A wide variety of optical techniques are available for making full field qualitative measurements of surface displacement. Their application to actual problems in an industrial environment leads to new and often unforeseen difficulties. This paper will describe recent experience gained using holographic interferometry, electronic speckle pattern interferometry CESPI), moire interferometry, and high resolution moire photography for measuring in-plane displacement, and hence strain. Work has been carried out in engineering studies and laboratory exercises, making direct comparisons between the techniques in order to highlight their relative merits in different applications.

This paper takes as its starting point the Bvs H (hysteresis) characteristics of steel in different directions relative to the applied stress. It predicts, and shows by means of tests, that there is a difference in directions between flux density B and the field strength H because stress makes steel anisotropic. This difference in direction (called the "rotation of magnetisation)" is the basis of a non-destructive method of estimating stress. An important point, that is emphasised now, is that the difference of principal stresses is measured, and not necessarily the absolute values of stresses.

Integrity assessment of electrical power generating plant requires a knowledge of both operating and residual stresses. For the more highly stressed components the commonly used mechanical methods of measurement, based on the observed strain changes when stressed material is removed, must be regarded as destructive and other, non-destructive techniques have to be used. Within the CEGB two non-destructive methods, based on X-ray diffraction and magnetic Barkhausen noise measurements, are employed. This paper describes the basis of these two techniques and gives two examples of their application to stress measurement on large components.

A new technique for the measurement of strain has been developed over the past few years. The sensor is a material, usually a highly crystalline polymer, in which the frequencies of the vibrational modes are proportional to the strain in the material. The frequencies of vibration of the material are measured using the light scattering technique of Raman spectroscopy. The sensor may be interrogated through any transparent medium such as epoxy resin or optical fibres.

The strain gauge using a speckle correlation method is described. A diffuse object is illuminated with a narrow laser beam and the speckle displacement caused by surface deformation is optoelectronically detected at two symmetrical scattering angles. For this purpose, the hardware correlators, which detects the cross-correlation peak between successive frames of a linear image sensor, have been newly developed. High stability in the measurement has been realized, which could not be attained by using the spatial filtering detectors used earlier. We applied the gauge to extension of a metal plate comparing with a resistance strain gauge and also to extension of polymer films comaring with a displacement transducer. The results are in good agreement at loading velocity between a few millistrain/s and a few tens of millistrain/s. The minimum sensitivity is equal to a few tens of microstrains using linear photodiode arrays of pitch 15 μm. There is no upper limit on the strain to be measured because an incremental strain is integrated. The gauge length, which is given by a beam diameter, is an order of one millimeter.

Stress measuring techniques have, for the last fifty years or so, been centred upon single point methods utilising the resistance strain gauge. Measurements over relatively large areas have been possible using photoelastic coatings and brittle lacquers but truly non-contacting techniques have had to await developments in optical technology to achieve widespread use. This paper reviews available techniques of stress measurement and makes comparison between them.

This paper describes an experimental technique whereby the behaviour of coatings, used to enhance surface properties of structures for thermoelastic stress analysis, may be assessed under a wide range of conditions. Established theoretical models are discussed and a novel description of the thermal response of coatings, with respect to changes in the substrate temperature, is presented. This description is then developed to include effects due to the thermoelastic response of the coating material and also the opacity of the coating to infra-red radiation. This work has substantial and wide ranging implications for the application of the thermoelastic technique. These are discussed briefly.

The determination of separate values of surface stresses from the SPATE stress sum data for the case of a small rotationally symmetrical pressure vessel end is described and discussed.

An enhanced theory for Thermographic Stress Analysis of isotropic materials is developed. This theory encompasses nonlinear thermoelastic, thermoplastic and specimen motion effects. With this theory, several new applications for Thermographic Stress Analysis are possible including cyclic plasticity analysis, high-temperature stress analysis, and residual stress analysis. Examples of each of these applications are presented.

The paper outlines a completely new technique, which has been given the name of TERSA, an acronym of Thermal Evaluation for Residual Stress Analysis, which could provide the basis of an instrument for the remote, non-contacting and non-destructive determination of static and residual stresses in engineering components, assemblies and structures and be applicable to both ferrous and non-ferrous metals and also to plastics. Some experimental work which has been carried out at ARE Portsdown over the past few years is described and results are presented which confirm the principle of the proposal.

The science of stress analysis has been applied extensively to human skeletal structures for many years by a variety of techniques and with varying degrees of success. The most modern of these techniques is computer Finite Element analysis (FE analysis) where the speed and power of the digital computer in combination with very sophisticated software can generate complex models and analyse both static and dynamic stress values. However the FE method in common with other techniques is only as good as the representative model and the knowledge of the many parameters required and must be evaluated against known and proven data.

