Stress Grading Research Articles

Reconstituted Bamboo Scrimbers Prepared from Bambusa tulda with Phenol Formaldehyde (PF) Adhesive for Structural Applications

Abstract: This study investigates the physical and mechanical properties of reconstituted bamboo scrimbers prepared from Bambusa tulda bamboo, also known as Indian timber bamboo or Bengal bamboo. The preparation involves varying pressure levels (21.0, 24.5, 28.0, and 31.5 kg/cm2 ) with phenol formaldehyde (PF) adhesive using a hydraulic hot press. The study explores the change in mechanical and physical properties of bamboo scrimbers under varying specific pressures, which significantly reduces moisture content, increases density, and enhances resistance to water absorption and swelling. Mechanically, it improves properties such as modulus of rupture (MOR), modulus of elasticity (MOE), compression parallel to the grain, hardness, and screw-holding capacity. The results position the reconstituted bamboo scrimbers in the super group (IS:3629-1986) in stress grading (MOR & MOE) at all tested specific pressures, surpassing raw bamboo strength properties as well as conventional standard timber, including Teak (Tectona grandis). The results demonstrate remarkable properties comparable to species like Sal (Shorea robusta), Khair (Acacia catechu), and Teak (Tectona grandis) in various mechanical properties. This research highlights the potential of bamboo-based materials over conventional timber in construction and for sustainable development. Future studies on different adhesives and pressure levels could further enhance their economic sustainability.

Theoretical and experimental studies on the bending properties of glued laminated timber manufactured with Chinese fir

China has been increasingly promoting the use of prefabricated timber structure buildings in recent years. However, unlike other countries, China lacks a diverse range of local tree species and engineered wood products suitable for load-bearing components in timber structures. This shortage poses a risk to the sustainable development of timber structures in China. This issue is addressed in this study, which focuses on the development and manufacture of glued laminated timber (GLT) using Chinese fir from plantation forests. The dynamic elastic modulus of 549 laminae was evaluated using the FAKOPP stress wave approach. A total of 28 laminae were randomly selected from Classes I, II, and III to assess the influence of stress grading on bending performance. This study included an analysis of failure modes, bending capacity, and the prediction model for bending stiffness of GLT with different layups and cross-sectional heights. The findings showed that the dynamic elastic modulus of the laminae followed a normal distribution. The bending strength and modulus of elasticity of finger-jointed laminae across different grades were 29.59 MPa to 37.05 MPa and 8.13 GPa to 10.94 GPa, respectively. The mechanical properties of both same-grade and mixed-grade composition GLT manufactured from Chinese fir met the TCT28 (MOR≥28 MPa, MOE≥8000 MPa) and TCYD24 (MOR ≥ 24 MPa, MOE ≥ 8000 MPa) garde requirements in GB/T26889, respectively. Failure modes in the GLT specimens typically began at the knots or finger joints of the bottom layer laminae. The cross-sectional height had no significant effect on the modulus of elasticity of GLT but exerted a significant effect on bending strength. The prediction model for bending stiffness, developed using the transformed section method, agreed with the experimental results. The outcome of this research provides a scientific and data-driven foundation for the application of Chinese fir in the field of structural materials.

Stress grading system optimisation for an inverter‐fed rotating machine

AbstractStress grading systems using non‐linear resistive coatings are a key component to suppress surface corona in the end‐windings of rotating machine. Compared to a sinusoidal‐fed motor, the high slew rate of the voltage at the flanks of the repetitive square voltages from the inverter cause large capacitive currents to flow in the main wall insulation. These large currents, if not properly considered in the design phase, lead to severe electrothermal stress of the grading system. Experiments and simulations were conducted on a stress grading system whose structure arises from limitation posed by the motor structure. Measurements performed with different rise times show that the maximum potential along the conductive armour tape (CAT) increases non‐linearly with increasing axial distance, and the potential at the edge of the CAT reached nearly twice the peak‐to‐peak voltage at 500 ns rise time, leading to corona inception. As metal plates are used in the machine to dampen vibrations in the end‐winding, similar plates were also fastened to the stress grading system, worsening the already inadequate corona suppression performance. The stress grading system was therefore modified, avoiding the surface corona while, at the same time, reducing the temperature in the grading system to acceptable levels.

