H. W. Ng

An experimental study on tearing energy in splitting square metal tubes

Abstract An experiment was performed to study the tearing energy in splitting square aluminium and mild steel tubes of thicknesses ranging from 0.47 to 1.67 mm. It was carried out by driving four rollers each attached to the side wall of the tube, leading to the bending of the wall to a constant curvature and, at the same time, the tearing along the four corners. By pre-cutting some corners to a different length, the tearing energy involved was determined. It was found that the tearing energy per unit torn area (R) may be related to the ultimate stress of the material (σu) and the fracture strain (ef) as R = 8.8σuef for mild steel and R = 37.2σuef for aluminium tubes; here the two coefficients have length dimensions in mm.

Non-destructive evaluation of plasma sprayed functionally graded thermal barrier coatings

Acoustic emission (AE) as a non-destructive evaluation technique has recently been used in a number of studies to investigate the performance and failure behavior of plasma sprayed thermal barrier coatings. The mechanism of coating failure is complex, especially when considering the composite nature of the coating. In the present paper, the thermal shock tests with in situ acoustic emission are used to study the cracking behavior of plasma sprayed functionally graded thermal barrier coatings. Each thermal cycle consists of 8 min heating in the furnace at 1000°C and 8 min cooling from 1000°C to the room temperature by a compressed air jet. The AE signals are recorded during the quench stage. Three, four and five layer functionally graded coatings have been tested. The results show that the five layer functionally graded coatings appear to have the best thermal shock resistance in the specimens tested, because of the gradual changes in material properties. Higher AE energy counts and cumulative counts recorded by the tests are associated with the macro-crack initiation and growth. On the other hand, micro cracking and phase transformation only give rise to lower AE signals.

Forming near net shape free-standing components by plasma spraying

Abstract The capability of plasma spraying for producing near net shape, free-standing components of metallic, ceramic and layered composites with alternate ceramic and metallic layer systems was studied. A simple and efficient mold mandrel with attachments to release the sprayed component was designed and machined. The plasma spraying was carried out, with powders of nickel and zirconia–8 wt.% yttria to form shapes such as nozzles, cones and cylinders. The plasma spray parameters and the mold mandrel operation were standardized by trial and error to produce the near net shapes. The microstructure, fracture morphology and microhardness of the sprayed components were studied. The study showed that the shapes constituting metallic, ceramic and layered composites with alternate layers of ceramic and metallic materials could be successfully formed by plasma spraying.

Influence of process parameters on the deposition footprint in plasma-spray coating

This paper presents an investigation of the influence of plasma spray process conditions on the in-flight particle behavior and their cumulative deposition to form a coating on the substrate. Three-dimensional computational fluid dynamics (CFD) analyses were performed to model the in-flight particle behavior in the plasma-spray process and their deposition on the substrate. The plasma spray was modeled as a jet issuing from the torch nozzle through the electrical heating of the arc gas. In the model, particles were injected into the plasma jet where they acquired heat and momentum from the plasma, some got melted and droplets were formed. By means of a droplet splatting model, the particle in-flight data generated by the CFD analyses were further processed to build up an imaginary three-dimensional deposition profile on a flat stationary substrate. It is found that the powder carrier gas flow rate influences the particle distribution on the substrate by imparting an injection momentum to the particles that were directed radially into the plasma jet in a direction perpendicular to the plasma jet. The larger sized particles will acquire higher injection momentum compared with the smaller sized particles. This causes particle distribution at the substrate surface that is elliptical in shape with the major axis of ellipse parallel to the particle injection port axis as illustrated in Fig. 1. Larger particles tend to congregate at the lower part of the ellipse, due to their greater momentum. The distribution of particle size, temperature, velocity, and count distribution at the substrate was analyzed. Further, based on the size and the computed particle temperature, velocity histories, and the impact sites on the substrate, the data were processed to build up a deposition profile with the Pasandideh-Fard model. The shapes of deposition profiles were found to be strongly driven by the segregation effect.

Role of particle injection velocity on coating microstructure of plasma sprayed alumina — validation of process chart

Abstract The role of particle injection velocity and size on the microstructure of plasma sprayed alumina coating was studied with reference to the process chart formulated by the computational fluid dynamics (CFD) study reported earlier. For this, three grades of alumina powder with mean sizes of 25, 40 and 76 μm were utilized and sprayed under different injection velocities. The coating was characterized using scanning electron microscopy (SEM) for surface structure, X-ray diffractometry (XRD) for phase changes and Micro-Vickers hardness test measurements. The particle states were found to change depending on the injection velocity and size in correlation with the process chart, as revealed in the form of unmelted particles or well spread splats in the coating surface microstructure. This was further confirmed by the XRD results which showed a steady decrease in α-alumina content with increase in the injection velocity indicating a rise in the extent of complete melting of alumina particles in the case of 25 μm mean size powder. The micro-hardness also varied considerably depending on the injection velocity, due to the changes in the extent of the melting of alumina particles. A close correlation between the particle states depicted in the process chart and the coating microstructure was observed indicating the validity of the process chart and its suitability to put into actual coating applications.

