Hugh M. Williamson

Control of shear cutting noise—effectiveness of enclosures

Abstract The effectiveness of noise abatement enclosures for roll former shears in the sheet metal industry has been investigated under the production environment. Measurements were taken over a considerable period of time on various machines before and after the installation of noise enclosures. Three generic noise abatement enclosures, which represent different types of construction and design, were assessed. Results indicate that noise abatement enclosures, when used to enclose roll former shears, achieve a noise reduction of 4–5 dB(A) at the operator’s position, virtually independent of the designs and materials. It has been shown that the rather poor performance of enclosures in this application is essentially due to structure-borne paths whereby vibrations are transmitted through the metal sheet product itself from the inside of the enclosure to the outside.

Surface mobility over a square contact area of an infinite plate: experimental measurements and numerical prediction

Abstract Both finite element and experimental techniques were investigated for the determination of the surface mobility of an “infinite plate” excited over a square contact area with a uniform conphase force distribution using the effective point mobility concept. Various factors affecting the accuracy of the finite element calculations are discussed. Comparison among the numerical, experimental and theoretical results shows similar trend in the behaviour of the surface mobility. Generally, the surface mobility decreases with increase in width-based Helmholtz number and the first two dips in the surface mobility are separated by approximately π in the Helmholtz number.

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.

Control of Shear Cutting Noise: Effectiveness of Passive Control Measures

In an attempt to reduce noise from roll former shears three noise abatement enclosures of different designs and sheet dampers were assessed. Results indicate that only a noise reduction of 5dB can be achieved at the operator position by noise abatement enclosures, virtually independent of their designs and materials. This rather poor performance of enclosures is basically caused by structure-borne paths whereby vibrations are transmitted through the metal sheet product itself from the inside of the enclosure to the outside. Sheet dampers which have been designed to reduce the impact induced vibrations of the sheet product achieves a similar noise reduction as the enclosures but at about one quarter of the cost.

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.

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.