Papers by Author: John A. Steel

Abstract: This research aims to characterise and quantify the acoustic emission (AE) generated during the high velocity oxy-fuel (HVOF) thermal spraying process, recorded using piezoelectric AE sensors. The HVOF process is very complex involving high temperature turbulent flow through a nozzle with entrained particles, the projection of these particles, and their interaction with the target surface. Process parameters such as gun speed, oxy-fuel pressure and powder specification affect various characteristics of the coating, including thermal residual stresses; the lamellar microstructure and the topology and geometry of pores, all formed when the fused powder hits the surface, forming “splats”. It is widely acknowledged in the thermal spray industry that existing quality control techniques and testing techniques need to be improved. New techniques which help to understand the effects of coating process parameters on the characteristics of the coating are therefore of value, and it was anticipated that recording the AE produced when the fused particles contact the surface would aid this understanding. As a first stage, we demonstrated here that AE associated with particle impact can, in fact, be discerned in the face of the considerable airborne and structure-borne noise. In order to do this, a new test method using a masking sheet with slits of varying size was developed. Thermal spraying was carried out for a range of spray gun speeds and process parameters. The AE was measured using a broad band AE sensor positioned on the back of the sample as the spot was traversed across it. The results show that the amplitude and energy of the AE signals is related to the spray gun speed, powder used and the oxy-fuel pressure. Using a simple geometrical model for particle impact, the measured AE was found to vary with the energy and number of particles impacting on the sample in a predictable way.

Abstract: This work presents the results of field measurements and laboratory studies carried out with a view to developing ways to monitor rail-wheel interaction using Acoustic Emission. It is known that impact, wear and cracking generate AE and it is therefore expected that axle loads, wheel out-of-roundness, speed and traction will influence the AE generated by an interaction. It is hoped that the extent of the effect might be sufficient to permit a measure of “interaction intensity” that could be used to quantify cumulative damage by wear and contact fatigue. In the field measurements, AE was acquired as a train with 20 moving sources of AE (20 wheels) passed a single sensor position and a laboratory rig has been devised which uses a single wheel whose condition, speed and loading can conveniently be modified. Simulated source tests have indicated that the AE wave characteristics on real rails are similar to those in the laboratory rig. A simplified analytical model, devised for AE waves propagating from a moving source(s), based on a ‘vehicle’ speed and wave damping coefficients, has been compared to measured results. As a wheel rolls towards a sensor and then away from the sensor the measured AE generally rises and falls in a predictable way. The effects of wheel and rail surface features appear to complicate the results by introducing sharp spikes in the signals. The numerical model for AE wave propagation from the moving sources (wheels) shows good agreement with the more slowly changing envelope of the signals.