Stochastic Aeroelastic Analysis of Composite Helicopter Rotor

The effect of uncertainty in composite material properties on the aeroelastic response, vibratory loads, and stability of a hingeless helicopter rotor is investigated. The uncertainty impact on rotating natural frequencies of the blade is studied with Monte Carlo simulations and first-order reliability methods. The stochastic aeroelastic analyses in hover and forward flight are carried out with Monte Carlo simulations. The flap, lag, and torsion responses show considerable scatter from their baseline values, and the uncertainty impact varies with the azimuth angle. Furthermore, the blade response shows finite probability of resonance-type conditions caused by modal frequencies approaching multiples of the rotor speed. The 4/rev vibratory forces show large deviations from their baseline values. The lag mode damping shows considerable scatter due to uncertain material properties with an almost 40% probability of instability in hover.

Effect of Constant and Cyclic Current Stressing on the Evolution of Intermetallic Compound Layers

In this paper the effects of constant and cyclic power loads on the evolution of interfacial reaction layers in lead-free solder interconnections are presented. Firstly, the differences in the growth behavior of intermetallic compound (IMC) layers at the cathode and anode sides of the interconnections are rationalized. This is done by considering the changes in the intrinsic fluxes of elements owing to electromigration as well as taking into account the fact that the growth of Cu3Sn and Cu6Sn5 are coupled via interfacial reactions. In this way, better understanding of the effect of electron flux on the growth of each individual layer in the Cu-Sn system can be achieved. Secondly, it is shown that there is a distinct difference between steady-state current stressing (constant current, constant temperature) and power cycling with alternating on- and off-cycle periods (accompanied by a change of temperature). The reasons behind the observed differences are subsequently discussed. Finally, special care is taken to ensure that the current densities are chosen in such a way that there is no risk for even partial melting of the solder interconnections.

Sliding wear studies in epoxy containing alumina powders

The effect of hard and refractory alumina additions on the mechanical properties of polymer in general and wear behavior in particular is not well studied. In this work, therefore, the changes in wear behavior of epoxy resin due to the additions of alumina powders have been looked into. Using a pin-on-disc set up, dry sliding wear tests were done on both filled (4, 8, & 11 wt. % alumina) and unfilled samples. A sliding velocity of 0.83 m/sec. and a sliding distance of 2 km were employed for the study. Load range used varied from 9.8 N to about 29 N. The experiments point to an increased resistance to wear with an increased presence of filler in the matrix. Further, higher loads result in larger loss of material irrespective of the filler level in the composite.

Shape optimization of RC flexural members

A natural velocity field method for shape optimization of reinforced concrete (RC) flexural members has been demonstrated. The possibility of shape optimization by modifying the shape of an initially rectangular section, in addition to variation of breadth and depth along the length, has been explored. Necessary shape changes have been computed using the sequential quadratic programming (SQP) technique. Genetic algorithm (Goldberg and Samtani 1986) has been used to optimize the diameter and number of main reinforcement bars. A limit-state design approach has been adopted for the nonprismatic RC sections. Such relevant issues as formulation of optimization problem, finite-element modeling, and solution procedure have been described. Three design examples-a simply supported beam, a cantilever beam, and a two-span continuous beam, all under uniformly distributed loads-have been optimized. The results show a significant savings (40-56%) in material and cost and also result in aesthetically pleasing structures. This procedure will lead to considerable cost saving, particularly in cases of mass-produced precast members and a heavy cast-in-place member such as a bridge girder.

