D. P. Almond

Thermal-wave detection and characterisation of sub-surface defects

Results of the analysis of air-gap and thermal-contact resistance defects are presented. The analysis is illustrated by model predictions of the influence of such defects in a number of important coated and uncoated materials. Experimental results of defects in steel and aluminium coated steel samples are presented and compared with theory. These results show the importance of the lateral extent of the defect and the presence of contact points within the defect.

The influence of coating properties on the sensitivity of thermal-wave-testing techniques

Abstract Thermal wave techniques have been shown to be potentially attractive for non-destructive evaluation of thermal spray coatings. Experimental and calculated thermal wave results for three sprayed coatings (zirconia, nickel chrome carbide and tungsten carbide) confirm the applicability of this technique to these coatings. The effect of translucency, a characteristic of zirconia, was of particular interest and it was found that although it does not adversly affect the sensitivity of the method it can cause an additional shift in the phase of the signal which unless allowed for could give anomalous results.

Modelling of Sample Translucency, Substrate Absorption and Interface Defects for Thermal Wave Infrared Inspection of Thermal Barrier Coatings

Thermal barrier coatings are generally applied as a discrete layer on to a thick metallic substrate or with a bond coat to promote its adhesion to the substrate [1]. Useful coating thicknesses range between 0.1–1 mm and the bond coat layer thickness may be up to 0.1 mm. Typical coating materials include yttria, alumina, calcia or magnesia stabilised zirconia.

Fibre Optic Laser Generation of Thermal Waves and Their Infrared Detection

The technique of photothermal radiometry (PTR) is ideally suited for many industrial non-destructive inspection applications such as non-contact thickness measurements, defect detection, thermal property determination and imaging [1]. In these applications laser powers of typically a few watts have been used to obtain reliable signal or phase angle measurements. These high powered laser sources which often require water cooling hinder the commercial application of the PTR technique especially in in situ examination of engineering components.

Thermal Wave Inspection Employing The Infrared Detection Technique

Thermal wave inspection is introduced with particular reference to the measurement and testing of surface coatings. Measurements are presented which demonstrate the effectiveness of the technique for assessing the thickness of plasma sprayed coatings and for detecting and imaging sub-surface defects. A new miniaturised testing system employing a high power semiconductor laser and a high collection efficiency infrared detector is described.

Analysis of Thermal Wave Reflectivity at Rough Surfaces in Contact

The detection of subsurface structure in opaque and translucent.materials by thermal waves has been widely reported over the past decade for fatigue cracks, subsurface channels, drilled holes, spherical and disc-shaped air-filled defects and partially bonded surface coatings /1,2/. These investigations have led to the development of two models for the characterisation of subsurface “defects”:- n ni) n nthe discrete air gap layer, and n n n n nii) n nthe thermal contact resistance.

The Non-Destructive Evaluation of Surface Coatings by Photothermal Imaging

A method for the NDE of thermal spray coatings by photothermal imaging is described. The principles of thermal wave interferometry are discussed in relation to spray coatings and it is shown how these can be applied to the detection of adhesion defects and coating thickness measurements. Examples of adhesion defects and thickness measurements in refractory and wear resistant coatings are given.

Thermal Wave Characterization of Translucent Ceramic Coatings

Thermal wave interferometry has been successfully used to measure the thickness of metallic plasma-sprayed coatings, and to detect the presence of defects such as debonded regions, inclusions or air gaps [1]. Such metallic coatings are opaque and relatively thermally conductive. In contrast to this, ceramic coatings such as Yttria-stabilised Zirconia (YSZ) or Alumina are translucent and relatively poor thermal conductors. The effect of translucency on the thermal wave response of ceramic coatings is described below.