A wave-based numerical scheme for damage detection and identification in two-dimensional composite structures

Previous studies on wave inspection in different propagation directions have focussed on the analysis of wave propagation and wave scattering from various types of joints in two-dimensional monolayered structures. In this work, a Finite Element (FE) based numerical scheme is presented for quantifying wave interaction with localised structural damage within two-dimensional layered composite structures having arbitrary layering, complexities and material characteristics. The scheme discretise a damaged structural medium into a system of N healthy substructures (waveguides) connected through a joint which bears the localised structural damage/discontinuity. Wave propagation constants along different propagation directions of the substructures are sought by combining Periodic Structure Theory (PST) and the FE method. The damaged joint is modelled using standard FE approach, ensuring joint-substructures mesh conformity. This is coupled to the obtained wave propagation constants in order to determine scattering coefficients for the wave interaction with damage in different propagation directions within the structure. Wave interaction coefficients for different damage types and structural parameters are analysed in order to establish an optimum basis for detecting and identifying damage, as well as assessing the orientation and extent of the detected damage. The main advantage of this scheme is precise predictions at a very low computational cost

Mid-frequency band gap performance of sandwich composites with unconventional core geometries

In this work novel unconventional core architectures are presented which are able to induce flexural band gaps while not being detrimental for structural bending stiffness of the sandwich structures. Two different core schemes are examined with both of them exhibiting low-frequency stop bands. While unconventional, the designs of the core offer a novel solution which can be easily manufactured in high volume parts using two-dimensional automated cutting machine. A hybrid finite element and periodic structure theory scheme is employed for the calculation of the stiffness and mass matrices, and periodic structure theory is used to obtain the wave propagation of the beams. Having acquired the wave dispersion curves and the finite element analysis' results, two specimens are manufactured using carbon fibre cured plates and commercially available PVC foam as core material. Experimental measurements of the dynamic performance of the structures are conducted using a laser vibrometer and electrodynamic shaker setup

Thermal effect on wave interaction in composite structures

There exist a wide range of failure modes in composite structures due to the increased usage of the structures especially in aerospace industry. Moreover, temperature dependent wave response of composite and layered structures have been continuously studied, though still limited, in the last decade mainly due to the broad operating temperature range of aerospace structures. A wave finite element (WFE) and finite element (FE) based computational method is presented by which the temperature dependent wave dispersion characteristics and interaction phenomenon in composite structures can be predicted. Initially, the temperature dependent mechanical properties of the panel in the range of -100◦C to 150◦C are measured experimentally using the Thermal Mechanical Analysis (TMA). Temperature dependent wave dispersion characteristics of each waveguide of the structural system, which is discretized as a system of a number of waveguides coupled by a coupling element, is calculated using the WFE approach. The wave scattering properties, as a function of temperature, is determined by coupling the WFE wave characteristics models of the waveguides with the full FE modelling of the coupling element on which defect is included. Numerical case studies are exhibited for two waveguides coupled through a coupling element

Phytosociological study of Hirschfeldia incana (L.) Lagraze-Fossat (Cruciferae) communities in mainland Greece

Using numerical analysis, the phytosociological study of Hirschfeldia incana communities in mainland Greece allowed their classification into the Rapistro rugosi-Hirschfeldietum incanae ass. nov., a new subnitrophilous association of the Hordeion leporini alliance. Three subassociations were distinguished (anthemidetosum incrassatae, hedypnoidetosum creticae and cardarietosum drabae), the distribution of which seems to depend on latitudinal alteration of rainfall. The new association has its optimum growth in habitats with moderate human influence, specifically in abandoned cultivations and wastelands. With respect to its floristic composition, the Rapistro rugosi-Hirschfeldietum incanae is close to anthropogenic vegetation with a high degree of naturalness, particularly to the therophytic, subnitrophilous vegetation of the Thero-Brometalia (Stellarietea mediae) and the perennial, subnitrophilous vegetation of Carthametalia lanati (Artemisietea vulgaris)

Systematic analysis of the evolution of electricity and carbon markets under deep decarbonization

The decarbonization of electricity generation presents policy makers in many countries with the delicate task of balancing initiatives for technological change with a commitment to market liberalization. Despite the theoretical attractions, it has become doubtful whether carbon markets by themselves can offer the desired solution. We address this question through an integrated modeling framework, stylized for the Great Britain (GB) power market within the EU ETS, which includes three distinct components: (a) long-term least-cost capacity planning, similar in functionality to many used in policy analysis, but innovative in providing the endogenous calculation of carbon prices; (b) short-term price risk analysis producing hourly dispatch and pricing outputs, which are used to test the annual financial performance metrics implied by the longer-term investments; and (c) agent-based computational learning to derive pricing behavior in imperfect markets. The results indicate that the risk/return profile of electricity markets may deteriorate substantially as a result of decarbonization, reducing the propensity of companies to invest in the absence of increased government support and/or more beneficial market circumstances. If allowed, markets may adjust by deferring investment until conditions improve, consolidating to increase market power, or operating in a tighter market with reduced spare capacity. To the extent that each of these “market-led” solutions may be politically unpalatable, policy design will need to sustain a delicate regulatory regime, moderating the possible increased market power of companies while maintaining low-carbon subsidies for longer than expected. This paper considers some of the modeling implications for this compromise

