28th ICAF Symposium – Helsinki, 3–5 June 2015 955 Towards the Virtual Fatigue Test: Hardware-in-the-Loop integrated Fatigue Test Simulation (HiLiFTS) Albert K. Wong Defence Science and Technology Organisation, Australia Abstract: The costs and times associated with Full Scale Fatigue Tests (FSFTs) are enormous, but to date, they have proved to be the only reliable means for verification and validation of aircraft durability and safety and are thus mandated by aviation regulators for any new aircraft type. At the same time, however, analyses or models have been playing an increasing role in the qualification/certification process, and the question beckons whether the FSFT can one day be dispensed with. In this paper, rationale for both sides of the argument is presented, but concludes that the important question is not so much whether the FSFT can be done away with, but rather how FSFT can be performed more efficiently? To this end, a new concept is proposed that is a hybrid of the conventional FSFT and the complete digital model. Borrowed from the control systems engineering field, the hardware-in-the-loop simulation (HiLS) approach is adapted to provide the best of both worlds – the realism of actual testing for the critical components, together with the flexibility and cost- effectiveness of modelling. Whilst this approach has been made possible by a unique DSTO-developed technology that allows fatigue hotspots to be identified, its eventual success will also require some other technical problems to be solved. However, whilst these hurdles may be challenging, it is shown that they are not insurmountable and some suggestions are presented on how these may be tackled. INTRODUCTION Full Scale Fatigue Tests form the cornerstone in the qualification and certification of new aircraft types in both the civil and military sectors. These tests consume enormous resources in time, effort and costs, but have thus far proved to be essential in providing independent verification and validation of the design and manufacture of aircraft in terms of safety and durability. But with the rising complexity and costs of physical testing, together with the ever increasing computational capabilities, great focus has been given towards increasing reliance on analysis and models. The NATO ATO Report titled “Qualification By Analysis” [1] wastes no time in setting the rationale for turning towards analyses by its Executive Summary’s opening sentence: “The time and cost associated with designing, developing and qualifying new military air vehicles is prohibitive…”; true to its title, the report then goes on to advocate the use of analysis for the qualification of all manners of aircraft systems and sub-systems to control both costs and time to market. The economics or benefits of qualification by analysis are alluring and as models become increasingly credible, terms such as “Virtual Tests” or more specifically, “Virtual Fatigue Tests” have emerged in the literature a little over a decade ago ([2], [3]). These early “tests” amount to nothing more than detailed models of materials or components which can produce results that compared well to physical tests under certain conditions. More recently, the concept of the virtual or digital aircraft assumes even greater prominence, where the aim is for high fidelity simulations and models of the entire aircraft and its environments to play a major role in the management of the aircraft over its entire life cycle. These are being pursued by European programs such as VIVACE 14 , MAAXIMUS 15 , CRESCENDO 16 , and the US led programs on the Digital Twin 17 , all of which have generated much excitement and interest in the aerospace world. Considering only the qualification/certification aspect, the scopes and expectations of these programs span a broad spectrum - from using simulations to simply improve efficiency of design, to forming the certification basis, as exemplified by the following: • “to improve the efficiency of aircraft systems design, integration and certification by using simulation widely.” (Homsi [6]) • “… to reduce need for repeat physical testing” (Coleman [7]) • “… will form a basis for certification of vehicles by simulation…” (Glaessgen & Stargel [5]) 14 Value Improvement through a Virtual Aeronautical Collaborative Enterprise (http://ec.europa.eu/research/transport/projects/items/vivace_en.htm) 15 More Affordable Aircraft through eXtended, Integrated and Mature nUmerical Sizing (http://www.maaximus.eu/) 16 Collaborative and Robust Engineering using Simulation Capability Enabling Next Design Optimisation (http://www.crescendo- fp7.eu/) 17 See USAF Broad Agency Announcement Airframe Solicitation Number: BAA-13-03-RQKP “Digital Twin Spiral 1” (https://www.fbo.gov/index?s=opportunity&mode=form&id=0b10f8d15837d4ad47ca81da9e97cfcd& tab=core&_cview=1); see also [4], [5].