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Wanhill R.J.H.,National Aerospace Laboratory Netherlands
Journal of Failure Analysis and Prevention | Year: 2011

This article discusses six cases of ancient silver embrittlement. The diagnostic techniques and results are surveyed, followed by the most likely explanations of embrittlement. There are two basic types of embrittlement: corrosion-induced, and microstructurally induced, which can act synergistically. Corrosion-induced embrittlement takes several forms, including what appears to be intergranular and transgranular stress corrosion cracking. The main intrinsic factor facilitating corrosion-induced embrittlement is retained cold-work in the silver. Microstructural embrittlement is characterized by intergranular fracture, most probably due to low-temperature segregation of lead to the grain boundaries. Knowledge of the details of embrittlement can be essential for optimizing the restoration and conservation of damaged artifacts. © 2011 ASM International.


Grooteman F.,National Aerospace Laboratory Netherlands
Probabilistic Engineering Mechanics | Year: 2011

In this paper, an adaptive directional importance sampling (ADIS) method is presented. The algorithm is based on a directional simulation scheme in which the most important directions are sampled exact and the others by means of a response surface approach. These most important directions are determined by a β-sphere enclosing the most important part(s) of the limit state. The β-sphere and response surface are constantly updated during sampling with information that becomes available from the exact evaluations making the scheme adaptive. Various widely used test problems, representing a broad range of complex limit states that can occur in practice, of which several that pose potential problems to stochastic methods in general, demonstrate the high efficiency, accuracy and robustness of the method. As such, the ADIS method is of particular interest in applications with a low probability of failure and medium number (up to about 40) of stochastic variables, for instance in aircraft and nuclear industry.


Barter S.A.,Defence Science and Technology Organisation, Australia | Molent L.,Defence Science and Technology Organisation, Australia | Wanhill R.J.H.,National Aerospace Laboratory Netherlands
International Journal of Fatigue | Year: 2012

A fatigue lifing framework using a lead crack concept, based on years of detailed inspection and analysis of fatigue cracks in many specimens and airframe components, has been developed by the DSTO for metallic primary airframe components. This framework is an important additional tool for determining aircraft component fatigue lives in the Royal Australian Air Force fleet. Like the original Damage Tolerance concept, developed by the United States Air Force, this framework assumes that fatigue cracking begins as soon as an aircraft enters service. However, there are major and fundamental differences. Instead of assuming initial crack sizes and deriving early crack growth behaviour from back-extrapolation of growth data for long cracks, the framework uses data for real cracks growing from small discontinuities inherent to the material and the production of the component. To this end, this paper examines the types of discontinuities that initiate fatigue cracks in typical metallic airframe structures. These discontinuities and the fatigue cracks that have grown from them are taken from coupon, component and full-scale tests, and also from service aircraft, including commercial transport aircraft and high performance military aircraft. © 2011 Elsevier Ltd. All rights reserved.


Van Dijk H.,National Aerospace Laboratory Netherlands
ISCRAM 2015 Conference Proceedings - 12th International Conference on Information Systems for Crisis Response and Management | Year: 2015

This paper describes an effort underway to develop an operational concept and technical implementation for a User Defined Operational Picture (UDOP). The purpose of the UDOP capability is to create, visualize, and share decision-focused views of the operational environment for decision-makers to support accurate situation awareness and timely decision-making. Unlike a traditional Common Operational Picture (COP), a UDOP allows the user to select what information should be included in-or excluded from the data set defining the operational picture at the source. This paper provides an overview of the UDOP capabilities, as well as a description of the initial prototype implementation in an operational setting.


Molent L.,Defence Science and Technology Organisation, Australia | Barter S.A.,Defence Science and Technology Organisation, Australia | Wanhill R.J.H.,National Aerospace Laboratory Netherlands
International Journal of Fatigue | Year: 2011

A fatigue lifing framework using a lead crack concept has been developed by the DSTO for metallic primary airframe components. The framework is based on years of detailed inspection and analysis of fatigue cracks in many specimens and airframe components, and is an important additional tool for determining aircraft component fatigue lives in the Royal Australian Air Force (RAAF) fleet. Like the original Damage Tolerance (DT) concept developed by the United States Air Force (USAF), this framework assumes that fatigue cracking begins as soon as an aircraft enters service. However, there are major and fundamental differences. Instead of assuming initial crack sizes and deriving early crack growth behaviour from back-extrapolation of growth data for long cracks, the DSTO framework uses data for real cracks growing from small discontinuities inherent to the material and the production of the component. Furthermore, these data, particularly for lead cracks, are characterized by exponential crack growth behaviour. Because of this common characteristic, the DSTO framework can use lead crack growth data to provide reasonable (i.e. not overly conservative) lower-bound estimates of typical crack growth lives of components, starting from small natural discontinuities and continuing up to crack sizes (thus encompassing short-to-long crack growth) that just meet the residual strength requirements. Scatter factors based on engineering judgement are then applied to these estimates to determine the maximum allowable service life (safe life limit). The aim of the paper is to present the framework of assumptions and observations used in conjunction with a unique measure of the initiating discontinuity and a simple crack growth law to predict a lower bound fatigue life estimate. © 2010 Elsevier Inc. All rights reserved.