A Proposed Process Control Chart for DC Plasma Spraying Process. Part II. Experimental Verification for Spraying Alumina

The role of particle injection velocity in influencing the nature of alumina coatings obtained by plasma spraying was studied. Previously reported process chart obtained by computational fluid dynamics (CFD) study on the particle states of alumina with respect to particle injection velocity and size was verified experimentally. For this purpose, alumina particles of three different size ranges with a mean size of 25, 40, and 76 μm were subjected to different injection velocities. The coating obtained was analyzed for cross-sectional microstructure and thickness by optical microscopy. In addition, the role of particle injection velocity and size in influencing the coating-deposition efficiency was studied. The experimental results agreed well with the CFD results, which had indicated the dependence of particle trajectory in the plasma plume on the particle injection velocity and size leading to the changes in the extent of melting. While a higher coating thickness and deposition efficiency was obtained with 25-μm particles, with further increase in particle size, a reverse trend was observed. This was attributed to the changes in heat-transfer characteristics of the particles with size, which governed the coating buildup and deposition efficiency.

Imaging diagnostics study on obliquely impacting plasma-sprayed particles near to the substrate

Real time close-up images of in-flight particles plasma sprayed onto a substrate and in freestream condition (without substrate present) are captured. Besides the images, particle behavior in terms of temperature, velocity, and heading are measured by the Spray Watch particle imaging diagnostics system. The monitoring and measurement of particle behavior have been performed for substrates inclined at various angles to investigate the effect of the substrate on particle behavior. The close-up images show that particles propelled from the torch travel with high momentum and are not affected by the substrate and inclination angle. Quantitative analyses of the particle average velocity and heading data with and without the different inclined substrates also lead to similar conclusions. The particle velocity is resolved into tangential and normal velocity components parallel and perdendicular to the substrate, respectively. The tangential velocity component controls the degree of splat elongation into elliptical shape from the circular shape seen in perpendicular impact. This is of practical importance in industrial spraying of engineering components of complex curvatures. A higher tangential velocity component also implies that more powders are lost through rebounding and overspraying and thus reducing the deposition efficiency. The normal velocity component decreases when substrate inclination increases, which tends to weaken the coating adherence.

Biaxial Ratcheting and Cyclic Plasticity for Bree-Type Loading—Part II: Comparison Between Finite Element Analysis and Theory

In Part 1 of this two part (Ng and Nadarajah, 1996), the results of an extensive program of finite element analyses were described. The problem being considered is the phenomenon of ratcheting and cyclic stress-strain hysteresis loop behavior in a thin-walled cylinder subject to cyclic thermal stress and sustained internal pressure. The purpose of Part 2 is to compare the finite element results with two analytical solutions and review the applicability of the latter as a design procedure for assessment of these types of structures. The comparison shows that the ratcheting to shake-down boundaries based on F.E. and analytical models are in close agreement. The hoop ratcheting rates predicted by the uniaxial model enveloped the F.E. and biaxial models, while for the axial ratcheting rates, the F.E. results are upper bound.

The Support of Horizontal Vessels Containing High-Temperature Fluids—A Design Study

This paper investigates the behavior of horizontal cylindrical vessels, subjected to thermal loading by high-temperature fluid, where the saddles are fixed to the supporting structure. In order to determine an optimum saddle design, three widely used saddle configurations, with differing saddle heights and top saddle plate extensions, are explored. Thereafter, one of the saddle designs is selected to illustrate a decoupling procedure, for the radial and axial expansions, whereby design charts are obtained to derive the maximum stress values for a range of vessel geometries. The finite element approach, using linear elastic, small displacement analysis, is used throughout.

An alternative way to support horizontal pressure vessels subject to thermal loading

When storing liquids at high temperature in horizontal vessels, the current design methods aim to minimise the thermal stresses by introducing a sliding surface at the base of one of the twin saddle supports. However, regular site maintenance is required to ensure that adequate sliding is achieved. This may be difficult and costly to carry out. The aim of the present work, therefore, is to dispense with the sliding support and to provide saddle designs which, although fixed to the platform or foundation, do not result in the storage/pressure vessel being overstressed when thermal loading occurs. This paper provides general recommendations for the most appropriate saddle geometries, and details the way in which design-by-analysis and fatigue-life- assessments may be carried out using the stresses that arise from these designs.