Multi-wavelength lasers using AWGs

Multiwavelength lasers using AWGs can be used as digitally tunable lasers with simple channel selection, and for generating multiple wavelengths simultanously. In this paper a number of different configurations is reviewed

Fatigue resistance of welded joints in aluminium high-speed craft: A total stress concept

Crew transfers, surveillance duties and {security, rescue, interception} operations at sea typically require high-speed craft. Aluminium is quite often selected as hull structure material because of its weight save potential in comparison to steel. The fatigue strength, however, may become a point of concern because of the decreased Young’s modulus. Bottom slamming is identified as a dominant type of repeated loading, meaning fatigue is a governing limit state in aluminium high-speed craft design. Particular attention in that respect is paid to arc-welded joints connecting the hull structure components, {plates, shells}, since the weld geometry introduces notches; fatigue sensitive locations. Fatigue physics cover an extensive range of scales and modelling may require a multi-scale approach. Adopting a structural response parameter S available at FSS level using global information only, however, seems attractive since S controls plasticity – required to facilitate fatigue damage: crack initiation, growth, propagation and fracture – at macro (structural)- as well as meso and micro (material) scale, but pays off in fatigue resistance data scatter and life time estimate uncertainty. Including physics at smaller scale, local information, improves the accuracy. A continuous increase of the considered scale range of physics as observed in fatigue assessment concepts developed over time – proposed to be classified according to approach, criterion, parameter and process zone – is however typically associated to increased (computational) effort and concept complexity. At the same time, similarity; proper scaling, meaning equal parameter values should yield the same fatigue resistance, seems still incomplete since all concepts available involve multiple fatigue resistance curves rather than one. From {MCF, HCF} design perspective, a local continuum mechanics approach seems sufficient and a total stress concept is proposed to balance accuracy, effort and complexity, improving similarity at the same time to obtain one aluminium arc-welded joint fatigue resistance curve. The weld geometry introduces at least a notch at the weld toe and depending on penetration level another one at the weld root. Cracks may initiate at both fatigue sensitive locations, grow principally in {plate, shell} thickness direction and continue to propagate in general either along or perpendicular to the weld seam through {plate, shell} because of the structure orthotropic stiffness characteristics, suggesting a {plate, shell} thickness based (detectable repair) criterion to be an appropriate fatigue design parameter. The total through-thickness weld notch stress distribution along the expected crack path {??_n?^T,??_nr?^T }, including both the ocean/sea waves induced cyclic remote mechanical loading- and welding process related quasi-constant thermal residual part, is assumed to be a key element. The predominant remote mechanical loading mode-I contribution {?_n,?_nr } has been examined to distinguish the involved stress components. A self-equilibrating weld geometry stress – consisting of a local V-shaped notch- and weld load carrying part – and equilibrium equivalent global structural field stress are identified; a refinement of a well-known definition. The semi-analytical formulations are related to the welded joint far field stress, calculated using a relatively coarse meshed {plate, shell} FE model as typically available for fatigue design purposes. Exploiting (non-)symmetry conditions, a generalised formulation demonstrating stress field similarity has been obtained and extends to the welding induced thermal residual stress distributions {??_n?^r,??_nr?^r }. Fatigue scaling requires both the (zone 1) peak value and (zone 2 notch affected and zone 3 far field dominated) gradient to be incorporated, meaning a damage criterion should take the complete distribution into account. The SIF K seems to meet this criterion, though, the intact geometry related notch stress distributions should be correlated to crack damaged equivalents; fatigue is assumed to be a crack growth (dominated) process. At the same time, hull structure arc-welded joints inevitably contain flaws or crack nuclei (defects) at the weld toe- and root notches, i.e. using the damage tolerant mode-I parameter K_I seems justified since fatigue associated to the {MCF, HCF} life time range at both locations will predominantly be a matter of micro- and macro-crack growth. The zone 3 related equilibrium equivalent stress contribution has been used to obtain a far field factor, distinguishing different type of cracks related to (non-) symmetry conditions for both (quasi) 2D- and 3D configurations. A notch factor incorporates the zone {1, 2} governing self-equilibrating stress. Remote mechanical weld toe- and weld root stress intensities show the zone {1, 2} notch affected- and zone 3 far field dominated parts define a micro- and macro-crack region, turning the stress field similarity into a stress intensity similarity. Each stress component dominates a certain crack length range: the notch stress the micro-crack region, the structural field stress the macro-crack region; the weld load carrying stress determines the transition (i.e. apex) location. The welding induced and displacement controlled mode-I residual stress intensity factor ?K_I?^r is acquired for both weld toe and weld root notches to complete the total weld notch stress intensity similarity factor formulation ?K_I?^T. Cyclic remote mechanical- and quasi-constant thermal residual loading turn ?K_I?^T into a crack growth driving force ??K_I?^T and defects may develop into cracks. The crack growth rate (da/dn) of micro-cracks emanating at notches show elastoplastic wake field affected anomalies, i.e. monotonically increasing or non-monotonic behaviour beyond the material threshold. Modifying Paris’ equation, a two-stage micro- and macro-crack growth law similarity is proposed to include both the weld notch- and far field characteristic contributions, elastoplasticity as well as remote mechanical- and thermal residual mean stress effects. Small/short crack growth data obtained using standard specimens including {SEN, DEN, CEN} in crack configuration – representing weld root notch geometries at the same time – available in literature has been reinvestigated for the alternating material zones in (aluminium) arc-welded joints: WM and HAZ zone containing respectively the weld root- and weld toe notch fatigue damage location, as well as BM for comparison. Fatigue testing series have been developed to identify crack growth behaviour at weld toe notches in aluminium arc-welded joints, adopting a typical fillet weld DS T-joint geometry. Using DIC, the required far field- and notch region parameters are obtained. Spatial displacement fields are estimated on a general kinematic basis using commercial DIC software (Istra4D, Dantec Dynamics). A posteriori, as a mechanical filtering process, the displacement fields are decomposed onto a selected kinematic basis, i.e. an Airy stress function. The displacement amplitudes, least squares solutions, present in a one-to-one correspondence the crack growth governing parameters: linear far field stress distribution, SIF and crack tip location. A sequence of images provides the temporal solution; weld toe crack growth data series showing both far field characteristics and notch affected (non-monotonic) anomalies. Crack growth model integration yields a (MCF) single slope resistance relation, a joint S_T-N curve correlating arc-welded joint life time N and the total stress parameter S_T; a line (equivalent point) criterion to estimate hull structure longevity ensuring {SSS, LSS, FSS} welded joint fatigue resistance similarity. A dual slope (i.e. random fatigue limit) formulation has been adopted to incorporate HCF taking the transition in fatigue damage mechanism (i.e. growth dominant turns into initiation controlled for decreasing load level), a slope change, into account. Regression analysis (i.e. a likelihood approach) is adopted to estimate model parameters, managing both complete- and right-censored data; failures and run-outs. Artificial fatigue test data of DS T-joints is investigated to determine the S_T parameter quality. The fatigue life uncertainty is about a factor 2 (T_S?1:1.2). As-welded SSS (T-T) CA data available in literature has been used to establish a family of (damage tolerant engineering) joint S_T-N fatigue resistance design curves to be able to estimate the fatigue life time N of welded joints (production quality is average) knowing the joint geometry and far field structural response. The MCF life time uncertainty bandwidth increases up to a factor 6, i.e. (T_S?1:1.6). In the hull structure (HCF) design region uncertainty is significant, predominantly because of lacking complete data. Full scale structure representative {T-T literature, T-C} CA LSS data has been examined to verify a SSS data scatter band fit. Since CA {SSS, LSS} fatigue resistance is principally used to estimate a VA FSS value adopting the Palmgren-Miner hypothesis, VA SSS data available in literature is examined and a scatter band fit is observed. The involved equivalent total stress parameter S_(T,eq) is obtained adopting an extended rain flow counting algorithm to capture the damage cube. Last but not least, hourly fatigue damage estimates D_h are obtained for some frame-stiffener connections in the slamming zone of an aluminium high-speed craft, using the FSS response as measured for several trials at the North Sea. The wave (loading) statistics induced D_h uncertainty is about a factor 2.5 comparing the measurement- and simulation structural response based values; quite close to the MCF fatigue design resistance value of 3 (R99Cxx – R50Cxx). The TS concept is implemented in a high-speed craft fatigue design tool, available to all research partners. Using the welded joints geometry- and loading induced far field structural response information, the fatigue damage estimate D(S_T ) of all notch locations is calculated and the governing one identified to obtain life time N.Maritime and Transport Technology (Ship Hydromechanics and Structures)Mechanical, Maritime and Materials Engineerin