Sijtsma P.,National Aerospace Laboratory Netherlands
18th AIAA/CEAS Aeroacoustics Conference (33rd AIAA Aeroacoustics Conference) | Year: 2012

Circular Harmonics Beamforming (CHB) is proposed as an alternative for Conventional Beamforming (CB) in axisymmetric set-ups. In CHB the steering vectors are composed of azimuthal mode amplitudes rather than Green's function values. In order to obtain these amplitudes, microphones have to be arranged in rings. An important feature of CHB is that the extension to rotating sources is straightforward and, therefore, easy to implement. Also the application to multiple rings of microphones is not too complicated. Thus, CHB can be applied to in-duct array measurements, when the microphones are mounted in rings, and stay away from bifurcations. Apart from a general discussion about the CHB method, this paper considers a few applications to simulated and measured in-duct array measurements. © 2012 by National Aerospace Laboratory NLR.


Stroeve S.H.,National Aerospace Laboratory Netherlands | Blom H.A.P.,National Aerospace Laboratory Netherlands | Blom H.A.P.,Technical University of Delft | Bakker G.J.,National Aerospace Laboratory Netherlands
Reliability Engineering and System Safety | Year: 2013

In the safety literature it has been argued, that in a complex socio-technical system safety cannot be well analysed by event sequence based approaches, but requires to capture the complex interactions and performance variability of the socio-technical system. In order to evaluate the quantitative and practical consequences of these arguments, this study compares two approaches to assess accident risk of an example safety critical sociotechnical system. It contrasts an event sequence based assessment with a multi-agent dynamic risk model (MA-DRM) based assessment, both of which are performed for a particular runway incursion scenario. The event sequence analysis uses the well-known event tree modelling formalism and the MA-DRM based approach combines agent based modelling, hybrid Petri nets and rare event Monte Carlo simulation. The comparison addresses qualitative and quantitative differences in the methods, attained risk levels, and in the prime factors influencing the safety of the operation. The assessments show considerable differences in the accident risk implications of the performance of human operators and technical systems in the runway incursion scenario. In contrast with the event sequence based results, the MA-DRM based results show that the accident risk is not manifest from the performance of and relations between individual human operators and technical systems. Instead, the safety risk emerges from the totality of the performance and interactions in the agent based model of the safety critical operation considered, which coincides very well with the argumentation in the safety literature. © 2012 Elsevier Ltd. All rights reserved.


Wanhill R.J.H.,National Aerospace Laboratory Netherlands
Metallography, Microstructure, and Analysis | Year: 2012

Discontinuous precipitation of copper has been held responsible for much of the embrittlement of ancient silver objects. The detailed characteristics of this precipitation have been suggested as possible indicators of an object's age and authenticity. These proposals are considered in the light of metallographic and analytical studies of several embrittled artifacts. These studies indicate that discontinuous precipitation is much less significant for embrittlement than residual stresses due to retained cold-work and externally applied stresses due to burial, and that the precipitation characteristics cannot be used for authentication. © 2012 Springer Science+Business Media New York and ASM International.


Taamallah S.,National Aerospace Laboratory Netherlands
Journal of Dynamic Systems, Measurement and Control, Transactions of the ASME | Year: 2016

We present a helicopter flight dynamics nonlinear model for a flybarless, articulated, pitch-lag-flap (P-L-F) main rotor (MR) with rigid blades, particularly suited for small-scale unmanned aerial vehicles (UAVs). The model incorporates the MR, tail rotor (TR), fuselage, and tails. This model is further applicable for high bandwidth control specifications and is valid for a range of flight conditions, including the vortex-ring-state (VRS) and autorotation. Additionally, the paper reviews all assumptions made in deriving the model, i.e., structural, aerodynamics, and dynamical simplifications. Simulation results show that this nonlinear model is in good agreement with an equivalent flightlab model, for both static (trim) and dynamic conditions. © 2016 by ASME.


Boelens O.J.,National Aerospace Laboratory Netherlands
Aerospace Science and Technology | Year: 2012

Accurate and cost-effective Computational Fluid Dynamics (CFD) methods play an increasingly important role, for example in the support of fighter aircraft operations. Prior to deployment of such CFD methods they should be well validated and evaluated against state-of-the-art wind tunnel and/or flight test data. The wind tunnel experiments on the detailed X-31 model performed in the DNW Low-Speed-Wind-Tunnel Braunschweig (DNW-NWB) provide an excellent data set for validation and evaluation purposes. This data set has been investigated in the framework of NATO RTO task group AVT-161 'Assessment of Stability and Control Prediction Methods for NATO Air & Sea Vehicles'. The National Aerospace Laboratory NLR participated in this task group using its in-house developed flow simulation system ENFLOW, which includes both grid generation tools and a flow solver. The focus of the present paper is on the question to what extend leading edge details, flap gaps, need to be taken into account for the X-31 wind tunnel model to properly simulate the flow around this configuration. To investigate this question, three leading edge configurations have been considered, i.e. one with all leading edge flap gaps, one with only the longitudinal flap gaps and one with no leading edge flap gaps. Results obtained for selected test conditions measured during test run VN01004 (M=0.18 and Rem. a.c.=2.07×10 6) of the wind tunnel experiments will be discussed. Properly modeling geometrical details of the wind tunnel model at the leading edge is essential in obtaining the vortical flow phenomena observed in the wind tunnel. Analysis of the pitching moment coefficient demonstrates how in case of not resolving geometrical details a seemingly correct behavior is obtained without, however, resolving the underlying flow physics correctly. © 2011 Elsevier Masson SAS. All rights reserved.

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