Institute of Fundamental Technological Research

Measurements of the state of stress in rocks are of vital importance in mining operations. These stresses are very difficult to measure. The reason for this is the necessity of making measurements inside the rock mass, usually at a distance of several meters from the rock face. Moreover, working conditions in mines are often arduous, which makes the application of more sophisticated measuring techniques difficult.

The paper briefly presents principles of strain determination in steel elements by using photoelastic coatings. Some innovations of the method are proposed, especially in the case of investigations on actual full-size structures. Examples of experimental data obtained by the authors in investigations of engineering structures (full-size elements) are given. Practical remarks on carrying on such investigations are given.

The authors have presented some results of the investigations of plastic deformations in component parts of steel structures obtained by means of photoelastic layer method. Attention was paid to characteristic features of the strain field exhibiting considerable nonuniformity with a stepwise yield zone propagation. Attempt was made to explain the sources of the phenomenon. Possible simplifications of experimental techniques have been indicated.

Describes three dimensional model investigations of a buttress dam. Coating techniques in metal moulds were used to make spatial models with holes and galleries. Gravity loading was investigated, by centrifuge and 'stress freezing' techniques. Non-uniform mass force effects were considered. Hydrostatic pressure at the stress freezing temperature was applied by a lever type loading arrangement. Stresses were determined from the 'frozen' photoelastic effects, demonstrated on slices from the models. Presents some characteristic fringe patterns, stress trajectories and boundary stresses.

This paper presents a theoretical analysis of the process of burst of a rock pillar. The analysis is based on a one-dimensional model of a pillar-roof system in which, due to mechanical softening of pillar rock and elastic unloading of roof rock, internal instability may occur. Instability results in a dynamic process, which is treated in the paper using both discrete and continuous approaches. In the latter case, instability of the system produces generation of an unloading stress wave in the roof. Equations for the velocities and accelerations of the bursting pillar, and for the time duration of the burst are derived. Assuming a simplified stress-strain relationship for the pillar rock, closed-form solutions for both discrete and continuous approaches are presented. Comparison of the above solutions is made, and numerical values for the maximum kinetic burst energy and its time duration are given for a wide range of mechanical constants for rocks.

A three-dimensional photoelastic model investigation of a body with voids loaded by uniform pressure on the external surface is presented. The model represents a certain rock region weakened by the crossing of a shaft with a horizontal excavation. The ″stress freezing″ technique was applied by loading the model hydraulically in a special container. The results are presented in the form of stress diagrams in the characteristic cross sections. In the introduction a short review of the most important prublications dealing with photoelastic rock investigations in mining is given.

The authors present a certain method of investigation of twodimensional photoelastic models with rigid constraints. It consists in the fact that cementing of constraints with the model is performed under
load, and photoelastic investigation — without external loads.
In the paper are presented advantages of the method , particularly when being used together with a conventional investigation.
The method is presented on an example of a halfplane under uniaxial load, the boundary of which is attached to a rigid constraint of finite length.

The paper contains an example of application of the photoelastic method to designing of a dam buttress. Testing procedures are described, special attention being paid to the method of realization of a uniform loading along the boundary of the model. Program of investigations is suggested rendering it possible to determine an optimum crosssection of the dam buttress.
In the paper is described the optimization procedure of the shape of the dam buttress based on the results of photoelastic investigations. As the optimization criterion is assumed the condition of minimum volume of the buttress at a determined stress value on the upstream side. Optimum section parameters calculated on the ODRA 1204 computer are presented in the form of tables and diagrams.

The problem of determination of shapes of plane structures satisfying the condition of constant boundary stresses is formulated in the paper. Survey of relevant literature on this subject is given. Moreover, an experimental method consisting in improving the initial model by adding or substracting some material is presented in a more detailed manner. The displacement of the boundary calculated on the basis of photoelastic investigations and an approximate value of the stress gradient give the new improved shape of the model. This procedure may be repeated as long as the uniform boundary stress is obtained. The method described is illustrated on the example of determination of the transition line between a column and a beam.

The paper concerns the method of determination of the stress distribution in a turbogenerator rotor by means of photoelastic methods. General formulation of the problem reduced to the plane
case is presented.
A short survey of the photoelastic methods is given as applied to the investigation of rotating elements, and the technique used by the authors is described. I t consists in making photoelastic
photographs of a rapidly rotating model by using flash sources of light. In describing the experimental stand, attention is drawn to certain improvements, e.g. free suspension of the rotating
model on an elastic shaft with oil damper and the coupling of the shutter of the camera with the photoelastic cell and flash lamp.
On the basis of the performed experiments it was proved that the method is applicable to this kind of investigations, although (this follows from the available literature) the method has not been widely used.

Photoelastic sensor with fringe image transfer by an image-guide to a computer for stress analysis was constructed for those applications where the place of measurements is hard to reach and electrical instruments are not permissible. Two types of sensors are presented: one, disc-like, for uniaxial stress observations and the second one, called a hollow glass cylinder, with axially bored stress concentrator for biaxial stress measurements. Sensor is a central cylinder glass cemented in a cubical concrete block. The sensor is connected to autocollimating polariscope, which allows the sensor to be lit and transport an isochrome image to digital image processor card. The numerical analysis of the fringes images is done in many ways. The original programs for the analysis are presented.

badania elastooptyczne, metody elastooptyczne, badania modelowe, metoda Cranza i Schardina