Plates VIII.-X.]ABSTRACT. l~umerous experiments have been performed on the strength of glass in the form of beams and plates. The beams, cut from sheet glass approximately 0.28 era. thick, were tested by a slow loading test and by the impact of a pendulum. The squares were tested both by pressing a steel ball against the square and also by the impact of a falling ball.It was shown that in the case of slow impact when using objects much heavier than the glass, the energy required to break the glass is greater than that needed under static conditions. The difference can be explained * The following symbols used in this paper generally follow the British Standard Handbook No. 560, 1934. Where they differ from those used by Professor S. Timoshenko, to whose book several references are made, his symbols are given in brackets afterwards. V, velocity ; t, the time ; E, Young's modulus of elasticity ; f, the modulus of rupture or bending strength ; P, the aggregate normal force ; ~, the deflection of a plate or beam ; a, (v) Poisson's ratio ; n, the frequency of vibration ; L, the length of a beam ; h, the thickness of a beam or plate ; a, the side of a square plate ; p, the specific gravity ; Pm (a max.), the ~standard deviation ) ma~mum stress ; v, the coefficient of variation \ average value × 100 for a series of measurements.if the known changes in the modulus of rupture of glass with time are taken into account. As the velocity of impact is increased, however, the energy required to shatter a glass plate generally diminishes markedly. This decrease in strength is ascribed to the operation of stresses, due to the weight of the glass, the initial value of which is given approximately by the relation V~/Ep. When glass is broken by small balls at high velocites characteristic cone or " shell '° fractures are produced, which appear to be an extension of the statically formed ring cracks along lines perpendicular to the maximum tension in the glass.The coefficients of variation of the measured quantities are given in a number of cases. Impact tests give a larger variation in the results than static tests since the former give estimates of the energy required to break and hence contain the term (f(l~v)) 2, where v is the coefficient of variation of the stress required to break.
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