Dynamic and energy analysis of frictional contact instabilities on a lumped system

When dealing with complex mechanical systems, the frictional contact is at the origin of significant changes in their dynamic behavior. The presence of frictional contact can give rise to mode-coupling instabilities that produce harmonic friction induced vibrations. Unstable vibrations can reach large amplitude that could compromise the structural and surface integrity of the system and are often associated with annoying noise emission. The study of this kind of dynamic instability has been the subject of many studies ranging from both theoretical and numerical analysis of simple lumped models to numerical and experimental investigation on real mechanical systems, such as automotive brakes, typically affected by such issue. In this paper the numerical analysis of a lumped system constituted by several degrees of freedom in frictional contact with a slider is presented, where the introduction of friction can give rise to an unstable dynamic behavior. Two different approaches are used to investigate the effects of friction forces. The first approach, the Complex Eigenvalues Analysis, allows for calculating the complex eigenvalues of the linear system that can be characterized by a positive real part (i.e. negative modal damping). The complex eigenvalues and eigenvectors of the system are investigated with respect to friction. In the second approach a non linear model has been developed accounting for the stick-slip-detachment behavior at the interface to solve the time history solution and analyze the unstable vibration. The effects of boundary conditions and of system parameters are investigated. Results comparison between the two different approaches highlights how nonlinearities affect the time history solution. The lumped model allows for a detailed analysis of the energy flows between the boundary and the system during self-excited vibrations, which are at the origin of the selection between the predicted unstable mode

Contact acoustic nonlinearity and local damage resonance for the detection of kissing bonds in structural adhesive joints

Adhesively bonded joints are susceptible to contamination of surfaces during manufacture and environmental deterioration in real operating conditions. These may cause the generation of so-called “kissing bonds” that can dramatically alter the strength of the joint leading to premature failure. Nonlinear acousto-ultrasonic (AU) techniques have shown great potential for monitoring kissing bonds with piezoelectric sensors permanently installed on the structural joint, thus enabling online and in-situ inspection. This paper investigated the combined contact acoustic nonlinearity (CAN) and local damage resonance (LDR) effects in the presence of kissing bonds in adhesive joints. Experimental nonlinear AU tests showed the formation of LDR frequency down-shifts and “jumps” of the fundamental damage resonance in adhesively bonded aluminium joints, with the kissing bond located internally to the overlapping region between the two adherends. These results were supported by a theoretical model based on the solution of the nonlinear Duffing's equation, under the assumption that the debonded region is a damped nonlinear harmonic oscillator subject to harmonic forcing. The Harmonic Balance Method was used to solve the nonlinear differential problem, showing the generation of frequency down-shifts via the dependence of the ratio between the excitation and LDR frequencies with the amplitude of the fundamental damage resonance. Additionally, two-dimensional finite element simulations using a reduced order model based on the Craig-Bampton technique were carried out to support experimental AU tests for the identification of the LDR frequency and the generation of nonlinear resonance effects. Good agreement between analytical, numerical, and experimental results revealed that a monitoring approach combining CAN and LDR is an extremely efficient and sensitive tool for ensuring integrity and safety of structural adhesive joints

Nonlinear substructuring in the modal domain: numerical validation and experimental verification in presence of localized nonlinearities

In many systems of interest, most of the structure is well approximated as linear but some parts must be treated as nonlinear to get accurate response predictions: significant nonlinear effects are due to the connections between coupled subsystems, such as in automotive or aerospace structures. The present work aims at predicting the nonlinear behavior of coupled systems using a substructuring technique in the modal domain. This study focuses on the effects of nonlinear connections on the dynamics of an assembly in which the coupled subsystems can be considered as linear. Each connection is instead considered as a quasi-linear substructure with stiffness that is function of amplitude or energy. The iterative procedure used here is enhanced with respect to previous works by enforcing a better control of the total energy at each iteration allowing to obtain the solution for a prescribed set of energy levels. Also, the initial guess and the convergence criterion have been modified to speed up the procedure. This technique is applied to a system made of two continuous linear subsystems coupled by nonlinear connections. The numerical results of the coupling are first compared to the ones obtained by using the Harmonic Balance technique on the model of the complete assembly to evaluate its effectiveness and understand the effects of modal truncation. Besides, a nonlinear connecting element, specifically designed in order to have a nearly cubic hardening behavior, is used in an experimental setup. Substructuring results are compared to experimental results measured on the assembled system, in order to evaluate the correlation between mode shapes and the accuracy in the resonance frequency at several excitation levels

Depletion of TDP-43 affects Drosophila motoneurons terminal synapsis and locomotive behavior

