Circular plates of float glass with thicknesses between 3 mm and 12 mm and clamped at a radius of 65 mm have been impacted with a 3 mm diameter hardened steel ball travelling at velocities in the range of 10-50 m S-l. The fractures can be classified as Hertzian ring cracks at the contact zone with or without conical fractures; crushed impact zones on the front face which may be accompanied by cone cracks, median, radial and lateral cracking and in some cases star cracks propagating from the rear face. Examples of these will be described together with a qualitative predictive analysis for their occurrence as a fknction of impact velocity, plate thickness and surface condition. Of particular interest is the observation of crack bifurcation within the cone fractures in plates of thicknesses below 6 mm.

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The determination of individual stress values in the multiaxial residual stress distributions found in toughened glass is a problem of major practical importance in the glass industry. In spite of many years of development, conventional experimental stress analysis techniques have not been able to provide a generally applicable means of obtaining this information. However, a unique method has now been developed for this purpose, based on the principles of magnetophotoelasticity, in which the combined optical retardation effects due to (i) the stress-birefringence of the material and (ii) a superimposed magnetic field, are utilized.The paper summarizes the basic theory of magnetophotoelasticity and describes experimental verification work using simple uniaxial systems of constant and linearly varying stress. Two important developments follow. First, a solution technique is developed whereby principal stress differences can be determined at any position through the thickness of a toughened glass plate, for an assumed biaxial residual stress distribution consisting of a combination of parabolic and linear stress variations. Second, an oblique incidence technique is employed to obtain individual stresses from the previously determined principal stress differences.A polariscope, designed and built for the implementation of the technique at any position over the area of an automotive windscreen, is described. Typical experimental results are presented and shown to compare well with the limited information available from conventional techniques.

Stress-concentration factors have been determined for oblique holes in flat plates by a method using frozenstress photoelasticity. The ellipses formed at the intersection of the hole and the plate surfaces had their major axes perpendicular to the direction of application of the load.The maximum stress-concentration factor was found to increase with increasing angle of obliquity with respect to the normal to the plate but the experimental factors were found to be considerably lower than those predicted from a theory of Ellyin et al.Graphs are given which enable maximum stress-concentration factors to be obtained for oblique holes having a ratio of hole diameter to plate width of 0.1. It was found that, for the models tested, elliptical hole data gave a reasonable estimate of the maximum stress-concentration factors based on net area.Removing the feather edge of the hole, by applying various edge radii, did not alter the stress concentration appreciably except in so far as load-carrying area was reduced.

the glassing process. have become larger and thinner, their base strength has not changed, rather, t h e i r format means that i t i s now easier t o induce greater stresses during handling. The higher breakage rates have already led to increased losses f o r both manufacturer and integrator which must lead to higher cost for such products.In f a c t , as coverglasses ABSTRACT As a result of the gradual advances in solar cell technology and the demand for lighter, more efficient solar arrays, i t i s necessary t o Droduce solar cell coveralasses which are larger and thinner than tho;e previously available. reduction i n strength which can be the cause of breakages during manufacture and integration. Associated w i t h t h i s i s the present method of visual inspection of coverglasses for cosmetic defects which does not necessarily ensure that tgey will be strong enough to survive glassing processes. Techniques have therefore been developed to produce high strength coverglasses where mean strength improvement factors of 12 have been obtained; and also apparatus manufactured t o provide a repeatable proof strength t e s t f o r standard and h i g h strength coverglassesThis means t h a t there i s an apparent INTRODUCTION