1,721,457 research outputs found
Damping contributions of coatings to the viscoelastic behaviour of mechanical components
No full textCoating layer technology is known in literature as an effective tool for modifying the viscoelastic behaviour of materials. In this work various coating solutions are investigated in order to estimate their contribution to the dissipative behaviour of mechanical components. Different production processes are used to apply single-layer and dual-layer coatings of Al oxide, Ti and TiO2 , Cr and CrN to uncoated specimens made of Al, stainless steel and harmonic steel. Force and displacement experimental data obtained from forced excitation dynamic mechanical measurements are used to find the specimen constitutive equation estimated parameters. A high order generalized Kelvin model is adopted to model the constitutive equivalent material relationship of coated specimens in the form of the ratio of two polynomials expressed as a function of frequency. The model parameters are numerically identified and the model reduced and optimized by means of a robust identification technique
Experimental identification of the material standard linear solid model parameters by means of dynamical measurements
Full text linkA procedure for the experimental identification of the material standard linear solid model parameters by means of dynamic mechanical analysis test instrument measurements is presented. Since the standard linear solid material stress–strain functional D(ω) relationship in the frequency domain formally depends on the standard linear solid material parameters, a procedure able to identify these parameters from test measurement estimates is proposed in this work. Nevertheless, a critical, nonlinear and non-parametric approach is to be followed since the number of the material standard linear solid block components is generally unknown, and the material D(ω) shows a highly nonlinear dependency on the unknown standard linear solid material parameters. For these reasons, measurement and test model noise is expected to strongly influence the accuracy of the identification results. A multi-step procedure is presented, consisting first in the non-parametric identification of a frequency dependent, two degrees of freedom model instrument frame by means of a polynomial rational function, where polynomial order and parameters, such as polynomial coefficients and pole-residue couples, are optimally identified by means of an algebraic numerical technique and of an iterative stabilization procedure. Another procedure able to identify the material D(ω) polynomial rational functional relationship in the frequency domain is also proposed, taking into account the dynamic contribution of the instrument frame, of the inertial contribution of the distributed mass of the beam and of the lumped mass of the instrument force measuring system. An effective procedure, able to identify the standard linear solid material model parameters in the time domain from the identified material physical poles, is finally proposed. Some application examples, concerning the identification of the standard linear solid model of a known material and of an unknown composite material, are shown and discussed as well
Experimental identification of the parameters of a multilayer coating architecture
No full textComposite components with specifically designed and optimized vibration damping behaviour can be obtained by means of multilayered coating technology solutions [1-3], and these authors presented a multilayer beam model that takes into account of the dissipative actions at the interface between different layers [3], modeled by means of a complex interlaminar impedance. This model can be used as a virtual prototyping tool in order to find and optimize innovative multilayer coating solutions. Since the model needs laminar parameters as input, i.e. the density and modulus of the layer materials and the interface parameters values that define the complex impedance between different coating materials, dynamic mechanical measurements and an optimization based numerical technique are used to find these laminar and interlaminar parameters. An identification procedure is applied to measurement data from three-layer and five-layer beam specimens in order to identify the unknown interface parameters modeling the interlaminar impedance.
Normalized estimator z [3] is used to evaluate the damping behaviour. Error vector e, function of the unknown parameters, is defined as the difference between z calculated from the model and z measured at the same frequency values. The objective function Ψ=0.5·e·eT is obtained. A multi-step optimization algorithm based on a quadratic, constrained non-linear optimisation technique is employed to identify the interface parameter values that minimize Ψ. Fig. (1a) shows examples of identification, i.e. measurements and the resulting model fit for five-layer beams, adopting Al and E1, E2 epoxy based layers.
Identified parameter values are then used as input to the multilayer beam virtual prototype model in order to simulate the damping behaviour of different multilayer composite architectures. Fig. (1b) shows the z model estimate from the C1 (E2-E1-Al-E1-E2) five-layer beam virtual prototype model, and the z experimental validation. Some different
coating solutions are then applied to gear pump Al casings to be tested and compared, by means of sound pressure measurements, in a real industrial application with real operating conditions, i.e. revolution speed n [1000-2000] rpm, fluid (oil) pressure p [50-230] bar.
