Corner Radius Research Articles

A Structural Analysis of Stress Concentration Factor of Rectangular Openings in Ships with Various Corner Radii

A Structural Analysis of Stress Concentration Factor of Rectangular Openings in Ships with Various Corner Radii

Cyclic fatigue testing of ProTaper Universal and ProTaper Next rotary instruments of different diameters

Introduction. Sudden fracture of instruments without previous warning signs, which occurs due to cyclic and torsional fatigue, represents the biggest problem and one of the most difficult complications during endodontic therapy. The aim of this research was to check the influence of diameter of the instruments on the appearance of cyclic fatigue in simulated canals in full rotation. Material and Method. The study analyzed 24 ProTaper Universal instruments (12 instruments with a diameter of 25 and 12 instruments with a diameter of 30) and 24 ProTaper Next instruments (12 instruments with a diameter of 25 and 12 instruments with a diameter of 30). The instruments were tested in an artificial canal stuffed in a metal block at an angle of 45? and a corner radius of 5 mm. The operating time of each instrument until fracture was measured, and then the number of cycles to fracture (NCF) was calculated. The length of the fractured fragments (FL) was measured with a Vernier caliper. Results. The number of cycles to fracture was higher (p < 0.001) in instruments of the ProTaper Universal group of diameter 25 (367.83 ? 17.00) compared to instruments of diameter 30 (329.33 ? 12.86) of the same group. The number of cycles leading to the fracture in instruments of the ProTaper Next group of diameter 25 (1189.33 ? 18.97) was higher (p < 0.001) compared to instruments of the same group of diameter 30 (971.08 ? 15.26). Conclusion. Obtained results indicated that with an increase in the diameter of rotating endodontic instruments, there is a decrease in the resistance to cyclic fatigue.

Steady three-dimensional unbounded flow past an obstacle continuously deviating from a sphere to a cube

We perform three-dimensional particle resolved direct numerical simulations of the flow past a non-spherical obstacle by a Finite Volume cut-cell method, a sub-class of non-body-conforming methods that provides a sharp description of the boundary, which is strictly mass and momentum conservative and can be easily extended to adaptive grids. The present research work discusses the effect of corner rounding and the incidence angle for a range of Reynolds numbers for which the flow exhibits a steady-state behavior. The obstacle is placed in a large cubic domain that properly models an unbounded domain. Hierarchically refined Cartesian meshes are used where the obstacle resides at the finest level of the mesh hierarchy, thus ensuring that the resolution of the boundary layer and the wake of the obstacle is highly accurate, along with significantly reducing the number of grid cells and the computing time. Specifically, we characterize the drag force and the main features of the flow past a bluff obstacle transitioning in shape from spherical to cuboidal through a superquadric geometrical representation. A superquadric representation is suitable for our study since it preserves geometric isometry, and our analysis, thus, focusses on non-sphericity caused by the level of curvature. We investigate a range of Re from 10 to 150, which spans the flow from attached to symmetric and separated past five different obstacle shapes, with the corner radius of the curvature of r/a=2/ζi=1,2/2.5,2/4,2/8 and 0 placed at incidence angles of α=0°, 15°, 30°, and 45° with respect to the streamwise direction. In general, our results show that the obstacle bluffness increases with α and ζi and this increase is more prominent at higher Re. Higher drag forces are a consequence of either higher viscous forces for more streamlined bodies and in less inertial regimes or higher pressure forces for more bluff bodies and in highly inertial regimes, depending on how the corners are contributing to the frontal and lateral surface areas.

Die Surface Design Method for Smoothing Shock Line in Sheet Metal Forming

Replacing the material of automobile body panels from steel sheets to aluminum alloy sheets is an approach to reducing the automobile weight. In the press forming of panel parts with aluminum alloy sheets, it is necessary to smooth stepped linear marks of about 5 μm to 50 μm generated during press forming to satisfy the surface quality of the panel parts. These linear marks are called shock lines, which are generated when the blank sheet slides on the die corner radius during press forming. In this research, the protrusion shape of forming dies that enables us to smooth a shock line is investigated using a 1.2-mm-thick 6000 series aluminum alloy sheet. The shock line is smoothed by applying a trapezoidal surface pressure to the blank sheet near the shock line at the bottom dead center. We propose a method of using finite element analysis to calculate the protrusion shape from the amount of elastic displacement of the dies that occurs when the target surface pressure is applied to the dies surface. The protruding shape calculated by the proposed method has been applied to the forming dies for the door inner panel of the 6000 series aluminum alloy sheet. As a result, in our experiments, the shock line has been smoothed by using these forming dies.