Experiment of electrothermal stress for different types of end turn grading in the inverter‐fed form‐wound windings

AbstractEnd turn grading with resistive–capacitive coupling experiences severe electrothermal stress when subjected to pulse width modulation (PWM) voltage. In this paper, several experiments and simulations were carried conducted for four types of end turn grading. First of all, the temperature rise in the end turn grading increased with a decrease in rise time. When the rise time was less than 500 ns, the temperature rise at the terminal was higher owing to the increased capacitive current coupled from the main wall insulation. Further, the current in the linear region exhibited minimal variation at different fundamental frequencies resulting in synchronized the temperature rise at the terminal and overlap. Furthermore, the jump voltage was the key factor influencing temperature rise in end turn grading, confirmed by comparing different voltage magnitudes. Finally, the transient behaviour of the maximum field in the stress grading material was determined at rise time. The experimental and simulation results indicate that balancing and interdependently addressing the electrical and thermal stress protection in end turn grading is crucial. The study aims to provide an experimental and theoretical foundation for an insulation system of inverter‐fed rotating machinery operating under PWM voltage.

Research on non-destructive testing of stress in ferromagnetic components based on metal magnetic memory and the Barkhausen effect

In this paper, the relationship between the effective field of metal magnetic memory (MMM) emission and the structural stress of the ferromagnetic components is studied. The relationship between the voltage of the received magnetic Barkhausen noise (MBN) and the excitation source is also studied. Moreover, the characteristics of the emission spectrum of the MMM and MBN detection signals, and the influences of the lift-off distance on the stress detection accuracy of the two detection methods are studied. The advantages and disadvantages of the two magnetic non-destructive testing (NDT) techniques are analyzed separately, and their functions are combined to enhance the advantages and weaken the disadvantages. Modern signal analysis methods such as wavelet packet transform and signal feature extraction are introduced to process MMM signals, and a classifier for stress grading is established in combination with machine learning methods. On this basis, graded detection of the temperature stress of the continuous welded rail (CWR) was carried out. The detection accuracies of the 30–50 MPa, 50–70 MPa and 70–90 MPa stress grade ranges of CWR reach 71.43%, 82.76% and 75.00% respectively. The rapid grading detection of the temperature stress of the CWR and the rapid judgment of the structural stability of the CWR are realized based on the MBN-MMM detection technique.

Optimal insulation design of form-wound stator winding with stress grading system under fast rise-time excitation

Optimal insulation design of form-wound stator winding with stress grading system under fast rise-time excitation

Assessment of Different Measurement Methods/Techniques in Predicting Modulus of Elasticity of Plantation Eucalyptus nitens Timber for Structural Purposes

The mechanical properties of plantation Eucalyptus Nitens timber are currently assessed by applying visual stress grading (VSG) designed for the sawn timber from the mature plantation and do not represent the actual characteristics of the resource. However, the well-known limitation of VSG application for this resource led to the discovery of other methods to grade the timber to its relevant structural grade. There is potential for hardwood plantations in Australia to supply wood to the timber industry and be used in structural applications. However, it is necessary to employ criteria to evaluate the structural properties of this resource before it could be satisfactorily used for structural purposes. This research aimed to assess the use of non-destructive technique (NDT) through acoustic wave velocity (AWV), machine stress grading (MSG), and multiple linear regression (MLR) model to predict the modulus of elasticity (MOE) as a grade-determining factor. The results showed that there was a strong correlation (R2 = 0.88) between the dynamic MOE (MOEdyn) and static MOE (MOEs) of the boards, proving the NDT as a reliable method for the MOE estimations of E. nitens timber. The results from the MLR model also showed that the density and AWV are effective parameters and their combination can be practical to estimate the MOE. There was a high correlation between the MOE obtained from MSG and MOE obtained from four-point bending, demonstrating that the MSG method through the flat-wise bending can be a suitable method for fast grading. The results also indicated that the measured MOE in the edgewise direction correlates with both the flatwise and longitudinal directions. The results also showed that the E. nitens timber resource has the potential to be used in structural applications with a wide range of MOE from 7 GPa to 21 GPa.

