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Using Multivariate Statistics

A fast-Fourier-transform method of topography and interferometry is proposed. By computer processing of a noncontour type of fringe pattern, automatic discrimination is achieved between elevation and depression of the object or wave-front form, which has not been possible by the fringe-contour-generation techniques. The method has advantages over moire topography and conventional fringe-contour interferometry in both accuracy and sensitivity. Unlike fringe-scanning techniques, the method is easy to apply because it uses no moving components.

Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry

A self-scanned 1024 element photodiode array and minicomputer are used to measure the phase (wavefront) in the interference pattern of an interferometer to lambda/100. The photodiode array samples intensities over a 32 x 32 matrix in the interference pattern as the length of the reference arm is varied piezoelectrically. Using these data the minicomputer synchronously detects the phase at each of the 1024 points by a Fourier series method and displays the wavefront in contour and perspective plot on a storage oscilloscope in less than 1 min (Bruning et al. Paper WE16, OSA Annual Meeting, Oct. 1972). The array of intensities is sampled and averaged many times in a random fashion so that the effects of air turbulence, vibrations, and thermal drifts are minimized. Very significant is the fact that wavefront errors in the interferometer are easily determined and may be automatically subtracted from current or subsequent wavefrots. Various programs supporting the measurement system include software for determining the aperture boundary, sum and difference of wavefronts, removal or insertion of tilt and focus errors, and routines for spatial manipulation of wavefronts. FFT programs transform wavefront data into point spread function and modulus and phase of the optical transfer function of lenses. Display programs plot these functions in contour and perspective. The system has been designed to optimize the collection of data to give higher than usual accuracy in measuring the individual elements and final performance of assembled diffraction limited optical systems, and furthermore, the short loop time of a few minutes makes the system an attractive alternative to constraints imposed by test glasses in the optical shop.

Digital wavefront measuring interferometer for testing optical surfaces and lenses

Approaches to learning

Biomechanics and wound healing in the cornea

The manufacture of aircraft components requires quality control procedures and mechanisms to identify potentially defective units during manufacture, testing stages as well as during service life. Defects can be formed due to random or systematic errors within the manufacturing process and comprise of tiny fatigue fractures within the wheel structure. Use of such defective wheel and tyre units by an aircraft operator could for example least to growth of these fractures, which in turn can generate sudden unexpected catastrophic failure on landing. Qualification and approval of all wheel units is mandatory under UK Civil Aviation Authority (CAA) and US Federal Aviation Authority America (FAA) regulations as well in the aircraft manufacturer standards. The regulatory authorities and manufacturers were requesting improvement in testing and traceability of, analysis methods during manufacture and during routine in service maintenance checks. They were thus considering alternative technologies. Speckle shearing interferometry is a non-contact wholefield sensor system used primarily within the aerospace industry, for analysis of defects in composite materials, including the analysis of manufacturing defects in aircraft tyres. This methodology has traditionally been used qualitatively to identify the location of faults such as disbands. Through laser technology and image processing technology advances, it has become more common for this type of instrumentation to deliver quantitative data. However there has been concern about data integrity, data quality and issues of defects being missed due to instrument sensitivity.

Residual lifetime prediction in aerospace structures using wholefield laser strain techniques