Spiders in the Web: Understanding the Evolution of REDD+ in Southwest Ghana

The implementation of the global programme on Reducing Emissions from Deforestation and Forest Degradation in developing countries, and the role of Conservation, Sustainable Management of Forests and Enhancement of Forest Carbon Stocks (REDD+) is lacks a robust financial mechanism and is widely criticized for producing too little positive impact for climate, nature, and people. In many countries with tropical forests however, a variety of REDD+ projects continue to develop on the ground. This paper fills in some of the gaps in our understanding of the dynamic relation between global policy making and implementation of REDD+ on the ground. Using the introduction of REDD+ in Southwest Ghana as an example, we apply a practice-based approach toanalyze the different roles that local actors and global-local intermediaries played in the introduction of REDD+. Our results show a more balanced picture than polarized debates at the global levels suggest. The logic of practice explains how REDD+ was translated to the local situation. Global actorstook a lead but depended on local actors to make REDD+ work. Together, they integrated elements of existing practices that helped REDD+ ‘land’ locally but also transformed REDD+ globally to resemble such local practices. REDD+ initiatives absorbed elements from established community-basedconservation, forest restoration, and sustainable agro-forestry practices. The evolution of REDD+ in Ghana reflects global trends to integrate REDD+ with landscape approaches

BEM–FEM coupling for the analysis of flexible propellers in non-uniform flows and validation with full-scale measurements