Stress Grading Performance Analysis of Stator Coil Considering the Manufacturing Method and the Insulation Design

In rotating machines, the approach of applying nonlinear conductive stress grading tape (SGT) has been applied to reduce the electric field stress in the coil’s overhang region. To characterize and verify the electric field grading effect of SGT in electric machines, electric field analysis using the nonlinear conductivity of SGT as a boundary condition must be considered. However, previous studies have shown limitations in the characterization and application of SGT materials in insulation system design for various reasons, such as lack of conductivity measurement standards for nonlinear materials or not considering possible changes in material properties during the manufacturing procedure. In this study, the nonlinear conductivity of SGT was measured by considering the rotating machine’s manufacturing procedure. A finite element method (FEM) electric field analysis was performed using the measured nonlinear conductivity. Based on the improved conductivity measurement method of SGT and FEM analysis, the effects of changing several insulation design parameters in the field grading performance were studied on a 13.8 kV stator coil. Moreover, the effect of preheating on the SGT conductivity was also examined. Based on analysis results, we realized the possibility of more precise insulation design of the stator coil. Our findings supplement the insufficient insulation design guide on the SGT part and contribute to establishing a systematic approach for insulation design. Furthermore, the proposed methods enable design optimization and detection of the cause of insulation failure.

An Automatic Light Stress Grading Architecture Based on Feature Optimization and Convolutional Neural Network

The identification of light stress is crucial for light control in plant factories. Image-based lighting classification of leafy vegetables has exhibited remarkable performance with high convenience and economy. Convolutional Neural Network (CNN) has been widely used for crop image analysis because of its architecture, high accuracy and efficiency. Among them, large intra-class differences and small inter-class differences are important factors affecting crop identification and a critical challenge for fine-grained classification tasks based on CNN. To address this problem, we took the Lettuce (Lactuca sativa L.) widely grown in plant factories as the research object and constructed a leaf image set containing four stress levels. Then a light stress grading model combined with classic pre-trained CNN and Triplet loss function is constructed, which is named Tr-CNN. The model uses the Triplet loss function to constrain the distance of images in the feature space, which can reduce the Euclidean distance of the samples from the same class and increase the heterogeneous Euclidean distance. Multiple sets of experimental results indicate that the model proposed in this paper (Tr-CNN) has obvious advantages in light stress grading dataset and generalized dataset.

One-Dimensional Model for the Nonlinear Resistive Electric Field Control in Medium-Voltage Rotating Electrical Machines

This study provides an insight in the analytical estimation of the electric field along the surface of a stress grading system (SGS) in the end-winding region of an electrical machine. An analytical approach is derived for the field grading materials (FGMs) having power-law electric-field-dependent conductivity. Despite some limitations, the introduced model can be used for the preliminary analysis of SGS with specific geometric and dielectric parameters, both during sinus and pulsed voltage condition. The analytical considerations are verified with finite element simulations for 15 markedly different nonlinear FGMs, which were previously reported in the applications related to the medium voltage rotating electrical machines.

Electromagnetic transient modeling of form-wound stator coils with stress grading system under PWM excitation

The insulation system of a machine coil includes several layers made of materials with different characteristics. The effective insulation design of machine coils, especially in the machine end winding, depends upon an accurate model of the stress grading system. This paper proposes a modeling approach to predict the transient overvoltage, electric field, and heat generation in machine coils with a stress grading system, considering the variation of physical properties in the insulation layers. A non-uniform line model is used to divide the coil in different segments based on material properties and lengths: overhang, stress grading and slot. The cascaded connection of chain matrices is used to connect segments for the representation of the complete machine coil. The resulting model is able to simulate the transient overvoltages due to the application of fast rise time pulses such as those observed with pulse width modulation from adjustable speed drives, considering coils with different insulation topologies and under pulses with different rise times. The parameters in each coil region are calculated using the finite element method (FEM). Additionally, the resistive heat and electric field distribution in the machine coil are calculated for excitations with different rise times by means of 3-dimensional FEM simulations.

