Triple correlation for detection of damage-related nonlinearities in composite structures

Nonlinear effects in vibration responses are investigated for the undamaged composite plate and the composite plate with a delamination. The analysis is focused on higher harmonic generation in vibration responses for various excitation amplitude levels. This effect is investigated using the triple correlation technique. The dynamics of composite plate was modelled using two-dimensional finite elements and the classical lamination theory. The doubled-node approach was used to model delamination area. Mode shapes and natural frequencies were estimated based on numerical models. Next, the delamination divergence analysis was used to obtain relative displacements for delaminated plies. Experimental modal analysis test was carried out to verify the numerical models. The two strongest vibration modes as well as two vibration modes with the smallest and largest motion level of delaminated plies were selected for nonlinear vibration test. The Fisher criterion was employed to verify the effectiveness and confidence level of the proposed technique. The results show that the method can be used not only to reveal nonlinearities, but also to reliably detect impact damage in composites. These results are confirmed using the statistical analysis

Identification and Modeling of a Variable Amplitude Fatigue Experiment Apparatus with Damaged Beam Specimen

The useful remaining life of engineering structures under variable amplitude (VA) fatigue loading remains a major unresolved engineering problem. The existing proposed life prediction models are usually based on empirical approximation from experimental results (Fatemi, Yang Int J Fatigue 20(1):9–34, 1998, Santecchia et al. Adv Mater Sci Eng 2016:1–26, 2016). The variable fatigue experiment apparatus in this extended abstract was designed for simulating structural fatigue with a high testing frequency, variable R-ratio as well as modifiable experimental layout (Falco et al. J Vib Acoust 136(4):041001, 2014). In previous studies, the inherent nonlinearity of the testing rig was detected, the obtained parameters allow one to properly use this testing rig within its linear region. As damage accumulates, however, the corresponding dynamic characteristics of the specimen alter accordingly. Therefore, proper modeling considering the interaction between the inherent nonlinearity and the damage induced nonlinearity for both (1) opening crack and (2) breathing crack is necessary for future fatigue life estimation under complex fatigue loading. Here, nonlinear system identification of the lately modified variable amplitude fatigue experiment apparatus is presented based on a combination of first-principles and data-driven modeling techniques. Eventually, structure-damage interaction dynamics will be described to model the underlying fatigue evolution and structural dynamics interactions

Study of ultrathin Pt/Co/Pt trilayers modified by nanosecond XUV pulses from laser-driven plasma source

We have studied the structural mechanisms responsible for the magnetic reorientation between in-plane and out-of-plane magnetization in the (25 nm Pt)/(3 and 10 nm Co)/(3 nm Pt) trilayer systems irradiated with nanosecond XUV pulses generated with laser-driven gas-puff target plasma source of a narrow continuous spectrum peaked at wavelength of 11 nm. The thickness of individual layers, their density, chemical composition and irradiation-induced lateral strain were deduced from symmetric and asymmetric X-ray diffraction (XRD) patterns, grazing-incidence X-ray reflectometry (GIXR), grazing incidence X-ray fluorescence (GIXRF), extended X-ray absorption fine structure (EXAFS) and transmission electron microscopy (TEM) measurements. In the as grown samples we found, that the Pt buffer layers are relaxed and that the layer interfaces are sharp. As a result of a quasi-uniform irradiation of the samples, the XRD, EXAFS, GIXR and GIXRF data reveal the formation of two distinct layers composed of Pt1-xCox alloys with different Co concentrations, dependent on the thickness of the as grown magnetic Co film but with similar ∼1% lateral tensile residual strain. For smaller exposure dose (lower number of accumulated pulses) only partial interdiffusion at the interfaces takes place with the formation of a tri-layer composed of Co-Pt alloy sandwiched between thinned Pt layers, as revealed by TEM. The structural modifications are accompanied by magnetization changes, evidenced by means of magneto-optical microscopy. The difference in magnetic properties of the irradiated samples can be related to their modification in Pt1-xCox alloy composition, as the other parameters (lateral strain and alloy thickness) remain almost unchanged. The out-of-plane magnetization observed for the sample with initially 3 nm Co layer can be due to a significant reduction of demagnetization factor resulting from a lower Co concentration

