Microwave near-field nondestructive detection and characterization of disbonds in concrete structures using fuzzy logic techniques

This paper presents a near-field microwave nondestructive testing technique for disbond/crack detection and evaluation in a concrete structure backed by an infinite half space of any material. A model describing the interaction of waves radiated out from an open-ended rectangular waveguide, in the near-field, with any layered medium will be utilized. The theoretical model calculates the effective reflection coefficient of the structure, at the aperture of the waveguide, as a function of the frequency of operation, the thickness and dielectric properties of the layers of the structures, including the standoff distance. The frequency of operation and standoff distance (the measurement parameters) can be optimized to achieve maximum sensitivity to the presence of the disbond. The presence of a disbond in a structure is viewed as an additional layer and will change the properties of the effective reflection coefficient (phase and magnitude). This change will depend on the thickness and location of the disbond. This fact will be used to investigate the potential of utilizing multiple frequency measurements to obtain disbond location and thickness information. A fuzzy logic model relating the phase of reflection coefficient, frequency of operation, and standoff distance to the disbond thickness and depth was generated and utilized.     

Estimating the depth and width of arbitrarily-oriented disbonds in laminates using shearography

Disbonds in a laminated plate are readily revealed from anomalies in the fringe pattern of a shearogram. The shearographic fringes represent loci of constant displacement derivatives of the deformed surface of the plate when subjected to a load increment such as vacuum stressing. In this investigation, a simple method is developed for easy estimation of the size and depth of arbitrarily-oriented square disbonds in laminated plates from a shearogram. The theory and experimental method presented may also be extended to the assessment of disbonds of arbitrary shapes.     

Cellular composites in lightweight sandwich applications

A novel sandwich element without separate joining layers as bonding between core and fibre reinforced face sheets is proposed. The core consists of a syntactic foam on the basis of cellular glass granules. Because of the cellular composite core and the proposed manufacturing process single as well as double curved sandwich elements can be produced. Experimental investigations show the significantly improved properties of such sandwich elements and the novel core material.     

Fracture analysis of facesheets in sandwich composites

There exist two fracture mechanisms in facesheets of sandwich composites consisting of the 0° and 90° plies, namely crack growth and crack blocking. The former is undesired since it may lead to failure of facesheets and even the core in sandwich composites. A shear-lag model is developed in this article and it gives a simple criterion governing these two mechanisms. It is established that, for a given ratio E_t/E_f of the elastic moduli in the transverse and fiber directions, there exists a critical facesheet thickness above which crack blocking is achieved and crack growth is prevented.     

Enhanced sandwich structure of powder-based composites

The enhanced sandwich structure of powder-based composite with relatively high volume fraction of powders composed of fine densified skins and periodically localized densified core-material for improving their mechanical properties and formability is presented. The structurized composites are fabricated by rolling process with the aid of a novel dynamic three-dimensional rolling technique. This methodology with various actuating phases exhibits different characteristics of forming schemes to fabricate desired structure of composites and provides assorted effects on the forming properties. Consequently, the enhanced sandwich structures of the composite sheet enable the tensile strength and the failure strain to be controllable and improved significantly. In addition, the bendability and the flowability of the composite sheets meaningfully related to their formability are greatly enhanced.     

Testing and analysis of ultra thick composites

For the development of a composite main landing gear fitting in carbon fiber reinforced plastics the behavior and performance of Ultra Thick Laminate components is investigated. Material thicknesses exceeds 60 mm. For the purpose of validation a test program is arranged using T-cross sections subjected to multiple load cases. The components are manufactured entirely with non crimped fabrics (NCF) using an adapted open mould manufacturing process. In addition to these T-Sections large full scale subcomponents of the entire fitting are manufactured and tested. As main topic of this paper standard FE methods are investigated and validated for thick structures using the generated test results. Due to the presence of transverse shear and normal stresses a 3D modeling approach is chosen. Transverse shear and normal stresses are indentified as main failure cause and failure is mainly initiated in the curved regions. Solid composite brick elements offer an efficient way to model thick structures. These are incapable of calculating accurate shear stresses on a ply level; usable results are however achieved by discretisation of the component with multiple elements over thickness. In addition stress gradients in the failure region are small; stress variations on a ply level are minimal. Out of plane material properties are not available and initial assumptions are made. Material correction factors (degradation) are introduced and discussed.     

