John P. Baird

Fringe-orientation estimation by use of a Gaussian gradient filter and neighboring-direction averaging.

We analyze the effect of image noise on the estimation of fringe orientation in principle and interpret the application of a texture-analysis technique to the problem of estimating fringe orientation in interferograms. The gradient of a Gaussian filter and neighboring-direction averaging are shown to meet the requirements of fringe-orientation estimation by reduction of the effects of low-frequency background and contrast variances as well as high-frequency random image noise. The technique also improves inaccurate orientation estimation at low-modulation points, such as fringe centers and broken fringes. Experiments demonstrate that the scales of the Gaussian gradient filter and the direction averaging should be chosen according to the fringe spacings of the interferograms.

Three-dimensional displacement measurement by a holospeckle interferometry method

The three-dimensional displacement of a structure may be measured by holographic interferometry and speckle interferometry. Under the general heading of holographic interferometry a number of distinct techniques are possible. These include the zero-fringe method, which uses three separate holographic plates, the fringe-localization method, and the fringe-counting method, all of which require a complicated recording and analysis system and high stability. These requirements make the techniques unsuitable for use in most industrial settings. In this paper we present a holospeckle interferometry method that couples objective speckle and reflective holographic interferometry and that is capable of obtaining the three-dimensional displacement of an opaque object with good visibility and resolution by a single holographic plate. Being inexpensive and portable, the recording and analysis system can readily be adapted to industrial use.

Life determination of riveted aircraft structure by holographic NDE

In a project funded by the Federal Aviation Administrations (FAA) Aging Aircraft Program, a Portable Holographic Inspection System (PHITS) has been further developed. The technique involves taking a double exposure white light reflection hologram of aircraft structures. Each exposure is taken at a slightly different load state, and the resulting interferogram shows the deformations that occur between the two load states. Results showed that the rivets in a simple lap joint, designed to simulate the longitudinal lap splice on a Boeing 737, behaved in two distinct and easily recognizable modes. The first mode occurred at low loads and was an indication that friction forces between the two sheets of the lap joint dominated the load transfer mechanism. Indications were that the second mode related to higher loads for which the friction forces played a much lesser role. The load at which the changeover begins to occur has been called the critical load. Preliminary experiments showed that structures with a high value of critical load had a fatigue life of order ten times that of a normally fastened splice. Critical load can be readily determined in the field using the PHITS system. Research designed to establish the relationship between fatigue life and critical load is continuing. An understanding of that relationship could lead to a technique capable of fatigue life determination in typical aircraft structures.

Optical examination of load transfer in riveted lap joints using portable holographic interferometry

In mechanically fastened single lap joints, such as those employed on aircraft fuselage skin splices, there are two distinct mechanisms of load transfer. At low values of load the transfer occurs primarily through friction between the component sheets while at higher loads the load is transferred by friction as well as through bearing at the fasteners. The load level at which the bearing mode of load transfer comes into action significantly affects the fatigue life of the joint, since the fasteners are stressed only at loads above this threshold load value. The portable holographic interferometry testing system (PHITS) is a robust, portable and sensitive non-destructive inspection system which produces contours of relative out of plane displacement by the method of superposition. PHITS is applied here to monitor the load transfer mechanism and identify the threshold at which the bearing mode comes into effect. In the friction mode there is no relative displacement between the fasteners and the skin panels. In the bearing mode the fasteners are loaded, causing a distinct tipping of the rivets, which is readily observable in the fringe pattern of deflection contours recorded by the holographic system.

Further understanding of aged composite and adhesively bonded structures

As the application of advanced composite materials and adhesively bonded components becomes increasingly numerous in aircraft structures, so is the number of aircraft containing such structures that can be classified in the aging aircraft category. The effect of environmental and in- service aging of such structures is not well known or understood, neither have NDE techniques been able to satisfactorily qualify and quantify the loss of structural integrity due to the aging process. This paper will present the latest developments in the practical use of a field portable holographic interferometric testing system. The system results, known as holographic interferograms, provide a better understanding of how a structure is behaving under realistic loads in the presence of defects, damage and material property aging. The system has been applied to a variety of defects in composite and adhesive bondlines, as well as artificial environmental aging of these materials. The holographic interferograms produced form these investigations will be briefly reviewed and their impact on structural integrity of the component discussed.

Beam-propagation method applied to the deflection of a laser beam across a hot plate subject to jet impingement

We describe an application of the beam-propagation method of Fleck et al. [Appl. Phys. 10, 129-160 (1976)], whereby we consider a laser beam propagating initially parallel to a hot plate subject to jet impingement; we also compute the intensity profile of the beam as it emerges from the boundary layer. A comparison with a calculation based on ray tracing illustrates the significant role of wave effects (as treated by the beam-propagation method) in this case. We describe a method for measuring the rate of heat transfer q????#x0307???? in the viscous sublayer that is applicable in the ray-tracing limit but that cannot be utilized here (other than to provide an upper limit to q????#x0307????) because of the substantial wave effects.

Real-time reflective holographic interferometry

A real-time reflective holographic interferometry system is described. It overcomes the vibration isolation problems associated with conventional real time holography, and presents living-fringe patterns which can give dynamic deformation maps of an object under study. Two set-ups used to produce real-time reflective holographic interferograms are presented. Its optical arrangement is very simple, comprising a low-cost C.W. laser (10mW He-Ne), a shutter and a spatial filter mounted on a camera tripod. This real-time technique is applied to measure the local buckling of a riveued lap joint.