Power Density and Electric Field Distribution at End-turn Stress Grading System of Inverter-fed Rotating Machine under PWM Waveform

Power Density and Electric Field Distribution at End-turn Stress Grading System of Inverter-fed Rotating Machine under PWM Waveform

Power Density and Temperature Rise Estimation at End-turn Stress Grading System of Large Rotating Machines under PWM Waveform

In this paper, the time-averaged power densities as well as the resulting temperature rise, on an end-turn stress grading (SG) system under PWM waveforms as functions of the carrier and the fundamental frequencies are systematically investigated by finite element method-based computations. A novel analytical approximation method to estimate the power densities and the temperature rises without time-consuming numerical computations are proposed. As a result, the maximum power density and the temperature rise increase nearly linearly with increasing fundamental and the carrier frequencies. It is suggested that power dissipations in the SG layer by the fundamental frequency component of a PWM waveform and those by the carrier frequency component can be clearly separated. Moreover, the estimated power densities and temperature rises based on the analytical approximations show reasonable agreement with the numerically computed ones, which indicates the validity of the proposed estimation method.

MFC-CNN: An automatic grading scheme for light stress levels of lettuce (Lactuca sativa L.) leaves

The identification and control of light stress is the key to the high-yield and high-quality production of leafy vegetables in a controlled environment. As a widely grown vegetable in the plant factory, lettuce is responsive to light intensity. Strong or weak light will seriously affect its yield and quality. These differences can be articulated by images and then classified by fine-grained classification methods. Deep learning is commonly used in crop image classification due to its fast and convenient advantages, but conventional fine-grained recognition approaches are still extremely challenging in dealing with this kind of inter-species classification. To address this issue, this work takes lettuce as the research object and established a set of leaf images for lettuce light stress grading. The leaves were divided into four categories depending on the changes in shoot fresh weight. Then, a hierarchical fusion convolutional neural network architecture (MFC-CNN) based on multi-scale input was constructed to grade the light stress. We firstly separate leaf patches from complete leaf and construct four-scale input to expand local leaf vein and texture information. The multi-scale dataset is fed into the main network at different depths according to the characteristics of CNN in feature extraction. Finally, the network is tested through comparative experiments. The results show that the proposed model has obvious advantages in light stress dataset and generalized dataset.

The correlation between Ampel bamboo (Bambusa vulgaris) dimension and geometry with its modulus of elasticity

Traditionally, Indonesian people are used to using bamboo as construction material. Whole bamboo stems (bamboo culm) which is round and hollow, has several segments and tapered. Regarding the development of bamboo utilization for construction material, testing standards and code of practices are needed to ensure the users safety. The quality of bamboo construction can be improved by using a stress grading system. Therefore, this research is needed to obtain the grading variable that influences the modulus of elasticity of Ampel bamboo (Bambusa vulgaris). Based on analysis of their variable, bamboo’s dimension and geometry are not affecting modulus elasticity value. This result shows that the further research for grading system is required to obtain identify characteristics parameter of Bambusa vulgaris that correlate with the modulus of elasticity.