Hybrid processing of thick skins for honeycomb sandwich structures

Carbon-epoxy prepregs are generally used to form the skins of honeycomb sandwich structures used in aerospace or racing yachts. For some applications, it is desirable to increase the thickness of the skins. In order to achieve an ideal core pressure level during cure for maximal skin-core bonding, the issue of air extraction from the honeycomb cells through the skin during processing thus becomes critical, in particular if vacuum only processing is used. In the present work, partially impregnated prepregs, called semipregs, having high initial transverse permeability to air, are combined with traditional prepregs to form a hybrid skin. Results are presented on the pressure change inside the honeycomb cells and the skin permeability to air during cure, as well as on skin-core adhesion. The final sandwich quality is assessed and found to be comparable to that obtained with prepreg skins.     

Measurement of coating physical properties and detection of coating disbonds by time-resolved infrared radiometry

This paper describes the principles and applications of time-resolved infrared radiometric (TRIR) imaging to characterization of coating systems. Examples are given of its application to the measurement of coating properties such as thickness and thermal diffusivity and to the detection of regions of coating disbond. Results are shown for coatings of different thicknesses, for test specimens containing artificial disbonds, and for thermal barrier coating specimens exhibiting real disbonds. A theoretical model describing the time development of the surface temperature of a coating during step heating is presented and the experimental results show good agreement with this model. Methods for applying the technique for inspection of large areas of coating as would be required in a process control or in service inspection environment are discussed and examples of parallel data acquisition using line heating sources are presented.     

Experimental determination of torsion and shear properties of sandwich panels and laminated composites by the plate twist test

A recently developed sandwich plate twist test is employed here for determination of the transverse shear modulus of the core and twist stiffness (D₆₆) of a sandwich panel consisting of a soft (H45 PVC foam) core and glass/vinylester face sheets. The shear modulus of the H45 PVC foam core extracted from the twist test was in good agreement with shear modulus obtained from ASTM plate shear testing of the foam core. D₆₆ values obtained from the sandwich twist test were in good agreement with predictions from classical laminated plate theory. In addition, the twist test was used to determine the in-plane shear modulus of glass/vinylester laminates isolated and as face sheets in sandwich panels with a stiff (plywood) core. The in-plane shear modulus of the face sheets, isolated and as part of a sandwich panel, was in good agreement with shear modulus determined using the Iosipescu shear test. The results point to the potential of the twist test to determine both in-plane and out-of-plane shear moduli of the constituents of a sandwich structure, as well as D₆₆.     

Compression properties of fire-damaged polymer sandwich composites

The effect of fire-induced damage on the edgewise compression properties of polymer sandwich composites is investigated. Fire tests were performed using a cone calorimeter on sandwich composites with high or low flammability. The highly flammable composite had a poly(vinyl chloride) foam core, while the flame resistant composite had a phenolic foam core. The residual edgewise compression properties of the burnt composites were determined after fire testing at room temperature. The compression stiffness and strength of the two sandwich composites decreased rapidly with increasing heat flux and heating time of the fire due to thermal decomposition of the face skin and foam core. A large reduction to the edgewise compression properties of the phenolic-based sandwich composite occurred despite having good flame resistance, and the reasons for this are described. Preliminary analytical models are presented for estimating the edgewise compression failure load of fire-damaged sandwich composites that fail by core shear or buckling.     

Dynamic analysis of sandwich cement-based piezoelectric composites

Theoretical analysis of a sandwich cement-based piezoelectric composite is presented based on the theory of piezo-elasticity. The steady-state responses of two kinds of this composite under different loading cases are obtained by the use of displacement method. The effects of piezoelectric phases on the performance of this kind of devices are simulated and discussed. The solutions are compared with both the numerical and experimental results, and good agreements are found. Sandwich cement-based piezoelectric composites have great application potential in civil structure health monitoring. The results obtained in this paper are beneficial to the design of this kind of smart devices.     