Role of heat accumulation in the multi-shot damage of silicon irradiated with femtosecond XUV pulses at a 1 MHz repetition rate

The role played by heat accumulation in multi-shot damage of silicon was studied. Bulk silicon samples were exposed to intense XUV monochromatic radiation of a 13.5 nm wavelength in a series of 400 femtosecond pulses, repeated with a 1 MHz rate (pulse trains) at the FLASH facility in Hamburg. The observed surface morphological and structural modifications are formed as a result of sample surface melting. Modifications are threshold dependent on the mean fluence of the incident pulse train, with all threshold values in the range of approximately 36-40 mJ/cm². Experimental data is supported by a theoretical model described by the heat diffusion equation. The threshold for reaching the melting temperature (45 mJ/cm²) and liquid state (54 mJ/cm²), estimated from this model, is in accordance with experimental values within measurement error. The model indicates a significant role of heat accumulation in surface modification processes

Identyfikacja parametrów modalnych układów niestacjonarnych z wykorzystaniem adaptacyjnego filtru falkowego oraz rekursywnyego algorytmu najmniejszych kwadratów

The paper presents a method of modal parameter estimation based on RLS (Recursive Least Square) algorithm, and wavelet filtering. The wavelet filtering gives possibility to decoupling frequency components of signal response of structure. This operation can also reduce the order of the signal model estimated by RLS algorithm. An additional advantage of this method is the possibility of adapting the wavelet filter parameters to the changing parameters of the system. Reduced model order significantly reduces the time of estimation of modal parameters, which enables the real – time implementation of the method. Due to recursively updated covariance matrix of model parameters, the confidence intervals of modal parameters can be also estimated. All routines have been implemented and tested in MATLAB®. The method have been tested on simulated data delivered by an AIRBUS team and on the test bed with a variable stiffness.Artykuł prezentuje metodę estymacji parametrów modalnych bazująca na algorytmie RLS (RLS (Recursive Least Square) oraz filtracji falkowej. Filtracja falkowa daje możliwość separacji składników częstotliwościowych sygnału. Ta operacja redukuje rząd modelu estymowanego przez algorytm RLS. Dodatkowa zaletą algorytmu jest możliwość adaptacji parametrów filtru falkowego do zmieniających się parametrów układu. Redukcja modelu znacznie skraca czas estymacji parametrów modalnych. Umożliwia to implementację algorytmu w czasie rzeczywistym. Dzięki rekursywnemu uaktualnianiu macierzy kowariancji parametrów modelu estymowane są również przedziały ufności otrzymanych wyników. Wszystkie procedury zostały zaimplementowane w środowisku MATLAB. Metodę przetestowano dla danych symulacyjnych (model samolotu dostarczony przez firmę AIRBUS), oraz dla układu ze zmienna sztywnością

Zastosowanie transformaty falkowej do badania własności dynamicznych układów nieliniowych

In the paper an application of time - frequency technique for nonlinearities detection is presented. The wavelet transform is used to transform an impulse response of the system into time - scale domain. Ridge and skeleton definitions of wavelet transform for backbone curve estimation has been used. Statistical approach for ridge detection of wavelet transform has been applied. Estimation of dynamic characteristic of the system with use of envelope of signal response has been presented. Based on properties of wavelet transform possibility of use the method for nonlinearities detection has been shown. The method also make possible define the nature of this nonlinearities. MATLAB package is used as numerical tool for computation of wavelet transform and dynamical characteristics of the system. The algorithm has been tested on simulated data and data from test bed with dry friction.W artykule przedstawiono metodę detekcji nieliniowości układów dynamicznych opartą o reprezentację czasowo - częstotliwościową sygnału, bazującą n przekształceniu falkowym. Do określenia charakterystyk dynamicznych wykorzystano definicję grzbietu i szkieletu transformaty falkowej. Pozwoliło to na wyznaczenie funkcji obwiedni sygnału. Algorytm został zweryfikowany z użyciem pakietu MATLAB na danych symulacyjnych oraz pochodzących ze stanowiska laboratoryjnego

