Uses and Abuses of Accelerated Age Testing of Pyrotechnic Devices

Abstract

This paper deals with age surveillance testing (AST) and its auxiliary test method accelerated age testing (AAT). The results of AAT are most often used to extend the service life of pyrotechnic devices used in aerospace systems. This life extension produces considerable savings. Reliability is also improved by avoiding frequent teardown and replacement of pyrotechnic devices, with the accompanying potential of damage and subsequent improper assembly. It also prevents acceptable, reliable components from being discarded prematurely. AAT is also a useful diagnostic tool that may expose design marginality and uncover flaws prior to production. Inappropriate AAT can result in failure of completely satisfactory components because of the imposition of unrealistic hightemperature exposures, or success of marginal designs, because of benign temperature limits that do not reflect the service life temperature extremes. A modification of the Arrhenius reaction rate equation is commonly used to define the AAT used to extend the lives of overaged pyrotechnic components. The accuracy and origin of this reaction rate factor (“3”) is challenged. I. Introduction ave you ever reached for that throat-soothing elixir only to discover that the expiration date was six months ago? Do you fret about using that carton of milk in the refrigerator with an expiration date of two days earlier? How about that precious jar of “Vegemite” so jealously guarded and tapped years after it was placed in the pantry? What does an expiration date mean? Is it based on tests or just a “crystal ball” guess? Or is it put there so that we buy more products? Do we replace a three-year car battery after three years or wait until it dies? Does a pilot think about the service life of his escape system before he pulls the arm/fire initiation handle in an emergency ejection scenario? Is this what makes aerospace components with limited lives so special? This paper deals with age surveillance testing and its cousin, accelerated age testing. Both test methods are designed to confirm the storage/service life of pyrotechnic components and highlight their design strengths or weaknesses. The uses, abuses, advantages and disadvantages of both are discussed. II. Pyrotechnic Service Life Assessment Programs (SLAPs) Since the early 1960’s, age surveillance testing has been conducted on aerospace pyrotechnic components to examine performance reliability versus design specifications after years of storage or service. The most significant early Service Life Assessment Programs (SLAPs) were conducted by General Dynamics (GD) on the F-111 program and by NSWC IH on cartridge- and propellant-actuated devices and missile components. SLAP results are used to determine whether any degradation has occurred and to evaluate the effect of aging by quantifying performance. The results establish the feasibility of life extension and also validate the current storage/service life of a component. In the past ten years, PSEMC has improved the validation concept by comparing age surveillance test results with the original lot acceptance results. This eliminates the bias of grouping results when lot-to-lot variability exists. However, to be statistically significant, sample sizes of at least five units must be available for analysis. The advantage of a SLAP is that it verifies reliability and safety of the components after exposure to service life environments. The components have been removed from aircraft or missiles in the field. The SLAP results identify