheater materials in simulated air-firing combustion conditions with oxy-firing combustion conditions (with hot gas recycling before flue gas de-sulphurisation). The gaseous combustion environment was calculated based on a specific co-firing ratio of CCP with Daw Mill coal. The fireside corrosion tests were carried out using the “deposit recoat” test method to simulate the damage anticipated in specific environments. A synthetic deposit (Na2SO4:K2SO4:Fe2O3=1.5:1.5:1mol.) which has commonly been used in fireside corrosion screening trials and is a mix that forms alkali-iron tri-sulphate (identified in many investigations as a cause of fireside corrosion) was used in these tests. The air-fired tests were carried out at temperatures of 600, 650 and 700°C and oxy-fired tests were carried out at temperatures of 600, 650, 700 and 750°C to represent the superheater/reheater metal temperatures anticipated in future power plants with and without synthetic deposits, with four candidate materials: T92, HR3C and 347HFG steels; nickel-based alloy 625 (alloy 625 was only tested with screening deposits). The progress of the samples during their exposures was measured using mass change methods. After the exposures, the samples were examined by SEM/EDX to characterise the damage. To quantify the metal damage, pre-exposure micrometre measurements were compared to the post-exposure image analyser measurements on sample cross-sections. The trends in corrosion damage in both air and oxy-firing conditions showed a “bell-shaped” curve, with the highest metal damage levels (peak) observed at 650°C for air-firing and 700°C for oxy-firing tests. However, at 600 and 650°C similar damage levels were observed in both environments. The shift in peak corrosion damage in oxy-firing condition is believed to be the presence of higher levels of SOx, which stabilised the alkali-iron tri-sulphate compounds. Generally, in both air and oxy-firing conditions the mean metal damage was reduced with increasing the amount of Cr in the alloys. However, at the highest temperatures in both air-firing (700°C) and oxy-firing conditions (750°C) the metal damage of nickel based superalloy 625 was higher than HR3C.

Determination of mode shapes using wavelet transform of free vibration data

In this article, a new method is proposed to determine the mode shapes of linear dynamic systems with proportional viscous damping excited by an impact force. The time signals of responses and a priori knowledge of the natural frequencies are required in this method. The method is particularly suitable for the wavelet techniques which can precisely estimate the natural frequencies. A previously proposed method based on a modified Morlet wavelet function with an adjusting parameter is used to identify the natural frequencies and damping ratios of system, and the mode shapes are estimated using the proposed method in this work. It is shown that the extracted mode shapes are not scaled. Therefore, mass change method is used for scaling the mode shapes. Moreover, the effect of noise on the extracted modal parameters is investigated. The validity of method is demonstrated using numerical and experimental case studies.

On exact and approximated formulations for scaling-mode shapes in operational modal analysis by mass and stiffness change

When operational modal analysis (OMA) is used to estimate modal parameters, mode shapes cannot be mass normalized. In the past few years, some equations have been proposed to scale mode shapes using the mass-change method, which consists of repeating modal testing after changing the mass at different points of the structure where the mode shapes are known. In this paper, the structural-dynamic-modification theory is used to derive a set of equations, from which all the existing formulations can be derived. It is shown that the known equations can be divided into two types, the exact and the approximated equations, where the former type does in fact fulfill the equations derived from the theory of structural modification, whereas the remaining equations do not, mainly because the change of the mode shapes of the modified structure is not properly taken into account. By simulations, the paper illustrates the large difference in accuracy between the approximate and the exact formulations. The paper provides two new exact formulations for the scaling factors, one for the non-modified structure and – for the first time in the literature – one for the modified structure. The simulations indicate the influence of errors from the measured natural frequencies and mode shapes on the estimation of the scaling factors using the two exact formulations from the literature and the new exact formulation proposed in this paper. In addition, the paper illustrates statistics of the errors on mode-shape scaling. All simulations were carried out using a plate with closely spaced modes.