A finite element assessment of flexural strength of prestressed concrete beams with fiber reinforcement

This paper presents an assessment of the flexural behavior of 15 fully/partially prestressed high strength concrete beams containing steel fibers investigated using three-dimensional nonlinear finite elemental analysis. The experimental results consisted of eight fully and seven partially prestressed beams, which were designed to be flexure dominant in the absence of fibers. The main parameters varied in the tests were: the levels of prestressing force (i.e, in partially prestressed beams 50% of the prestress was reduced with the introduction of two high strength deformed bars instead), fiber volume fractions (0%, 0.5%, 1.0% and 1.5%), fiber location (full depth and partial depth over full length and half the depth over the shear span only). A three-dimensional nonlinear finite element analysis was conducted using ANSYS 5.5 [Theory Reference Manual. In: Kohnke P, editor. Elements Reference Manual. 8th ed. September 1998] general purpose finite element software to study the flexural behavior of both fully and partially prestressed fiber reinforced concrete beams. Influence of fibers on the concrete failure surface and stress-strain response of high strength concrete and the nonlinear stress-strain curves of prestressing wire and deformed bar were considered in the present analysis. In the finite element model. tension stiffening and bond slip between concrete and reinforcement (fibers., prestressing wire, and conventional reinforcing steel bar) have also been considered explicitly. The fraction of the entire volume of the fiber present along the longitudinal axis of the prestressed beams alone has been modeled explicitly as it is expected that these fibers would contribute to the mobilization of forces required to sustain the applied loads across the crack interfaces through their bridging action. A comparison of results from both tests and analysis on all 15 specimens confirm that, inclusion of fibers over a partial depth in the tensile side of the prestressed flexural structural members was economical and led to considerable cost saving without sacrificing on the desired performance. However. beams having fibers over half the depth in only the shear span, did not show any increase in the ultimate load or deformational characteristics when compared to plain concrete beams. (C) 2002 Published by Elsevier Science Ltd.

Constraint loss under dynamic loading in rate independent plastic solids

The objectives of this paper are to examine the loss of crack tip constraint in dynamically loaded fracture specimens and to assess whether it can lead to enhancement in the fracture toughness at high loading rates which has been observed in several experimental studies. To this end, 2-D plane strain finite element analyses of single edge notched (tension) specimen and three point bend specimen subjected to time varying loads are performed. The material is assumed to obey the small strain J(2) flow theory of plasticity with rate independent behaviour. The results demonstrate that a valid J-Q field exists under dynamic loading irrespective of the crack length and specimen geometry. Further, the constraint parameter Q becomes strongly negative at high loading rates, particularly in deeply cracked specimens. The variation of dynamic fracture toughness K-dc with stress intensity rate K for cleavage cracking is predicted using a simple critical stress criterion. It is found that inertia-driven constraint loss can substantially enhance K-dc for (K) over dot > 10(5) MPa rootm/s.

Fabrication and evaluation of 450 F electrochemical redox supercapacitors using inexpensive and high-performance, polyaniline coated, stainless-steel electrodes

Redox supercapacitors using polyaniline (PANI) coated. stainless-steel (SS) electrodes have been assembled and characterized. PANI has been deposited on SS substrate by a potentiodynamic method from an acidic electrolyte which contains aniline monomer. By employing stacks of electrodes, each with a geometrical area of 24 cm(2), in acidic perchlorate electrolyte, a capacitance value of about 450 F has been obtained over a long cycle-life. Characterization studies have been carried out by galvanostatic charge-discharge cycling of the capacitors singly, as well as in series and parallel configurations. Various electrical parameters have been evaluated. Use of the capacitors in parallel with a battery for pulse-power loads. and also working of a toy fan connected to the charged capacitors have been demonstrated. A specific capacitance value of about 1300 F g(-1) of PANI has been obtained at a discharge power of about 0.5 kW kg(-1). This value is several times higher than those reported in the literature for PANI and is, perhaps, the highest value known for a capacitor material. The inexpensive SS substrate and the high-capacitance PANI are favorable factors for commercial exploitation. (C) 2002 Elsevier Science B.V. All rights reserved.

Interactions of interfacial arc cracks

The interaction of two interfacial arc cracks around a circular elastic inclusion embedded in an elastic matrix is examined. New results for stress intensity factors for a pair of interacting cracks are derived for a concentrated force acting in the matrix. For verifying the point load solutions, stress intensity factors under uniform loading are obtained by superposing point force results. For achieving this objective, a general method for generating desired stress fields inside a test region using point loads is described. The energetics of two interacting interfacial arc cracks is discussed in order to shed more light on the debonding of hard or soft inclusions from the matrix. The analysis based on complex variables is developed in a general way to handle the interactions of multiple interfacial arc cracks/straight cracks.