Conditions for vertical pulling of semi-finished leather products under driving rollers

The conditions for the vertical pulling of a semi-finished leather product laid bent on a transporting base plate between rotating squeezing rollers are theoretically investigated in the article for the first time. At that, an important role is played by the value of the radii of the squeezing rollers, installed in one row in the horizontal plane, and the value of the corner radius of the front part of the transporting base plate. If the value of the corner radius and the thickness of the base plate are large, then when it contacts the squeezing rollers, a significant frontal resistance arises. Thus, to derive the conditions for gripping the semi-finished leather product between the rotating squeezing rollers, the pulling-in force of the traction chains, the normal pressure force of the squeezing rollers, the coefficient of friction between the rotating squeezing rollers and the leather semifinished product are taken into account. The condition for the vertical pulling of the semi-finished leather product laid on the base plate is determined, taking into account the coefficient of friction between the semi-finished leather product and the squeezing rollers when they come into contact.

A Study of Stamping Die Design of Autobody Part Using Software Simulation

This paper presents a study of stamping die design of car backdoor reinforcement using software simulation. The existing process of stamping die design and manufacturing in the company being observed involves seven to ten trials before getting an acceptable die and therefore increase the cost of dies. By using software, it is expected that the number of trials is reduced as the design is analyzed in advance to identify unwanted defects in stamping process, such as wrinkle, split, and springback and develop a proper die design. The scenario started with comparing a final stamped part with its design. By using 3D scanner, the final part geometry is obtained and compared with the original 3D CAD model to examine the deviation of the part from its original design. The comparison between the final stamped part and the original design shows discrepancies, it is obvious that most probably the design has been modified to prevent stamping defects. Next, the original design is simulated by applying blank holder force of 109.7 kN. The first simulation on the original design part shows that the minimum thickness strain in -14.4% and has strong wrinkle tendency, therefore the design was modified then run a simulation again on this modified design part. The modification was mainly focus on essential corner radius and increasing blank holder force to 129.2 kN to prevent wrinkle and reduce springback. The second simulation yields a good formability indicated by better forming limit diagram, safety zone, and springback, despite the minimum thickness strain was increased to -19.2%. So, by using software simulation the correct die geometry can be obtained prior to manufacture the dies as to avoid unnecessary cost of modifying the dies during the trial processes. In addition, the number of trials can be minimized resulted in shorter lead time of producing stamping dies.

Experimental Study of Hydroformed Al6061T4 Elliptical Tube Samples under Different Internal Pressures

In order to achieve crack free elliptical shape under controlled conditions, an experimental set-up was designed and fabricated. This setup consists of three hydraulic cylinders, an intensifier, a hydraulic power pack, storage tanks, forming die, and all parts are controlled by a Programmable Logic Controller (PLC) system. The elliptical samples can be achieved through proper control of internal pressure and axial force with proper sealing. Experimental work has been carried out with different magnitudes of internal pressure and constrained conditions of axial force. Initially die of elliptical shape has been designed and modeled in Abaqus to successfully achieve the particular shape of the Al6061T4 tube under different internal pressure. The fabricated tube hydroforming machine set-up is highly effective for forming 0.5 mm-2 mm thick Al6061T4 alloy tube samples. The Experimental test has been carried out at 12.7 mm outer diameter, 175 mm length and 0.5 mm thick Al6061T4 samples. Bulge height parameters measured at different points of regular distance gap on the axial direction of the tube length and corner radius found at different pressures range of the samples are plotted under different internal pressures. Samples having an 18.7 mm major elliptical bulge were achieved during the experiment. The experimental data was validated by simulation results.