The first part of the paper presents a partitioned fluid–structure interaction (FSI) coupling for the non-uniform flow hydro-elastic analysis of highly flexible propellers in cavitating and non-cavitating conditions. The chosen fluid model is a potential flow solved with a boundary element method (BEM). The structural sub-problem has been modelled with a finite element method (FEM). In the present method, the fully partitioned framework allows one to use another flow or structural solver. An important feature of the present method is the time periodic way of solving the FSI problem. In a time periodic coupling, the coupling iterations are not performed per time step but on a periodic level, which is necessary for the present BEM–FEM coupling, but can also offer an improved convergence rate compared to a time step coupled method. Thus, it allows to solve the structural problem in the frequency domain, meaning that any transients, which slow down the convergence process, are not computed. As proposed in the method, the structural equations of motion can be solved in modal space, which allows for a model reduction by involving only a limited number of mode shapes. The second part of the paper includes a validation study on full-scale. For the full-scale validation study a purposely designed composite propeller with a diameter of 1 m has been manufactured. Also an underwater measurement set-up including a stereo camera system, remote control of the optics and illumination system has been developed. The propeller design and the underwater measurement set-up are described in the paper. During sea trials blade deflections have been measured in three different positions. A comparison between measured and calculated torque shows that the measured torque is much larger than computed. This is attributed to the differences between effective and nominal wakefields, where the latter one has been used for the calculations. To correct for the differences between measured and computed torque the calculated pressures have been amplified accordingly. In that way the deformations which have been computed with the BEM–FEM coupling for non-uniform flows became very similar to the measured results.</p

Boundary element modelling aspects for the hydro-elastic analysis of flexible marine propellers

Boundary element methods (BEM) have been used for propeller hydrodynamic calculations since the 1990s. More recently, these methods are being used in combination with finite element methods (FEM) in order to calculate flexible propeller fluid–structure interaction (FSI) response. The main advantage of using BEM for flexible propeller FSI calculations is the relatively low computational demand in comparison with higher fidelity methods. However, the BEM modelling of flexible propellers is not straightforward and requires several important modelling decisions. The consequences of such modelling choices depend significantly on propeller structural behaviour and flow condition. The two dimensionless quantities that characterise structural behaviour and flow condition are the structural frequency ratio (the ratio between the lowest excitation frequency and the fundamental wet blade natural frequency) and the reduced frequency. For both, general expressions have been derived for (flexible) marine propellers. This work shows that these expressions can be effectively used to estimate the dry and wet fundamental blade frequencies and the structural frequency ratio. This last parameter and the reduced frequency of vibrating blade flows is independent of the geometrical blade scale as shown in this work. Regarding the BEM-FEM coupled analyses, it is shown that a quasi-static FEM modelling does not suffice, particularly due to the fluid-added mass and hydrodynamic damping contributions that are not negligible. It is demonstrated that approximating the hydro-elastic blade response by using closed form expressions for the fluid added mass and hydrodynamic damping terms provides reasonable results, since the structural response of flexible propellers is stiffness dominated, meaning that the importance of modelling errors in fluid added mass and hydrodynamic damping is small. Finally, it is shown that the significance of recalculating the hydrodynamic influence coefficients is relatively small. This fact might be utilized, possibly in combination with the use of the closed form expressions for fluid added mass and hydrodynamic damping contributions, to significantly reduce the computation time of flexible propeller FSI calculationsShip Hydromechanics and Structure

Mode-III fatigue of welded joints in steel maritime structures: Weld notch shear stress distributions and effective notch stress based resistance

The predominant mode-I response of maritime structures can be multiaxial, involving out-of-plane mode-III shear components. Semi-analytical mode-III notch stress distribution formulations have been established for critical details like welded T-joints and cruciform joints, reflecting (non-)symmetry with respect to half the plate thickness. Using a stress distribution formulation based effective notch stress as fatigue strength criterion, the mode-III welded joint mid-cycle fatigue resistance characteristics have been investigated. In comparison to mode-I, the material characteristic length and resistance curve slope estimate suggest the fatigue damage process to be even more an initiation related near-surface phenomenon. Mean shear stress effects seem insignificant.Ship Hydromechanics and Structure

Mode-{I, III} multiaxial fatigue of welded joints in steel maritime structures: Effective notch stress based resistance incorporating strength and mechanism contributions

The response of maritime structures can be multiaxial, involving predominant mode-I and non-negligible mode-III components. Adopting a stress distribution formulation based effective notch stress as fatigue strength parameter for mixed mode-{I, III} multiaxial fatigue assessment purposes, a mode-I equivalent von Mises type of failure criterion has been established at the critical fracture plane. Counting includes a cycle-by-cycle non-proportionality measure and damage accumulation is based on a linear model. Distinguished mode specific and material characteristic strength and mechanism contributions in terms of respectively the resistance curve intercept and mean stress induced response ratio coefficient, resistance curve slope and material characteristic length, have been incorporated. Evaluating the mid-cycle fatigue resistance, the outperformance is impressive. The analysed multiaxial mode-{I, III} data fits the uniaxial mode-I reference data scatter band and a single resistance curve can be used for fatigue assessment.Ship Hydromechanics and StructuresShip and Offshore Structure