A systematic approach on service life prediction of a model aerostat envelope

Abstract

Lighter-than-air (LTA) helium aerostats are receiving renewed attention for scientific as well as defense applications. Designing of new and improved materials for this kind of structure that consistently survive in the harsh atmospheric condition is one of the critical issues. However, durability of a newly developed material, in the actual field of application, is often unknown. Field experiments to know about its service life take prolonged time. In this study, a systematic approach has been proposed to predict the in service weatherability of a model aerostat envelope. A reliability model with two stress types (UV radiation and temperature) has been achieved to predict the service life of a model aerostat envelope material concerning the gas permeability through the material. Accelerated aging tests at higher stress levels have been performed to obtain a life stress relationship from which service life of the material has been determined at the use level conditions. Validation of the proposed model has also been performed using the actual field test data. It will be worthy to apply this approach in various fields of application to predict the service life of newly developed materials before their commercialization.

Introduction

Aerostats are advanced inflatable aerial delivery systems having the shape of an aircraft that floats in the troposphere region for aerial surveillance and remain exposed in the harsh atmospheric weather. Unlike fixed-wing aircraft or helicopters, aerostats are ‘lighter-than-air (LTA)’, typically use helium gas to stay aloft and are tethered using a mooring system operated from a fixed location. Degradation due to the severe weathering stresses (mainly UV radiation and temperature) affects the barrier property of the aerostat hull or envelope and thus limits its service life causing a great financial loss. A typical aerostat envelope consists of a strength layer generally made up of woven textiles, to provide a flexible high strength base to the structure and a protective layer which acts as a gas barrier layer for maintaining the inflated condition of the structure. In such cases, the lifetime of the ultimate inflatable structure is primarily determined by the protective properties of the polymeric layer. Thermoplastic polyurethane (TPU) has recently attracted a lot of interest in this regard [1], [2], [3]. However, similar to other polymer materials, exposure of TPU to aggressive environments for prolonged time causes loss of its use value [4], [5], [6], [7].

When any material is exposed to the sun, its properties start deteriorating. The harmful ultra-violet (UV) part of the solar radiation is capable to break the chemical bonds present in the material and thus initiates the degradation process. With continued exposure, the properties of the material continue to degrade impairing the functioning of the material. When at least one, or the most important property drops to a pre-designated minimum value, the material is said to have failed. The time corresponding to the failure is called the service life of the material. However, the field experiments (natural exposure) to find out the service life of the material take prolonged time, which is not acceptable in an industrial set up. Besides, natural weathering exposure is a variable function of the exposure site and the actual ever changing weathering conditions due to the day cycle, seasonal and year to year variability. Natural conditions have too many uncontrolled and unpredictable variables that make analysis difficult. Thus, focus has been on artificial weathering where the exposure conditions are maintained in a controlled manner. In artificial weathering, acceleration can be achieved via two different approaches: by enhancing of exposure parameters over use level conditions and the time-lapse approach that repeats episodes with higher effect at shorter frequency. Due to the time constraints, it is highly desirable to investigate short-term data of the responses at accelerated levels [8], [9] and analyze the data effectively leading to the most accurate forecast of the lifetime of a material [10], [11], [12], [13].

In the literature, several attempts can be found to model degradation of coatings exposed to weathering. The state of the art in modeling techniques of weathering degradation of polymeric coatings can be categorized in two general practices: stochastic (statistical) modeling and mechanistic modeling [14], [15], [16], [17]. The mathematical model based on a stochastic approach is generally articulated as reliability theory that fundamentally deals with probability hypothesis; while, mechanistic models are particularly based on knowledge of the underlying degradation mechanisms. Critically analyzing the previous investigations, it can be concluded that, though requiring systematic laboratory experiments, the statistical models based on reliability theory seem to be the most direct way to proceed for prediction of service life. In terms of understanding and analysis, mechanisms of photoinitiated coating degradation have not been quantified satisfactorily to an extent that allows a mechanistic analysis of weathering data. As different coating system can follow varying degradation mechanism, it is difficult to mechanistically predict service life separately for each system [18]. Very few models have been developed so far and moreover, several important rate phenomena and kinetics of photoinitiated reactions have not been incorporated in those existing mechanistic models. In addition, validation of the existing models against a sufficiently large set of experimental data is also lacking. Another obvious disparity can be noted in the literature related to lifetime prediction modeling is to precisely define the failure of a coating.

Therefore a need exists for a systematic methodology which is capable of making reliable service life predictions from accelerated experiments. However, performing these methods and their analysis is a complex task. In this context, reliability prediction model can be a good approach. The ability of the reliability technique to consider test variability as part of a distribution makes it a very valuable tool for weathering studies.

In this paper a simple and systematic approach has been proposed to predict the service life of a model aerostat envelope material, on the basis of direct evaluation of the functional property. Previous work reports [19] helium gas permeability (rather than tensile strength, tearing strength etc.), as the most critical input for estimating the useful life of aerostat envelope system. In this work, a reliability model with two stress types (UV radiation and temperature) has been applied for the estimation of the service life concerning the loss of helium gas barrier property. Validation of data of the proposed model has also been done with the natural weathering. An approach to evaluate the material's property including seasonal variation of natural weathering, has also been taken into consideration.

Although several works have been done on modeling service life of materials used in different domains [20], [21], [22], mainly in Civil and Mechanical engineering, no work has been reported so far particularly for the coatings of aerostat envelope. For the very first time reliability theory has been applied to aerostat coatings and an approach has been exhibited for estimating its service life.