Estimating Forming Conditions for Deep Drawn SUS304 Square Cups Having no Delayed Cracks Using FE Simulation

Forming conditions for deep drawn SUS304 square cups having no delayed cracks are determined using finite element (FE) simulations based on the threshold % of wall thickening for delay cracking obtained from deep drawing tests of SUS304 cylindrical cups. The maximum drawing ratio and the initial blank thickness are fixed at 1.87 and 0.8 mm, respectively in this study taken into the considerations of the capacity of the press machine i.e., 100 kN. The blank holding forces (BHF), punch corner radius, rp and die corner radius, rd are varied to obtain 75 mm x 75 mm square cups having % of local thickening less than the threshold value i.e., 47% in the cracking zones located adjacent to the top corners of the cups. A quarter 3D FE simulation model of the deep drawing process is constructed, the wall thickness distributions of the cups are calculated. The simulation results showed that the forming conditions for the crack-free cups could be obtained with BHF ≥ 47 kN, rp ≥ 8.5 mm and 6.8 mm ≤ rd ≤ 9.5 mm. A method for designing the forming conditions of deep drawn SUS304 square cups is proposed in this study.

The Effect of the Extrusion Ratio on Load and Die Wear in the Extrusion Process

In this paper, regarding the example of concurrent extrusion of a round bar, the results of a comparative analysis of the impact of the extrusion ratio R on process parameters such as die wear intensity, extrusion force, stress intensity in the die, and its elastic deformation during extrusion are presented. The tests were carried out in a wide range of the R (1.5-11), assuming a constant outer diameter of the die. A comparative analysis of the wear was carried out in the area of the die corner radius and the calibration strip since the wear is the highest there. Based on the result obtained, it was shown that the increase in the extrusion force was not proportional to the increase in the extrusion ratio, and this could be described by a logarithmic equation. On the other hand, the value of equivalent stresses in the area of the die corner depends, to a relatively small extent, on the value of the extrusion ratio. However, due to the deformation of the die, it is most advantageous to use an extrusion ratio greater than or equal to three. Based on the wear analysis, it was estimated that the increase in the extrusion ratio R from 3 to 7 caused a more than 2.5 times increase in tool wear. In turn, the implementation of the extrusion process with the use of R = 11 caused a 4.5-fold increase in the average depth of die wear compared to its wear using the extrusion ratio R = 3.

Electric discharge drilling of blind holes in titanium alloy Ti6Al4V

PurposeThis experimental study aims to deal with the improvement of process performance of electric discharge drilling (EDD) for fabricating true blind holes in titanium alloy Ti6Al4V. Micro EDD was performed on Ti6Al4V and blind holes were drilled into the workpiece.Design/methodology/approachThe effects of input parameters (i.e. voltage, capacitance and spindle speed) on responses (i.e. material removal rate, tool wear rate and surface roughness [SR]) were evaluated through response surface methodology. The data was analyzed using analysis of variance and multi-optimization was performed for the optimized set of parameters. The optimized process parameters were then used to drill deeper blind holes.FindingsBlind holes have few characteristics such as SR, taper angle and corner radius. The value of corner radius reflects the quality of the hole produced as well as the amount of tool roundness. The optimized process parameters suggested by the current experimental study lower down the response values (i.e. SR, taper angle and corner radius). The process is found very effective in producing finished blind holes.Originality/valueThis experimental study establishes EDD as a feasible process for the fabrication of truly blind holes in Ti6Al4V.

Effect of stress-strain models and sectional analysis methods on the axial-flexural interactions of discontinuously FRP strengthened square RC columns

This study analytically investigated the axial-flexural (P-M) interactions of quare reinforced concrete (RC) columns discontinuously strengthened swith fibre reinforced polymers (FRP). Six FRP confined concrete stress-strain models and two sectional analysis methods were used to develop the analytical P-M interactions of discontinuously FRP strengthened square RC columns. The developed analytical P-M interactions were validated against experimental data available in the literature. It was found that the selection of the stress-strain models had a substantial effect on the analytical P-M interactions, while the influence of the sectional analysis methods heavily depended on the selected stress-strain models. It was also found that the three available design codes for concrete structures externally strengthened with FRP yielded different analytical P-M interactions of discontinuously FRP strengthened square RC columns. A parametric analysis was undertaken and found that the corner radius and confinement ratio significantly affected the P-M interactions of discontinuously FRP strengthened square RC columns. Also, discontinuously FRP strengthened square RC columns with a higher number of FRP layers and smaller corner radius obtained lower flexural and axial strengths than their counterparts with a lower number of CFRP layers and higher corner radius.