Optimum design of a helicopter rotor for low vibration using aeroelastic analysis and response surface methods

An aeroelastic analysis based on finite elements in space and time is used to model the helicopter rotor in forward flight. The rotor blade is represented as an elastic cantilever beam undergoing flap and lag bending, elastic torsion and axial deformations. The objective of the improved design is to reduce vibratory loads at the rotor hub that are the main source of helicopter vibration. Constraints are imposed on aeroelastic stability, and move limits are imposed on the blade elastic stiffness design variables. Using the aeroelastic analysis, response surface approximations are constructed for the objective function (vibratory hub loads). It is found that second order polynomial response surfaces constructed using the central composite design of the theory of design of experiments adequately represents the aeroelastic model in the vicinity of the baseline design. Optimization results show a reduction in the objective function of about 30 per cent. A key accomplishment of this paper is the decoupling of the analysis problem and the optimization problems using response surface methods, which should encourage the use of optimization methods by the helicopter industry. (C) 2002 Elsevier Science Ltd. All rights reserved.

Role of in situ generated tribofilm on the tribological characteristics of monolith and TiB2 reinforced MoSi2 intermetallic

Wear experiments performed on steel disc with increasing load for monolithic MoSi2 of different densities and its composite with TiB2 showed three distinct wear regimes. The specimens exhibited severe wear rate below the lower and above the upper critical loads and mild wear in between the two critical loads. The increase in density of the monolith and the reinforcement of TiB2 were effective in reducing the coefficient of friction and the specific wear rate. The wear experiments have been performed in these three regimes (15, 50 and 75 N). The tribofilm formed on the pin surface was found to contain both pin and disc materials. The temperature of the pins during the sliding against EN-24 disc was calculated using one dimensional heat transfer equation at different loads for each composition. The composite experiences lower temperatures compared to the monoliths. (C) 2002 Elsevier Science B.V. All rights reserved.

Static Balancing of Spring-loaded Planar Revolute-joint Linkages without Auxiliary Links

We present a method to statically balance a general treestructured,planar revolute-joint linkage loaded with linear springs or constant forces without using auxiliary links. The balancing methods currently documented in the literature use extra links; some do not apply when there are spring loads and some are restricted to only two-link serial chains. In our method, we suitably combine any non-zero-free-length load spring with another spring to result in an effective zero-free-length spring load. If a link has a single joint (with the parent link), we give a procedure to attach extra zero-free-length springs to it so that forces and moments are balanced for the link. Another consequence of this attachment is that the constraint force of the joint on the parent link becomes equivalent to a zero-free-length spring load. Hence, conceptually,for the parent link, the joint with its child is removed and replaced with the zero-free-length spring. This feature allows recursive application of this procedure from the end-branches of the tree down to the root, satisfying force and moment balance of all the links in the process. Furthermore, this method can easily be extended to the closed-loop revolute-joint linkages, which is also illustrated in the paper.

Performance evaluation of nonthermal plasma reactors for NO oxidation in diesel engine exhaust gas treatment

The discharge plasma-chemical hybrid process for NOinfinity removal from the flue gas emissions is an extremely effective and economical approach in comparison with the conventional selective catalytic reduction system. In this paper we bring out a relative comparison of several discharge plasma reactors from the point of NO removal efficiency. The reactors were either energized by ac or by repetitive pulses. Ferroelectric pellets were used to study the effect of pellet assisted discharges on gas cleaning. Diesel engine exhaust, at different loads; is used to approximately simulate the flue gas composition. Investigations were carried out at room temperature with respect to the variation of reaction products against the discharge power. Main emphasis is laid on the oxidation of NO to NO2, without reducing NOx concentration (i.e., minimum reaction byproducts), with least power consumption. The produced NO2 will be totally converted to N-2 and Na-2 SO4 using Na-2 SO3. The ac packed-bed reactor and pelletless pulsed corona reactor showed better performance, with minimum reaction products for a given power, when the NO concentration was low (similar to 100 ppm). When the engine load exceeds 50% (NO > 300 ppm) there was not much decrease in NO reduction and more or less all the reactors performed equally. The total operating cost of the plasma-chemical hybrid system becomes $4010/ton of NO, which is 1/3-1/5 of the conventional selective catalytic process.

