Andrzej Katunin

Damage assessment in composite plates using fractional wavelet transform of modal shapes with optimized selection of spatial wavelets

Damage assessment is one of the crucial topics in the operation of structural elements made of polymers and polymeric composites. From the wide range of diagnostic methods and techniques the vibration-based damage evaluation seems to be effective, useful for application in industrial conditions and the low-cost. In order to improve the sensitivity of such class of methods the advanced signal processing techniques should be applied.` One of such techniques is the wavelet transform, which ensures the high sensitivity to the singularities in the measured vibration signals. The paper deals with a method for damage detection and localization in composite plates using the fractional discrete wavelet transform applied to the displacements of the modal shapes. The optimized selection of the parameters of spatial wavelets makes possible to improve the sensitivity of a proposed method. The presented studies cover the discussion on the various types of heuristic optimization algorithms that can be used for searching the best values of parameters of the fractional wavelet transform. The problem of selection of the most effective optimization algorithm, which allows to find the most suitable parameters of wavelets for the structural diagnostics of composite plates, is also considered. The method was tested on the data obtained from numerical experiments with various types of simulated single and multiple damages. Case studies were also conducted experimentally using non-destructive and non-invasive measurements of the displacements of modal shapes. The main advantages, difficulties and limitations of the presented method were discussed.

Influence of heating rate on evolution of dynamic properties of polymeric laminates

Abstract The most common description of the mechanical behaviour of viscoelastic materials is presented using complex dynamic properties. These properties depend on many external factors (temperature, excitation frequency, etc.) and determine the evolution of derivative effects in the structure. The purpose of the conducted investigations is the determination of the influence of heating rate on the dynamic mechanical behaviour of a polymer based composite material under different excitation frequencies and temperatures. The glass transition temperatures and the activation energies of the phase transition were calculated, which allow extending the Arrhenius equation by the heating rate factor. The obtained dependencies could be useful for the estimation of the mechanical and thermal degradations of polymeric composites and, among others, for the determination of the self-heating temperature distributions and their evolution during cyclic loading. The presented results could also be applied for the determination of fatigue, crack growth and residual life of composite structures.

Thermal fatigue of polymeric composites under repeated loading

The paper deals with results of experimental study of thermal fatigue of polymeric composites caused by self-heating effect under repeating loading conditions. The tests were carried out on laboratory stand, which provides synchronization of all measured parameters. It was noticed that the process of thermal fatigue consists of three phases and the end of the second phase coincides with critical self-heating temperature, which causes rapid degradation until breakdown. During experiments, composite specimens were loaded for achieving a given temperature value, then cooled and loaded again. Based on experimental results, it was shown that the reloading caused cross-linking process, which causes hardening of a composite. The parametric analysis of self-heating curves was carried out for single-, double- and triple-loaded specimens, and fatigue life for investigated cases was determined and compared.

A Concept of Thermographic Method for Non-Destructive Testing of Polymeric Composite Structures Using Self-Heating Effect

Traditional techniques of active thermography require an external source of energy used for excitation, usually in the form of high power lamps or ultrasonic devices. In this paper, the author presents an alternative approach based on the self-heating effect observable in polymer-based structures during cyclic loading. The presented approach is based on, firstly, determination of bending resonance frequencies of a tested structure, and then, on excitation of a structure with a multi-harmonic signal constructed from the harmonics with frequencies of determined resonances. Following this, heating-up of a tested structure occurs in the location of stress concentration and mechanical energy dissipation due to the viscoelastic response of a structure. By applying multi-harmonic signal, one ensures coverage of the structure by such heated regions. The concept is verified experimentally on artificially damaged composite specimens. The results demonstrate the presented approach and indicate its potential, especially when traditional methods of excitation with an external structure for thermographic inspection cannot be applied.