Optimization of double-layer stress grading system for high voltage rotating electrical machines by electric field and thermal coupled analysis

Silicon carbide particle-loaded semi-conductive materials with electric field-dependent conductivity have been used for end-turn stress grading (SG) systems of high voltage rotating electrical machines for decades. Particularly, high voltage class turbogenerators use double-layer SG systems, at which two SG materials with different conductivities are applied, to reduce power dissipations within the SG materials. To prevent the excessive local heating of the SG material and resulting flashover, the field-dependent conductivities of the two SG materials, as well as length of the SG layer in the longitudinal direction along a coil, were optimized, by a nonlinear transient electrical field and thermal coupled analysis. The field-dependent conductivities of the SG materials were adjusted to the suitable ones by changing the average diameter of silicon carbide particles. Then, the appropriate length of the SG layer was selected to minimize the temperature rise in the system. As a result, the newly optimized double-layer SG system showed a 20% lower temperature rise, and more than 15% higher flashover voltage than the conventional ones. Therefore, the effectiveness of the optimized double-layer SG system is successfully confirmed.

Study on the Measurement of Volt Ampere Characteristics of Silicon Carbide Stress Grading Materials

In order to optimize the design of the stress grading structure for the end-windings of high voltage (HV) electric machines, a suitable and convenient method for accurate measurement of Volt ampere characteristics became significant. In this paper, the volt ampere characteristics of the Stress grading coating (SGC) before and after impregnation were studied. The experiment found that the resistivity of SGC after the immersion paint was significantly improved. There were two orders of magnitude differences in resistivity before and after impregnation. For the sake of obtaining more accurate volt ampere characteristics of SGC after impregnation, a new method of pre electrode was proposed for the first time in this paper. The results showed that the current obtained by the method with pre electrode was larger and more accurate.

Temperature and electric field distributions along a form wound coil of an inverter-fed rotating machine with micro-varistor stress grading system

Temperature and electric field along the stress grading system of a form wound coil under repetitive impulse voltage are studied using 2D axisymmetric and 3D models in COMSOL¯ 5.3a. The validity of the 3D model is shown by comparing the results with the measured temperature profile along the stress grading system. The evaluation of both temperature and electric field shows that using a stress grading system based on a micro-varistor characteristic reduces the maximum temperature; however, with an increase in the electric field, which can be reduced to an acceptable level. An optimization on the initial conductivity of the micro-varistor characteristic confirms that desired electric field and temperature rise are achievable by selecting an optimized conductivity characteristic. 2D and 3D simulations show that a proposed stress grading system, which is a combination of an optimized stress grading tape conductivity and minimum conductive armor tape length, has a good temperature and electrical performances under repetitive impulse voltage.

Experimental Investigation of the Spatial and Temporal Evolution of the Tangential and Normal E-Field Components along the Stress Grading System of a Real Stator Bar

This paper presents results based on direct experimental measurements of tangential (Et) and normal (En) E-field components along the stress grading system (SGS) of a real stator bar (Roebel type) for different AC 60 Hz applied voltages. These measurements were made with a new electro-optic system allowing for the study of both spatial distributions of two E-field components along the bar and their temporal evolution at critical points. The results obtained allowed us to calculate the correlation between the distribution of En and Et along the SGS. In particular, it was demonstrated that the En distribution presents a characteristic minimum, which can be used to identify the zone of partial discharge inception. Moreover, it was possible to observe an enlargement of the Et component distribution followed by a saturation in magnitude with the applied voltage increase. Moreover, the results have demonstrated that the waveform of the En component is mostly affected by the SG material used, producing a greater distortion in its waveform than those obtained for the Et component. The more significant distortion was obtained at the end of the outer corona protection (OCP) material, corresponding to the first maximum of the En component and characterized by the appearance of a third harmonic of large amplitude.

Information-based model reduction for nonlinear electro-quasistatic problems

We suggest a nonlinear model reduction approach for transient electro-quasistatic field simulations of high-voltage devices that comprise strongly nonlinear electric field stress grading material. The singular value decomposition is employed to obtain the proper orthogonal decomposition modes, while nodes at which interpolation constraints are imposed, are sampled according to a greedy approach and to information criteria. More precisely, in addition to the greedy approach, at each node of the computational mesh the spectral Shannon entropy of the electric potential is computed and interpolation constraints at high-entropy nodes are introduced. Numerical experiments validate that this maximal information node sampling strategy results in improved reduced models, in terms of nonlinear iterations and, in cases, also in terms of accuracy.