Spectrum estimation for diagnostic purposes with the use of wavelet transform

W artykule przedstawiono koncepcję estymacji widm sygnałów o wysokim poziomie zakłóceń i szumów generowanych przez procesy niestacjonarne, z wykorzystaniem transformaty falkowej i falką Morleta. Przedstawiono wyniki uzyskane dla danych symulowanych i rzeczywistych przebiegów sygnałów akustycznych, charakteryzujących się złożonym widmem.In this paper the problem concerning estimation of a spectrum of signals generated by non-stationary processes, with high level of extraneous noise is presented. The continuous wavelet transform has been used to eliminate the noise. The results of analysis of data collected from a simulated process as well as real signals, having rich spectrum, were presented and discussed

Analysis and research of polymer-gypsum mortar with increased flexibility

Intensywne poszukiwania kompozytów i mieszanin polimerowych wynikają z potrzeby otrzymania nowych materiałów o polepszonych właściwościach lub specjalnych cechach użytkowych. Do gipsu lub cementu mogą być wprowadzane różne napełniacze. Z uwagi na to, że gips i cement są materiałami wiążącymi, powszechnie stosowanymi w budownictwie, w modelarstwie, stomatologii, chirurgii i sztukatorstwie, mogą one wpływać na poprawę wybranej cechy lub określonych właściwości wyrobu. Zaprawy gipsowe mają określoną wytrzymałość mechaniczną, którą można oznaczyć podczas badań zginania i ściskania lub wyznaczając czas wiązania (twardnienia). W celu rozszerzenia przydatności spoiw gipsowych do zastosowań w budownictwie, podjęto próbę zmiany ich wytrzymałości na zginanie oraz przyspieszenie czasu wiązania, poprzez zastosowanie domieszek polimerowych. Domieszki polimerowe w postaci regranulatu polietylenowego oraz regranulatu gumy wprowadzono w różnych proporcjach do mieszaniny z gipsem.Intensive research for composites and polymer blends arising from the need to obtain new materials with improved properties and special functional properties. Accessories made of recycled materials or rubber can be incorporated into a variety of materials, including gypsum or cement. Due to the fact that these binding materials are commonly used in construction, scale-modeling, stomatology, surgery and stucco work they can affect the improvement of selected features or specific properties of parts. Gypsum mortars have a certain mechanical strength which can be adjusted during bending and compression tests or by determining the setting time (hardening). In order to expand the usefulness of gypsum binders for their application in the construction industry, an attempt has been made to increase the value of selected indicators of strength and to shorten setting time due to the use of polymer additives. Polymer admixtures in the form of rubber granulate and polyethylene granulate have been added in different proportions into mixtures with gypsum

Zastosowanie transformaty falkowej do identyfikacji parametrów modalnych układów niestacjonarnych

In the paper an application of a time-frequency signal analysis technique for modal parameters identification is presented. Procedures of wavelet filtering, which allow estimating mechanical parameters for systems with non-constant parameters have been shown. The wavelets are used to detect natural frequency of the system and transform system time response into the time-scale domain. Presented method has been verified on simulation data for a two degrees of freedom discrete system. Two cases have been considered, with a constant and with a varying damping. The procedure has been applied to a real data recorded during a flight of ISKRA airplane.W artykule przedstawiono możliwość zastosowania transformaty falkowej do identyfikacji parametrów modalnych układów mechanicznych. Pokazano procedury filtracji falkowej pozwalającej na wyznaczanie parametrów dynamicznych układów wykazujących niestacjonarność. Metodę zweryfikowano na danych symulacyjnych dla układu o dwóch stopniach swobody ze zmiennym tłumieniem. Opracowaną metodę zastosowano do badania zmian tłumienia w czasie lotu samolotu ISKRA