Flow around an Oscillating Cylinder at Low Reynolds Number with Forced Convection: Effect of Corner Radius and Reynolds Number

This numerical study investigated the flow-induced vibration (FIV) on non-heated and heated cylinders with different normalized corner radii (r*) at different Reynolds numbers (Re). Four different values of r* were considered (i.e., 0 (square cylinder), 0.5, 0.75, and 1.0 (circular cylinder)) at three different Re: 100, 150, and 200 within the laminar regime. The cylinder constrained in the axial direction and oscillated transversally was considered for a fixed nondimensional cylinder mass (m*) of 10 and a reduced velocity (Ur) of 4.92. The effect of r* and Re could be seen in the vibration modes of cylinders. The two-dimensional incompressible Navier–Stokes and energy equations were solved together with Newton’s Second Law governing the motion of the cylinder with the help of a computational solver. Four different modes were observed in this study: Mode-I characterized by exceptionally low amplitude; Mode-II characterized by fluctuating amplitude known as hysteresis (beating); Mode-III characterized by high amplitude due to synchronization or lock-in; and Mode-IV characterized by the monotonic oscillation of fixed amplitude. For r* = 1, synchronization phenomenon/lock-in was observed. For the heated cylinder cases, due to the change in the normalized corner radius, a notable change in nondimensional vibrational amplitude A/D and the average Nusselt number Nuavg was seen. It was observed that A/D was higher when lock-in occurred (at Re = 100 and r* = 1), leading to a rise in Nuavg by 47.9% compared to Re = 100 and r* = 0. Due to the change in r*, a shifting phenomenon was observed at Re = 150, r* = 0.75 and Re = 200, r* = 1. A major change in Nuavg was observed from the circular cylinder to square cylinder at different Re. The beating phenomenon was observed at Re = 100 for r* = 0.75, which was similar to that occurring at Re = 150 and r* = 0.5, and those at Re = 200 and r* = 0. Heat transfer and wake structure parameters were found to be dependent on r* and Re.

Structural Optimization of a Circular Symmetric Threaded Connection System Based on the Effect of the Upper Stabbing Flank Corner Radius

In order to boost shale gas drilling efficiency, oil and gas exploration and drilling distances have risen along with the number of horizontal definitions and directions. As a result, the joints of oil wells and core gas pipes are under continual stress and compression, internal and external pressures, and extreme bending stresses. This will negatively impact the drilling system’s sustainability and the performance requirements are therefore reinforced. Good thread design is crucial because premium threaded connections are typically employed in joints, and the construction of reliable premium connections to external limitations is closely related to the choice of pertinent parameters as optimal values. In this article, new premium connection systems will be created and constructed that can show good vibration and durability under such stressful conditions. The amount of von Mises stress that is received from the threaded connection depends on how each parameter is changed. In particular, the values of the “upper stabbing flank corner radius” were used to examine the von Mises stress variations in the entire structure. An optimized design for the premium connection system is proposed with the best value of upper stabbing flank corner radius according to minimized stress on the teeth of the premium connection against compressive forces. The study’s findings suggest that it is feasible to create a premium connection that can lower stress following the genuine demand of the industry.

Controlling Lattice Misfit and Creep Rate Through the γ' Cube Shapes in the Al10Co25Cr8Fe15Ni36Ti6 Compositionally Complex Alloy with Hf and W Additions

Trace elements play an important role in the fine-tuning of complex material properties. This study focuses on the correlation of microstructure, lattice misfit and creep properties. The compositionally complex alloy Al10Co25Cr8Fe15Ni36Ti6 (in at. %) was tuned with high melting trace elements Hf and W. The microstructure consists of a γ matrix, γ' precipitates and the Heusler phase and it is accompanied by good mechanical properties for high temperature applications. The addition of 0.5 at.% Hf to the Al10Co25Cr8Fe15Ni36Ti6 alloy resulted in more sharp-edged cubic γ′ precipitates and an increase in the Heusler phase amount. The addition of 1 at.% W led to more rounded γ′ precipitates and the dissolution of the Heusler phase. The shapes of the γ' precipitates of the alloys Al9.25Co25Cr8Fe15Ni36Ti6Hf0.25W0.5 and Al9.25Co25Cr8Fe15Ni36Ti6Hf0.5W0.25, that are the alloys of interest in this paper, create a transition from the well-rounded precipitates in the alloy with 1% W containing alloy to the sharp angular particles in the alloy with 0.5% Hf. While the lattice misfit has a direct correlation to the γ' precipitates shape, the creep rate is also related to the amount of the Heusler phase. The lattice misfit increases with decreasing corner radius of the γ' precipitates. So does the creep rate, but it also increases with the amount of Heusler phase. The microstructures were investigated by SEM and TEM, the lattice misfit was calculated from the lattice parameters obtained by synchrotron radiation measurements.