Optimum amount of additive mass in scaling of operational mode shapes

Recently, identification of modal parameters using the response only data has attracted considerable attention particularly where the classic modal testing methods is difficult to conduct. One drawback of the response only data, also known as Operational Modal Analysis (OMA), is that only the unscaled mode shapes can be obtained which restricts the applications of OMA. The Mass change method is a usual way to scale the operational mode shapes. In this article a new method is proposed to optimize the additive mass for scaling of the unscaled mode shapes from OMA for which a priori knowledge of the Finite Element model of structure is required. It is shown that the total error of the scaled mode shapes is minimized using the proposed method. The method is validated using a numerical case study of a beam. Moreover, the experimental results of a clamped-clamped beam demonstrate the applicability of the method.

Mass–stiffness change method for scaling of operational mode shapes

In Operational Modal Analysis (OMA) the structure is excited by unknown forces and only the response signals are measured. Therefore, the mode shapes cannot be scaled straightforward from the test. The mass change method is a well-known way to estimate the scaling factors of mode shapes by adding mass to the selected points of structure. In the mass change method, the scaling factors are estimated using the frequency shift due to the added masses to the structure. However, in the mass change method the frequency shifts for the first modes of structure are less than those of the higher modes based on the sensitivity analysis. In the conventional scaling methods, the mode shapes are required to change slightly by the mass change for all the modes under investigation. As a result, the first modes do not experience a significant frequency shift and they cannot be scaled precisely. In this paper a method is applied for scaling of the operational mode shapes on a basis of both mass and stiffness change of the test structure. It is shown that the advantage of the applied method is that the first modes of the structure are more affected by the stiffness changes and consequently the corresponding scaled mode shapes are more accurate. The numerical and experimental results show that the accuracy of the scaled mode shapes improves by using the proposed mass–stiffness change method.

Fireside corrosion of superheaters: Effects of air and oxy-firing of coal and biomass

The growing world population requires increasing quantities of electricity from sources that have reduced environmental impact, especially CO2 emissions. For pulverised coal-fired power plants, reduced CO2 emissions can be achieved by increasing the operating temperatures/pressures of the steam systems (which increases the efficiency of power generation), the use of ‘carbon-neutral’ fuels such as biomass, and/or the implementation of CO2 capture technologies. The use of small quantities of biomass fuels (typically up to 5% of the energy content of the fuels) is already widely practised in the UK. As an alternative, oxy-firing pulverised fuel power plants provides a route to enable CO2 capture technologies to be introduced; but this route also requires the use of higher temperature/pressure steam systems to counter-act the efficiency penalty of CO2 capture and handling systems. All of these options to reduce CO2 emissions result in significant changes to the operating conditions (e.g. metal temperatures, gas temperatures, gaseous environments and deposits) that are anticipated for superheaters in these power systems. Such changes in operating conditions are expected to result in higher rates of fireside corrosion and so reduce the potential lives of these critical components.This paper reports the results of investigations that have been carried out to assess the effect on fireside corrosion of operating superheaters at higher metal temperatures in power systems using coal/biomass fuels fired with air or oxygen. A series of fireside corrosion experiments has been carried out using the ‘deposit recoat’ test method to simulate the damage anticipated in different specific environments. Using an alumina-lined controlled-atmosphere furnace, the gaseous and deposit environments were simulated for superheaters in (a) air-fired and (b) oxy-fired (hot gas recycle system variant) power plants using cereal co-product and a UK coal. The corrosion tests were carried out at temperatures of 600 and 650°C, to represent the metal temperatures anticipated in power plants in the near future. The potential superheater materials used in these tests were T92, 347HFG and HR3C steels, as well as one nickel-based material, alloy 625. The progress of the exposures was monitored using traditional mass change methods. Destructive examinations of the samples were carried out after their exposures, using optical microscopy and SEM/EDX methods to characterise the damage observed. The performance of the materials was determined using dimensional metrology; pre-exposure micrometer measurements and post-exposure image analyser measurements on polished cross-sections. The resulting metal loss distribution data for each sample exposed are being used for the development of statistical models of the fireside corrosion of superheater materials in novel operating environments.