Results concerning the application of the E1-E2 coating architecture are reported in Fig. (2). A discussion and a critical analysis of the results is presented
Multi layer composite coating technology for high damping mechanical structural applications
No full textMulti layer coating technology can be an effective tool to design composite materials with specifically designed damping behavior and some applications are recently known in the aerospace and in the automotive industry. Dynamic mechanical measurements were made on thin-walled components, in the form of multi layered beam specimen, to investigate the composite dissipative properties. Conventional beam theories, can be unsuitable to accurately describe the complex damping behavior of multi layered beams and cannot take into account of contributions such as the frictional actions and slipping at the interface between layers. In this work, multi layered composite beams are modelled, by means of a modified, third order, zig zag model where the contribution of the stiffness of the coating layer materials and of the hysteretic actions at the layer interfaces is taken into account. Third order polynomial functions are adopted to describe the longitudinal displacement of the beam and the continuity of the shear stress across the multi layered beam depth is enforced. The number of kinematic variables used to define the beam motion may depend on the
number of layers of the beam, and on the imposed kinematic and dynamic continuity conditions. Such a model is used to get optimal coating solutions with specific dissipative properties at the modelling stage. Some applications are presented and results are discussed
A novel approach for the fractional SLS material model experimental identification
Full text linkA multi-step, iterative technique for the local non-parametric identification of the standard linear solid (SLS) material model employing fractional order time differential operators is presented. Test input data consists of a set of identified material complex modulus values estimated at different frequency values, obtained from input–output experimental measurements made on a material specimen by means of forced harmonic excitation and from experimental measurements made on the same specimen in quasi-static relaxation conditions. The proposed technique is mainly based on an algebraic procedure leading to the solution of an overdetermined system of linear equations, in order to get the optimal value of the model unknown parameters. The procedure is non-parametric, since the SLS model order is initially unknown. The optimal model size can be found by evaluating the stability properties of the solution associated to any model size and by automatically discarding computational, non-physical contributions. The identification procedure is first validated by means of numerically simulated test data from within known model examples, and then it is applied to some experimentally obtained test data associated to different materials
Material model robust identification procedure from dynamical measurements made on a flexible specimen-frame system
Full text linkA procedure for the identification of the material model of beam specimens by means of harmonic force and displacement measurements in flexural condition is presented. Input-output frequency response function (FRF) evaluations are used in standard DMA instruments to estimate the material stress–strain frequency response function. Nevertheless, the contribution of the instrument frame model coupling and of the inertial contribution of the excitation moving substructure to the input–output FRFs can make such estimates meaningless in most practical applications, especially if a wide excitation frequency range is taken into account. In this work the instrument frame model contribution is estimated by means of doing calibration measurements on some reference beams and processing them by a procedure based on optimization algorithms. A signal processing-based procedure is also proposed to identify the optimal frame model rational function fit by eliminating computational and noise related contribution. The identified rational frame model is used to obtain the material model in the frequency domain being filtered from the contribution of the experimental system. The proposed technique robustness is tested on some numerical model cases. The same technique is then applied to some dynamical measurements made on specimens of different materials. The results are shown and critically discussed
IDENTIFICATION OF A FRACTIONAL SLS MATERIAL MODEL BY MEANS OF DYNAMIC AND QUASI-STATIC MEASUREMENTS
No full textA non-parametric algebraic procedure for the identification of a generalized Standard Linear
Solid (SLS) fractional material model, starting from E(w) complex modulus experimentally
estimated values in wide frequency range, is proposed. The complex modulus is experimentally estimated from within forced excitation dynamical measurements in the high [f1, f2] frequency range and from relaxation creep measurements in the [0, f1] frequency range.
The proposed model identification procedure is able to find the SLS model order and the
parameters associated to each SLS element, including the local fractional differentiation order.
The proposed identification approach is validated by means of some numerically simulated test cases from a known material model test case. The identification of the optimal equivalent material model of some experimentally tested polymeric materials, to be used in vibration damping coating applications, is also presented
On the damping behaviour of single and multi-layer coatings
No full textMechanical components with high stiffness, resistance and vibration damping specifications are tipically required in most aerospace and industrial mechanical applications. Composite materials such as multi layer materials can be properly designed and optimized to this aim. Most coating layer deposition technologies can be effectively used to increase the component dissipative behaviour, as it is also shown by some experimental results reported in this work. This result can be obtained by applying coating layers with high internal hysteresis or by maximizing frictional actions between the different layer interfaces. Different deposition techniques, such as the reactive plasma vapor deposition and the anodizing process, are considered in this work. Single-layer and multi-layer coatings are applied on metallic substrates. A theoretical model for multi-layered beams is proposed and critically discussed. Conventional beam theories theories, like the Timoshenko-Bernoulli beam theory, can not be employed to accurately describe the complex behavior of multi-layer beams and even higher order theories show some shortcomings when dealing with nonlinear contributions such as interface slipping and friction. A third order zig-zag layer-wise model approach is considered in this work, where the number of kinematic variables is not dependent on the number of layers considered in the model. The contribution of the frictional actions at the layer interfaces and of the viscoelastic behavior of the coating layer materials is considered.
