Shear Properties of the Re-Entrant Auxetic Structure Made via Electron Beam Melting

While the tensile/compressive mechanical properties of the re-entrant auxetic cellular structure have been relatively well modeled, their shear properties including the shear modulus and shear strength have not been investigated. This paper focuses on the analytical modeling of the shear properties of this auxetic structure utilizing beam analysis. The modeling results were further compared with results from both simulation and experimentation. It was found that in addition to the effective length reduction effect, the size effect also becomes significant for the shearing of this re-entrant auxetic structures. Due to the size effect, it was expected that the re-entrant auxetic structure could not be effectively homogenized based on the developed analytical property model, and additional design factors must be considered in the future.Mechanical Engineerin

Laser Beam Welding of Stainless Steels

Obiettivo principale di questa tesi di dottorato è quello di studiare la saldatura laser degli acciai inossidabili. Durante gli esperimenti un laser Nd-YAG in onda continua da 1.1kW è stato utilizzato per saldare rispettivamente acciai martensitici in configurazione di piena penetrazione e combinazioni di acciai austenitici/ferritici in configurazione d’angolo. È stata studiata l’influenza di vari parametri di processo come potenza del laser, velocità di saldatura, diametro della fibra, angolo di incidenza e defocalizzazione nonché le loro interazioni sulla geometria del giunto e sulle sue proprietà meccaniche. Si sono analizzati, inoltre, gli effetti della densità di energia e dell’energia per unità di lunghezza sulle caratteristiche del giunto di saldatura in modo da evidenziare la dipendenza del processo dai fenomeni di scambio termico. Successivamente si è studiato la microstruttura della solidificazione del giunto e la relativa distribuzione degli elementi di lega per diversi valori della densità di energia correlandole con la variazione locale della microdurezza. Durante il corso della ricerca, sono state utiliizzate tecniche di DOE come il FFD ed il RSM con l’obiettivo di modellare ed ottimizzare il processo di saldatura laser. In questa fase, per ogni materiale saldato sono stati elaborati dei modelli in grado di determinare i fattori chiave che governano il processo. Tali modelli, inoltre, sono stati ottimizzati prendendo in considerazione la combinazione dei parametri di processo che consente di avere giunti di qualità superiore in termin di geometria e caratteristiche meccaniche. È stato, infine, elaborato un modello teorico per la determinazione della geometria di un giunto ottenibile dalla saldatura in piena penetrazione di acciai ferritici. Tale modello si basa sul concetto che la geometria risultante è funzione del tipo di scambio termico che si genera durante il processo e che a sua volta tale scmbio termico vari in funzione della densità di energia fornita dla laser. Il modello ha dimostrato una corrispondenza con i dati sperimentali con una maggior accuratezza nel caso di saldatura per conduzione

A Quasi-3-D Theory for Impedance Eduction in Uniform Grazing Flow

A 2-D impedance eduction methodology is extended to quasi-3-D sound fields in uniform or shearing mean flow. We introduce a nonlocal, nonreflecting boundary condition to terminate the duct and then educe the impedance by minimizing an objective function. The introduction of a parallel, sparse, equation solver significantly reduces the wall clock time for educing the impedance when compared to that of the sequential band solver used in the 2-D methodology. The accuracy, efficiency, and robustness of the methodology is demonstrated using two examples. In the first example, we show that the method reproduces the known impedance of a ceramic tubular test liner. In the second example, we illustrate that the approach educes the impedance of a four-segment liner where the first, second, and fourth segments consist of a perforated face sheet bonded to honeycomb, and the third segment is a cut from the ceramic tubular test liner. The ability of the method to educe the impedances of multisegmented liners has the potential to significantly reduce the amount of time and cost required to determine the impedance of several uniform liners by allowing them to be placed in series in the test section and to educe the impedance of each segment using a single numerical experiment. Finally, we probe the objective function in great detail and show that it contains a single minimum. Thus, our objective function is ideal for use with local, inexpensive, gradient-based optimizers