Pathological modifications in the highly conserved and ubiquitously expressed heterogeneous ribonucleoprotein TDP-43 were recently associated to neurodegenerative diseases including amyotrophic lateral sclerosis (ALS), a late-onset disorder that affects predominantly motoneurons [Neumann, M. et al. (2006) Ubiquitinated TDP-43 in frontotemporal lobar degeneration and amyotrophic lateral sclerosis. Science 314, 130-133, Sreedharan, J. et al. (2008) TDP-43 mutations in familial and sporadic amyotrophic lateral sclerosis. Science 319, 1668-1672, Kabashi, E. et al. (2008) TARDBP mutations in individuals with sporadic and familial amyotrophic lateral sclerosis. Nat. Genet. 40, 572-574]. However, the function of TDP-43 in vivo is unknown and a possible direct role in neurodegeneration remains speculative. Here, we report that flies lacking Drosophila TDP-43 appeared externally normal but presented deficient locomotive behaviors, reduced life span and anatomical defects at the neuromuscular junctions. These phenotypes were rescued by expression of the human protein in a restricted group of neurons including motoneurons. Our results demonstrate the role of this protein in vivo and suggest an alternative explanation to ALS pathogenesis that may be more due to the lack of TDP 43 function than to the toxicity of the aggregates. © 2009 Federation of European Biochemical Societies

Efficacy of Conventional and Organic Insecticides against Scaphoideus titanus: Field and Semi-Field Trials

Scaphoideus titanus is the main vector of phytoplasmas associated with Flavescence dorée (FD), one of the most serious threats to viticulture in many European countries. To minimize the spread of this disease, mandatory control measures against S. titanus were decided in Europe. In the 1990s, the repeated application of insecticides (mainly organophosphates) proved to be an effective measure to control the vector and the related disease in north-eastern Italy. These insecticides and most of the neonicotinoids were recently banned from European viticulture. Serious FD issues detected in the recent years in northern Italy could be related to the use of less effective insecticides. Trials aimed at evaluating the efficacy of the most used conventional and organic insecticides in the control of S. titanus have been performed in semi-field and field conditions to test this hypothesis. In efficacy trials, carried out in four vineyards, etofenprox and deltamethrin proved to be the best conventional insecticides, while pyrethrins were the most impactful among organic insecticides. Insecticide residual activity was evaluated in semi-field and field conditions. Acrinathrin showed the most significant residual effects in both conditions. In semi-field trials, most of the pyrethroids were associated with good results in terms of residual activity. However, these effects declined in field conditions, probably due to high temperatures. Organic insecticides showed poor results in terms of residual efficacy. Implications of these results in the context of Integrated Pest Management in conventional and organic viticulture are discussed

Hyperphosphorylation as a Defense Mechanism to Reduce TDP-43 Aggregation

Several neurodegenerative diseases including amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration with ubiquitinated inclusions (FTLD-U) are characterized by inclusion bodies formed by TDP-43 (TDP). We established cell and transgenic Drosophila models expressing TDP carboxyl terminal fragment (ND251 and ND207), which developed aggregates recapitulating important features of TDP inclusions in ALS/FTLD-U, including hyperphosphorylation at previously reported serine403,404,409,410 residues, polyubiquitination and colocalization with optineurin. These models were used to address the pathogenic role of hyperphosphorylation in ALS/FTLD-U. We demonstrated that hyperphosphorylation and ubiquitination occurred temporally later than aggregation in cells. Expression of CK2α which phosphorylated TDP decreased the aggregation propensity of ND251 or ND207; this effect could be blocked by CK2 inhibitor DMAT. Mutation of serines379,403,404,409,410 to alanines (S5A) to eliminate phosphorylation increased the aggregation propensity and number of aggregates of TDP, but mutation to aspartic acids (S5D) or glutamic acids (S5E) to simulate hyperphosphorylation had the opposite effect. Functionally, ND251 or ND207 aggregates decreased the number of neurites of Neuro2a cells induced by retinoic acid or number of cells by MTT assay. S5A mutation aggravated, but S5E mutation alleviated these cytotoxic effects of aggregates. Finally, ND251 or ND251S5A developed aggregates in neurons, and salivary gland of transgenic Drosophila, but ND251S5E did not. Taken together, our data indicate that hyperphosphorylation may represent a compensatory defense mechanism to stop or prevent pathogenic TDP from aggregation. Therefore, enhancement of phosphorylation may serve as an effective therapeutic strategy against ALS/FTLD-U

Steady state of modal coupling instabilities as a dynamic energy equilibrium

Contact interaction between different parts of mechanical system is at the origin of phenomena known in literature as friction induced vibrations. In this paper friction induced unstable vibrations are analyzed by an energy point of view. The analysis is here developed on a polycarbonate disc, which is constrained on the external circumference and in frictional contact with a steel cylinder rotating at its inner circumference. In a classical approach the stability of the system can be evaluated by a complex eigenvalue analysis, performed with the commercial finite element code ANSYS. Then, the steady state of the system can be evaluated by a transient non linear analysis, performed with the explicit finite element code PLAST2D. Power flows are analyzed during the transient response to observe the effect of the local friction coefficient and the boundary conditions on the different terms of the energy balance