The application of fracture mechanics methods for assessing the significance of defects in welded structures or components may require accurate determination of stress intensity solution for typical joint configurations (1). At present, numerical methods such as finite element analysis are most widely used (2). Thermoelastic technique (SPATE) is a new experimental technique based on the measurement of infra-red radiant flux emitting from the surface of a body under cyclic stress. It has been shown from Refs. 3 and 4 that accurate stress intensity solutions can be derived from SPATE results obtained by scanning a cracked body with simple geometry under mode I and mode II loading. Hence the SPATE method offers an attractive alternative to numerical analysis, or a means of validating the numerical methods. In addition, it is anticipated that the new technique could be used for analysing a cracked body with complex geometries not easily analysed by numerical methods, for example semi-elliptical surface cracks in plates and cylinders, or tubular connections with weld toe cracks.

Fatigue cracks have occurred on fabricated "T"-piece welds on some of the pipes supplying a bearing block labyrinth seal with cooling water. In some instances the weld quality was poor, which gave stress concentration factors (SCF) higher than would be expected for the component geometry. In addition, the vibration damping inherent in the design was extremely low, leading to a significant magnification in stress under resonant conditions. A programme of work was initiated to determine the cause of the cracking and to implement design changes to alleviate the problem; a section of this work involved the use of the CEGEr:s SPATE 8000 system to determine SCFs for "T"-piece/pipe weld geometries.

The thermoelastic effect, first quantified by Lord Kelvin, describes the change in temperature of a body as it undergoes a change in stress under adiabatic conditions. The thermoelastic equation as described by Kelvin is given in eqn(1.1),

This paper discusses digital signal analysis techniques suitable for the measurement, by thermoelastic techniques, of full-field surface stress distributions on structures subjected to loading conditions which may not be of a uniform cyclic nature. A computer-aided test system has been programmed to acquire and analyse both sinusoidal and non-periodic signals from a SPATE 8000 infra-red scanning radiometer. The use of the system to measure stress and estimate damping under modal conditions is described. Sinusoidal data analysed by a digital technique together with results from a standard SPATE system are presented. The digital technique produced a clear reduction in edge effect compared with the data from the SPATE 8000 analogue instrumentation. Signal analysis procedures are presented which are suitable for the estimation of the quasi-static and modal behaviour of structures subjected to non-periodic, variable-amplitude loading. Stress patterns pre-dicted by the finite element method are also displayed for two simple structures and compared with thermoelastic data measured using wide-band random loading.

When attempting to control vibration in structures, the identification of the significant paths of vibration transmission from sources, through the structure to some point of interest is desirable. The concept of power flow, a vector quantity related to vibration energy transport, has been developed to allow evaluation of the significance of the various transmiision paths in a typical, complex structure. For flexural-wave, power flow measurements in beam-like structues, existing schemes employ two accelerometers, which suffer from inter-transducer phase differences, inducing measurement errors, particularly significant in the prescence of standing waves. This paper proposes a real-time, laser power flow meter with negligible inter-transducer phase differences and thus improved accuracy. Laser point velocity sensors offer remote, non-contact vibration measurements whose use will further extend employment of the technique to hot, light or inaccessible surfaces. Further improvement in measurement accuracy and simplification in the measurement scheme may be obtained by utilising a laser velocity gradient system to obtain normal-to-surface velocity spatial derivatives. A scheme is proposed for measuring longitudinal wave power flow in beams, in the prescence of flexural waves.

Designers of rotating machinery seek to limit torsional vibrations in order to ensure smooth transmission of power and to minimise problems of coupling and bearing wear, excessive noise, fatigue etc. The principal disadvantage of traditional methods of torsional vibration measurement [1] is that they involve physical contact with the rotating member, requiring both potentially costly machinery "downtime" and space for installation in locations where access may already be restricted. Development of laser-based transducers [2] has produced solutions to these problems. In particular, the laser torsional vibrometer [3] provides non-contact measurement on rotating components of arbitrary cross-section and has the additional advantage of insensitivity to solid body motions of the component.

For more than twenty years, the laser has provided the opportunity of making non contact velocity and vibration measurements at long range and with very great sensitivity using the heterodyne or Doppler principle. For a given wavelength, the sensitivity depends on the amount of light received back from the target and the measurement bandwidth. Thus, at ranges up to 300m, sub micron sensitivity can be obtained in bandwidths up to 1 Khz, while at the other extreme at very close range, picometer vibration levels of mirror like targets can be measured in bandwidths of several Mhz.

A standard 660MW turbo-alternator, operated by the CEGB, runs at an energy conversion efficiency of about 38%. In addition to the 660MW electrical power, 600MW of waste thermal power is generated which has to be dissipated via water cooled heat exchangers. A typical 2000MW station has a requirement of about 1.3 billion gallons of cooling water per day. This is more than the daily throughput of most of our rivers and so inland stations are equipped with cooling towers to dump heat from the coolant.

An optical displacement transducer using pulsed laser light and a triangulation principle has been developed to measure the lateral vibration of Advanced Gas-cooled Reactor (AGR) fuel stringer tie bars in a laboratory pressurised flow rig. Three pairs of orthogonally mounted units provide positional data to a resolution of < lOμm in the frequency range 0-600Hz.

An alternative to strain gauge technology is proposed using the adoption of a laser vibrometer to measure dynamic surface response. This data can then be applied to evaluate the bending strain in the structure. The simple theory is presented for beams and plates undergoing discrete frequency harmonic motion and random vibration. The sources of errors are analysed and a judgement on the method is given for future applications.