Modeling the anchoring and performance of downhole equipment using an extended Gurson model

In oil and gas (O&G) exploration the well casing, in the form of a long steel tube, maintains the opening of the drilled well hole. Mechanical equipment is often inserted into the well for the purpose of well monitoring, pressure control and various operations. This downhole equipment may be mechanically connected to the pipe casing by the outward radial motion of anchoring teeth such that the inner wall casing is indented. The connection between the tool and the casing must support significant mechanical loads in the longitudinal (axial) direction of the casing, i.e. transverse to the direction of indentation, while minimizing the indentation depth in order to preserve the stiffness and strength of the casing. Consequently, a determination of the ultimate strength of the connection is of critical importance. Failure of this connection involves intense shear of the inner wall of the casing, akin to a machining operation. The critical load for axial slip can be determined experimentally or numerically (or by combination of both). In this study, detailed simulations are performed using the shear-extended GTN (Gurson-Tvergaard-Needleman) model. The choice of model is motivated by the need to accurately the extensive plastic deformation associated with indentation as well as shear-dominated ductile failure on a sub-millimeter scale. The shear-extended GTN model requires a careful calibration of the model parameters by an accurate measurement of the material response. Accordingly, the casing steel was characterized by appropriate measurements under a range of stress states. The calibrated model was used to investigate an idealized two-dimensional representation of the anchoring problem, with a focus on the effect of indentation depth upon connection strength. Both the indentation of the casing inner wall by the anchoring teeth and the subsequent shear of the casing wall were simulated in detail to determine the load required to initiate and progress slip of the anchoring teeth. The results of these analyses show that the connection strength increases linearly with increasing indentation depth

Production of Hybrid Tubular Metal-Fiber-Preforms: Material Characterization of Braided Hoses with a Binder

Hybrid shafts or rods, where the area of load introduction is metallic (e.g. steel or aluminium) and the area of load transfer is made of fibre reinforced plastics (FRP), are an established concept for lightweight parts. Besides the monolithic FRP and the metallic areas, the overlap area of both materials is particularly important. Such parts can beneficially be produced by the use of liquid composite moulding (LCM), where the bonding process takes place during the resin curing. This is called intrinsic hybridization. Beforehand it is crucial to produce a near-net-shape preform in which the metallic end fittings for the load introduction are already integrated. To manufacture such parts constantly with a high quality, a process model of the joining by draping the braided preform is necessary. In this paper an approach for the production of hybrid preforms made of braided hoses and metallic fittings is presented in order to develop a process model. The process starts with a cylindrical multi-layer preform made of braided hoses, in which the layers are bonded by a thermoplastic binder powder. The decisive process step is the draping of the preform onto the metallic fitting. For this forming step, the material characterization of the hybrid preform plays an important role. Several material tests to determine the textile parameters of the preform are therefore evaluated and performed. Finally, the results of these tests are presented and discussed

Discrete Stiffness Tailoring: Optimised design and testing of minimum mass stiffened panels

Discrete Stiffness Tailoring (DST) is a novel manufacturing concept where stiffness tailoring is achieved using discrete changes in ply angle to favourably redistribute stresses. Resulting performance increases can be exploited to potentially achieve lightweight rapidly manufacturable structures, uninhibited by the minimum tow-turning radii which limit continuous fibre steering approaches. An efficient two-stage optimisation routine is implemented to design a DST minimum-mass stiffened aircraft wing panel subject to buckling and manufacturing feasibility constraints. The panel is manufactured and compression tested to failure, extending the DST design concept to component level for the first time. A weight reduction of 14.4% is achieved compared to a constant stiffness optimum, through redistribution of load to the stiffener region. The optimum design removes material from the skin, between stiffeners. Experimentally, the optimised tailored panel achieved a buckling load, without failure, within 5% of that predicted, validating both the methodology and modelling