The increase of the damping behaviour of coated specimens can be obtained by properly designing the interface frictional actions at the modeling stage, by adopting a consistent coating deposition technology and then by experimentally validating it. Different material solutions are tested for both the substrate and the coating technology, i.e. aluminum alloy, structural steel and stainless steel for the substrate and metal, metal oxide and metal nitride for the coating layers. Dynamical measurement data in a wide range of excitation frequency are obtained from slender beam specimens by means of a dynamic mechanical analyzer, in a flexural forced excitation experimental set-up, with clamped sliding boundary conditions. Homogeneous, uncoated specimens are also tested and experimental measurements are used to compare the effectiveness of the different coating solutions. The complex material modulus is estimated from force and displacement experimental data taking into account of the contribution of the inertial actions. The material constitutive relationship is modeled by means of a high order generalized Kelvin model, and this results in non-trivial constitutive material equations. A robust identification procedure resulting from previous work of the authors of this paper is used to identify the optimal material model and its parameters, and to eliminate the non-physical model components. The ratio of imaginary and real part of the estimated complex modulus is considered as a measure estimate of the material dissipative behaviour. Results related to different technologies are presented and compared. Some engineering test cases are considered and critically discussed in this work
Optimal design of high damping coated mechanical components
No full textIn this work a numerical procedure for the design of highly damped multi-layer coating architectures, to be applied in thin-walled mechanic components, is proposed. Optimization restraints with respect to vibration and acoustic emission reduction in operating conditions are considered. The procedure makes use of a multi-layer beam model that takes into account of dissipative actions at the interface between different layers. Dynamic mechanical measurement tests on uncoated and coated symmetric beams are used to identify the unknown material stiffness and interface parameters needed as input by the multi-layer beam model. A virtual prototyping procedure for the simulation of the dissipative effect of different multi-layer coating architectures is proposed and experimentally validated. The damping behaviour of coated slender beam specimens is tested by means of experimental measurements. Coating solutions, based on epoxy organic polymeric materials, and some architectures employing them are presented in this work. Real application examples, concerning the acoustic emission of mechanical gear pumps adopting uncoated and coated casings are presented, by means of the comparison of sound pressure measurements, related to these different configurations, in operating conditions
"Prospective comparative trial of bone marrow transplantation and postremission chemotherapy for childhood acute myelogenous leucemia"
No full textPURPOSE: This study was conducted to assess the comparative values of allogeneic bone marrow transplantation (BMT) and autologous bone marrow transplantation (ABMT) with sequential postremission chemotherapy (SPC) in children with acute myelogenous leukemia (AML) in first remission.
PATIENTS AND METHODS: From March 1987 to March 1990, 161 assessable patients younger than 15 years of age with newly diagnosed AML were treated uniformly with two courses of daunorubicin and standard-dose cytarabine. After initial consolidation with a course of daunorubicin, cytarabine, and thioguanine (DAT), patients in complete remission (CR) were randomized to receive either ABMT or SPC, except for those with an HLA-matched sibling who were assigned to undergo BMT. SPC consisted of three additional courses of DAT, followed by three pairs of drugs administered sequentially for a total of six cycles.
RESULTS: Overall, 127 of 161 patients attained CR (79%). The estimated probabilities of survival and event-free survival (EFS) at 5 years for all patients were 42% and 25%, respectively (median follow-up, 28 months). For the 127 complete responders, the 5-year probability of disease-free survival (DFS) was 31%, with a cumulative risk of relapse of 64%. For the purpose of this study, all complete responders were evaluated for analysis of disease outcome according to the intent-to-treat principle, regardless of whether they actually received the intended therapy. The 5-year DFS was 51% for the BMT group (n = 24), significantly higher (P = .03) than that observed for the other cohorts: 21% for ABMT (n = 35), 27% for SPC (n = 37), and 34% for a group of 31 nonrandomized (NR) patients. Bone marrow relapse was the most frequent cause of postremission failure in all therapeutic subgroups, including the BMT cohort, in which no deaths attributable to the toxicity of the procedure were recorded
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