Unsteady Structure of Compressor Tip Leakage Flows

Abstract Direct numerical simulations (DNS) are performed of a cantilevered stator blade to identify the unsteady and turbulent flow structure within compressor tip flows. The simulations were performed with clearances of 1.6% and 3.2% of chord. The results show that the flow both within the gap and at the exit on the suction side highly unsteady phenomena controlled by fine-scale turbulent structures. The signature of the classical tip-leakage vortex is a consequence of time-averaging and does not exist in the true unsteady flow. Despite the complexity, we are able to replicate the flow within the tip gap using a validated quasi-three-dimensional (Q3D) model. This enables a wide range of Q3D DNS simulations to study the effects of blade tip corner radius and Reynolds number. Tip corner radius is found to radically alter the unsteady flow in the tip; it affects both separation bubble size and shape, as well as transition mechanisms in the tip flow. These effects can lead to variations in tip mass flow of up to 10% and a factor of 2 variation in dissipation within the tip gap.

Modeling of size effect on compressive behavior of CFRP-confined rectangular RC columns: Influence of corner radius and aspect ratio

Taking into consideration the concrete heterogeneity, a three-dimensional meso-scale approach was established to model the failure of axially loaded rectangular reinforced concrete (RC) columns confined with carbon FRP (CFRP). This meso-scale method was first verified by the available test data. The investigation on the influences of edge sharpness from 0.07 to 0.20 and aspect ratios of 1.0, 1.5 and 2.0 on compressive behavior of CFRP-confined rectangular RC columns with different cross-sectional heights (300 mm, 600 mm, and 900 mm) was performed using the verified numerical model. Similar failure model was observed from the geometrically-similar columns, while a significant size effect was exhibited in the compressive strength. As the structural size increased, the effective confinement area ratio calculated from axial stress distribution decreased, illustrating that the confinement of CFRP jackets on concrete existed a size effect. Moreover, as the edge sharpness increased, the size effect on confinement was weakened, and this weakening effect was reduced with the aspect ratio increasing. Finally, considering the influence of the new arch action model and structural size, a modified model was proposed to predict compressive strengths of rectangular RC columns strengthened with CFRP.

Microstructure Observation, Accumulated Equivalent Stress, and Creep Strain Simulation of SPD Disconnector Solder Joint Based on Isothermal Aging

Surge protective device (SPD) is a widely used type of overvoltage protection device, and the good resistance characteristics of metal oxide varistor (MOV) inside determine its ability to suppress transient overvoltage. However, the deterioration of MOV after long-term operation will make the temperature rise gradually. The disconnector made from disconnection mechanism and low-temperature solder joint can prevent the burning of MOV caused by increasing temperature, and its solder joint inside is vital to the successful disconnection. However, the cracks and voids will gradually appear inside the low-temperature solder joint during the operation, which seriously affects the reliability of SPD. Microstructures of SPD disconnector solder joint were assessed at 20 °C (natural operation) and 135 °C (aging operation) in this article. Besides, the energy-dispersive spectrometer (EDS) is used to compare the compositions and element migration at 20 °C and 135 °C. Furthermore, the accumulated equivalent stress and creep strain distribution on cracks and voids in solder joint are analyzed by the finite-element analysis (FEA). It is shown that the solder joint at 135 °C not only changes its microstructures and compositions compared 20 °C but also appears larger voids and cracks. Their appearance is due to the large accumulated equivalent stress and creep strain by analyzing the 2-D simulation model of the solder joint. Moreover, the increase of the radius of electrode corners can effectively reduce the accumulated equivalent stress and creep strain, and it will slow down the generation of cracks and voids and improve the reliability of SPD operation.