Micromechanics of local viscoelastic buckling in thick composites

The time-dependent, local microbuckling of multilayered viscoelastic media is modeled with emphasis on bifurcation and imperfection sensitivity. The instability or ‘growth-of-waviness’-type analysis is carried out for initially straight fibers (bifurcation), as well as for initially wavy fibers (imperfection-sensitivity), assuming a perfect-slip condition at the fiber-matrix interface. The concept of dominant wavelength (i.e. the fastest growing wavelength within the Fourier spectrum), previously defined for viscoelastic bifurcation (Biot, M. A. ‘Mechanics of Incremental Deformations’, John Wiley and Sons, Inc., New York, 1965), is extended from the single fiber analysis (Bhalerao, M. and Moon, T., On the growth-of-waviness in fiber-reinforced polymer composites: viscoelastic bifurcation and imperfection sensitivity, ASME J. Appl. Mechanics, in press, 1995) to multiple fiber analysis for a multilayered medium. A parametric study is presented which covers a wide range of physically relevant values. The results for the multiple fiber analysis are found to be analogous to those for single fiber analysis. The bifurcation dominant wavelength depends negligibly on matrix properties, yet highly on the applied load. On the other hand, the imperfection-sensitive dominant wavelength is predicted to depend strongly on matrix properties, while negligibly on load. The imperfection-sensitive wavelength is the one important in composites processing and in-service behavior.     

Accurate buckling load calculations of a thick orthotropic sandwich panel

This paper is concerned with the buckling of thick sandwich panels with orthotropic elastic face sheets bonded to a linear elastic orthotropic core. When such panels are analyzed for axial load carrying capacity, it is now commonplace to adopt the finite element method to carry out computations. The accuracy of the numerical results will depend not only on roundoff and algorithmic errors, but additionally on the approximations made in computing the incremental (second order) work associated in computing the change of configuration from the unbuckled to the buckled state. Here we show that, particularly for orthotropic thick sandwich structures, large errors can be incurred in computing buckling loads with available commercial software, unless the proper work conjugate measures of stress and strain with their stress-dependent tangential moduli are used in the buckling formulation.     

Localised formulations for thick “sandwich” laminated and composite structures

A “sandwich” panel under axial compressive loading is studied over its post-buckling range. Alternative formulations were developed to cover solutions other than the deflection shape given by the classical Rayleigh-Ritz analysis. General elasticity principles are used to derive the total potential energy function in terms of the displacement field. Given a constant modal amplitude displacement field, which is introduced in the total potential energy function, the results are discussed having as background models studied earlier. Techniques for the analysis of localized solutions are presented with reference to the study of the beam on nonlinear elastic foundation. The concepts of localization are introduced by using dynamical analogy and the double scale perturbation method and applied to the sandwich panel, to derive the governing amplitude modulation equations. The resulting formulations are discussed using some examples, and by solving analytical and numerically the amplitude differential equations.     

夹层结构复合材料低速冲击试验与分析

设计了聚甲基丙烯酰亚胺(PMI)泡沫、 交联聚氯乙烯(X-PVC)泡沫、 NOMEX蜂窝、 缝合PMI以及开槽PMI泡沫等形式的玻璃布面板夹层结构复合材料, 研究了芯材种类和厚度、 面板玻璃布层数以及缝合和开槽等因素对夹层结构低速冲击性能的影响。结果表明, PMI泡沫芯较X-PVC泡沫芯和NOMEX蜂窝芯具有更高的冲击破坏载荷和吸收能量。随着泡沫密度及面板厚度的增加, 夹层结构复合材料的冲击破坏载荷和破坏吸收能量增大。合理的缝合和开槽, 能够增加PMI泡沫夹层结构的强度、 刚度及界面性能, 提高冲击承载能力。     

Neural-fuzzy optimization of thick composites curing process

This article addresses the optimization of curing process for thick composite laminates. The proposed methodology aims at the evaluation of the thermal cycle promoting a desired evolution of the degree of cure inside the material. At the same time, temperature overshooting as well as excessive temperature and cure degree gradient through the thickness of the material are prevented. The developed approach is based on the integrated application of artificial neural networks and a fuzzy logic controller. The neural networks promptly predict the behavior of composite material during curing process, while the fuzzy logic controller continuously and opportunely adjusts the proper variations on the imposed thermal cycle. The results highlighted the efficiency of the method in comparison with the cure profiles dictated by the material suppliers. For thick laminates, a reduction of 35% of cure time and improvements of approximately 10% of temperature overshooting was obtained compared to conventional curing cycles. The method was validated by experimental tests.