Jordanka Ivanova

Aging and filler effects on the creep model parameters of thermoset composites

Abstract The effect of marble mineral filler on the creep response of polyester thermoset matrix before and after physical aging has been investigated. A number of composites with different filler content (0.29, 0.38, 0.45, 0.55) have been studied in order to understand the role of the filler for the creep response. A mathematical model has been chosen for interpreting the experimental results, its parameters having a definite physical sense and being related with the single components of deformation under creep conditions. This model represents the generalized equation of Maxwell for relaxing isotropic medium in the form presented by G.I. Gourevich (Maxwell–Gourevich equation). The influence of the filler and physical aging on the parameter values of the model has been discussed, respectively on the elastic and rubberlike elastic (inelastic) deformation. The possibilities of the proposed model to describe the creep response of the matrix and the composites before and after physical aging are shown.

Interfacial debonding of a piezoelastic bi-material structure applicable for wind rotor blades.

The main goal of the article is to model the straight line part of a wind blade by an analytical 1D shear lag model in order to analyze and detect possible interface delamination through the change of the voltage along the interface. A unit cell presented by a bi-material structure consisting of a first piezoelastic layer and an elastic second layer under static mechanical and electric load is considered. Numerical examples are provided and illustrated by figures and discussed. A criterion for detecting the interface elastic-brittle delamination and a safety zone for combined electrical and static loading is proposed.

Cracked bi-Material Plates under Thermomechanical Loading

A bimaterial structure composed of two elastic plates bonded together by an interface with a normal (transverse) crack in the first plate and subjected to monotonically temperature and tension loading is considered. The interface is assumed to exhibit brittle failure at critical shear stress value or progressive damage in a cohesive zone preceding delamination. Using modified Shear lag model, the analytical solution is provided specifying the length of debonding. The critical lengths of a partial debonding along the interface are calculated and the limit value of temperature at full debonding is obtained. The analytical predictions are compared with experimental data and numerical results of Lemaitre and Song. The comparison shows a good agreement and proves the validity of the model used.

Design sensitivity analysis for shape optimization by the BEM

Abstract In this paper the shape optimization problem for an inelastic case by the hybrid utilization of the BEM and design sensitivity analysis is examined. Gourevichs model for describing the inelastic behaviour of the solid is used. As a numerical example, the problem for determining the optimal shape of the free boundary of a hole in a rectangular elastic and inelastic metal plate unaxial tension is solved.

Interface Behavior of a Bimaterial Plate under Dynamic Loading

The paper deals with interface behavior of bimaterial ceramic-metal composites under dynamic time-harmonic load. The first plate is precracked with a normal crack touching the interface between the plates. It is assumed that the respective restriction for the ratio of energy release rates of the plates allowing the occurrence of an interface single delamination before the initiation of the normal crack in the second plate is satisfied. The growth of interface delamination is not considered. The used approximate shear-lag dynamic approach gives a possibility to obtain solutions in a closed form for axial and shear stresses of the structure. At an elastic-brittle interface behavior theoretical predictions for single debond length of two bimaterial structures are calculated. The parametric analysis reveals the sensitivity of the interface single debond length and shear stress to the type of bimaterial structure and to the characteristics of the dynamic load—in particular its frequency and amplitude. All results are illustrated in figures and tables and are discussed.

Influence of Interface Gap on the Stress Behaviour of Smart Single Lap Joints Under Time Harmonic Load

Abstract Adhesive joints are frequently used in different composite structures due to their improved mechanical performance and better understanding of the failure mechanics. The application of such structures can be seen in aerospace and high technology components. The authors developed and applied modified shear lag analysis to investigate the hygrothermalpiezoelectric response of a smart single lap joint at environmental conditions (with/without an interface gap along the overlap zone) and under dynamic time harmonic mechanical and electric loads. The main key is the study of the appearance of possible delamination along the interface. As illustrative examples, the analytical closed form solution of the structure shear and the axial stresses response, as well as the interface debond length, including influence of mechanical, piezoelectric, thermal characteristics and frequencies is performed and discussed. All results are presented in figures. The comparison of the shear stress and electric fields for both cases of overlap zone (continuous or with a gap) is also shown in figures and discussed.

THERMOELASTIC STRESS ANALYSIS FOR LINEAR ELASTIC BODIES

The stress analysis for elastic cylinders made of polymethylmethacrylate (PMMA) under cyclic loading is provided by using FEM. Under the assumption that the elastic moduli depend linearly on the temperature, new fomulae, generalizing the well known Kelvins formula, are given such that we can estimate the temperature variation on the external surface of a homogeneous ideal cylinder. Using suitable numerical methods we obtain theoretical results which fit the experimental values for the temperature variation

Instability of Thin Elastic and Elasto-Plastic Orthotropic Shells under Combined Static and Dynamic Loading

This chapter investigates the dynamic instability of thin elastic and elasto-plastic orthotropic shells under the action of combined loading by a static, load acting permanently and by a suddenly applied short time-duration dynamic impulse. The loading thus applied defines the problem as a dynamic one, due to the action of the dynamic impulse. To clarify the features of the shell response to such a combined load, we shall briefly discuss some theoretical and experimental studies.

Postcritical Deformations of Thin Anisotropic Shells

The purpose of this book is to extend the so-called “geometric method” to the local and global stability analysis of physically and geometrically nonlinear thin shells, subjected to static, dynamic, and combined loading. The results presented here should have a wide range of applications in engineering practices due to the widespread use of technologically new thin-walled structures, and the importance of necessary conditions for their stable performance.

Postcritical Deformations of Thin Elastic Anisotropic Shells with Linear Memory

The objective of this chapter is to apply the geometric method to nonlinear stability problems of anisotropic visco-elastic shells. An analytical-asymptotic solution of the corresponding differential equations of nonlinear stability problems is worth researching due to the wide practical use of shells made from new technological materials (polymers reinforced with glass fibres, etc.). For this purpose an analogue of the variational principle A for studying postcritical deformations of anisotropic visco-elastic shells will be derived. It is based on the geometric description of the shell postcritical shapes and on some physical concepts that preserve the anisotropy during postcritical deformations. The final form of the variational principle will be presented in section 2.2 in terms of Laplace transforms with respect to time of the deformation postcritical energy and work done by the external staticn load.