Axial behavior of basalt FRP-confined reinforced concrete columns with square sections of different corner radii

Axial compressive tests were conducted on fourteen BFRP-confined square reinforced concrete (RC) columns, involving variations of the corner radius and FRP layers. The test results demonstrate thatthe Basalt-FRP confinement leads to strength improvement, such as 109% gain at a corner radius ratio of 0.6, and also improves the ductility of the RC column. The stress–strain response has a bilinear type, and the difference in confinement level is mainly reflected in the inelastic range. Furthermore, the observation of the distribution of hoop strain values shows that the confinement pressure uniformity increases with thecornerradius. Finally, a prediction equation for the strain efficiency factor is proposed to improve the accuracy of existing square section strength models. A modified strength model of Wu and Zhou shows good agreement with the experimental results.

Single-stage single point incremental square hole flanging of AA5052 material

ABSTRACT Single-point incremental forming process is a cost-effective die-less process to form sheet metal parts. To implement the forming method in industrial applications, it is required to study forming of various shapes. The available literature on hole flanging by single-point incremental forming is mostly focused on cylindrical flanges and very few studies on the square flange are available. The available study on the square flange is based on multistage experiments. The single-stage forming can reduce the forming time significantly. Therefore, in this work, the single-stage single-point square hole flanging of AA5052-H32 sheet material has been studied. A process-specific fracture forming limit line is obtained by single point incremental hole flanging experiments to analyze the strain behavior during the forming of flanges. A definition of Limit Forming Ratio for square flanging has been proposed and its value for the flange forming has been obtained experimentally. The effect of precut square hole length, pre-cut square hole corner radius, and tool rotation on flange formability has been studied and presented. Moreover, forming forces evolving during the process, the microstructure of the material, and the surface roughness of the flange are analyzed and presented. The results show that the flange formability increases with increase in the pre-cut square hole corner radius; the tool rotation reduces the flange formability and increases the surface roughness.

Investigation and machine learning-based prediction of parametric effects of single point incremental forming on pillow effect and wall profile of AlMn1Mg1 aluminum alloy sheets

Today the topic of incremental sheet forming (ISF) is one of the most active areas of sheet metal forming research. ISF can be an essential alternative to conventional sheet forming for prototypes or non-mass products. Single point incremental forming (SPIF) is one of the most innovative and widely used fields in ISF with the potential to form sheet products. The formed components by SPIF lack geometric accuracy, which is one of the obstacles that prevents SPIF from being adopted as a sheet forming process in the industry. Pillow effect and wall displacement are influential contributors to manufacturing defects. Thus, optimal process parameters should be selected to produce a SPIF component with sufficient quality and without defects. In this context, this study presents an insight into the effects of the different materials and shapes of forming tools, tool head diameters, tool corner radiuses, and tool surface roughness (Ra and Rz). The studied factors include the pillow effect and wall diameter of SPIF components of AlMn1Mg1 aluminum alloy blank sheets. In order to produce a well-established study of process parameters, in the scope of this paper different modeling tools were used to predict the outcomes of the process. For that purpose, actual data collected from 108 experimentally formed parts under different process conditions of SPIF were used. Neuron by Neuron (NBN), Gradient Boosting Regression (GBR), CatBoost, and two different structures of Multilayer Perceptron were used and analyzed for studying the effect of parameters on the factors under scrutiny. Different validation metrics were adopted to determine the quality of each model and to predict the impact of the pillow effect and wall diameter. For the calculation of the pillow effect and wall diameter, two equations were developed based on the research parameters. As opposed to the experimental approach, analytical equations help researchers to estimate results values relatively speedily and in a feasible way. Different partitioning weight methods have been used to determine the relative importance (RI) and individual feature importance of SPIF parameters for the expected pillow effect and wall diameter. A close relationship has been identified to exist between the actual and predicted results. For the first time in the field of incremental forming study, through the construction of Catboost models, SHapley Additive exPlanations (SHAP) was used to ascertain the impact of individual parameters on pillow effect and wall diameter predictions. CatBoost was able to predict the wall diameter with R2 values between the range of 0.9714 and 0.8947 in the case of the training and testing dataset, and between the range of 0.6062 and 0.6406 when predicting pillow effect. It was discovered that, depending on different validation metrics, the Levenberg–Marquardt training algorithm performed the most effectively in predicting the wall diameter and pillow effect with R2 values in the range of 0.9645 and 0.9082 for wall diameter and in the range of 0.7506 and 0.7129 in the case of the pillow effect. NBN has no results worthy of mentioning, and GBR yields good prediction only of the wall diameter.