Evaluation of the Multiple Load Sequence methodology for GFRP fatigue life prediction

Abstract

The current preferred static certification method used in the sailplane industry as defined by aviation governing bodies limits the allowable service life to 12 000 Flight Hours (Fh) which is problematic for current Australian sailplanes in the field that is approaching the service life boundary faster compared to European sailplanes (Patching & Wood, 1991, pp. 100-104; Van der Walt, 2020, p. 2). The current certification legislation allows for the use of a dynamic fatigue certification procedure to extend the service life past the 12 000 Fh barrier where a life factor of at least three is required to prove structural operational safety for the desired flight hour amount. (Pommera, 2000, p. 3; M&D Flugzeugbau, 2020, p. 4). The problem with the dynamic fatigue certification procedure is the extremely long certification test duration which can be a daunting task to perform. Alternative methods are henceforth required to decrease the costly dynamic fatigue testing for Fibre Reinforced Polymer (FRP) composite material sailplanes that require more than 12 000 Fh. (Van der Walt, 2020, pp. 2, 9-10). Researchers such as Kensche (2002(a), pp. 32-43), Rodzewics (2009, pp. 452-460), Trappe (2010, pp. 94-99) and Van der Walt (2020, pp. 1-122) proposed alternative methods where a certain amount of Variable Amplitude (VA) load sequences could be exchanged with Constant Amplitude (CA) load sequences to determine the fatigue life of composite sailplane structures at an accelerated rate. The proposed methods are uncertain about the correlation between CA and VA load sequencing at sailplane loading conditions. The research of Kensche (Kensche, 2002(a), pp. 32-43) initially hypothesized that the relationship between CA and VA load sequencing is linear, irrespective of the loading sequence. Early research performed by Kensche (2002(a), p. 42), concluded that a certain amount of VA cycles may, in practice, interchange with a certain amount of CA cycles due to the equivalent damage state of the two spectrums. An exchange of the spectrums could result in sailplane manufacturers shortening the fatigue certification duration of sailplanes requiring a flight hour barrier exceeding 12000 Fh. Research conducted by Kensche (2002(a), pp. 32-43) resulted in an empirical technique capable of comparing the different load spectrums to be proposed by Van der Walt (2020, pp. 1-122) for the extension of sailplane service life past the 12000 Fh barrier.

The research outcome of Van der Walt (2020, pp. 88-92) was that the Multiple Load Sequence (MLS) method showed promise and required further investigation. Further investigation into the MLS method commenced with testing similar to that was done by Van der Walt (2020, pp. 1-122) with additional Residual Strength and Stiffness (RSS) testing for validation purposes of the MLS method. CA and VA fatigue testing was done at 6Hz for a coupon type specimen at 50, 65 and 75 % of the specimen’s Ultimate Tensile Strength (UTS). The VA fatigue testing test interval quantities was increased compared to Van der Walt (2020) to achieve a more accurate MLS data curve. The residual strength method was applied to determine the load-carrying capability of the specimen during the fatigue life cycle. The Residual stiffness method was used to determine the deformation resistance capability of the specimen subjected to loading during the testing cycle. The RSS methods yielded equations that converged linearly as according to the PM assumption (Nijssen, 2007), whereas, the MLS methodology yielded a 4th degree polynomial equation which is not the linear function desired by Van der Walt (2020, p. 88), who obtained a quadratic equation with less testing interval points. The outcome of the experimental testing yielded an estimated result of 22920 certifiable Fh compared to the current 12000 Fh for a UTS of 50 %. The obtained 22920 Fh translates to either 48605 CA or 3189674 VA cycles which is 5,7 KoSMOS II cycles for the coupon type specimen. The evaluation concludes that although the MLS methodology equation was found to be non-linear, it is just as accurate compared to the validation and verification methods. There is henceforth merit in CA testing instead of the conventional VA testing for accelerated certification of sailplanes after an adequate Building Block Approach (BBA) database is established according to sailplane certification legislation requirements.