Crack formation during the laser surface treatment of materials with low coefficients of thermal conductivity values, such as ceramics, is one of the most intractable problems encountered in the field of laser materials processing. Likewise, porosity formation during the laser surface treatment of ceramics presents a problematic subject. Any analysis, therefore, of these issues would be most informative. It is widely accepted that to prevent cracking in ceramics during laser surface treatment some form of pre- and post-treatment heating must take place. This study examines the formation of cracks and porosities during the CO2 laser surface treatment of a magnesia partially stabilized zirconia (MgO-PSZ) ceramic when pre-treatment heating was applied externally and post treatment heating was achieved using firstly a plain diverging beam, and secondly using a diffractive optical element (DOE) to produce a “tailored” laser beam profile. The results of this study revealed that the pre-treatment heating regime and the laser beam profile played a vital role in determining the occurrence, number and scale of microcracks and porosities in the CO2 laser surface treated MgO-PSZ.Crack formation during the laser surface treatment of materials with low coefficients of thermal conductivity values, such as ceramics, is one of the most intractable problems encountered in the field of laser materials processing. Likewise, porosity formation during the laser surface treatment of ceramics presents a problematic subject. Any analysis, therefore, of these issues would be most informative. It is widely accepted that to prevent cracking in ceramics during laser surface treatment some form of pre- and post-treatment heating must take place. This study examines the formation of cracks and porosities during the CO2 laser surface treatment of a magnesia partially stabilized zirconia (MgO-PSZ) ceramic when pre-treatment heating was applied externally and post treatment heating was achieved using firstly a plain diverging beam, and secondly using a diffractive optical element (DOE) to produce a “tailored” laser beam profile. The results of this study revealed that the pre-treatment heating regime ...

An analysis of crack and porosity formation in laser surface treated magnesia partially stabilized zirconia (MgO-PSZ) and methods for alleviation

A technique of simultaneously measuring all three Cartesian components of displacement in holographic interferometry is proposed. A single camera is used to record four interferograms of different sensitivities on a single video frame with the aid of mirrors mounted behind the holographic plate. A transformation algorithm has been developed to transform the two-dimensional fringe centres on the interferograms onto the object surface in spatial coordinates. The transformation takes into account perspective distortion and lateral inversion of the images. The accuracy of the transformation is verified by transferring grid points on a graph paper fixed around a cylinder. The measurement technique has been verified by measuring the surface displacement of a thermally deforming cylinder and the results are compared with the prediction of a finite element model of the cylinder.

Simultaneous measurement of x, y, z components of displacement in holographic interferometry using a four-mirror technique

Optical whole-field testing techniques have been carrots dangled in front of engineers' noses for a considerable period of time. The promise of acquiring meaningful data without upsetting the component nor its environment, has significant attractions. ESPI technology has been modified and pursued with these goals in mind. This paper presents some of the recent work containing several developments which now make the engineering realisations a near term possibility. An overview of the correlation imaging mechanism is presented with a discussion on how this principle type of optical interferometer can be configured to provide the data necessary for analytical use. Attempts to produce instrumentation able to function outside the laboratory have required replacement of continuous wave lasers with Nd.YAG pulsed lasers. The new pulsed lasers are able to be combined with the computer based fringe pattern analysis which has been produced to suit the requirements of the engineer. Experimental results using such equipment are presented and further work is included which demonstrates the ability for speckle interferometry to produce three-dimensional analysis with the data being presented in conventional cartesian form.

Structural Analysis Using Phase-Stepped, Double Pulsed ESPI

Secondary school, age range 11–14, technology and engineering education in England has been delivered mainly within Design and Technology (D&T). This inadvertently makes D&T teachers responsible for pupils’ engineering education and motivation. This paper analyses D&T teachers’ (N = 33) technology subject knowledge through self-assessment competency questionnaires, before and after developing a Science, Technology, Engineering and Mathematics (STEM)-focused project of their choice for their classroom. Participants were least confident in teaching the areas of technology that required mathematics and scientific knowledge. The results analyse a suggested misalignment between teachers’ Creative Arts background subject knowledge compared to the technology subject knowledge required for engineering education. Suggested causes of this issue are Initial Teacher Training standards and curriculum flexibility, not teacher capability. The paper concludes that teachers have been unaware of some elements of STEM education and that continuing professional development interventions are required to assist teachers and improve their engineering knowledge in order to better equip their pupils for engineering.

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John R. Tyrer

Papers

Residual lifetime prediction in aerospace structures using wholefield laser strain techniques

An analysis of crack and porosity formation in laser surface treated magnesia partially stabilized zirconia (MgO-PSZ) and methods for alleviation

Simultaneous measurement of x, y, z components of displacement in holographic interferometry using a four-mirror technique

Structural Analysis Using Phase-Stepped, Double Pulsed ESPI

The effect of teacher’s confidence on technology and engineering curriculum provision