Nondestructive Damage Assessment of Composite Structures Based on Wavelet Analysis of Modal Curvatures: State-of-the-Art Review and Description of Wavelet-Based Damage Assessment Benchmark

The application of composite structures as elements of machines and vehicles working under various operational conditions causes degradation and occurrence of damage. Considering that composites are often used for responsible elements, for example, parts of aircrafts and other vehicles, it is extremely important to maintain them properly and detect, localize, and identify the damage occurring during their operation in possible early stage of its development. From a great variety of nondestructive testing methods developed to date, the vibration-based methods seem to be ones of the least expensive and simultaneously effective with appropriate processing of measurement data. Over the last decades a great popularity of vibration-based structural testing has been gained by wavelet analysis due to its high sensitivity to a damage. This paper presents an overview of results of numerous researchers working in the area of vibration-based damage assessment supported by the wavelet analysis and the detailed description of the Wavelet-based Structural Damage Assessment (WavStructDamAs) Benchmark, which summarizes the author’s 5-year research in this area. The benchmark covers example problems of damage identification in various composite structures with various damage types using numerous wavelet transforms and supporting tools. The benchmark is openly available and allows performing the analysis on the example problems as well as on its own problems using available analysis tools.

Electrical percolation in composites of conducting polymers and dielectrics

Abstract This article deals with the electrical conductivity of a composite of two polymers, one of which is a conducting polymer, whereas the second is a dielectric. The problem was formulated within the framework of electrical percolation, i.e., the percolation thresholds, which allow for a high electrical conductivity, is under investigation. For this purpose, a numerical model was developed, and its parameters were analyzed and discussed. Based on the determined thresholds, it was possible to evaluate the weight ratios of the conducting-dielectric polymers in a composite. The proposed approach allows for reducing the manufacturing cost of composite material with respect to conducting polymers with simultaneous retaining of high conductance properties of conducting polymers, as well as durability and flexibility of dielectrics.

The conception of the fatigue model for layered composites considering thermal effects

Polymer-based layered composites are widely used in many responsible applications, such as turbine blades, helicopter propellers etc. Therefore its behaviour must be predictable in different conditions and operational states. Dynamic excitation of polymer composites obliges to consider it as a viscoelastic material, which introduce additional phenomena, e.g. self-heating effect. This effect occurred into two states: steady and non-steady. The strain of the structure subjected to the self-heating could be evaluated basing of DMA measurements and using Arrhenius kinetic concept together with time-temperature superposition principle. For obtaining the fatigue model the pseudo-strain approach was used. The fatigue model was based on damage function with taking into consideration self-heating effect.

Modeling and simulation of longwall scraper conveyor considering operational faults

Abstract The paper provides a description of analytical model of a longwall scraper conveyor, including its electrical, mechanical, measurement and control actuating systems, as well as presentation of its implementation in the form of computer simulator in the Matlab®/Simulink® environment. Using this simulator eight scenarios typical of usual operational conditions of an underground scraper conveyor can be generated. Moreover, the simulator provides a possibility of modeling various operational faults and taking into consideration a measurement noise generated by transducers. The analysis of various combinations of scenarios of operation and faults with description is presented. The simulator developed may find potential application in benchmarking of diagnostic systems, testing of algorithms of operational control or can be used for supporting the modeling of real processes occurring in similar systems.

Concept of a Conducting Composite Material for Lightning Strike Protection

Abstract The paper focuses on development of a multifunctional material which allows conducting of electrical current and simultaneously holds mechanical properties of a polymeric composite. Such material could be applied for exterior fuselage elements of an aircraft in order to minimize damage occurring during lightning strikes. The concept introduced in this paper is presented from the points of view of various scientific disciplines including materials science, chemistry, structural physics and mechanical engineering with a discussion on results achieved to-date and further plans of research.

Structural Diagnostics of Composite Beams Using Optimally Selected Fractional B-spline Wavelets

The method of structural diagnostics of composite structures presented in this paper is based on discrete wavelet transform of displacements of modal shapes with usage of fractional B-spline wavelets. An application of such wavelets makes possible to improve the sensitivity of the method, which allows for detection and identification of even small damages occurred in the structure. In order to select the most sensitive wavelet bases for the analysis the optimization study was carried out, where the highest ratio of peak, which indicated the damage, to other values of detail coefficients and possibly short support of a peak in the location of the damage were chosen as the optimization criteria. Obtained results allow for the evaluation of the possible best wavelet bases for the damage identification of one-dimensional composite structures.