Reliability-based load and resistance factors for soil-nail walls

Existing soil nailing design methodologies are essentially based on limit equilibrium principles that together with a lumped factor of safety or a set of partial factors on the material parameters and loads account for uncertainties in design input parameter values. Recent trends in the development of design procedures for earth retaining structures are towards load and resistance factor design (LRFD). In the present study, a methodology for the use of LRFD in the context of soil-nail walls is proposed and a procedure to determine reliability-based load and resistance factors is illustrated for important strength limit states with reference to a 10 m high soil-nail wall. The need for separate partial factors for each limit state is highlighted, and the proposed factors are compared with those existing in the literature.

Time dependent deformations in concrete: A multi-scale approach

Estimation of creep and shrinkage are critical in order to compute loss of prestress with time in order to compute leak tightness and assess safety margins available in containment structures of nuclear power plants. Short-term creep and shrinkage experiments have been conducted using in-house test facilities developed specifically for the present research program on 35 and 45 MPa normal concrete and 25 MPa heavy density concrete. The extensive experimental program for creep, has cylinders subject to sustained levels of load typically for several days duration (till negligible strain increase with time is observed in the creep specimen), to provide the total creep strain versus time curves for the two normal density concrete grades and one heavy density concrete grade at different load levels, different ages at loading, and at different relative humidity’s. Shrinkage studies on prism specimen for concrete of the same mix grades are also being studied. In the first instance, creep and shrinkage prediction models reported in the literature has been used to predict the creep and shrinkage levels in subsequent experimental data with acceptable accuracy. While macro-scale short experiments and analytical model development to estimate time dependent deformation under sustained loads over long term, accounting for the composite rheology through the influence of parameters such as the characteristic strength, age of concrete at loading, relative humidity, temperature, mix proportion (cement: fine aggregate: coarse aggregate: water) and volume to surface ratio and the associated uncertainties in these variables form one part of the study, it is widely believed that strength, early age rheology, creep and shrinkage are affected by the material properties at the nano-scale that are not well established. In order to understand and improve cement and concrete properties, investigation of the nanostructure of the composite and how it relates to the local mechanical properties is being undertaken. While results of creep and shrinkage obtained at macro-scale and their predictions through rheological modeling are satisfactory, the nano and micro indenting experimental and analytical studies are presently underway. Computational mechanics based models for creep and shrinkage in concrete must necessarily account for numerous parameters that impact their short and long term response. A Kelvin type model with several elements representing the influence of various factors that impact the behaviour is under development. The immediate short term deformation (elastic response), effects of relative humidity and temperature, volume to surface ratio, water cement ratio and aggregate cement ratio, load levels and age of concrete at loading are parameters accounted for in this model. Inputs to this model, such as the pore structure and mechanical properties at micro/nano scale have been taken from scanning electron microscopy and micro/nano-indenting of the sample specimen.

Robust Aeroelastic Optimization Of Composite Helicopter Rotor

A robust aeroelastic optimization is performed to minimize helicopter vibration with uncertainties in the design variables. Polynomial response surfaces and space-¯lling experimental designs are used to generate the surrogate model of aeroelastic analysis code. Aeroelastic simulations are performed at the sample inputs generated by Latin hypercube sampling. The response values which does not satisfy the frequency constraints are eliminated from the data for model ¯tting. This step increased the accuracy of response surface models in the feasible design space. It is found that the response surface models are able to capture the robust optimal regions of design space. The optimal designs show a reduction of 10 percent in the objective function comprising six vibratory hub loads and 1.5 to 80 percent reduction for the individual vibratory forces and moments. This study demonstrates that the second-order response surface models with space ¯lling-designs can be a favorable choice for computationally intensive robust aeroelastic optimization.