The impact of temperature on wave interaction with damage in composite structures

The increased use of composite materials in modern aerospace and automotive structures, and the broad range of launch vehicles’ operating temperature imply a great temperature range for which the structures has to be frequently and thoroughly inspected. A thermal mechanical analysis is used to experimentally measure the temperature-dependent mechanical properties of a composite layered panel in the range of −100 ℃ to 150 ℃. A hybrid wave finite element/finite element computational scheme is developed to calculate the temperature-dependent wave propagation and interaction properties of a system of two structural waveguides connected through a coupling joint. Calculations are made using the measured thermomechanical properties. Temperature-dependent wave propagation constants of each structural waveguide are obtained by the wave finite element approach and then coupled to the fully finite element described coupling joint, on which damage is modelled, in order to calculate the scattering magnitudes of the waves interaction with damage across the coupling joint. The significance of the panel’s glass transition range on the measured and calculated properties is emphasised. Numerical results are presented as illustration of the work

Hyper-damping properties of a stiff and stable linear oscillator with a negative stiffness element

A simple, stiff, statically and dynamically stable linear oscillator incorporating a negative stiffness element is used as a template to provide a generic theoretical basis for a novel vibration damping and isolation concept. This oscillator is designed to present the same overall static stiffness, the same mass and to use the same damping element as a reference classical linear SDoF oscillator. Thus, no increase of the structure mass or the viscous damping is needed, as in the case of a traditional linear isolator, no decrease of the overall structure stiffness is required as in the case of ’zero-stiffness’ oscillators with embedded negative stiffness elements. The difference from these two templates consists entirely in the proper redistribution and reallocation of the stiffness and the damping elements of the system. Once such an oscillator is optimally designed, it is shown to exhibit an extraordinary apparent damping ratio, which is even several orders of magnitude higher than that of the original SDoF system, especially in cases where the original damping of the SDoF system is extremely low. This extraordinary damping behaviour is a result of the phase difference between the positive and the negative stiffness elastic forces, which is in turn a consequence of the proper redistribution of the stiffness and the damping elements. This fact ensures that an adequate level of elastic forces exists throughout the entire frequency range, able to counteract the inertial and the excitation forces. Consequently, a resonance phenomenon, which is inherent in the original linear SDoF system, cannot emerge in the proposed oscillator. The optimal parameter selection for the design of the negative stiffness oscillator is discussed. To further exhibit the advantages that such a design can generate, the suggested oscillator is implemented within a periodic acoustic metamaterial structure, inducing a radical increase in the damping of the propagating acoustic waves. The concept may find numerous technological applications, either as traditional vibration isolators, or within advanced composite materials and metamaterials

Electromagnetic shielding effectiveness of carbon fibre reinforced composites

This paper reports results on the shielding effectiveness parameter of laminated epoxy composites with carbon fibre reinforcements. Measurements of shielding effectiveness were carried out with a coaxial transmission line testing chamber according to ASTM 4935 standard and epoxy-matrix composites with continuous carbon-fibres were proven to be an excellent electromagnetic interference shielding material, where a composite slab made of 4 layers of prepregs provided more than 99.9% of electromagnetic attenuation. It was found that the reflection mechanism of the shielding material was mainly influenced by the fibre volume ratio, and that an increase in the number of layers of the composite resulted in higher shielding effectiveness due to a greater absorption mechanism. Calculations of the shielding effectiveness parameter of the material used were made by means of commercial electromagnetic simulation tools, having determined experimentally the overall resistivity of the composite. The findings presented in this work suggest that in presence of a greater number of interfaces at different impedance the separate modelling of matrix and fibres at mesoscopic scale must be taken into account

The impact of mesoscale textile architecture on the structural damping in composite structures

In this article, a method allowing for prediction of the effective structural damping given the mesoscale architecture of a composite is presented. The method enables a fast and accurate calculation of the loss factor as a function of the direction of the wave propagation and the frequency in any periodic textile composite. The scheme combines two reduction methods, the first being the wave and finite element method that employs periodic structure theory to reduce the size of the problem by allowing for modelling only a unit cell of a periodic structure, while the second reduction approach is a mode-based component mode synthesis that reduces the size of the dynamic stiffness matrix of a unit cell. The exhibited methodology allows for structural damping prediction and optimisation through judicious design of the textile architecture. Numerical examples several complex architectures are exhibited

Optimal internal pressurisation of cylindrical shells for maximising their critical bending load

AbstractThe paper studies the influence of internal pressure on circular thin-walled pipes (D/t>150) subjected to pure bending. Both straight pipes and curved pipes are analysed. Both yield and buckling failures are considered. It is shown that internal pressure decreases the limiting load for yield but increases the limiting load for buckling.The study is mainly FEA-based. A formula to predict critical moment given by linear buckling analysis is proposed. Comments on difference between linear and non-linear analysis results are given. It is shown that a pipe curvature opposite to the bending moment can increase the critical load. It is shown that cylindrical thin-walled shells have an optimal value of internal pressure to which limiting load for yield and critical buckling moment are equal, corresponding to an optimal use of material