Advanced Materials Research Vols. 891-892

Abstract: The Australian Defence Science and Technology Organisation (DSTO) was recently tasked with evaluating an in-service fatigue crack discovered in the primary structure of an Australian Army Black Hawk helicopter. This crack, discovered early in the affected components service life, was quite large and subsequent fractographic analysis generated substantial information with respect to its growth characteristics. Consequently, it was decided to use this in-service fatigue crack to test the ability of those methods and technologies being developed at DSTO to model and assess a ‘real’ example of helicopter airframe fatigue cracking. This paper details the process of analysing an in-service fatigue crack using methods and technologies developed and/or improved at DSTO in the areas of visualisation, fatigue spectra generation, load and stress analysis, and crack growth prediction. Results will highlight the effectiveness of the methods and techniques in modelling the observed fatigue damage and areas where further improvements are required.

Abstract: Helicopter airframe fatigue cracking is a cause of significant and growing cost of ownership and operational readiness concerns for the operators of (primarily) metallic airframe helicopters. Airframe fatigue has often had relatively low priority for helicopters, with research and design concentrated on the fatigue of flight critical rotating structural components such as rotor blades and pitch links. The Australian Defence Science and Technology Organisation (DSTO) and Naval Air Systems Command (NAVAIR) of the US Navy are collaborating to develop improved methods and technologies that can be used to assess the fatigue damage accrued by ageing helicopter airframes. The flight load sequences, or fatigue spectra, experienced by a helicopter airframe in its lifetime contain many billions of load cycles due to rotor revolutions. The application of spectra containing such vast numbers of load cycles is often impractical for reasons of test duration and cost, therefore spectra simplification techniques must be employed. To this end, truncation is a technique that is used to eliminate non-or lesser-damaging load cycles, producing spectra equivalent in terms of theoretical fatigue damage but with substantially fewer load cycles. This paper describes several truncation techniques that have recently been developed at DSTO specifically to deal with the very large numbers of load cycles that are characteristic of helicopter airframe fatigue spectra. These techniques, which include both sequence and frequency based approaches, feature tunable levels of truncation and allow for large reductions in numbers of turning points while maintaining high-fidelity and realistic fatigue spectra. Also detailed are preliminary results from a comprehensive coupon test program, which DSTO is using to experimentally verify that truncated and un-truncated spectra are approximately equivalent in terms of the fatigue damage that they produce.

Abstract: Helicopter airframe fatigue cracking is a cause of significant and growing cost of ownership and operational readiness concerns for the operators of (primarily) metallic airframe helicopters. Airframe fatigue has often had relatively low priority for helicopters, with research and design concentrated on the fatigue of flight critical rotating structural components such as rotor blades and pitch links. The Australian Defence Science and Technology Organisation (DSTO) and the US Naval Air Systems Command are collaborating to develop improved methods and technologies that can be used to assess the fatigue damage endured by ageing helicopter airframes. The flight load sequencesor fatigue spectraexperienced by a helicopter airframe in its lifetime contain many billions of load cycles due to rotor revolutions. Fatigue spectra developed for helicopter airframe certification tests are heavily simplified for reasons such as computational efficiency, test practicality and cost. Real airframe fatigue spectra are likely to be influenced by the modes of vibration that might be present on the airframe, the attenuation of the vibratory loading that is introduced at the main and tail rotors and the relative magnitudes and influences of both quasi-static (manoeuvre induced) and vibratory loading. To better capture such complexity, more realistic, higher fidelity fatigue spectra are required. Fatigue spectra generation involves creating realistic flight-by-flight sequences of flight conditions and assigning high-fidelity flight loads data to those sequences. This paper details DSTOs development of a novel computer-automated process which pseudo-randomly generates realistic sequences of flight conditions to match a known or assumed usage spectrum.

Abstract: Abstract. The objective of this paper is to probabilistically evaluate the effects of mistuned sectors on the dynamic characteristics of an integrally bladed disk (blisk). Small blade to blade physical variation in a disk is termed as mistuning. In this study, the dynamic characteristics of the perfectly tuned blisk were firstly analysed as a baseline. Secondly, a probabilistic approach is used for the mistuning analysis of a blisk. A reduced-order method named as the subset of nominal modes (SNM) was used to generate modes for a mistuned blisk from a cyclic perfectly tuned FE model without creating the full model. Furthermore, as only the modes with natural frequencies close to the modes of interest were considered, a relatively shorter computational time and a much smaller model size than a full blisk model is used. Therefore, the dynamic characteristics of the forced response for random mistuned blisks were obtained.

