Patent Grant
H630
The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to us of any royalties thereon. Estimating the useful life of production materials or a combination of these material is necessary, and particularly even of a higher magnitude of importance is the estimating of the effects of aging on solid propellant materials of solid propellant rocket motors. Presently, items such as solid propellant rocket motors are stored in thermal environments which have little or no control and are seldom monitored. However, when monitored, devices such as recording thermometers are used. These thermometers can record the temperature extremes and temperature-time profile, and by integrating under a temperature curve, the temperature and total heat effects can be measured. Such equipment is costly, requires a constant energy source to operate, and monitors a general area and not a specific item in total. Of particular interest and benefit would be a low cost item which can monitor specific items. For example, an aging device which can be attached directly to a specific item would be attractive. An aging device which can be attached directly to a specific item, and an aging device which is passive, would be particularly attractive since it requires no operating energy source. Therefore, an object of this invention is to provide an aging device which requires no operating energy source. Another object of this invention is to provide an aging device which can be attached directly to individual items for which aging data or information is desired. Still another object of this invention is to provide an aging device which can provide an irreversible color change which can indicate the maximum temperature to which the item is exposed. An additional object of this invention is to provide an aging device which can be employed with an item to correlate natural aging or an accelerated aging based on calibrated data obtained on the aging device and the item with which the aging device is stored. The aging gauge of this invention comprises a container with a cap having a fixed opening or a variable sized vent which can be opened to control the sublimation rate of a thermally sublimational material contained within the container. In use, the aging gauge is stored with an item to determine total heat to which the item has been subjected and also the maximum temperature to which the item has been exposed. The aging gauge container contains a thermally sublimational material such as naphthalene or similar material which has a low sublimation rate over the temperature range from about 70.degree. F. to about 160.degree. F. The amount of sublimation over a time period is calibrated from comparison values along with the amount of aging products found to have taken place with a propellant composition subjected to a like temperature range. The calibration data serves to correlate propellant aging with the amount of sublimation from the aging gauge. This correlation gives a temperature versus time relationship which serves to predict the life of an item such as a propellant composition where chemical reaction rates are known, and their effects are known with respect to temperature and total heat exposure. The aging gauge is additionally provided with a series of thermocolor pigments which are formulated to yield irreversible color changes at a maximum temperature to which it is subjected. These pigments can be placed on the inside or outside of the aging gauge. The container for the aging gauge can be metal, glass, or plastic; and when clear glass or plastic, the quantity of low temperature sublimational material remaining can be observed. Likewise, the thermocolor pigment can be observed for irreversible color changes when placed on the inside of the container when the container is transparent.