A new method for extracting quantitative information from a double exposure holographic interferogram is presented. Dual reference beams are used to produce continuously variable phase differences between the two images of the object at the recording stage of the hologram. Image reconstruction at three known phase differences via a CCD camera and digital framestore allow automatic image processing methods to calculate the three dimensional surface deformation.

A comparative study between several full field optical techniques for the visualization of the vibrational modes of aero engine components is presented. The methods considered are holography, electronic speckle pattern interferometry (ESPI) and laser Doppler velocimetry. The relative merits of each technique are assessed with regard to their application in an industrial environment.

There are many situations in vibration analysis where it is necessary or desirable to make three-dimensional measurements. By using three different illumination geometries around a single imaging system, Electronic Speckle Pattern Interferometry (ESPI) can be used to measure the orthogonal components of vibration amplitude independently. These can be combined to determine the three-dimensional mode shape. The theory is given for making quantitative measurements, and a practical system is described. Examples of experimental results are presented for volume vibrations of a thick cylinder, identification of vibration modes of a turbocharger blade, and observation of in-plane modes in a thin plate.

Electronic Speckle Pattern Interferometry (ESPI) used with cw lasers has so far proved useful only as a laboratory research tool, requiring restrictive environmental conditions and specialist knowledge at the data interpretation stage. A pulsed laser eliminates the need for such requirements mainly due to its shorter pulse duration. The incorporation of a pulsed laser into ESPI provides the high quality fringe data needed to meet the stringent demands of industrial modal analysis. Results obtained with ESPI incorporating a pulsed Neodinium/YAG laser (working at 0.532 μm) are reported in this paper, where the technique was applied to deformation and vibration studies of object areas up to 0.5 m2 in size. The fringe patterns produced are analysed by a phase reduction method to yield out-of-plane amplitude data. The method is easily extended to in-plane sensitivity with very little additional optics. These data may be presented in the form of three dimensional surface displacement maps which may be manipulated by computer for combination with other modal analysis equipment. With the introduction of pulsed laser and a dedicated fringe analysis software and hardware package, ESPI has -now become a commercially viable industrial measurement and analysis tool. A complete description of the system is presented here emphasizing the relevance of the above points and their direct practical applications.

The application of structural dynamics principles and procedures to power station plant integrity assessment and condition monitoring encompasses both theoretical and experimental methods. In recent years structural dynamic response measurement has been developed to include non-contacting full-field measurement techniques such as laser holography, Stress Pattern Analysis by measurement of Thermal Emission (SPATE), and more recently Vibration Pattern Imaging (VPI). These full-field techniques have complemented the conventional point measurement methods with a degree of structural dynamic visualisation which was historically felt to be unachievable. This paper presents an assessment of one of the latest techniques, Vibration Pattern Imaging, applied to modal testing utilising a specially designed 'T' section plate as the test specimen. A comparison of the dynamic behaviour of the plate was performed using the following techniques: (i) Vibration Pattern Imaging, (ii) finite element modelling, (iii) frequency response function measurement, (iv) pulsed holography, and (v) Stress Pattern Analysis by measurement of Thermal Emission. In addition, the capability of the VPI to operate as a non-contacting vibration transducer for use in a standard modal analysis is compared with the performance of a conventional piezoelectric accelerometer.

The advantages of Scanning Laser Doppler Vibrometry as a non-contacting method of vibration measurement are reviewed, with particular emphasis on industrial vibration measurement and modal testing. The measurement principle embodied in commercially available equipment is described, together with external features and user facilities incorporated in the system. Application examples drawn from work carried out in industrial and university laboratories are used to illustrate the broad capabilities of the technique.

Building a computer model that simulates the dynamic behaviour of a structure is one of the most powerful techniques available to the vibration control engineer. For, having developed a model that correctly predicts the vibration problem, he can easily use it to explore the effect of many different trends in design which would be difficult, lengthy and expensive to determine by hardware changes. In addition, he can link his model to optimisation algorithms which will find the best combination of design variables - a goal not possible to achieve by trial-and-error methods.

In dB(A) terms most practical structures (machines, engines, etc.) radiate the major proportion of their noise in the mid frequency range approximately 800 - 2500 Hz. In general, this is due to multimodal vibrations with a relatively high modal order and as such modelling using finite element or modal analysis techniques can be excessively complex. A much simpler, less detailed approach, using energy balance methods (Statistical Energy Analysis, etc.) is much more applicable to this type of vibration.

Double-pulsed holography is an established technique in which the interference pattern between two holographic images shows a fringe pattern which is representative of a vibrating object's response shape. The technique has many industrial applications, a number of which would benefit from an accurate and reliable means of converting the fringe information into numeric data. By the use of a seperate reference beam for each of the measurement pulses it is possible to apply the Quasi-heterodyne technique, also known as fringe-shifting, to convert the fringe information directly into displacement information. This paper demonstrates a numerical simulation of the Quasi-heterodyne process and its use to investigate the error sources which may be present in a real optical system.