Abstract: The fatigue critical structures of military aircraft are generally subjected to variable amplitude flight spectrum loading. Maintaining aircraft structural integrity to ensure safe operation of the fleet is critically dependent on accurate analysis and reliable prediction of fatigue crack growth in those structures under service operating conditions. To achieve this goal, laboratory experimental methods that can accurately measure and monitor fatigue crack growth under variable amplitude loading are required. This can be challenging as no test standard exists to guide the process of fatigue crack growth measurement under variable amplitude loading conditions to ensure the accuracy of the test results. This challenge was addressed by developing a modified compliance method as described in this paper. The results presented employed a modified compliance method complemented with a travelling microscope technique and marker band loads. The modified compliance method developed is able to measure in-situ, fatigue crack growth of standard compact-tension specimens under a fighter flight spectrum loading. The marker band loads and microscope readings were used to assist the post-test validation using quantitative fractography. The results from this study have demonstrated that the modified compliance method can produce consistent and accurate fatigue crack growth data under variable amplitude loading conditions.

Abstract: LAU-7 missile launcher housings, which are fitted to most Royal Australian Air Force (RAAF) F/A-18A/B aircraft, can experience cracking in the guide rail. This paper covers the design, manufacture and validation of a life extension repair for cracked launcher housings. The repair development uses DSTO's rework shape optimisation technology and fatigue testing capabilities. The rework design reduces peak stresses by 33 %, resulting in significant fatigue life enhancements, as demonstrated by representative coupon testing. A special manufacturing jig has been designed and transitioned to the RAAF, which has used it to repair housings. These housings have performed well in flight tests, with no cracking detected.

Abstract: The Mechanistic empirical pavement design method for flexible pavements is based on modelling certain modes of failures for different pavement materials. In the Australian and New Zealand guidelines, the mechanistic empirical pavement design is based on modelling fatigue and permanent deformation as the two major modes of failures. The Austroads guidelines use the Shell fatigue performance transfer function to model the fatigue behaviour of asphalt mixes. In this research, the fatigue behaviour of different mixes AC10, AC14 and AC20 with different types of binders 80/100 and 60/70 was thoroughly investigated. The Shell model significantly underestimated the measured fatigue life for all mixes. A wide range of properties of the examined mixes was considered; percent of air voids ranges from 1.2% to 11.4%, binder content (at optimum, ± 0.5 from optimum), and the flexural modulus ranges from 1600 to 4576 MPa. A new fatigue model was developed at the University of Canterbury. The Canterbury model was based on the bending beam fatigue results of 78 beams tested at constant strain mode at different strain levels range from 300 to 600 microstrains. The new model provides a much better matching to the measured data with no observed bias and it accounts for percentage of air voids in the total mix and the effective binder content instead of the total binder content that is currently included in the Shell Model.

Abstract: The multistage strength degradation theory, which has recently emerged from studies on the material and structural behaviour of concrete, provides a clear description of the mechanism of fatigue. According to this theory, fatigue is caused by the sporadic sudden change of cracking behaviour in a system under cyclic loading, leading to intermittent strength reduction of the system and its eventual failure. As metal is the main engineering material plagued most by fatigue failure, this newly-established theory needs to be experimentally verified on metal, which is the aim of this study. The obtained test results present strong experimental evidence for the existence of multistage strength degradation processes in metals under cyclic loading, and the strength degradation is clearly triggered by the abrupt change of cracking behaviour. These tests confirm the relevance of the multistage strength degradation theory on metal fatigue, and the engineering implications of the study are discussed.

Abstract: Gas turbine operating cycles at high temperatures often consist of load reversals mixed with hold times; the latter occurring either as cruise for aero engines or at continuous power output for land based turbines, but also at low frequency loading conditions, e.g. slow “ramp up” of engine thrust. The hold time conditions cause the crack to grow by intergranular fracture due to material damage near the crack tip, thus rapidly increasing the crack growth rate. Since the damaged zone will affect the crack propagation rate due to cyclic loadings as well, the complete load history of a component therefore has to be considered. The crack propagation model presented in this paper is based on the damaged zone concept, and considers the history effect in the form of damaged zone build up during hold times, and subsequent destruction as the crack propagates onwards by rapidly applied load reversals. By incorporating crack closure for handling different R-values, an aero engine component spectrum is evaluated for a surface crack at 550 °C. The result shows a good correlation to model simulation, despite the complexity of the load spectrum.

Abstract: In this study we focus on the effect of hydrogen gas on the cracking resistance of metals. The main objective is to predict the fatigue crack propagation rates in the presence of hydrogen. For this purpose, a Cohesive Zone Model (CZM) dedicated to cracking under monotonic as well as cyclic loadings has been implemented in the ABAQUS finite element code. A specific traction-separation law, adapted to describe the gradual degradation of the cohesive stresses under cyclic loading, and sensitive to the presence of hydrogen is formulated. The coupling between mechanical behaviour and diffusion of hydrogen can be modelled using a coupled temperature - displacement calculation available in ABAQUS. The simulations are compared with fatigue crack propagation tests performed on a 15-5PH martensitic stainless steel. They show that while the proposed model is able to predict a lower resistance to cracking in presence of hydrogen, at this stage it cannot fully account for the detrimental effect induced by high pressure of gaseous hydrogen.