Dynamic Photoelasticity and Its Application to Stress Wave Propagation, Fracture Mechanics and Fracture Control

Since research in the field of dynamic photoelasticity was initiated by Tuzi(1) in 1928, there has been a continuous development of new and improved high-speed photographic systems. With the development of new higher speed films, more intense light source

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James W. Dally Mechanical Engineering Department University of Maryland

A. Lagarde (ed.), Static and Dynamic Photoelasticity and Caustics © Springer-Verlag Wien 1987

J.W. Dally

248

CHAPTER 1 RECORDING SYSTEMS FOR DYNAMIC PHOTOMECHANICS

1.1

Introduction

Since research in the field of dynamic photoelasticity was initiated by Tuzi( 1 ) in 1928, there has been a continuous development of new and improved high-speed photographic systems.

With the development of new

higher speed films, more intense light sources, ingenious camera designs, and reliable electronic circuitry, continuous improvement has been made in the quality of the photographs of dynamic events with objects or images propagating at high velocity.

Three different photographic systems

have been specially adapted for application involving dynamic photomechanics which provide whole field representation of the data.

All three of

these systems may be considered adequate for photographing high-density fringe patterns propagating at veloeitles as high as 100,000 in/sec (2540 m/sec), but each system exhibits advantages and disadvantages. The first system utilizing a high speed framing camera such as the Beckmann and Whitley was introduced by Feder et al. ( 2 ) in 1956. The of this camera to dynamic photoelasticity has been markedly imprbved by Flynn et al. ( 3- 5 ) and used in a number of interesting

appli~ation

problems.

The optical system for this camera is illustrated in Fig. 1.1

and a view of the back of the assembly is given in Fig. 1.2.

The light

from the object is collected by a relatively long focal length lens and focused on a rotating mirror.

This mirror is driven at high speed by a

light gas turbine and the image of the object is swept about the drum. Framing is accomplished by using a series of slits and relay lenses which receive the image from the rotating triangular mirror.

The slit width is

adjusted so that an image is transmitted to the film plane for about 1/3

Dynamic Photoelasticity

Fig. 1.1

Schematic of Optica1 System of a Framing Camera

Fig. l. 2

View of Back Framing Camera Showing Major Comr ,,ne nts

249

J.W. Dally

250 of the interframe interval.

The relay lenses serve to focus the image of

the object from the mirror to the film plane.

The film usually 35 mm in

width is positioned around the drum-like case for the camera. Framing rates are controlled by adjusting the speed of the turbine. Framing rates R from about 100,000 to 2,000,000 frames/sec are common. Exposure times, te, are re1ated to framing rates by (1.1)

Since exposure times of less than 1

~sec

are usua1ly desirable in

recording stress wave and crack propagation, the camera should be operated at rates exceeding 333,000 frames/sec. The nurober of frames which can be recorded vary with the individual camera manufacture and the model; however, 30 to 80 frames is common. These cameras are expensive with costs usually in the range of $100,000 to $200,000 for a complete system which includes lighting and synchronization circuits. The second high-

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