The effects of thickness and temper on the resistance of glass to small missile impact

With regard to window glass breakage, there are two predominate modes of failure: uniform wind pressure and missile impact. This paper is concerned with the latter and deals with the effects of sample thickness, temper, and missile weight on the impact resistance of glass.

Included in this research are (1) a discussion of past research as it relates to the present study, (2) presentation of data with relevant statistical analyses, and (3) discussion of appropriate theories of failure.

Surveys of previous research revealed that a relatively small number of investigators have studied the effects of missile impact on window glass. The survey also showed that missiles are a potential hazard to glass during windstorms and revealed that the size of the missile is major controlling factor in causing glass failure.

Impact tests were run on six different thicknesses of glass ranging from 3/16 in. to 3/4 in. at four temper levels and with two missile weights, 5.55 gmf and 28.14 gmf. Linear regressions were performed on the experimental data to relate thickness, missile weight, and amount of heat treating to the velocity required to break a sample of glass.

Confidence bands were determined from the regressions. From observations of both the data and the statistical analyses, four conclusions were drawn: (1) Within the range of thicknesses tested, the lowest velocity required to break a sample of glass did not occur in the smallest thickness. (2) This minimum missile velocity is not the same for annealed and heat treated glass. (3) The missile weight has a more pronounced effect on annealed glass than on heat treated glass. (4) The degree of heat treatment appears to affect the missile resistance of glass.

Relevant impact theories were employed to explain trends in the experimental data. The stress wave theory appears to explain the trends in the data best.